The viscoelastic effect during acoustoelastic testing of polyethylene

The aim of this work is to investigate the viscoelastic effect during acoustoelastic testing of a thermoplastic (polyethylene) for a reliable stress evaluation, which is critical for the processing and in-service life of thermoplastic products. The strain accumulated under step loading-holding stages was firstly characterized by a stepped isostress method, which can be integrated into the acoustoelastic equations. A nonlinear acoustoelastic term was also introduced in order to explain the viscoelastic effect from the increasing stress/strain. Although different holding times and loading rates will change the ultrasonic wave velocity profiles, their influences can be correlated and included in the improved acoustoelastic equations. Furthermore, the nonlinear problem can be circumvented by using an acoustic birefringence technique. Finally, the traditional and nonlinear acoustoelastic coefficients of polyethylene were calculated and verified.