Impact of time-dependency on long-term material testing and modeling of polyethylene

Jeffrey E. Bischoff
Mechanics of Time-Dependent Materials, Volume 12, Number 3, September 2008

Ultra-high molecular weight polyethylene (UHMWPE) has an important role in orthopaedic implants because of its favorable properties as an articulating surface. UHMWPE component testing often focuses on measuring the long-term fatigue or wear response of the material that could be realized during many years of use. However, the impact of time-dependent properties of UHMWPE on such tests is not well characterized. In particular, altering the frequency of loading and allowing for material creep or relaxation can significantly alter the stress/strain state of the material, and therefore affect long-term mechanical properties (e.g. wear, fatigue) that are dependent on the constitutive state. The goal of this work was to use advanced, validated material modeling of UHMPWE that incorporated time-dependent properties to explore the effects of frequency and rest time on the mechanical response of UHMWPE. Various approaches could be used for the parameter regression including gradient-based techniques (Bischoff 2007) and simplex techniques (Bergström et al. 2004). The optimization software HEEDS 5.1 was used in this study. This software affords use of the proprietary SHERPA algorithm, which incorporates both local and global optimization techniques, as well as standard gradient-based approaches.
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