Influence of moisture-induced stress on in situ fiber strength degradation of unidirectional polymer composite

A three-phase concentric cylinder model is used to estimate moisture-induced internal stress in unidirectional polymer composite. It is found that moisture-induced tensile stress on the fiber is substantial when absorbed moisture level in the polymer matrix is high. It is also shown that the stress-state of the fiber is not significantly changed by inclusion of an interphase layer with properties similar to the matrix. The consequence of moisture-induced internal stress is that glass fibers are subjected to stress corrosion, even under no externally applied stress. It is found, by measuring the size of the fracture mirrors of failed glass fibers in model composites, that the in situ tensile strength of E-glass fibers in an epoxy matrix composite was degraded at a faster rate compared to the case of stress-free aging of glass fibers. Thus it is concluded that moisture-induced tensile stress in glass fibers plays an important role in strength degradation of unidirectional composite under environmental aging.