PhD Thesis

Development of a Computational Method of Low Cycle Fatigue Prediction for Multi-Layer Surfaces under Rolling/Sliding Contact Conditions

Abstract

This thesis documents some significant progress in improving the understanding of how complex multilayer surfaces respond under mixed rolling-sliding contact. By utilising advanced finite element simulation techniques a unique method for modelling multilayer contact was developed. Capable of simulating mixed rolling-sliding contact in surfaces with up to eight layers; this method was applied to model the response of three complex coated systems commercially available from Oerlikon Balzers Coatings Ltd. The simulation method allows a direct comparison of each coating, quantifying the benefits achieved through the application of the coatings and monitoring the effect on the stresses and strain experienced by the underlying substrate material. The model simulation was then further developed to investigate the effects of low cycle fatigue on the failure mode of coated components. Through the development of a unique subroutine working concurrently with the Abaqus solver, shakedown principles and the critical crack plane approach were applied to simulate both the location and orientation of the point of crack initiation in the subsurface material, and the potential beneficial effects attainable through the addition of tribologically advanced surface coatings. Twin disc testing was also employed to compare the relative wear rates of a number of commercially available multilayer coatings when subjected to mixed rolling-sliding contact under dry and heavy loading. Through these tests, carbon based coatings were shown to offer significant improvements on the surface life of the coated samples from rolling contact fatigue wear. A further detailed study was then conducted into the specific failure regime of BALINITC, a multilayer Tungsten Carbon-Carbide coating, when subjected to cyclic rolling contact fatigue conditions. Detailed optical inspection over a range of load cycles found the failure of the coating occurred through subsurface crack initiation induced by the build up of coating material in substrate surface recesses remaining prior to surface coating.

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