An Investigation on the Fatigue Fracture of Notched Specimen (On the Region of Plastic Deformation ahead of the Tip of the Crack)

It is considered that the process of fatigue fracture could be arranged the three classes, i.e., crack initiation, crack propagation and fracture. Of these the second process is important from the following point of view. (1) As has so far been reported in many publications, the importance of crack propagation process in total fatigue life increases in proportion to the increase in the stress concentration factors. (2) The non-propagating crack is out of all consideration so far as is concerned. Summarizing the studies that have so far been made relevant to the propagation process under cycling stress, it may be divided into two classes. (1) The macroscopic approach concerning the relationship of the velocity of crack propagation and the applied stress. (2) The theoretical approach which deals with the analysis of the modelled mechanism of crack propagation. Neither the macroscopic nor the theoretical approach is adequate, however, to give full accounts of such phenomena as crack initiation and crack propagation. For this reason studies on the crack propagation in the course of fatigue process have been made from the microscopic point of view by Holden, Grosskreutz, McEvily, Wood, Taira, at al., using the electron microscopy or the back-reflection X-ray micro beam techniques. Holden reported the relation of the crack length and the extension of plastic region at the tip of the crack. Grosskreutz, Wood and Taira revealed the relation of the mechanism of the crack