Low Cycle Fatigue Behavior and Seismic Assessment for Pipe Bend Having Local Wall Thinning-Influence of Internal Pressure

Abstract

One of the concerned technical issues in the nuclear piping under operation is pipe wall thinning caused by flow accelerated corrosion. This paper focuses on influence of internal pressure on low cycle fatigue life of pipe bends with local wall thinning and evaluation of safety margin against seismic loading in order to apply the obtained knowledge to the nuclear piping. In-plane bending fatigue tests under several constant internal pressure magnitudes were carried out using carbon steel pipe bends with local wall thinning at the extrados. Also finite element analysis, code-based seismic evaluation and fatigue analysis based on calculated strain range were carried out. Obtained main conclusions are as follows: (1) the tested pipe bends with local wall thinning at the extrados have a strong resistance against fatigue failure based on nuclear seismic piping design in Japan at least up to 12 MPa. That is, the tested pipe bends with severe local wall thinning (eroded ratio = 0.5 and eroded angle = 180 deg) at the extrados have margins against fatigue failure, even though the wall thickness is less than the code-required minimum value based on the nuclear piping seismic design in Japan. (2) Combination of the conventional B2 index and the Ke factor provided in the JSME Design and Construction Code, which is referred by JEAC 4601-2008 overestimates fictitious stress amplitude, when sum of the primary and secondary stress is much greater than 3 Sm.

Evaluation of the Limit State of a Six-Inch Carbon Steel Pipe Elbow in Base-Isolated Nuclear Power Plants

“…In general, leakage is examined, and the influence of the internal pressure is considered in the experimental studies in order to evaluate the limit state of the nuclear power plant piping system. The inside of the pipe is filled with room temperature water and the internal pressure is applied and maintained using a hydraulic machine [36][37][38][39].…”

Effect of wall thinning on deformation and failure of copper‐nickel alloy elbows subjected to low cycle fatigue

“…This problem was also emphasised during investigations of the integrity and residual strength of piping containing wall thinning defects after recent powerful earthquakes in Japan. 11 In the case of extreme event (eg, earthquake or shut down), the piping elements are subjected to high amplitude loading excursions, leading to material cycling in low cycle fatigue regime. 12,13 There were many analytical, numerical, and experimental studies in this area with the ultimate aim of developing a method for the evaluation of low fatigue strength of various piping elements with various wall thinning defects with and without internal pressure.…”

“…12,13 There were many analytical, numerical, and experimental studies in this area with the ultimate aim of developing a method for the evaluation of low fatigue strength of various piping elements with various wall thinning defects with and without internal pressure. 2,10,11,[14][15][16][17][18][19][20][21][22][23][24][25] For steel elbows, the recent studies have demonstrated that when the elbows are subjected to high amplitude cyclic loading, the cross-section geometry distorts and bulges progressively with number of cycles, while the failure mechanism is largely associated with the initiation and propagation fatigue cracks. 11,18 The focus of the current study is the effect of the localised wall thinning on the low cycle fatigue resistance of elbows made of copper-nickel alloy.…”

“…Therefore, it is important to understand the effect of these structural defects on the strength in order to maintain the safety and integrity of hazardous structures. This problem was also emphasised during investigations of the integrity and residual strength of piping containing wall thinning defects after recent powerful earthquakes in Japan …”

“…In the case of extreme event (eg, earthquake or shut down), the piping elements are subjected to high amplitude loading excursions, leading to material cycling in low cycle fatigue regime . There were many analytical, numerical, and experimental studies in this area with the ultimate aim of developing a method for the evaluation of low fatigue strength of various piping elements with various wall thinning defects with and without internal pressure . For steel elbows, the recent studies have demonstrated that when the elbows are subjected to high amplitude cyclic loading, the cross‐section geometry distorts and bulges progressively with number of cycles, while the failure mechanism is largely associated with the initiation and propagation fatigue cracks …”