X線残留応力測定に及ぼす試料表面の影響

抄録

It is a recent remarkable tendency that the method of X-ray stress measurement by means of X-rays has come to be widely utilized in many laboratories. It is also a fact, however, that some experimenters concerned with this measurement have still some doubts about its accuracy.
The results of the experiments carried out in our laboratory, as was reported last year in the special issue of this society pertaining to the measurement of stress by X-rays, showed a high accuracy in the measurement. This high accuracy is considered to have been established as a result of the confirmation that the condition of the specimen surface has an important effect on the accuracy in the measurement of stress by X-rays, which was obtained through a number of investigations in our laboratory.
Some literature concerning the measurement of stress by X-rays, however, treat the influence of the surface condition lightly. Accordingly, in order to make this point clear, experiments were carried out using the specimens with various surface conditions. As a result, it was found that the stress measured in the specimens with fine surfaces is in good agreement with the load stress, while that of the specimens with rough surfaces show discrepancies. Besides, as regards the residual stress, the measured values of the specimens with fine surfaces were found to be larger than those of the specimens with rough surfaces. The difference between both the measured values became larger as residual stress increased.
The reason why the measured values show differences according to the conditions of the specimen surface might be ascribed to the circumstance that they may have some connection with the depth of penetration of X-rays. In case the specimen surface is rough, the real stress value may not be obtained for the shallow penetration of X-rays, because the stress in the projected area is relieved. Therefore, the specimen surface must always be flat and smooth However most specimens in our use are far from satisfying this condition. Consequently, polishing method is used to obtain a smooth surface, but in the course of polishing stress is introduced, resulting in the shift of the true stress value. In order to eliminate this stress in processing, etching methods by means of chemicals such as hydrochloric or nitric acid have been employed so far. Experiments, however, have shown no good results on the chemically etched surfaces.
In our laboratory, therefore, taking the flatness of the specimen surface after the removal of stress in processing into consideration, electrolytic polishing method has been adopted, as was reported in the special issue of this society concerning the measurement of stress by X-rays. Besides, an electrolytic polishing apparatus was manufactured which is capable of eliminating the stress in processing in any specimen in a short time and which is used in our laboratory.
This report contains the results obtained for the case in which Cr-K_α rays were employed as characteristic X-rays. If Co-K_α rays with a different penetration depth from that of Cr-K_α rays were used, the results might be somewhat different from this report. But it remains a common requirement that the specimen surface should be flat and smooth. As regards the relation between the roughness of the specimen surface Co-K_α rays, the authors will further their researches and await another opportuuity to report the result.