In the fatigue test, material fatigue is evoked through a cyclic load with corresponding test frequency. This may involve a pulsating load test in tension or compression as well as an alternating load test with tension and compression components.
Material failure in fatigue testing often occurs well below the static strength limits.
Results from fatigue tests are commonly presented in the form of stress-load cycling diagrams. Here the number of cycles to specimen break is plotted against the amplitude of the cyclic stresses.
On one hand, fatigue tests are used for characteristic value determination and on the other to determine fatigue life.
Material fatigue is described as the damage or failure of a material or component under time-varying, frequently repeated stress.
Material fatigue is caused by plastic deformation, which in its smallest form is called microplastic deformation. This damage can grow with continued stress (crack growth), ultimately leading to definitive failure of the material or component.
Investigation of many cases of damage lead to the following findings:
- Components that have been in use for some time without any problem, can suddenly fail.
- The failure is not caused by one single overload.
- The failure occurs well below the static strength limits.
- The load varies over time and is often repetitive.
The fatigue life of cyclically stressed components is limited. Therefore, before critical components are implemented, a fatigue life evaluation, fatigue life calculation or fatigue test are performed to provide a durability estimate of the component (determination of durability).