Fatigue Crack Threshold of Bearing Steel at a Very Low Stress Ratio

The aim of this study is to elucidate the effects of compressive mean stress on the fatigue crack threshold in SAE52100 bearing steel with Vickers hardness of 703. Tension-compression fatigue tests were carried out on specimens which have semi-circular notches, each measuring 2a of 300 μm in length. The threshold maximum stress, σ _{max, th} , was seen to decrease at the corresponding reductions in stress ratio, R, of-5,-3 and-1, at which points the fatigue limit was determined by the crack initiation limit. On the other hand, σ _{max, th} at R =-10 was unexpectedly higher than that measured at R =-5. The unbroken specimen at R =-10 possessed non-propagating cracks, in which the maximum length was 2a of 535 μm, at both ends of the notch. In order to clarify the reason for such a peculiar phenomenon, the crack closure behavior of the non-propagating crack was measured, using the unbroken specimen at R =-10. The results indicated the existence of crack closure which can arrest crack growth.