PSI - Issue 51

Juraj Belan et al. / Procedia Structural Integrity 51 (2023) 109–114 J. Belan et al. / Structural Integrity Procedia 00 (2022) 000–000

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Fig. 2 Micrographs of fatigue surfaces - overall view and detail of fatigue crack initiation of samples with cycle asymmetry parameter R  1: a) samples with high value of load amplitude  a , LFHC fatigue, f = 151 Hz, N f = 357 871, starting stage, b) detail of one of initiation site, starting stage, c) samples with a value of load amplitude  a close to fatigue limit, LFHC fatigue, f = 151 Hz, N f = 4 520 474, starting stage, d) detail with single one initiation site, e) samples with high value of load amplitude  a , LFHC fatigue, f = 149 Hz, N f = 402 978, annealed at 800 °C/72 hrs., f) samples with value of load amplitude  a close to fatigue limit, LFHC fatigue, f = 144 Hz, N f = 1 550 260, annealed at 800 °C/72 hrs., SEM. The stated fatigue crack initiation mechanism on fine-grained materials with a grain size d ≤ 5  m  10  m is in accordance with the findings of the authors Alexandre et al. (2004). In their work, the authors were concerned with modelling the optimal grain size for the low-cycle fatigue life of the IN718 superalloy and investigated the presence of two possible crack initiation sites. They found that in the material with a small grain size (5 ÷ 10 µm), the fatigue crack initiation sites were systematically related to the presence of a second phase particle with a size of about 10 ÷ 20 µm. In many cases, these particles were niobium carbides NbC, which were strongly oxidized and transformed into an oxide of the type Nb 2 O 5 . In the case of grains with a larger size, d ≈ 150 µm, cracks were initiated by the classic mechanism by creating the so-called PSB  s. By comparing the fatigue crack initiation, the minimal influence of the stress method represented by the cycle asymmetry parameter R (R = -1 or R  1) was found. In all cases, the fatigue crack initiated on the surface, or on TiC (NbC) particles expelled just below the surface in the case of fatigue tests at ambient temperature, Fig. 1b, d. Fatigue crack initiation at the test temperature of 700 °C took place in the same way on the surface, but from significantly oxidized TiC (NbC) particles, Fig. 1f. Likewise, the influence of the loading frequency (e.g. 64 Hz vs. 20 132 kHz) did not have a significant effect on the nature of fatigue crack initiation. It is related to the number of cycles to fracture, where at a value of N f  10 5 the influence of frequency is negligible because the influence of the loading amplitude is dominant.