The influence of fracture mechanism on the stage 2 fatigue crack growth resistance in center-notch plate and compact type specimens of high-strength steel 4340 has been studied in relation to the different microstructure and stress intensity, particularly in a wide range of growth rate, by means of fracture mechanics and fractography. The fractographic examination has shown three general mechanisms of growth, namely a structure sensitive mechnism (stage 2a), a striation mechanism (stage 2b) and a pseudo-monotonic fracture mechanism like microvoid coalescence or intergranular cracking (stage 2c). Departure from the exclusively striation mechanism to include the pseudo-monotonic fracture mechanisms has been found to result in accelerated crack growth rates. The roles of microstructure, strength level and fracture mechanics parameters in promoting the different crack growth behaviour was also discussed.
