Moisture diffusion and hygrothermal aging in bismaleimide matrix carbon fiber composites: part II—woven and hybrid composites

Moisture absorption and hygrothermal aging in a woven and woven/uni-weave composites are studied. Different from the neat resin and uni-weave composite, the short term diffusion in the woven composite is non-Fickian, in that the uptake initially increases rather rapidly but quickly slows down. A dual-diffusivity model that can successfully describe the observed weight gain curves has been developed. This model is composed of two independent diffusion processes with very different diffusivities. The fast process is associated with diffusion in cure-induced voids and cracks, while the slow process is the intrinsic diffusion in the matrix. Two different hybrids, namely, uni-weave/woven/uni-weave (U-W-U) and woven/uni-weave/woven (W-U-W), are also investigated. The short term absorption curves of the U-W-U and W-U-W composites can be described by Fickian diffusion and the dual-difftisivity model, respectively. The resemblance between the U-W-U and uni-weave composites, as well as that between the W-U-W and woven composites suggest that the diffusion properties of a composite are controlled by the outer plies. During prolonged moisture absorption, the swelling stresses eventually cause the formation of interfacial cracks in the composite.