Finite element analysis of surface cracks with applications to tribological coatings and polyethylene wear

Hard coatings, such as TiN and diamondlike carbon films are commonly applied to contacting machine elements and high speed milling tools to extend their life. A common mode of failure observed in the coating materials is rolling contact fatigue (RCF), often revealed by the formation of a spall or pit on rolling surfaces. In knee prostheses, pitting wear is a dominant form of polyethylene surface damage in total knee replacement (TKR) articulation and may originate from surface cracks that propagate under repeated tribological contact. Knowledge of the stress intensity factors is one of the useful concepts in fracture mechanics and describes crack tip conditions in a linear elastic material. In the present study, stress intensity factors, K{dollar}sb{lcub}rm I{rcub}{dollar} and K{dollar}sb{lcub}rm II{rcub}{dollar}, were calculated for a surface crack in two dimensional, layered systems subject to rolling contact and/or sliding contact pressure, using finite elements. A series of preliminary models, containing a vertical crack in a thick plate and a surface breaking crack in a nonlayered tribological system, were developed to validate the finite element methodology. The "stress method" was used to determine K{dollar}sb{lcub}rm I{rcub}{dollar} and K{dollar}sb{lcub}rm II{rcub}{dollar} at the crack tip. Linear and second order extrapolation techniques were investigated for accurate calculations of K{dollar}sb{lcub}rm I{rcub}{dollar} and K{dollar}sb{lcub}rm II{rcub}{dollar}. A series of models of modern tribological coatings (TiN, DLC, MoS{dollar}sb2{dollar}) with a surface crack were studied with variations in crack length, layer thickness, and load location. Novel advances in analyses containing crack face friction were made in the coating models. Furthermore, rolling and sliding articulation between a CoCr cylinder and a flat polyethylene tibial component containing a vertical surface breaking crack was simulated. In this study variations in crack length and load location were investigated to determine possible crack propagation mechanisms and driving modes in a polyethylene tibial component.