An Investigation of the Fatigue Induced Failure Modes of Fiber/Elastomer Composites as Bearing Surfaces in Total Hip Joint Prosthesis

It is proposed to use elastomeric composites as artificial joint bearing surfaces to increase their fluid film lubrication and therefore reduce the number of failures which have been attributed to wear mechanisms related to frictional forces. The fluid generated between the surfaces supports part of the load and prevents direct contact of the bearing surfaces. In order to obtain optimum elastohydrodynamic lubrication, elastomeric materials have been selected based on their static mechanical properties such as modulus of elasticity. However, permanent deformation and failure of both elastomers alone and model elastomeric composites has been observed. In the composite materials the failure was attributed primarily to debonding between the fibers and matrix. It was hypothesized that the operating conditions such as frequency (velocity) and temperature at the contact between the elastomeric composite and the reciprocating counter part may contribute significantly to the failure of the elastomer. Since the mechanical properties of elastomers, as of other polymers, are highly dependent on strain rate, frequency, and temperature a fundamental investigation into the effects of these parameters on the frictional properties was initiated. Polyurethane thermoplastic elastomers of three different hardnesses (85A, 93A, 100A) were characterized with dynamic mechanical thermal analysis. The coefficient of friction between these materials and metal was measured using reciprocating motion. Data showed that the lubrication mode of a metal-elastomer contact in tribological conditions observed in artificial joints is highly dependent on temperature and frequency. In this respect, dynamic mechanical analysis can be used in the selection of an optimal elastomeric material for reciprocating bearing conditions. The next step of this project will consist of using this model to analyze the effect of friction on the failure of elastomeric composites composed of the polyurethane matrix and fibers. Introduction Low modulus elastomeric (LME) coatings were proposed and investigated by several authors as a method to improve the tribological performance of bearings (Unsworth et al., 1987; Dowson et al., 1991). By deforming under pressure, elastomeric layers enhance lubrication by the formation of a fluid film through elastohydrodynamic and micro-elastohydrodynamic lubrication (Dowson and Jin, 1986). This occurs when the asperities of these materials flatten due to local pressure perturbations. However, when the fluid film breaks down, adhesive friction increases in the presence of smoother elastomeric materials (Fuller and Tabor, 1975). Consequently, lower modulus materials provide more effective micro-elastohydrodynamic lubrication but fail due to an increase in shear strain at the elastomer surface. Dowson et al. (1991) suggested that the elastic modulus should not be lower than necessary to provide effective micro-elastohydrodynamic lubrication. However, these authors did not address the National Textile Center Annual Report: November 1996 71