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Using Full-Field Measurement Capability of Digital Image Correlation to Assess 3D Properties of Composites



The ability to accurately determine the basic material properties including 3D stress-strain constitutive relations, fully characterizing material behavior under mechanical loads, is essential for understanding complex deformation and failure mechanisms of structures manufactured from highly anisotropic polymeric composite materials. Non-contact full-field deformation measurement techniques, such as digital image correlation (DIC) allowing for assessment of all surface strain components on the entire specimen surface, enable simpler experimental setups and material specimen designs for more efficient and accurate material characterization. This work presents a method which uses the full-field measurement capability of DIC for a simultaneous assessment of transverse tensile behavior and non-linear shear stress-strain relations for composites in all three principal material planes. The method, which employs a small rectangular plate torsion specimen, takes full advantage of the full-field measurement and advances our ability to measure 3D material properties in composite materials in a single experiment. Small rectangular plate torsion (plate-twist) specimens are placed in a custom test fixture and loaded in a servo-hydraulic load frame at a constant crosshead displacement rate until failure. DIC data including the in-plane as well as out-of-plane strain components is monitored simultaneously in the three principal material planes until failure using a system of 6 high-resolution cameras. An iterative FEM updating method is developed to solve the inverse problem of deriving the material constitutive properties using the DIC full-field strain information. The basis of the inverse method is the minimization of the weighted regularized least square error between DIC-measured strains and FEM-predicted strains by adjusting the parameters of the material constitutive model. Results include stress-strain curves in all three principal material planes and transverse tensile properties simultaneously captured for a practical carbon/epoxy tape material system. Non-linearity in the transverse tensile response is discussed.

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