PSI - Issue 51

Milan Uhríčik et al. / Procedia Structural Integrity 51 (2023) 166 – 172 M. Uhrí č ik et al. / Structural Integrity Procedia 00 (2022) 000–000

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Crack initiation mechanisms were the same for both types of samples. The crack started to propagate from the corner of the sample and crack initiation occurred on the surface of the sample, from the bar manufacturing imperfection. Crack initiation and propagation for both types of samples are shown in Fig. 5.

a b Fig. 5. Fractures of austenitic steels AISI 304 after the fatigue test – start of crack initiation and direction of fatigue propagation; (a) for austenitic steel in the initial state; (b) for austenitic steel after plasma nitriding. In the fatigue area, for the sample in the initial state (Fig. 6), fatigue striations, a large number of dimples formed by sulfide particles, and secondary cracks that were perpendicular to the fatigue crack propagation direction, were observed.

a b Fig. 6. Fractures of austenitic steel AISI 304 in the initial state after the fatigue test; (a) secondary cracks; (b) striations and dimples. In the fatigue area, for the sample after plasma nitriding (Fig. 7), fatigue striations (which were a much greater amount than in the sample in the initial state), secondary cracks and dimples (which were formed again after sulfide particles), were also observed.