Presence and role of the third body in a wheel–rail contact

Abstract Note : Persons concerned by modelling wheel–rail contacts should take into account the existence of the third body as a boundary condition of their models. Similar articles have been published in two journals: one dealing specifically with this subject in Fatigue Fract. Eng. Mater. Struct . while the other, in this special issue of Wear , describes a more general experimental approach. Some parts of this article, which describe in details the reality of the wheel–rail contact, have been used in the synthesising article: Y. Berthier, S. Descartes, M. Busquet, E. Niccolini, C. Desrayaud, L. Baillet, M.C. Baietto-Dubourg, The role and effects of the third body in the wheel–rail interaction, Fatigue Fract. Eng. Mater. Struct. 27 (5) (2004) 423–436. doi: 10.1111/j.1460-2695.2004.00764.x Published by Blackwell. This paper focuses on the presence of the third body, a solid interfacial layer, in the wheel–rail contact. A phenomenological analysis is carried out as thoroughly as possible of the real tribological behaviour of this contact. To improve the understanding of the wheel–rail contact reality and the reconstitution of contact dynamics, this paper is presented a synthesis of different studies coming from: analysis of specimens taken out periodically from rails and wheels in service, and thus under real contact conditions, test laboratories, allowing us to impose rolling–sliding conditions with very high precision. The results show the presence of natural third body ranging in thickness from a few micrometers to several dozen micrometers on the rail and wheel. Initially composed of particles stemming from wheels and rails, it flows into the contact to accommodate the sliding between wheel and rail while absorbing and digesting solid and fluid contaminants. Up to now, the third body is a means of tracing local conditions in the wheel–rail contact, in order to pass through the difficulties of in situ instrumentation. From all these studies and results, a better understanding of the role of the third body and its influence on friction is reached. It also controls the rail's lifetime, the lubrication as well as the wheel–rail adherence via its “degradation and/or reformation” mechanisms.