Effect of surface finishing and loading condition on competing failure mode of clean spring steel in very high cycle fatigue regime

The effect of surface finishing and loading condition on competing failure mode of a newly developed spring steel, SUP7-T386, in very high cycle fatigue (VHCF) regime was experimentally studied. As a result, this steel for ground specimen represents the duplex S–N characteristics under rotating bending and the continuously descending S–N characteristics under axial loading, respectively, and all corresponding to surface and interior failure, whereas that for electro-polished specimen represents traditional S–N characteristics with a clear fatigue limit, only corresponding to surface failure. Owing to the combined effect of stress gradient and surface residual compressive stress, as well as the larger size of inhomogeneous microstructure (IM), the interior inclusion-induced failure with fine granular area (FGA) and the interior IM-induced failure without FGA become the predominant failure mode of this steel for ground specimen under rotating bending and under axial loading in VHCF regime, respectively. The threshold stress intensity factor for controlling surface and interior small defect-induced crack propagation of this steel is evaluated to be 2.86 MPa m1/2 and 2.04 MPa m1/2, respectively, the short crack effect plays a key role in causing the small defect-induced failure of this steel in VHCF regime. Considering the effect of surface residual stress, the surface fatigue limit of this steel can be well evaluated by the size of inclusion or IM. For the evaluation of interior fatigue limit of this steel, it is suitable by the size of IM under axial loading but it is all conservative by the size of inclusion, or IM and or FGA under rotating bending.