Influence of Fabrication Strain on Low Cycle Fatigue of Pipe Fittings

Abstract In recent years, elastic-plastic analysis has been increasingly used in the seismic piping design of nuclear industry. However, in the case of using elastic plastic analysis to actual piping design, it is necessary to take into consideration the need to limit residual strain in the welded joints and pipe fittings to be evaluated as much as possible. On the other hand, it is known that the in-production strain of pipe fittings (elbows, tees, reducers) used in nuclear piping is large, exceeding 50% in the case of elbows [1]. Although large residual strain (pre-strain) is likely to be a factor in reducing the low cycle fatigue life of pipe fittings, few studies have investigated the relationship between residual strain in pipe fittings and low cycle fatigue life of pipe fittings. In this paper, we explain the manufacturing method of pipe fittings/ bend pipe and the mechanical properties such as tensile strength and true fracture strain of elbows before and after forming were investigated by monotonic tensile strength testing. Also, we conducted low cycle fatigue testing on test specimens taken from 18B elbow to confirm the influence by residual strain of hot forming elbow. In conclusion, we report that the mechanical properties of elbows before and after forming are almost the same, and that the true fracture strain does not decrease significantly, and hot forming process causes no decrease of fatigue life of elbow.