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.