A Generalized Couple Theory of Hygrothermal-Elasticity: Transient Effects in Composite Laminate with Circular Cavity.

Abstract : A system of coupled differential equations involving moisture, temperature and stress are derived within the framework of classical thermodynamics. Analyzed are the transient character of the stress and energy density distribution around a circular cavity in a cross-ply graphite T300/5208 epoxy resin laminate assumed to behave in a quasi-isotropic manner. The laminate is stretched uniaxially such that a mutual interaction of moisture, temperature and stress depend on whether the laminate is in a state of plane strain or plane stress. Unlike the case of moisture-temperature and stress being uncoupled, the diffusion coefficients for plane strain must be distinguished from those for plane stress when stress coupling effects are included. Time-dependent numerical results are obtained from a finite element formulation and reveal that stress coupling can have a marked influence on the solution, both qualitatively and quantitatively. This is particularly significant near defects or cavities where the stress and energy state experience high elevation. The neglect of stress coupling can lead to unconservative predictions. Possible failure sites are predicted from the strain energy density criterion which assume that excessive dilation is associated with material separation while excessive distortion with yielding.