Evaluation for fracture resistance curves of nuclear real pipes using curved equivalent stress gradient (curved ESG) specimens

- Curved equivalent stress gradient (curved ESG) specimen was proposed.
- Instead of J-Q theory, the crack-opening stress gradient was used to design specimen.
- J-R tests were performed on real pipes and various types of specimens.
- FEA method was applied to obtain the parameters for calculation of J-R curves.
- The J-R curves of nuclear real pipes were evaluated more precisely with curved ESG specimen.

The leak-before-break (LBB) concept was usually applied to design and estimate the piping system in nuclear power plant, which is based on the fracture resistance (J-R) curves of nuclear real pipe. However, the real pipe test is costly and difficult. Instead of the real pipe test, the J-R curves of standard specimens suggested in the ASTM E1820 were used. Nonetheless, the J-R curves obtained from the standard specimens are too conservative, owing to the differences in specimen geometries and loading configurations (constraint effect). Thus, in this study the equivalent stress gradient (curved ESG) specimens were developed to evaluate the J-R curves of nuclear real pipes (SA-312 TP304L) precisely. The curved ESG specimens were designed to show the same crack-opening stress gradients as those of the real pipes at the driving force J = JIc. The JIc for stainless steel SA-312 TP304L was obtained by the J-R tests on standard compact tension (CT) specimens. Comparison of the J-R curves obtained from the curved ESG specimens and those obtained from the real pipes show that the curved ESG specimen can be used in the LBB concept to design and estimate the piping systems.