There are several ways of approach in the study of problems concerning the strength of metallic materials. The strength of materials is discussed for its basis in the scope of atoms composing the materials with respect to their order. In order to eliminate the factors affecting the complex character of the strength, single crystals or specimens of extremely coarse grains of pure metals or metals of simple chemical composition have been used as the test materials in this line of works. We are amply equipped with knowledges of the strength of materials.
A great number of studies, on the other hand, have been made in application of engineering science. In this line of work, the materials to be subjected to experiments are mainly engineering metals of rather complex composition, and the strength of materials is observed or interpreted from macroscopic or submacroscopic viewpoint. This side of approach is, of course, of immense importance to machine designers and metallurgists.
Various criterions in the strength of materials have been proposed. These criterions have in general been derived from probable models of the strength of materials that have been introduced from the findings in the fundamental study in atomic order. Although some of the criterions are useful for the prediction of strength of materials, we are seldom convinced of the mechanism because of lack of vivid proof of the strength of materials on engineering materials. In this sense, the authors believs that there still remains a gap to be fulfilled between the study of strength in engineering field and those in physical aspects. It is desirable therefore to make efforts in search of practical experimental means to find the change in microstructure of engineering materials, to enable us to recognize the property of the strength of materials.
The application of X-ray diffraction is no new technique as means of experimental study. It has often been adopted for direct and non-destructive observation of changes in the micro-structure of crystalline materials in fundamental researches. It has been the plan of the authors to adopt this technique of applying X-ray diffraction to the experiment to elucidate the minute variation of micro-structure that has occuried in engineering materials.
The authors have hitherto adopted the technique of applying a series of X-ray diffraction in the study of the strength of engineering materials where X-ray stress measurement and its half-value breadth have been adopted as the measure of changes in the structure. The X-ray measurement has been applied to various kinds of stresses. Especially its application marks one of our most successful work in measuring the residual stress of cold worked metals under the conditions of various kinds of cold work by using sin2φ method. The relation of half-value breadth and number of cycles of fatigue stresses has been studied in detail for various sorts of engineering metallic materials. As the results of a number of experiments, it has been found that the variation in half-value breadth shows very regular relation with number of cycles.
The technique of X-ray diffraction that used to be employed in studies of earlier days was, however, of usual back-reflection, being a merely conventional means in industry for practical estimate of the strength of the materials, and so the findings obtained thereby were not sufficient for microscopic interpretation of the strength of the materials. In place of the ordinary back-reflection technique that is commonly used for the study of strength of engineering metallic materials, as being of insufficient resolving power, the X-ray microbeam technique has recently been introduced, whereby it is expected that engineering metallic materials will reveal more clearly the feature of variation in micro-structure, and the understanding of the findings in the early studies will be supported by the observation of the micro-structure.
