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. 1 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 , HFHC fatigue, f = 20 150 kHz, N f = 60 460, RT, b) detail of one of initiation site with broken carbide particle close to surface, RT, c) samples with a value of load amplitude  a close to fatigue limit, HFHC fatigue, f = 20 150 kHz, N f = 73 749 000, RT, d) detail with single one initiation site, e) samples with high value of load amplitude  a , LFHC fatigue, f = 64.1Hz, N f = 37 770, T = 700 °C, f) detail of initiation site signed as 1. with oxide inclusion surrounding the TiC primary carbide close to the specimen surface, T = 700 °C, SEM. The initiation itself is primarily accelerated by increased temperature and the presence of an aggressive environment, and the frequency of loading is also related to this because reducing it increases the total time of exposure to thermal load and corrosion attack (Almaraz et al., 2020 and Almaraz et al., 2022), Fig. 1f. The effect of frequency on the initiation of fatigue cracks, especially in high-cycle fatigue where N f > 10 5 , is negligible. The fatigue cracks initiation after the three-point bending fatigue test at RT on specimens in starting stage and after applied annealing are reported in Fig. 2. The fatigue crack initiation appeared on the free surface of the sample in starting stage, Fig. 2a, c, the occurrence of initiation on carbide particles or other inclusions was not observed, Fig. 2b, d. With the higher amplitude, multiple fatigue crack initiation sites are presented, and the number of fatigue crack initiation sites with decreasing stress amplitude decreases. The effect of frequency on the initiation of fatigue cracks, especially in high cycle fatigue where N f > 10 5 , is negligible. The same observations were made on the annealed samples. Fatigue crack initiation took place from only one preferential location, which corresponds to initiation on carbide or oxide particles on the surface of the samples. The effect of frequency on the initiation of fatigue cracks, especially in high-cycle fatigue where N f > 10 5 , is negligible. However, the number of initiation sites at specimens after annealing was not affected by load amplitude, Fig. 2e, f. The reduced number of initiation sites is related to the increased volume fraction of the acicular δ-phase in the structure, which acts as a fracture initiator in subsurface areas and represents the main phase for fatigue crack propagation (Rafiei el al. 2021). The reason why fatigue cracks initiate on the free surface of the sample is that dislocations are more mobile on the free surface of the material compared to the internal volume, the surface is a preferential place for dislocation nucleation, and the surface is exposed to the effects of an aggressive environment.