Hygroscopic Behavior Study On Resin Matrix Composite Involving Damages

The discrepancy between moisture expansions of fiber and resin matrix could cause local mismatch stress in composites during moisture absorption or desorption. Mismatch stress may introduce damages of fiber/matrix interface debounding and matrix cracking if it is big enough, and might further lead final failure of composite. In this master degree dissertation, the effect of damage on moisture diffusion and hydrothermal residual stress were studied, and details are as follows:A three-phase steady-state moisture diffusion model for unidirectional composites was developed in which the effect of fiber/matrix inter-face damage was considered. Transverse effective moisture diffusivity of unidirectional composites(UCTEMD) was investigated under conditions of various fiber volume fraction, thickness of fiber/matrix inter-face as well as the diffusion properties of fiber and inter-face. Meanwhile, unit cell FEM approach with regular hexagonally fiber package was built to valid the theoretical model. Study revealed that both interface and fiber possess critical diffusivities which are independent to inter-face thickness. UCTEMD decreases with fiber volume fraction increasing if diffusivity of inter-face or fiber smaller than their critical values, otherwise UCTEMD increases.A steady-state moisture absorption model, which contains random distribution of fiber/matrix interface damage densities, was conducted. Micro FE approaches including one thousand randomly packing fibers were modeled and for them interface damage were also present in random. The numerical simulations have proven the validation of the theoretical model, and found the influence from randomness of damage distribution on UCTMED is neglectable if damage density is invariant.One and two-dimension moisture absorption mathematical formulas were derived for both steady and transient boundary conditions. Based on these solutions, hydrothermal residual stress in lamina and laminate were achieved during transient absorption process. The studies reveal that, residual compressive stress, induced by moisture diffusion in surface area, will decrease during absorption. Residual tensile stress in the central area, however, is observed to increase initially and then decrease finally to zero. As for laminate, residual stress appears to ascend everywhere, and hydro residual stress can be counteracted, in some degree, by thermal residual stress.