IAEA coordinated research project on Master Curve approach to monitor fracture toughness of RPV steels: Effects of bias, constraint, and geometry

There is strong interest from the nuclear industry to use the precracked Charpy single-edge notched bend, SE(B), specimen (PCVN) to enable determination of the reference temperature, T₀, with reactor pressure vessel surveillance specimens. Unfortunately, for many different ferritic steels, tests with the PCVN specimen (10×10×55 mm) have resulted in T₀ temperatures up to 25°C lower than T₀ values obtained using data from 25-mm thick compact specimens [1TC(T)]. This difference in T₀ reference temperature has often been designated a specimen bias effect, and the primary focus for explaining this effect is loss of constraint in the PCVN specimen. The International Atomic Energy Agency has developed a three-part coordinated research project (CRP) to evaluate various issues associated with the fracture toughness Master Curve for application to light-water reactor pressure vessels. One part of the CRP is focused on the issue of test specimen geometry effects, with emphasis on the PCVN bias. Participating organizations for this part of the CRP are performing fracture toughness testing of various steels, including the reference steel JRQ (A533-B-1) often used for IAEA studies, with various types of specimens under various conditions. Additionally, many of the participants are taking part in a round robin exercise on finite element modeling of the PCVN specimen. Some preliminary results from fracture toughness tests are compared with regard to effects of specimen size and type on the reference temperature T₀. In agreement with a number of published results, the results do generally show lower values of T₀ from the PCVN specimen compared with the compact and larger bend specimens. They also clearly show higher apparent fracture toughness for the shallow crack compared with the deep crack configuration. Moreover, the SE(B) specimens exhibit a tendency for decreasing T₀ with decreasing specimen size (thickness and/or remaining ligament). Additionally, as shown in previous CRPs, the results also exhibit a dependence on test temperature. Following completion of all testing, the results will be evaluated relative to existing proposed models with a view towards developing an understanding of the reasons for the observed differences.