Fatigue and Creep Rupture of an Acetal Copolymer

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Abstract
Part 1 of this paper describes the uniaxial fatigue testing of injection-moulded cylinders made of acetal copolymer at various frequencies between 0.167 and 10 Hz. The influence of mean stress and cycle control (stress or strain) has also been studied. Two mechanisms of failure were observed: cyclic thermal softening, above certain limits of stress and frequency, and traditional crack propagation fatigue, which was found to be independent of cyclic frequency. Strain cycle control tests were thermally stable and stress range became stable after the initial 1 per cent of the total endurance. Creep rupture tests were also performed. Part 2 considers the fatigue and creep rupture of tubular specimens containing geometrical stress raisers. These were injection moulded under the following conditions: (a) single-end gated, (b) side gated, (c) double-end gated, (d) ‘rough’ mould and polished mould. Moulding defects such as weld lines, presence of voids and sprue, poor surface finish, etc., are compared for fatigue-strength reduction with machined circumferential notches and transverse holes of known theoretical stress concentration.

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