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Creep studies have been carried out under uniaxial tension and uniaxial compression on poly(oxymethylene). Measurements under tension have been modelled using a function with four material parameters. One of these parameters, related to a mean retardation time for the relaxation process responsible for creep, decreases with increasing stress, and this gives rise to non-linear creep behaviour. Measurements of creep under compression indicate that the retardation time parameter is determined by the stress state as well as the stress magnitude. A theoretical extension of the model has been proposed that relates stresses and strains under situations where the stress is not constant but varies with time. This is necessary for the model to be implemented in a finite element system to carry out design calculations that take proper account of the time dependent properties of polymers. The validity of the theory is evaluated through the use of the creep model to predict deformation for a variety of simple stress and strain histories under uniaxial tension. The close agreement between calculation and measurement supports the development of code, in future work, to obtain solutions for multiaxial stresses and strains using a finite element system.

A nonlinear viscoelastic model is described that can predict the time-dependent deformation of a polymer under a multiaxial stress. The main material property requirements are tensile creep compliances over a suitable time range and at different stress levels over which creep behavior is nonlinear. Some creep data under uniaxial compression are also required. Relationships are derived between multiaxial stress and strain components for arbitrary stress or strain histories. The model has been coded into a finite element system to enable stress analyses to be carried out on objects of complex geometry. The validity of the model is explored through comparisons of predicted and measured results from a test that is designed to produce a predominantly shear stress state in the gauge region of the specimen. Results are presented for loadings under constant stress and constant deformation rate. POLYM. ENG.

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