Modifications to a fretting-fatigue testing apparatus based upon an analysis of contact stresses at complete and nearly complete contacts

An earlier form of fretting-fatigue test apparatus employs two collinear hydraulic actuators to impose bulk tension and shear. This apparatus employed a Hertzian contact to induce well-defined tractions. However, it is very difficult to make use of information culled from such tests in designing contacts which are either complete or almost complete in nature. A modified apparatus for investigating experimentally fretting fatigue under complete and nearly complete contact conditions is described. For these cases, any misalignment in the plane of the contact would alter the contact pressure distribution significantly leaving the results of the test questionable. Additionally, when applying a shearing force, it should be in the plane of the contact, eliminating the risk of a rocking moment which would otherwise have a similar effect. Here, we describe a testing apparatus that allows the above two issues to be addressed and resolved. The contact then sits in its “natural position” on the test specimen and the applied shearing force is in the plane containing the contact. Further implications for the development of the coefficient of friction are discussed.