Propagation of small cracks under RCF: a challenge to Multiaxial Fatigue Criteria

ABSTRACT. RCF is traditionally a very critical load condition for fatigue and, moreover, material defects (inclusions, inghomogeinities) play a significant role in determining the service life of materials exposed to the out-of-phase stresses which typically occur at the interface and below the surface of contacting bodies. In this paper we summarize the fatigue test results obtained for three different steels (a bearing, a gear and a railway wheel steels) that have been subjected to out-of-phase multiaxial fatigue tests simulationg RCF conditions in presence of small shallow pre-cracks. The results show that the fatigue resistance domain is characterized by the presence of two different phenomena. In the region of σh < 0, tests simulating RCF for deep defects show a peculiar co-planar propagation driven by shear while in torsional tests the fatigue strength appears to be controlled by the onset of Mode I propagation. The experimental results have then been discussed adopting the Dang Van criterion showing that the dependence on crack size can be easily predicted in terms of Mode I and Mode III thresholds at the tip of the micro-cracks