13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- 5. Discussion and conclusions The parameters of both Beremin models clearly show dependencies on irradiation level and test temperature. In the relevant range of the load-displacement curve of the SPT, large parts of the specimen are subjected to a biaxial stress state which is characterized by approximately equal first and second principal stresses. Accordingly, the maximum principal stresses at the onset of void coalescence and consequently the determined Weibull reference stresses are mainly determined by the current yield stress of the material. Figure 4 shows the almost linear relationship between calculated Weibull reference stresses and identified yield stresses. The strong influence of the test temperature and irradiation level on the hardening properties of the matrix material thus transfers to the particular Weibull reference stresses: for higher temperatures, the Weibull reference stresses decrease. This result is consistent with the temperature dependence of principal stresses leading to cleavage fracture that were experimentally determined on notched tensile specimens [35]. Note that there are several temperature-dependent modifications of the Beremin model, e.g. [36]-[38]. However, in these modifications the Weibull reference stress must be increased with higher temperatures to get reasonable results, which lacks a micromechanical motivation [39] and contradicts with our results. The predicted fracture toughness values agree well with experimental results in the brittle region and in the ductile region. In the brittle-ductile transition region, the values determined from the SPT cannot be transferred to the fracture mechanics specimen. The calculated fracture toughness values are much smaller than the experimentally determined values. It is therefore questionable whether the assumptions made in the Beremin model are sufficient to capture size effect and influence of stress triaxiality. Particular in the transition region, the assumption of the failure of the whole structure by the unstable crack growth of a micro-crack (weakest link assumption) seems to be not justified for the fracture mechanics specimen. Acknowledgements The financial support by the German Federal Ministry of Economics and Technology (BMWi) is gratefully acknowledged (projects 1501298 and 1501343). References [1] Pineau, A.: Development of the local approach to fracture over the past 25 years: theory and applications. In: International Journal of Fracture 138 (2006), 139–166. [2] Abendroth, M.; Kuna, M.: Determination of deformation and failure properties of ductile materials by means of the small punch test and neural networks. In: Computational Materials Science 28 (2003), Nr. 3-4, 633-644 [3] Abendroth, M.; Kuna, M.: Identification of ductile damage and fracture parameters from the small punch test using neural networks. In: Engineering Fracture Mechanics 73 (2006), 710725 [4] Linse, T.; Kuna, M.; Schuhknecht, J.; Viehrig, H.-W.: Usage of the small-punch-test for the characterisation of reactor vessel steels in the brittle-ductile transition region. In: Engineering
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