Stress intensity factors for a vertical surface crack in polyethylene subject to rolling and sliding contact

Fitting wear is a dominant form of polyethylene surface damage in total kne e replacements, and may originate from surface cracks that propagate under repeated tribological contact. In rite present study, stress intensity fact ors, K-iota and K mu, were calculated for a surface crack in a polyethylene -CoCr-bone system ln in the presence of rolling or sliding contact pressure s. Variations in crack length and load location were studied to determine p robable crack propagation mechanisms and modes, The crack tip experienced a wide range of mixed-mode conditions that varied as a function of crack len gth, load location, and sliding friction. Positive K-I values were observed fur shorter cracks in rolling contact and for all crack lengths when the s liding load moved away from the crack. K-II was greatest when the load was directly adjacent to the crack (g/a = +/-1), where coincidental Mode I stre sses were predominantly compressive. Sliding friction substantially increas ed both K-I(max) and K-II(max). The effective Mode I stress intensity facto rs, K-eff, were greatest at g/a = +/-1, illustrating the significance of hi gh shear stresses generated by loads adjacent to surface cracks. K-eff tren ds suggest mechanisms far surface pitting by which surface cracks propagate along their original plane under repeated reciprocating rolling or sliding , and turn in the direction of sliding under unidirectional sliding contact .