Fatigue life is a term used in fatigue testing and refers to the deformation and failure behavior of materials under cyclic loading.
A fatigue life test with constant amplitudes is referred to as S-N test. The tolerable stress amplitude is determined using the corresponding number of cycles.
In fatigue life tests the material or component is subjected to load changes. This reduces the permissible stress in a material: A break can occur before the tensile strength is reached; in part even without having exceeded the linear-elastic range.
In general, load changes are interpreted as cycles, which can be carried out under different load ratios.
Calculation of load ratio: R=σu / σo
From the S-N diagram you can read the maximum number of load changes for a certain load amplitude. It is dependent on the material properties, the force and the load application type (pulsating compression load, pulsating tensile load or alternating load).