Effect of relative humidity on uniaxial cyclic softening/hardening and intrinsic heat generation of polyamide-6 polymer

Strain-controlled uniaxial cyclic tests on a polyamide-6 (PA6) polymer with various relative humidity (RH) levels were performed in a room environment. The effects of RH on the cyclic softening/hardening and intrinsic heat generation of the PA6 are discussed by using the experimental data obtained in the cyclic tests with various applied strain amplitudes, mean strains and strain rates. It is observed that the cyclic softening/hardening of PA6 depends greatly on the applied strain amplitude and strain rate, and the cyclic softening observed in the cyclic tests with a larger strain amplitude and at a higher strain rate is mainly caused by the remarkable temperature rise due to the intrinsic heat generation. The variation of relative humidity does not change the cyclic softening/hardening feature of PA6, but does influence the extent of cyclic softening/hardening; an apparent mean stress relaxation occurs in the asymmetrical strain-controlled cyclic tests, and its extent increases with increasing mean strain but decreases with increasing relative humidity.