Robust Determination of Fatigue Crack Propagation Thresholds from Crack Growth Data

The robust determination of the threshold against fatigue crack propagation [Formula: see text] is of paramount importance in fracture mechanics based fatigue assessment procedures. The standards ASTM E647 and ISO 12108 introduce operational definitions of [Formula: see text] based on the crack propagation rate [Formula: see text] and suggest linear fits of logarithmic [Formula: see text] – [Formula: see text] test data to calculate [Formula: see text]. Since these fits typically suffer from a poor representation of the actual curvature of the crack propagation curve, a method for evaluating [Formula: see text] using a nonlinear function is proposed. It is shown that the proposed method reduces the artificial conservativeness induced by the evaluation method as well as the susceptibility to scatter in test data and the influence of test data density.