Recently, the experimental procedure of X-ray stress measurement has been notably improved because of improvement of the equipment for use as well as of revision of theoretical background. However, there are a few problems that want further investigations. One of them is the question whether the stress values obtained from the measurement of a state of stress by using different lattice plane coincide or not. This problem of diffraction plane dependence has been discussed to some extent up to

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... e, but its real feature is still in the vague. Consequently, considering from the stand point of engineering application, it is important to determine the most favourable diffraction plane for the measurement of stress through the use of characteristic X-ray. In order to clear up the problem, the author performed the following experiments. Two sorts of materials were used in this experiment, one being annealed specimens of 0.07% carbon steel and the other a plastically extended specimens of 0.13% cabon steel plate. All specimens were etched before subjecting to X-ray photography. The characteristic X-ray of mm in dia. and stresses were measured by adopting the diffractions from (310), (211) and (220) atomic planes. Since it is important in stress measurement to have an accurate measure of the distance between film and specimen surface for each exposure of film, the standard substance was used and both diffraction lines of standard substance and objective materials were taken on the same film. Intensity distribution curve was taken from each pattern by using an automatic recording type microphotometer. The distance between peak position of intensity of diffraction from standard material and specimen was measured from these curves. This process led us to very easy detection of the peak of X-ray diffraction and also to exact measurement of length on the film. Two series of experiments were carried out as follows; (A) A special miniature bending testing machine was used and surface stress on specimens of annealed 0.07% carbon steel was measured by X-ray at several stages of bending within elastic range. The mechanical stress and the value of X-ray stress which was obtained from each lattice planes were compared. (B) Experiments were made concerning the diffraction plane dependence in the measurement of residual stress that was produced by plastic deformation. Annealed 0.13% carbon steel specimens were stretched beyond yield point. At several stages of loading the load was removed and the surface residual stresses were measured using (310), (211) and (220) atomic planes obtained by