Quantifying cross-shear under translation, rolling, and rotation, and its effect on UHMWPE wear

• A multidirectional tribo-system was used to study wear under different cross-shear motions.
• Cross-shear for each motion was calculated using parameters presented in the literature.
• All parameters of cross-shear ratio had good to strong correlations with wear factor.
• Some parameters showed special promise, but we were not able to distinguish a clearly superior method.

Wear of ultra-high molecular weight polyethylene is highly dependent on cross-shear, but the fundamental relation between them is not well understood. Over a half-dozen parameters for quantifying cross-shear have been proposed in the literature. The parameters are either cycle-based, defined using a cyclic motion path, or memory-based, defined only by motion history. All of the parameters have never been compared under the same test conditions to see which has the best correlation to wear rate and wear factor. This paper reviews and summarizes the different parameters and compares how well they correlate with wear test results from a multi-direction tribosystem under physiologically realistic, ball-on-flat test conditions.Experimentally, most cross-shear magnitudes correlated well to wear rate, with coefficients of determination ranging from 0.64 to 0.96. All cross-shear ratios correlated strongly to wear factor, with coefficients of determination ranging from 0.95 to 0.98. The memory-based definitions are robustly formulated, phenomenologically grounded, and show promise, but some of the cycle-based parameters had equally strong correlations. These results were not able to distinguish the optimal parameter, but they do reiterate the significant effect that cross-shear has on wear.