PSI - Issue 23

Roman Petráš et al. / Procedia Structural Integrity 23 (2019) 209–214 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The profile of the crack developed from the specimen surface is shown in Fig. 6a. The middle area of the cross section revealed the net of the connected internal cracks as documented in Fig. 6b. Moreover, the detailed inspection of the internal cracks proved the cavity formation and subsequent cavity coalescence resulting in internal crack nucleation, see the higher magnification of the area in Fig. 6b marked by rectangle.

4. Conclusions

The influence of an environment on the damage mechanisms under LCF loading conditions was investigated in Sanicro 25 steel. The identical loading procedures were implemented for both procedures conducted in the air and in the vacuum chamber. Preferential grain boundary oxidation under LCF condition in air resulted in early cracking of the grain boundaries leading to rapid micro-crack formation and intergranular propagation of the fatigue crack. Even though the process of the oxide layer formation is fully suppressed in the case of LCF vacuum loading, the grain boundaries represent the crack nucleation sites. The main reason is quite likely the accumulation of the vacancies at the grain boundaries resulting in cavity formation and subsequent development of the crack. Moreover, the cavity formation along with the internal cracks occurs in the interior of the material as well. Bearing in mind the lower fatigue life of the specimen subjected to LCF in the air; the early cracking of the oxidized grain boundaries and further crack propagation illustrates the crucial role of an environment. The rapid crack growth can be supported by the mechanism known as SAGBO explaining the premature intergranular failure based on the preferential grain boundary oxidation and brittle fracture of the oxide developed.

Acknowledgements

The work presented was realized in CEITEC-Central European Institute of Technology with research infrastructure supported by the project CZ.1.05/1.1.00/02.0068 financed from European Regional Development Fund. The work was conducted in the frame of IPMinfra supported through the project No. LM2015069 and the project CEITEC 2020 No. LQ1601 of MEYS. The support by the project RVO: 68081723 and grant 13-23652S of GACR is gratefully acknowledge. The part of the research introduced was conducted at the University of Siegen, Germany supported by VUT Erasmus program.

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