Nutting creep in mono- and bi-layer polymers

Abstract

The Nutting law, commonly used to model creep in metals, but originally proposed for non-metals, has been employed to describe non-linear tensile creep in polymer composites. The creep observed within both series and parallel, dissimilar polymer arrangements is characterised within the time and stress exponents of this law. While the law remains valid over a wide stress range, the time interval is restricted to the region in which the viscoelastic strain rate diminishes with time, i.e. the primary region. Secondary creep strain did not appear within the time intervals allowed at moderate stress levels; the creep rate diminished with time to become negligible. At higher stress levels a brief, constant strain rate appeared. Under stepped loading conditions, a change to the stress exponent occurred under stress levels high enough to promote an unstable geometry as with necking and very rapid viscous flow; the accompanying strain rates increased in a manner reminiscent of tertiary creep in metals. The phenomenological approach is potentially useful for modelling creep of composites. The total strain appears with an instantaneous strain added to a creep strain. The latter is characterised by one coefficient and two exponents: one of stress and the other of time. The instantaneous strain depends either linearly or exponentially upon stress. Creep strain is characterised with constant time and stress exponents but with limits placed upon their ranges. Strain measurements, made as the material recovered following unloading, suggest that non-linear, viscoelastic creep strain may be separated into permanent plastic and recoverable anelastic components. The simple Nutting description of viscoelastic strain employs far fewer materials constants than a classical mathematical approach to non-linear viscoelasticity. As a consequence, various non-linear, superposition principles are readily constructed to describe creep under incremental loading.