13th International Conference on Fracture June 16–21, 2013, Beijing, China Figure 7. SEM micrographs. (a)Crack initiation. Arrows indicate the crack initiation zones. (b) Crack initiation. Arrows indicate the fatigue striations. (c)Crack growth at the position 2249μm away from main crack origin. Arrows indicate the fatigue striations. (d)Oxidation features. Experimental life data of the five turbine components is shown in Table 1. It is shown that crack growth rate increases with flight time at early stage of crack propagation. Then crack growth rate decreases with flight time. The reason is that the fir-tree mortise, a typical multiple load path structure. When the crack propagated at the second tooth, the loading in the fir-tree attachment will be reallocated, which caused the crack growth rates decreased. Based on experimental life data, the relation between flight life and the crack length is shown in Figure 8. According to material’s fracture toughness referred to [10], the critical crack growth life Nc is 1017.1hours. If the overhaul period is 300hours and 500hours, the critical crack size are 23.5mm and 17mm respectively. Considering twice overhaul period, we can obtain critical crack size 14.5mm for 300hour-overhaul period and 7.7mm for 500hour-overhaul period. Table 1. Experimental results Sample No. Crack growth life N/hours Crack size a/mm Δa/ΔN(mm/hours) 1 331.0 13.5 — 2 582.8 16 0.0098 3 762.2 25 0.0510 4 771.8 26 0.1042 5 886.8 31 0.0435 Figure 8. The relation between crack growth life and crack size 3. Crack growth life prediction A 3-D finite element model of a turbine component including a blade and a sector of the disc was created, as shown in Figure 9. Most of the significant geometric features were modeled and a relatively finer mesh was used for the region of the blade-disc connection. The turbine blade is made of GH4033, a ferrum-base wrought superalloy and the turbine disc is made of GH2036, a nickel-based wrought superalloy. FM analysis was conducted using the commercial code MARC. The singular finite elements at the crack tip are shown in Figure 10. The HLCCF loading spectrum and the temperature distribution are similar to experimental loads. The stress intensity factor range (SIF) ΔKblock under HLCCF loading can be expressed as [11] block LCF HCF K K n K Δ =Δ + ⋅Δ (1) where ΔKLCF is the SIF range under LCF loading, ΔKHCF is the SIF range under HCF loading, n is the number of HCF cycles at a period. -6-
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