Modeling of skin/rivet contact: Application to fretting fatigue

Fretting is a contact, damage mechanism arising from microslip associated with small-scale oscillatory motion of nominally clamped structural members. Fretting has been observed near aircraft skin fastener holes. The present work, focusing on frett ing as a crack micleation mechanism in lapjoints, analyzes the contact at a typical skin/rivet interface using both plane and shell finite elements. Contact elements implementing the Coulomb friction law keep track of contact status between interacting surfaces. The shell model accounts for bending, contact between the skin panels and rivet clamping pressure. Elastic supports control the load transferred and simulate various rivet configurations. No interference is considered. There is 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. The tensile stress decays rapidly away from the edge of contact. 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, while for high remote stresses, life decreases with increase in coefficient of friction. Increasing rivet head clamping pressure increases the life. Plasticity blunts effects of friction coefficient and clamping. An approximate solution, that does not require finite element analysis, estimates the crack nucleation life for any rivet, configuration quickly. * Professor, Senior Member A I A A 1 Copyright ©1997 by the American Inst i tute of Aeronautics and Astronautics, Inc. All rights reserved. INTRODUCTION Fretting is a damage process resulting from a combination of wear, corrosion and fatigue driven by micro-motion and stresses at the contact zone. The first analysis of Hertzian contact with the additional complexity of a tangential force having magnitude less than that required to produce gross sliding was done by Mmdlin and independently by Cattaneo.' These analyses reveal the division of contact into regions of stick and slip, which is relevant to fretting observations. Fretting fatigue is observed in roller bearings, riveted lap-joints, dovetail notches of turbine blades and even artificial hip joints.' In the recent past, various researchers have studied the fretting of lapjoints in the laboratory." Fretting in engineering materials is a three-stage mechanism. The first stage involves the removal of thin oxide layer on the material surface through wear. As the oxide degrades in the first few cycles, the underlying material of the contact surfaces forms microwelds through an adhesive process. This causes the accumulation of wear debris between the contact surfaces. The coefficient of friction increases in the first few hundred cycles of fretting contact, coincidentally with wear debris accumulation. ~ As the cycles keep accumulating there is plastic deformation near the surface and additional wear promoting more oxide formation. This near surface plastic deformation promotes the nucleation of cracks. Fretting fatigue results when one or more of these microcracks penetrate into the bulk of the material. Fretting can cause an order of magnitude change in expected fatigue life. Fretting fatigue crack nucleation has generated renewed interest of late, especially with regard to aging aircraft. The United States Air Force (USAF) is looking to extend the lives of some of its aging fleets (e.g. KC-135 and C-141). Also, some of the