Interaction between wear and rolling contact fatigue in pearlitic rail steels.

The work presented in this thesis is aimed at investigating the interaction between wear and rolling contact fatigue, which are two of the most serious forms of deterioration caused by the wheel on rail contact stresses. Wheel-rail contact conditions were simulated by a two disc contact using the LEROS (LEicester university ROlling-Sliding wear testing machine). Investigation of the wear behaviour of BS11 pearlitic rail steel showed that steady state wear behaviour is established after a certain number of rolling-sliding cycles. Contact surface failure by ratchetting (accumulation of unidirectional plastic strain) was found to be the dominant failure mechanism during the period leading to the steady state. This mechanism was confirmed by the drop in the wear rates when the direction of rolling-sliding; i.e. strain in the surface layer, was reversed at predetermined numbers of cycles. The effect of repeated rolling direction reversals on crack morphology, propagation and rolling contact fatigue (RCF) life of BS11 rail steel was also investigated. It was established that rolling direction reversal has a beneficial effect on RCF life. A new mechanism, the "variable crack face friction mechanism", was proposed to explain this effect. Interaction between wear and RCF fatigue was investigated through rolling-sliding experiments where specimens were run dry for certain number of cycles, to induce different levels of wear damage, before the fatigue performance was investigated. It was shown that initial dry cycles above a critical number causes sudden and significant deterioration in RCF life. This deterioration has been explained in terms of the role of the accumulation unidirectional plastic strain (ratchetting) in initiating and propagating the early cracks during the dry phase. A strong correlation was found between the total ratchetting strain induced during the dry phase and the deterioration in RCF life. An empirical relationship to estimate this deterioration was concluded.