Fatigue crack propagation and threshold for shallow micro-cracks under out-of-phase multiaxial loading in a gear steel

Most of the recent studies on shear propagation have been devoted to the description of growth rate and growth mechanisms under shear, even though the state of stress caused by RCF is a more detrimental condition. The aim of the paper is to obtain the growth rate and threshold condition for small cracks under RCF conditions in a gear steel, whose properties under torsion had been previously studied.Two series of out-of-phase fatigue tests were carried out on specimens containing small cracks (EDM micro-notches pre-cracked under Mode I). Results show that compared with simple torsion shear crack propagation under RCF conditions is enhanced and that the threshold condition (characterized by a discontinuous crack advance) is lower than it is under simple torsion. Moreover, the experimental data confirmed the existence of a ‘short crack effect’ under RCF conditions. The crack growth mechanism is characterized by an intense rubbing and plastic deformation of the crack lips which, unlike simple shear, enhances crack propagation.