PSI - Issue 13

Florian Fehringer et al. / Procedia Structural Integrity 13 (2018) 932–938 Fehringer, F., Schuler, X., Seidenfuß, M. / Structural Integrity Procedia 00 (2018) 000 – 000

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(NRB), tensile specimens). For medium and high stress triaxialities, the semi-analytical curves fit the limit strain values for the different specimens quite good. Also for small stress triaxialities (torsion test, stress triaxiality approx. 0 and Lode angle parameter  = 1) the limit strain curve for  = 1 predicts the limit strain acceptable. Nevertheless, the plotted limit strain curves describe a mean value of the real fracture strains. For industrial application a safety factor needs to be applied to the limit strain curves. The definition of a suitable safety factor is part of the current research project. Fig. 6 (b) shows the results from the investigation of size effects on notched round bars at three different stress triaxialities. Larger specimens lead to smaller limit strains. But, as already observed in paragraph 4, the size effect has only a small impact on the limit strains. Furthermore, the size effect even decreases with increasing specimen sizes. For even very large specimens, the limit strains approach more and more certain threshold values (Krieg and Seidenfuß (2003)).

Fig. 6. Limit strain curves for 20MnMoNi5-5 (a) different stress triaxiality values (b) different sizes.

6. Conclusion

To evaluate safety margins of power plant components in case of postulated design exceeding incidents, a new strain based structural integrity assessment concept was introduced. For the required damage mechanics simulations, the Rousselier model was extended for low stress triaxialities, a Lode angle dependency and kinematic hardening. With the enhanced model, the prediction of failure at low stress triaxialities is possible. In addition, the damage behavior of non-proportional loading situations can be described. From the experimental and numerical results, limit strains at different stress triaxialities can be derived. Regarding limit strains, the specimen size has only a small impact, which even decreases with increasing specimen size.

Acknowledgements

This research work is funded in the framework of the German reactor safety research program by the Federal Ministry for Economic Affairs and Energy (BMWi) under contract No. 1501474. The support is gratefully acknowledged.

References

Armstrong, P.J, Frederick, C.O., 1966. A mathematical representation of the multiaxial Bauschinger effect. C.E.G.B., Report RD/B/N 731. ASME BPVC III, 2013. Rules for Construction of Nuclear Facility Components, Division 1 – Subsection NB. ASME BPVC VIII, 2013. Rules for Construction of Pressure Vessels, Division 2. BMWi-project No. 1501474. Quantification of load carrying capacities of metallic components in case of multiple incidents, with a limit strain based safety assessment concept. Current research project at MPA University of Stuttgart. Bao, Y., Wierzbicki, T., 2004. On fracture locus in the equivalent strain and stress triaxiality space. International Journal of Mechanical Sciences, Vol. 46, No. 1, pp. 81 – 98.