Flattening of Pyramidal Asperities Under Combined Normal Loading and In-Plane Biaxial Straining

In metal forming processes, contact mechanics in the tool-workpiece interfaces determine the resulting friction and thus influence the quality and stability of the process. In models representing dry, thin film and boundary lubrication conditions in metal forming, friction may be assumed proportional to the ratio of the real contact area to the apparent contact area. It is therefore important to obtain a fundamental understanding of as many of factors influencing asperity flattening as possible. This paper presents asperity flattening of pyramidal model asperities loaded normally by a plane tool. The experiments are performed in a recently developed test setup, which allows for different in-plane strain ratios in combination with the normal load. The in-plane deformation is known, through the yield criterion, to influence the necessary yield pressure to flatten the asperities and hence the resulting real contact area. In-plane balanced biaxial stretching is shown to result in further asperity flattening than plane strain. When comparing to previous results, it is shown that in-plane strains reduce the effect of the asperity shape on the resulting real contact area ratio.