Fatigue at high number of cyclic loads: Testing methods and failure mechanism

A significant proportion of machinery and equipment is operated up to a number of cycles greater 10⁸, which is in the range of conventionally fatigue limit design. For materials with a face‐centred cubic crystal lattice and for high‐strength steels with a body‐centred cubic crystal lattice fatigue failures were observed even in the Very High Cycle Fatigue (VHCF) regime of load‐cycles greater N = 10⁷. To reduce the testing time in the VHCF regime, one possibility is to perform the tests at a higher frequency. In addition to the typical servo‐hydraulic testing machines or resonant fatigue testing machine with test frequencies up to f = 400 Hz, ultrasonic fatigue testing machines with frequencies up to f = 20 kHz were used. In different comparative investigations it was shown that the testing method has a significant influence on fatigue life and fatigue strength. In this paper the influence of the testing method and test frequency on fatigue behaviour in the VHCF regime is presented using the example of steels and aluminium alloys and different hypotheses for the decrease in fatigue strength in this area are discussed.