抄録
In fatigue crack growth, crack tip blunting and resharpening occur within every cycle. Thus the first requisite for fatigue threshold may be considered as fulfilment of the condition of dislocation group emission from the crack tip. On the other hand, it has been experimentally observed that fatigue threshold stress intensity depends on the ferrite grain size. Therefore, the second requisite for fatigue threshold may be to satisfy the mechanical relation between the emitted dislocation group and the grain boundary. Thus, herein firstly the mathematical formula for fatigue threshold has been derived based on the first requisite mentioned above as the rate determining process. The criterion for fatigue threshold is proposed based on the condition of fulfilment of both the first and the second requisites. The model in this criterion is as follows. The fatigue threshold is considered as the stress intensity required to let the lead dislocation after emitted from the source reach the grain boundary. That is, when this condition is fulfiled, a micro crack initiates at the grain boundary by piled-up stress concentration, and, then, the main crack joins with this micro crack. Thus the fatigue crack growth will occur. Finally the criterion proposed is compared with the experimental results in literatures and also with other criteria, and the considerations were made.
