Cyclic and relaxation behavior of crosslinked ultra-high molecular weight polyethylenes

Highly crosslinked UHMWPEs have been introduced in an effort to improve the wear resistance of total joint replacements. More accurate constitutive models for conventional and crosslinked UHMWPEs are needed to better predict performance of joint replacements in finite element analyses. A useful constitutive model for UHMWPE should be able to predict monotonic, cyclic, and stress relaxation behavior. The objectives of this study were to examine the cyclic stress-strain and stress relaxation behavior of crosslinked UHMWPEs compared with conventional UHMWPE. Specimens were tested under fully-reversed uniaxial cyclic straining or loading. Stress relaxation tests were also conducted. It was found that the conventional and crosslinked UHMWPEs exhibited a comparable amount of cyclic softening under cyclic straining. Under load-controlled cycling, there was greater softening in tension than in compression and greater asymmetry was found for the crosslinked UHMWPE materials. The stress relaxation time constant of conventional UHMWPE was significantly greater than that of the crosslinked UHMWPEs. The findings in this study will be used to validate constitutive models for UHMWPEs that are currently under development.