Laser-Generated Transient Lamb Wave in Thin Plates

The laser-generated transient Lamb wave in thin isotropic plates is studied theoretically by employing the method of eigenfunction expansion.The thermoelastic displacement field is expressed by symmetric and antisymmetnc Lamb wave modes,so that it is convenient to investigate thg transient excitation of the lowest and higher order Lamb wave modes.The characteristics of transient Lamb waves in thin materials with different thicknesses are discussed in details.

Broadband Acoustic Transmission Enhancement through a Structured Stiff Plate with Locally Resonant Elements

Broadband acoustic transmission enhancement(ATE)is realized for a periodically structured stiff plate without any opening that is conventionally thought to be only capable of supporting narrowband ATE,by introducing locally resonant(LR)elements.This exotic phenomenon is interpreted by analyzing the vibration pattern of the structure-induced LR modes,and is well modeled by a simple"spring-mass"system which reveals the contribution of the LR effect to the important broadband performance.Our findings should help to better understand the physical mechanism of ATE and may have potential impact on ultrasonic applications such as broadband acoustic filters or compact acoustic devices in subwavelength scale.

Concealing a Passive Sensing System with Single-Negative Layers

We propose a multi-layer structure for concealing an electromagnetic sensing system (a sensor is wrapped witha transparent protective layer),using single-negative (SNG) materials whose material parameters are completely independent of those of the host matrix as well as the concealed system.The numerical results show that only three different kinds of SNG materials are sufficient to yield the cloaking effect even in the presence of weak loss.This may significantly facilitate the experimental realization of a well-performing sensor-cloaking device.

Nondestructive Characterization of Quantitative Bonding Strength at a Bonded Solid-Solid Interface

We propose a nondestructive method to characterize the quantitative bonding strength at a bonded solid-solid interface by a contact acoustic nonlinearity(CAN)microscope.The principle of the CAN microscope is briefly described.The vibration amplitude of the incident focusing wave at the bonded interface is calculated,the standard bonding strength with a complete bonding state is established by the tension test,and the CAN parameter is calibrated.The quantitative contour of bonding strength at the interface could be obtained.The experimental contours of two samples are also presented.

A New Inverse Method of Elastic Constants for a Fibre-Reinforced Composite Plate from Laser-Based Ultrasonic Lamb Waves

A new inverse method based on the wavelet transform and artificial neural networks (ANN) is presented to recover elastic constants of a fibre-reinforced composite plate from laser-based ultrasonic Lamb waves. The transient waveforms obtained by numerical simulations under different elastic constants are taken as the input of the ANN for training and learning. The wavelet transform is employed for extracting the eigenvectors from the raw Lamb wave signals so as to simplify the structure of the ANN. Then these eigenvectors are input to a multi-layer internally recurrent neural network with a back-propagation algorithm. Finally, the experimental waveforms are used as the input in the whole system to inverse elastic constants of the experimental material.

Thermoelastic Excitation of Transient Lamb Wave in an Orthotropic Thin Plate

A numerical method to simulate thermoelastic excitation of transient Lamb wave propagating along arbitrary directions in orthotropic plates is presented by employing the expansion method of the Lamb wave modes.The displacement is expressed by a summation of the symmetric and antisymmetric modes in the surface stress-free orthotropic plate,and it is particularly suitable for waveform analyses of Lamb wave in orthotropic thin plates because one needs only to calculate the lowest few modes.The characteristics of dispersion and transient wave-forms are analyzed for a transversely isotropic plate.The results show that this method provides a quantitative analysis to Characterize anisotropic properties and elastic stiffness properties of the orthotropic plates by the laser-generated Lamb wave detection.

Anisotropic Behavior Analyzed by Laser-Generated Lamb Waves for Fiber Composite Plates

Analyses of the anisotropic behavior of a Uber composite plate from the laser-generated ultrasonic Lamb waveforms propagating in different directions are presented by employing wavelet transform techniques.The time-frequency representation plots of the waveforms clearly exhibit the anisotropic behavior of the composite plate.The plots also indicate that the waveform mainly consists of three or four lower-order Lamb wave modes,and other higher-order modes can be neglected in extracting elastic stiffness properties by comparison of theory with experimental waveforms.The group velocity of each mode is calculated according to the group delay and the source-receiver distance.

Microstructure Effects of Ultrasonic Waves in a Fiber Reinforced Composite Plate

The microstructure effect of ultrasonic waves in a unidirectional titanium graphite composite is analyzed by the mode energy conversions of the laser-generated ultrasonic Lamb wvaves.The carrying energy of each mode in the Lamb wvaves is calculated quantitatively by the time-frequency filtering technique of the Wigner distribution.We found that the energy conversions among modes have happened in the process of propagation of ultrasonic Lamb wave.These energy conversions are attributed to microstructure scattering of ultrasonic wave by the fibers in a fiber reinforced composite material.This work will provide a quantitative method of ultrasonic characterizationof microstructure feature of the composite materials by the laser-generated Lamb wave technique.