Fatigue in a material is defined as the progressive localized plastic deformation process that occurs in a material that is subjected to cyclic stresses and strains at high stress concentrations, resulting in cracks or complete fracture after a sufficient number of fluctuations.
Fatigue life is defined as the number of cycles of loading (stress) of a given character that a sample continues before failure in a particular structure occurs. The number of revolutions is related to the engine speed. It can be converted to equivalent endurance watches. Fatigue life is affected by cyclic stresses, residual stresses, material properties, internal defects, grain size, temperature, design geometry, surface finish, oxidation, corrosion, and so on. The fatigue life of a component can be listed under different fatigue mechanisms.
The total fatigue life is equal to the lifetime of crack formation and crack propagation. Fatigue life depends on the cycle history of the loading magnitude because crack initiation requires greater stress than crack propagation.
The fatigue life of the component can be determined by the strain, stress or energy approximation. Fatigue is a very complex process influenced by many factors. It is often more effective to use a macro method rather than a microscopic approach to model the effects of fatigue mechanisms on fatigue life.
In manufacturing, fatigue is a failure mechanism that involves cracking of materials and structural components due to cyclic (or fluctuating) stress. While the applied stresses can be tensile, compressive, or torsion, crack initiation and propagation are due to the tensile component. One of the interesting factors regarding fatigue development is that fatigue cracks can be initiated and propagated at stresses well below the yield strength of the building material. These stresses are generally related to elastic deformation, not plastic deformation.
Fatigue failure generally consists of three stages: crack initiation, crack propagation, and failure. Fatigue cracks almost always start on a free surface and near a stress amplifier. Once a crack is initiated, continued cyclic stresses repeat the process, slowly growing the microcrack, which becomes a threat to structural integrity. The eventual failure event may be ductile or brittle depending on the material, thickness, temperature and applied stress.
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