Fretting wear-fatigue study for tribologically-induced damage in simple and complex geometries
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This thesis presents the development of a computational and experimental methodology for predicting fretting wear-fatigue interaction in simple and complex geometries, specifically targeting the taper-lock assembly in modular hip implants. Nonlinear continuum damage mechanics is first coupled with fretting fatigue analysis. This method shows similar results to the critical plane Smith-Watson-Topper (SWT) approach, which can also capture the dependence of fretting fatigue life on slip amplitude. An energy-based wear approach combined with the critical plane SWT, together with a non-linear kinematic hardening model for cyclic plasticity, is implemented for Hertzian round-on-flat and rounded punch-on-flat fretting configurations. The effect of contact geometry on fretting performance is addressed and a plastic ratchetting failure is shown to be a possible failure mechanism in partial slip cases, competitive with fatigue damage accumulation. Tribological and profilometry tests are carried out on two hip implant candidate material combinations, namely CoCr/forged Ti-6Al-4V and CoCr/DLMS Ti-6Al-4V. A scanning electron microscope (SEM) based technique for surface wear measurement is developed. Both coefficient of friction and energy wear coefficient for DLMS Ti-6Al-4V are found to be lower than for forged Ti-6Al-4V. A fretting wear-fatigue analysis methodology for the taper-lock assembly in modular hip implants is developed and presented. A global model is built for fretting analysis, assuming a flat-on-flat contact. Wear evolution is predicted on the stem surfaces for both material combinations, with CoCr/DLMS Ti-6Al-4V giving superior wear performance. A micro-scale sub-model for frictional contact and wear-fatigue analysis is developed. Surface undulations (for enhanced frictional locking effects) on the stem and femoral head play an important role in fretting fatigue life. The inclusion of undulations in fretting wear-fatigue simulations will avoid non-conservative design. It is also found that to simulate wear in the micro-scale models can avoid over-conservative designs. Therefore, the combination of fretting wear-fatigue modelling with rough surface profile is a key to accurate fretting performance prediction. The predicted service time, which is comparable with the published test results, shows that the hip joint modelled here meets the National Health Service (UK) requirement.
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