International round robin test on Master Curve reference temperature evaluation utilizing miniature C(T) specimen

The Master Curve (MC) method is a promising technique for evaluating the fracture toughness of ferritic steels. It enables the determination of the reference temperature, T_o, of a probabilistic fracture toughness curve using small specimens. The Central Research Institute of Electric Power Industry (CRIEPI) investigated this capability of the MC method using different size of C(T) specimens, and it was found that 0.16T-C(T) specimen with the dimensions of 4 by 10 by 9.6 mm, hereafter called "Mini-CT specimen," can be used to obtain valid T_o values. The advantage of this Mini-CT specimen technique is that multiple specimens can be machined from one of the broken halves of Charpy size specimens, which are used in a standard surveillance capsule of a reactor pressure vessel (RPV). In order to ensure the robustness of this technique, a round-robin test was planned. The idea is to perform MC tests using Mini-CT specimens by different investigators to see if consistent T_o values can be obtained. All the specimens used were machined and pre-cracked by one fabricator from unique RPV material to avoid any possible effect of specimen preparation on T_o values. Seven institutes participated in this exercise, and obtained valid T_o values. No specific difficulty was found in the MC tests performed in accordance with the ASTM E1921-10el protocol. The scatter of the obtained T_o values was well within the uncertainty range defined in Appendix X4.2 of ASTM E1921, indicating the robustness of the Mini-CT specimen technique in terms of the testing procedure. Throughout this activity, we could obtain 182 K_JC(1Teq) for a single material. We investigated the statistics of this large database, and found that there is no remarkable difference not only in the T_o values, but also in the fracture toughness distribution between the Mini-CT specimen and the standard 1T-C(T) specimen results.