Modelling of Lamb Waves in Composite Laminated Plates Excited by Interdigital Transducers

The technique of permanently attaching interdigital transducers (IDT) to either flat or curved structural surfaces to excite single Lamb wave mode has demonstrated great potential for quantitative non-destructive evaluation and smart materials design. In this paper, the acoustic wave field in a composite laminated plate excited by an IDT is investigated. Based on the discrete layer theory and a multiple integral transform method, an analytical-numerical approach is developed to evaluate the surface velocity response of the plate due to the IDT's excitation. In this approach, the frequency spectrum and wave number spectrum of the output of IDT are obtained directly. The corresponding time domain results are calculated by applying a standard inverse fast Fourier transformation technique. Numerical examples are presented to validate the developed method and show the ability of mode selection and isolation. A new effective way of transfer function estimation and interpretation is presented by considering the input wave number spectrum in addition to the commonly used input frequency spectrum. The new approach enables the simple physical evaluation of the influences of IDT geometrical features such as electrode finger widths and overall dimension and excitation signal properties on the input-output characteristics of IDT. Finally, considering the convenience of Mindlin plate wave theory in numerical computations as well as theoretical analysis, the validity is examined of using this approximate theory to design IDT for the excitation of the first and second anti-symmetric Lamb modes

Analysis of a Piezoelectric Sensor to Detect Flexural Waves

In this paper the electro-mechanical transfer characteristics of adhesively bonded piezoelectric sensors are investigated. Using dynamic piezoelectricity theory, Mindlin plate theory for flexural wave propagation and a multiple integral transform method, the frequency response functions of piezoelectric sensors with and without backing materials are developed and the pressure-voltage transduction functions of the sensors calculated. The corresponding simulation results show that the sensitivity of the sensors is not only dependent on the sensors' inherent features such as piezoelectric properties and geometry, but also on local characteristics of the tested structures and the admittance and impedance of the attached electrical circuit. It is also demonstrated that the simplified rigid mass sensor model can be used to successfully analyse the sensitivity of the sensor at low frequencies, but that the dynamic piezoelectric continuum model has to be used for higher frequencies, especially around the resonance frequency of the coupled sensor-structure vibration system

Experimental Investigation of the Acousto-Ultrasonic Transfer Characteristics of Adhesively Bonded Piezoceramic Transducers

For the development of optimised health monitoring systems and smart materials, the understanding of the interaction of piezoelectric transmitting and receiving transducers with the structure is essential. This paper reports on the development of an experimental technique to determine the acousto-ultrasonic transfer characteristics of adhesively bonded piezoceramic transducers. Five millimeter diameter piezoceramic discs with and without brass backing were glued to one and two millimeter thick aluminium plates. Narrow and broad band excitation pulses were applied to the transducers in a frequency range between 50 kHz and 1 MHz, a frequency regime suitable for guided wave ultrasonic non-destructive evaluation applications. The electro-mechanical transfer properties of the ultrasonic transmitter elements were determined using a heterodyne Doppler laser vibrometer as a noncontact receiver device and Rayleigh-Lamb wave theory to describe the propagation behaviour of the waves in the structure. It is found that the transfer characteristics are extremely complex including sharp and narrow as well as broader but less pronounced frequency regimes of high energy transfer. It is shown that the major features of the transfer functions for different experimental configurations are similar, but the magnitudes of the peaks and their locations in frequency space are different for individual transducer/substrate combinations

Dynamic response and energy absorption of functionally graded porous structures

This paper is focused on the in-plane crushing of two-dimensional (2D) porous structures with a special attention on the effect of functionally graded (FG) porosities. The dynamic response and energy absorption of closed-cell metal foams with different porosity distributions are investigated by using finite element (FE) analysis. Two symmetric, two asymmetric and one uniform distributions of internal pores along the impact direction are constructed with Voronoi tessellation. The proposed porous structure is crushed under the impact of a rigid panel with a constant velocity. The deformation of cell walls is simulated using a plastic kinematic material model. The erosion criteria and hourglass control are applied to ensure the accuracy of numerical results, which are validated against the experimental data from open literature. The effects of varying parameters on the energy absorption, deformation pattern, and stress-strain curve of the FG porous structure are discussed. The dynamic response is found to be influenced by different random cell geometries, porosity gradients, cell wall thicknesses, internal pore numbers, and impact velocities. The effective way to improve the energy absorption capability of the porous structure under a constant-velocity impact is proposed, shedding new insights into the deformation mechanism of the FG porous structure for engineering design