The aim of this research was to develop a reliable predictive fatigue damage model for adhesively bonded structures. It was necessary for such a numerical model to be independent of geometry of the structure and capable of considering different fatigue damage phases, of simulating the experimentally measured damage evolution and of predicting the effect of main fatigue loading characteristics. Three different adhesively bonded joints, namely the single lap joint, the laminated doubler in bending and the mixed-mode flexure specimen manufactured with the same adhesive system were considered for experimental and numerical investigations. The bonded joints were tested under quasi-static and fatigue loading and the failure responses of the bonded joints were studied experimentally and modelled numerically. To assess static and fatigue progressive damage in the bonded joints, experimental approaches, including backface strain and video microscopy techniques were employed. The effect of important fatigue loading parameters including the maximum fatigue load level and the load ratio on the failure behaviour of the bonded joints was examined experimentally. A cohesive zone model with a bi-linear traction-separation response was used to simulate the progressive damage in the adhesively bonded joints. This cohesive zone model was integrated with a damage mechanics based fatigue model to simulate the deleterious influence of fatigue loading. The proposed fatigue damage model was able to account for the effects of fatigue loading characteristics including the maximum fatigue load and fatigue load ratio. The static and fatigue damage models were calibrated, validated and optimised against the experimental results obtained and other published experimental data. The fatigue damage model was applied to adhesively bonded joints subjected to constant and variable amplitude fatigue loading. The model was able to successfully predict the detrimental effect of the variable amplitude fatigue loading as well as the constant amplitude fatigue loading. The proposed fatigue damage model was generally found to be a significant improvement on other damage models available for adhesively bonded structures

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The present work made use of existing thermal stress (fast moving distributed Hertzian heat source) solutions available to extend the isothermal stress solution to take into account effects of the flash temperature rise and the corresponding thermo-elastic stress components. The highlight of this new analytical model was the development of a thermo-mechanical yield factor, γ-factor, whereby critical conditions for which inelastic behaviour occurs can be predicted and the likelihood of failure estimated. The results of this work showed that failure by formation of welds can occur. This presumes that the prerequisite conditions of the failure of the fluid film and boundary lubrication and sufficient asperity interactions occur so that the average friction is high enough to produce a bulk surface stress which is sufficient to exceed the yield limit. Thus, an engineering explanation of how macro sliding/rolling contacting components (as opposed to lubricating systems) fail is available for the first time. A mechanism of weld-formation was also proposed. Initial scuffing is seen to be caused by the inability of the surface patch to shrink back elastically when flash temperature effects disappear. Subsequently, on the next contact encounter, it will lead to stress concentration and cause local melting and shearing of the patch, resulting in metallic welds smearing onto the faster moving body and thereby became what is known as a 'scuffed mark'. Experimental work on roller-on-disc dry sliding wear tests confirmed that the level of metal-to-metal friction experienced by rubbing hardened steels, even for mild wear conditions was within the range of friction coefficients required by the present analytical model to cause bulk plastic flow of the concentrated sliding/rolling contacting surfaces.

The main concern of this thesis was to investigate the running-in wear of automotive cam-follower systems used in the testing of lubricants and to develop wear particle analysis methods using image processing with particular reference to shape determination. Two kinds of cam-follower test rig were used to investigate the behaviour. The initial tests were carried out using a motored Peugeot TU3 cylinder head. The change in surface topography occurring through wear was examined as a basis for establishing an objective method of wear evaluation. The second series of tests was performed using a specially instrumented MIRA cam and tappet test machine. The effects of variations in the initial surface finish and hardness of the cams and followers were evaluated in which the effect of hardness difference between cam and follower and the implications of surface finish variation were highlighted. When compared with previous work, the measurement of the worn surface profiles of followers were similar to those predicted by a specific wear model for the situation where the cam was harder than the follower. Quantitative methods to analyse wear particle shape using image processing were developed. Appropriate image analysis hardware and associated computer software facilities were established to process data and provide quantitative information about particle size and shape, particularly edge detail. Wear particles taken from different sources were analysed. In the MIRA tests, when the cam was harder than the follower, particles generated under two body abrasion conditions exhibited a large aspect ratio and positive skewness, coupled to high kurtosis. This situation was contrasted by the smaller, rounder, rougher edged particles generated in the case where the cam was softer than the follower which exhibited a milder abrasive polishing type wear.

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