Advances in Heterogeneous Material Mechanics 2011
Advances in Heterogeneous Material Mechanics (ICHMM 2011)
Resolving Deformation Modes of Nanocrystalline Metals using Continuum Metrics Based on Atomistic Simulations
Deformation processes in nanocrystalline (NC) metals such as dislocation nucleation/absorption and grain boundary shuffling/sliding are mediated at interfaces. Grain boundaries and their junctions fundamentally control NC material inelastic deformation mechanisms. However, material deformation is inherently multiscale and quantifying failure criteria and scaling relations across multiple length/time scales has proven to be difficult. We apply recently developed volume-averaged kinematic variables from continuum mechanics as metrics to assess results of atomistic simulations. These metrics are computed using nearest neighbors and include kinematic quantities such as microrotation and dilatation. Useful insight into the origins of plastic deformation in NC metals is gained by their application, enabling resolution of the contributions of competing mechanisms to inelastic material deformation.
Continuum Modeling of Dislocation Plasticity: Theory, Numerical Implementation and Comparison to Discrete Dislocation Simulations
The development of advanced materials is driven by continuous progress in the synthesis and control of materials microstructure on sub-micrometer and nanometer scales. Confined to these length-scales, many materials show strikingly different physical properties from their bulk counterparts, like a strong increase in flow stress with decreasing size. This calls for an increased effort on physically motivated continuum theories which can predict size-dependent plasticity by accounting for length scales associated with the dislocation microstructure. An important recent development has been the formulation of a Continuum Dislocation Dynamics (CDD) Theory which provides a kinematically consistent continuum description of the dynamics of curved dislocation systems [1]. Here we present a brief overview of the CDD method and illustrate the implementation of the CDD by numerical examples, the bending of a thin film, the torsion of a wire, and the plastic flow around an elastic inclusion. Results are compared to three-dimensional discrete dislocation dynamics simulations.
Cyclic Deformation Response of ARBed Copper with Composite- natured Ultra-fine- Microstructures and its Comparison with Cyclic Deformation Response of ECAPed Copper
The composite microstructure found in ARBed materials has yielded cyclic softening and cyclic creep response, both of which, especially the former, have been attributed to the grain coarsening phenomenon. Surface damage due to shear banding was also found. Grain coarsening was located near and along the pre-existent shear bands; whereas the already coarsened grains from processing were found to accommodate some plastic strain. Comparison between ECAPed and ARBed copper, specifically the micro-mechanisms involved in accommodating cyclic plastic strain, will be carried out. Similarities between the two types of materials are analyzed to arise from the similar microstructures; whereas the dissimilarities arise from the composite nature, which gives rise to the sequential activation of different micro-mechanisms, in the ARBed microstructure.
Studies on Self-assembled ZnO Nanorods/Nanowires Grown on Silicon Substrates by Electrochemical Deposition
ZnO nanowires have promising applications in photonics, chemical, gas ionization, field emission, and piezoelectrical sensors due to their wide bandgap, large exciton binding energy, and piezoelectrical properties. Electrochemical growth is the low cost and short time fabrication method that is used for fabrication of ZnO nanorods/ nanowires on n- and p-type Si substrates, on Au and Al metallic layers, on seed layer, on patterned surface, and in anodic alumina membrane. The morphology modifications are reported based on changes in temperature of electrolyte, on effects of the conductivity, the surface roughness, and area of the substrate.
Interfacial Property in Nano-materials due to Van Der Waals and Electrostatic Coulombic Interactions
The interfacial properties of some nano-materials simulations due to van der Waals and electrostatic Coulombic interactions are investigated using molecular mechanics. Three objects, i.e., the pull-out process of some outer walls against other nested inner walls in a multi-walled carbon nanotube, the pull-out processes of a carbon nanotube from a polyethylene polymer and an alumina (Al2O3) matrices, respectively, are studied. It is found that the pull-out force, is independent of nanotube length, but is proportional to nanotube diameter at the sliding interface. A theory is proposed to predict this pull-out force based on the nanotube diameter at the sliding interface.
Atomistic Modeling of Dislocation-interface Interactions
Using atomic scale models and interface defect theory, we first classify interface structures into a few types with respect to geometrical factors, then study the interfacial shear response and further simulate the dislocation-interface interactions. The results show that the structural characteristics of both heterophases and homophases interfaces play a crucial role in (i) their mechanical responses and (ii) the ability of incoming lattice dislocations to transmit across them.
Finite Element Analysis of the Contact between Rough Surfaces
This paper is devoted to present a Finite Element Analysis (FEA) of the frictionless normal contact between a rough metallic surface and a rigid plane. The computation takes into account large plastic deformation and a fine mesh. Cumulative plastic deformation and von Mises stress distribution close to the contact surface are studied. Evolution of height distribution and the influence of contacting asperities on their neighborhood are also described.
Atomistic Simulation of Deformation and Failure Mechanism for Intermetallic Compound Mg17Al12
The deformation and failure processes of the composition of Mg17Al12 magnesium alloy near defects at atomic scale level are examined by means of molecular dynamics (MD) simulation. With the increase of temperature, the maximum elastic deformation and the yield stress of Mg17Al12 are reduced, obviously, .i.e. “Softeningâ€. The effect of different radii of voids on deformation of Mg17Al12 is analyzed by means of the center of symmetry parameters. The dislocations have been observed along with the [110] crystal direction, which reveal that the dislocation is the main deformation mechanisms of Mg17Al12.The “neckingâ€ of intermetallic compound Mg17Al12 between the two voids show that it has well capable of plasticity.
Numerical Simulation on the Mechanical Behavior of Water-contained PEM
In this paper, at first, MD simulation is performed to clarify the effect of water content on the distribution of density of molecular chains of PEM under uniaxial tension. It is found that the lower water content the more remarkable change of the density of molecular chains of PEM, which may lead to a high possibility of the nonaffine movement of the physically entangled points in the molecular network. Continuously, a computational model, in which the development of the deformation resistance is assumed to be a function of the number of the physically entangled points, is employed to discuss the effect of the water content on the mechanical behavior of PEM under various loading condition.
The Pressure Dependence of Elastic Stiffness of Granular Materials: A Binary Model in Effective Medium Theory
Granular media have many interesting and unusual properties, different from those of either solids or liquids. The study of sound propagation and nonlinear elasticity in unconsolidated granular matter is an interest topic. In the simplest experiments, a packing of spherical glass beads is confined under hydrostatic conditions and the compression and shear sound speeds p V and s V are measured as functions of static confining pressure P. Experimentally it is exhibit that the bulk modulus K and shear modulusG of a granular assembly of elastic spheres increase with pressure p faster than the p1 3 . This result is law predicted by effective medium theory (EMT) based on Hertz-Mindlin contact forces. To understand the origin of these discrepancies, we treating contact stiffness as a variable, extend the effective medium approximation used to obtain elastic stiffness of a random pack of spherical grains. More specifically, we show that EMT can describe the moduli pressure dependence if introducing a binary model for grain contacts and a parameter t f , which represents the amount of non-slipping contacts in the mixture. The simple extension of the theory provides a better fit for many laboratory test result.
Nanometer Aluminum Oxide Prepared by Detonation of Low Density Water-Gel Explosive
The water-gel explosive with low density 0.4 g/cm3 is made of aluminum nitrate, carbamide, RDX and plastic foam. The nanometer aluminum oxide is prepared by detonation of it. The results of transmission electron microscope morphology analysis and x-ray diffraction analysis indicate that all granules of nanometer aluminum oxide are spherical and homogeneous, the average dimension of granules is about 22.66nm, and the aluminum oxide is γ phase. According to experimental phenomenon, the imperfect detonation is generated. The phenomenon is explained by coherent theories of thermodynamics and explosion mechanics. The research provides favorable guide for the more works on nanometer aluminum oxide prepared by detonation.
Strain Rate and Temperature Effects on Compressive Buckling of Carbon Nanotubes and Intramolecular Junctions
Intramolecular carbon nanotube junctions under compression tend to buckle or exhibit structural instability, which will cause structural or electronics failure. Thus it is important to study the mechanical behavior of intramolecular junctions under compression. In this study, molecular dynamical simulations are carried out to study the compressive response of intramolecular junctions, as well as that of carbon nanotubes for comparison. The simulation results indicate that, within low loading rate, the strain rate effect can be ignored. If the strain rate increases further, the critical strain also increases. Simulation results reveal that local deformation may be introduced at high environmental temperature. It is also found that with increasing tube length the deformation modes transfer from shell buckling to column buckling for both junctions and carbon nanotubes.
Molecular Dynamics (MD) Simulation of Uniaxial Tension of Ceria and Gadolinia Doped Ceria (GDC)
In this paper, molecular dynamics (MD) simulations are employed to investigate the mechanical behavior of monocrystal ceria and GDC. The effects of doping content and temperature are taken into consideration. The results show that phase transition can be observed for pure ceria or GDC with low doping content. However, for GDC with high doping contents, no phase transition appears any more. In addition, the fracture stress and Young’s modulus are reduced for all samples by increasing the doping contents, the similar effect can also be observed with the temperature increases.
Atomistic Study of Tip Radius Effect of Nanoindentation on Nanocrystalline Cu
In current study we investigate deformation mechanisms of nanocrystalline Cu subjected to nanoindentation using atomistic simulation. Our results show that in addition to dislocation activities, grain boundaries associated mechanisms and deformation twinning are also observed during deformation process. Subsequent evaluation of indentation tip radius effects shows the maximum load for elastic-plastic deformation transition increases with increasing tip radius. There is a critical tip radius for mediating deformation mechanisms: below which dislocation density increase with increasing tip radius, while above which grain boundary associated mechanisms is more pronounced.
Interface Structure and Mechanical Properties of the Nanolayered Al/Pd Thin Films
Al/Pd multilayer system is studied by using the first-principles calculations and molecular dynamics (MD) simulations. Negative interface energies are found for different Al/Pd interface structures. Through atomic simulation, the Young’s modulus of the Al/Pd multilayers is found largely limited by the weak one. Moreover, the remaining interface areas after shear failure are not smooth but covered with two Al atomic layers, showing the strong bond strength between Al and Pd atoms has important influence on the atomic behaviour of Al/Pd interface
Deformation Twinning Mechanisms in FCC and HCP Metals
We report recent work on twinning and detwinning in fcc and hcp metals based on the in situ and ex situ TEM observations and molecular dynamics simulations. Three aspects are discussed in this paper. (1) Detwinning in single-phase Cu with respect to growth twins, (2) deformation twinning in Ag-Cu composites, and (3) deformation twinning mechanisms in hcp metals. The main conclusion is that the atomic structures of interfaces (twin boundaries, two-phase interfaces, and grain boundaries) play a crucial role in nucleating and propagating deformation twins.
Influence of Strain Rate and Temperature on Interface Misfit Dislocation Networks in Ni-based Superalloys
The three-dimension dislocation networks play an important role in the superior mechanical properties of Ni-based superalloys. In this paper, the effects of strain rate and temperature on the deformation and damage of dislocation networks in Ni-based superalloys are studied by molecular dynamics simulations. The deformation mechanisms of the phase interfaces at different strain rates and temperatures are explored. The results indicate that the dislocation networks of three different types of phase interfaces show different deformation and damage mechanisms at different strain rates and temperatures.
Molecular Dynamics Simulation of the Deformation of Al Nanopillars
Molecular dynamics are employed to simulate the compression process of Al nanopillar with both free and rigid side surface boundary conditions. In both conditions, the results show that plastic deformation behaviors are governed by intense dislocation activities including nucleation, propagation, annihilation, and interaction. In addition, rigid surface can trap dislocations inside the pillar and finally increase the yield strength. The results are in agreement with recent compression experimental results.
Active Cathode of Lithium Battery for Electric Vehicle
Lithium manganese oxides with a fine spherical morphology different from that of the normal composite oxides are formed after detonation wave treatment due to the very high quenching rate. Free metal atoms are first released with the decomposition of explosives, and then theses metal and oxygen atoms are rearranged, coagulated and finally crystallized into lithium manganate during the expansion of detonation process. The growth of lithium manganate via detonation reaction was investigated with respect to the presence of an energetic precursor, such as the metallic nitrate and the degree of confinement of the explosive charge. The detonation products were characterized by scanning electron microscopy. Powder X-ray diffraction and transmission electron microscopy were used to characterize the products. Lithium manganate with spherical morphology and more uniform secondary particles, with smaller primary particles of diameters from 10 to 20 nm and a variety of morphologies were found.
The methodology assumes that the heat of detonation of an explosive compound of composition can be approximated as the difference between the heats of formation of the detonation products and that of the explosive, divided by the formula weight of the explosive. For the calculations in which the first set of decomposition products is assumed, predicted heats of detonation of slurry explosives with the product H2O in the gas phase have a deviation of 866 kJ/kg from results with the product H2O in the liquid state. Fine-particle lithium manganate has been prepared by the detonation of slurry explosives of the metal nitrates, M (NO3) x (M = Li, Mn) as oxidizers and glycol as fuels, at high temperature and short reaction time. The detonation soot was identified from X-ray powder diffraction (XRD) patterns, transmission electron microscope (TEM) and scanning electron microscopy (SEM) measurements. XRD analysis shows that nanoparticles of lithium and manganese oxides can be produced from detonation of slurry explosives due to fast quenching as well as appropriate detonation velocity, pressure and temperature. The results showed that the calcination temperature largely influenced crystalline phase and size of the powders.
The present situation of detonation synthesis and emulsion explosives both at home and abroad is summarized in this paper. To solve the problems in research for lithium ion batteries, we suggested that zinc and lithium oxides should be used as cathode materials for lithium ion batteries. So, we design special emulsion explosives, and synthesize zinc and lithium oxides from an emulsion explosive. It is concluded that nanoparticles of lithium zincate can be synthesized from the unconventional emulsion explosive. Unconventional emulsion explosives for synthesis of zinc and lithium oxides are designed firstly. Furthermore, we succeeded in synthesizing nano-powders of zinc and lithium oxides by explosion at the fist time in the final analysis. Zinc and lithium nanooxides are successfully collected and validated by XRD, FTIR, and TEM.
Effect of Interface Energy on Stress Field Around a Spheroidal Nano-inclusion Embedded in a Semi-infinite Elastic Medium
Based on the surface elasticity, we investigate the elastic fields of semi-infinite body containing a nanosized spheroidal inclusion subjected to biaxial tension paralleling to the plane boundary applied at infinity. Using Boussinessq-Sadowsky’s potentials and transform relations between special functions, the stress fields of the elastic half-plane are obtained. Numerical results show that the intensity of the influence depends on not only the size and the shape of the inclusion but also the proximity of the inclusion to the edge and the surface stress.
Elastic Analysis of Interaction between a Screw Dislocation and a Coated Nano-inhomogeneity Incorporating Interface Stresses
The interaction between a screw dislocation inside a coated nano-inhomogeneity and the matrix incorporating interface stresses is investigated. The analytical expressions of the complex potentials, stress fields and the image force are obtained by means of the complex variable function method. The results indicate that the influence of the interface stress on the movement and equilibrium positions of the dislocation becomes significant when the radius of the inhomogeneity is reduced to nanometers. An additional repulsive force or attractive force is produced and the normalized image force depends on the size of the inhomogeneity.
The Influences of Residual Surface Stress on the Thermo-elastic Bending of Simply Supported Nanoplates
When thickness of plates is in nanoscale, the surface energy effect will become prominent, which renders the effective mechanical behaviors of plates to be size-dependent. In this paper, the classical plate theory is modified, by taking the surface energy effect into account, to investigate the size-dependent thermal bending of simply supported nanoplates. Results show that not only the surface elasticity but also the surface residual stress has significant effects on the thermal bending deformations.
Orientation Dependence of the Torsional Behavior of Copper Nanowires: An Atomistic Simulation Study
The mechanical properties and behaviors of <100>, <110> and <111> orientation copper nanowires under torsional loading are investigated using molecular dynamics. The relations of the normalized strain energy and virial torque with twist angle are computed. The orientation dependent plasticity in the single crystal copper nanowires is analyzed by the different modes of nucleation and propagation of dislocation from nanowires surface.
Molecular Dynamics Simulation of Microstructure Evolution and Deformation Mechanism for X80 Steel
X80 steel which was used in buried gaspipeline mainly consists of ferrite and pearlite. Experiments show that two kinds of micro defects exist inevitably in the manufacturing process of X80 steel including the micro-void and micro-crack, which affect the mechanical properties of X80 steel significantly. In this study, the deformation and failure processes of the composition of X80 steel near defects at atomic scale level were examined by means of molecular dynamics (MD) simulation. The EAM/FS potential is employed to characterize the interatomic interactions of the composition of X80 specimens. The deformation and failure processes of single crystal structures forï€ ï¡-Fe, Fe3C and the Fe3C-Fe interface are observed around the crack. The results reveal that microstructure evolution and deformation mechanism under external loading are correlated with crystal structure and crystal orientation intrinsically.
Simulation on Tensile Mechanical Properties of Single Crystal Copper Nanostructures with Different Hole Configurations
In this paper, the tension process of the single crystal copper nanostructures with holes was simulated by the large-scale parallel molecular dynamics simulation technique. Based on centrosymmetry parameter method and combined with the dislocation nucleation theory, the effects of holes configuration and holes number on the tensile deformation mechanisms of nanostructures were investigated, and thus their influences on tensile mechanical properties of nanostructures were revealed. Obtained results show that hole defects configuration affect the anisotropy of nanostructures. With the same total volume of holes, holes configuration and holes number alter the atoms arrangement near holes which lead to different patterns of holes defect evolution, and thus affect the mechanical properties of nanostructures in terms of dislocations activity.
Unusual Mechanical Behaviors of CrN Polymorphs from First Principles
Mechanical properties and electronic structures of different CrN phases have been studied using density functional theory calculations. It is found that the trend of bulk modulus is completely different from that of shear modulus (and elastic constant C44), which shows that the underlying sources of incompressibility and hardness are fundamentally different. These unusual mechanical behaviors can be understood from the different roles of Cr 3d electrons in hydrostatic compression and shear deformation. The present work provides important insights into the origin of hardness, and thus can help us to design new superhard materials of transition metal compounds.
Adhesive Mechanisms of a Bio-inspired Nano-film
Inspired by geckos’ spatulae, the adhesive behavior of a finite nano-film is investigated theoretically and numerically in this paper, considering the effects of adhesive length as well as surface roughness. It is found that (1) an effective adhesive length controls the peeling-off force, beyond which the peeling-off force will keep a constant for a determined peeling angle; (2) An intermediate surface roughness exists, at which the normal adhesion force between a finite nano-film and the rough surface achieves the weakest. All the results are qualitatively consistent with the experimental observations, which may be helpful for understanding geckos’ micro-adhesion mechanisms.
Microvoids Pressures of Tritium and Helium-3 in Vessel Wall with Their Accessional Effects
Tritium and helium-3 in the steel wall of a spherical pressure vessel (SWSPV) will cause hydrogen and helium embrittlements of the wall material, respectively, and thereby change the carrying capacity of the vessel. In this paper, the formulas of tritium and helium-3 pressures within microvoids which naturally exist in SWSPV were deduced, their spatio-temporal distributions laws were analytically obtained, and by using a representative volume element (RVE) model to perform finite element simulations, the effects of gas pressures on equivalent mechanical properties of the material were studied. It is found that the higher the gas pressures are, the lower the material ultimate strength, specific elongation and percentage contraction of area are, which is in good accordance with the phenomena of hydrogen embrittlements.
Reducing the Lithiation Induced Stress in Lithium Battery by Shape Design
this manuscript, a numerical method was formulated to calculate the stress induced by lithium ion insertion into the electrode of lithium battery. Stress distribution in two kinds of electrodes, i.e. film deposited on substrate and nano-particle deposited on substrate, was investigated. The calculation showed that reducing the size of the electrode would help to reduce the stress of the electrode during lithiation/delithiation. It is suggested that lithium ion battery with longer duration could be obtained by the application of the electrodes with optimized nanostructu
Twin Boundaries in Nanowires: Knowledge-based Fabrication & Mechanics
This short paper focuses on two elements. First, the authors show that twin boundaries are beneficial to mechanical properties of nanowires. Second, the authors present a mechanism of controllably introducing the twin boundaries
Multi-physics Analysis of Nano-components by First-Principle Simulation
In recent years, materials in a nano-scale such as nano-films, nano-wires, and nano-dots have been manufactured not only by scientific interest but also by anticipation of future practical application. They possess different characteristic strength from their macro-scale counterparts (effect of external structure; shape). On the other hand, it is well known that the understructure (e.g., grain boundary) affects the material strength. At atomic level, the crystallographic understructure means the distortion of atomic arrangement; grain boundary; e.g. domain wall. In order to understand the strength of nano-components, the both effects have to be clarified. Thus, we have investigated the mechanical behavior of nano-components by ab initio computational simulations1-3. The electrical (magnetic) property is never constant when materials are subjected to external load. This coupling effect between mechanical deformation and electrical properties is called the “multi-physics propertyâ€. Since this stems from the electronic structure at the atomic level, the external structure as well as the understructure in nano-components eminently influences the characteristics. The ab initio simulation is powerful tool to analyze the behavior as well2, 4-11. In the below, we show some examples on the ferro-electric behavior of PbTiO3 in nano-components5-
Response of Bulk Cu-Nb Nanolamellar Composites to Severe Rolling Strains
In bulk multi-phase composite metals containing an unusually high density of heterophase interfaces, the bi-metal interface controls all defect-related processes. Quite unconventionally, the constituent phases play only a secondary role. With the ‘right’ characteristics, these bi-material interfaces can possess significantly enhanced abilities to absorb and eliminate defects. [1-3] Through their unparalleled ability to mitigate damage accumulation induced under severe loading and/or severe environments, they will provide their parent composite with a highly effective healing mechanism and consequently a robustness not possible in existing advanced structural materials. Today’s bulk synthesis techniques and materials models, however, are unprepared to treat materials that are dominated by bi-metal interfaces. Consequently we cannot fabricate interface-dominant composites with the needed interfacial properties. In this work, we approach this problem by developing a predictive and experimentally validated model that holds the unique capability to predict the evolution of the interfacial structure and behavior during bulk large strain deformation. To succeed in this pioneering effort, we will spearhead new and creative ways to overcome the areas in which current methodologies are the weakest—in linking length scales over the so-called ‘micron gap’ and accounting for the special roles of bi-metal interfaces. This innovative multi-scale predictive capability will for the first time offer a tool to address materials falling within the new paradigm of interface dominance and to predict the synthesis routes needed for a targeted set of desired interfacial properties
Mechanical Properties of Heterogeneous Nanostructured Metals
Mechanical behaviors of polycrystalline metals with nano-sized grains, or called as nano-grained or nanocrystalline metals, have been extensively investigated for several decades after the pioneering study of H. Gleiter in 1980s. However, measured mechanical properties of the metals with homogeneous nano-sized grains are rather disappointing: very high strength accompanied with very limited ductility. It means the traditional strength-ductility trade-off still holds in the homogeneous nanostructures. In recent years, several types of heterogeneous nanostructured metals with hierarchical structures have been synthesized. It is interesting to find that extremely high strength can be obtained without compromising tensile ductility or fracture toughness in the heterogeneous nanostructured metals. For instance, in a tempformed low-alloy steel containing ultrafine elongated ferrite grains strengthened with nano-sized carbides, toughness increases along with an increasing strength when temperature decreases from 60 to –60°C. Nano-twinned metals provided an alternative example of hierarchical nanostructured metals with extraordinary mechanical behaviors: strength is elevated to 10 times higher than that of coarse-grained form, accompanied with an obvious increased ductility
Probing Mechanical Properties of Nanostructured Materials via Large Scale Molecular Dynamics Simulations
The rapid development of synthesis and characterization of nanostructured materials as well as unprecedented computational power have brought forth a new era of materials research in which experiments, simulation and modeling are performed side by side to probe the unique mechanical properties of nanoscale materials [1,2]. We have recently reported a dislocation source controlled mechanism in the newly-developed nano-twinned metals in which there are plenty of dislocation nucleation sites while dislocation motion is not confined. We show that dislocation nucleation plays the governing role in the strength of such materials, resulting in their softening below a critical twin thickness. Large-scale, fully 3-D, molecular dynamics simulations and a kinetic theory of dislocation nucleation in nano-twinned metals show that there exists a transition in deformation mechanism, which occurs at a critical twin boundary spacing where the strength is maximized, from the classical Hall-Petch type of strengthening due to dislocation pile-up and cutting through twin planes to a dislocation nucleation governed softening mechanism with nucleation and motion of partial dislocations parallel to the twin planes (twin boundary migration). The simulations indicate that the critical twin boundary spacing for the onset of softening in nano-twinned Cu and the maximum strength depend on the grain size: the smaller the grain size, the smaller the critical twin spacing, and the higher the maximal strength of the material.
