X-ray stress measurement is a technique which has been widely applied to residual stress. However, a satisfactory method for measuring residual stress in springs has not been appropriately developed yet. Thus, a research committee in Japan Society of Spring Engineers was established to research the X-ray based stress measurement of springs in 2008. The first item accomplished by the committee was an exploratory investigation to understand the measuring conditions of X-ray stress measurement being used in spring industries. By referring to these results obtained, four subjects were investigated as follows: a) development of a method of residual stress measurement in shot-peened plate, b) development of a method of residual stress measurement in shot-peened round-bar, c) evaluation method for 2
θ—sin
2ψ diagrams, d) characterization of an X-ray stress constant in ferrous spring steels.
The following results were obtained:
(1) The errors for 68.3% confidence limits in the residual stress of the shot-peened plate specimen, which were observed by X-ray stress measurement under routine measurement conditions in participating companies, were within 6% of the average residual stress values obtained. Therefore, X-ray stress measurement conditions based on a 68.3% confidence limit were proposed.
(2) In a sample with a curved surface, such as a coil spring, the diameter of an X-ray collimator should be less than 20% of the diameter of the spring wire so that the error of the residual stress can be less than 10%. It turned out that the nonlinearity of the 2
θ—sin
2ψ diagrams and wave-like fluctuation in diffraction intensity—sin
2ψ diagrams were observed in the surface of fine-shot-peened round-bars.
(3) An evaluation method for 2
θ—sin
2ψ diagrams showing the pattern of bowing or waving using two statistical approaches and simulations in tri-axial stress states has been investigated. Two parameters C
B and C
w representing degrees of the bowing and waving in 2
θ—sin
2ψ diagrams have been suggested. At least seven diffraction angles in the 2
θ—sin
2ψ diagram are necessary to evaluate waving. A simulation of the 2
θ—sin
2ψ diagram enables us to confirm effectiveness of the evaluation technique with C
B and C
W.
(4) A round robin test (RRT) for characterization of an X-ray stress constant in ferrous spring steels such as SUP9A and SUP12 was carried out. As a result, it appears that the average X-ray stress constant K in the two spring steels was -338 ± 13.1 MPa/deg.
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