Further Considerations Concerning a Mechanism of Fatigue Crack Propagation

X-ray microbeam diffraction technique is a useful one to investigate the features of the crystal deformation in a localized area. That is, this method have been adopted to examine the density and array of dislocation, microscopic lattice strain and macroscopic residual stress. And so, the informations obtained from the tip of the crack during the fatigue process have been correlated with the behaviours of the initiation and propagation of the crack. The authors, in the present paper, investigated a relation between the distributions of the microscopic lattice strains which are calculated and measured by the technique, and suggested the sort of dislocation at the tip of the crack that relate to the fatigue crack propagation. The crack initiated at the notch root of the specimen which was composed of the coarse grain and propagated along the grain-boundary in the early stage under fatigue process of the alternating stress 4.1 kg/mm(2). Thereafter, it changed the propagating direction toward the inside of the grain. The distributions of the micro lattice strain in each reflecting plane which were measuerd at the plastically deformed zone in the vicinity of the grainboundary and at the crack tip agreed well with modes of the strain distribution due to a screw and a edge dislocations by the calculation, respectively. From these results, the authors concluded that the fatigue crack propagation would relate closely to the changing in the sort of the dislocation from the screw to the edge.