X-Ray Investigation on the Residual Stress of Metallic Materials (On the Residual Stress of Stretched Carbon Steel) by Shuji TAIRA and Yasuo YOSHIOKA (Faculty of Engineering, Kyoto University, Kyoto) The X-ray method of stress measurement is taken as a unique method of non-destructive measurement of local stress. Recently the experimental technique and the theory of stress measurement by X-rays have been greatly improved. From the results of recent study, it is understood that the mechanically applied stress with in the elastic limit can be measured by X- rays with sufficient accuracy. In the case of measurement of residual stress by means of X-rays, however, there remains a question whether the stress measured by X-rays represents the macroscopic residual stress itself. Since the lattice spacing is taken as the gauge length for measurement of strain, it is plausible that the microscopic nature as well as the macroscopic nature are included in the value of the stress obtained. In the present study, experiments were carried out to find the influence of microscopic nature on the valuation of residual stress by X-rays using stretched plate specimens of carbon steels. Eight sorts of carbon steels with carbon content ranging between 0.14 and 1.00 percent were used as the material of the specimens. All the specimens were annealed after the forming. The plastic tensile strain of a quarter and a half of the strain at the maximum load in tensile test was given for every material, and residual stress measurement was made by X-rays and mechanical methods for the stretched specimens. It was intended to compare the distributions of residual stress determined by both these methods. As the X-ray diffraction apparatus, an automitic X-ray stress analyzer of the parallel beam gave diffraction from the (211) crystal planes of carbon steel. For the purpose of determining the distribution of residual stress by X-rays, a thin layer was successively removed on both the surfaces of the plate specimen, and the stress on the revealed surface was measured. On the other hand, the macroscopic residual stress was measured by the conventional mechanical method of successively removing of thin layers only on one side of the specimen. The obtained results are summarized as follows: - 1) Compressive residual stress is observed in the surface layers of the stretched specimens, and its value seems to be independent of the stretched strains. reliable. 3) The residual stress on the surface of the stretched specimens by X-rays shows non-uniformity from place to place of the surface. It is considered that the non-uniformity of stress value by X-rays must be taken into consideration in the comparison of the stress measured by X-rays with that measured by mechanical means. 4) The compressive residual stress measured by the X-ray method increases with the increase
of carbon contents both on the surface layers and at the core of the specimen. If the stress value by X-rays represents only the macroscopic residual stress, it must converge to zero at an infinitely small thickness after the successive removal of layers from both the surfaces. However, the experimental result shows the existence of compressive residual stress in such a case. Therefore, it is considered that the residual stress measured by X-rays in the stretched carbon steel includes the macroscopic residual stress caused by the surface effect and also the microscopic nature due to the plastic deformation, and the cause of the discrepancy between the X- rays and the macroscopic residual stress in the stretched carbon streel cannot be explained by the criterion of the phase stress (Gefugespannung) alone. (Received Sep. 10, 1963) Table 1 Chemical composition and annealing temperature of test materials
A B 60 170 For X-ray method 150 260 For mechanical method Fig. 1 Dimension of specimen
Table 3 Results of tension test Fig. 2 Curvature gage for residual stress measurement Fig. 3 Stress-strain diagram and X-ray redidual stresses in the specimen surface due to stretching of 0.14% carbon steel Table 2 Measuring apparatus, method and condition of stress measurement by X-ray method
(a) (b) Distance from center (mm) Fig. 5 Residual stress distribution in the specimen surface before and after stretching (c)
Depth from surface a (mm) (d) (h) (e) (f) (i) Fig. 6 Residual stress distributions in the stretched plate specimens measured by the X-ray and the mechanical methods
Fig. 7 Base line stresses for varions carbon steels
Fig. 8 Schematic diagram of residual stress distribution a) Macroscopic residual stress crystals crystals c) Superposition of these stresses 3) T. Nishihara and S. Taira, Mem. Fac. Eng. Kyoto Univ., 12, 90 (1950)
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