Pitting formation under the conditions of fretting fatigue

On the basis of a numerical-analytic model used for the analysis of durability and fracture of bodies whose contact occurs under the conditions of fretting fatigue, we predict and study the process of pitting formation in these bodies. It is assumed that pitting is initiated by a surface macrocrack formed in the contact region somewhat earlier and propagating as a result of its filling and wedging by the products of fretting. It is also assumed that the crack propagates according to the mode-I mechanism. On the basis of the analysis of the stress intensity factors, it is shown that surface cracks located near the boundaries of the contact region and oriented toward the unloaded surface of the body are most favorable for the development of pitting. The paths of propagation are constructed for cracks whose location is especially favorable for the formation of pitting. It is shown that the higher the pressure of the products of fretting upon the crack lips, and the smaller the angle made by the crack with the surface of the body, the faster the process of spalling of the surface (the paths turn to the boundary more sharply). The increase in the amplitude of the friction coefficient in contact cycles also intensifies the process of spalling. The theoretical results are in good agreement with the experimental data obtained for specimens made of TS-5 turbine titanium alloy and the well-known data of observations and conclusions made by Waterhouse.