Nonlinear viscoelastic contribution to the cyclic accommodation of high density polyethylene in tension: Experiments and modeling

Viscous polymers tackle the prediction of the accommodated cycle from which fatigue criterion parameters should be computed, within a formalism adapted to structure calculation. This work focuses on viscoelasticity, shown to be predominant in the high cycle fatigue stress range. Stress-controlled cyclic, recovery and creep tensile tests in polyethylene at room temperature allow characterizing the frequency and the stress ratio effect on the ratcheting strain and the loop parameters. An isothermal nonlinear viscoelastic model is derived for small strains in a Thermodynamics of Irreversible Processes framework. Nonlinearity arises from the description of the relaxed state towards which viscous internal variables evolve. Both the short (loop) and long (1000 cycles) time scales are captured, with a ratcheting strain globally underestimated but features of the accommodated loop satisfyingly predicted.