FATIGUE-CRACK PROPAGATION BEHAVIOR OF ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE UNDER MIXED-MODE CONDITIONS

Analytical studies of the stresses on and within ultra high molecular weight polyethylene joint components suggest that damage modes associa ted with polyethylene fatigue failure are caused by a combination of s urface and subsurface crack propagation. Fatigue crack propagation tes ts under mixed mode loading conditions were conducted on center-cracke d tension specimens machined from extruded blocks of sterilized polyet hylene in an attempt to determine how fatigue cracks change direction in this material. Cyclic testing was performed using a sinusoidal wave form at a frequency of 5 Hz and an R-ratio (minimum load/maximum load ) of 0.15. Specimens had the notch oriented perpendicular to the direc tion of applied load and at angles of 60 degrees and 45 degrees to the loading direction. Numerical analyses were used to interpret the expe rimental test and to predict the fatigue behavior of polyethylene unde r mixed mode conditions. It was found that all cracks eventually propa gated horizontally, regardless of the initial angle of inclination of the notch to the direction of applied cyclic load. In fact, the extent of the curvilinear crack growth was quite Limited. An effective range of cyclic stress intensity factor was calculated for correlation with the rate of crack growth. The results followed a Paris relation, with crack growth rate linearly related to a power of the range of stress intensity, for all three crack orientations. The numerical analyses ad equately modeled the experimental fatigue crack growth results. (C) 19 94 John Wiley and Sons, Inc.