Quantitative Limit State Assessment of a 3-Inch Carbon Steel Pipe Tee in a Nuclear Power Plant Using a Damage Index

Abstract

Seismic motions are likely to cause large displacements in nuclear power plants because the main mode of their piping systems is dominated by the low-frequency region. Additionally, large relative displacement may occur in the piping systems because their supports are installed in several places, and each support is subjected to different seismic motions. Therefore, to assess the seismic performance of a piping system, the relative displacement repeated by seismic motions must be considered. In this study, in-plane cyclic loading tests were conducted under various constant amplitudes using test specimens composed of SCH 40 3-inch pipes and a tee in the piping system of a nuclear power plant. Additionally, an attempt was made to quantitatively express the failure criteria using a damage index based on the dissipated energy that used the force–displacement and moment–deformation angle relationships. The failure mode was defined as the leakage caused by a through-wall crack, and the failure criteria were compared and analyzed using the damage index of Park and Ang and that of Banon. Additionally, the method of defining the yield point required to calculate the damage index was examined. It was confirmed that the failure criteria of the SCH 40 3-inch carbon steel pipe tee can be effectively expressed using the damage index.

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

“…In a previous study, the representative yield point of each test specimen was defined by using the tangent intersection (TI) method in the mean regression curves of their first cycles [42]. For the TI method, different yield points can be calculated depending on the method of defining the tangent of the slope in the plastic region [45]. This indicates that the position of the yield point may vary, depending on the subjective judgment of the engineer for the TI method.…”

“…In addition, a line was drawn with an angle 𝛷 that became 2tan𝜃 using Equation (6). The point at which this line met the P−D curve was the collapse load point: In a previous study, the yield point had one representative value, and it was applied to all the test specimens in the same manner [42,45]. Banon's damage index, however, is based on the nonlinear combination of the ductility index and the energy dissipation index.…”

“…In addition, the collapse load point, which is the limit load for seismic design, was found to have been exceeded when the loading amplitude was ±40 mm. In a previous study, the yield point had one representative value, and it was applied to all the test specimens in the same manner [42,45]. Banon's damage index, however, is based on the nonlinear combination of the ductility index and the energy dissipation index.…”