Doctoral thesis English OPEN
Ashkanfar, Ariyan

A challenge in engineering coupling design is the understanding of performance of contact geometry for a given application. “Wear” is one of a number of mechanical failures that can occur in mechanical coupling design. “Fretting wear” occurs where surfaces in contact are subjected to oscillating load and very small relative motion over a period of time. Fretting has been observed in many mechanical interactions and is known to be a reason for failure in many designs.Recent evidence suggests that fretting wear occurs at the taper junction of modular total hip replacements and leads to failure of the implants. Experimental testing to determine the wear behaviour that occurs in mechanical devices is time consuming, expensive and complicated. Computational wear modelling is an alternative method which is faster and cheaper than real testing and can be used in addition to testing to help improve component design and enhance wear characteristics. Developing an algorithm that can accurately predict fretting wear considering linear wear, volumetric wear and surface wear damage is the main focus of this thesis.The thesis proposes a new computational methodology incorporating published wear laws into commercial finite element code to predict fretting wear which could occur at the taper junction of total hip replacements. The assessment of wear in this study is solely based on mechanical wear (fretting) as being the primary mechanism causing surface damage. The method is novel in that it simulates the weakening of the initial taper ‘fixation’ (created at impaction of the head onto the stem in surgery) due to the wearing process. The taper fixation is modelled using a contact analysis with overlapped meshes at the taper junction. The reduction in fixation is modelled by progressive removal of the overlap between components based on calculated wear depth and material loss.The method has been used for three different studies to determine surface wear damage, linear and volumetric wear rates that could occur at taper junction of total hip replacements over time. The results obtained are consistent with those found from observation and measurement of retrieved prostheses. The fretting wear analysis approach has been shown to model the evolution of wear effectively; however, it has been shown that accurate, quantitative values for wear are critically dependant on mesh refinement, wear fraction and scaling factor, wear coefficient used and knowledge of the device loading history. The numerical method presented could be used to consider the effect of design changes and clinical technique on subsequent fretting wear in modular prosthetic devices or other mechanically coupled designs.
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