Stiffness degradation resulting from 90-deg ply cracking in hygrothermal aged cross-ply laminates is analyzed. First, the material properties of the composite are affected by the variation of temperature and moisture and are based on a micromechanical model of laminates. Consequently, the hygrothermal conditions degrade the stiffness of the laminate. On the other hand, when this latter is subjected to tensile mechanical loading, the early stage of damage is dominated by matrix cracking in the transverse plies; its presence causes stiffness reduction and can be detrimental to the strength of the laminate. In this investigation, a modified shear-lag analysis, taking into account the hygrothermal effect on the material properties of the laminate, is employed to evaluate the effect of transverse cracks on the stiffness reduction in the hygrothermal aged cross-ply laminated composites. The results represent well the dependence of the degradation of elastic properties on the crack density and hygrothermal conditions.
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