Fretting and Contact Fatigue Studied with the Aid of Fretting Maps

Fretting fatigue refers to fatigue life shortening in bulk testing, induced by the simultaneous application of surface fretting, that provokes nucleation and growth of surface fatigue cracks. This paper suggests controlling fretting fatigue by controlling the conditions of the fretting contact. Fretting is known for its complexity of parameters and variables. The present work has focused on a systematic representation of the surface effects of fretting, called fretting maps. By dynamic recording of tangential force and displacement for convex circular contact between metallic specimens, different fretting regimes are distinguished. The corresponding modes of surface damage are identified by metallographic studies of the fretting scars. For increasing displacement amplitude, three regimes can be identified. 1. The stick regime is characterized by very limited interfacial slide and surface damage. A tangential force (FT) versus displacement (δ) plot for a complete load cycle is a straight line, indicating predominantly elastic deformation and no sliding losses. 2. The mixed stick-slip regime contact surface is subdivided into a central stick area, surrounded by an annular yield and slip area. For soft materials contact fatigue cracks are observed around the stick-slip boundary. The FT(δ) curve displays a narrow hysteresis loop, indicating some plastic deformation and relaxation by sliding. 3. The gross slip regime (transition) is recognized by a sudden drop in the recorded FT(δ) curve, corresponding to a transition from static to kinetic friction (the “point of incipient gross slip”). All adhesive asperity contact bridges are broken during every half-cycle and plastic deformation takes place in the bulk of the contact zone. The scars are characterized by sliding wear grooves in the fretting direction. The FT(δ) hysteresis loop is deformed by the incipient gross slip drop in FT. Fretting test series were performed at varying test parameters, and fretting maps were constructed by pairwise combinations of the recorded critical parameter values for transition from one regime to another. Generally, there is a gradual transition from the stick regime to the mixed regime, whereas, the mixed-to-gross-slip transition is sharp. The mixed stick-slip regime is characterized by high cyclic surface stresses at the boundary between the stick zone and the slip annulus, promoting contact fatigue. Thus, conditions of mixed stick-slip will aggravate bulk fretting fatigue. Similarly, the sliding conditions of the gross slip regime are characteristic of fretting wear, during which surface fatigue cracks will be successively worn away and prevented from contributing to fretting fatigue. Once the relevant fretting maps for a given tribosystem are established, it is, in principle, possible to control the fretting process by choosing the active parameters so as to promote or suppress a certain type of fretting damage.