SLOW FATIGUE CRACK GROWTH UNDER STEP AND RANDOM LOADING.

Fatigue crack growth rates of the structural steel StE 620 V were measured under random loading and under eight-step loading at a mean stress equal to zero in the high-cycle regime. The results are compared with constant-amplitude loading crack growth rates. As reference values, the maximum cyclic stress intensity attained during the whole sequence and its root mean square value are tested; their applicability is discussed. For random loading in the tested range neither reference value leads to a threshold in the conventional sense. This is in contrast to the results on eight-step and constant amplitude loading. Crack growth predictions are made on the basis of linear damage accumulation not taking into account load history effects. The calculations roughly approach the experimental results.