PREDICTIVE MODEL FOR TENSILE TRUE STRESS-STRAIN BEHAVIOR OF CHEMICALLY AND MECHANICALLY DEGRADED ULTRAHIGH MOLECULAR-WEIGHT POLYETHYLENE

The gamma radiation sterilization of ultrahigh molecular weight polyet hylene (UH-MWPE) components in air generates long-lived free radicals that oxidize slowly over time during shelf storage and after implantat ion, To investigate the combined effects of chemical and mechanical de gradation on the mechanical behavior of UHMWPE, sterilized tensile spe cimens were immersed in 0.5% hydrogen peroxide solution at 37 degrees C for up to 9 months and concurrently subjected to cyclic stress level s of 0 (control), 0 to 5, and 0 to 10 MPa. After chemical and mechanic al preconditioning, specimen density was measured using the density gr adient column technique. The true stress-strain behavior was measured up to 0.12 true strain and characterized using a multilinear material model, the parameters of which were found to vary linearly with densit y and cyclic stress history, The mechanical behavior of as-irradiated and degraded UHMWPE was accurately predicted by an analytical composit e beam model of the tensile specimens. The results of this study suppo rt the hypothesis that chemical and mechanical degradation affect the true stress-strain behavior of UHMWPE. In the future, the material mod el data presented in this study will enable more accurate prediction o f the stresses and strains in UHMWPE components following gamma steril ization in air and subsequent in vivo degradation. (C) 1998 John Wiley st Sons, Inc.