PSI - Issue 2_A

Jenni Herrmann et al. / Procedia Structural Integrity 2 (2016) 2951–2958 Jenni Herrmann et al./ Structural Integrity Procedia 00 (2016) 000–000

2956

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initial crack has a depth of 2 mm the propagation is less fast in the GJS-600-3 and the GJSF-SiNI30-5 rotor shaft. But at approximately 1E+05 load cycles the propagation rate increases, thus the crack depth exceeds the depth in the GJS 1000-5 shaft. From the beginning the crack in a shaft made of GJS-400-18-LT grows the fastest.

2c

1E-03

a

- a/c + a 0

42CrMo4 GJS-400-18-LT GJS-600-3 GJS-800-10 GJSF-SiNi30-5 GJS-1000-5

+ -

α + -

1E-05

crack depth a

+ -

a 0

1E-07

crack propagation rate da/dN

4

40

load cycles N

stress intensity range ΔK [MPa m 0.5 ]

Fig. 7. Forman/Mettu curves of different materials

Fig. 8. Influence of different parameters on crack growth – with a 0 as the initial crack depth, α as the stress concentration factor and R as the stress ratio

In this consideration a semi-elliptical crack in the shaft hotspot is assumed. Though, in sharp notched or hardened rotating bending loaded shafts, a circumferential crack is also possible. In the present case this assumption is very unlikely, but still has to be investigated. A closer look on a selection of the considered materials shows a much lower number of load cycles at the same initial crack depth than for a semi-elliptical crack geometry (see Fig. 9). Despite equal external stress and equal initial crack depth, the deciding reason for the higher stress intensity factor for a cir cumferential crack, is the higher value of the geometry function, due to the higher cross section reduction. However, a material specific difference is not apparent.

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GJS-400-18-LT (semi-ellip.) GJSF-SiNi30-5 (semi-ellip.) GJS-600-3 (semi-ellip.) GJS-400-18-LT (circum.) GJSF-SiNi30-5 (circum.) GJS-600-3 (circum.)

GJS-800-10 GJS-1000-5

1 %

27 % 44 % 45 % 76 % 89 % 99 %

20

11 %

15

42CrMo4 GJS-400-… GJSF-… GJS-600-3

24 %

55 %

10

56 %

5

crack depth a [mm]

73 %

0

1E+04

1E+06

1E+08

1.E+03

1.E+04

1.E+05

fatigue life remaining life

load cycles N [-]

Fig. 9. Assumption of semi-elliptical and circumferential crack

Fig. 10. Total life of a rotor shaft made of different materials

4.1. Remaining life estimation at one-stage loading on the test rig Subsequently the remaining life of the rotor shaft made of the considered materials is examined for the test environ ment at different constant load amplitude (Fig. 11). Therefore, two different load stages are simulated. In the following investigation a 2 mm initial crack depth is assumed. The crack in the forged shaft has the lowest propagation rate independent of the loading level. The crack growth in cast shafts is considered more differentiated. At high load levels the austempered ductile iron is the most resistant material, in contrast to GJS-400-18-LT, where sudden, unstable crack growth occurs (see Fig. 11 c)). In the beginning,