13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 4. Application and results 4.1. Identified Hardening Parameters The explained identification strategy was applied to determine yield curve parameters from loaddisplacement curves of SPTs carried out for the two materials, both in non-irradiated and irradiated state, at different temperatures. The influences of test temperature and irradiation level on the hardening characteristics of the reactor steels are clearly visible in the measured load-displacement curves of SPT [4][5]. They are also found in the identified yield curves. Lower temperatures and higher irradiation levels lead to a significant increase in yield stresses in the entire strain range. Figure 1 shows the identified initial flow stresses for the material JFL. The results for the material JRQ are not shown here as they are comparable in quality; however JRQ shows a much higher sensitivity to radiation than the steel JFL. The identified values agree well with data measured with standard tensile test. 4.2. Determined Weibull Parameters Weibull parameters were determined for those temperatures and irradiation levels, respectively, for which the tested specimens failed in a brittle manner. Using the maximum likelihood method, unreasonable high Weibull modules were obtained for some test series. Therefore, it was decided to set the Weibull module to a constant value of m=30. The parameters of the original Beremin model were identified for reference purpose. Figure 2 represents the determined Weibull reference stresses for the material JFL. Both the parameters of the original and the modified Beremin model show dependencies on irradiation level and test temperature. The influence of temperature on the Weibull reference stress is best visible for the non-irradiated materials: Within the same irradiation level, the specific Weibull reference stresses decrease for higher test temperatures. 4.3. Critical Fracture Toughness values Subsequent to the identification of the yield curves and the damage parameters critical fracture toughness values were indirectly quantified by numerical simulation of fracture mechanics tests. The developed non-local ductile damage model was used in combination with the original Beremin model or the modified version after Bernauer to calculate the probability of cleavage fracture. The fracture toughness values from FEM calculations of CT specimens using the identified parameters are shown in Figure 3 for the non-irradiated material JFL. In the brittle region and the ductile region, the calculated values agree well with experimental results. For the transition region, the calculated fracture toughness values and their scatter are clearly too small.
RkJQdWJsaXNoZXIy MjM0NDE=