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The robust determination of the threshold against fatigue crack propagation ΔKth is of paramount importance in fracture mechanics based fatigue assessment procedures. The standards ASTM E647 and ISO 12108 introduce operational definitions of ΔKth based on the crack propagation rate da/dN and suggest linear fits of logarithmic ΔK– da/dN test data to calculate ΔKth. Since these fits typically suffer from a poor representation of the actual curvature of the crack propagation curve, a method for evaluating ΔKth 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.