X-Ray Measurement of Stress by Multiple Exposure Technique

Abstract

Hereunder report is made of the studies pursued regarding application of multiple exposure technique to X-ray measurement of stress. In this technique, a film is repeatedly exposed to the diffracted X-ray in two or three positions. The distances from the specimen to the positions of exposure (R1, R2 and R3) are chosen as follows: R3-R2=R2-R1=R. If we express the distance from diffraction line obtained at R1 to that obtained at R2 by P, the diffraction angle θ is given by the following relation: θ=1/2tan-1(-P/R).In the present paper special emphasis is given on this technique which was employed in the X-ray measurement of stress in a coarse grained specimen, which gave spotty diffraction patterns on the film. An apparatus was devised for double exposure with interchangeable soller or double pin hole slit. The value R of the apparatus was put at 30mm. The value P was determined as the mean of several distances Pi between respective pairs of spots on the film which were diffracted from the same crystal. The standard deviation of diffraction angle θi of the spots is given by the following relation: θs=√1/iΣi1(θi-θφ, ψ)2.The origin of the error which can be induced in the measurement with multiple exposure technique was also discussed. The error in the measurement has been caused mainly by the divergence of X-ray, missetting of the film and inadequate measurement of Pi on the film. In the measurement of coarse grained specimen, divergence of X-ray is the main origin of the error. It is desirable therefore that the divergence angle in the use of soller slit will be as small as possible.The stress in the mild steel specimen with 50μ grain diameter was determined under bending load. The stress value obtained with this technique showed good agreement with that obtained by counter method using oscillation technique of X-ray optical system.Since there was no requirement for the use of reference material the diffraction line intensity from the specimen was free from any unfavourable affectation. The peak density of diffraction lines was subject only to the control of exposure time.