Cyclic stress-strain curves at finite strains under high pressures in crystalline polymers

Abstract

The cyclic torsional stress-strain behaviour for crystalline polymers of polyethylene (PE), polypropylene (PP) and polyoxymethylene (POM) was investigated at finite strain amplitude under hydrostatic pressure up to 2000 kgf cm⁻². The following features for the cyclic stress-strain curves were indicated. (1) Two types of cyclic stress-strain curve were observed: one was a PE type where the present hysteresis loop was not so affected by the strain histories, and the other was a PP type where the hysteresis loop was greatly affected by the previous maximum strain. (2) In the pressure ranges tested, the shape of the cyclic stress-strain curves for the polymers used was not essentially altered by the hydrostatic pressure. (3) The stress-strain curves after the first strain reversal showed an unusual shape which has not been observed for metals. (4) The decrease in cyclic softening caused by the stress amplitude with increasing number of cycles at a constant strain amplitude test occurred irrespective of the hydrostatic pressure. (5) The stress-strain behaviour at a partly reversed cyclic loading was different from that expected at a fully reversed cyclic loading.