Development of a low-frequency piezo-ceramic transducer optimized for the generation of a plane fundamental shear horizontal guided wave

摘要

Structural health monitoring (SHM) consists of continuously assessing structural integrity using integrated testing systems. One well-known method to achieve such a goal is ultrasonic testing. This technique has considerably evolved through non-destructive testing (NDT) by the use of, initially bulk ultrasonic waves and, subsequently, guided ultrasonic waves. One of the main challenges that emerges from the use of ultrasonic testing for SHM applications is to minimize the overall footprint of the specifically designed transducers. Another important challenge, either for NDT or SHM applications, is the requirement for complex signal processing due to the multimodal and dispersive nature of the guided waves. One simple and extensively used method to avoid the former case is to use frequencies under the first cut-off frequency to avoid high-order modes. A second complementary method to avoid the latter case is to use the fundamental shear horizontal (SH) guided wave mode (SH0), which is the only non-dispersive guided wave mode propagating in a thin plate.ududThis masters thesis focuses on the development of a method to design a piezo-ceramic ultrasonic transducer optimized for the generation of the SH0 mode and the minimization of both fundamental Lamb modes (A0 and S0). The proposed methodology consists of first choosing the proper piezoelectric material based on the best suited vibrational mode, which is PZT-5H in thickness-shear mode respectively. The second step is to optimize two geometrical parameters, the width and the length of the rectangular active element. Both parameters have a direct influence on the relative amplitude of the three generated modes as well as the aperture and the number of generated mode directivity lobes. Based on these criteria, four combinations have proven to be of sufficient interest, and their behaviour was validated using an analytical wave propagation simulation. Such modeling was used because the required three-dimensional (3D) finite-element (FE) simulations are very computationally intensive when it comes to wave propagation. The results and analytical model accuracy were then validated using Abaqus finite element software. The final optimal geometry and frequency combination, 25.4 mm long, 3.7 mm wide, and 1 mm thick, centered at 425 kHz, was finally experimentally validated using a laser Doppler vibrometer system to obtain the resulting complete 3D wave field. The results show that it is possible to generate a plane SH0 wave at a relative amplitude of at least 16.4 dB above both fundamental Lamb modes in any direction, and of 23.0 dB within an aperture of 20 degrees.