Some fatigue characteristics of thermoplastics

Abstract

Uniaxial and rotating bending fatigue tests were carried out on polypropylene, polycarbonate, poly(methyl methacrylate) (PMMA), poly(tetramethylene terephthalate) (PTMT) and glass filled PTMT to establish the general régimes of thermal softening and fatigue types of failure observed in earlier tests on acetal copolymer. In the uniaxial testing PMMA exhibited both types of behaviour whereas polypropylene and PTMT were particularly prone to the thermal type of failure over a wide range of cyclic frequency and stress and fatigue failure could not be achieved for endurances up to 10 7 cycles. However, for PC and filled PTMT only the cracking type of fatigue failure was obtained as there was only a slight temperature rise which stabilized and hence did not lead to thermal softening. It was evident that the two types of failure were related to the magnitude of the loss tangent value for the material. Under uniaxial loading the effect of a sharp notch in each material was to reduce the thermal effect so that, for example, fatigue failures were produced in polypropylene and PTMT under stresses which would only produce thermal failures in plain material. In general fatigue endurances were reduced owing to the presence of the notch. Rotating bar fatigue tests using the same specimen as in the uniaxial tests exhibited both thermal softening and fatigue type failures, but at higher stress amplitudes and frequencies compared with the uniaxial tests since only the outer surface was subjected to the maximum stress. The results of constant strain rate and creep rupture tests on these materials have also been included for comparison.