PSI - Issue 76

Xabat Orue et al. / Procedia Structural Integrity 76 (2026) 3–10

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Table 4 shows the results of FCGtests executed by K-Decreasing procedure according to ASTM E647 [18]: Table 4. Results of FCG tests in H-H and H-V directions. Direction ΔK th,eff [MPa·m 1/2 ] ΔK th,lc [MPa·m 1/2 ] C p [-] n p [-] H-H 1,90 ± 0,05 4,14 ± 0,19 (1,80 ± 1,70) ·10 -10 5,76 ± 0,57 H-V 2,00 ± 0,05 2,70 ± 0,07 (2,50 ± 0,71) ·10 -8 3.12 ± 0,20 Where, ΔK th,eff is the effective fatigue crack growth threshold (R0.7), ΔK th,lc is the fatigue crack growth threshold in the long crack regime (R0.1), and C p and n p are Paris fitting parameters (R0.1). The values correspond to the means and standard deviations. The directions analyzed in FCG tests are representative of crack growth direction of horizontally extracted samples. Due to the implemented deposition strategy (see section 2), the obtained properties are orthotropic (L=T≠S) , i.e., the properties in horizontal directions are similar (L=T) and the only difference is in vertical direction (S). Therefore, the notation was simplified with H and V for horizontal and vertical directions respectively. According to the results obtained (Table 4), it could be stated that H-V direction is more critical than the H-H direction, because the long cracks propagation threshold ( ∆ ℎ, ) is lower (being ∆ ℎ, similar) and the Fatigue Crack Growth Rate (FCGR) is higher. This is related to the low distribution of defects [2] and the columnar microstructure obtained (Fig. 2), where the alignment of prior-beta grains with H-V direction facilitates the propagation of cracks. In H-H direction on the other hand, the crack is faced with more grain boundaries that impedes its propagation, and that is why better FCG results are obtained. Considering all these results, the K-T diagram and the modifications by El Haddad, Murakami and Chapetti are presented for each crack growth direction (Fig. 4):

H-H

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log Δσ w (MPa)

100,00

1,00

10,00

100,00

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log √area ( µ m)

H-V

1000,00

log Δσ w (MPa)

100,00

1,00

10,00

100,00

1000,00

log √area (µm)

Fig. 4. K-T diagram and de modifications of El Haddad, Murakami and Chapetti in H-H (up) and H-V (down) directions.