Hygrothermal effects on structural stiffness and structural damping of laminated composites

The hygrothermal effects on structural stiffness and damping of laminated composites are investigated. Since the hygrothermal influence on properties of composite materials is primarily matrix dominated, the effects of temperature and moisture on the storage modulus, Poisson's ratio and material damping of the epoxy matrix are first determined. With the experimentally determined properties of the epoxy material, the complex moduli (E exp * sub L , E exp * sub T , G exp * sub LT and v exp * sub LT ) of unidirectional glass--epoxy and graphite--epoxy composites are then determined. The structural stiffness (extensional and flexural) and damping of symmetric angle-ply laminates of glass--epoxy and graphite--epoxy are then investigated both analytically and experimentally for temperatures of 20 and 80 deg C, respectively. Three moisture contents, which are the dry, saturated and a non-uniform moisture gradient states corresponding to each temperature case, are considered. Numerical and limited experimental results show that the effects of moisture on the real part of A exp * sub 11 , A exp * sub 66 , D exp * sub 11 and D exp * sub 66 at room temperature, 20 deg C, are negligible for all the considered cases; but as temperature increases, the moisture and temperature combined influence induces significant changes in the complex stiffness A exp * sub 11 , A exp * sub 66 , D exp * sub 11 and D exp * sub 66 , especially for the matrix dominated terms. . 11 ref.--AA