Fatigue crack propagation behaviour derived from S–N data in very high cycle regime

ABSTRACT The crack propagation law was derived from the S–N data in the very high cycle fatigue of a bearing steel. The propagation rate, d a /d N (m/cycle), of surface cracks was estimated to be a power function of the stress intensity range, Δ K (MPa√m) with the coefficient C s = 5.87 × 10 −13 and the exponent m s = 4.78. The threshold stress intensity range was 2.6 MPa√m. The crack propagation from internal inclusions was divided into Stages I and II. For Stage I, the coefficient of the power law was C 0 = 3.44 × 10 −21 and the exponent m 0 = 14.2. The transition from Stage I to II took place at Δ K = 4.0 MPa√m. For Stage II, the coefficient was C i = 2.08 × 10 −14 and the exponent m i = 4.78. The specimen size and loading mode did not influence the surface fatigue life, while the internal fatigue life was shortened in larger specimens and under tension–compression loading. For ground specimens, the surface fatigue life was raised by the compressive residual stress, while reduced by the surface roughness introduced by grinding. For shot‐peened specimens, fatigue fracture did not take place from the surface because of a high surface compressive residual stress. The internal fatigue life was reduced by the tensile residual stress existing in the interior of the specimens.