Molecular Dynamics Simulation of Plastic Deformation of Metallic Glass at Strain Rates Varying Between 10-3 s-1 to 101 s-1
Molecular Dynamics simulation at low strain rates suggests a diffusive type of mechanism which is characterized by flow softening due to structural changes. The structural changes show up as transition in the radial distribution function. Initially stress rises to a high level followed by drastic drop. This results in localized shear banding. The simulations rule out the role of shear band movement as the specific mechanism of plastic deformation and establish a criterion to demarcate the atomic movement in low strain rate and high strain rate plastic deformation. During the plastic deformation nano-crystallization is known to occur [1-3]. The simulation relates the plasticity with the precipitation in the light of above.
Nano-chemo-mechanics of Advanced Materials for Hydrogen Storage and Lithium Battery Applications
Material failures due to coupled chemo-mechanical effects are the serious roadblocks in the development of advanced nanostructured materials for renewable energy technologies. Currently, hydrogen and lithium-ion batteries are being considered as the promising candidates of efficient energy sources and carriers. However, the adsorption of hydrogen can drastically degrade the structural materials for its containment, and the intercalation of lithium in electrodes often causes the mechanical failures and capacity fading in batteries. These issues arise owing to the large electro-chemical-mechanical driving forces in systems, high mobilities of hydrogen and lithium as well as their strong chemo-mechanical coupling with the host materials. In this talk, our recent nano-chemo-mechanical studies of the hydrogen embrittlement and lithium intercalation phenomena will be presented. We show that multiscale simulations can well explain and predict the experiments of hydrogen embrittlement in grain boundary engineered metals and the electrochemically induced mechanical failures in lithium-ion nano-batteries
The Deep Link between FCC and BCC Metals
Sample size effect is currently a hot research topic in a world wide range [1-4]. In this talk, I will report our recent research progress on submicro-sized single crystal Ni and Mo pillars. It was found that prior to the compression tests, the <111> orientated single crystal face centered cubic (FCC) nickel pillars fabricated through Focused Ion Beam (FIB) contained a high density of defects. However, quite unexpectedly, the dislocation density was observed to decrease dramatically during the deformation process and, in some cases, even resulted in a dislocation-free crystal. The phenomenon, which we termed as “mechanical annealingâ€, is the first direct observation of the dislocation starvation mechanism and sheds new light on the unusual mechanical properties associated with submicron- and nano- scale structures [2]. However, due to the dislocation core structure difference between the screw and edge dislocations in body centered cubic (BCC) metals, it was believed that BCC metals are incapable of mechanical annealing and the sample size strengthening behavior should be less that in FCC metals. By in situ compression of nanopillars inside a transmission electron microscope, we demonstrate that with the pillar diameter decreasing to hundreds of nanometers, significant mechanical annealing does occur in BCC Mo. In addition, there exists a critical size (DC ~ 200 nm for Mo at room-temperature) below which the strengthening exponent ï¡ in Hall-Petch like regression increases dramatically to that similar to FCC metals. Thus, a new regime for size effects in BCC is discovered that converges to that of FCC, revealing deep connection in the dislocation dynamics of the two systems. We attribute the observed phenomena to the diminishing importance of lattice friction at high stresses, when the size-enhanced flow stress exceeds a single screw dislocation’s lattice friction
The Closure Problem in Density-Based Modeling of Dislocation Dynamics
Development of density-based models of dislocations is a critical step towards predicting the collective dislocation dynamics in deforming metals. In the recent years, a few approaches have been developed to accomplish this objective, one of which is the statistical approach to dislocation dynamics. This approach is based on the concepts of statistical mechanics, and it treats the evolution of the dislocation population using kinetic equations, the completion of which requires dealing with a closure problem similar to the case of classical kinetic theories of particle systems. Such a closure problem can be approached by modeling the statistics of dislocations and their internal fields. This communication focuses on this task. Specifically, we present modeling and simulation results for (a) the spatial and orientation statistics of dislocations, (b) the statistics of dislocation velocity, resolved shear stress on dislocations and resolved shear stress in the crystal, and (c) the temporal statistics of cross slip and short range reactions in both FCC and BCC crystals. The statistics of the velocity and resolved shear stress are required to develop a density-based dislocation mobility law for use in the kinetic equations. The spatial statistics is required to model the dislocation correlations, and the temporal statistics is needed to model all source (rate) terms in the kinetic equations for the dislocation density evolution. The method of dislocation dynamics simulation is used to conduct all numerical simulation of all relevant statistical measures. The utility of the statistical measures presented here in the context of density-based models of dislocations is discussed.
Effects of Ultrafine Nanograins on the Fracture Toughness of Nanocrystalline Materials
For the purpose of evaluating the effects of ultrafine nanograins (UFNGs) on the fracture toughness of conventional nanocrystalline (nc) materials, we developed a theoretical model comprised of UFNGs (with a grain size d between 2 and 4 nm) evenly distributed in the conventional nc matrix. The UFNGs could be treated as a part of triple junctions, denoted as super triple junctions (STJ). In the framework of our model, stress concentration near crack tip initiates intergrain sliding that leads to the generation of edge dislocations at STJ. The dependence of critical crack intensity factors on grain size was calculated. It was demonstrated that the existence of the UFNGs approximately doubles the critical crack intensity factors.
Recovering the Full Dislocation Tensor from High-Resolution EBSD Microscopy
A novel stereological method is reported which infers experimentally inaccessible components of the Nye geometrically necessary dislocation density tensor. It is possible to measure five of nine components of the Nye tensor by high-resolution electron backscatter diffraction (HR-EBSD), but electron-opacity of metals obscures the remaining four components. This paper illustrates that it is possible to use equilibrium equations in connection with the known (measured) strain field of dislocations to infer the four unknown dislocation density components. Once the full Nye tensor is obtained, it can be transformed into a crystal coordinate frame and interpreted in a physical se
On the Interaction of Material Heterogeneity and Geometric Discontinuity in Serrated Plastic Flows
Some new experimental results are presented on the evolution of jerky plastic flow and associated localized plastic strain fields in Al-Mg sheet metals under quasi-static loading conditions. Effects of plasticity-induced material heterogeneity on the nucleation and growth of deformation bands over a nominally homogenous macroscopic stress field were first characterized using the standard uniaxial tension coupon geometry. Subsequently, test coupons with U- and V-shaped notches were used to characterize the notch-tip deformation fields subjected to a heterogeneous macroscopic stress field due to geometric discontinuity of notches. Experiments were carried out using both hard and soft test machines and limitations and uncertainties in the experimental measurements were critically examined. These experimental observations motivated a possible new modeling approach towards plausible explanations of the observed serrated plastic flow phenomena.
Micro Heterogeneous Deformation and Strain Localization Behavior in Austenitic Ferritic Duplex Stainless Steels
This paper provides a review on the recent work done on the micro heterogeneous deformation and strain localization behavior in austenitic ferritic duplex stainless steels studied using TEM, SEM and in-situ X-ray diffractometer and neutron diffractometer and the simulation using multi-scale material modelling. The results from both studies show that the difference in the elasto-plastic properties of the austenite and ferrite phases has caused different amounts of plastic deformations occurring in these two phases, and consequently different static and cyclic stress strain behaviours and substructures. From the simulations it is found that the coupling effect plays a key role in producing the changes in the cyclic stress-strain behaviour and also on the substructure evolution. TEM investigation also shows that the dislocation slipping behaviour and substructures strongly depend on the elasto-plastic properties of the individual phases and also on the coupling effect. The study indicates that the material damage and crack initiation in a two phase metal start mainly in the weakest phase if the deformation hardening is considered.
Two-scale Analysis for the Composite Plate with 3-D Small Periodic Configuration under Condition of Coupled Thermoelasticity
In this paper, the second-order two-scale analysis method for dynamic propagation of mechanical and thermal disturbances of the plate made from composites with 3-D periodic configuration is presented by means of construction way. It can capture the microscopic 3-D coupled mechanical-thermal behaviors caused from 3-D micro-structures. First, directly starting from the 3-D elastic plate model of composite materials with 3-D periodic configuration, three cell models are defined, and correspondingly the three classes of cell functions only defined on the normalized cells are constructed. And then, the effective homogenization parameters of composites are calculated from those local functions, it leads to a homogenized laminar plate problem. Next, to solve it the homogenization solution is obtained. Finally, the second-order two-scale solution for dynamic coupled thermoelasticity problems of periodical composites plates in three dimension case is constructed from the micro-cell functions and the homogenization solution.
Phase Field Simulation of Static Recrystallization for Deformed Alloys
A multi-state free energy function for deformation alloy with storage energy is proposed to simulate the microstructure evolution of static recrystallization with phase field model. The transformation kinetics curve of recrystallization and the time index of Avrami curve are systematically analyzed. The simulation results are in good agreement with other theoretical or experimental results.
A Calculation Approach of the Stiffness Tensor of Spherical Particulate Composites: The Effects of the Reinforcements Arrangement
The architecture of microstructure, i.e., volume fraction, size, shape, orientation and arrangement geometries of constituents, affect the stress and strain partitioning of the constituents. A numerical method was presented in this paper for calculation of stiffness tensor of particulate reinforced composites having continuous matrix including spherical particulate. The arrangement effects of spherical particulate on the stiffness tensor of particulate composites were estimated. The estimation of numerical calculations were very good agreement with results of finite element method (FEM) and Mori Tanaka Method (MTM). Numerical method presented can consider arrangement effects of reinforcement particulate via a simple parameter named as geometrical factor (Gf). The important advantage of numerical method proposed in this work is to allow the systematic and parametric investigation of the effects of microstructural geometry on the elastic behavior of particulate composites.
The Influence of Linear Defect on Twin Movement under a Shear Loading in Magnesium Single Crystal
Due to the hexagonally close-packed (hcp) structure of magnesium, magnesium and its alloys exhibit a strong anisotropy and an unlike yielding in tension and compression. Twinning is always regarded as the dominant deformation mode at low temperatures in magnesium and its alloys. Therefore it is important to study the influence of vacancies and linear defects on the movement of twin for a further understanding of the plastic deformation mechanisms of magnesium alloys. In this work, molecular dynamics simulation has been applied to investigate the movement of the {1012} twin. A linear defect is created near the twin boundary. The simulation results indicate that the linear defect impedes twin movement. However, when the applied strain is increasing, the twin can pass through the linear defect, and a similar phase transition region is left behind the twin boundary.
Physical Aging of Glassy Polyvinyl Chloride in Non-uniform Stress Field
Creep experiments have been used to measure the viscoelastic property of glassy PVC under tensile and dual cantilever loading modes at 25~35 K below Tg. Characteristic creep rate decreases with aging time and increases with temperature, and the value under tensile loading mode is more than that of dual cantilever loading mode. In primary stage of aging, the evolution of characteristic creep rate under tensile loading mode is faster than that of dual cantilever loading mode. However, in the later stage of aging, the evolution is the same although the value of characteristics creep rate is different.
Mesh Generation of Nomex Honeycomb Material
The aim in this paper is to discuss the geometrical modelling of a Nomex honeycomb material using the concept of near-regular textures. The starting point is a scanned image of a representative cellular structure, which is processed to get for the texture description a structural unit and mappings. These can be taken as a concise description or recipe of a material having microstructure. Then multiple samples with the same statistics but slightly different geometry can be generated from a single scanned sample.
Microstructure and Damage Analysis of Solid Propellant
According to a Micro-CT based tomogram, the particles in solid propellant was considered as ellipses. A numerical method called random ellipses packing algorithm was used to model the 2D microstructure of the solid propellant. Then, a bilinear cohesive law, which relates the stress tractions and displacement jumps, was adopted to simulate the debonding process of particle/binder interfaces, and a representative volume element of the propellant microstructure under uniaxial tension was analyzed. The results indicate that the debonding of particle/binder interfaces is an important reason to nonlinear macroscopic constitutive responses, and the larger particles are debonding earlier than that of small ones.
Properties of RF Sputtered ZnO Thin Films under Different Oxygen Flux
The microstructures, optical, and mechanical properties of zinc oxide (ZnO) thin films deposited on glass substrates by rf magnetron sputtering were studied. Microstructures were examined using atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. Optical property was measured by photoluminescence spectrum. The mechanical properties were measured by nanoindentation. The crystalline structures of ZnO thin films are well ordered with high c-axis (002) orientations and the crystallinity is strongly affected by O2 flux. Surface morphologies of ZnO thin films are smooth and grains grow and distribute uniformly. The hardness and Young’s modulus of ZnO thin films are ranged from 8.2 to 10.4 GPa and 105 to 120 GPa, respectively.
Determination of Cement Paste Strength by Nano-indentation on Thin Section
In this research a new method is developed to measure material properties of cement paste that makes use of nano-indentation. Thin sections of cement paste are prepared on a glass plate with a small circular hole. These specimens are loaded with a nano-indenter at the location of the hole in the glass plate. The failure mechanism is of a punching shear type. These tests are simulated with a lattice model and in this way the local mechanical properties of the cement paste can be determined. In this paper the method is explained and examples of experiments and simulations are presented.
Quantitative Phase-Field Modeling of the Ferrite-to-Austenite Transformation
A quantitative phase-field model is developed to simulate the ferrite-to-austenite phase transformation. The proposed model can be reduced to the mixed-mode model for solid-solid phase transition in a sharp-interface limit or thin-interface limit. The matched asymptotic expansion for the phase-field equations is performed at the second order with a small parameter. An anti-trapping current, suitable for two-sided diffusion, is introduced in the solute conservation equation to eliminate anomalous interface stretching and surface diffusion effects due to unequal diffusivities in both phases. Thus, the local equilibrium at the interface can be recovered. Data for model validation is presented.
Numerical Study of Mechanical Anisotropy of Extruded Magnesium Alloy AZ31
The recently developed Elastic Visco-Plastic Self-Consistent (EVPSC) model is applied to study the mechanical behavior of extruded magnesium alloy AZ31. Different specimen orientations with respect to the extrusion direction are used to study the mechanical anisotropy under both uniaxial tension and compression. It is found that the uniform strain and 0.2% proof strength under tension are sensitive to the loading direction. Under uniaxial compression, the 0.2% proof strength is almost independent of the loading direction. Numerical results are in good agreement with the experimental observations found in the literature.
Thermoelastic Contact of the Homogeneous Materials Coating Half-plane
This paper investigates the thermoelastic contact of homogeneous materials coating half-plane under plane strain deformation. A rigid punch is sliding over the surface of the homogeneous materials coated half-plane with the constant velocity V. Frictional heating is generated at the interface between the punch and homogeneous materials coated half-plane. The Fourier integral transform method is employed to cast the problem to the Cauchy singular integral equations, which are then solved numerically to obtain the unknown contact pressure and the in-plane component of the surface stress. The effects of the Peclet number and friction coefficient of the coating surface are discussed and shown graphically in detail. Numerical results show that the change of the Peclet number and friction coefficient of the coating can influence the distributions of contact pressure.
Determination of Fracture Properties in Thermal Barrier Coating Systems by Non-standard Bending Tests
The fracture properties of plasma sprayed thermal barrier coatings have been investigated. In this work, specimens in as-sprayed condition and after thermal oxidation have been prepared. Non-standard four point and three point bending tests that can generate a single interface crack, have been conducted at room temperature and at the ambient temperature of 100°C, respectively. Examinations of fractured surfaces by scanning electron microscopy and energy disperse spectroscopy have been used to probe the failure modes. From the load-deflection curves of specimens, the fracture toughness for stable crack propagation, in terms of energy release rate, was estimated reliably from analytical considerations.
Analyses of Adhesive Contact Problem for Non-homogeneous Medium Based on a Multi-layer Model
The present paper is aimed at a more systematic study of the adhesive contact of non-homogeneous medium coatings with arbitrary spatial variation of material properties. We consider a non-homogeneous medium coating perfectly bonded to an elastic half-space and indented by a rigid spherical indenter. A Maugis type of adhesion model is used to describe the effect of the presents of adhesion. We will build multi-layer computational models to simulate the non-homogeneous medium with arbitrary spatial variation of material properties. By using the Hankle integrate transform technique and transfer matrix methods, the adhesive contact problem of non-homogeneous medium is reduced to a integrate equation.
Study on the Heterogeneous Structure of Composites Bulletproof and the Protecting Mechanism
To the deficiency of the homogeneous bulletproof material, this paper puts forward the heterogeneous composite bulletproof structure. It combines the ceramic, metal, fiber, STF together in certain proportion and thickness to multi-layer structures, obtains different bulletproof results through different associations. This text carries research on the composition, shelter mechanism and application in four kinds of compound structure such as ceramic/ metal, the ceramic/ hard fiber reinforced material, ceramic/ fiber, STF/ Kevlar,etc. Through studying and comparing, the bulletproof structure of heterogeneous, its protective capacities are obviously improved than traditional homogeneous protective materials.
Application of Granite in Asphalt-Treated Base
Granite Asphalt-Treated base not only solve the lacking of the high quality aggregate, but also improve the stress distribution in the highway structure and extend the service life. In this study, the effects of different modifiers (hydrate lime, active mineral powder, and cement) on the reforming properties of ATB were assessed by testing Marshall stability (MS) and splitting tensile strength (STS). The results indicated that all types of modifiers improved the ATB mixture’s MS, Marshall stability ratio (MSR), tensile strength ratio (TSR). In particular, the MSR and the TSR values increased up to 95.2% and 83.2% by using hydrate lime, respectively.
Design of Microstructure for Porous Material with Maximum Bulk Modulus
This paper presents a new approach to designing periodic microstructures of cellular materials. The method is based on the bidirectional evolutionary structural optimization (BESO) technique. The optimization problem is formulated as finding a micro-structural topology with the maximum bulk under a prescribed volume constraint. Using the homogenization theory and finite element analysis within a periodic base cell (PBC), elemental sensitivity numbers are established for gradually removing and adding elements in PBC. Numerical examples demonstrate the effectiveness of the proposed method for achieving convergent microstructures of cellular materials with maximum bulk. Some interesting topological patterns have been found for guiding the cellular material design.
Validation of a Cellular Micromechanical Model
Heterogeneity in cellular structures has obviously a crucial effect on strength. Considering the fact, a micromechanical model employing heterogeneous specimen geometry consisting of hexagonal cells has been generated. In order to describe the mechanical behavior of each individual cell wall, elastic Bernoulli beam model is utilized. The model uses two parameter statistics for cell wall length and thickness, i.e. mean value and standard deviation. By means of the micromechanical model, virtual experiments for plane stress state are carried out. In the meantime, physical experiments for various honeycomb cores are conducted in order to compare and validate parameters of macroscopic constitutive equation obtained through virtual experiments.
Micro-inertia Effect and Dynamic Poisson’s Ratio of Closed-cell Metallic Foams under Compression
A significantly decreasing influence of lateral constraint on the crushing stress with increasing loading rates has been found for a closed-cell aluminum foam. This interesting phenomenon may be interpreted by the micro-inertia effect and/or the decrease of Poisson’s ratio in dynamic compression. In this paper, the three-dimensional Voronoi technique and the explicit finite element method are utilized to investigate the micro-inertia effect and dynamic Poisson’s ratio of closed-cell metallic foams. The simulation results indicate that the micro-inertia effect plays little role in enhancing the plateau stress of metallic foams. The Poisson’s ratio varies with the nominal strain and its peak value decreases as impact velocity increases.
Impact Behaviour and Energy Dissipation in Metallic Thin-walled Hollow Spheres
Metallic foams made of thin-walled hollow spheres are attractive in applications where weight saving is a critical issue. In this work, an experimentally validated finite element model was developed to investigate the impact behaviour and energy dissipation capacity of single hollow spheres, both of which were found to be significantly influenced by loading velocity. An initial crushing strength that increases with the loading velocity occurs due to inertia effects of wall materials at higher loading velocity (e.g. v > 80 m/s) and related deformation modes. The impact energy dissipated by a hollow sphere increases linearly with the loading velocity.
Vibration Analysis of Embedded Microtubules within Elastic Medium
The embedded microtubules, within the elastic medium, responsible for maintaining cell shape, are the essential part of the cytoskeleton in eukaryotic cells. Based on the fact that the microtubules are surrounded by the elastic medium, we developed an orthotropic shell Winkler- like model to investigate the vibrational behavior of the microtubules within the elastic medium. Modeling the microtubules as anisotropic elastic shell, we derive the Eigenfrequencies and Eigenmodes of elastic vibration of the embedded microtubules.
Homogenization Analysis of Plane Structures with Three Dimensional Periodic Microstructures
A rational homogenization method is presented for characterizing the in-plane material response for plane structures which have three dimensional periodic microstrcutures. This method is based upon the classical asymptotic expansion homogenization theory but with a novel multiscale displacement field expansion that is capable of reflecting the plane structure properties, i.e., the plane stress condition. Rigorous asymptotic expansion derivations yield the local unit cell problem. The in-plane material response is systematically computed via homogenization analysis of the local unit cell problem. The efficacy of the present method is verified by a numerical example.
Evaluation of Mechanical Properties of Ductile Films on Brittle Substrates Based on the Representative Strain
A new approach to extract the mechanical properties of ductile thin films on brittle substrates based on the representative strain (corresponding to the representative stress) has been presented in depth-sensing indentation with finite element method (FEM). Forward and reverse analyses of high deformation symmetry numerical simulation of Berkovich indentation tests are used to determine mechanical properties of ductile films. The proposed reverse analysis provides a new solution for the initial yield stress (or the representative stress) and the strain-hardening exponent, given that the Young’s modulus is predetermined from the Sakai method, e.g., as [1]. The validity of the reverse analysis method is checked using the numerical nano-indentation test.
Mechanical Behavior of PE Cylindrical Shells under Axial Impact Load
Impact response of the pseudo-elastic phase transformation cylindrical shells (PTCS) is systematically investigated experimentally and numerically. Result shows that unloading and recovery process of PTCS is not consistent with the local stress redistribution and concentration. It can be divided into four phases: 1. local stress unloads first in the middle of PTCS; 2. stress continues to unload near the incident bar. 3. Stress continues to unload near the transmission bar, the first and second layer PT buckling rumple disappear; 4. the whole PTCS vibrates elastically. Martensite fraction distribution along the wall thickness of CPTH is the most non-uniformly, that of axial phase transformation hinge takes second place, and that of the phase transformation diagonal hinge is the most uniform.
Activated Slip Systems in Copper Polycrystal under Multiaxial Fatigue Loadings
This paper present an experimental investigation of the plastic slip activity on the surface of Oxygen Free High Conductivity (OFHC) copper specimens. Surface topology of the specimen is investigated during uniaxial and multiaxial fatigue testing. Persistent slip bands (PSBs) topology evolution, kinetics of onset and progression are characterized using AFM. The slip systems that can be considered as activated are also discussed based on the evolution of the resolved shear stress (RSS) under cyclic loading.
SHPB Experiment and Numerical Simulation of Concrete under Room Temperature and High Temperature
Concrete was a typical strain-rate sensitivity material. In this paper, the concrete specimen was tested in the Split Hopkinson Pressure Bar (SHPB) experiment system under room temperature. Besides, with the microwave heating technique, the concrete specimens were also tested under high temperature. The stress-strain curves, got from numerical simulation in LS-DANY with the specific H-J-C constitutive model, were compared with the stress-strain curves from experiment. The results showed that the specific H-J-C constitutive could simulate the dynamic characteristics of concrete under both room temperature and high temperature.
Band Gap Computation for Mixed Solid-Fluid Phononic Crystals of Triangular Lattice by Dirichlet-to-Neumann Maps
A numerical method based on Dirichlte-to-Neumann (DtN) map for calculating the band gaps of two-dimensional (2D) phononic crystals with mixed solid and fluid components is presented. It is applied to a triangular lattice of circular solid cylinders in a fluid matrix, for which the conventional plane wave expansion method may fail due to different wave modes in the solid and fluid. The DtN map is constructed by using the cylindrical function expansions. Linear eigenvalue problems are formulated, from which the band structures can be determined. Two numerical examples are illustrated to show the efficiency of present method.
A numerical method is presented that simulates explosion in soil medium. A code M-MMIC2D based on this method can be used to simulate the spherical charge exploding in semi-infinite soil medium. The simulation results are found to be in full agreement with the principle of physics and experimental data. Which indicates that the model and algorithm presented in this paper are reasonable and the simulation method can be used for simulating the formation process of cavity through blasting in soil.
Finite Element Analysis of Mechanical Properties of 3D and 5-D Tubular Braided Composites
In this paper, a finite element solid model was established based on the unit cell model of 3D and 5-D tubular braided composites, as well as the assumption that the yarns possess elliptic cross-section. Periodical boundary condition was employed. Using this model, average stress and strain corresponding to a given displacement load could be obtained with the implementation of ANSYS. The elastic constants of material and effective properties should be gained subsequently. The calculation results showed good agreements with the experimental results.
