Experimental and Numerical Investigation on Shear Propagation of Subsurface Cracks under Rolling Contact Fatigue

Shear propagation of subsurface cracks under rolling contact fatigue was investigated on quenched and tempered SAE 5135 gear steel. The formation of subsurface cracks almost parallel to the contact surface was observed, highlighting the de-attachment of micro-particles from the cracks surfaces due to shear. Subsequent wear and plasticization processes lead to fragmentation of these particles, which transform in a smaller incoherent structure, and to the formation of a gap between the crack faces. The gap is maximum in the central part of the crack, with a dimension of some microns, whereas it tends to zero approaching the crack tips. Moreover, wear reduces the roughness of the crack faces by smoothing the asperities. A FEM model was developed to simulate the process of crack propagation in presence of asperities along the crack path, by taking in to account their shape variation due to wear. For this aim, the crack was divided in several zones, assigning different faces roughness to each of them according to the experimental evidence. The significant influence of the roughness severity in determining the crack driving force could be quantified by this model, putting in evidence its locking effect for varying crack length.