Plastic deformation and damage accumulation below the worn surfaces

Abstract

Delamination of material layers adjacent to the worn surfaces is a commonly observed form of wear in unlubricated or poorly lubricated surfaces. In ductile materials, the delamination process usually involves large plastic deformation and subsurface damage. In this study, metallographic techniques have been used to determine the extent of plastic deformation and strain localization events during the sliding wear of annealed OFHC copper samples. Tests were performed using a block-on-ring type wear machine under constant load and constant velocity conditions. Subsurface displacement and microhardness gradients were measured as a function of sliding distance. It was observed that both the magnitude of plastic strain (and stress) gradients and the depth of highly deformed layers vincreased with the sliding distance. The flow stress and strains at the subsurface regions are shown to obey a Voce type constitutive equation. Wear proceeded mainly by a mechanism of delamination via subsurface crack growth. It is proposed that the competition between the plastic strain which enhances void growth and the hydrostatic pressure which suppresses it is responsible for the generation of a damage gradient so that the delamination takes place at a certain depth where the damage accumulation rate is maximum. A model based on the Rice and Tracey analysis of ductile void growth is developed and used to determine the location of subsurface crack propagation.