Numerical Prediction for the Elastic Properties of 3D Full 5-directional Braided Composites Considering Adhesive Layers
Considering the effect of adhesive layers between adjacent yarns, a new FEM model of representative unit cell is established, which more really simulates the inner meso-structures of the 3D full 5-directional braided composites. Then, the effective elastic property parameters are obtained by FEM calculation and compared with the corresponding results of FEM calculation without considering the effect of adhesive layers. The results show that effective elastic property parameters decreases slightly because of the impact of adhesive layers.
Elasticity Solutions for Functionally Graded Circular Plates
England (2006) proposed a novel theory to study the bending problem of isotropic functionally graded plates subject to transverse biharmonic loads. His theory is extended here to functionally graded plates of transversely isotropic materials. Using the complex variable method, the governing equations of three plate displacements appearing in the expansions of the displacement field are formulated based on the three-dimensional theory of elasticity for a transverse load satisfying the biharmonic equation. The elasticity solutions of an FGM circular plate under biharmonic polynomial loads are derived. Numerical comparison with available solution indicates a good agreement. The effects of related parameters are also discussed.
Phase Field Method Equations Derived From Thermodynamic Extremal Principle
The thermodynamic extremal principle (TEP) formulated in discrete parameters [1] represents a handy tool in modeling of thermodynamic processes in solids. As a further concept the phase field method (PFM) has gotten an established method for simulation of microstructure evolution and allows accounting parallel action of a number of thermodynamic processes in an extremely easy way. The PFM goes back to the seminal work by Ginzburg and Landau [2], dealing with the ordering of atoms within unit cells, see the discussions by Penrose and Fife [3] and Gurtin [4]. In this contribution it is shown that the elementary equations of the PFM can be directly derived from the TEP. For doing this it is necessary to express the total Gibbs energy consisting of the chemical Gibbs energy of a mixture of individual phases (two different crystalline phases and one phase characterizing the amorphous structure in the interface) in the system and of a penalty term due to square of gradient of the order parameter. The order parameter also represents an indicator determining, how the thermodynamic quantities or kinetic parameters of individual components, e.g. the chemical potentials or diffusion coefficients, must be calculated (i.e. to which phase the thermodynamic quantities are addressed). If no diffusion is assumed in the system, the dissipation is expressed exclusively as quadratic form of the rate of the order parameter.
Imaging Interfacial Stress and Strain in Heterogeneous Films
Interface failure is common in structures consisting of dissimilar materials. Interface stresses arise due to the mismatch of mechanical properties during processing or usage which often results in fracture. While much progress has been made in understanding the interface fracture in hard materials, relatively little is known about the fracture mechanics of soft matter. A familiar yet challenging example is the drying of a colloidal film.
Micro/Macro-Analysis of Lamellart Composites Based on Discrete Dislocation Plasticity and Periodic Homogenization
In this study, micro/macro-analysis of periodic lamellar composites consisting of elastic and elastic-plastic layers is performed using a discrete dislocation plasticity approach [1]. In this approach, periodic lamellar composites are assumed to be subjected to macroscopic uniform deformation described by macroscopic stress ï“ij and/or macroscopic strain Eij, and to be defined by infinitely periodic arrangements of a periodic unit V. Here elastic and elastic-plastic layers are regarded as inclusions VI and matrices VM, respectively, which are assumed to be isotropic elastic materials, but the motion of dislocations is allowed in the matrix area.
Void Formation and Surface Roughening in Amorphous Li-Alloy Electrodes
Lithium alloys with metallic or semi-metallic elements are attractive candidate materials for the next generation negative electrodes of lithium ion batteries due to their large specific and volumetric capacities. The key challenge with lithium alloy electrodes, especially Li/Si, has been their large volume changes during insertion and extraction of lithium. These volume changes can lead to pulverization and debonding. Recent efforts to improve the cyclability of Li-alloy based electrodes are highlighted by the utilization of nano-structured materials, including Si nanowires (NWs), crystalline-amorphous Si core-shell NWs, sealed Si nanotubes and nano-structured carbon/silicon composites. It is widely accepted that nano-sized materials provide better accommodation for diffusion-induced stress (DIS) and thus improve a battery’s cyclability.
Understanding of Interface Structures, Defects, and Mechanical Properties at General Fcc-bcc Interfaces Using “Tunableâ€ Potentials
Nanolayered Cu-Nb composites exhibit high strength and enhanced radiation damage tolerance [1, 2]. To understand the relevance of interface structures to interface properties in general fcc-bcc systems, “tunableâ€ potentials offer a fairly simple way to selectively vary parameters independently [3]. Atomistic modeling of Cu-Nb interfaces reveals that the atomic structure of the interface and the energetics of point defects at the interface play vital roles in the enhanced radiation damage tolerance of these composites [2]. However, in the previous work, the relative effects of interface geometry (crystallographic orientation relations and lattice mismatch strain) and thermodynamic properties (heat of mixing, interface energetics, etc) were not deconvoluted. In this work, the parameterization of the EAM interatomic potentials [4,5] in fcc-bcc system is modified to understand the interface structures, defects, and mechanical properties.
Crystal Symmetry Dependent Deformation in Pb(ZrxTi1-x)O3 Ceramics
Ferroelectric ceramics had been widely used as sensors, actuators, transducers, etc., due to their peculiar electromechanical coupling properties, ultrafast response and compact size. Among all the ferroelectric ceramics, lead titanate zirconate (PZT) family ceramics got the most widely applications and were systematically studied for years. PZT ceramics will show linear piezoelectric behavior under a low electric field or a small stress, while exhibit intensively nonlinearities when subjected to a large electric field or a high stress which is thought to be caused by domain switching. Domain switching in ferroelectric ceramics is crystal symmetry related. There exists only 1800 and 900 domain switching in tetragonal ceramics, and 1800, 1090 and 710 switching in rhombohedral ceramics. In ferroelectric single crystals, perfect alignment of polarization can be achieved and a single domain state can exist after poling by a strong DC field. While this is not the case in ferroelectric ceramics where the crystallite axes arrange in a random way and multiple domain states could exist after poling. In a real ferroelectric ceramics, domain switching will be constrained by neighboring domains and may not accomplish completely even a very large electric field is applied. The constraint level of domain switching in ferroelectric ceramics is also crystal symmetry dependent and it may be well known that tetragonal PZT ceramics is difficult to pole, while rhombohedral PZT and PZT ceramics near the morphotropic phase boundary (MPB) are easier to.
On the Mechanism of Anomalous Slip in BCC Metals
Computer simulations and empirical studies of the core structure of single dislocation in bcc metals over the last few decades have made enormous contributions to interpret many abnormal mechanical behaviors of bcc metals: tension/compression stress asymmetry, high Peierls (friction) stress for the motion of screw dislocations, and strong strain-rate and temperature dependence of yield and flow stresses at low temperatures [1]. However, the single-dislocation core model remains inconclusive to elucidate a peculiar anomalous slip behavior of bcc metals, which occurs on planes for which the Schmid factors are fifth and sixth in the order of largest Schmid factors for the {110} <111> slip systems, and for which the resolved shear stress is less than half that on the (101) [111] primary system. Note that the anomalous slip behavior is also known as the violation of Schmidï‚¢s law, which states that plastic deformation of a single-crystal metals would begin on a slip system (a combination of the slip plane and the slip direction) when the resolved shear stress on the slip plane and in the slip direction reached a critical value (i.e., critical resolved shear stress). The resolved shear stress (ï´) is given by ï´ = ï³ cos ï¦ cos ï¬, where ï³ is applied stress, ï¦ is angle between the stress axis and the normal to the slip plane, and ï¬ is angle between the stress axis and the slip direction. The factor cos ï¦ cos ï¬ is usually called the Schmid factor (m). Schmidï‚¢s law in general is well obeyed by close-packed face-centered cubic (fcc) and hexagonal closed-packed (hcp) metals, which deform by slip in close-packed directions on planes that are close-packed planes. Bodycentered cubic (bcc) metal is however not a close-packed structure, which deforms by slip in the most closely packed direction:
Experimental and Crystal Plasticity Investigations of Initial Stress in Thermally Treated Titanium and Nickel Alloys
A crystal plasticity model for hcp crystals has been developed which takes due account of the anisotropy which is known to exist in the thermal expansivity. The c-axis expansivity is generally thought to be lower than that for the a-directions and on cooling of a polycrystal (in processing, for example), this potentially leads to the establishment of type II strains and initial stresses which therefore exist prior to in-service loading. Figure 1 below shows a schematic of a model hcp polycrystal in which the crystallographic orientations may be specified within our crystal plasticity finite element formulation (Dunne et al, 2007).
Plastic deformation in small volumes whose dimensions fall in the range of micrometer and nanometer length scales can be very different from that in the bulk. Large surface (or interface) to volume ratio imposes limitations on dislocation generation and motion, leading to increase in strength and alterations in deformation behavior. We have investigated three such cases, namely, composites nanowires, nanoscale multi-layer metallic composites, and micropillar single crystals, using either molecular dynamics or dislocation dynamics simulations or both. The results of these investigations are summarized as follows.
Hierarchical Failure Analysis and Optimal Toughness Design of Carbon Nanotube-reinforced Composites
Hierarchical analysis of the fracture toughness enhancement of carbon nanotube (CNT)-reinforced composites is herein carried out on the basis of shear-lag theory and facture mechanics. It is found that neither longer reinforced CNTs nor stronger CNT/matrix interfaces can definitely lead to the better fracture toughness of these composites. In contrast, the optimal interfacial chemical bond density and the optimal CNT length are those making the failure mode just in the transition from CNT pull-out to CNT break. The process to optimize the fracture toughness of the CNT-reinforced composites is proposed.
Network Interfacial Dynamics of Microstructure Evolution
Modeling the evolution of microstructure in heterogeneous materials requires an efficient computational description of complex interfaces that delineate crystal domains of uniform or homogeneous properties. The motion and reconfiguration of such interfaces change the extent of the uniform domains, and is known to be responsible for most of the mechanical and physical properties at the macro-scale. Examples of the complex dynamics of interfaces include the dynamics of dislocations during plastic deformation, grain growth dynamics during growth or in response to mechanical loads, the evolution of thin film microstructure during CVD or PVD processes, the kinetics of precipitation involving coalescence of growing phases, the evolution of porosity during sintering and consolidation, etc. The largest challenge in all these fundamental phenomena is the topological reconfiguration of the interfaces, including atomic-level constraints. We present here a new computational method that is based on network and graph theories, where the microstructure is described by a network composed of dynamic sub-networks. Each subnetwork contains vertices or (nodes) and a collection of edges (or segments) that connect pairs of vertices, with all the topological properties associated with the mathematical structure of graphs and hyper-graphs. The implementation of this graph theory based model for parallel computing will be discussed, along with a variety of applications at various length scales for modeling the atomic structure of dislocations during crossslip, plastic deformation in small systems (micro-pillars), and the evolution of grains in polycrystals.
Nanomechanical and Strength Properties of Heterogeneous Composite Materials Scale Factor
According to numerous investigations the variation, within certain limits, of the mechanical and strength properties of composite materials can be attained by filling them with various kinds of disperse particles (technical, carbon, silicates, organic clays, etc.). It is known that the reinforcement effect, that is, the improvement of one mechanical characteristic or another, is directly connected with the sizes of filler particles, i.e., with their specific surface Su.
A Kind of Physics Interpretations of Material Length Scales
In the present research, a new mechanics model is presented and used to characterize the trans-scale mechanical behavior from nano-scale to macroscopic scale. In the mechanics model, both surface effect and strain gradient effect are considered. In order to interpret the physics meanings of the material length parameters included in the mechanics model, a new method will be used to represent the length parameters and a clear physics interpretation is obtained. Specifically, we use both the strain gradient theory and the Cauchy-Born rule to characterize the mechanics behaviors and then make a linkage for both kinds of results according to equivalence of strain energy densities. The values of the materials length parameters included in the strain gradient theory are expressed with the physics parameters included in the Cauchy-Born rule. As the example, for the face-centered cubic metals, the values and variation trends of the length scale parameters are obtained.
A Multiscale Method of Plasticity Enabling Dislocations to Seamlessly Pass Scale Interfaces
In the concurrent multiscale analysis, it is difficult to have truly seamless transition between the atomistic and continuum scale. This situation is even worse when defects pass through the interfaces between different scales. It seriously limits the development of multiscale plasticity. This work proposes a new method to circumvent these notorious difficulties so that defects such as dislocations can seamlessly pass through scale interfaces. Originally proposed by the first author in 2009[1] the generalized particle (GP) multiscale modeling method serves as the foundation for the new method. It is substantially different from the so called DC multiscale methods which link the atomic and continuum region by direct coupling.
Wrinkling of Graphene Sheet under Constrained Extension via Atomistic and Continuum Mechanics Simulations
Graphene is the thinnest materials in the universe with the thickness of one carbon atom only. Here, we demonstrate the wrinkling behaviors of graphene sheets under constrained extension using both atomistic simulation and continuum mechanics method. Due to the constraint of the loading edges, the deformation in the graphene is not uni-axial extension. Compression stress perpendicular to the extension direction occurs in the middle region, and this local compression results in the wrinkling pattern, as illustrated by our molecular dynamics simulations. We find that the slenderness ratio plays a very important role in the wrinkling pattern. When the ratio exceeds a critical value, the parallel lines break at the middle and separate to two parts on the left and right region. This has also been explained by deformation analysis using the finite element method. The wavelengths and magnitudes of the wrinkling are dependent on the mechanical properties of graphene and the applied extension strain. Our results can be of great significance to control the wrinkling pattern of suspended graphene, and further to design nano-meter sized mechanical sensors.
The Reflection of Synergetic Features in the Response of Geological Medium on Outer Force Actions
In our research of no stationary geological medium in a frame of natural experiments in a real rock massifs, which are under heavy man-made action it had been showed, that the variation of the state can be distinguished with use of synergetic approach in hierarchic media. The change of the system state on the used space bases and times can be observed in the parameters, which are linked with the structure medium peculiarities of the second or higher rank, using geophysical methods, set on a many ranked hierarchic non stationary medium model.
Numerical Simulation of Crack Propagation and Deformation Mechanism Around Crack Tip across Atomistic/Meso/Continuum Scales
In this paper, an across scale numerical model with five regions is proposed to simulate the deformation around crack tip and the crack extension at atomic scale, which carry out the coupling transmission of mechanical parameters among 5 different scales for single side crack of ï€ ï¡ï€ iron. Under macro-scale loading, the processes of deformation near crack tip and crack propagation were simulated. The crack propagation and deformation mechanisms included dislocation and phase transfer were described in discrete atomic region and the crack fork was observed. The special displacement connection method named as “the piecewise linear fittingâ€ was proposed to carry out the transition of mechanical parameter both in discrete atomistic region and continuous media.
Molecular Mechanics and Continuum Mechanics Study of Buckling of Pre-Stressed Multi-Walled Carbon Nanotubes
The buckling behaviors of pre-stressed multi-walled carbon nanotube (PS-MWCNT), where the interlayer distance is less than 0.34 nm, are studied using both molecular mechanics (MM) simulations and continuum mechanics. Due to the combined effect of interlayer distance and tube diameter, the buckling membrane force increases as the interlayer distance decreases for PS-MWCNTs with a fixed intertube chirality difference. Similar to normal MWCNTs, the buckling membrane force of the PS-MWCNTs increases with the number of walls for a given innermost tube size. Moreover, the buckling membrane forces predicted by the MM simulations are qualitatively consistent with those from the continuum mechanics model.
Coupling Quantum and Continuum Scales to Predict Dislocation Nucleation from an Aluminum Crack Tip in the Presence of Oxygen and Hydrogen
Atomistic simulations of deformation processes have long been plagued by the challenge of accurately and efficiently describing the complexities of multispecies bonding. In the case of metals, this has led to the majority of atomistic modeling efforts focusing on pure elemental metals in vacuum, rather than more technologically relevant problems involving alloys with impurities and surface oxides in realistic environments. At the root of the challenge is a trade-off between accuracy and computational expense. Here we use a concurrent multi-scale approach to address this long-standing challenge in the context of crack tip behavior in Al. We couple an atomistic region whose forces are calculated via Kohn-Sham Density Functional theory to a continuum region described by linear elasticity. This approach enables us to accurately capture complex chemistry at the crack tip while properly accounting for long-range elastic fields via a large simulation domain. The method is used to predict the critical stress intensity factor for both crack tip dislocation nucleation and propagation in the presence of oxygen and hydrogen. Interpreting the mechanical results alongside of charge analysis reveals new insight into the role of chemistry at a crack tip.
Heterogeneous Deformation of Polycrystals Simulated with Micropolar Single Crystal Plasticity
The influence of grain size on the flow stress of small scale statistical volume elements (SVEs) is investigated using micropolar single crystal plasticity. In a micropolar single crystal, the presence of couple stresses work-conjugate to the lattice torsion-curvature leads to a non-symmetric Cauchy stress that when resolved on a given slip system gives rise to kinematic hardening during heterogeneous deformation. The scale-dependent response of the micropolar single crystal is dictated by both energetic (higher-order elastic constants) and dissipative (plastic lattice torsion-curvature) constitutive length scales. In this work, we investigate how variations in these constitutive length scale parameters affect the scaling behavior of the macroscopic (ensemble average) flow stress.
Length-Scale Effect in Stability Problems for Micro-Periodically Stiffened Cylindrical Shells
Thin linear-elastic cylindrical shells having a micro-periodic structure along one direction tangent to the shell midsurface (uniperiodic shells) are object of considerations. The aim of this paper is to investigate the effect of a periodicity cell size on the stationary stability of such shells. In order to take into account the length-scale effect in special stability problems, a new averaged non-asymptotic model of uniperiodic shells, proposed by Tomczyk in [2], is applied. In the framework of this model not only the fundamental “classicalâ€ critical forces but also the new additional higher-order critical forces depending on the period of heterogeneity will be derived and discussed.
Electromagnetic Properties of Sandwich Materials with Hexagonal Honeycomb Core
electromagnetic properties; FD-TD; S Parameters; sandwich materials
Nanoscale Cutting Simulation of Single Crystal Copper Based on Multiscale Method
A multiscale simulation method based on quasicontinuum (QC) theory was adopted to study the nanometric cutting process of single crystal copper. The cutting mechanisms were analyzed in atomistic view. Dislocation of workpiece in cutting process was examined in detail. The generation and disappearance of dislocation cause the fluctuations of tangential and normal cutting forces during the nanometric cutting process of single crystal copper. In addition, the effects of cutting depth on cutting force had also been investigated. Typical rule for cutting force in nanoscale cutting of single crystal copper was obtained by using multiscale simulation method. By comparison with the results in molecular dynamics (MD) simulation, it is shown that multiscale simulation can increase the modeling size without increasing of time consumption and accurate results can be found in the simulation.
A Strategy to Multi-scale Modeling of Stochastic Heterogeneous Material
The behavior of heterogeneous materials, especially concrete, is often dominated by the coupling of randomness and nonlinearity. In order to describe these behaviors, a class of microscopic stochastic damage model is developed. In present paper, the stochastic harmonic function, which is proposed by the authors’ research group, is compared with the K-L Decomposition. And the comparison results show that the stochastic harmonic function can represent the fracture strain random field with least random variables. Based on this, the stochastic fluctuation of heterogeneous material response in macro-scale can be conducted based on the microscopic stochastic damage model.
An Elastic Constitutive Model for Frequency-dependent Anisotropy in the Medium with Multi-scale Fractures
An elastic constitutive model was proposed according to the combination of high-frequency Hudson model and low-frequency Gassmann model. The effects of fluid flow between the pores and the fractures with multi-scale distribution have been considered. The combined model was validated with synthetic seismograms. The results reveal that the anisotropic dispersion can occur at seismic frequency. P-wave velocity dispersion and attenuation are significant over a frequency range which spans at least two orders of magnitude. Acceptable results are obtained within the low-frequency and high-frequency limits, using the model we have presented. The properties agree well with anisotropic Gassmann model at low frequency, while the velocity is hardly affected by the fractures and the behavior of fractures corresponds to the results for isolated cracks filled with a fluid at high-frequency limit.
Multiscale Modeling of Fracture Processes for Hardened Cement Paste
The paper studies the fracture processes of hardened cement paste and its mechanical performance through a multiscale lattice fracture model. The fracture process of cement paste is modeled at microscale and mesoscale respectively, and the bridge between these two scales is established by employing a parameters passing scheme. The shape of cement paste specimen is cube, and its side length is 100 μm in the microscale model and 1 mm in the mesoscale model. The corresponding mesh sizes for these two scales are 1 μm and 0.1 mm respectively. Hence, the length 100 μm is the point which connects microscale model to mesoscale model. At microscale the spheres based model is employed to represent the microstructure of hardened cement paste, but at mesoscale the cement paste is modeled as continuum material with some heterogeneity, which is implemented during the lattice network construction by introducing some geometry randomness. It is assumed that the cement paste material behaves linear-brittle locally at microscale, while some softening is allowed locally at mesoscale and is determined by the output of the microscale model. A numerical experiment of uniaxial tensile test is presented to illustrate the modeling procedures and the parameters passing scheme.
Effective Modulus of Nano-particle Composites with Different Particle Size Distributions
A combination of the differential scheme and a mapping method is applied to predict the effective modulus of nano-composites with different particle size distribution. The nano-particles with interface effect are replaced with common particles resort to an energy equivalency approach. Both discrete and continuous particle size distributions are analyzed. As a numerical example, lognormal distribution is analyzed in this paper. The results demonstrate that the effective elastic properties of nano-composites are particle size distribution depend
Ultrasonic Dispersion and Cryogenic Tensile Properties of Poly-dicyclopentadiene Composites with Carbon Nanofibers or Nanocoils
Cryogenic tensile properties of poly-dicyclopentadiene (poly-DCPD) composites with carbon nanofibers (CNFs) or carbon nanocoils (CNCs) have been investigated experimentally. Flat tensile specimens of poly-DCPD composites with 0.25, 0.5, 1.0 and 1.5vol% CNFs or CNCs were fabricated by an ultrasonic method and tested at room temperature (RT) and liquid nitrogen temperature (77K). The effect of ultrasonication time on the cryogenic tensile properties of poly-DCPD composites with CNFs or CNCs was discussed. A scanning electron microscope (SEM) was also used to observe the fracture surface morphology of poly-DCPD composites.
Investigation on Tensile Properties and Size Effect of 3D Braided Composites
The elastic properties of the braided composites are obtained by volume average method. The variation of shear modulus and Poisson’s ratio is obtained as well. It is shown that the elastic modulus increases with the increase of number of interior cells. The predicted results are in agreement with the experimental results. The results may provide experimental basis for further study on strength prediction and the establishment of strength criterion for 3D four-directional braided composites.
Composite Materials Method for Studying Mechanical and Deformation Properties of Layered Salt Rock
In this paper, layered salt rock was seen as the layered composite materials, composite materials method was proposed to study the mechanical and deformation properties of layered rock salt. the sandwich was taken as the "reinforcement" in the laminated Composite materials, and the salt rock was taken as the "Matrix"，Then， Creep mechanism of fiber reinforced composite materials and layered rock salt were compared; Dislocation motion from the composite materials was used to analyze stress transfer process, stress concentration and the fracture of layered rock salt, finally, layered rock salt was considered heterogeneous at the microscopic level and continuous and uniform at the macro scale, representative volume element could be adopted to study the effective properties of layered rock salt. The proposed study consideration in this paper provided a new approach for the deformation and mechanical properties of layered rock salt geological materials.
Investigation of the Effective Properties of 3D Braided Composites with Distributed Pores by the Finite Element Method
In this paper the effective mechanical property of a type of 3D braided composites with equally distributed pores is studied. A representative element model for the 3D braided composites is built. The nonhomogeneous finite element method is adopted in this paper. When the volume fraction of the pores is constant, the moduli and the Poisson ratio of the 3D braided composites with equally distributed pores are obtained. It is found that the pores can weaken the mechanical property of the 3D braided composites. And the effects of the pores on different components of the mechanical property of 3D braided composites are different.
Investigation on Static Shear Creep Behavior of Asphalt Mastic Containing Recycled Cement Mortar Powder
The static shear creep behavior of asphalt mastic containing recycled cement mortar powder (CMP) is investigated in this paper. The used mastic consists of asphalt and filler at a mass ratio of 1:1. The static shear creep tests were performed by Dynamic Shear Rheometer (DSR) at 0°C. The deformation and stiffness of CMP asphalt mastic were discussed. Experimental results showed that CMP results in smaller deformation, higher static shear creep stiffness modulus of asphalt mastic at 0°C.
Dynamical Model for Evolution of Rock Massive State as a Response on a Changing of Stress-Deformed State
For research of the medium response with hierarchic structure we had developed an iterative algorithm for electromagnetic and seismic fields in the problem setting similar to analyzed higher for layered-block models with homogeneous inclusions. From the received results we did a conclusion, that the higher the rank of hierarchy of the medium the hardly diminishes the similarity between seismic and electromagnetic results, and the obtained information has a nondependent sense, which underlines the complexity of the researched medium.
