X-Ray Investigation on Fatigue Fracture of Notched Steel Specimen

Abstract

The mechanism of fatigue fracture in metallic material is thought to be independent of its initial condition in itself. In other words, a monotonously varing parameter, which represents the fatigue progress of material with any initial treatment, should be considered. Considering this, the authors intended to study on crystal deformation during the fatigue process of low-carbon steel specimens, which were treated in a different manner before the fatigue test, by means of the back-reflection X-ray microbeam diffraction technique because this technique made it possible to perform an quantative investigation on deformation of grain and subgrain distinctively.In this publication, they dealt with the results of investigation on the fatigue process preceding the macro crack initiation of cold-rolled low-carbon steel specimens with different reduction percent. They were itemized as follow.(1) The number of sharp spots in an arc diffracted from a grain increased and the intensity of its back ground darkness decreased relatively, as the fatigue process progressed. On the other hand, an arc diffracted from a specimen subjected to cyclic stress under the fatigue limit made no considerable change.This was observed to de regardless to the reduction percent of cold-rolling.(2) Simultaneously with the appearance of sharp spots in a diffracted arc, persistent slip bands came within the scope of an optical microscopic examination during the fatigue process.(3) During the fatigue process the micro lattice strain range within one grain of a heavily cold-rolled specimen changed differently from that of a lightly cold-rolled one. In the former case it showed a rapid decrease in the early stage and then its diminution became gradual. On the contrary, in the latter case it increased abruptly first and it was followed by gradual increase.The micro lattice strain range within one subgrain of lightly cold-rolled specimens tended to decrease with some fluctuation of first stage. While that of heavily cold-rolled ones showed an abrupt decrease in the early stage and then it made no considerable change.(4) The subgrain size tended to be smaller and the total misorientation within one grain increased during the fatigue process, although those changes were not outstanding.(5) Crystal deformation differed from grain to grain in the degree of its progress during the fatigue process.In the next publication, models of crystal deformation during the fatigue process are to be proposed and the mechanism of fatigue fracture of cold-rolled low-carbon steel specimens is to be discussed in relation to the case of annealed ones, being based on the above-mentioned results.