Any engineering components under rolling contact fatigue loadings (i.e. gears, bearings, rails and etc.) exhibit
mode III crack propagation. Since the fatigue failures of the mechanical components previously mentioned
inevitably result in costly engineering damage/loss and down-time, it is of some importance to deep inside the
mode III fatigue crack behaviour. Fundamental understanding of the mechanics of shear crack grow is necessary in the
formulation of multiaxial fatigue crack growth laws.
Mode III fatigue crack behaviour have been investigated by some researchers: however almost all mode III crack growth
studies, considered a circumferentially notched cylindrical crack growth specimens [1] submitted to torsional loading.
Tschegg experiments [2] revealed that there is a critical applied stress intensity factor value under which mode III crack
propagation is no more stable, resulting in a mode I branching. The reason of the unstable mode III crack growth can be
attributed to the frictional effect. Moreover Nayeb-Hashemi et al [3] observed that a superimposed static axial load on the
fatigue in torsion can promote stable mode III crack growth, since it reduces the sliding mode crack growth effect.
Nevertheless few experiments have been performed in order to investigate the mode III crack growth behaviour under
multiaxial fatigue loading. The aim of the present work is to present a new experimental method in order to promote
mode III co-planar crack propagation under mixed-mode loading conditions: out of phase fatigue tests were carried out in
specimens containing shallow micro defects of different sizes. OOP tests have been done in order to better investigate the
effect of a superimposed cyclic compression on the stable co-planar crack growth as well as the role of friction under
mixed mode I+ III loading condition.