Abstract
The impact fatigue tests were carried out using a rotating disk type impact fatigue testing machine. The influence of ductile-brittle transition temperature and test temperature on the impact fatigue crack growth rate was investigated by means of fracture mechanics and fractography in quenched and tempered Cr-Mo alloy steel in which the prior austenite grain size was varried from 8.3 to 25.4μm, and the results of impact fatigue tests were compared with those of non-impact fatigue tests. The results obtained were as follows.
(1) The impact fatigue and non-impact fatigue crack growth rates associated with striation formation at stage IIb were insensitive to the change in ductile-brittle transition temperature and test temperature.
(2) In impact fatigue, it was found that the impact loading gave rise to transition of dominant fracture appearance from striation formation at stage IIb to intergranular cracking and cleavage at stage IIc as the Charpy impact value was decreased.
(3) In impact fatigue, the transition of dominant fracture appearance from exclusively striation formation to intergranular cracking and cleavage was found to result in the acceleration of crack growth rate. The relation between the m value in Paris's equation and the ratio of test temperature, T, to ductile-brittle transition temperature, TM, was expressed as follows.
m=1.12(T/TM)-1.99.