The Numerical Simulation Investigation on Penetration Capability of New Nose Shape Projectile Penetrating into Concrete Target
Based on the new structure earth penetrating warhead (EPW) validated by MBDA, the new nose shape projectiles with multi-teeth shape were established in this paper. Numerical simulations were conducted on the penetration capability of projectiles with tooth and those with general nose shape penetrating into concrete. The effects of tooth nose shape parameters tooth height(TH) and nose pole diameter(D) on penetration capability of projectiles is discussed. The approaches were brought forward for enhancement of penetration of EPW with tooth nose shape by increasing TH and decreasing D under a certain extension, and achieved a reasonable combination.
Cumulative Plastic Deformation for Improved Soil Subgrade under Cyclic Loadings
Based on an improved dynamic model, a method is proposed to predict the cumulative plastic deformation for improved weathered granite subgrade under long-term cyclic loadings. The formulas for parameter quantification in the method are derived by regression of repeated load triaxial test results. To determine the deviator stresses used in the method, a fully non-linear simulation model of an improved weathered granite subgrade in Wu-Guang PDR is developed by employing Lagrangian difference method and free-field boundaries. The settlements of the improved soil subgrade are predicted and analyzed. The applicability of the proposed method is verified by comparisons between predicted and field test results.
Scale Effect on Failure Characteristics of Sandy Pebble Soil
Because of discontinuity and heterogeneity of sandy pebble soil, the mechanical characteristic is different from the other homogeneous mediums`. Scale effect should be consisted. By means of the direct shearing test and heavy compaction test, shear strength characteristics of sandy pebble soil with different gradation and maximum particle size are analyzed. The results are as follows: (1) The large particle size has important influence on the shearing indexes; (2) The cohesion becomes increased with the well gradation. The friction angle is on the contrary; (3) The cohesion will be reduced if improving the maximum particle sizes. However the friction angle will be increasing.
Damage Process Analysis of Jointed Rock Mass Using FLAC/PFC Coupled Numerical Modeling of AE
By capturing the Acoustic emission (AE) events, underground opening induced rock mass degradation or damage can be located and evaluated. In this study, a coupled numerical method is used to study AE at the jointed rock. Two codes, Fast Lagrangian Analysis of Continua (FLAC), a finite difference code and Particle Flow Code (PFC), a distinct element code, are coupled. The motive to apply the FLAC/PFC coupled approach is to take advantage of each modeling scheme while at the same time minimizing the requirement for computational resources. The coupling is realized through an exchange of displacements and forces in each cycling step. The rock mass surrounding an AE sensor is modeled using PFC and the remaining rock mass is modeled with FLAC to consider the geological complexity and the excavation sequence. This approach takes account of stress redistribution and provides stress and displacement patterns in the rock mass that are consistent with AE observations. The most significant value of the work will eventually be realized through various simulations to aid the AE data analysis, interpretation, and construction management.
Multi-scale Analysis on the Characteristics of Coal Intensity Distribution
In order to study the discipline of the coal damage, the intensity distribution model of coal microcell is established. The research comes up with the characteristics of coal intensity which is based on the result of uniaxial compression experiment. The research indicates that the disorder distribution of the coal microcell determines the damage threshold’s randomness in the system. The coal’s brittle fracture strength mainly meets the normal distribution, which is able to explain the status of the stress concentration and the stress fluctuation.
Significance Analysis of Weld and Processing Parameters on Wall Thinning and Cross-sectional Deformation of Welded Tube in NC Bending Process
The mechanical behaviors of different kinds of microstructure in weld joint reduce the ductility of the welded tube. In this study, the influences of these inhomogeneities and processing parameters on the deformation behaviors of welded tube are determined by the combination of orthogonal experimental design and finite element analysis. Five factors viz., weld position, push assistant level, mandrel shank diameter, number of balls and mandrel extension length are selected. The analysis of variance shows that push assistant level and mandrel shank diameter have significant influence on wall thinning △t, and number of balls has highly significant influence on cross-sectional deformation △D. The weld shows little effect on the deformation behaviors of the QSTE340 welded tube.
Multi-scale Analysis Method for Combined Conduction-Radiation Heat Transfer of Periodic Composites
A Multi-scale analysis method is developed for the Rosseland equation, the most common model for the combined conduction-radiation heat transfer. This equation is strongly nonlinear and the composites bring forth rapidly oscillating coefficients. Under some physical conditions, we prove the existence, uniqueness and the estimation of the maximum and lower bound. We first define the asymptotic expansion formulas. Then the effective coefficients and predictor- corrector finite element algorithms are obtained. Numerical results compared with both fine mesh and volume-averaged method show that it can be applied to predict the meso-scale thermal behavior of periodic composites with good accuracy.
Multiscale Modeling for Discussion about the Anisotropism of Spruce Wood
Geometry models were constructed based on the micron scale structures of the spruce wood showed by the EM photos. To focus on the anisotropy caused by the micron structures, the material that formed by wood fibers was considered isotropic here. The failure envelops obtained by simulations with and without pits structures considered are quite different, shows that these micron structures have considerable effects on the material properties in macroscale.
Elastic Analysis for Contact Problems with Surface Stress Effects
Based on surface elasticity, the influence of surface stress on contact problems at nanoscale is considered. The complex variable function method is adopted to derive the fundamental solutions of the contact problem. As examples, the deformations induced by a uniformly distributed traction are analyzed respectively. The results reveal some interesting characteristics in contact mechanics, which are distinctly different from those in classical elasticity. At nanoscale, the displacement gradient on the deformed surface transits continuously across the uniform distributed loading boundary as a result of surface effects. In addition, for nano–indentation, the indent depth depends strongly on the surface stress.
Atomistic Predictions of Dislocation Nucleation at Experimental Loads and Temperatures with Transition State Theory
The combined use of atomistic simulations and transition state theory (TST) has enabled significant improvements in the prediction of thermally activated processes in a wide range of applications, e.g. conformation changes in molecules, chemical reactions, kinetic phase transitions, solid state diffusion. However, such an approach is still in its infancy with regard to mechanics of materials applications. Focusing here on dislocation nucleation in Al, we examine the utility of five TST-based approaches. Using the finite temperature string method, we interpret the success/failure of each approach in terms of a full energetic analysis of the nucleation processes. After showing that advanced TST approaches such as variational TST can accurately predict nucleation rates, we employ variational TST to study dislocation nucleation from a free surface under ordinary laboratory conditions. The predictions (1) demonstrate that nucleation will only occur under very high stresses at room temperature, (2) provide an upper bound on the shear strength of Al in dislocation starved contexts, and (3) provide a rate sensitivity signature of the nucleation process.
Multi-Time Scaling Image Based Crystal Plasticity FE Models Dwell Fatigue Initiation in Polycrystalline Ti Alloys
This work develops a crystal plasticity finite element simulation based prediction of fatigue crack nucleation in polycrystalline alloys. The novel framework paves the way for a paradigm change in fatigue modeling of metals and alloys. Two specific topics are at the core of this development. The first is a non-local fatigue crack nucleation model at the scale of grains in a polycrystalline microstructure based on post-processing results of crystal plasticity finite element simulations under cyclic loading. The second development is a unique wavelet transformation based multi-time scaling algorithm for accelerated crystal plasticity finite element simulations. It is motivated by the large number of cycles that may be required to initiate a fatigue crack in a polycrystalline sample. Simulating such large number of cycles remains intractable to conventional single time scale finite element analysis. The unique aspect of the developed method is that the algorithm does not require inherent scale separation as with other conventional methods that assume averaging, periodicity or near periodicity
A Multiscale Study of Energetic Composite Responses
In recent years, energetic composites containing metallic fuel and inorganic oxidizer are gaining strong interests due to their outstanding combustion characteristics at nanoscale, such as high energy density, tunable energy release rate, ignition sensitivity and benign explosion products [1-4; among others]. Since simple physical mixing could lead to a nonhomogeneous distribution of fuel and oxidizer nanoparticles, a better fuel and oxidizer assembly is desired for the optimal performance of the nanothermite during the detonation process. Hence, various kinds of nanothermite materials, such as the self-assembled nanothermite comprising CuO nanorods and Al nanoparticles [5, 6], have been produced for the higher chemical reaction rate. On the other hand, the multiscale model-based simulation procedure for the detonation process of these kinds of energetic composites remains at its infant stage, and an integrated analytical, computational and experimental effort is required to better understand the combustion mechanisms of the nanothermite
Material Heterogeneity and Emergent Length Scale Due to Non-equilibrium Phase Transition
Most processes in both material fabrication (e.g., solidification in casting of alloys, diffusion and displacive phase transition, high speed metal forming and cutting, etc) and service (e.g., field-induced phase transitions in structural/functional materials) are intrinsically thermodynamic non-equilibrium. These processes involve, at one stage or another, different versions of material nonlinearity, instability and multi-field coupling at different length and time scales, and constitute the main physical origins of material heterogeneity observed in various microstructure and pattern evolutions under non-equilibrium conditions [see 1-3]. In this talk I will present recent experiment and modeling on domain patterns and emergent length scales during stress-induced phase transition process, emphasizing the roles of loading time scale and thermo-mechanical coupling. Their effects on the stress-strain curves and domain patterns observed in NiTi strips in the strain rate range of 10-4/s ~ 102/s are reported (see Fig. 1). We found that, with the increase in loading rate, the deformation mode changed from the localized-propagation mode with a few domains to the heterogeneous multiple-nucleation mode with fine domains and eventually to homogeneous deformation. The domain spacing, as the emergent length scale of the structure, decreased with the applied stretching rate and follows a power-law strain rate dependency which originates from the strong coupling among the material’s strong nonlinear property, temperature dependence of transition stress and the heat transfer. The physical origin of material heterogeneity is explored and the important competition of heat transfer time and loading time in imposing the emerging length scales of the material’s heterogeneity is demonstrated. Scaling laws for the rate dependent domain spacing is developed which agree well with the experimental data over the strain rate range of 10ï€­5 / s ~ 102 / s
Surface Stress and Surface Energy of Solids and Size-dependent Young’s Modulus
Following Gibbs’ approach to interfacial excess quantities, Müller and Saúl analyzed the excess of elastic energy at an interface by decomposing stress and strain tensors into perpendicular and parallel contributions [1]. Based on traction continuity and non-gliding at the interface, they defined interfacial stress tensor,
Multi-scale Modelling of Void Growth and Coalescence within Heterogeneous Void Clusters
The ductility of advanced high strength automotive alloys is limited by their complex microstructures that contain heterogeneous distributions of reinforcing particles. To describe the ductile fracture mechanism in these alloys, Butcher [1] proposed a multi-scale material model that accepts measured particle distributions to capture void nucleation and evolution from the individual particle-scale to the macro-scale. While this model is promising, it is limited by the use of nucleation and evolution rules that assume the voids and particles to be isolated, thereby neglecting the critical interactions within the void and particle clusters that ultimately govern fracture. In the present work, we propose an attractive solution to this issue by implementing a non-local averaging scheme into the model of Butcher [1] to incorporate a microstructure-based lengthscale into the evolution submodels. For each void, the nearest neighbour information is used to compute the average morphology in the region defined by the lengthscale. This information is used to evaluate the evolution models for isolated voids to approximate the interactions within the clusters.
Multiscale Crystal Plasticity Modeling of Microtwinning Induced Deformation in y-y¢ Nickel based Superalloys
Nickel-based superalloys such as alloy 600 have been widely used in nuclear industry as steam generator tubes, through-wall nozzles and weld material due to their superior strength and corrosion resistance properties. Alloy 600 provides better resistance to stress corrosion cracking compared to SS304 grade of austenitic stainless steel. Alloy 182/82 have been used as welding electrode/ filler material and these are capable of depositing defect-free weldments in alloy 600. Recently, it was observed that alloy 600 is still subject to stress-induced reheat-cracking after long exposure to high purity reactor steam and primary water. As a result, a new nickel-based superalloy, i.e., alloy 690 along with alloy152/52 welding electrodes has been developed to replace alloy 600. Similarly, for intermediate heat exchangers of high temperature gas-cooled reactors, nickel-based superalloys are crucial due to their superior high-temperature strength and creep resistant properties.
Origin of Plasticity Length-Scale Effects in Fracture and Deformation
Engineering design of essentially all metallic components used in structural applications – aircraft structures, cars, bridges, pacemakers, hip implants, computer chip packages, turbine disks, among others - relies heavily on the long-standing framework of continuum plasticity. However, many experiments now show that the plastic flow stress in metals increases in material volumes on the micron scale and below. Micro and nano indentation hardness of metals, the flow strength of nanocrystalline metals, nano- and micro- pillar, nano-asperities, and thin films, all obey the mantra of “smaller is strongerâ€ and highlight the failure of conventional continuum plasticity. These issues have spurred new approaches to plasticity such as phenomenological strain-gradient plasticity (SGP) models and discrete-dislocation models, aimed at capturing the effects of “geometrically necessary dislocationsâ€. While these models show size effects, there is no clear physical identification of the material length scales controlling size-dependence, in spite of wide speculation on possible length scales. Here, we use a new discrete-dislocation/cohesivezone model to unambiguously demonstrate that the spacing between obstacles to dislocation motion is one dominant material length scale controlling the fracture toughness of plastically deforming metals. Our results support one SGP model and provide a physical interpretation for that model’s phenomenological length scale. We then propose a new “stress gradient plasticityâ€ concept based on the behavior of dislocations in a “pile-upâ€ at an obstacle under a stress gradient. “Stress gradient plasticityâ€ helps rationalize our fracture results. More importantly, applied to bending, it predicts an increase in the initial flow stress and is consistent with discrete-dislocation simulation studies. We then implement “stress gradient plasticityâ€ within a low-order continuum plasticity framework and demonstrate size effects in bending, torsion, indentation, and void expansion, and explain observed size effects—both strengthening and hardening—in recent bending and torsion experiments.
Size Effects at Nano-Scale: Governing Mechanism and Efficient Simulations
Materials at nano-scale exhibit interesting behaviors in mechanical and other aspects. For instance, the Young’s modulus of nano-scale samples demonstrates distinct size effects in both experiments and simulations, either increases or decreases with decreasing sample size. So, the challenging issue is: what mechanism underlies the opposite size effects and how to simulate the size effects correctly and efficiently in various possible applications In our work on size effect at nano-scale, it is found that the intermolecular potentials can significantly affect surface lattice and result in different size effects. For example, since L-J potential produces a repulsive equilibrium state in bulk, it would lead to a looser surface structure than bulk once a new surface is created, then results in the decrease of Young’s modulus when sample size decreases. While Buckingham pair potential (with Coulomb interaction) induces the opposite size effect, owing to a denser surface resulting from an attractive equilibrium state. These findings have been justified with 1D and 2D analysis or the comparison to experimental observations, [1-3]. For larger nano systems with more complex potentials, efficient simulations are badly needed. For this sake, a coupled method, MST/CST (Molecule and Cluster Statistical Thermodynamics) has been developed [3-4]. This method could bridge the gaps in both spatial and temporal scales involved in size effects. Practically, the MST/CST method is equal to MD to characterize the deformation of atomic lattice, phase transformation, etc, but it can beat the latter in computational efficiency.
A Consistent Thermodynamic Theory of Inelasticity Resulting in Gradients in Dislocation Density with Naturally Evolving Length Scales
A structure for an internal state viable description of inelastic deformation of crystals is developed. The deformation gradient is multiplicatively decomposed into an elastic deformation resulting from externally applied loads and the deformations associated with each density of defects to be included as internal state variables. The deformations associated with these defects or foreign atoms such as diffusing species, may further be decomposed into elastic and plastic parts depending upon the structure of the defect. In many cases, either the elastic or plastic part of a particular deformation gradient will be negligible, depending upon the situation. The appropriate elastic strain-like variable associated with the defects is included in the free energy resulting in conjugate thermodynamic forces (internal stresses) that must be included in the dissipation inequality. In addition, these forces (stresses) are required to satisfy micro or meso scale linear and angular balance laws. All transport equations (e.g. heat conduction or diffusing species) are derived from a combination of the energy balance and these force balance laws. This is in contrast to classic state variable theories in which only temporal evolution equations were specified for the internal state variables. Constraint equations are required for the extra kinematic degrees of freedom that are introduced. These are based upon the physics of the associated defect density/state variable an example given by the flow rule or plastic velocity gradient based upon the Orowan equation relating plastic strain rate to mobile dislocation density and velocity. Examples are given for theories ranging from simple statistically stored dislocations, coupled transport theories for hydrogen and micropolar type theories for asymmetric defects
Electromechanical Response of Multilayer Piezoelectric Actuators for Fuel Injectors from Room to Cryogenic Temperatures
We study the electromechanical response of multilayer piezoelectric actuators for fuel injectors from room to cryogenic temperatures. A thermodynamic model was used to predict a monoclinic phase. A shift in the boundary between the tetragonal and monoclinic phases with decreasing temperature was determined, and the temperature dependent piezoelectric coefficients were evaluated. A nonlinear finite element analysis was then carried out to examine the voltage induced displacement of the multilayer piezoelectric actuators from room to cryogenic temperatures. Electromechanical fields were also calculated, and the results were discussed in detail. In addition, experimental results on the voltage induced displacement, which verify the model, were presented.
Surface Piezoelectricity for a Plane Boundary
Material at or near a surface usually possesses different properties than the bulk material. The effect of such material heterogeneity may become remarkable when the surface-to-volume ratio of a body is large. In this paper, a plane boundary of a piezoelectric body is modeled as a thin layer with specified material properties. The state-space formalism is employed to give a transfer relation between the state vectors at the top and bottom surfaces of the layer. Using the power series of the transfer matrix, the equations of surface piezoelectricity for different orders are derived. A simple example is given to demonstrate the surface effect on wave propagation in a half-space.
Irradiation-induced Thermo-mechanical Coupling in Dispersion Nuclear Fuel Elements
Dispersion nuclear fuel elements are widely used in the research and test reactors. Due to their high burnup, they have promising prospects to be used in GEN IV advanced reactors, and to be an alternative to transmute the long-lived actinides in nuclear wastes. Besides irradiation experiments, numerical simulation and theoretical analysis become important methods to optimize design and interpret the failure mechanism. The components in the nuclear reactors are exposed in an extreme environment, so the irradiation damage interactions must be considered in order to predict their life-time. A review is presented of our recent studies.
Recursive Algorithm to Study Transmission Coefficients for P-SV Waves Propagating in 1D Inhomogeneous Phononic Crystals
A recursive algorithm based on stiffness matrix is proposed to study the P-SV wave propagation in a one-dimensional phononic crystal of continuous inhomogeneous media. It could avoid the numerical instability of the transfer matrix method for the high frequency range. A power law profile is proposed for the cell of phononic crystals, where the density and modulus change continuously. It is found that the inhomogeneity could reduce the size of the bandgaps of the transmission or reflection coefficients of the wave.
A Modified Micromechanical-based Model for Characterizing Magneto-mechanical Behavior in NiMnGa Alloys
As one of typical ferromagnetic shape memory alloys, NiMnGa alloy at room temperature is a heterogeneous material with martensitic variants and high magnetic anisotropy property. By taking the influence of the material anisotropy on the martensite variants reorientation into account, a modified micromechanical model, which follows the Eshelby equivalent inclusion method and the Mori-Tanaka scheme, is proposed to investigate the effect of morphology of the martensite variant inclusion on magneto-mechanical behaviors of the FSMA during martensite variants reorientation process. The simulation results show a good agreement with the experimental data.
Effects of Heterogeneous Distribution of Fuel Particles on the Irradiation-Induced Mechanical Behaviors in the Dispersion Nuclear Fuel Plate
The three-dimensional finite element models for different particle distances are developed. The virtual temperature load method is adopted in simulating the swelling effect of fuel particles and irradiation growth of the cladding. The irradiation-induced mechanical behaviors in the dispersion nuclear fuel plate are numerically obtained. The research results indicate that the maximums of the first principal stresses between the close fuel particles are obviously affected by the particle distance variation and may be the main factor that induces the matrix failure.
Wave Propagation in Heterogeneous Materials with Secondary Substructure
We study numerically the influence of the presence of a complex internal structure of laminates consisting of layers of different properties on the dynamic response of a material. The influence of the mutual position of different layers on wave profiles is demonstrated by example of double periodic laminates.
Analysis of Thermo-mechanical Interaction in Membranes
Considering high temperature gradient, the non-Fourier heat conduction and the couple stresses，the simulation results of one dimensional transient for finite thick membrane shows that the thermal shock is always company with the high temperature gradient and high couple stresses. The larger delay time, the larger stress and couple stresses on the surface layer. So it is a basic principle to choose material with shorter delay time as the exterior surface layer and that with longer delay time as subsurface layer in thermal insulation design.
Effect of UV Aging on the Rheological Properties of Asphalt Mortar
Ultraviolet radiation (UV) aging of asphalt is an inevitable process in the practical application of asphalt pavement. This paper presented a comprehensive study on the effect of UV aging on rheological properties of asphalt mortar. Asphalt mortar was prepared with a filler to asphalt ratio of 1.0. Thin film asphalt mortar samples were pressed using a dynamic shear rheometer (DSR) instrument. The complex shear modulus (G*) and phase angle(δ) of asphalt mortars were determined using DSR through temperatures sweep tests from -10°C to 35°C. The results indicate that film thickness has a slight influence on the UV aging properties of asphalt mortar. Higher UV aging temperature would result in more serious aging.
Confinement Effects on the Dynamic Compressive Properties of Granite
The behavior of rock under dynamic loading has gained great attention in many fields, such as exploration, rock fragmentation, military defense and rock-burst. Experiments on granite under uniaxial and confined compressive loading are conducted with the split Hopkinson pressure bar(SHPB). By adding the soft material mass as the pulse shaper, it can ensure the stress uniformity in the specimens before fracture and avoid the fluctuation of test data due to input stress pulse. The experimental results show that the compressive strength and the fragment numbers of granite increase with the strain rate. The fragment size decreases with the strain rate. The research method and conclusions can be used to analyze the dynamic behavior of the other brittle materials.
Study on the Molding Technology of Bonded NdFeB Magnets from Injection Molding
For the composite powders with different NdFeB powder content , the effect of processing parameters including injection temperature, injection pressure, holding pressure and injection velocity on the sizes and properties of injection molded bonded NdFeB magnet was investigated, and the reasons of effect were also analyzed. The injection temperature and injection pressure have the greatest impact on the performance. The injection molding bonded NdFeB magnet with high performance is obtained with magnetic properties Br=0.539T, Hcb=345.37kA/m, Hci=681.02kA/m, (BH)max=47.37kJ/m3 and density 5.07g/cm3 respectively
A Mechanochemical Model of the Microtubule Growing Dynamic
Microtubule dynamics is largely influenced by the hydrolysis of nucleotides bound on tubulins and the associated configuration changes. The growth-stabilizing mechanism considering the hydrolysis effect is supposed as a “GTP capâ€ model. Meanwhile, there exists an open sheet structure curving outward at the end of the microtubule, and the growth process experiences a “sheet-to-tubeâ€ curvature transition. This growth mode may also help stabilize the growth, which is referred to as a “conformational capâ€ model. Here, we present a novel mechanochemical model to simulate the microtubule growth process at the resolution of monomer, and look into the stabilizing mechanism of the microtubule growth.
A Family of Hamiltion-type Variational Principles for Linear Thermo-Magneto-Electro-Elastic Solids
A general Hamiltion-type variational principle of linear thermo-magneto-electro -elastic solids is presented which contains stress, strain, displacement, electric displacement, electric field, electric potential, magnetic displacement, magnetic field and magnetic potential, temperature and heat flux as the independently assumed field variables. The stationary conditions of the functional are all governing equations in linear magneto-electro-elastic solids. Using a set of constitutive equations, a family of Hamiltion-type variational principles of linear thermo-magneto-electro-elastic solids is presented which are deduced from the general variational principle.
Optically-driven Reversible Deformations in Nematic Liquid Crystal Polymer Networks
The bending behavior of the polydomain liquid crystal polymer (LCP) plate upon linear polarized light is studied. Considering the photoisomerization and light absorption process, we obtain the photo strains in polydomain LCPs by using the Mori-Tanaka mean field method. We found that under the irradiation of light, effective bending moments will be produced. The bending moments are affected by the direction of polarized light. The bending behaviors of plates with different boundary conditions are studied. The recent experimental phenomena such as directionally dependent bending of polydomain films can be effectively described in our results.
Modeling Stress Dependent Hysteretic SMA Behavior Based on a Block-spin-approach from Statistical Mechanics
Hysteresis behavior of shape memory alloys (SMAs) is highly important for design and applicability of these materials in active structures. Especially the damping behavior depends strongly on the hysteretic behavior. Experimental investigations show the influences of stress on the hysteretic cycle according to phenomena like elastic stored energy, interfacial energy, elastic energy related to an external stress field and work of friction. The current study shows a model which is capable to investigate the influence of an external applied stress field on the hysteretic cycle according to a recently developed statistical mechanics method.
