Surface-crack propagation in plane-bending fatigue of smooth specimen of low-carbon steel

Long-life fatigue tests of smooth specimens of low-carbon steel were conducted under cyclic plane bending with zero mean stress. The propagation rate of small surface cracks nucleated at the corner of the specimen was measured and analysed from the viewpoints of linear and elastic-plastic fracture mechanics. The rates of crack growth both on the surface and at the depth were higher at larger amplitudes of the applied stress, when they were correlated to the stress intensity range. The J integral range evaluated from the elastic-plastic stress-strain distribution in the depth of the specimen was found to be a unique parameter controlling the growth rate of fatigue cracks both on the surface and at the depth of the specimen, and it was also found that the relation between the growth rate and the J integral for small surface cracks agreed with that for long through-thickness cracks.