13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Application of damage models to the ductile-brittle transition region of Reactor steels Thomas Linse1,*, Meinhard Kuna2,* 1 TU Dresden, Institute for Solid Mechanics, 01062 Dresden, Germany 2 TU Bergakademie Freiberg, Institute of Mechanics and Fluid Dynamics, 09596 Freiberg, Germany * Corresponding author: meinhard.kuna@imfd.tu-freiberg.de Abstract Two German ferritic pressure vessel steels (JFL, JQR) are examined in the brittle to ductile transition regime as function of temperature and irradiation. The experiments are done by a miniaturized Small-Punch-Test (SPT) in hot cells within the temperature range of -185oC up to 70oC. From the load-displacement curve of the SPT, the yield curves and parameters of both a non-local GURSONTVERGAARD-NEEDLEMAN ductile damage model and the BEREMIN-model are identified. The influence of temperature and irradiation on the determined model parameters is analyzed. All parameters are verified by comparison with results from standard test methods. The parameters, identified from SPT, are used to simulate the failure behaviour in standard fracture mechanics specimens. In the upper shelf, the non-local GTN-model is applied to simulate J-∆a-resistance curves, from where the fracture toughness data JIc could be successfully predicted. In the lower shelf, the WEIBULL-stress of the specimens was computed to find out the statistics of KIc - values. Finally, a modified BEREMIN-model and the non-local GTN-model were combined to evaluate the failure of fracture specimens in the brittle-ductile transition region. This way, an acceptable agreement with Master-curve data for nonirradiated steels could be achieved over the whole temperature range. Keywords non-local GTN-model, Beremin-Model, small-punch-test, brittle-ductile transition 1. Introduction The quantification of irradiation effects on ferritic steels is an essential task to warrant the safety of existing nuclear power plants. While a large number of tests is needed for the description of the brittle and transition region, the available volume of irradiated material for testing is very limited. Damage mechanics can be used to assess the integrity of mechanical structures with micro-defects through the evaluation of local criteria. In contrast to the methods of linear-elastic and elasticplastic fracture mechanics, this approach, the concept of the Local Approach [1], can in principle be applaied to any structure. The application of the Local Approach to determine fracture mechanical properties requires the exact knowledge of the hardening and softening behavior of the material. A promising method for the description of the failure behavior of ferritic steels in the brittle, transition and ductile region consists in minimal invasive sampling and testing of miniaturized specimens followed by the numerical simulation of standard fracture mechanical tests to predict fracture mechanical properties. The small punch test (SPT, [2]-[6]) is a miniaturized deep drawing process that requires only tiny volumes of material. Its load displacement curve contains information about the elastic, hardening and softening properties of the tested material. Cleavage fracture is the typical failure mechanism observed when testing in the brittle and transition region. The probability of cleavage fracture depends on the distribution of microcracks resulting from microplastic deformation, which causes a large scatter of the fracture mechanical properties. Therefore, a probabilistic model such as the Beremin local approach model [7][8], is required to
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