Study on Microwave Curing of Epoxy Matrix Composites.
Microwave has been studied as an efficient alternative energy source which can significantly speeds up the curing process of polymer and polymer-based composites comparing with conventional heating systems, the ability of rapid and volumetric heating can dramatically reduce processing times and save energy. In this paper, the fundamentals of electromagnetic transmission, heat generation and transfer and cure kinetics of microwave curing process of polymer-based composites are discussed, and the latest research and practical applications of microwave curing are reviewed.
Effect of Phase Change Material on Temperature Susceptibility of Asphalt
High-temperature is a major problem for asphalt pavement in summer. A possible answer to this problem is the application of phase change material (PCM), thereby increasing thermal energy storage and reducing temperature-peak of pavement. This research aims to analyze the influence of PCM on temperature susceptibility of asphalt. Asphalt mortars were prepared with PCM contents of 3%, 5% and 7% respectively. Penetration indexes (PI), viscosity-temperature susceptibility (VTS) were introduced to evaluate temperature susceptibility of asphalt. The results indicate that the addition of PCM can result in a slight increase in the temperature susceptibility of asphalt, while there is an optimum content that can minimize the influence. Thus, it is promising to use PCM in asphalt concrete to solve the problem of pavement high-temperature.
Test Study on Influence of Soaking in Acid/base Solution on Normal Deformation of Rock Fracture
Since engineering rock mass always exist in natural weathering or erosion environment, it’s interesting to study the influence of soaking in acid/base solution on strength and deformation features of rock and its fracture. Strength and deformation characteristics of four types of cylindrical rock samples soaked in five kinds of solution and initial normal stiffness and maximum aperture of its fracture were experimental studied in this paper. The results show that both strength and average modulus of rock samples that have been soaked in acid/base solution are lower than fresh ones; initial normal stiffness of soaking rock fracture is also lower, but its maximum aperture is bigger than fresh ones.
Application of NCS Soil Agent in Treating the Super-wet Soil
The composite solidified material—NCS soil agent is composed of lime, cement and SCA.Thereinto,SCA is the inorganic material of high performance and strong water-absorbent. According to the treatment projects, the super-wet soil in Ya’an area of Sichuan province is improved by the lime, cement and NCS soil agent. Based on the results of indoor test, the engineering characteristics of the improved super-wet soil such as compaction characteristics, unconfined compressive strength are studied. Furthermore, the relations between the engineering characteristics of the improved super-wet soil and the quantity of the additive are analysized. The researches show that:(1)the water content of the improved super-wet soil decreases in a large extent;(2) the unconfined compressive strength improves obviously; (3)the optimum water content is bigger than the initial soil.
Experimental Research on Creep and Hydrate Laws of Heterogeneous Soft Rock under Deep Mining
Along with practical situation that the Chinese mines are getting into the deep mining, the creep deformation of heterogeneous soft rock increased greatly. According to the experimental research on creep and hydrate laws of deep soft rock, the creep and hydrate laws of heterogeneous soft rock under water are found. The results show the effect of water ratio on creep deformation of soft rock is obvious. The creep dependent variable of soft rock advanced gradually with increasing of water ratio. The effect of hydrate on soft rock strength is projecting. Because of good water absorbability, the uniaxial compressive strength of hydrated rock could reduce to 12.5% of nature on max. There are two situations after hydration: First, the strength of soft rock debases. Secondly, the soft rock dissolves directly. The experimental results could provide reference on creep and hydrate laws of heterogeneous soft rock under deep mining conditions.
Effect of Active Mineral Filler on Asphalt Mixtures with Acidic Aggregates
Active mineral filler’ (AMF) was introduced and applied, with the aim to reduce the level of damage due to moisture in asphalt mixture. Four types of asphalt mixtures (gneiss aggregate and mineral filler, gneiss aggregate and AMF, granite aggregate and mineral filler and granite aggregate and AMF) were prepared. The moisture susceptibilities of all mixtures were evaluated by Marshall stability and immersion rutting test. The results show that the addition of AMF improves the moisture sensitivity of asphalt mixture. Furthermore, the asphalt mixtures consisting of granite are found to be superior to the ones consisting of gneiss for the greater improvement of moisture sensitive.
Experiment Modeling of the Finite Thermo-hyperelasticity for Fiber-reinforced Rubber Composites
Mechanical behaviors for a kind of two-family fiber-reinforced rubber composite are studied by uniaxial tension tests with a varying temperature fields using a Zwick020 material test machine. Stress-strain relation curves for the rubber composite and the pure rubber are attained under different temperatures. The effect of temperature on the deformation of the material is analyzed. A strain energy function to catch the finite thermo-hyperelastic behaviors of the rubber composite material is developed using the least square method. Material constants are also determined from the experimental results. The agreement between the calculated and the experiment results are good. The thermo-hyperelastic strain energy function proposed here may have a practical use for the rubber products with two-family fiber-reinforced rubber composites.
Corrosion Behavior of AA6061-T6 Alloy in the Ethylene Glycol-Water Solution
The corrosion behavior of AA6061-T6 alloy was investigated in ethylene glycol (EG)-water solutions by the electrochemical measurement. After long term immersion test, the AA6061 alloy suffers more severe corrosion in stimulated corrosion water with EG than that without EG. The addition of glycolic acid (GA) to the EG-water solution renders the corrosion potential to more negative direction and increases the corrosion current density. There are no evident changes for the cathodic Tafel slopes in the EG-water solution with and without GA. The presence of GA strengthens the anodic dissolution of AA6061 alloy in the EG-water solution. The electrochemical impedance spectra (EIS) showed that the polarization resistance of AA6061 decreased with the increasing of the concentration of GA. The enhanced corrosion of AA6061 alloy is attributed to the oxidation of EG, which can form a dissolved complex with aluminium ions.
Discussion on the Relation between Fractal Distribution of Slope Loose Deposit and Its Mechanics Principle
The fractal law of loose slope deposits around the State Road 317 Dujiangyan to Wenchuan, was studied by employing fractal theory. The research shows that: The fractal dimension of particle size distribution of the slope loose deposit is between 2 and 3. Density, porosity and internal friction angle was studied experimentally with large relative densitometer and direct shear apparatus. The result shows that, as fractal dimension increases, density and porosity of the slope loose deposits increases, while the internal friction angle decreases. On this basis, a fitted formula which can approximately reflect the relation between fractal dimension, density, porosity and internal friction angle was obtained.
Experimental Study on Rock Salt Dissolving Characteristics with Stress Effect
Through a lot of rock salt dissolving characteristics tests with stress effect, difference between rock salt dissolving characteristics with and without stress effect has been studied. Under stress effect, especially after plastic strain has been occurred, rock salt dissolving characteristics has been changed obviously; the relationship between dissolving mass and axial plastic strain and dissolving time can be used to quantitatively describe the variation of rock salt dissolving velocity with stress effect; the development of rock salt cracks can be used to analyze the mechanism of variation of rock salt dissolving velocity with stress effect. The achieved results on this paper can provide a foundation for the further analyses of coupled stress-dissolving mechanism of rock salt.
Manufacture and Properties of Bonded NdFeB Magnets from Injection Molding
The injection molding bonded magnets were prepared by using the MQP-B rapidly quenching NdFeB magnetic powder and polyamide 12. The manufacture technology of bonded NdFeB magnets was described. The effects of content of binder, additives and magnetic powder on the magnetic property, mechanical property and fluidity of injection molding magnets were investigated. Furthermore, the effect mechanism was revealed microcosmically. The magnetic properties are :Br=0.539T, Hcb=345.37kA/m, Hci=681.02kA/m, and (BH)max=47.37kJ/m3.
Design of Polymeric UHMWPE-based Composites with Increased Tribotechnical Properties by Mechanical Activation, Ion Implantation, Chemical Modification and Nanofiller Enforcement
Influence of Mechanical Activation (MA), Chemical Modification (ChM), Ion Implantation (IIm) and adding different size fillers (nano- and micro) onto physical-mechanical and tribotechnical properties of Ultra High Molecular Weight Polyethylene (UHMWPE) are investigated. By means of X-ray diffraction, IR-spectroscopy, DSC and SEM the structure and wearing pattern for a number of UHMWPE-based composite enforced by Ð¡ and Al2O3 nanofibers, Ð¡u Ð¸ SiO2 nanoparticles, Al2O3 Ð¸ AlO(OH)) microparticles were examined. Comparison of different ways of modification of UHMWPE binder to obtain composites on its basis is carried out. The mechanism to explain high wear resistance of composites with high-molecular weight matrix is discussed.
Numerical Simulation for Heat-solid Coupling of Coal under Injecting Heat
Based on the mechanism for heat-solid coupling of coal under injecting heat, the numerical model for heat-solid coupling of coal under injecting heat has been established. The temperature field and stress field laws of coal body nearby the well casing with exploitation are found. The results show that the temperature of coal nearby the well is higher than it far away the well. The heat action on the boundary spread to interior of coal gradually along with time. The velocity of gas desorption increase tremendously because of augmenting of temperature gradient, and the gas will release hand over fist from matrix of coal and the surface of coal body. The reservoir stress extends from well to surrounding rock along with time. The simulation results could provide reference on heat-solid coupling of coal under injecting heat.
Modeling Displacive-Diffusional Coupling in Solid State Processes
Microstructural evolution in materials often involves coupled displacive and diffusional processes as a rule rather than as an exception. For example, structural phase transformations often involve coupled lattice shear, shuffle, and diffusion. Dislocation shearing of ordered precipitates can couple strongly to chemical reordering. In these processes it is the coupling that governs the rate of transformation and deformation. Mechanistic studies of these mechano-chemically coupled processes require modeling capabilities at atomistic length scales but diffusional time scales. In this presentation, we introduce a new approach, called diffusive molecular dynamics (DMD), which can capture diffusional time scale while maintaining atomic resolution, by coarse-graining over atomic vibrations. DMD combines long-range elastic and short-range atomic coordinate interactions simultaneously with gradient chemical thermodynamics. Kinetically it solves the master equation for diffusion on a moving atom grid. Examples will be given to demonstrate applications of DMD to various solid-state processes that involved coupling between displacive and diffusional atomic movements, including nanoindentation and climb of extended dislocations
Multimodal Nanostructured Titanium Using Several Plastic Deformation
In the present study, multimodal nanostructured titanium was engineered using severe plastic deformation. The multimodal structured titanium exhibits an ultrahigh strength of over 940 MPa and a large failure elongation of 24%. The ultrahigh strength is mainly derived from the nanostructured structures; whilst the exceptional ductility originates from the large fraction of high angle grain boundaries, micro-scale structures, and the non-equilibrium grain boundary configuration. It is worth noting that apart from dislocation slip processes, the formation of deformation twins reduced the effective slip distance and increased the strain hardening capacity via the Hall-Petch mechanism, leading to high ductility of the multimodal structured titanium.
Strengthening Mechanism in Metals with Nano-scale Twins
Methods used to strengthen metals typically also cause a pronounced decrease in ductility. Here we review a novel approach to realize significant strengthening while maintaining a reasonable ductility by nanoscale coherent twins. By introducing a high density of nano-scale twins in pure Cu, high strength and good ductility are achieved simultaneously. Particularly, a maximum strength was found in the nanotwinned Cu at a twin thickness of 15 nm, followed by a rapid softening and a monotonic enhancement in both the strain-hardening and ductility at smaller twin thicknesses. These unique properties and related dislocation–twin boundary strengthening mechanism differ fundamentally from the dislocation-grain boundary strengthening mechanism.
Combined Self-Consistent and Mori-Tanaka Approach for Mechanical Properties of Particulate Composites and Applications
A combined self-consistent and Mori-Tanaka approach is developed for the evaluation of the effective property of particulate composites. The conventional Mori-Tanaka and self-consistent schemes can be obtained as its two special cases. The effective elastic property of some typical particulate composites is analyzed and compared with experimental results. This approach can be extended to the evaluation of the elastoplastic properties of particulate composites. The introduced parameter n can be associated with experimental data for a more accurate evaluation, and can play a role to describe indirectly the relative size-effect of particles.
Numerical Modeling of Microstructure Evolution in Alloy Solidification
During the last two decades, extensive efforts have been dedicated to develop numerical models to simulate the microstructures of solidification. In this paper, the cellular automaton and lattice Boltzmann based models are presented. The models can produce a graphic output of microstructure evolution, transforming the computer into a dynamic microscope. The capabilities of the models are addressed by presenting examples encompassing a broad variety of issues such as dendritic growth in two and three dimensions, the formation of hydrogen pores in aluminum castings, and the effect of convection on dendritic solidification. Also presented is liquid phase separation in monotectic alloys.
Preparation and Characterization of WO3 Nanocrystalline by Selected-dissolving of WO3/SiO2 Composites
WO3/SiO2 was first synthesized by incorporating silicotungstic acid into amorphous SiO2 substrate via impregnation-sintering method. The nanocrystalline tungsten trioxide (WO3) particles were achieved through selected-dissolving as-prepared WO3/SiO2 composites in dilute fluoric acid solution. The crystallizing formation procedure of WO3 was monitored by BET and thermogravimetric-differential thermal analysis (TG-TGA), respectively. The resultant WO3 products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results indicated the obtained particles belonged to the high pure orthorhombic phase, and possessed a small particle diameter and a narrow particle size distribution.
Viscoelastic Relaxation of Bulk Metal Glass Zr46.75Ti8.25Cu7.5Ni10Be27.5 at High Temperature
The internal friction of the bulk metallic glass (BMG) Zr46.75Ti8.25Cu7.5Ni10Be27.5 (at. %) originates from the viscoelasticity. The viscoelastic internal friction increases with decreasing vibration frequency at higher temperatures and increases with increasing frequency at lower temperatures. It gives rise to a minimum as a function of frequency. The proposed viscoelastic relaxation three-parameter model can be used to describe the viscoelastic internal friction of the present BMG.
Effect of Zeolite 4A on the Aging and Rheological Properties of SBS Asphalt Binders
Effect of zeolite 4A on the properties of styrene-butadiene-styrene (SBS) asphalt binders was studied. The thin film oven test (TFOT) and pressure aging vessel system (PAV) were used to investigate the effect of zeolite 4A on the short-term and long-time aging properties of SBS asphalt. The effect of zeolite 4A on the rheological properties was studied using dynamic shear rheometer (DSR) test. The variation of the softening point, penetration, ductility of asphalt binder indicated that the addition of zeolite 4A can improve the aging property and high temperature stability of SBS asphalt, especially when the content of it was 6wt%. Furthermore, the results of the temperature sweeps by the DSR on SBS asphalt and that modified by 6wt% zeolite 4A showed that the rutting resistance and high temperature stability of SBS asphalt were improved when the zeolite 4A had been added.
Lamb Wave Propagation in a Functionally Graded Piezoelectric- piezomagnetic Material Plate
Propagation of Lamb waves in a functionally graded piezoelectric-piezomagnetic material plate with material parameters change gradually along thickness direction is investigated. The power series technique is employed for solving the governing differential equations with variable coefficients. In numerical examples, the influence of the variation of each parameter on dispersion curves and cut-off frequency for electrical and magnetic open case is discussed. The results obtained indicate that graded elastic parameters and mass density affect the phase velocity remarkably. Some other graded electrical and magnetic parameters affect the phase velocity a little. The cut-off frequency is close related to two elastic parameters and mass density, while other material parameters almost can’t affect it. All the results may provide theoretical guidance not only for the analysis and design of high performance electro-magnetic transducers but also for the ultrasonic nondestructive evaluation (NDE) of functionally graded piezoelectric-piezomagnetic materials.
Deformation Characteristic of Metal Foam under Different Strain Rate and Temperature
The non-uniformity of metal foam structure affect the deformation models significantly. Different sizes of metal foam have effect the mechanical properties of materials, each microscopic structure deformation models corresponds to different macro impact energy-absorption ability of metal foam. Compressive deformation behavior of a closed-cell Aluminum foam was assessed under different strain rate (0.001/s to 2000/s) and temperature(-100℃ to 500℃). High strain rate experimental have proceeded by using the Split Hopkinson Pressure Bar (SHPB), and the Quasi-static tests have conducted on the Material testing machine. Through the experiment data analyzed the influence of thermal-mechanics coupling. It’s found that with the increase of the strain rate, the collapse stress and plateau stress increased. With the increase of the temperature, the plateau stress decreased. And with the increasing of the specimen size, the platform of the stress-strain curve are linearly reduced, the reason of this phenomenon are mainly influenced by the inhomogeneous of metal foam.
Ultimate Bearing Capacity of RC Beams Reinforced by Prestressed AFRP Sheet under 4-point Bending
The computation results of a layered shear-lag model on the stress redistributions of prestressed AFR sheet-reinforced RC beams with a crack under 4-point bending and the stress distributions of each sub-layer were adopted. On the basis of this, an existing modified shear-lag model was developed and used to further study the stress concentrations induced by aramid bundle fractures in sheet; and then the statistical strength and ultimate bearing capacity of beams strengthened with prestressed AFRP sheet under 4-point bending were obtained by using a micro-statistical failure theory. The results obtained relatively coincide with the existing test values.
Characterization of In-Plane Fiber Architecture in Z-Pinned Unidirectional Carbon-Fiber Preforms
In this study a comprehensive investigation on the changes of microstructure in z-pinned unidirectional carbon-fiber preforms caused by the z-pinning by dry process is carried out. Analysis of digital images of the local fiber architecture in a unit cell is taken. Distortion length and distortion width which characterize the local microstructure are measured. The fiber distortion model is used to describe the inhomogeneous fiber volume fraction and in-plane fiber deflection angle due to z-pinning.
Effect of Adherent Thickness on Strength of Single-lap Adhesive Composites Joints
Tension tests on single-lap T700/EXOPY unidirectional laminates adhesively joints of two kinds of adherent thicknesses under uniaxial tensile loading are performed, and the failure modes and shear stress—strain curves of them are analyzed and compared. Furthermore, finite element simulations are carried out to analyze the peel/shear stress fields along joint interfaces. The simulation results show that the joint strength is not linear with the adherent thickness. The maximum peel/shear stress of the interface of lap zone increases with the increase of adherent thickness, and the joint strength decreases with the increase of thickness difference of adherents. Good agreements between the present simulations and the experimental results are found.
Numerical Analysis of the Residual Stresses in Z-Pinned Composite Laminates during the Curing Process
In this study the residual stresses in z-pinned composites induced during process are analyzed by FE model. The model is composed of resin-rich region, composite and pin. The chemical shrinkage and thermal expansion of the matrix during curing are considered in this model. The interface between resin-rich region and composite or pin is studied in detail. The higher stresses are located at the vicinity of the interface. The stresses decrease quickly with distance away from the interface.
Evaluation of the Effective Properties of High Silica/Phenolic Composite under Volumetric Ablation Condition
The volumetric ablation problem of high silica/phenolic composites was studied by microstructure analysis of the internal material before and after ablation. The mesophysical models considering matrix and fiber were established. The effect of pore gas pressure on the material properties was considered in the Arrhenius equation. The macroscopic thermo-mechanical and thermo-physical behavior for high silica fiber and phenolic resin associated with the mesostructure were discussed by the Mori-Tanaka method. Finally, the elastic modulus and thermal expansion coefficient of high silica/phenolic composites under high temperature were predicted according to the effective properties of component materials.
Study on Impact Resistance Properties of Key Structure with the Dynamic Photoelastic Experimental Method
The impact resistance properties of structure play an important role in designing. The dynamic photoelasticity has been widely used to investigate impact resistance properties of structures. The dynamic parameters of structures, such as propagation of stress wave and dynamic stress concentration, are obtained through this method, which provide guidelines for structure design and optimization. In this paper, this method is applied in the research on impact resistance properties of a ship key structure. An improved digital dynamic photoelastic system is also adopted to calculate elastic wave propagation in the medium, in which the diode-pumped solid-state green laser and high-speed CCD are used as light source luminaries and recording system respectively. This method has been adopted in this project successfully, and better results have been obtained.
Variation of Electro-conductive Property of SSF/PP System under Impact Loading using a Modified SHPB
In order to detect the variation of electro-conductive property of short steel fiber (SSF) filled polypropylene (PP) under impact loading, a modified split Hopkinson pressure bar (M-SHPB) is suggested. Such M-SHPB is constructed by designing a new test electrocircuit, and connecting it to the specimen and oscillograph. On the other hand, a copper foil cover is designed and placed on the whole SHPB equipment for avoiding disturbance of electromagnetic wave existing in the testing environment. By means of M-SHPB, the relation between the resistance and dynamic stress or dynamic strain is effectively detected. The experimental results indicate that the resistance of the material decreases with the increase of strain or stress.
Material-technological Modeling of Various Holding Times at the Partitioning Temperature in AHSS with Different Alloying Strategies
New methods of heat treatment usually require precise control of processing parameters. Therefore, a new type of heat treatment, the Q-P (quenching and partitioning) process, was carried out on a thermomechanical simulator, which enables application of rapid and precisely controlled temperature and deformation load. The most important Q-P process parameters are quenching and partitioning temperatures and partitioning hold, which affect not only the fraction of retained austenite but also precipitation of carbides. Q-P strategies with different partitioning conditions were tested on three different high strength low alloyed steels with different contents of Mn and Si. Strength of over 2000MPa with a ductility A5mm of about 14% was obtained for the optimal combination of partitioning hold conditions.
Porous Material Modeling and Electrical Characteristics Parametric Analysis of A Planar Solid Oxide Fuel Cell with Gradual Porosity Based on Finite Element Method
In this paper, a 3D conventional planar Solid Oxide Fuel Cell (SOFC) unit porous model had been built using finite element method (FEM). Types of single-unit cell models with different porosity gradient were created for the purpose of investigating the effect of porosity of the anode on cell current-voltage (J-V) characteristics. The porosity of anode (ï¥ ) ranges between 0.2 to 0.5, and the anode was divided to most four layers with variable thicknesses. Simulation results were discussed comparing with experimental data from the literature. And the comparison shows that unit cell anode with two levels porosity gradient have the best performance. This paper finds a FEM way to study the single-unit cell electrical properties, and can be used as a tool for cell structure optimization.
Mechanical Properties of Modified Epoxy Resins and Their Glass Fiber Reinforced Composites
Diglycidyl ether of bisphenol-F (DGEBF)/diethyl toluene diamine (DETD) was used as the starting matrix. The reactive aliphatic diluent (n-butyl glycidyl ether, BGE) was employed as the soft modifier and the multiwalled carbon nanotubes (MWCNTs) were employed as the stiff modifier to modify the epoxy matrix. The mechanical properties of the modified epoxy system were examined first for optimizing its formulation. Then, the modified epoxy system was used to prepare glass fiber reinforced composites and the tensile, flexural and interlaminar shear properties of the fiber reinforced composites were investigated. It is shown that the introduction of BGE and MWCNTs led to the remarkable enhancements in the mechanical properties of the composites. This observation is explained by the variance in the fiber-matrix interfacial adhesion strength before and after the introduction of the modifiers.
Modelling the Microstructure Morphology and Material Properties of High Performance Alloys
The macroscopic behaviour of metallic components is governed by the morphology and behaviour of the microstructural constituents. To optimise the performance of modern engineering materials it is essential to gain a physical based understanding of the relevant deformation and damage mechanisms. Due to complex interdependencies during material processing, it is impossible to study the effect of different microstructural features on the global component behaviour solely by experimental characterisation methods. Furthermore saving time and costs, the validated and verified computationally aided simulation of material behaviour plays an increasingly important role in modern material design. Within the scope of this contribution, special emphasise is dedicated to the widely used Ni-based cast alloys, AHS steels and Ti-based alloys Spatially inhomogeneous solidification conditions within Ni-based cast parts cause the formation of micro-voids. Capturing the local curvature of pores and the overall void volume fraction by distributions of primitive geometries facilitates to quantify the deterioration of properties of porous material as compared to pore-free material. The remarkable macroscopic behaviour of dual-phase steels is due to placement of hard martensite within a soft ferritic matrix. Modelling the grain morphology by a Voronoï tessellation and the phase specific constitutive behaviour by Hall-Petch type relations enables to predict the formability and damage tolerance of these AHS steels. The Johnson-Mehl tessellation provides an adequate morphological description of Ti-base alloys featuring a duplex microstructure. Studying the distribution and redistribution of residual stresses during processing allows improving the accuracy of life time prediction of engineering components subjected to cyclic loading. Hence, multiscale material modelling techniques combined with a fully automated computer generation of microstructures provide a flexible and robust tool to study material and component behaviour by taking into account processing characteristics and microstructural features.
Ultrasonic Scattering in Polycrystalline Materials under Applied Loads
Previous research on acoustoelasticity has focused primarily on homogeneous materials (isotropic or crystalline) for which ultrasonic wave speed changes as a function of applied stress have been determined. However, the majority of structural materials of interest are heterogeneous such that ultrasonic scattering occurs when the wave length is on the order of the length scale of the microstructure. In this article, scattering from polycrystalline materials under applied stress is examined both theoretically and experimentally. The scattering is governed by the second-order orientation statistics of the grain moduli which depends quadratically on the applied stress. Applications for stress measurements are discussed.
