Fatigue and Cyclic Thermal Softening of Thermoplastics

Abstract

Thermoplastic components subjected to cyclic loading may distort because of localized rises in temperature and/or fracture because of incremental (fatigue) crack propagation from microscopic cracks or defects produced during the manufacturing process or by service conditions. The object of the present paper is to show quantitatively how the first mode of failure, which is referred to as cyclic thermal softening, can be related to loss compliance, specimen geometry and the frequency and magnitude of the cyclic load. The results of cyclic bending tests on polymethylmethacrylate (p.m.m.a.) and Polyvinylchloride (p.v.c.) beams of rectangular and circular cross-section are analysed. For the service conditions considered the p.m.m.a. specimens showed both failure modes whereas the p.v.c. specimens failed as a result of incremental (fatigue) crack propagation.