Mechanical behaviour of various machine elements, such as gears, brakes, clutches, rolling bearings, wheels, rails, and screw and riveted joints, are influenced by interaction between contact elements and surfaces. Surfaces in rolling and/or sliding contact are exposed to material contact fatigue. Contact fatigue can be defined as a kind of damage caused by changes in the material microstructure which results in crack initiation followed by crack propagation, under the influence of time-dependent rolling and/or sliding contact loads. The contact fatigue process can in general be divided into two main parts:
initiation of micro-cracks due to local accumulation of dislocations, high stressesat local points, plastic deformation around inhomogeneous inclusions or other imperfections in or under the contact surface;
crack propagation, which causes permanent damage to a mechanical element.