Spectrum fatigue life assessment of notched specimens based on the initiation and propagation of short cracks

This contribution deals with the initiation and growth of short fatigue cracks in cyclically loaded notched specimens made of 0.15 wt% carbon steel SAE1017. On the basis of experimentally determined data, a damage model based on cyclic crack growth has been developed which accounts for the anomalous behaviour of short fatigue cracks. In this model the initiation and propagation of a critical, naturally produced crack is regarded as damage. This approach allows the calculation of fatigue life for constant amplitude tests as well as for multi-level tests and irregular loading. Deviations from Minerʹs rule, which have often been observed for two-level tests are attributed to the varying fraction of crack initiation and propagation phase for different loads. The impreciseness of Minerʹs rule deduced from two-level tests is of secondary importance for service life calculation when compared with the negligence of amplitudes below the fatigue limit. The proposed model yields shorter service life than the elementary version of Minerʹs rule.