A time-of-flight mapping method for laser ultrasound guided in a pipe and its application to wall thinning visualization

Piping systems in nuclear power plants (NPPs) are susceptible to flow-accelerated corrosion (FAC) that results in local wall thinning of the piping system. FAC can cause severe piping failure such as leakage and rupture. In this study, a reliable ultrasonic time-of-flight (ToF) mapping system in the pipe is proposed with the goal of improving ultrasonic NPP pipe inspection. The system consists of a high-speed laser ultrasonic scanning system (50 mm/s at intervals of 0.5 mm) and two ultrasonic sensors. The ToF mapping algorithm uses mode identification based on wavelet transform, an automatic threshold setting method based on the statistics of the spatial noise map, and two-dimensional (2-D) ToF extraction based on the threshold crossing time method. The Lamb-like waves guided in the wall of a stainless steel (SUS) straight pipe were simultaneously interrogated by the proposed two-channel laser ultrasonic system. The proposed ToF mapping algorithm enabled the generation of a smooth ToF distribution within the beam incidence angle of ±60° in the curved surface of the pipe. Then a ToF mapping method was applied to a wall-thinned pipe, and an ultrasonic wave propagation movie was generated to help understand the complex ultrasonic propagation pattern. Based on an understanding of the ToF change in the wall-thinned region, and on the mode collapse and attenuation that occurred because of the ultrasonic bottleneck phenomenon, the wall-thinned region was evaluated. Since this method allows in-situ and automatic field inspection, it can contribute to NPP pipe health management. ► Ultrasonic time-of-flight mapping system is used for NPP pipe inspection. ► Wavelet transform is used for mode identification for time-of-flight mapping. ► Threshold level is selected based on the statistics of the spatial noise map. ► 2-D time-of-flight is extracted based on the threshold crossing time method. ► Pipe with 60% wall-thinning rate can be visualized and evaluated through a ToF map.