PSI - Issue 4

Hans-Jakob Schindler / Procedia Structural Integrity 4 (2017) 48–55 Author name / Structural Integrity Procedia 00 (2017) 000 – 000

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(b)

(a)

Fig 3: (a) Near-surface stress profiles at three different circumferential positions of x according to Fig. 2(b); (b) residual stress distribution along x-axis for two different circumferential positions. The near surface stresses were measured at three positions around the circumference (at 0°, 120°, 240°) by using one straingage (M1 according to Fig. 1) for each measurement. The cut to a depth of 3 mm is introduced by a diamond cutting disk. One of the cuts and the corresponding straingage can be seen on Fig. 2(a). The measured stress-profiles are shown in Fig. 3(a). the stress profiles at 0° and 120° are roughly the same, the compressive stresses at 240° are significantly lower for x < 2.5 mm. In order to measure the stress distribution through the entire cross-section, a plate of about 15 mm thickness in radial/axial orientation was cut from the piece shown in Fig. 2(a). Subsequently, this plate was cut in half by an axial cut along the central axis, such that the width W of the two resulting plates was the radius of the axle. In each of these two plates a cut was introduced by electric discharge machining (EDM), which enabled the stress distribution to be measured across the corresponding radii (150° and 330°). The results are shown in Fig. 3(b). The two curves are nearly identical. Disregarding a sharp compressive stress peak right at the surface, the curves are in relatively good agreement with the near-surface measurements shown in Fig. 3(a). In the following the effect of the measured residual stresses on the fatigue life predicted by eq. (5) for the axle shown in Fig. 2 is considered as an example. It is assumed that a pre-crack of 2.5 mm length is present at the section of maximum surface stress due to the service load, i.e. at the location where the residual stress measurement was performed (x-axis in Fig. 2(b)). The service load is given in Table 1 by the histogram of the nominal stress (i.e. the bending stress in the cylindrical part). The local stress along the x-axis normalized by the nominal stress is shown in Fig. 4(a). Moreover, stationary stresses imposed by the press-fit between the wheel and the axle are taken into account. They represent a stationary pre-stress that can be superimposed to the measured residual stress shown in Fig. 3(b). They depend linearly on the difference d between the diameter of the central bore in the wheel and the diameter of the cone at the corresponding axial position. The stress distribution on the x-axis as obtained from an axisymmetric FEM-model for the nominal value of d = 0.3 mm is shown in Fig. 4(b). 5. Effect of residual stress on residual life calculations