FUNDAMENTALS OF FATIGUE AND CREEP RUPTURE OF A THERMOPLASTIC.

Experimental study showed that cyclical uniaxial stressing of an acetal copolymer results in two mechanisms of failure thermal softening which is markedly frequency dependent and normal fatigue cracking which is largely frequency independent over the range from 30 to 600 c/m. Thermal equlibrium in the specimen under reversed stressing which allows fatigue cracks to develop, can be predicted from an energy balance equation. Fatigue cracks do not develop from the polished free surfaces but from a small internal flaw developed during injection molding. Creep rupture data can form the end point of fatigue in the sense of zero stress range and maximum tensile mean stress. Three distinct deformation and failure mechanisms arise in creep rupture depending on the stress level.