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Limit State Design of Composite Structures Based on Characteristic States in Damage Evolution



Limit state design (LSD) of composite structures is a common practice in industry where the strategy is to keep the loads on a structure such that the material in the structure does not exceed a selected limiting state. In composite laminates, one can identify certain states in damage evolution that can be used as limiting states depending on the structural design function. For instance, if the structure is to have no cracks at all, then the initiation of first crack in any of the plies of the laminate can serve as the limiting state. If the structure is stiffness critical, but can tolerate a certain loss of stiffness, then the limiting state corresponding to the maximum attainable crack densities in all plies of the laminate is a useful limiting state. This state is known as the characteristic damage state (CDS) and has been a focus of many studies. For strength-critical structures, the load bearing capacity of the laminate is a design variable, and its limiting values are given by an unstable progression of damage and total failure. In this work, we firstly address the CDS under static and fatigue loading for symmetric laminates containing plies in transverse as well as other off-axis orientations. Following previous studies of transverse cracking in cross ply laminates, we use attainment of a compressive stress between cracks as a criterion for the limiting state of cracking also for other off-axis cracks. We develop a beam-on-elastic-foundation model that analytically calculates the minimum crack spacing of transverse cracks in cross ply laminates and in combination with a three-dimensional finite element (3-D FE) analysis also calculates this spacing for other off-axis orientations. The model predictions agree well with available test data. For fatigue loading, it has been observed that the crack saturation may vary with the loading history. We discuss the mechanism of such a CDS in the plies. Finally, the potential application of the studied CDS for LDS is proposed.

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