X-ray stress and residual stress analysis of Charpy steel specimens

Carrying out the X-ray analysis on pronounced notches, on transitions of transverse sections or on components with small cylindrical profiles using the sin(2)psi-method demands the appropriate experience, special testing equipment and attention to various circumstances. The choice of a suitable primary gate is significant. Glass capillaries with significantly higher intensity than conventional aperture gates are employed. The primary beam's diameter is derived from the gate type, gate diameter, distance of the primary gate from the diffractometer centre and the type of beam. The ratio of the primary beam's diameter to the notch radius should preferable not exceed 1/5 to 1/4. However, the local irradiated measuring area must, in any case, be small enough such that a mainly homogeneous stress condition with a linear distribution of interference line locations is measured. Furthermore, owing to the grain size statistic, the variations of the measurement values along the notch root and the lower intensity, the primary gate's diameter must not turn out be too small. By using an X-ray beam with a high lateral resolution, one can partially resort to a translation along the notch. A precise and reproducibly adjustable X-ray beam, a diffractometer with both a high precision axis and a small error sphere and both a definite and a reproducibly locatable specimen are of equally substantial importance. Errors in adjusting the Xray beam or the notched specimen become noticeable from the geometric W-separations. Verification of the measuring procedure is essential for reliable analyses. As the investigations here confirm, feasible results are attainable with the available X-ray analysis using the sin(2)psi-method for the V-notch of a Charpy impact specimen. This is demonstrated by the measurements of the loading stresses on a V-notch specimen. The concentration factors determined here for the case of the X-ray method are in very good agreement with the FEM calculations. This contrasts with the concentration factors determined with strain gauges, which turn out to be significantly smaller. The residual stresses at the notch root, which were produced by manufacturing the steel V-notched specimens under different grinding conditions, by up-cut milling and after stress relieving, are largely comparable with the results for flat and for less pronounced notch geometries found in the scientific literature. Thus, specific and reproducibly different manufacturing states at the notch root of Charpy impact specimens can be produced to determine the implications for the behaviour of the Charpy impact test. An increasing practical demand for local residual stress analysis methods for examining even smaller geometries in the future, underlines the significance of reliable X-ray stress analyses with high lateral resolution.