The Effects of Geometry, Rate and Temperature on the Mode I, Mode II and Mixed-Mode I/II Interlaminar Fracture of Carbon-Fibre/Poly(ether-ether ketone) Composites

Abstract

A detailed study on the interlaminar failure of carbon-fibre/poly(ether-ether ketone) unidirectional composite (termed PEEK composite: "APC-2" from ICI (UK) plc) is presented. A fracture mechanics approach has been adopted and Mode I, Mixed-Mode I/II and Mode II tests have been conducted and the effects of specimen geometry, test rate and test temperature have been investigated. It is shown that for the interlaminar fracture of the PEEK composite the value of the interlaminar fracture energy, G,, generally in creases as the crack propagates through the composite, i.e., a rising "R-curve" is observed. Thus, it is not usually possible to assign one unique value to the interlaminar fracture energy, G_c, for any given Mode of loading for the PEEK composite. We have therefore defined both an initiation value, G_c(init), and a steady-state propagation value, G_c(s/s prop). The variation of these parameters with the Mode of loading, method of precracking and the test temperature is described in detail. From optical and electron microscopy studies it is shown that in Mode I the "R-curve" behaviour mainly arises from the degree of fibre-bridging increasing as the interlaminar crack grows, whilst in Mode II it appears to mainly arise from the increasing degree of microcracking and plastic deformation damage which develops around the tip of the advancing crack. The failure loci for the in terlaminar fracture of the PEEK composite have also been established, and various theo retical models to describe these data are considered.