Effect of work hardening on dynamic friction

Abstract

We have studied dynamic friction by impacting a copper plate onto a target consisting of a tapered, annealed, aluminium alloy plug mounted into a matching hole in a stainless steel outer sheath. The velocity of the back surface of the plug was measured using velocity interferometry. Unfortunately it was found that apparently identical configurations gave significantly different velocity profiles. Therefore a series of experiments were conducted in which the manufacturing tolerances were tightened and the assembly procedure was more carefully controlled than previously. Five experiments have been performed at the same impact velocity, (300m/s). Comparing the recent results with those fired previously it has become apparent that work hardening processes in the aluminium alloy near the sliding interface play an important role in the development of frictional forces. It is thought that work hardening generated at those regions of the cone surface in contact with the outer steel component during the assembly process leads to localised shear and, therefore, low friction when the system is loaded dynamically. By contrast those regions of the aluminium surface which remain soft give high friction following dynamic loading. We believe these effects explain the variability in the observed velocity traces.

Towards the role of interfacial shear in shock-induced intermetallic reactions

“…It has been suggested that this dynamic behaviour is due to thermal softening processes occurring in the interfacial region due to the high temperatures generated, in excess of 350 K from our models. Recent analysis of final state deformation at aluminium -stainless steel sliding interfaces has suggested the formation of regions of deep and shallow deformation alongside each other, of millimetre and micrometre order respectively [7]. The presence of regions of highly elongated shallow deformation can likely be attributed to thermal softening, allowing sub-surface sliding to occur in the aluminium in addition to any relative motion at the material interface.…”