Paper 42: Preliminary Study of the Influence of Stress and Deformation in the Substrate on Junction Growth and Friction

Abstract

The change in surface texture and the associated increase in ‘real’ area of contact at the surface in contact with the punch during stretch forming is very much greater than that predicted by Tabor's theory. It was suggested initially that this large change could be due to the effect of the biaxial tensile stress in the bulk material. Subsequent tests on model asperities show that the pressure to cause yielding of the asperities is reduced when tensile stresses are present in the bulk material resulting in a larger ‘real’ area of contact for a given normal load. However, an even greater increase in contact area is produced if the bulk material is allowed to deform plastically during the application of a constant normal load. Other stress states were examined on a basis of indentation testing. It is suggested that the greatly reduced apparent yield pressure of the asperities gives rise to high friction coefficient over the punch nose. Application of Tabor's theory, together with the already large initial real area of contact, seems to account for the observed large junction growth.