Deformation conditions - Nonlinear viscoelasticity relationship for HDPE solid-state extrudate under cyclic fatigue

Fatigue behavior of HDPE extrudates was investigated on the basis of nonlinear dynamic viscoelastic analyses. The HDPE extrudate was prepared by a solid-state extrusion at an extrusion temperature of 353 K with an extrusion ratio of 11.1. Fatigue tests were carried out under the conditions of strain-controlled frequency of 11 Hz and environmental temperature of 303 K. Three deformation conditions were applied to fatigue tests: tension-tension, tension-compression and compression-compression deformation. Relationship between stress and strain amplitudes during fatigue process indicated that in the cases of tension-compression and compression-compression deformations, fatigue lifetime remarkably decreased with increasing dynamic strain amplitude. Nonlinear viscoelasticity during fatigue process was evaluated quantitatively by nonlinear viscoelastic parameter NVP. With increasing imposed strain amplitude, nonlinear viscoelasticity under compression-compression deformation became more remarkable because kink-bands were formed and stress distribution became inhomogeneous. Analyses of hysteresis loss during fatigue process indicated that nonlinear viscoelasticity was closely related to hysteresis loss consumed for irreversible structural change. In particular, in the case of compression-compression deformation, nonlinearity of viscoelasticity increased remarkably with hysteresis loss consumed for structural change due to kink-band formation.