SURFACE ASPERITY DEFORMATION UNDER BULK PLASTIC STRAINING CONDITIONS

The paper is concerned theoretically and experimentally with the flatt ening of asperities by tools in metalforming processes, a topic import ant to lubrication in metal forming. It specifically studies the plane strain compression process. It proposes kinematically admissible velo city fields for the crushing of asperities on a plastically flowing fo undation and by adjusting independent variables of the fields predicts the actual flows by an energy minimization method. The theoretical pr edictions are compared with plane strain compression tests on aluminiu m samples with surfaces abraded in the direction of bulk plastic flow (the condition that the modelling describes). Both theory and experime nt predict that asperities are crushed at the start of bulk plastic co mpression, to generate a real area of contact a certain fraction of th e tool area depending on the contact friction and aspect ratio of the flow field, between 0.75 and 0.95 according to theory and between 0.6 and 0.8 according to experiment; but that with further bulk compressio n these contact fractions do not change. Despite these quantitative di fferences, it is felt that the method developed in this paper could us efully be extended to study asperity behaviour in other forming proces ses such as drawing or rolling. (C) 1998 Elsevier Science Ltd. All rig hts reserved.