Strain-rate effects on the plastic indentation and abrasion of elastohydrodynamic contacts by debris particles

Abstract

A numerical model of plastic indentation and abrasion of elastohydrodynamic contacts by debris particles previously developed by the author is extended to include the dependence of material flow stress on strain rate. Using the Johnson–Cook viscoplasticity model, the flow stress of all materials involved in the indentation process is expressed as a function of plastic strain, strain rate and temperature. This complements other elements of the model, including strain-gradient plasticity, work-hardening, frictional heating from particle extrusion, thermal softening, melting and material loss due to adhesion. Following a laborious programme of experimental validation and numerical comparisons, the predictions of the model are shown to be in excellent agreement with the experimental results on soft and hard particles in rolling and rolling–sliding elastohydrodynamic contacts. The incorporation of strain-rate effects further improved the agreement between theoretical and experimental results previously established with simpler versions of the model that ignored the strain-rate factor. Strain rate is also shown to affect several parameters in the process of surface damage, including the magnitude of contact stresses and flash temperatures, as well as the behaviour of a particle in a concentrated contact. It is also shown that for an optimum contact velocity linked to strain-rate effects and fluid film thickness in lubricated contacts, surface damage is minimised, particularly for large and hard particles.