The role of microstructure during fatigue crack growth in poly(aryl ether ether ketone) (PEEK)

Abstract

The fatigue crack growth performance of injection moulded poly(aryl ether ether ketone) (PEEK) has been characterized using standard compact tension specimens and computer controlled photomicroscopy for crack length measurement. The influence of the materials parameters molecular weight and degree of crystallinity have been assessed together with experimental variables such as loading waveform shape and specimen orientation. Plots of crack growth rate against stress intensity factor range have been constructed from the experimental measurements. A quantitative measure of degree of crystallinity was obtained for each material using wide-angle X-ray diffraction. Post-fracture investigations using scanning electron microscopy were carried out to elucidate fatigue crack growth mechanisms. The results indicated that waveform shape had no effect on fatigue crack growth response. No anisotropic behaviour was evident, but an increase in molecular weight significantly improved the resistance of PEEK to fatigue crack growth. Similarly, increased crystallinity enhanced fatigue performance to a small extent. It is suggested that cyclic modes dominate fatigue crack growth at low rates of crack growth. However, with increasing speed, static processes begin to interact with cyclic mechanisms. At the onset of instability, failure is essentially dominated by static fracture modes.