Normal displacements in non-Coulomb friction conditions during fretting

Non-Coulomb friction may occur in gross sliding fretting conditions, in which the tangential force increases as the fretting movement approaches its extreme position and produces 'hooked' fretting loops. Uncertainties in frictional behaviour make the design of highly loaded contacts against fretting a challenging task. Experiments were made with quenched and tempered steel, and cyclic normal displacements were discovered during non-Coulomb friction conditions. Normal displacement and non-Coulomb friction were caused by tangential fretting scar interactions between protrusions and depressions formed by material transfer. Tangential interlocking leads to inclined sliding conditions, which produce loading components in both tangential and normal directions; this explains most non-Coulomb friction.