A B S T R A C T An overview of our research performed during the last 15 years is presented to improve the understanding of fatigue crack propagation mechanisms. The focus is devoted to ductile metals and the material separation process at low and intermedial crack propagation rates. The effect of environment, short cracks, small-scale yielding as well as large-scale yielding are considered. It will be shown that the dominant intrinsic propagation mechanism in ductile metallic materials is the formation of new surface due to blunting and the resharpening during unloading. This process is affected by the environment, however, not by the length of the crack and it is independent of large-or small-scale yielding.a = crack length c = Manson-Coffin exponent CTOD = crack tip opening displacement da/dN = fatigue crack propagation rate K max , K min = maximum and minimum stress intensity factor N f = number of cycles to failure R = stress ratio K min /K max CTOD = cyclic crack tip opening displacement ε pl = plastic strain range J = cyclic J-integral J eff = effective cyclic J-integral K = stress intensity rang W e = elastic component of the remote strain energy density range W p = plastic component of the remote strain energy density range σ y = yield stress Figure 1 shows the fatigue crack propagation behaviour of austenitic steel. Such fatigue crack propagation rate, da/dN , versus stress intensity range, K, curves are typical for ductile metals. The fatigue crack propagation behaviour of such materials is characterized by: a steep drop of the crack propagation rate at the threshold of stress intensity range, K th , an extended Paris regime with a Correspondence: R. Pippan. I N T R O D U C T I O N

The propagation and non‐propagation conditions of cracks with crack closure and without crack closure are investigated in various materials. It was found that there exist lower limits for the crack‐driving force at which cracks do not propagate (da/dN is smaller than 10−10 mm cycle−1) for constant and variable amplitude loading. Finally, possible reasons for the growth of cracks below such limits, which may occur in the very high‐cycle regime, are discussed.