Investigation of the performance of wedge type ultrasonic transducers for excitation of Lamb waves in steel plates

Abstract

Performing non-destructive testing of a large constructions consisting of metal plates with lateral dimensions up to 100 meters, the most effective are techniques exploiting guided waves. The main factor which limits the measurement possibilities are the signal losses due to the long propagation path, therefore low frequencies signals should be used (about 50 kHz). The presented examples of the simulated fields in pulse and transient modes illustrate, that the effective generation of S₀ mode Lamb waves in a low ultrasonic frequency range (about 50 kHz) using the wedge type transducers is not possible. The amplitude of the generated longitudinal component of the plane wave, suitable for generation of the S₀ mode, is too small. The reason for this is that the diameter of the transducer is smaller than the wavelength and therefore the transducer is generating a spherical wave and, not as expected, the plane wave. Therefore, transformation of the modes in a steel plate occurs and not only as expected the S₀ mode of the Lamb wave is generated, but also the A₀ mode.
The transverse component of the A₀ mode is generated better. The received S₀ mode signal was very weak, and the only strong A₀ mode signal was received. The reason for that was trapping of the S₀ mode in the plate, which acted as the planar waveguide. Therefore, the S₀ mode was not generated efficiently by a wedge type transducer. The experimental results are in good agreement with the simulation results of the excited field in pulse and transient modes.