Near-threshold fatigue-crack propagation in steels

The characteristics of fatigue-crack propagation in metals and alloys have been the subject of several extensive reviews in recent years, but in very few instances have the details of ultralow growth rate, nearthreshold fatigue-crack propagation been similarly discussed. In this review the effects are examined of various mechanical, microstructural, and environmental factors which influence fatigue-crack propagation in steels at growth rates less than 10-6 mm/cycle, where the alternating stress intensity ∆K approaches the so-called threshold stress intensity ∆K0 below which crack growth cannot be experimentally detected. The marked influences of load ratio, material strength, and microstructure on such near-threshold growth are analysed in detail and rationalized in terms of possible environmental contributions and crackclosure concepts. These effects are contrasted with crack-propagation behaviour in other engineering materials and at higher growth rates.