Selective Generation of Lamb Wave Modes in a Finite-Width Plate by Angle-Beam Excitation Method.

A Lamb wave in a plate with a finite width has both thickness and width modes, whereas only thickness modes exist in an infinitely wide plate. The thickness and width modes are numerously formed in a finite-width plate, and they all have different cut-off frequencies, wave velocities, and wave structures. These different characteristics can be utilized in various applications, but a selective generation method for a particular Lamb wave mode in a finite-width plate has not been sufficiently studied, and only a method using multiple elements has been reported. This paper presents the selective generation of a certain Lamb wave mode in a finite-width plate by an angle-beam excitation method using single or dual wedges. In the proposed generation method, a specially designed wedge with grooves or a patch having insulation layers is employed for partial acoustic insulation of the ultrasonic energy incident into the plate. The feasibility of the proposed method was investigated through finite element method (FEM) simulations for Lamb wave excitation and propagation, and then experimentally demonstrated by the measurement of Lamb wave propagation using a laser scanning vibrometer.

Effects of beryllium coating layer on performance of the ultrasonic waveguide sensor.

Under-sodium viewing is one of the critical technical issues and requirements for the in-service inspection of the sodium-cooled fast reactor (SFR) that is currently under development. The waveguide sensor that uses leaky A(0) mode Lamb waves has shown its potential for high-resolution viewing/scanning of the reactor core and in-vessel structures. However, a few problems arise under a liquid sodium environment due to high sound speed in liquid sodium and dispersion in the long waveguide plate, which simultaneously deteriorate the reconstructed C-scan images. This paper proposes coating the surface of the waveguide sensor plate with a thin layer of material that has a very high ultrasonic wave velocity. It is shown that this coating layer can largely reduce the size (width) and radiation angle of the acoustic beam from the waveguide sensor. This paper precisely analyzes the effects of coating parameters on the beam quality. The proposed idea is validated through ultrasonic experiments in which the radiation beam profiles and group velocities in waveguide sensors with different surface treatments are measured and compared.