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

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at a lower load level, it is just the opposite. The fatigue crack growth in the normal strength ductile iron is slower than in GJS-800-10 and in GJS-1000-5 (Fig. 11 a)). At this loading, till approximately 4E+05 load cycles, also the brittle higher strength cast iron shaft (GJS-600-3) is more insensitive to fatigue cracking than the GJS-1000-5-shaft. Espe cially GJSF-SINI30-5 is suitable at external stresses beneath the endurance limit. But, with the increasing number of load cycles, the crack propagation rate in normal and higher strength ductile iron shafts rises and unstable crack growth starts at a lower cycle numbers than in the GJS-800-10 shaft. Fig. 10 shows the total life as a sum of fatigue life till crack initiation and remaining life (at initial thumbnail crack with a depth of 2 mm) till component failure, when the hotspot stress amplitude is 240 MPa. Here again the high discrepancy of the resistance with regard to fatigue and to fatigue crack growth is obvious.

a)

b)

10 15 20 25 30

10 15 20 25 30

100 120 140 160 180

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

300 MPa

160 MPa

GJS-800-10 GJS-1000-5

0 20 40 60 80

0 5

0 5

crack depth a [mm]

1.E+00 1.E+02 1.E+04 1.E+06

1E+00

4E+05

0E+00

5E+04

1E+05

load cycles N [-]

Fig. 11. Crack propagation at cyclic-single-stage stress amplitude - a) 160 MPa, b) 300 MPa

4.2. Fracture mechanical investigation at real variable loading in a wind turbine Furthermore, besides the assessment of remaining life after a fatigue crack initiation, the potential risk of an unde tected imperfection in the rotor shaft has to be investigated. On this account, initially a conservative assumption for surface defects with different depths in the hotspot region of the rotor shaft from the beginning of the operation are done. The crack length increases as a function of load cycles at a realistic frequency distribution of the wind turbine bending moment as pictured in Fig. 12. The maximum value of the cumulative frequency distribution of stresses at the real notch geometry of the considered shaft is below the endurance limit (approximately 70 %). Furthermore, the high proportion of cycles with far smaller amplitudes leads to a much lower crack propagation rate in the wind turbine than at the test rig. For the crack growth calculation the cumulative frequency distribution of stress of the whole wind turbine life is divided by the cycle number of the least occurring class, sorted in descending order and then successively repeated till the whole cycles of lifetime (6E+08 cycles correspond to 20 years of service life) are passed through.

stress spectrum GJS-800-10 GJS-400-18-LT

42CrMo4 GJS-600-3

GJS-1000-5 GJSF-SiNi30-5

160

100

110

50

60

a 0 = 20 mm

crack depth a [mm]

0

10

stress amplitude σ [MPa]

1E+00 1E+01 1E+02 1E+03 1E+04 1E+05 1E+06 1E+07 1E+08 1E+09

load cycles N [-]

Fig. 12. Crack growth at real variable loading out of Fig. 1 c)