An Innovative System for Fretting Wear Testing Under Oscillating Normal Force
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Material damage caused by fretting wear is of significant concern in many engineering applications. This paper describes the design and performance of a new machine for the laboratory investigation of fretting wear under oscillating normal force (fretting mode II). The test machine uses an electromagnetic actuator to impose an oscillating normal force between the contacting bodies at a constant force amplitude over a wide range of frequencies. The principle of the actuation mechanism and the fretting wear induced with this particular wear test configuration are outlined in detail. Normal force and electrical contact resistance were measured on-line during fretting mode II wear tests. The performance of the wear test machine is illustrated by data obtained for different materials combinations, namely, hard materials, such as high-speed steel and (Ti,Al)N coatings oscillating against alumina ball counterbodies, and soft materials, such as a tin coating oscillating against the same. In general, wearing of the counterbodies was observed in the slip region. It has been observed that hard coatings and bulk ceramics are prone to fretting fatigue cracking. The evolution of electrical contact resistance in the case of the self-mated soft tin coatings tested under fretting mode II conditions is also reported.
I. INTRODUCTION
Fretting is an interfacial material degradation process occurring when surfaces of contacting solid bodies are subjected to small-amplitude oscillatory relative motions.1 The fretting damage results from a simultaneous or subsequential occurrence of two or more different wear processes, such as adhesion, oxidation, fatigue, and abrasion.2,3 The practical importance of this material degradation process attracts widespread concern in industry. Fretting is the modern plague of industrial machinery4,5 and occurs in a number of applications such as aircraft components (e.g., splines, cables, bearings, hinges, seals, actuating devices, rocket propulsion fuel tanks, etc.),6 nuclear power plants,7 automobiles, electrical contacts, and surgical implants.8 Such contact vibrations are often induced by cyclic accelerations, cyclic stresses, acoustic noises, or thermal cycling. The displacement field of an arbitrary vibration can be divided into three basic “fretting modes” characterized by a linear, radial, or circumferential trajectory.9 The three fretting modes are schematically shown in Fig. 1 for a ball-on-flat contact condition. In mode I, fretting is induced by a linear relative displacement with constant frequency. Laboratory fretting experiments reported in literature up to now have concentrated on this mode I testing.1,10 J. Mater. Res., Vol. 15, No. 7, Jul 2000
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FIG. 1. Schematic representation of the three basic fretting modes and the corresponding displacement trajectories in the case of a ball-on-flat contact geometry.
Fretting under radial displacement, classified as fretting mode II, occurs when the radius of the contact area oscillates due to
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