Fatigue damage mechanisms in unidirectional carbon-fibre-reinforced plastics

The fatigue life behaviour and the underlying micromechanisms have been stu died in two different types of unidirectional carbon-fibre-reinforced plast ics loaded in tension-tension along the fibre direction. The carbon fibres (AS4) were the same in the two composite systems. One thermoplastic matrix (polyetheretherketone, PEEK) and one thermosetting matrix (epoxy toughened with a thermoplastic additive) were used. The macroscopic fatigue behaviour was characterised by fatigue life diagrams. Surface replicas were taken in termittently during the course of the fatigue tests to monitor the active f atigue damage micromechanisms. The thermoset based composite showed a highe r fatigue resistance with few microcracks initiated at distributed fibre br eaks growing at a decelerating rate. The thermoplastic composite had a more pronounced fatigue degradation with a steeper fatigue life cur ve, which w as caused by widespread propagating debonds and matrix cracks. The use of a tougher and more ductile matrix results in an inferior fatigue life perfor mance, due to a more widely distributed accumulation of damage that propaga tes at a higher rate. (C) 1999 Kluwer Academic Publishers.