Fatigue and Long Term Strength of Thermoplastics

Abstract

Thermoplastics are complex materials in their response to applied loads. They do not obey Hooke’s law, being non-linear in their relationship between stress and strain. They are also viscoelastic in nature. This property is seen either as a time dependent response in stress (stress relaxation), when held under conditions of constant strain, or as a time dependent response in strain (creep), when subject to a constant continuously applied load. As a general rule their mechanical properties are more dependent upon temperature than metals which they seek to displace in engineering components. As a class of materials they have, in their unmodified form, comparatively low modulus and low strength. In addition to this complicated response to continuously applied loads, which results in a progressive decrease in strength with elapsed time under load, thermoplastics still suffer from ‘fatigue’ under cyclic loads. Hence the qualification implied within the title. At one time¹ these responses were known as static and dynamic fatigue, but this terminology led to some confusion with engineers. Consequently, as a matter of convention nowadays, the term ‘fatigue’ is restricted to the effects of cyclic or intermittent loads only, as is the case for metals, and ‘long-term strength or creep rupture’ to the effect of continuously applied loads.