Ultrasonic high-frequency guided waves for the inspection of shell weld in core support structure of fast breeder reactors

Abstract

This article reports an ultrasonic high-frequency guided wave based methodology for inspection of shell weld of core support structure of the main vessel in 500-MWe sodium-cooled prototype fast breeder reactor. The high-frequency guided waves are generated in 30-mm-thick plate of core support structure at 2 MHz (frequency–thickness product of ∼60 MHz-mm). These wave modes are fairly nondispersive over considerable distance of propagation through the material. In order to understand the generation and propagation characteristics of these high-frequency guided waves, a series of two-dimensional plane strain finite element models are developed using explicit scheme. The optimum transducer placement for generating efficient high-frequency guided wave is achieved by sweeping the transducer position along the curvature of the main vessel plate with respect to the centerline of the vertical shell support plate. The generated high-frequency guided waves are analyzed and identified with the help of dispersion curves and time–frequency analysis. The simulation results are found to capture the features observed in the experimental measurements rather well, and the time of flight of the reflected waves in the simulation is found to be in excellent agreement with the experimental results. The optimum transducer locations for inspection of a general T-joint configuration with different angles between the web and the base have been studied in detail using finite element simulations. With optimum transducer position, the high-frequency guided waves are also used to inspect the artificially introduced defects in the connecting weld of shell support plate. It has been demonstrated that the high-frequency guided wave could detect defects of 20% wall thickness (6 mm) in the connecting shell weld.