What Does Fatigue Wear Mean?
Fatigue wear of a material is caused by a cycling loading during friction. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. Fatigue occurs if the applied load is higher than the fatigue strength of the material.
Fatigue wear is caused by contact between asperities with very high local stress, and are repeated during sliding or rolling with or without lubrication. The result of fatigue wear is severe plastic deformation. Repeated, alternating mechanical stresses lead to the formation and propagation of cracks under the stressed surface, which is thus destroyed.
Fatigue wear prevention must be a consideration in planning and design phases.
Corrosionpedia Explains Fatigue Wear
Fatigue wear occurs when surface and subsurface cyclic shear stresses or strains in the softer material of an articulation exceed the fatigue limit for that material.
One of the types of fatigue wear is fretting wear, caused by cycling sliding of two surfaces across each other with a small amplitude (oscillating). The friction force produces alternating compression-tension stresses, which result in surface fatigue.
Fatigue wear in rolling bearings generally starts with micro-pitting—small areas on the bearings’ surface where material has been removed due to repetitive stress. At its terminal point, surface fatigue causes significant surface spalling—large craters often several hundred microns across, which are easily visible to the naked eye. While the effects of fatigue on bearings are well-documented, wear debris analysis offers a unique insight into fatigue failure.
Fatigue cracks start at the material's surface and spread to the subsurface regions. Under repeated or cyclic loading conditions, subsurface delamination and cracking can occur. The cracks may connect to each other, resulting in separation and delamination of the material pieces. For example, fatigue of an overlay of an engine bearing may result in the propagation of the cracks up to the intermediate layer and total removal of the overlay.
When fatigue wear occurs during sliding:
- At a high coefficient of friction, there is a high degree of plastic deformation.
- Shear strain is very high.
- Surface material is shifted in the sliding direction.
- Grains are drawn out, and oriented parallel to surface.
- Submicron-sized dislocation cells occurs, similar to highly cold-worked metals.
- High stacking fault energy promotes cell formation (Al, Cu, Fe).