An experimental method to study high speed sliding characteristics during forward and reverse slip

This paper introduces an experimental technique to investigate dynamic fric tion characteristics of sliding interfaces at normal pressures up to 125 MP a, slip speeds up to 15 m/s and slip distances of approximately 10 mm. This new apparatus involves a novel modification of the conventional torsional Kolsky bar apparatus employed extensively in the past to investigate high-s train-rate behavior of engineering materials. The experimental configuratio n allows critical frictional parameters such as the friction-stress, slip s peed and slip displacement to be resolved on a micro-second time scale with out the use of transducers at the frictional interface. Moreover, the exper iment provides information on the evolution of dynamic friction stress duri ng forward as well as reverse slip. Using this experimental configuration dynamic friction experiments were con ducted on 6061-Al (T6)/1018 steel and 7075-Al (T6)/tool-steel (D3) tribo-pa irs. The results of experiments on 6061 - Al (T6)/1018 steel indicate that steady-state kinetic friction is obtained within the rise time of the torsi onal loading pulse. The coefficient of kinetic friction is observed to incr ease with the roughness of the tribo-pair surfaces. For a soft/hard tribo-p air inter-face (7075 Al/tool-steel (D3)) the surface roughness of the harde r material (tool-steel (D3)) constituting the tribo-pair is observed to con trol the frictional force. The measured coefficient of kinetic friction is approximately independent of the interfacial slip velocity for sliding velo cities in the range of 2-10m/s and normal pressures of approximately 100MPa . Moreover, it is observed that the dynamic friction stress during reverse slip is almost twice as large when compared to friction stress in the forwa rd direction. This increase in friction stress is understood to be due to f rictional contact between fresh metallic surfaces formed by the breakdown o f oxide and other surface films and anisotropy in frictional surfaces devel oped during the forward slip. (C) 2001 Published by Elsevier Science B.V.