Design of mode selective actuators for Lamb wave excitation in composite plates

Abstract

Structural Health Monitoring based on Lamb waves, a type of ultrasonic guided waves, is a promising method for in-service inspection of composite structures. In this study, mode selective actuators are developed to excite a particular Lamb wave mode in quasi-isotropic CFRP (carbon fibre-reinforced polymer) plates. Different manufacturing technologies based on monolithic piezoceramics and piezocomposites are described. The actuators are based on interdigital transducer design in order to control the frequency as well as the wavelength of the desired mode within the excitation. To determine the wavelength of the desired Lamb wave mode, experimental dispersion diagrams of the CFRP plates are measured using air-coupled ultrasonic scanning technique. The dispersion diagrams show the A0 and S0 mode in a frequency range of 25–400 kHz. A mode selective actuator is designed to amplify the A0 mode and to attenuate the S0 mode at 40 kHz. Within experimental tests the actuator and circular piezoceramic sensors are applied on a CFRP plate in order to determine the mode selectivity. These tests are accompanied by 2D finite element simulations. On the basis of simulations and experimental test the influence of different parameters such as number and width of electrode segments, excitation signals and apodization is investigated. The results show that a mode selectivity of the A0 mode in CFRP plates can be achieved by the designed actuator.