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Rolling contact failures are classified according to th eir failure mode as (a) wear, (b) plastic flow, (c) fat igue, and (d ) bul k failures, with th e last class arising outside th e im m ediate contact area. Th e wear failures are subdivided in to mild wear by loose particle removal and sme aring in vo lvin g m etal transfer. Plast ic failures may arise du e to overload or to temperature imbalance. Fat igue is of th e spalling or th e surface dist ress t yp e. An analy sis of th e st ress conditions in a rolling contact is ske tched ; th e ste p'wise refinem en ts of th e con tact mo del are considered starting from H ertz th eory, and progressively including surface traction, plasticity, elastohYdr~dynamic lub ricant film s, surface m icrotopography , and the in homo geneities of real m etals, E ach failure m ode is associated with th e relevan t sev erity param et ers of th e contact stress con dition and a descri ption of its mo de of [ormation is given. Gu idelin es are provid ed for the identifi~ation of th at failure m ode which, amo ng competing m od es, is mo st likely to rend er a rollin g contact inoperative in a given operating en vironme n t . Nomenclature w = Wear ratẽ o = Elastohydrodynamic film thickn ess-roughness ratio a = Area of real asperity contact ao = Area of Hertz contact K = Rolling wear constant h» = "Uniform" elastohy drodynamic film thi ckness 0" 1.2 = rm s roughness of the contacting surfaces ]V = Load S = Sliding velocity f = Constant exponent (smearing load vs. speed equation) P = Probability to failure TO = Maximum al ternating shear stress range Zo = Depth below surface, of st ress TO N = Number of stress cycles e = Weibull dispersion paramet er V = Volume (of material under st ress) f = Function sign h, c = Exponents in life equ ation

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