Development of Measurement Methods for Vibration-Induced Stress of Small-Bore Piping

The vibration-induced fatigue failure of small-bore piping is one of the common causes of failure trouble at nuclear power plants (NPPs). Therefore, the purpose of this study is to develop the measurement methods of vibration-induced stress for the screening to prevent from fatigue failure mechanism of small-bore piping. First, a measurement method using a single-mass model was introduced, and then, a measurement method using a two-mass model developed as an improved calculation model was proposed. These two kinds of models were validated by vibration tests using mock-up with small-bore branch piping. The results showed that the single-mass model could be used as the coarse screening. Additionally, the two-mass model was found to be suitable to the fine screening due to more accurate measurement of vibration-induced stress. Next, for small-bore piping with typical pattern configurations consisting of several masses and supports, the model considering the supports and the center of gravity being out of pipe centerline was developed and put into practical use. Finally, for the more complex small-bore piping with general piping configurations consisting of many bends, branches, or joints, the method based on the finite element analysis and using the measured values was developed. In the developed method, the differences between the natural frequency and the response acceleration obtained by the measurement and those values calculated using the analysis model are optimized to be enough small, and then, the vibration-induced stress is estimated by superposing the vibration modes of the small-bore piping with the static deformation representing the main piping vibration. In this study, the usability of the developed method was confirmed by the comparison with the numerical results without the measurement error, which were assumed to be the true values. The peak stress induced by vibration frequently occurs at the filet weld part between the small-bore piping and the main piping. The developed methods can be used for various weld geometries although the measurement method using strain gauges cannot be used for such weld parts. The failure possibility by vibration-induced fatigue can be evaluated by comparing the nominal stress measured by the methods in this study with the fatigue threshold stress divided by the stress concentration factor appropriate for the weld geometry.