Probing Viscoelastic Properties of Polymer Boundary Layers in Solutions
Viscoelastic properties of polymer boundary layers in solutions can be drastically different from that of the corresponding polymer solutions due to the interactions of the polymers and the solid surfaces and the confinement on the interfaces. We have used quartz crystal resonator technique to systematically investigate mechanical behaviors and viscoelasticities of boundary layers of polymer solutions under different conditions. The quartz crystal resonator technique probes the solution boundary layer with a thickness approximately equal to the viscous penetration depth adjacent to the surface of the quartz crystal. In most cases relevant to chemical and biological applications, the penetration depth ranges from nanometers to a few micrometers. In this talk, we will present results on polyethylene glycol (PEG) solutions and polystyrene sulfonate sodium (PSSS) solutions with different molecular weights and different concentrations. The results show that the viscosity and the shear modulus of a polymer solution boundary layer increase with the polymer concentration in the solution. The increase depends on the molecular weight and the molecular configuration. The results are analyzed in terms of the scaling theory proposed by de Gennes for polymer solutions. For solutions with large PEG molecules the viscoelastic properties of their solution boundary layers are distinctly different from that of corresponding bulk solutions. These behaviors can be explained in terms of solid-like behaviors due to an increased concentration of PEGs next to the quartz crystal and the interactions between polymers and the interface.
Thermal-elastoplastic Constitutive Model of Particle-filled Composites
A thermal-elastoplastic constitutive model is proposed for particle-filled composites in this paper. Particles are assumed to be linear thermoelastic while the matrix follows the thermal-elastoplastic responses with the generalized Ramberg-Osgood relation. Based on the micromechanics methodology and homogenization procedures, the effective thermal-mechanical constitutive functions are derived including the macroscopic Helmholtz free energy and the macroscopic yield function. First, it is assumed that in the case of plastic unloading or stress-strain state being in the macroscopic yield surface, the constitutive relation of the composites is linear thermoelastic expressed by the macroscopic Helmholtz free energy. The micromechanics-based thermoelastic properties of the composite are obtained including the effective elastic moduli, thermal expansion coefficients, and specific heats. Furthermore, with the concept of linear comparison composites, the variational principle is extended to consider the thermal effect, from which the lower bound of the macroscopic stress potential for the nonlinear composites can be computed. The associated macroscopic plastic strain is defined, and the macroscopic yield function in the temperature-strain space is therefore determined. Finally, the above two constitutive functions are combined with the thermal-elastoplastic constitutive theory proposed by Huang (1994) to develop the loading-unloading criterion in the temperature-strain space and the incremental thermal-elastoplastic constitutive relations for particulate composites. The results can be useful in the study of the thermomechanical behavior of particle-filled composites at elevated temperatures.
Interfaces in Size-Dependent Crystal Plasticity
Plasticity in heterogeneous materials with small domains is governed by the interactions and reactions of dislocations and interfaces. These include reactions of existing dislocations, as well as the nucleation of dislocations at an interface. The rational for interface dominated plasticity is simple: dislocations glide through the single crystal domain with relative ease, but pile-up at interfaces, so that interface reactions become a critical step in continuing plastic deformation. While the details of dislocation reactions at interfaces take place at the atomic scale, and the behavior of dislocations in bulk is most accurately modeled by discrete dislocation dynamics, both of these models are much too expensive and impractical for analyzing the resulting bulk behavior. The need for a continuum framework for describing the plasticity across crystal interfaces, including the ubiquitous size effects, is acute. Recently developed size-dependent crystal plasticity theory [Mesarovic et al. 2010 J. Mech. Phys. Solids 58, 311-29] employs the representation of the singular part of dislocation pile-up boundary layers as superdislocation boundary layers, or equivalently, as jumps in slip at the boundary, but internal to the crystal. These boundary superdislocations exist on two sides of an interface and react or combine to lower the total energy under certain conditions. Using this theory, we obtain solutions to simple problems of single-slip and double-slip shear of sandwiched thin film. Then, we compare the results with available discrete dislocation simulations.
Developing Multi-scale Model to Study Atypical Crystal Plastic Behavior
To study atypical plasticity behaviors for single-crystals at submicron-to-nanometer scales, a three-dimensional discrete dislocation dynamics (3D-DDD) model coupled with finite element method (FEM) is developed. With this new formulation, 3D discrete dislocation plasticity is completely handled under a continuum mechanics framework. It has a competitive advantage in solving large and nonlinear deformation problems by communicating deformation variables between DDD and FEM. As an application, an uniaxial compression test for Cu micro-pillars is simulated to investigate the process of dislocation propagating, starvation hardening and the effect of boundary conditions.
A Phenomenological Mean Field Model for Coupling Phase Transformation with Plasticity
The interaction between martensitic phase transformation (MPT) and plasticity is highly relevant for describing the thermomechanical behavior of maraging steels. Contrary to materials undergoing coherent phase transformations the challenge of modeling maraging steels is the generation of plastic strains through high internal stresses triggered by MPT. In dependence of different thermomechanical multiaxial load cases the material behavior changes dramatically according to the diversity of selected martensitic variants, which influences the transformation strain as well as the induced plastic deformation. A phenomenological mean-field-model based on the β-scale transition rule has been developed for coupling MPT and TRIP. Furthermore, the evolution of the transformation surface as well as its convexity have been studied. The model predicts the material behavior for several thermomechanical multiaxial load cases very well as is demonstrated by a large experimental database.
Uniaxial and Multiaxial Ratchetting of SiCp/6061Al Composites: Constitutive Modeling and Finite Element Simulation
Based on the Eshelby equivalent inclusion theory and Mori-Tanaka (MT) averaging method, a meso-mechanical cyclic elasto-plastic constitutive model used for the simulation of uniaxial and multiaxial ratchetting of particle-reinforced metal matrix composites was proposed. The uniaxial and multiaxial rachetting behaviours of SiCP/6061Al composites are predicted by the proposed model. Then the simulations by the model are compared with those by the finite element analysis with the help of the representative volume element (RVE) of the composites. It is shown that: the proposed meso-mechanical constitutive model can describe the multiaxial ratchetting of the SiCP/6061Al composites reasonably; while the predicted ratchetting by the meso-mechanical model is fairly weaker than that by the finite element method due to the different choices of homogenization methods.
Some Modifications in Continuum Thermodynamics and their Applications
The continuum thermodynamics is one of the foundations of the continuum mechanics. In this paper some modifications in thermodynamics and their applications will be given. It is found that the first law of thermodynamics includes energy conservation law and physical variational principle. The physical variational principle is also a basic physical law and can be used to derive the governing equations in mechanics and physics. The accelerative variation of the temperature needs the extra heat which will produce the inertial entropy. Applying the inertial entropy theory the temperature wave equation with finite phase velocity is obtained.
Dynamic Response of Circular Sandwich Plate Subjected to Blast Loading
The dynamic response of the clamped metal foam core circular sandwich plate to blast loading is analytically and numerically studied. Assuming a constant core strength in compression, membrane mode solution for the problem is obtained. Also, finite element calculation is carried out to numerically study the problem. Comparisons of the analytical solution with the numerical result are presented and good agreements are found.
Research on Creep Characteristic of Nano-Montmorillonite Modified Asphalt Binder
The requirement for modified bitumen are increasing according to the high load condition and worse serving environment in modern highway construction. The aim of this work was the comparison of aging resistance capability from the creep stiffness modulus curve, for original asphalt and asphalt modified with different amounts organic nanometer montmorillonite (OMMT). Repeated creep and recovery experiments using dynamic shear rheometer (DSR) at different temperatures were carried out. The result showed that the OMMT introduction can reduce the creep stiffness modulus of the asphalt aged and 5wt% OMMT introduction reduce it most obviously, which indicates that the OMMT power can effectively improve the aging resistance capability of the asphalts.
Nearly Frictionless Plastic Flow within Annular Micropores
Plastic flows associated with very large deformations for glassy (amorphous) matter confined in microscopic domain at room temperature regime was investigated using a transition-state dependent model considering the shear thinning behavior which means, once material being subjected to high shear rates, the viscosity diminishes with increasing shear rate. By using the boundary perturbation method, we obtained a class of plastic flows for certain activation energy and activation volume with the presumed small wavy roughness distributed along the walls of an annular micropore. Results show that there might be nearly frictionless plastic flows due to the almost vanishing viscosity in micropores.
A Constitutive Model for Coupled Transformation and Plastic Behavior of Shape Memory Alloys
A constitutive model is developed for shape memory alloys. It is based on the concept that an SMA is composed of austenite and martensite, and its constitutive behavior is the combination of the individual behavior of each of its two phases. The deformation of the martensite phase is separated into thermal, transformation, elastic and plastic parts, and that in the austenite phase is separated into thermal, elastic and plastic parts. The main features of SMAs such as shape memory effect, pseudoelasticity and other typical features can be well replicated with the model.
Numerical Analysis of Elastoplastic Behavior of Functionally Graded Foams
The objective of this work is to numerically investigate the elastoplastic behavior of functionally graded foams. We propose a 3D model of functionally graded foams based on the tetrakaidecahedral open cell. The variation in the cell wall thickness induces density gradient. Based on the finite element method, macroscopic elastic constants and initial yield stress against the relative density and grade variation are discussed in this paper. Through a systematic study we find that the grade variation significantly affects the Young’s modulus and the initial yield stress of the foams.
Creep Damage Calculation Model of Quasi-soft Soil
Quasi-soft soil is one of the special soft soils that a suit of carbonaceous soft rocks. It is very important that how to describe the creep damage behavior of quasi-soft soil. For the Bingham creep model, a nonlinear function is introduced into initial creep phase and steady creep phase, and damage is introduced into accelerating creep phase; then, the creep damage constitutive equation is presented on the basis of Xu’s model. To check the validity of the new model, the creep curves are calculated and compared with experimental data. The results show that the new model has a good consistency with the experimental results.
Research on a Dynamic Constitutive Model of Concrete in High Temperature
Based on ZWT visco-elastic constitutive model and damage evolution of concrete submitting Weibull distribution, the damage thermo-visco-elastic constitutive equation at high strain rates is proposed. Through numerical fitting, the parameters of the constitutive equation are obtained. A comparison between model prediction and experimental results show that both of them agree well with each other.
Calculate of the Fatigue Life for Concrete Pavement
Based on combination of the critical plane method and the energy method, the fatigue damage parameters are defined as the total strain energy density on the critical plane. The fatigue damage accumulation equations and the prediction model of the fatigue life are built. The fatigue lives under different load locations are predicted. Different cases corresponding to standard loads and overload conditions are analyzed. According to the numerical results and the three-stages law of concrete fatigue damages, it is found that some repairing measures can be adopted so that further damages may be avoided.
In-Situ Measurement of Local Zones’ Stress-strain Curves in Welded Joint by using Instrumented Double Holes Micro-shear Test
A in-situ test instrumentation, that related to the double holes micro-shear test approach, was presented to determine local zones’ stress-strain curves for the heterogeneous welded joint. The instrument is fixed by double holes drilled in material to be tested. The measured material bridge between the double holes is loaded by an indenter，until it is broken. According to measured load-displacement curve, the stress-strain curves of the material could be obtained. The accuracy of this technique was validated by using aluminium alloy. The plastic deformation properties of local zones in aluminum alloy welded joints were evaluated.
Nonlinear FEM Analysis and Experimental Research on the Mechanical Behavior of Beam-Column Joint of Concrete-Filled Tube Composite Material
Concrete-filled tube composite material is a new building material that is made by concrete and steel tubes. In this paper, the beam-column joints made by composite material were studied in low cyclic loading by finite element analysis and full-scale test. The mechanical properties and failure modes about the semi-rigid joints under low cyclic loading were investigated. The results were compared with each other and indicate that the joint shows good energy dissipation performance.
Study on Size Effects of Coal Intensity Based on Weibull Distribution
In order to realize reasonable application from standard coal body strength of laboratory test to the engineering project, a lot of uniaxial compressive strength experiments are conducted with five kinds of different sizes of coal samples from the domestic and international coal mine engineering using RFPA software numerical simulation. And deformations and damage forms of coal samples are researched. The fitting analysis is proceed by means of size effect experiment of uniaxial compressive strength and numerical test results of Weibull distribution, The attenuation law of coal samples macroscopic strength with coal intensity and homogeneous degree is obtained. The regression formula of standard coal sample strength reduction to coal intensity is gotten from test results, and satisfactory results comparing with the local data can be obtained.
Effect of the Material Absorptive to Laser on Temperature and Stress Field during Laser Braze Welding Processing
The absorptivity to laser energy of the different material is different. The temperature, pressure, surface roughness can all affect the absorptivity. The energy which the welding sample and the weld joint absorb are different， because the properties of the welding sample and the weld joint are different and their absorptive to the laser energy are different during the braze welding. In this paper, the numerical simulation method to the laser braze welding process is instructed firstly. The fluid’s fluxion effect and the latent heat of material fusion are considered. Secondly, the material absorptivity to laser is analyzed. The calculation results show that the effect of the different absorptivity to laser energy on the welding temperature and stress distribution is obvious during laser braze welding processing. The heat affect region, the welding temperature and stress distribution can all change with the different absorptivity.
Nonlinear Numerical Analysis of Large Deflection of FGM Circular Plate
The fundamental objective in this paper lies in finding out numerically large deflection of axisymmetrical homogeneous and functionally graded material (FGM) circular clamped plates under uniformly distributed load. The governing equations of large deflection of FGM plate were first derived and the dimensionless ones with boundary conditions were given. The nonlinear boundary-value problems for ordinary differential equations (ODEs) were solved using ODEs solver, COLSYS. The numerical results are obtained. The research method in this paper is effective and the results provide basis for the non-linear analysis or engineering applications of functionally graded circular plate problem.
Fracture Analysis of Nonhomogeneous Materials Using an Interaction Integral Method
For nonhomogeneous media without body force, the interaction integral method developed by Yu et al. [1] has become a promising technique for evaluating the stress intensity factors (SIFs) since it does not contain any term related to the derivatives of material properties and its validity is not affected by material interfaces. In this paper, the interaction integral method with these advantages is extended for a nonhomogeneous media under body force. Finally, the interaction integral method is employed to analyze the influences of Poisson’s ratio on the SIFs.
Implementation of Stress Loading Repetitive Unit Cell Finite Element Model
Finite element modeling of repetitive unit cell (RUC) with imposed periodic boundary conditions (PBCs) is a widely used approach for not only determining effective macroscopic material properties but also studying the damaging mechanism of composite materials. In this paper, a stress loading RUC model with PBCs was developed. The procedures for obtaining the effective homogeneous material properties are presented in detail. Thus the proposed model can be easily reconstructed by interested readers.
Application of XFEM in Metal Fracture Analysis
Metal fracture is a complex discontinuous mechanical problem, having geometric mutation in the object. The extended finite element method (XFEM) follows the frame of conventional finite element method (CFEM) and succeeds in solving the problems of contour stress and strain concentration at the crack tip, which is the effective method to analyze the metal fracture problems recently. In this paper, the basic theory of XFEM is introduced firstly. And then the two metal fracture examples are simulated by XFEM. The whole failure process, including initiation, propagation and abruption of crack, is simulated successfully, which indicates the feasibility and advantage of applying XFEM to the metal fracture analysis.
Fracture Mechanics of Composite Materials with Complex Interfaces
In order to analyze the fracture problems of composite materials with complex interfaces, such as particle reinforced composite materials (PRCMs), we developed a new interaction integral method by which the stress intensity factors (SIFs) can be solved using an integral domain with arbitrarily complex interfaces. The interaction (energy) integral method[1] was derived from the J-integral by considering a composition of two admissible states (the actual and auxiliary fields) to obtain mode I and mode II SIFs separately for homogeneous materials. Subsequently, the interaction integral method was successfully used to solve the crack problems in functionally graded materials (FGMs)[2]. Generally, the contour integral should be converted into an equivalent domain integral in numerical computations. Since divergence theorem can not be used in a domain with material interfaces, the material properties in the integral domain are assumed to be continuous in previous studies.
Stress-dependent Chemical Potentials and Their Applications Crystaline Solids
Crystalline solids (e.g., metals and ceramics) used in modern applications (electronics, MEMS, fuel cells, nuclear rectors, etc.) are often subjected to multi-physics driving forces (electrical, chemical, radiological, thermal, mechanical, etc.). The interactions of these different fields often determine the reliability and durability of the crystalline solids. To fully understand how the different driving forces interact requires theories and models that are capable of accounting the coupling of multi-physics processes.
Coupled Crystal Plasticity-Phase Field Formulation to Describe the Microstructural Evolution in Polycrystalline Aggregates During Recrystalisation
During thermo-mechanical processing, the strain energy stored in the microstucture of an FCC polycrystalline aggregate is generally reduced by physical mechanisms which rely, at least partially, on mechanisms such as dislocation cell or grain boundary motion which occur during recovery, recrystallisation or grain growth. The aim of this work is to develop a constitutive framework capable of describing the microstructural evolution driven by grain boundary curvature and/or stored energy during recrystallisation and grain growth. As recrystallisation processes depend primarily on the nature of the microstructural state, an accurate prediction of such phenomena requires that the microstructural heterogeneities which develop just before recrystallisation, such as dislocation cells and pile-ups, shear and twin bands, be properly described. The microstructural characteristics present in a polycrystal aggregate at the onset of thermal recrystallisation are first reproduced numerically. The constitutive behaviour of each grain in the aggregate is described using a non-local dislocation mechanics-based crystallographic formulation. Different measures of stored internal strain energy are determined based on the dislocation density distribution in the aggregate.
Computer-aided Design of Filled Polymeric Composites for Tribotechnical Applications
The paper deals with two scale approach to design of filled polymeric composites being based on computational mechanics and physical mesomechanics concepts. 1. Computer aided design—direct and inverse problems of deformation-strength design. Within this part of actuality are two following problems of deformation-strength properties of materials under design: i) Determination of effective material characteristics by values of control parameters—properties of phases to compose them, internal geometry of the material, pattern of interphase interaction (direct problems); ii) Finding out values of the control parameters to provide formation of materials with specified deformation-strength properties. Problems of two-scale analysis of filled polymeric composite coatings for tribotechnical application. First level is responsible for analysis of a representative mesovolume, where presence of different phases is considered in explicit form. At the second level the material is considered as a homogeneous medium. The obtained results are compared with experimental data with discussion of further prospects as well as for solving practical problems.
Multiaxial Fatigue Life Criteria and Microstructure Modeling of Metals
Years of active research since the end of the 19th century lead multiaxial fatigue strength methods to a mature state. However, some key experimental features are still not well understood and captured (non-proportional loadings, non-linear damage accumulation under variable amplitude loadings for example). This paper proposes to analyze some fundamentals of well known multiaxial fatigue life prediction criteria in the HCF domain in the light of FEA results using polycrystalline plasticity modeling. The local stress state developing in FCC artificial microstructures under complex loading has been investigated. The heterogeneity of the local mechanical response will be discussed
Dynamic and Nonlinear Behaviour of Deformation and Fracture of Quasicrystalline Materials
It is well-known that there are two displacement fields in the quasicrystals, one is the conventional displacement field named phonon field according to the terminological words of physics, the other is the phason field, the unusual displacement field. This leads two strain tensors and corresponding stress tensors. In the dynamic problems there are different points of view since the new solid phase discovered. This article discusses the arguments and compares the results coming from the different frameworks. Though the material presents brittle behaviour at low and medium temperature, it can dramatically undergo plastic deformation at high temperature. Because here is lack of any constitutive law of the plasticity so far, the study is confronted with fundamental difficulty. By using some simple physical models, the analytic solutions for deformation and fracture of plasticity of quasicrystalline have been constructed.
A Robust Cyclic Elastoplastic Constitutive Framework for Fatigue Life Analysis
The paper describes present capabilities for modelling the progressive mean stress relaxation observed under strain control. One difficulty is associated with the incomplete mean stress relaxation for low strain amplitudes, though the classical models based on Armstrong-Frederick kind of rules lead to a rapid vanishing of the mean stress. Coupling with the cyclic softening effect is another specific difficulty for the materials under application.
Fatigue Behavior and Short Crack Growth in Ferritic-pearlitic-bainitic Steel
Fatigue behavior of ferritic-pearlitic-bainitic steel, in particular the initiation and short crack growth has been studied. Smooth cylindrical specimens and cylindrical specimens having a shallow notch were produced from a massive forging. Fatigue cracks initiate preferably in persistent slip markings in areas of proeutectoid ferrite. Fatigue cracks grow with a contribution of crack linkage. The kinetics of short crack growth has been described using the concept of an “equivalent crackâ€. The plastic strain amplitude plays a dominant role in crack initiation stage, in the growth of short cracks and in the fatigue life evaluation.
Numerical Modeling of Mechanical Damage in Short Fiber Reinforced Thermoplastic Composites
The mechanical damage of carbon fiber reinforced thermoplastic (CFRTP) composite material is simulated using a periodic-cell simulation which can address the effect of composite microstructure, matrix viscoplasticity, and microscopic damage (fiber break and matrix crack). Especially, the effect of fiber length on the mechanical damage of composites is studied. When the fiber length is varied, the damage mode is divided into two modes: fiber-breaking mode and matrix cracking mode. This damage mode transition is consistent with the brittle-ductile transition in behavior of composite failure.
Study on Microstructure Variation of Different Areas of Fractured Rock under Triaxial Compression with Chemical Corrosion and Permeation
The physical and mechanical properties of different areas of fractured rock are different under external loading, so it has important significance to study the microstructure variation of different areas of fractured rock. Real-time computerized tomography (CT) scanning experiments in the failure process of sandstone specimens under triaxial compression with chemical corrosion and permeation are conducted. Based on the test results of standard deviation of CT number, the characteristics of microstructure variation of different areas of fractured rock effected by chemical corrosion and permeation are analyzed. The research results indicate that the anisotropy degree is increased under permeation and water pressure environment. Whether under penetration or under without penetration environment, the change rule of standard deviation rates in the level of crack tip has large difference with other scan layers.
Compression Responses of Brittle Granular Materials Subjected to Dynamic Loading
In order to reveal the damage and fracture property of brittle granular materials subjected to dynamic loading, the 3-D Particle Flow Code (PFC) is employed. Three scales, for example stiff balls at the micro scale, particles at the meso scale and specimen at the macro scale, are involved. Platen tests on aggregates indicate the consistency of Weibull modulus between numerical and experimental results, which proves the effectiveness of the cluster model. Based on 1-D-strain compression numeric tests of 300 aggregates, compression behavior and the evolution process of cluster size distribution are obtained. The results are helpful for the understanding of dynamic fragmentation for brittle materials.
Some Experimental Investigations on Laser Welding of AISI 904 L Stainless Steel Joints
Laser beam welding experiments have been carried out on 3 mm thick super austenitic stainless steel specimens in butt-joint configuration. The experiments were conducted according to the central composite design approach. The laser weld bead profiles mainly depends on various parameters such as beam power (BP), travel speed (TS) and focal position (FP) of the laser spot and types of shielding gas. These parameters are to be selected suitably to obtain the desirable output. The bead profiles were measured using optical microscopy. The laser weld joints were tested through tension test, impact test and hardness test. Microstructural studies were also carried out.
Inclusion Problem with Dislocation-Like Imperfect Interface
The exact solution to spherical inclusion problem with imperfect interface is presented. Using the dislocation-like imperfect interface model, a spherical inclusion embedded in an infinitely extended body with uniform stress at infinite is considered. Based on the direct method developed by Luré to solve symmetrically loaded sphere problems, spherical harmonic functions are introduced to express the displacement components. With the dislocation-like boundary conditions, the displacements fields inside and outside the inclusion are obtained in explicit form, respectively. Then the stress field in the whole domain is concluded relevantly
Experimental Investigation on the Energy Absorption of Foam Aluminum Cylinder at Moderate Deformation Rate
The steel shell, foam aluminum cylinder and foam aluminum filled steel shell were tested using drop hammer to study their energy absorption characteristics at moderate deformation rate. The results showed that the foam aluminum-copper alloy exhibits typical brittleness. The energy absorption capability of the steel shell with circumferential notches was weaker than that without notches. The foam aluminum filled steel shell possessed strong capability of energy absorption due to the combination of steel shell and filled materials.
Fracture Behavior of Three-dimensional Braided Composites
Internal defects in three-dimensional (3D) braided composites may affect the mechanical properties greatly and result in fracture failure. Due to the complexity of the crack problems of braided composites, it is significant to develop some numerical simulation methods to study the crack-tip fields. In this paper, the fracture behaviors of 3D four-directional braided composites are investigated. In order to calculate the stress intensity factor of the crack in 3-D braided composites, this work adopts a modified interaction integral method in conjunction with the finite element method (FEM). This interaction integral method can deal with complex interfaces in 3D braided composites.
