복합하중 조건에서 파열시험을 통한 감육곡관의 손상압력 평가

Abstract

Piping systems in nuclear power plants (NPPs) are subject not only to internal pressure but also to bending loads induced by deadweight, thermal expansion, and internal pressure under normal operating conditions. Bending is thus considered to be an important factor in evaluating the integrity of defective piping components. wall thinning defect due to flow-accelerated-corrosion(FAC) is known as a main degradation mechanism of carbon steel pipe in nuclear power plant, so the bending load effect on the integrity evaluation of wall-thinned piping components has become an important issue. Therefore, this study performed burst test using real-scale pipe specimens with local wall-thinning under combined internal pressure and constant bending load, to evaluated the bending load effect on the failure pressure of local wall-thinned pipe bends. As a bending load, in-plane closing- and opening-mode bending were considered in the experiment. Both modes of bending were applied to the specimen in load-control and displacement-control. Also, the specimens with local wall-thinning located at intrados and extrados of elbow were used in the experiment. The results showed that the bending load effect was more significant when a constant bending load was applied than when a constant displacement corresponding to the constant bending load was applied. This is associated with the relaxation of bending load which was applied by constant displacement during the pressurization. Thus, the bending effect is relatively minor when applying the displacement controlled bending compared to the load controlled bending. In the load-controlled bending condition the bending load effect was more significant under opening-mode bending than under closing-mode bending. Also, the effect was considerable for intrados rather than extrados wall-thinning case. All specimens were failed by bulging followed by axial cracking, which is a typical failure mode for wall-thinned piping components subjected to internal pressure. The shape of the crack and degree of bulging were virtually the same for all specimens, regardless of the wall-thinning location or applied bending load. This indicates that the failure mode of wall-thinned elbows is less affected by in-plane bending load. The finite element simulation showed that the predicted failure pressure for local wall-thinned elbow specimens under combined loading condition agreed well with the experimental one. This indicated that the conventional finite element analysis reasonably predicts the failure pressure of wall-thinned pipe bends.