Shear force effects on fretting fatigue behavior of Ti-6Al-4V
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I.
INTRODUCTION
FRETTING induces damage on or near the contact surface of an assembly of two bodies when exposed to microscale relative motion between them due to vibratory forces. This surface damage greatly reduces the initiation and early propagation lives of fatigue cracks compared to those where fretting is absent. Premature failures therefore are commonly seen in several structural components due to fretting, e.g., riveted and bolted joints, metal ropes and cables, and dovetail joints of blade/disk attachments in gas turbine engines. Many variables such as shear or tangential force, normal load or contact load, bulk stress (i.e., external applied load), relative slip, contact geometry, coefficient of friction, and several other factors all seem to have effects in some unknown collaborative way on fretting behavior. For example, there have been several studies in which it has been shown that these factors influence fretting fatigue life/strength.[1] Further, it is generally agreed that these variables either directly or indirectly play a role in fretting damage process and behavior, but the exact individual contribution of these factors is not well characterized. Furthermore, the fretting test system can also contribute to the interaction among these variables. As an example, when a compliance-based fretting system involving a fatigue test machine with one servo-hydraulic actuator[2,3,4] is used, shear force depends on the axial load applied to the test specimen. This has made it a difficult task to quantify the influence of a single variable on fretting fatigue behavior/process. Therefore, approaches based on the same local mechanistic parameters in the contact region such as critical plane approach,[5] fracture mechanics parameters,[6] and O. JIN, Research Scientist, and S. MALL, Professor, are with the Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson AFB, OH 45433-7765. Contact e-mail: shankar.mall@ afit.edu Manuscript submitted March 7, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
combination of stresses and relative slip (i.e., Ruiz parameter)[7] have been used to characterize fretting fatigue crack initiation behavior. In other words, these approaches sum up the effects of these variables; they do not isolate their effects. However, it is desirable to investigate the influence of these variables, as independently as possible, on the fretting fatigue behavior/process as well as to characterize the interaction among these variables. Among many variables, shear force plays a major role in fretting behavior. Shear force originates from friction between contacting bodies and primarily results from the mechanical interlock of surface asperities and adhesive bonding. Shear force along with bulk stress has a great influence on the stress state in the contact region between two mating bodies. Nix and Lindley[8] found that the size of defects produced during fretting fatigue increased with increasing shear force and was independent of applied bulk stress. They fu
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