Interplay of fretting wear, fatigue and damping: Influence of contact shape and mode of loading

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Fretting wear and fretting fatigue represent a considerable and long standing problem in applications using frictional contacts subjected to vibrations as e.g. fretting of tubes in heat exchangers, joints in orthopaedics, or dovetail blade roots of gas turbines. The physical reason for the fretting is partial sliding in the vicinity of boundary of a frictional contact. It is due to the vanishing normal pressure at the contact boundary in contacts with curved surfaces. This partial slip and the resulting fretting wear can only be prevented by using a contact with sharp edges (as e.g. a flat-ended cylinder). However, in this case, both normal and tangential stresses will be singular at the boundary, and the high oscillating stress will lead to fretting fatigue. Thus, applications with frictional contacts under vibrations should find an optimal path between Scylla of fretting wear and Charybdis of fretting fatigue. One way to avoid fretting damage could be the introduction of phase shifts between different oscillation modes which leads to a moving slip zone and may qualitatively change the character of wear and fatigue. Another way could be to completely suppress vibrations. However, in many structures this suppression can only occur due to damping caused by micro slip. Nevertheless, high structural damping due to friction (without damaging the contacting surfaces) can also be a positive constructive goal for itself (as e.g. in space applications).In the present project we study this triad “wear-fatigue-damping” for the fretting contact of steel-on-steel. We will study the transition from fretting wear to fretting fatigue by continually changing the shape from parabolic to flat-ended and by considering the superposition of normal and tangential oscillations with various amplitudes and phase shifts. The intended research work aspires to reduce fretting damages and thereby increase the life span of tribological systems. The project will comprise extensive empiric investigations, ranging from short-term model experiments to very-long-term endurance and fatigue tests. The experimental investigations will be supported by physical models and numerical simulations of the underlying contact mechanics and the governing damage mechanisms.The department "System Dynamics and Friction Physics" of Technische Universität Berlin, where the project is to be carried out, has excellent research experience in the field of tribology and is one of the world's leading institutions in the investigation of dynamic contact problems.