Initial Cracking of RC Beams Strengthened by Prestressed AFRP Sheet under Four-point Bending
On the basis of the feature of 4-point bending failure for prestressed aramid fiber sheet-reinforced RC beams, a layered shear-lag model with flaw was established to study the stress redistribution of RC beam strengthened by AFRP sheet under 4-point bending, and then the stress distributions for all sub-layers are obtained. Combined with the energy method, the initial cracking of RC beams reinforced by AFRP sheet was studied, and initial cracking loads were got. The results obtained relatively coincide with the existing test values. The present method provides a new research approach for the bending failure of prestressed reinforced components.
Concrete Crack Propagation Study with Experiment and Extended Finite Element Modeling
In this work, both experiment and the extended finite element method are employed to investigate the failure procedure of concrete which is treated as heterogeneous material. The extended finite element model represents effects of aggregate particle size and prosperity on trajectory of crack propagation in concrete. Crack faces of commonly observed in many concrete specimens are rough and rugged instead of plane surfaces in homogeneous modeling. The extended finite element model reproduces roughening transitions of the crack surfaces of concrete. The model successfully explains the trajectory of crack propagation, roughening transitions of the crack surfaces investigated in the experiment.
Research Progress of Honeycomb Core Failure Analysis for Sandwich Structures
The failure analysis of honeycomb cores is necessary for honeycomb sandwich structures’ quantitative design. It is related to the method of creating the numerical model and the failure mode and corresponding failure criteria. There are mainly two ways to simulate honeycomb cores at present. One is micromechanical model, in which the honeycomb wall is modeled by shell elements. The other is macro model, in which the honeycomb core is considered as an equivalent orthotropic body. Shear failure and compressive failure are honeycomb cores’ main failure mode. At present the failure analysis results do not correspond with the test results precisely. The failure analysis methods under complex loads still need to be further investigated.
Effect of Chosen Damage Initiation Criteria of Constituents on the Predicted Strength of 3D Fine Woven Punctured C/C Composites
Two different strength criteria for fibers, Hashin and Hoffman criteria, of 3D fine woven punctured C/C composites are adopted in the computation. The maximum stress criterion is utilized to judge the damage initiation of matrix. The progressive damage process of the composites is obtained by finite element analysis of a representative volume element applied periodic boundary conditions. The results find that the damage model with Hoffman criterion of fibers is suitable to predict the mechanical response of the composites.
Numerical Simulation of the Impact Response on Concrete Target under Penetration
By using the FEM code LS-DYNA, the penetration of concrete target by a rigid projectile was simulated. Three material models in LS-DYNA (MAT 72R3, 111 and 159) were used to model the impact response of concrete material respectively. The deformation region and failure zone in the concrete target, and the residual velocity of the projectile for different constitutive models was discussed and compared. The simulation results showed that the concrete model of MAT 72R3 was harder than the other two models and the residual velocity of projectile for three concrete models were almost approximate.
Research on the Influence of Pores and Micro-cracks on the Macro-mechanical Properties of Concrete
Concrete is typical porous medium permeated by an interconnected network of pores and micro-cracks. In the micro level, concrete is considered as a three-phase composite with aggregates, pores and mortar matrix. In the paper, the random multi-scale mechanical analysis model is adopted to study the macro-mechanical properties of concrete with pores. The results indicate that the existence of pores and micro cracks has great influence on macro elastic modulus, effective strength, residual strength and other mechanical properties of concrete, which should not be neglected when investigating the mechanical properties and damage fracture of concrete.
The Concrete Model of the Penetration Problems on Meso-level
On meso-level, the concrete was heterogeneous because of kinds of aggregates. And the heterogeneity of the concrete would affect the ballistic responses. To study the influence, the meso-model of concrete (‘numerical concrete’) should be conducted. Most of the ‘numerical concrete’ was constituted with mortar, aggregates and the interface between them. The ‘numerical concrete’ in the paper was composted with mortar and aggregate. And the interfaces were replaced by connected strength. From the simply study of the normal penetration into concrete target, it was founded that: the stress symmetry affected by the randomly-distributed aggregates would change the missile trace, which couldn’t be alternated in the homogeneous model, and that the residual velocity of the missile decreased with the increase of the connected strength of the interfaces.
Analysis of the Effective and Internal Cyclic Stress Components in Cast y–TiAl Based Alloy
Cyclic strain controlled multiple step tests in air have been performed on cylindrical specimens of cast ï§-based alloy Ti–48Al–2Nb–2Cr–0.82B at. % with nearly lamellar microstructure of ï§ï€ andï€ ï¡2 phases at 23 and 750 °C. The objective was to analyze the internal and effective cyclic stress components which were derived from the hysteresis loops analyzed according to the statistical theory of hysteresis loop. Dislocation structures in cyclically strained specimens at room temperature were observed using TEM. Cyclic response at both temperatures was compared and discussed in relation to changes of internal and effective stress components and dislocation modes in ï§ phase observed and referred in the literature.
Numerical Computation and Analysis on Damage Effects of Multilayer Medium under Explosive Loading
A multilayer medium consists of compacted soil, gravel and concrete layers. It is simulated damage effects of multilayer medium under explosive loading using ANSYS/LS-DYNA. It is researched the fluctuation of peak pressure about concrete layer. And it is analyzed the anti-explosion capacity of multilayer medium about different initiation point depths.
Dynamic Viscous-damage Constitutive Model for Concrete
Starting from the continuum static damage energy release rate-based elastoplastic damage framework, a novel elastoplastic viscous damage model for the nonlinear analysis of concrete structures under dynamic loading was developed in the present paper. Based on the fiber bundle model, regularization was employed to account for the rate-sensitivity of damage evolution, and the uniaxial static damage evolution was extended to the rate-dependent case. The multi-axial dynamic damage evolution was established by substituting the energy equivalent strain into the uniaxial dynamic damage evolution function. As illustration of application, the dynamic nonlinear behavior of simply supported reinforced concrete beams under impact loading was simulated using the model. The agreement between numerical results and experimental data verifies the validity of the proposed model.
The Microstructure and Mechanism of Fatigue Crack Initiation of 2024-T3 and 2524-T34 Aluminum Alloys
The micro structure and mechanism of fatigue crack initiation of 2024-T3 and 2524-T34 Al alloys were investigated at the present paper. Four-point bending and tension-tension fatigue tests were conducted on the test alloys along the rolling direction at room temperature and frequency of 15Hz, R=0.1. It was found that the flat grain was elongated along the rolling direction, showing the laminar grain structure. The majority of fatigue cracks of 2524 were initiated at coarse particles, a few of them was formed on sites of material defects or slip bands. The intrusion and extrusion induced by slip band in the Al cladding layer provided principal fatigue crack initiation sites for 2024 Al-cladding aluminum alloys.
Fatigue Analysis of Tension Shear Spot Welds Based on Material Property Gradient
Fatigue life analysis was carried out on spot welds, taking into the effect of welding process on local metal mechanical properties within heat affected zone. Considering the relationship between material strength and cyclic stress strain parameters, life prediction with Pollard’s equation gave results with errors in 2 factors, as well as Barkey’s data. Experimental results were further studied with Manson-Coffin damage equation. Results showed that the metal mechanical properties gradient have implicit effect on crack initiation site, cannot be neglected in fatigue life prediction.
Effects of Design Parameter Changes on the Fatigue Life of Light-Blocked Component
The front end light-blocked components of the third generation of synchrotron radiation (SR) facility are subjected to intense thermal cycles from the high intensity X-ray beams. This work studies the effects of design parameter changes on the stress-strain responses and at last on the thermal fatigue lives of a simplified light-blocked component. A thermal elastoplastic finite element method (FEM) was used to simulate the stress-strain variation of the component in the fatigue process. Three significant design parameters, i.e. heat flux distribution, beam line incidence angle and beam spot drift position, were analyzed to estimate their influences on the fatigue life on the light-blocked component. The predicted results based on Smith-Watson-Topper (SWT) model provide reference data for future thermal fatigue tests.
Stress Intensity Factors of Periodic Crack Arrays with Eigen COD Formulation of Boundary Integral Equation
In this paper, the concept of eigen crack opening displacement (COD) is introduced to investigate the problem of periodic crack arrays in infinite plates. With the proposed approach, the SIF of crack arrays can be solved accurately with the conventional displacement discontinuity boundary integral equations (BIE) with a small size of system matrix. Some numerical examples are presented and compared with those in literature to show the accuracy of the proposed approach.
Generalized Stress Intensity Factors in the Interaction between Two Polygonal Holes in Anisotropic Materials
A novel hybrid finite element method based on a full numerical procedure is proposed in order to compute the singular field near the angular corner in the interaction of two polygonal holes in an anisotropic material. Numerical examples are given for anisotropic plates containing interacting double square holes, generalized stress intensity factors (GSIFs) at the angular corner tip are systematically calculated for material properties and spacings. The numerical results show that present method yields satisfactory results with fewer elements. Compared with conventional finite element methods and integral equation methods, present method are more suitable to deal with micromechanics of anisotropic materials.
Numerical Simulation on Failure Process of Ring-shape Specimen of Rock
Crack propagation and collapse in engineering materials including rock and concrete can cause failure. Rock masses often contain a large number of natural fractures and cracks, which are usually known as candidate locations of rock failure. Knowledge of the rules of crack growth and coalescence is very important for the stability of rock masses or materials and for the safe design of structures in or on rocks. Numerical modeling in this study is accomplished using the finite element program RFPA2D (two-dimensional Rock Failure Process Analysis), which is fully parallelized with OpenMP directives on Windows to utilize the increased computational power of PCs. Modified ring shape specimens with two pre-existing cracks are used to studying the crack growth and the failure process. Effect of the crack orientation angle on the failure modes is discussed and the related rock failure mechanisms are clarified. Compared with the test results, the failure modes show in good conformity.
Damage Monitoring in Composite Structures by Embedded Fiber Bragg Grating Sensors
We present a novel methodology for modeling and detecting the anomaly evolution in composite structures by fiber Bragg grating sensor (FBG). The strain/ transverse force and temperature change effects on the embedded FBG sensor area are separately modeled by introducing the change of the refractive indices and grating period along the fiber to the transfer matrix formulation method (‘measured’ spectrum). Furthermore, the use of genetic algorithm for reconstruction of non-uniform applied anomaly from the ‘measured’ spectrum is presented. The presented method is verified through the numerical simulations and the results show that our methodology can effectively represent and model the profile of the applied perturbation profile.
Stress Level Monitoring in Steel Tension Rod Based on Ultrasonic Guided Wave Technique
Aim at the deficiency of steel structure online stress monitoring, a new method of online stress monitoring based on theory of nonlinear acoustoelasticity guided ultrasonic waves was proposed. The disperse curves longitudinal L (0, 1) mode and flexural F (1, 1) mode in different stress level were drawn by using acoustoelasticity motion equation and Pochhammer frequency equation. According to the disperse curves, the optimum frequencies of excitation wave were obtained, which are ranging from 30 kHz to 75 kHz, and group velocity of L (0, 1) mode and F (1, 1) mode in different stress level were calibrated for 30 kHz, 45 kHz, 60kHz and 75 kHz. The results show that both L (0, 1) mode and F (1, 1) mode guided wave group velocities decrease linearly with the increasing of stress in the steer bar. Acoustoelastic effect of the above mentioned four frequencies in different stress level were simulated by finite element; the results are consistent well with the theoretical curves, it prove that stress level in steel bar can be tested by ultrasonic guided wave technique
Observation of Three-dimensional Crack Path in a Dual Phase Steel
Fatigue cracks were initiated in a dual-phase ferritic-martensitic steel using smooth specimens. Tiny sub-specimens containing typical crack configurations were then cut out by spark erosion which could be put in a synchrotron beam-line (spring8 in Japan). This allowed determining the three-dimensional shape of the cracks. Afterwards, the local microstructure in the vicinity of the crack front was investigated in the SEM including the crystallographic orientations via EBSD. This was done on the specimen surface as well as in depth direction by polishing in well defined steps. Consequently, the local microstructure along the crack front was known and its influence on three-dimensional crack could be analyzed.
Numerical Simulation of Progressive Failure in Transversely Isotropic Geomaterials
The mechanical behaviors, especially, strength of geomaterials such soils and rocks are significantly related to their inherent microstructures, which are of anisotropy or transversely isotropy, therefore, the behaviors of geomaterials should be described by anisotropic or transversely isotropic constitutive relation. In this study, a modified Drucker-Prager yield criterion for transversely isotropic materials is developed, and the corresponding consistent return mapping algorithm is formulated. The transversely isotropic yield function and isotropic yield function are, respectively, adopted in the numerical examples for investigating the different of the pattern of strain localization of panel. Numerical results illustrate the validation and performance of the present model in modeling progressive failure in geomaterials, and show that the influence of yield criterion on pattern of strain localization is important.
Analysis on a Thermal Fracture Problem in Packaging Structures
In this paper, the interaction integral method combine with the finite element method (FEM) are introduced to study the thermal fracture behaviors of packaging structures. The modified interaction integral is provided for solving the fracture problems of packaging materials subject to thermal loading and the path-independence of the integral is still kept. Some numerical examples are checked to verify the present method. The temperature fields and the thermal stress fields are calculated and the thermal stress intensity factors (TSIFs) are extracted. The influences of the crack lengths and temperature fields on the TSIFs are also considered.
The Refined analysis of Bi-layer Beams without ad hoc Assumption
Cheng’s refined theory is extended to investigate bi-layer beams, and an extension of recent analyses for bi-layer beams is given. Expressions are obtained for all the displacements and stress components in terms of displacement and stress of the interface of upper layer and lower layer. Using Lur’e operator functions, the refined analysis of the bi-layer beams is given from continuity conditions. On the basis of the refined analysis developed in the present paper, solutions are obtained for bi-layer beams under transverse surface loading. The refined analysis provides exact solutions which satisfy all of the governing equations
Discrete Element Modeling of Three-Dimensional Impact Problems Including Elastoplastic Deformations and Severe Damages
In this paper, a scheme of the discrete element method (DEM) for numerical simulation of three-dimensional (3-D) impact problems including elastoplastic deformations and severe damages is proposed. Based on the basic principles of continuum mechanics, a 3-D DEM framework is established for modeling the elastoplastic behavior of solid materials. Failure criteria are introduced for modeling the transformation from continuum to discontinuum. The numerical results obtained by the proposed scheme agree well with those obtained by the finite element method and the corresponding experiment respectively, which proves the feasibility and the reliability of our scheme.
A Fracture Mechanics Model for an Arbitrarily Oriented Crack in Functionally Graded Materials
The general solution to an arbitrarily oriented crack in FGMs is investigated in this paper. Applying the superposition principle and Fourier integral transform method, the expressions of the displacement fields and stress fields are obtained. Then the general crack problem is turned into a group of singular integral equations by introducing auxiliary functions defined in the cracked layer. The final singular integral equations are solved by using the theory of residues and numerical method. Finally, the variation of the mixed-mode stress intensity factors (SIFs) with the crack angle is discussed.
Analytical Solution of Non-Uniform Settlement for Embankments on Soil Foundations
A novel model for analyzing non-uniform settlement of embankments on heterogeneous soil foundations is proposed. The embankment is constructed on a foundation which consists of two different soil segments. Model for analysis is established using stress diffusion method. Basic differential equations and pertinent boundary conditions are given. Fourier transform technique is employed for finding the analytical solution of the non-uniform settlement. Numerical result of this solution is provided.
Failure Mechanisms and Life Prediction of Thermal Barrier Coatings
Thermal barrier coatings (TBCs) have great potential for many applications due to its good thermal protective feature, high hardness and wear resistance. However, as one of the most complex coating systems, TBCs often show a poor service life. Here, an acoustic emission (AE) real-time monitoring technique and a non-destructive digital image correlation (DIC) technique were used to quantify the damage evolution and analyze the failure mechanism of TBCs. The dangerous regions and the life time of TBCs were predicted based on FEM.
Strain Gradient Solution for the Eshelby-Type Problem of an Anti-Plane Strain Cylindrical Inclusion in a Finite Elastic Matrix
Eshelby’s equivalent eigenstrain method and fourth-order strain transformation tensor [1] are essential for homogenization schemes including the Mori-Tanaka and self-consistent methods. However, Eshelby’s tensor originally provided in [1] is based on classical elasticity and is for an ellipsoidal inclusion embedded in an infinite elastic matrix. As a result, homogenization methods based on this classical Eshelby tensor cannot capture particle (inclusion) size effects or account for boundary effects. Hence, there has been a need to obtain Eshelby tensors for an inclusion in a finite matrix using higher-order (non-classical) elasticity theories. In this study, such an Eshelby tensor is provided for the finite-domain anti-plane strain inclusion problem of a finite elastic matrix containing a cylindrical inclusion prescribed with a uniform eigenstrain and a uniform eigenstrain gradient using a simplified strain gradient elasticity theory (SSGET) [2]. This SSGET involves only one material length scale parameter and has been applied to analytically solve several Eshelby-type inclusion problems [3–7]. In the current formulation, the SSGET-based Green’s function for an infinite anti-plane strain elastic body is first derived using the Fourier transform method. The extended Betti’s reciprocal theorem and Somigliana’s identity based on the SSGET and suitable for anti-plane strain problems are then used to determine the displacement field in the finite matrix in terms of this Green’s function. The displacement solution reduces to that of the infinite-domain anti-plane inclusion problem when the boundary effect is suppressed.
Low-cycle Fatigue Behaviors of Small Cracks of Glidcop Al-15
Glidcop Al-15, a dispersion strengthened copper alloy containing 0.15% of submicroscopic alumina particles, is a particularly attractive material for high temperature components because of its excellent thermal conductivity, strength retention and micro-structural stability at elevated temperatures [1-3]. As fatigue damage requires to be expressed in terms of a crack, observations are made on Glidcop Al-15 that relate to the initiation zones and the propagation of small cracks subjected to low cycle fatigue and these observations are based on surface replica studies [4, 5]. On the other hand, the effect of prior fatigue history on the growth rate of a small crack is investigated systematically using special specimens containing an artificial small hole of various diameters, i.e. 40, 100 and 200 μm at three different load levels equivalent to cyclic stress levels separately. Similar experiments for a medium carbon steel NO.45 are also operated for comparison with the results of Glidcop Al-15 [6]. Hour-glass shaped specimens which have a minimum diameter of 8 mm at the central cross-section are used in the fatigue tests. These specimens are called ‘plain specimens’. To simulate a defect or an inclusion, a very small hole was drilled on the periphery of the central portion of a plain specimen. The diameter d of the hole and its depth were identical, as indicated in Figure 1. Three hole diameters were used, with d equal to 40, 100 or 200 μm. These specimens are called ‘holed specimens’.
Exploring the Physics of Dynamic Fragmentation through High- Performance Computing
Dynamic fragmentation of brittle materials is of great importance in several applications (transportation safety, ballistic impact, asteroid impact, to name a few). The rapid breakage of a body is governed by complex physics. The initiations of microcracks at materials defects, their propagation at high velocities, and the subsequent branching and coalescence, yielding fragments, are all influenced by stress wave interactions. In this presentation, we review past experimental and theoretical work on dynamic fragmentation. We then describe a novel methodology in which the Discontinuous Garlerkin framework coupled to cohesive elements [1] is used to conduct highly scalable simulations of three-dimensional bodies. We conduct massively parallel calculations to generate converged fragmentation results. We show that for fine enough meshes, the average fragment size becomes independent of mesh resolution. Our simulations also reveal the existence of simple scaling laws for the distribution of fragment sizes. Our numerical results agree with some conclusions of the well-establish energy models [2]. For instance, we find a -2/3 exponent dependence of fragment size with strain rate. However, we illustrate the power of scientific computing by obtaining statistics, which are not within reach of analytical work. In particular, we discuss recent dome-explosion calculations, in which fragment shapes are shown to depend on material defects and the extent of crack branching.
Design and Mechanical Characterization of Lattice Sandwich Cylindrical Shells
Lattice sandwich cylindrical shell has superior mechanical performances and relatively light structural weight, and is thus widely used in aerospace and aviation engineering. In this paper, deformation mechanisms are systematically investigated and failure analyses are conducted for the lattice cylindrical shells with various core topologies. Analytical models are proposed to predict the axial stiffness, critical elastic buckling load or effective yield strength of these shells. Finite element (FE) simulations are carried out to identify the validity of the models. Based on the homogenization theory of lattice materials, the overall buckling behavior of lattice sandwich cylindrical shells is analyzed through energy method and first-order shear deformation shell theory (FSDT). The buckling loads are calculated for different shell thicknesses and also compared with FE simulations, which yields good agreements. Furthermore, filament winding and twice co-curing processes are proposed to make a carbon fiber reinforced composite (CFRC) lattice sandwich cylinder with Kagome cores. Axial compression experiments are carried out to reveal the stiffness, load capacity of the fabricated sandwich cylinder.
A Nonlinear Constitutive Model for Nematic Gels
Nematic gels are materials that combine the properties of conventional gels and liquid crystals. Compared with conventional gels, nematic gels exhibit more interesting properties due to their liquid-crystalline behaviors. For example, nematic gels may deform spontaneously in response to external field, enabling them to be ideal materials for actuators and sensors. In this paper, we developed a constitutive theory of nematic gels based on free energy minimization. With the resulting theory, we modeled a miniaturized cylindrical actuator made of nematic gels and showed that the cylindrical nematic gel actuator would undergo a substantial elongation along its longitudinal direction in response to a cooling environment.
Two-scale Asymptotic Computations for Piezoelectric Problem in Perforated Structure Composites
The piezoelectric materials play an important role in countless areas of modern life. By means of two-scale and coupled boundary layer method, the two-scale asymptotic computation for the displacement and the potential for structure of perforated composites under piezoelectric condition are constructed firstly, then the approximate solutions and corresponding approximate errors of the displacement and the potential are given, and some numerical results show the effectiveness of the method lastly.
Nonlinear Analysis of New Type Concrete-filled Steel Tubular Column-beam Joint
The concrete-filled steel tubular column had freedom from restraint of axial compression ratio, so it had been widely used in high-rising structures recently. In order to meet the reqirement of earthquake resistant design code, the connection structures were very complex and it was difficult to construct. In this paper, the mechanical behavior of a new steel plate joint consisting of concrete-filled steel tubular column and SRC beam was studied by the three-dimensional finite element method. The results shows that the joint had excellent integrality, larger initial stiffness, better seismic resistance performance, favorable ductility deformation. At the same time, the joint had long yield platform and the strenth fell little after yield. The stress in the column pipe was small and there was almost no yielding after damage. Most parts of concrete in the core of column were compressed, but the stress was very small. Both panel zones and column had little deformation. In the damage process, plastic angle moved away from joint so brittle destruction was prevented.
Optimal Design of Energy Absorption Structures against Blast Resistance
An optimization model of energy absorption structures composed of bi-material under impulse loads is established. The dissipated energy introduced by the cracks’ growing is chosen as the objective function. Distributions and the volume fraction of the bi-material are set to be the design variables and the constraints, respectively. A swapping material criterion (SMaC) and an improved genetic algorithm are proposed to optimize the distribution of the bi-material in the structural design domain and improve the ability of energy absorption of structures respectively. The effectiveness of the proposed two algorithms is verified by numerical examples, and some suggestions for the optimal design are provided in the conclusion of the paper.
Polygonal Inhomogeneity in Anisotropic Full-Planes
An exact closed-form solution for Eshelby problems of polygonal inhomogeneity in anisotropic full-planes is presented. The induced elastic fields are expressed in terms of the line integral on the boundary of inhomogeneity with the integrand being the Green’s functions. As numerical examples, our iteration algorithm is applied to both triangle- and square-shaped quantum wires within GaAs full-planes.
Heterogeneous Objects Design and Manufacturing: Multi-materials Three Dimensional Printing System
An integrated CAD/CAE/CAM framework for the design and manufacture of heterogeneous objects is presented. The primary goal of the research is to develop systematic methodologies to represent, visualize, and fabricate heterogeneous objects. The tetrahedral solid cell representation is used to establish the relationships among structures, materials and colors. A color STL based mapping with CMY mode is applied to visualize the CAD model of heterogeneous objects. The proposed integrated scheme has been implemented and tested in our Multi-materials Three Dimensional Printing (MTDP) system based on a micro droplets dispensing process.
Strengthening Mechanisms and Yield Strength Reduction of a Hot Rolled Enameling Steel
The microstructures, strengthening mechanisms, and reasons for yield strength reduction of a hot rolled enameling steel were studied by combining experiments and thermodynamic calculations. The results show that before the enamel-fire anneal (EFA), the microstructure consists of ferrite, pearlite, and free cementite. The EFA decreases dislocation density and increases grain size and pearlite volume fraction. The main strengthening mechanisms of the enameling steel are grain size strengthening, solution strengthening, and dislocation strengthening. The dominant factors responsible for yield strength reduction are grain growth and a reduction in dislocation density.
