PSI - Issue 43

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Ivo Šulák et al. / Procedia Structural Integrity 43 (2023) 209–214 Author name / Structural Int grity Procedia 00 (2022) 000 – 000

Fig. 2 . Fatigue hardening/softening curves of the B1914 superalloy (a) 800 °C; (b) 900 °C.

Fig. 3b shows the fatigue life curves of B1914 in the dependence of ε ap vs. 2N f . The plastic strain amplitude ε ap was calculated from the hysteresis loop width at half-life. Experimental data were approximated by Coffin-Manson law: log(2 ) = ( 1 ) log − ( 1 ) log ´ . (2) The parameters fatigue ductility coefficient ε f ´ and fatigue ductility exponent c were evaluated using linear regression analysis and are listed in Table 1. In this case, the slopes of the fatigue life curves are slightly different, leading to their intersection for the highest values of plastic strain amplitudes. However, the cyclic loading at 800 °C is associated with lower plasticity and therefore the corresponding fatigue life data are shifted to lower values.

Table 1. LCF parameters of B1914 tested at 800 °C and 900 °C . Temperature (°C) ´ (MPa) b (-) 800 1850 ± 170 - 0.14 ± 0.01

´ (-)

c (-)

1.31 ± 1 .62 0.24 ± 0.08

-1. 08 ± 0 .15 - 0.79 ± 0. 04

900

1620 ± 110

- 0.16 ± 0.01

Fig. 3. Fatigue life curves in (a) Basquin representation; (b) Coffin-Manson representation.

Typical LCF damage of the B1914 superalloy tested at 800 and 900 °C can be observed in Fig 4. At 800 °C, fatigue cracks initiate at the surface without any specific weak point and propagates transgranularly into the material (Fig. 4a). With an increase in the testing temperature, the process of fatigue crack initiation at grain boundaries and