切欠き材の疲労破壊に関する研究 : き裂先端における塑性域について

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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 propagation or the fatigue fracture and the substructure surrounding the tip of the crack. But as the direct observation of the crack tip is very hard even by means of electron microscopy technically, there have been but few observations so far published on it. Considering it, the authors have intended to study on the crystal deformation at the tip of the propagating and non-propagating cracks during fatigue process of pure copper and low carbon steel specimens with notches by means of electron microscopy and the back-reflection X-ray micro beam diffraction technique. The electron microscopy technique has been used to obtain the qualitative information on the subject, and the back-reflection X-ray micro beam diffraction technique has been used to perform quantitative investigation on it. The synopsis of this paper is as follows; (1) Cracks started in the trace of the slip stratum, and eventually spread along the persistent slip band. Non-propagating cracks stopped at the first grain or its boundary. (2) The extension of the plastic region ahead of the propagating crack tip observed by the electron microscopy is larger than that of non-propagating crack. (3) The quantitative investigation by means of the back-reflection X-ray micro beam diffraction technique points out that the value of the micro lattice strain and of the misorientation ahead of the propagating crack tip decreases steeply at the place about 0.2mm of distance from the crack tip. This value is equal to that ahead of the non-propagating crack tip. (4) The plastic region at 0.2mm of distance from the tip of the propagating crack correlates well with the place at the tip of the propagating crack observed by means of replication electron microscopy.
社団法人日本材料学会の論文

切欠き材の疲労破壊に関する研究 : き裂先端における塑性域について

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