Surface interaction between elastically loaded bodies under tangential forces

Abstract
This paper describes an experimental investigation of the micro-displacement between two bodies in contact under the action of a tangential force less than that of limiting friction. Experiments have been performed for the unlubricated contact of a hard steel ball with a hard steel flat, under a range of normal loads, using balls of varying diameter. Measurements of micro-displacement have been made under the action of both steady and oscillating tangential forces. In the latter case vibrational energy is dissipated at the interface and fretting of the surfaces occurs. The mechanism of these processes has been studied and is discussed. The quantitative results of the experiments provide considerable support for Mindlin's theoretical elastic analysis. For small tangential forces the displacements are almost entirely elastic and the compliance of the two bodies is given exactly by the elastic theory. As the tangential force approaches the value to cause slipping, the displacement exhibits a non-elastic component arising from the relief of an infinite shear stress which ideally elastic behaviour predicts at the boundary of the circle of contact. Observation of the surfaces after subjection to a sustained oscillating force showed that intimate metal-to-metal contact occurs only at the crests of the surface asperities as suggested by Bowden & Tabor. Fretting takes place over an annular area round the boundary of the contact circle and spreads radially inwards with increasing tangential force. Measurements of the energy loss suggest that for small amplitudes of oscillating force the theoretically infinite stress is accommodated by predominantly elastic distortion of the surface asperities. Under larger forces, however, the asperities are deformed plastically through large strains, which leads to eventual fatigue and fretting of the surfaces over the annular region of high stress. The amount of energy so dissipated agrees quantitatively with that calculated by Mindlin, showing that the degree of plastic strain occurring at the interface is governed by the stress distribution in the elastic hinterland.

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