PSI - Issue 46

T.J. Gschwandl et al. / Procedia Structural Integrity 46 (2023) 17–23 T.J. Gschwandl et al. / Structural Integrity Procedia 00 (2021) 000–000

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The second step is the measurement of the out-of-plane displacements of the cut and deformed surfaces. Finally, the numerical calculation of the residual stress distribution is conducted. Therefore, an FE-based software tool called VRS (Visible Rail Stress ©MCL) was developed in-house to numerically calculate the residual stresses by mapping the measured displacements. The calculation of the stresses is valid, as the deformation of the surfaces from planarity is presumably originating from elastic relaxation of residual stresses. (Pagliaro (2008)) In comparison to other residual stress measurement methods, the contour method delivers a two-dimensional distribution of the longitudinal residual stresses, perpendicular to the cross-section, in the component. In this work, the residual stresses of a rail with squat defects in the surrounding area of the cut have been measured with the contour method. Squats naturally initiate and propagate in the rail head and therefore it was decided to start the cutting process from the rail foot leading to the head. This enables a precise cut with EDM in the rail head and the influences from the cutting onset may be neglected. Subsequently, the displacements of the deformed surfaces have been measured with a Coordinate Measurement Machine (CMM) and then entered as input for the stress calculation in VRS. Fig. 3(a) shows the set-up of the cutting process whereas (b) displays the measurement of the out-of-plane displacements.

Fig. 3 (a) Wire erosion set-up to cut the rail starting from the foot to the head. (b) Surface measurement of the cut rail with a coordinate measurement machine.

2.3. X-ray Diffraction Method (XRD) The X-ray diffraction method is a commonly used (mostly) non-destructive approach to investigate the residual stresses in crystalline components. Simply explained, due to the assumption of a linear elastic distortion of the crystal lattice, the strain in the crystal lattice is determined from which the residual stress can be calculated (Prevéy (2019)). In principle, the X-ray diffraction method is limited to the measurement of the specimen’s surface. However, also subsurface measurements can be conducted after electropolishing the surface layer. In this work, the residual stress depth profiles of four positions on the rail head have been investigated along a path for a depth of 3 mm with 10 measurement steps – see positions (P1 – P4) in Fig. 4(b). 3. Results and Discussion 3.1. Measurement Fig. 4 shows the results of the measured out-of-plane longitudinal stresses within the rail. Fig. 4(a) displays the stress distribution in the rail head computed from the contour method. In the middle of the rail head a tensile stress area with stresses in the range of approximately 300 MPa is present. The compressive stresses of up to -200 MPa indicate the primary wheel contacting area on the left side of the rail. In Fig. 4(b) the near-surface stress profiles