Increment Poisson’s Ratio for Frozen Clay with Thermal Gradient
The increment Poisson’s ratio for frozen clay with thermal gradient both in K0DCGF (K0 drained consolidation, freezing with non-uniform temperature under loading) and GFC (freezing with non-uniform temperature, isotropic consolidation) triaxial compression tests were investigated. The results indicated that the increment Poisson’s ratio decreases with the increasing in axial strain at various consolidation pressures and thermal gradients, but that increases both in K0DCGF test and GFC test with the further increasing in axial strain. The μinc,i (initial increment Poisson’s ratio)and the μinc,f (failure increment Poisson’s ratio) increase as the thermal gradient increased both in K0DCGF test and GFC test. However, the effects of consolidation pressure to μinc,i and the μinc,f present the inverse law in K0DCGF test and GFC test
Design of Composite Materials with Optimal Stiffness and Thermal Conductivities
This paper presents the bidirectional evolutionary structural optimization (BESO) method for the design of two-phase composite materials with optimal properties of stiffness and thermal conductivities. The composite material is modeled by microstructures in a periodical base cell (PBC). The inverse homogenization method is used to derive the properties of effective bulk modules and thermal conductivities. BESO procedures are presented to optimize the two individual properties and their various combinations. Two numerical examples are studied. The effectiveness of BESO has been demonstrated by good comparisons to the benchmark microstructures and the Hashin-Shtrikman bounds.
Cavitated Bifurcation for Two-family Fiber-reinforced Incompressible Hyper-elastic Materials
Cavitated bifurcation for the sudden formation of cavity in a solid two-family fiber-reinforced anisotropic incompressible hyper-elastic sphere subjected to a uniform radial tensile boundary dead load and the growth of a pre-existing cavity are studied. There exists a critical value for the load computing the differential equation between the cavity radius and the applied load. A cavity may form at the center of the sphere when the load is larger than this critical value. The bifurcation occurs locally to the left and the subsequent growth of the cavity is without bound. The stability of solutions is discussed through energy comparison.
Static Deformations of Annular Sectorial Plates with Material Inhomogeneity
Based on the three-dimensional elastic theory and assuming the material properties having an arbitrary variation in the radial direction and having an exponent-law variation along the thickness, the static analysis of annular sectorial plates with simply supported radial edges and arbitrary circular edges is achieved by employing a semi-analytical method. A parametric study is carried out to show the effects on the static behavior of different variations of material properties along the thickness, radial direction and both directions, respectively.
Mechanical Response and Failure Mechanism of Fiber Reinforced Composite Truss Core Sandwich Structures
Sandwich panels are widely used in lightweight construction especially in aerospace industries because of their high specific strength and stiffness. In present work, we introduced a kind of sandwich structure made from carbon fiber reinforced composite truss core. The sandwich structures are fabricated with a molding hot-press technique. In this method all the continuous fibers of composite are aligned in the direction of struts and thus the truss structure can fully exploit the intrinsic strength of the fiber reinforced composite. Analytical and experimental investigation has been carried out to explore the mechanical response of carbon fiber composite truss core sandwich panel. The failure mechanisms of such sandwich structure were also considered in this study.
Experimental Studies and Numerical Simulation of Compressibility of Al—10% Cu Powder Metallurgy Composites
The density level of the part produced by the Powder Metallurgy route is purely controlled by the parameter such as the amount of pressure applied; ram velocity, friction between the powder and the die surfaces. In this research work, an attempt has been made to investigate experimentally the powder compaction of Al-10%Cu metal matrix composites and numerical method was used to simulate the compressibility study of the above mentioned PM process. The experimental and numerical results revealed that the effect of the second phase particle in the void ratio development and the level of enhancement in the relative density of PM compacts. ABAQUS, a finite element tool has been used for performing the numerical simulation utilizing the CAM—CLAY and Porous Metal Plasticity models for the prediction of void ratio and relative density enhancements of the material considered.
SHPB Test on Dynamical Mechanical Behavior of Concrete with High Temperature
An experimental system is designed by combining the split Hopkinson pressure bar (SHPB) with microwave heating device, based on numerical simulation and stress wave theory, availability of the experiment technique is analyzed. Tests of concrete whose temperature changes from room temperature to 650°C and impact velocity from 5m/s to 12m/s are completed and for the first time high-temperature dynamical damaging phenomena of concrete are obtained. Based on data analysis, the dynamical mechanical behavior of concrete with high temperature is affected by not only the strain rate effect whose influence keeps on decreasing with temperature increasing, but also the high temperature weakening effect. And the strain rate hardening effect is coupled with high temperature weakening effect, but the latter has greater influence.
Nowadays, Friction Stir Welding (FSW) process has been widely used in the area of space, aircraft, marine, fuel tank, food saving industry. Joining of composites is necessary to realize structural applications. In this paper authors discussed not only the welding of Metal Matrix Composite (Al-SiC) plates, but also joining of polymeric composites through FSW. Forces exhibited along the tool axis and weld direction during FSW joining of MMC were monitored and analyzed. The FSW processed MMC joints were analyzed with mechanical and metallurgical studies. Modified linear FSW process is used to join Polymeric Matrix Composite (PMC).
Theoretical Investigation into the Thermo-mechanical Stresses in Cross-ply Laminated Composite Cylinder with Finite Length
Using series solving method of differential equations, analytical solutions for temperature and thermo-mechanical stresses in cross-ply laminated composite cylinder are presented in this paper. The cylinder is simply supported at its two ends and subjected to non-uniform thermo-mechanical loadings on the inner and outer surfaces. As an example, numerical results of temperature distribution, stress fields of a four-layer laminated composite cylinder are presented and briefly discussed.
Three-dimensional Exact Solution for the Thermo-mechanical Stresses in Cross-ply Rectangular Laminated Plates
The 3D thermo-mechanical analysis of isotropic cross-ply rectangular laminated composite plates is made in this paper in terms of variable separation approach and series solving method for ordinary differential equation. The plate is simply supported at four edges and is subjected to thermo-mechanical loadings on its top and bottom surfaces respectively. Furthermore some numerical results for the temperature distribution, the displacement and stress fields of a three layers alumina fiber reinforced laminated composite plate are shown in figures and briefly discussed.
New Optimal Design Concept of Sandwich Structures with Modified Pyramidal Lattice Cores
For the pyramidal lattice truss cores sandwich structures, its mechanical properties depend on not only the lattice truss cores, but also the facesheets and the interface. In this paper, in order to enhance the equivalent shear strength of the pyramidal lattice truss cores sandwich structures, one new optimal design concept, which is based on the performance harmony among the cores, the facesheets and interface, is brought forward to design the sandwich structures with modified pyramidal lattice truss cores. Analytical results based on the new optimal design concept show that the equivalent shear strength of the modified pyramidal lattice truss cores sandwich structure may be evidently increased owing to optimum structure of the pyramidal lattice cores.
Strengthening Design for Carbon Fiber Reinforced Composite Lattice Structure
Carbon fiber reinforced composite lattice structures often buckle in ultra lightweight situation. This study developed an approach for designing lattice structures with enhanced buckling strength. Novel composite pyramidal lattice structures with wood-core trusses and hollow trusses were explored respectively. Then the strengthening mechanisms of these structures were analyzed, and compression experiments were subsequently carried out for demonstration. It was observed that Euler buckling was no longer happened again for either wood-core truss or hollow truss lattice structures. The strength of hollow truss lattice structures tested was lower than analyzed, which might be ascribed to their fabrication process.
Prediction of Tensile Modulus of 3D Braided Composites with Interior Cracks
To a laminate model of 3D braided composites with periodic interior cracks, six new normal stress functions in weave direction are present. In these functions, variations of stress with x and y are considered. By using the principle of minimum complementary energy, stress components in the representative unit are obtained. Tensile modulus of 3D braided composites with periodic interior cracks is calculated by utilizing Hashin’s average theory and the principle of minimum complementary energy. To demonstrate the solution, some examples are analyzed.
Hydro-elastic Vibration of Partially Liquid-filled or Partially Liquid-surrounded FGM Cylindrical Containers
Vibration characteristics of partially liquid-filled or partially liquid-surrounded functionally graded cylindrical shells are investigated in this paper. Using Rayleigh-Ritz energy method and Love’s shell theory, eigenvalue equation of the problem is carried out, and the polynomial for natural frequencies of such shells is obtained. The external work by the hydrodynamic pressure, which is introduced by liquid sloshing, is taken into account in the energy function. Natural frequencies of two types of FGM cylindrical shells are investigated and graphically presented.
Performance Properties of Engineered Cementitious Composites (ECC) as Concrete Repair Materials
Maintenance, repair and strengthening of concrete structures have become a very important part of the activities of the concrete industry. The paper presents the results of an experimental investigation to determine some typical properties of engineered cementitious composites (ECC) as concrete repair materials, which are of significance to the subsequent structural behavior of repaired concrete members. It also compares the performance of this kind of materials with plain concrete mixes of similar strength and stiffness, which are also suitable for repair applications. Data on strength, stiffness, porosity, permeability and shrinkage under different temperature and humidity exposure are presented. In addition, water permeability coefficients of the materials have been determined. The results show that permeability and shrinkage of the repair materials is significantly lower than the normal mortar.
Composite Propellant Dewetting Simulation based on CZM Model and Contact Angle Measurements
The composite propellant is the high filled particulate elastomeric matrix composite. Debonding of particle/matrix interfaces can significantly affect its macroscopic mechanics behavior. How to model the propellant material and describe damage processes has been a long standing quest. This paper provides an effective method to link the macro-scale to the meso-scale through combining contact angle measurement experiments and multiscale finite element simulation based on cohesive model, the effect of microstructural damage on the macroscopic constitutive response of propellant is shown.
Optimization of End Milling Parameters for Glass Fiber Reinforced Plastic (GFRP) Using Grey Relational Analysis
Milling of GFRP Glass Fiber Reinforced Plastic (GFRP) is required for fabricating structures. Analysis of the behavior of cutting tools (solid carbide end mills) in the machining of GFRP is very important in manufacturing industries. In this paper, optimization of the machining parameters on milling of GFRP with Grey Relational Analysis (GRA) is carried out. This work attempts to minimize the surface delamination, machining forces, cutting torque and surface roughness. GFRP was fabricated with 33% fiber and 66% volume general purpose resin. Experiments were designed and carried out as per Taguchi technique and parameters were optimized using Grey Relational analysis (GRA).
Mesoscopic Constitutive Modeling of Corroded Concrete Based on CT
The constitutive modeling of corroded concrete is difficult due to the nonuniform corrosion over the section of specimen. In the present paper, the behaviors of (uniformly and nonuniformly) corroded concrete are simulated with the help of the CT. Firstly the mesoscopic constitutive law of corroded zones is modified according to the Fick’s Second Law. Then, the strength reduction ratio of mortar and Interfacial Transition Zone (ITZ) are identified by comparing the experiment results with the simulation results. Finally, the constitutive law of uniformly corroded concrete is simulated with the corrosion homogenization approach.
Analytical Solutions for Vibration near the Buckled Configuration of FGM Beams under an In-Plane Thermal Loading
Analytical solution is obtained for the dynamic responses around the buckled configuration of FGM beams subjected to a uniform in-plane thermal loading. The equations governing the axial and transverse deformations of FGM beams are derived based on the classical beam theory and the physical neutral surface concept. The two equations are reduced to a single nonlinear fourth-order integral-differential equation governing the transverse deformation. By assuming that the amplitude of beam’s vibration is infinitesimal, and its response harmonic, the non-linear partial differential equations are reduced to two sets of coupled ordinary differential equations; one for postbuckling and the other for linear vibrations of the beam superimposed upon the buckled configuration. The nonlinear equation is directly solved and analytical solutions for static and dynamic responses are obtained.
Transient Investigations of Functionally Graded Materials with Arbitrary Properties
The transient response of functionally graded materials of arbitrary properties with a crack vertical to the free surfaces is analyzed theoretically. The impact loading condition is considered. Based on the piecewise-exponential model (PE model), a new analytic method is developed. The dynamic stress intensity factors (DSIFs) are calculated by using Laplace and Fourier integral transforms, singular integral equation methods, the theory of residues and the Laplace numerical inversion technique. Some representative samples with different kinds of nonhomogeneous properties are analyzed and the corresponding DSIFs are presented. It is shown that the properties varying forms have greatly influences on the DSIFs.
Influences of Applied Electric Fields on Thermoelectric Properties of P-type Bulk Polycrystalline Materials
We presented a theoretical model to predict thermoelectric properties of P-type bulk polycrystalline materials under a temperature gradient and an external electric field. The model is based on the Boltzmann transport equation for holes, and particularly, in which the hole scattering at the grain boundary is taken as the boundary conditions. The influences of external electric fields on the Seebeck coefficient and the power factor have been studied respectively. The results indicate that the applied external electric fields can impose non-trivial influences on the values of Seebeck coefficient and power factor, and the influences on Seebeck coefficient become more significant as the size of grain decreases.
Heterogeneous Topologically Interlocked Materials: A New Class of Heterogeneous Materials
Topologically interlocked materials are a class of materials in which the load carrying capacity emerges from load transfer across a multitude of contact surfaces between a set of identical solid unit elements. Both biological [1] as well as man-made material systems [2, 3] based on this principle have been described in the past. In particular, man-made topologically interlocked materials can advantageously be constructed as assemblies of unit elements in the shape of platonic solids. Here, we discuss heterogeneous materials based on the concept of topological interlocking. Heterogeneous topologically interlocked materials can be constructed by the use of two (or more) different classes of unit elements, say a high modulus ceramics and a low modulus silicone rubber. We will describe manufacturing approaches for this class of materials and give a description of their properties. Heterogeneous topologically interlocked materials can also be constructed by the use of a single class of unit elements, but with each unit element possessing a distinct microstructure. We will describe manufacturing approaches for this class of materials and give a description of their properties. As described in [4] the concept of segmentation and assembly as underlying both homogeneous and heterogeneous topologically interlocked materials holds the promise to enable the design of materials outside the envelope of current material properties. We will discuss such opportunities.
Sequential Approximate Optimization Based Robust Design of SiC-Si3N4 Nanocomposite Microstructures
microstructures of SiC-Si3N4 nanocomposites for desired high temperature toughness is presented. The focus is on finding robust nanocomposite microstructures with maximum toughness at two temperatures: 1500 oC and 1600 oC. Within this context a sequential approximate optimization algorithm under uncertainty is applied to six different test problems addressing different aspects of robust microstructure generation. During optimization, statistical uncertainties inherent to the computational microstructural generation are quantified and introduced in the optimization framework. The results show that the SiC volume fraction, the number of Si3N4 grains, the grain size distribution of the Si3N4 grains, and the grain size of the SiC grains have varied effects on the microstructure toughness at different temperatures. At 1500 oC, the preferred microstructure is the one with higher Si3N4 volume fraction. On the other hand, at 1600 oC, the preferred microstructure is the one with higher SiC volume fraction.
Mechanics of Natural Honeycomb as Paradigm for Design of Unconventional Cellular Materials
Engineering honeycombs mimic the macroscopic geometry of natural honeycombs, but have yet to achieve the micro-structural sophistication of their natural counterparts. We studied the hierarchical structures, and measured the mechanical properties, of natural honeycombs. We show that the honeycomb walls are progressively self-stiffened with the time of usage, by adding reinforced layers reminiscent of modern fibre-reinforced composite laminates. The overall shear modulus of the comb increases more than threefold in one year and fourfold in two years. We also calculated the effective coefficient of thermal expansion of the honeycomb. The properties of conventional engineering porous or cellular media including honeycombs with homogeneous walls are bounded by two inherent constraints, namely, low stiffness and un-tunable coefficient of thermal expansion (CTE) by their microstructures. The natural honeycombs may serve as a prototype of truly bio-mimetic unconventional cellular materials with improved stiffness and a tunable coefficient of thermal expansion via pore surface coating, as well as an example of cellular materials with structural transformation. Stiff, strong, and lightweight porous materials with tunable CTE are ideal for applications in environments with large temperature fluctuations.
Particulate-reinforced Titanium Composites for Orthopaedic Implants
In the present study, to enhance the strength of porous pure titanium scaffolds with high porosity, new particulate-reinforced Ti-based composites with the addition of biocompatible oxide particles such as TiO2, SiO2, ZrO2 and Nb2O5 were prepared using a powder metallurgical method. The strengths of the new particulate-reinforced titanium composites were found to be significantly higher than that of pure titanium with an excellent biocompatibility. SaOS-2 osteoblast-like cells grew and spread well on the surfaces of the new particulate-reinforced titanium composites. The present study illustrated the feasibility of using the particulate-reinforced titanium composites as an orthopaedic implant material.
Multiscale Biomechanical Behaviors of Heterogeneous Airway Smooth Muscle Cells
The lung is a complicated organ with dramatically different structures such as airways at different length scale, and thus forms a very complex and heterogeneous material. Embedded in the airway wall are airway smooth muscle cells (ASMCs) that typically respond to contractile stimuli with exaggerated shortening in asthma. To understand such complex system, a multiscale approach is needed to integrate experimental observations and theory. In this paper, we address some multiscale behaviors of the ASMCs, which may help to an improved understanding of the link between cellular pathophysiology and the whole lung response in both health and airway diseases such as asthma.
Nonlinear Flexoelectric Model for Nanomechanical Analysis of Double-Stranded DNA Chips
Nonlinear flexoelectric effect in double-stranded DNA (dsDNA) chips is studied. Based on nonlinear Poisson-Boltzmann equation (NLPB) and flexoelectric theory of liquid crystal thin films, the relation among dsDNA molecule structure feature, salt solution concentration and biofilm electrostatic potential, chip macroscopic deflection is formulated with the help of Fogolari’s modified formula. The hybridization parameters are obtained by curve fitting based on Wu’s experimental data. The linear and nonlinear predictions for chip deflection are compared.
Friction Enhanced Adhesion in Biomimetic Nanostructures
Recently hierarchical carbon nanotube (CNT) arrays with entangled tips become a promising candidate to fabricate gecko inspired dry adhesives. However detailed deformation and interfacial behaviors of the CNT arrays are not fully investigated, which is of great importance to understand the switching mechanism between strong attachment and easy detachment. Here, a fully atomistic molecular dynamics simulation was implemented to characterize the friction and adhesion behaviors of CNTs peeled from rough substrates. The result shows that friction enhanced adhesion exists in the nano sized fibrillar structures pertaining to the spatula level in gecko.
Research on the Elastic and Plastic Deformation of CoCr Alloy Used for Coronary Stents
The Percutaneous Transluminal Coronary Angioplasty (PTCA) assisted with stenting has become one of the effective means to cure heart disease. In practice, when the balloon and stent are assembled and the stent expands after being inputted, the stent will be compressed and stretched with elastic and plastic deformation twice, which affects the limit load and fatigue strength. In order to grasp the elastic or plastic deformation of CoCr alloy used for coronary stents, this paper proposed solutions based on compression and expansion experiments and analyzed the effect that pre-deformation does on material’s property, since stents were compressed before being stretched. The results suggested that pre-compression plastic deformation have almost no effect on the tensile strength of the material, but the tensile elongation of the material decreased.
Experimental Study on the Cooling Effect around Large Vessels While Tissue Heating by Using U-tube Phantom
Heat transfer in living biological tissues is a very complex phenomenon. The ultrasonic tissue phantom has the similar ultrasonic characteristics with the living soft tissue, and it can be used to simulate the living tissue in vitro. The aim of the paper is to investigate the cooling effect of blood flow and vascular structures on living tissue heating by perfusion phantom experiments. The phantom was immersed in water bath and heated in order to simulate the environment of hyperthermia. Meanwhile, the thermocouples were employed to measure temperatures of interest locations. Finally the experimental results were visualized by AVS (Advanced Visual System) software. The results showed that the water flow and vascular structures had a significant effect on the heat transfer in the phantom. It is expected that the work may be helpful in designing hyperthermia protocol of tumor treatment.
Preliminary Study to Dynamic Mechanical Properties of the Fresh Swine Thighbone
Mechanical property of the bone is an important part of biomechanical study. Compared with the static performance of the bone, its dynamic response characteristic has vital significance. In this paper we tested the dynamic mechanical properties of the fresh swine thighbone with the Split Hopkinson Pressure Bar (SHPB). The collected data were processed automatically by the software with Tow-wave and Tri-wave methods, and then the dynamic compression stress-strain curve was achieved. It is more meaningful to do the experimentation in this way than we used to do in a cell test specimen.
Turning Weakness Into Strength– how Biological Materials Combine Strength, Toughness and Adaptability
Biology exquisitely creates hierarchical structures, where initiated at nano scales, are exhibited in macro or physiological multifunctional materials to provide structural support, force generation, catalytic properties, or energy conversion [1, 2]. This is exemplified in a broad range of biological materials such as hair, skin, bone, spider silk or cells. For instance, despite its simple building blocks spider silk is one of the strongest, most extensible and toughest biological materials known, exceeding the properties of many engineered materials including steel [3, 4]. This is particularly puzzling since despite its great strength, spider silk is made of some of the weakest chemical bonds known, H-bonds. We have discovered that the great strength and extensibility of spider silk can be explained based on its particular structural makeup, which involves several hierarchical levels from the nano- to the macro-scale. Thereby, the structural confinement of H-bonds into ultra-small beta-sheet nanocrystals with dimensions of only a few nanometers is the key to overcome the intrinsic limitations of H-bonds, creating mechanically strong, tough and resilient cross-linking domains between a semi-amorphous phase composed of 31 protein helices [5-8]. Our work unveils a material design strategy that enables silks to achieve superior material properties despite its simple and structurally inferior material constituents. Exploiting this concept could lead to a novel materials design paradigm, where enhanced functionality is not achieved using complex building blocks but rather through the utilization of universal repetitive constitutive elements arranged in hierarchical structures. We discuss analogies with other protein materials such as collagen and intermediate filaments, and present approaches towards the design of adaptable, mutable and active materials using a computational materiomics approach [9].
In this paper, a linear elastic fracture mechanics framework is developed for investigating the toughening and strengthening mechanism observed in nacre. A half-infinite crack configuration is employed to obtain the toughening ratio. An equation governing the stress intensity factor is proposed and a general framework for solutions is established. The toughening ratio, the ultimate material strength and the flaw-tolerance of nacre and nacre-like composites are then investigated with the effects of platelet thickness and length, the composite stiffness and interfacial mechanical properties. The results demonstrate that the presence of platelet bridging in nacre is an important contribution to its high toughness, high strength and superior flaw-tolerance. In addition, it was shown that the aspect of platelets, nacre stiffness and interfacial strength in nacre significantly contribute to those outstanding mechanical properties mentioned above. These results provide detailed insights into the platelet bridging features that make nacre tough, stiff, strong and flaw-tolerant. Based on these results, some design guidelines for novel materials mimicking nacre have been also proposed.
Dynamic Shear Behavior of Soft Biological Tissues
The characterization of the mechanical behavior of soft biological tissues has been an active area of research for many years. In particular the determination of the mechanical properties of brain tissue, especially at high strain rates, has become of considerable importance due to the increased incidence of traumatic brain injury due to automotive, sports and military accidents. Compressive response of soft biological tissues at high strain rates was determined using a Kolsky compression bar. The high-rate elastic modulus was found to be significantly greater than the shear modulus. An analysis indicates that modulus discrepancy is due to very small perturbations in the radial strain in the Kosky-bar experiments due to factors such as non-ideal slippage at the specimen-bar interface or transverse inertia. The analysis also shows that the discrepancy is a function of the ratio of the bulk modulus to the shear modulus, where the shear modulus can be orders-of-magnitude lower than the bulk modulus in soft tissues. The analytical results were verified by experiments using a lightly cross-linked poly(dimethyl siloxane) based polymer gel, which was developed with similar mechanical properties to soft biological tissues, where the modulus of the gel can be adjusted via the amount of low molecular weight diluent in the synthetic gel.
Application of Cell Mechanics in Disease Detection & Diagnosis
During the onset of a disease, a cell may experience changes in the composition and organization of its cellular structures. These changes may then lead to alterations in its geometrical and biomechanical properties such as cell size and shape, deformability and adhesion [1]. In fact, these cellular physical and mechanical property changes are known to directly contribute to the pathology of diseases [1]. By conducting cell mechanics studies, not only can we elucidate changes in the structure-property-function relationship of diseased cells [2,3], we can also exploit such knowledge to develop mechanics based microdevices for better disease detection and diagnosis [4]. Here, we will illustrate with examples of two diseases, malaria and cancer, and show how we can exploit the cell mechanical property changes as biomechanical markers for detection and diagnosis. In particular, we will also showcase the development of 1D, 2D and spiral microfluidic devices that harness the changes in cell mechanical properties for us to effectively and efficiently detect and isolate these diseased cells [5].