Multiple cracking, crack coalescence and fatigue lifetime - Model and experiments on an austenitic steel and on a nickel base alloy

In many metals and alloys, low-cycle fatigue (LCF) and thermomechanical fatigue (TMF) generate a large number of surface microcracks. Using the replica technique, we document their growth and coalescence in the austenitic steel 1.4550 and the nickel base alloy Inconel 100. A model based on elastic-plastic fracture mechanics is developed, which considers the growth and coalescence of many coplanar semi-elliptical surface cracks. The results of the model are consistent with the experiments. An important conclusion is that the fatigue lifetime predicted by the multi-crack model is not substantially shorter than that predicted by a corresponding model assuming only a single semi-circular surface crack. This means that lifetime assessment can still be based on single-crack data, if one considers a factor 2 to include scatter and multiple-crack effects.