Stress gradients in fretting fatigue

Abstract

Launched by the engine manufacturer SNECMA (SAFRAN group), this thesis focuses on the damage by fretting fatigue. The control of this phenomenon is of major importance in determining the lifetime of fan's disc. When two contact parts are subjected to a load of fretting fatigue, there is a strong stress gradient from the surface. In these cases, local approaches based on the level of stress at the most critical point, are not appropriate. Non-local approaches have been developed empirically, which are based on critical distance and the stress gradient in the immediate vicinity of the critical point. A second difficulty is the multiaxial nature and non-proportional loading. Fatigue-fretting loading has three main components, the fatigue loading (cyclic), the normal force (usually assumed to be constant) and fretting loading (cyclic but also at a frequency which may differ from that of fatigue). The third point concerns the mechanisms. The stress gradients are seen as beneficial in the sense that a crack initiated at the most critical point can stop due to the presence of the gradient, without leading to the breakdown. To quantify the effect of the gradient, tests on specimens with different gradient were carried out on a fatigue testing contact bench developed at the University of Brasilia. Mechanisms of damage were studied using post-mortem analysis and optical microscopy. The simulation of the fretting fatigue life was done using different approaches. The first one is based on the Critical Distance Method and a fatigue criterion. The second is based on a ΔK-based short crack arrest method. Finally, a new criterion was proposed. This method considers a generalized Von Mises yield criterion for the crack tip region and accounts for the T-stresses in the asymptotic LEFM development.