X-Ray Measurement of Stress by the Double Exposure Method

Abstract

There are two ways to measure stresses by means of X-rays, namely the counter method and the photographical method. There is tendency in recent years that the counter method has been gradually being employed in laboratories and factories because of its affording possibilities of easy handling and quick measuring. It is well known, however, that the photographical method has hither to been more frequently applied than the counter method to the measurement of residual stress on local and small positions. There are several difficult problems about the measuring technique of the conventional photographical method, for example, the difficulty in powdering the specimen surface with calibrating material and the difficulty in determination of peak positition on the broadened diffraction lines, since the distance between the diffraction lines from the specimen the calibrating material is close on the film.In the present study a new method of determining X-ray stress from this point of view by the photographical method without using calibrating material has been tested and those features are mentioned. The distance between the X-ray film and the surface of specimen can be precisely determined by the diameter of diffraction ring of the calibrating material which powders the specimen surface by the conventional method. Bragg's angle of specimen is obtained by using the value of the distance and the radius of the diffraction ring of the specimen. On the other hand, by a new method which is called the double exposure method, the first exposure is given on the X-ray film from which the distance to the specimen is l1, and when the distance between the specimen is set at l2, the second diffraction ring of the specimen is photographed. Bragg's angle 2θ is calculated from Δl, the difference between the distance l1 and the distance l2, and Δr, the difference between the radius r of the diffraction rings at the first and second exposures, as 2θ=tan-1[-Δr/Δl1].As was expected, it has been made clear from the theoretical and the experimental discussions that this double exposure method has many advantages as follows:(1) If the difference between the radius of both the diffraction rings, Δr, is larger than a quarter of diameter of the diffraction ring of the specimen on the conventional method, the accuracy of the measured value of stress by the double exposure method is better than that by the conventional method.(2) The two diffractions forming the similar patterns, the distance between the peaks of diffraction intensity curves, have been easily measured.(3) Irradiation of X-ray beams can be easily pointed at the measuring position on the specimen without using the metallic powder as calibrating materials.(4) There is a difficult problem in measuring by the conventional method, the stress on the specimen with broad diffraction lines, because these diffraction lines of the specimen interface with those of the calibrating material. In contrast with above mentioned case, the stress measurement of such a specimen is made possible by employing this double exposure method.