Fundamental relationship between rolling contact fatigue driven surface damage and torsional fatigue

Abstract

This paper presents fundamental findings on equivalence of rolling contact fatigue (RCF) driven surface damage and torsional fatigue modes of failure. Fatigue experiments were conducted for both modes of failure using AISI 4130 case carburized steel specimens. A Thrust Bearing Surface Pitting Rig (TBSPR) was designed and developed to perform RCF tests on flat test specimens using thrust ball bearings. The experiments were conducted in boundary lubrication regime, simulating the contact conditions in tribological machine components. The current investigation presents a novel approach where the effect of contact pressure and number of fatigue cycles on the initiation of surface damage was characterized by surface profilometry of the wear track after each test to generate a stress-life (S-N) map. The S-N map provides unique insights to early detection of RCF driven fatigue damage. The S-N map was utilized to generate the surface damage S-N curve. Torsional fatigue experiments were conducted on the same material, and the S-N curves of both failure modes were compared. The comparison shows key similarities between the two S-N curves, as they differ by a simple scaling factor. This investigation demonstrates that RCF driven surface damage characteristics can be predicted by conducting much simpler, and faster, torsion fatigue tests. This paper also presents a novel methodology to determine contact pressures to avoid incipient micropitting/microdenting damage.