To achieve a rational maintenance program for aged light water reactor components, it is important to establish and to improve the flaw evaluation criteria. The current flaw evaluation criteria such as ASME Boiler and Pressure Vessel Code Section XI are focused on Class 1 piping, which usually shows relatively higher toughness. On the other hand, flaw evaluation criteria suitable for Class 2 and Class 3 piping with moderate-toughness are also required because some Class 2 and Class 3 piping systems are as important to plant risk analysis as Class 1 piping. In this study, fracture experiments by four-point bending were conducted for circumferentially cracked pipes. Tested pipes were made of moderate-toughness carbon steel, STPT410 carbon steel. Ductile fracture behavior and load-carrying capacity were compared with those from elastic-plastic fracture mechanics analysis. As a result, it was ascertained that ductile fracture behavior could be well predicted by fracture mechanics analysis using J-integrals. A Z-factor approach applicable to moderate-toughness pipes was also discussed.

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