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, near-threshold 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^-6mm/cycle, where the alternating stress intensity AK 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 crack-closure concepts. These effects are contrasted with crack-propagation behaviour in other engineering materials and at higher growth rates.