PSI - Issue 51

Kamila Kozáková et al. / Procedia Structural Integrity 51 (2023) 145–151 K. Kozáková et al. / Structural Integrity Procedia 00 (2022) 000–000

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approximated by Wöhler curves. For both materials, the specimens with the smaller diameter reached better fatigue properties. In the case of 1.4306, a more significant effect of the diameter can be seen for whole the lifetime, while in the case of 1.4307, the increase in the lifetime is observed only for lower number of cycles, while beyond 10 � cycles the measured data show similar fatigue response of both geometries of the specimens. However, for both material types, a higher fatigue endurance limit was determined for specimens with lower diameter. Differences in the slope of the fatigue curves could be dedicated to the acting fatigue fracture mechanism, which is changed in the case of 1.4306. Also, considering only the same fracture mechanism, the slope of the Wöhler curve would be steeper for 1.4306 in comparison to 1.4307. The results correspond to the risk volume determination, lower risk volume was calculated for specimens with a diameter of 3 mm. The higher fatigue endurance limit was determined for 1.4307, in the case of specimens with a diameter of 5 mm (no. 2), what agrees with the material tensile properties. Higher tensile strength and yield stress of 1.4307 stainless steel in comparison to 1.4306 were presented in (Klusák et al., 2022), and it is probably connected with slightly lower grain size. Distribution of -ferrite is more homogeneous for the 1.4306., The amount of -ferrite is also slightly higher for 1.4306 (1.8 vs. 1.2 for 1.4307, determined by EBSD), which can also contribute to the higher fatigue response of the material. The difference in the fatigue behavior of the specimens with a diameter of 3 mm, could be dedicated to the fact that in the case of 5 mm diameter specimens , there is a larger proportion of -ferrite on the surface of the specimens in comparison to the 3 mm diameter specimens. The -ferrite particles can act as the stress concentration places and as the fatigue crack initiation sites. Thus, in the case of specimens with the diameter of 5 mm, there is a higher probability of crack initiation. 3.4. Fractographic analysis In the case of specimens from 1.4306 stainless steels, two different fatigue fracture mechanisms were observed. The transgranular fatigue fracture mechanism was characteristic for the low and partially high cycle fatigue region (specimen with a red circle in Fig. 8), while a combination of transgranular and intergranular mechanism was observed in the very high cycle fatigue region (specimen with a green circle in Fig. 8). The change in the mechanism was observed for both the specimen diameters and corresponds to the data discontinuity in Fig. 8. In the case of stainless steel 1.4307, the only transgranular fracture mechanism was observed. In the case of low and partially high cycle fatigue regions of 1.4306, the single fatigue crack initiation on the surface was a characteristic feature, see Fig. 9. on the other hand, multiple crack initiation on the surface was typical for the combined crack propagation mechanism, see Fig. 10.

Fig. 9. Fracture surface of specimen, transgranular crack propagation.