Shell Modeling of Fretting in Riveted Lap Joints

Abstract

Fretting is a contact damage mechanism arising from microslip associated with small-scale oscillatory motion of nominally clamped structural members. Fretting has heen ohserved near aircraft skin fastener holes. Shell finite elements are used to model the contact at a typical skin/rivet interface, with emphasis on fretting as a crack nucleation mechanism. Contact elements implementing the Coulomh friction law keep track of contact status between interacting surfaces. The model accounts for bending, contact hetween the skin panels, and rivet head clamping pressure. Elastic supports control the load transferred and simulate various rivet configurations. No interference is considered, leading to loss of contact between the skin and rivet on loading. The distinct stick-slip zones, combined with high tensile stresses at the edge of contact, are indicative of fretting, resulting in crack nucleation at the edge of contact. Away from the edge of contact, the tensile stress decays rapidly. The slip displacement has values typically associated with fretting fatigue. The interface between the two skin panels is also a region of fretting damage. Crack nucleation lives are predicted using a multiaxial fatigue theory. The top row of rivets has the smallest predicted life. For low remote stresses, increase in friction coefficient increases life, whereas for high remote stresses, life decreases with increase in coefficient of friction. Increasing rivet head clamping pressure increases the life. Plasticity blunts the effects of friction coefficient and clamping pressure. An approximate solution that does not require finite element analysis estimates the crack nucleation life for any rivet configuration quickly.