Fatigue accumulated damage in notched quasi-isotropic composites under high-temperature conditions: A discussion on the influence of matrix nature on the stress energy release rate

Abstract

In order to investigate the contribution of matrix nature to the fatigue behaviour, the purpose of the present work is to establish the correlation between material toughness and macroscopic damage accumulation during tensile cyclic loading in the brittle (C/epoxy) and ductile (C/Polyphenylene sulfide) matrix systems. More specifically, this article presents a fracture mechanics-based approach to compute the strain energy release rate during fracture along with the macroscopic transverse crack growth in fatigue. The knowledge of energy-absorbing processes is important as they are responsible for the toughness of the composite. Woven-ply laminates are characterised by matrix-rich regions which may stop or slow down the growth of fatigue cracks by absorbing the mechanical energy through local plastic deformations at the cracks tip depending on matrix nature. With respect to C/epoxy laminates, the local plastic deformations at the cracks tip are prominent in highly ductile composite systems (e.g. C/Polyphenylene sulfide), and ultimately results in fatigue behaviour virtually independent of the applied stress level under high temperatures T > Tg. To evaluate the influence of matrix ductility and toughness on fatigue damage, a damage variable d based on the measurement of longitudinal stiffness at each cycle was used. A model derived from a Paris law and a fracture mechanics criterion were combined to: (i) evaluate the fatigue crack growth – (ii) compare the changes in the strain energy release rate G and the macroscopic damage d during cyclic loading. Macroscopic damage appears to be well correlated with the strain energy released during fracture.