Effects of Annealing and Swelling to Initial Plastic Deformation of Polyethylene probed by Nonlinear Ultrasonic Guided Waves

Abstract

Identification of precursor events related to incipient plastic deformation in polyethylene parts using a nondestructive technique is investigated in this study. A pair of ultrasonic transducers mounted on the surface of a test sample were used to propagate ultrasonic pulses of varying frequency while progressively small flexural deformations below yielding were applied. The evolution of higher order harmonics were observed in association with increasing micro-structural modification. Three different polyethylene grades were molded using different thermal treatments. Results showed that different crystalline networks could be correlated to different mechanisms of plastic deformation that was observed by a defined ultrasonic parameter under proposed method. Variation of the ultrasonic parameter was similarly observed with the residual stresses associated with solvent swelling, as studied by penetration of toluene in the bimodal and copolymer grades. Results and discussion presented in this study connect this non-destructive characterization method with mechanisms of incipient plastic deformation in polyethylene.