An ultrasonic guided wave excitation method at constant phase velocity using ultrasonic phased array probes

Abstract

High-order ultrasonic guided wave modes have recently been attracting interest in a variety of nondestructive testing applications, ranging from thickness gauging to bond characterization. Accurate control of the transmitted ultrasonic guided wave mode is paramount when working at frequencies above the cutoff of the first high-order mode. The high number of modes available makes this range of frequency-thickness products difficult to exploit in practice. Many papers and textbooks have showed that multielement probes, such as comb transducers, are able to target a specific wavelength which depends on the elementary pitch. This method can be enhanced by adding an elementary delay law. However, this method of excitation has major drawbacks as the areas of excitation in a dispersion curves depends on the frequency and the technique is not unidirectional. This paper demonstrate that a conventional phased array transducer for which the elementary pitch is small relative to the targeted wavelength is able to excite high order guided wave modes at a constant phase velocity (independently of the frequency). The aim is to excite different regions of the dispersion curves by controlling the input signal bandwidth and the angle of the generated beam. The paper describes the theoretical background and details the differences between the various methods of excitation of ultrasonic guided waves, especially with the comb transducer method. Finite element simulations are presented to verify the analytical predictions and quantify the unidirectional and diffraction properties of the transmitted beam. Experiments conducted on an aluminum plate show striking agreement with finite element simulations, including the possibility of exciting a single mode in a narrow region at high frequency-thickness products. Experiments conducted on a CFRP plate demonstrates that the method can be adapted to other materials.

Keywords: Beam steering; Excitation method; Guided waves; Mode selection; Phase velocity excitation.