This paper presents an analysis of the conditions for the transition from stable to unstable fatigue crack propagation and of fracture toughness characteristics under static,dynamic and cyclic loading.The analysis is based on the results of investigations on heat resistant steels in an embrittled state and at low temperatures,as well as on previous results.It has been shown that for cyclically softening steels in an embrittled state the upper limit of the fatigue fracture toughness (final fracture) is the dynamic fracture toughness,while the lower limit is about 0.7 of this value.The fatigue fracture toughness that corresponds to the appearance of the first cleavage zones has even lower values.No correlation is observed between the magnitudes of static and fatigue fracture toughnesses.The results obtained are in fair agreement with a previously proposed model for the transition from fatigue to brittle fracture.
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