Transverse failure initiation in polymer composites

Abstract: Transverse failure is one of the most important failure modes in polymer composites. The phenomenon often causes the first deviations from nonlinear laminate behavior. Also, in pressure vessels and pipes, fluid leakage through a path of transverse cracks is often the limiting design criterion. In the present work, experimental and theoretical studies focused on the micromechanical level have been carried out. The objective was to investigate transverse failure initiation in the matrix. The other major mechanism of failure initiation, fiber/matrix debonding, was not considered. The triaxial nature of the matrix stress state in glass fiber/epoxy was confirmed by finite element analysis. Experimental results for glassy epoxies subjected to composite-like stress states demonstrated large reductions in strain to failure as compared with uniaxial loading. The triaxial stress state is therefore by itself a sufficient explanation for the low transverse strain to failure in polymer composites. Plastic yielding in the matrix was demonstrated not to be the cause of failure initiation. Instead cavity induced cracking was suggested as a failure mechanism. A criterion was proposed based on a critical value for the dilatational energy density. Comparison with experimental results for epoxies subjected to a variety of multiaxial load-cases supported the criterion. Additional support was obtained from comparison with experimental results in the literature for transverse failure of glass fiber/epoxy at different fiber contents. Although the epoxy matrix was different from those in the present study, general trends in data were supported by predictions based on the criterion and finite element analysis. Thermal residual stresses were found to be important for high fiber contents. Based on the criterion, a conservative estimate of composite strain to failure was obtained. This is reasonable since the criterion predicts initiation, not final failure. Based on the model, effects from changes in constituent properties were examined in a parametric finite element analysis. Fiber modulus was found to strongly influence transverse failure. Introduction of a third phase interphase between fiber and matrix was also investigated. Beneficial results on transverse failure strain caused by matrix initiation was observed for thin rubbery interphases.