PSI - Issue 4

Stefan Kolitsch et al. / Procedia Structural Integrity 4 (2017) 95–105 Stefan Kolitsch/ Structural Integrity Procedia 00 (2017) 000 – 000

100

6

The endurance limit in the stress based analysis is reduced by several factors considering notches, surface roughness etc, Hence, regarding rail production and the resulting surface roughness in the as-rolled condition, a reduction factor K surface , following FKM (2012) has to be taken into account:

surface K n    e,0





.

(11)

skin bending, e,

In the rail industry, for the stress based analysis, the Smith diagram is commonly used for prediction of the endurance limit. This diagram is exemplarily displayed in Fig. 2 for one material under tension/compression (red line), bending load (blue line) and bending load including an assumed surface roughness of the rolling skin (green line). In addition, the black dotted line represents the mean stress and the dashed lines the different stress ratios R .

Fig. 2. Smith diagram of the pearlitic material for tensile load (red line) and bending load (blue line) assuming a polished surface condition, for bending including an assumed surface roughness from the rolling skin (green line) and fatigue strength experiments (crosses).

To validate the predicted endurable stress amplitudes, several experiments to determine the endurance limit at different loadings, stress ratios and surface conditions have been conducted. The results are additionally displayed by crosses in the Smith diagram in Fig. 2. For tension/compression loading at stress ratios R = -1 and R = 0.1 the prediction correlates almost perfectly with the experiments. However, the predicted blue line for the bending load is lower than the experimental result. Nevertheless, the difference is in an acceptable range and implies a conservative prediction. Also the prediction and the experiment for the endurance limit with the surface roughness correlate well. Finally, Fig. 3 shows the predictions for all different materials investigated under bending including the influence of the surface roughness. Especially at higher mean stresses, the minimum and maximum endurable stress amplitudes are almost identical for all materials.