The fatigue test serves for determining the endurance strength of a component or material sample (fig. 1).
For this purpose, the sample is exposed to quick mechanical vibrations in such a way that defined tensile and pressure forces alternately act on the component.
The difference is made between the upper stress level, i.e. the highest value of stress for each cycle, and the lower stress level, i.e. the lowest value of stress for each cycle (fig. 2).
Loads are distinguished by alternating loads (oscillating around the zero point) and pressure threshold or tension threshold load with the average stress level in the pressure or tension range.
The test determines the number of cycles N until fatigue failure. In order to determine a Wöhler curve (figure 3), several tests with different stress amplitudes δa are necessary (DIN 50100) while the average stress is kept at a constant level. In the diagram of a defined average stress, the stress amplitude is applied over the respective number of cycles at which the failure occurs. In order to make a clear statement, several tests are to be carried out under the same load conditions.
Each test run is regarded as successful after ten million cycles without fatigue failure. If the entire series of tests is successful, the material is regarded as fatigue endurable for the applied force. If the component fails before reaching the end of the test, this defines the finite fatigue strength of the component for the applied force.