Fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values that cause such damage may be much less than the strength of the material typically quoted as the ultimate tensile or yield limit.
Factors that affect fatigue failure are cyclic stress state, geometry, surface quality, material type, residual stresses, size and distribution of internal defects, direction of loading, temperature, etc.
Internal method, various standards performed on BOSE ElectroForce 3330