Nutting creep in polymer composites

The Nutting’s law [Proc. ASTM 21 (1921) 1162] has been used extensively to model creep in metals though it was originally proposed for the minerals pitch and asphalt. It is employed here for various polymer composites that exhibit non-linear visco-elasticity. The creep observed within a parallel connection of two dissimilar polymers appears 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 a region where the visco-elastic strain rate diminishes with time. A marked change in the two exponents arises under stress levels high enough to promote an unstable geometry as with necking and very rapid viscous flow. Here the accompanying strain rates increase as the stress levels continuously change. Within a limited constant stress range both exponents assume constant values which enable an application of the Nutting equation. This is potentially useful for modelling creep of non-linear, visco-elastic composites. The creep strain is characterised by one coefficient and two exponents. Descriptions of the instantaneous strain are given when it is elastic, inelastic and rate-dependent. Constants may be determined from a load–unload programme or one in which load is increased in a step-wise manner. This empirical account of the total strain response to stress at a given time employs far fewer material constants than a classical mathematical approach to non-linear visco-elasticity [F.J. Lockett, Non-linear Visco-elastic Solids, Academic Press, New York, 1972].