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

Abstract

This paper introduces an experimental technique to investigate dynamic friction characteristics of sliding interfaces at normal pressures up to 125 MPa, 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-strain-rate behavior of engineering materials. The experimental configuration allows critical frictional parameters such as the friction-stress, slip speed and slip displacement to be resolved on a micro-second time scale without the use of transducers at the frictional interface. Moreover, the experiment provides information on the evolution of dynamic friction stress during forward as well as reverse slip. Using this experimental configuration dynamic friction experiments were conducted on 6061-Al (T6)/1018 steel and 7075-Al (T6)/tool-steel (D3) tribo-pairs. The results of experiments on 6061-Al (T6)/1018 steel indicate that steady-state kinetic friction is obtained within the rise time of the torsional loading pulse. The coefficient of kinetic friction is observed to increase with the roughness of the tribo-pair surfaces. For a soft/hard tribo-pair interface (7075 Al/tool-steel (D3)) the surface roughness of the harder material (tool-steel (D3)) constituting the tribo-pair is observed to control the frictional force. The measured coefficient of kinetic friction is approximately independent of the interfacial slip velocity for sliding velocities in the range of 2–10 m/s and normal pressures of approximately 100 MPa. Moreover, it is observed that the dynamic friction stress during reverse slip is almost twice as large when compared to friction stress in the forward direction. This increase in friction stress is understood to be due to frictional contact between fresh metallic surfaces formed by the breakdown of oxide and other surface films and anisotropy in frictional surfaces developed during the forward slip.