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A B S T R A C T Fretting fatigue fracture of industrial machines is sometimes experienced after a long period of operation. It has been a question whether the fatigue limit which means infinite life really exists in fretting fatigue or not. Fretting fatigue tests in ultra high cycle region up to 10 9 cycles were performed. Test results showed that the S-N curve had a knee point around 2 × 10 7 cycles and a clear fatigue limit was observed in the giga-cycle regime for partial slip conditions. An electropotential drop technique was applied to detect the crack growth behaviour under the contact pad. The real-time measurement of crack depth during the fretting fatigue test at the fatigue limit showed that a crack initiated at an early stage and then ceased to grow after 2 × 10 7 cycles and the crack became a non-propagating crack. These results indicated that the fatigue limit exists in fretting fatigue and infinite endurance is achieved by the mechanism of forming a non-propagating crack. a = crack depth HV = Vickers hardness p = mean value of contact pressure N = number of cycles R = stress ratio (σ min /σ max ) V = potential drop V 0 = initial potential drop without crack σ a = stress amplitude K = stress intensity factor range K th = threshold stress intensity factor range

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