PSI - Issue 19

T. Kato et al. / Procedia Structural Integrity 19 (2019) 238–248 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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100 120 140 160 180 200

-80 -60 -40 -20

 eq,max ,  (MPa)

 n,max (MPa)

Δτ eq,max Δτ σ n,max Before During After

-120 -100

Tread braking

Fig. 13. Equivalent maximum shear stress range   eq,max with tread braking.

4. Critical defect size for subsurface crack initiation The critical defect sizes for subsurface crack initiation in the wheels are estimated by applying  eq,max and the fatigue limits of the wheel steel as shown in Figure 14. It can be predicted that the subsurface cracks will be initiated in the wheels with defects larger than the critical size, and RCF failures will occur in these wheels. Figure 15 shows the relationship between the critical defect size and wheel size. B38 and H36 wheels have almost the same critical defect size, however, J33 wheel has a larger one. The estimated critical defect size of the J33 wheel is 1.5 mm, which is approximately equivalent to 1.6 mm, the allowable internal defect size in AAR specification. This indicates that the RCF crack initiations from internal defects can be prevented in the J33 wheel with the loading condition of this analysis. On the other hand, RCF cracks are predicted to initiate from the internal defects in the B38 and the H36 wheels. Decreasing the allowable internal defect size or reducing the maximum car weight are considered as the way to prevent RCF crack initiations in the B38 and the H36 wheels. The critical defect sizes in the case of applying tread braking are shown in Figure 16. The critical defect sizes during braking and after cooling are larger than before braking. This result suggests that the stop braking in this analysis condition does not have negative effects on RCF crack initiations from internal defects. However, the fatigue limits of wheel steel in elevated temperature or other tread braking conditions such as the drag braking should also be evaluated regarding the effect of tread braking in the future.

100 150 200 250 300 350 400

Fatigue limit prediction lines (El-Haddad)

Critical defect size

 eq,max

㻜㻚㻜㻜㻝 㻜㻚㻜㻝 Shear fatigue limit (MPa)

㻜㻚㻝

㻝

㻝㻜

Defect size (mm)

Fig. 14. Critical defect size estimation method.