PSI - Issue 57

Tobias Pertoll et al. / Procedia Structural Integrity 57 (2024) 250–261 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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a

b

Figure 6. Threshold crack depth a th for different deep rolled surface conditions and a 0 / c 0 ratios visualised in (a) as a 2D diagram and (b) as a 3D diagram

3.2. Load spectrum

The significant influence of deep rolling force on the number of load-cycles up to a crack depth of 8 mm is confirmed by the results of the second investigation, shown in Figure 7. It can be seen that the different effective depth of the process application due to the different applied deep rolling forces has a significant influence on the remaining service life of the railway axle. For the determination of the remaining service life the afore described load spectrum is applied. In this case, the investigation is limited to an a 0 / c 0 ratio of 1.0. The starting crack depths considered are defined according to the threshold crack depth a th determined previously. For the sake of simplicity and conservatism, the starting crack depths are rounded up to the nearest half millimetre. The initial crack depth values of 3.0, 4.5, 5.5 and 6.5 mm are therefore obtained. If the railway axle is not deep rolled, the remaining load-cycles for the considered crack depths are comparably short. With an occurring crack depth of 3 mm, Figure 7 (a), crack growth can be expected in the not deep-rolled and in the deep rolled condition with a used deep rolling force of 5 kN. The remaining service life is increased by the process application from 8 . 10 5 to just under 4 . 10 7 number of load-cycles. If deep rolled with 10, 15 or 20 kN, no crack growth with a starting crack depth of 3 mm can be assessed. With a starting crack depth of 4.5 mm, Figure 7 (b), crack growth can be assessed for the surface conditions not deep rolled and deep rolled with 5 and 10 kN. The tolerable number of load-cycles in the not deep rolled condition is 4.5 . 10 5 load-cycles, deep rolled with 5 kN 2.1 . 10 6 and deep rolled with 10 kN 6.3 . 10 7 load-cycles. Only with a deep rolling application with 20 kN, no crack growth is to be expected with an initial crack depth of 5.5 mm, Figure 7 (c). At this crack depth, the following load-cycles result for the respective post-treatment conditions up to a crack depth of 8 mm: Not deep rolled - 3 . 10 5 ; Deep rolled 5 kN - 1 . 10 6 ; Deep rolled 10 kN - 9.3 . 10 6 ; Deep rolled 15 kN - 1.8 . 10 8 . From a crack depth of 6.5 mm, Figure 7 (d), cracks with all considered surface conditions are capable of propagation. However, the surface condition has a very large influence on the number of load-cycles that can be carried up to a depth of crack of 8 mm. The following numbers of load-cycles are tolerable: Not deep rolled - 1.7 . 10 5 ; Deep rolled 5 kN - 5.5 . 10 5 ; Deep rolled 10 kN - 4.8 . 10 6 ; Deep rolled 15 kN - 1.7 . 10 7 ; Deep rolled 20 kN - 1.1 . 10 8 .