Kohei Kojima

Determination of Residual Stress of Low Carbon Steel Plates by X-Rays

During the recent years, much progress has been made in the metallurgical field, more particularly in investigation to detemine with X-rays the internal stress of metals and in the accuracy of its detection with the improved experimental procedure and the equipment for measusement of stress with X-rays. But, there are some problems relative to the microstress obtained by X-rays and the macrostress by mechanical methods.Considerable agreement of both the stresses in the macro-elastic range is recognized. In the measurement of residual stress in the material subject to plastic deformation the diffraction plane dependency, the broadening of the diffraction pattern, “Gefugespannungen” etc. are important factors in the measurement of stress with X-rays to determine the residual lattice strains of certain grains in the specimen selectively. Many investigators pointed out that, after uniaxial plastic deformation, the surface of metals shows residual compressive stress, the so-called surface effect.These factors must be considered even in determining the surface stress by X-rays in the low carbon steel after various plastic detormation. At each stage after the uniaxial plastic deformation, the residual stress obtained by the lattice strains determined by the diffraction lines of the(310), (211) and (220) planes using Cokα1, CrKα1 and FeKα1, respectively on the surface of the low carbon steel plates is compressive, and increases rapidly up to plastic strain e≅6%, and thereafter increases slowly. The breadth of the diffraction line increases correspondently to the residual stress. The residual axial stress distributed over the cross section of the plate was determined by means of etching, and measured by X-rays or by strain gauge. The results showed that the compressive residual stress on the surface layers balanced to the tensile stress in the interior of the specimen when we used strain gauge, but lost balance when we used X-rays. When the thickness of the specimen approached zero after successive etchings, the residual compressive stress obtained by X-rays diminished, but some compressive stress remained still on their layers. The maximum compressive stress was not observed on the surface, but was latent in the interior near the surface. It is considered from these results that “Gefugespannungen” due to the existence of the cementite and ferrite phase seems to play an important role.In determining the breadth of the diffraction line, measurement was made from the differential curve of the diffraction contour by the twin pole Geiger-Muller counter as mentioned in The Third International Conference on Non-destructive Testing 1960. A new method is mentioned. The differential curve obtained by the twin pole Geiger-Muller counter method shows zero on the peak position of the intensity curve of the X-ray diffraction and a peak on the position of the maximum slope of the intensity curve. The angle between these two positions corresponds to half the width of the intensity curve. Thus the influence of the variation of the intensity of the X-ray source which is one of the most serious problems in usual counter method became small, and we could measure the width easily even though the diffraction line was broad in the plastic deformation.

Effect of Rate of Tension on the Yield Point on Rolled and Irradiated Polyethylene

Specimens applied to tests were made of the Ziegler type polyethylene rolled and/or irradiated with gamma-ray from cobalt 60 under immersion in water and then were heat-treated in boiling water for one hour to stabilize the polyethylene materials. An Instron tester was employed to measure the yield points of all specimens at the cross-head speeds of 0.5, 3.0 and 20 cm/min at a temperature of 30°C. Finally, concluding remarks are as follows : the yield point of polyethylene increases with increasing cross-hear speed, but the yield point ratio (which is defined as the ratio of yield point of irradiated polyethylene to the yield point of non-irradiated polyethylene) becomes smaller when tensile test is performed at higher speed than it is done at lower speed.

Plastic Deformation of Metals and the Octahedral Shearing Stress Theory (2nd Report)

The experiments were carried out to study the stress-strain relation in the metal under combined stress condition. In the case concerning the 1st report, the tubular specimens of 0·4%C steel with the open ends or the closed ends were at the first time deformed up to plastic range by internal oil pressure, which was removed soon, and then the specimens were subjected to tensile force in the axial direction. Experimental results : (1) The more severely the specimens with the closed ends were strained in the pressure tests, the greater the material was strain-hardened by the tensile strain except the non-prestrained specimen. (2) In the case of the specimens with the open ends, the elastic limits became lower with increase in the amount of the prestrain, while strain-hardening by tensile strain was greater in the range of large strain.