반복하중 조건에서 원전 배관재의 파괴거동에 미치는 하중속도의 영향

Abstract

The piping components of nuclear power plants (NPPs) should be designed and maintained to ensure their structural integrity under seismic condition as well as normal operation condition. Thus, the reliability of integrity assessments of piping components under the seismic loading condition is an important issue. In the current integrity assessment procedures, seismic loading is treated as monotonic and applied once; the mechanical properties obtained under the monotonic and quasi-static loading conditions are used for the assessment. However, the seismic load has both dynamic and cyclic characteristics and the mechanical properties under dynamic and cyclic loading conditions are different from those under monotonic and quasi-static loading conditions. Hence it is important to clearly understand the dynamic and cyclic loading characteristics in the mechanical properties of materials. Although a number of studies have investigated the cyclic loading effect on the fracture behavior of materials, the loading rate effect on the fracture behaviour under the seismic loading condition is not clear yet. This study investigated the loading rate effect on the fracture resistance under cyclic loading conditions to clearly understand the fracture behavior of piping materials under seismic conditions. J–R fracture toughness tests were conducted under monotonic and cyclic loading conditions at various displacement rates at room temperature (RT) and the operating temperature of NPPs(316°C). SA508 Gr. 1a low-alloy steel (LAS) and SA312 TP316L stainless steel (SS) piping materials were used for the tests. The fracture resistance under a reversible cyclic load was considerably lower than that under monotonic load regardless of test temperature, material and loading rate. Under both cyclic and monotonic loading conditions, the fracture behavior of SA312 TP316L SS was independent of the loading rate at both RT and 316°C. For SA508 Gr. 1a LAS, the loading rate effect on the fracture behavior was appreciable at 316°C under cyclic and monotonic loading conditions. However, the loading rate effect diminished when the cyclic load ratio of the load (R) was –1. Thus, it was recognized that the fracture behavior of piping materials, including seismic loading characteristics, can be evaluated when tested under a cyclic load of R = –1 at a quasi-static loading rate.