PSI - Issue 39

Lucia Morales-Rivas et al. / Procedia Structural Integrity 39 (2022) 515–527 Author name / Structural Integrity Procedia 00 (2019) 000–000

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In a previous work, these authors investigated the influence of the microstructural features of nanobainitic steels on high cycle fatigue in U- shaped specimens [Rementeria, et al.(2015)], where the crack seemingly propagated according to stage I along the activated single slip bands over about 20 μm. In that sense, crystal plane traces corresponding to the activated slip system of bainitic ferrite correlated with the observed crack path. A homologous crystallographic study on initiation and growth of surface cracks such as the one observed in this study is the subject of current research

by these authors. 4. Conclusions

A nanobainitic steel was subjected to 100,000 cycles of an interrupted fatigue test with R=0.1 under Sa=250 MPa. An initial defect was theoretically designed to ensure that it will imitate the physically short-crack regime, and in practice the defect was induced by means of FIB. After the fatigue test, the crack path was examined by SEM. The following conclusions can be derived from the morphological analysis of the crack path: • Occurrence of multiple crack initiation, about 7 µm away from the FIB defect tip (classified in the text as section II), was inferred. Stage I of crack propagation and/or formation of en-passant cracks would explain the observed crack morphology. • In regions adjacent to the defect root and far away from that (classified in the text as section I and section III), the crack grew basically perpendicular to the direction of the maximum principal stress, which supports the idea of the stage Ib and stage II of crack propagation. Acknowledgements This research was funded by the German Research Foundation (DFG) program: Research Grants; project number: 411091845. The authors would like to thank Dr. Thomas Löber and Dr. Sandra Wolff for the FIB work at the Nano Structuring Center-TU Kaiserslautern (Germany); and to Dr. Carlos Garcia-Mateo for the heat-treatments at CENIM-CSIC (Spain). Appendix A. Stress intensity factor solution for a quarter elliptical corner crack in a flat plate for a < c proposed by Anderson(2017).

Fig. A.1. A quarter elliptical crack embedded at the corner of a flat plate assuming < , where and specify the crack depth and length, respectively, and and t indicate the component width and thickness, respectively [Anderson(2017)]. Fig. A.1 corresponds to the gauge section of the flat fatigue specimens shown in Fig. 3 subjected to the tensile loading. Therefore, parameters w and t specify the actual gauge width and thickness, respectively, of the studied