Mechanics of fretting fatigue crack formation

Fretting is a contact damage process arising from surface microslip associated with small-scale oscillatory motion of clamped structural members. The fretting damage process is a synergistic competition among wear, corrosive and fatigue phenomena driven by both the microslip at the contact surface and cyclic fretting contact stresses. Fretting fatigue is one mechanism of the formation of cracks in many common structural members, often leading to multi-site damage in riveted lap joint assemblies in aging aircraft. Thus a criterion for prediction of fretting fatigue crack nucleation is needed. A detailed analysis of the microslip distribution at the contact surface and the subsurface stress field is required for such a prediction. Relevant closed-form solutions for the 2-D elastic stress fields are adapted for reduced loading configurations modeled in a recently constructed fretting fatigue experiment that applies loads relevant to aircraft lap joints. The resulting stress field is combined with a multiaxial fatigue theory that combines strain-life ideas with a maximum normal stress to predict both the initiation site and life of fretting fatigue cracks. In particular, the theory predicts formation at the trailing edge of contact-not the location of the maximum shear stress traditionally associated with crack formation in contact fatigue. The fretting fatigue crack nucleation theory is validated through comparison with data in the literature. Once validated, the model is used to investigate the effects of coefficient of friction, load intensity and fatigue properties on life. It is shown that increases in coefficient of friction and surface microslip sharply reduce the number of cycles required to nucleate cracks. Application of the fretting fatigue crack nucleation model to actual loading configurations in common structural members such as riveted lap joints can lead to a tool for evaluating fatigue life of those members.