The effect of cyclic true strain on the morphology, structure, and relaxation behavior of ultra high molecular weight polyethylene

Cyclic loading of total joint replacement bearing materials made of ultra high molecular weight polyethylene (UHMWPE) can lead to fatigue failures and the generation of wear particles resulting from the accumulation of plastic strain. Susceptibility to damage processes can be further complicated by choice of sterilization method and shelf aging prior to implantation. The objective of this study was to characterize the effects of cyclic true tensile strain on the morphological evolution and mechanical relaxation behavior of both non-sterile and radiation sterilized medical grade UHMWPE. A gamma -radiation sterilization process with several years of shelf aging was utilized in order to discern morphological changes and mechanical behavior resulting from oxidative degradation coupled with cyclic loading. A video-based true stress-strain measurement system was developed and validated to characterize the true stress-strain behavior of the UHMWPE. Non-sterile and sterilized specimens were each subjected to a range of true strain over a number of loading cycles. Following each test, strain recovery data was collected and final residual plastic strain was determined. Density gradient column measurements were conducted to provide evidence of structural changes that resulted from the cyclic strain tests. Scanning electron and field emission microscopy were utilized to provide further evidence of the morphological evolution of the UHMWPE. This study showed that the morphological evolution of both the non-sterile and sterile material groups could be correlated with the amount of true strain and the number of cycles. Furthermore, these differences in the behavior of the two materials could be attributed to their distinct initial microstructure. The results of this study have important implications for the development of constitutive and phenomenological models that may incorporate morphological evolution in UHMWPE and other semi-crystalline polymers. (C) 2001 Elsevier Science Ltd. All rights reserved.