No abstract

Clamped contacts subjected to vibratory loading undergo cyclic relative tangential motion or micro-slip near the edges of contact. This cyclic micro-slip, known as fretting, leads to removal of material through a mechanism known as fretting wear and formation and growth of cracks through a mechanism known as fretting fatigue. In aircraft, fretting fatigue occurs at the rivet/hole interface leading to multisite damage which is a potential failure mechanism for aging aircraft. A finite element model of a current fretting fatigue experiment aimed at characterizing fretting in riveted joints is detailed. A non-symmetric bulk tension is applied to the specimen in addition to the loads transferred from the fretting pad. The model is verified through comparison to the Mindlin solution for a reduced loading configuration, in which the bulk tension is not applied. Results from the model with the bulk tension show that the distribution of micro-slip in the contact is not symmetric and that for some loads reversed micro-slip occurs. Finite element results are given for the effects that four different sets of loading parameters have on the maximum tensile stress induced by fretting at the trailing edge of contact. It can be shown using multiaxial fatigue theory that this stress controls fretting fatigue crack formation. This maximum tensile stress is compared to that of the Mindlin solution for a symmetric distribution of micro-slip. This stress is also compared to that of a variation based on the Mindlin solution for the cases with a non-symmetric distribution of micro-slip. It is concluded that the solution based on the Mindlin variation and the full finite element solution lead to similar predictions of the maximum tensile stress, even when the shear traction solutions differ significantly.

Fretting fatigue is a serious concern for the aviation industry. Understandins why and how these cracks form and propagate is t,he key to scheduling proper maint,enance inspect.ions and avoiding accidents. Knowledge of the surface stresses and stress intensity factors which are present can aid in this underst,anding. Surface stresses are calculated for two different pad geometries. The cylindrical pad geometry can be correlated with _the rivet/skin contacts on the fuselage while the round-edged flat punch geometry is similar to the dovetail notch in the engine, where disk and blade meet. Also, two different met,hods of obtaining stress intensity factors are compared. Finite element models and numerical solutions of singular integra,l equations are utilized in the study of t#he stresses and stress intensity factors. It is found that a great potential for fretting fatigue is present in both geometries. The most severe effect of fretting on the nucleation and fatigue processes occurs very close to t,he surface at the trailing edge of contact. MOTIVATION