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 2θ—sin2ψ 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 2θ—sin2ψ diagrams and wave-like fluctuation in diffraction intensity—sin2ψ diagrams were observed in the surface of fine-shot-peened round-bars.
(3) An evaluation method for 2θ—sin2ψ diagrams showing the pattern of bowing or waving using two statistical approaches and simulations in tri-axial stress states has been investigated. Two parameters CB and Cw representing degrees of the bowing and waving in 2θ—sin2ψ diagrams have been suggested. At least seven diffraction angles in the 2θ—sin2ψ diagram are necessary to evaluate waving. A simulation of the 2θ—sin2ψ diagram enables us to confirm effectiveness of the evaluation technique with CB and CW.
(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.