Evolution of fretting fatigue damage and relaxation of residual stress in shot-peened Ti-6Al-4V
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I. INTRODUCTION
FRETTING fatigue is a surface-damage mechanism capable of dramatically reducing the normal fatigue life of material. It occurs between two contacting surfaces in the presence of a load normal to the contacting surfaces when subjected to cyclic loading. The relative oscillatory displacement between the contacting surfaces is of the order of 20 to 50 m.[1] The near-surface damage developed between the contact surfaces enhances the probability that a crack would initiate and, thereby, reduce the fatigue life of the component. The premature failure of the engine components in some advanced fighter aircraft has been attributed to fretting fatigue damage.[2] Most often the fretting fatigue damage is observed to occur, in particular, in the turbine-disk fir tree. In order to enhance the fretting fatigue life of the components, surface enhancement techniques are used. The surfaces are shot peened or laser shock peened to impart a near-surface compressive residual stress in the material. This near-surface, compressive, residual stress is expected to retard the process of crack initiation at the surface. In other instances, components are coated to reduce the friction between the contacting surfaces. The reduced friction is expected to improve the fretting fatigue life of the components. Although the surface enhancement techniques are known to provide an extension of fretting fatigue life, the engine components are still inspected during scheduled maintenance, to prevent potential catastrophic failures. Significant progress S.A. MARTINEZ, Aerospace Engineer, and M.P. BLODGETT, Research Engineer, are with the Materials and Manufacturing Directorate, Air Force Research Laboratories, NonDestructive Evaluation Branch, Wright Patterson Air Force Base, OH 45433. Contact e-mail: [email protected] S. SATHISH, Senior Researcher, is with the University of Dayton Research Institute, Dayton, OH. S. MALL, Professor of Aerospace Engineering, is with the Air Force Institute of Technology, Wright Patterson Air Force Base, OH 45433. Manuscript submitted April 21, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
has been made in understanding the mechanisms that lead to fretting fatigue damage, but the development of techniques that allow the severity of the damage to be evaluated are still at primitive stages. For the past several decades, visual inspection methods have been the only tools that have been used for evaluation during maintenance. In visual inspections, the surface roughness of the area affected by fretting fatigue is determined using visual observation, supplemented by tactile information obtained by running a finger across the surface. A classification of the damage into light, moderate, and severe categories is made.[3] The components that have severe surface roughness are then rejected from service. While cost effective, this method relies heavily on the experience levels of the individual evaluators and does not provide quantitative information about the damage found in the material. Making a de
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