Zenghua Liu

Delamination detection in composite beams using pure Lamb mode generated by air-coupled ultrasonic transducer

The interaction of Lamb wave A0 mode with delamination and delamination detection in 16-ply carbon fiber–reinforced epoxy composite beams are investigated through three-dimensional finite element simulation and experimental studies in this article. Wave propagation in composite beams with delamination with different lengths and located at different interfaces are investigated in finite element simulations, and some unique mechanisms of interaction between A0 mode and delamination are revealed in detail. Experimental results obtained with air-coupled ultrasonic transducers are well in accordance with finite element simulation results. In an experimental study, an air-coupled ultrasonic transducer is oriented at a coincidence angle such that it generates a pure fundamental antisymmetric Lamb wave mode A0 for delamination detection in laminated composite beams. The receiving transducer can be oriented either to detect the transmitted wave propagating in the same direction as incident wave or to detect the reflected wave in contrast to incident wave. The location and size of delamination can be evaluated quantitatively using the time-of-flight of reflected wave from both ends of the delamination.

Baseline-free delamination inspection in composite plates by synthesizing non-contact air-coupled Lamb wave scan method and virtual time reversal algorithm

In the paper, we combined air-coupled Lamb wave scan method and virtual time reversal (VTR) algorithm and proposed a composite baseline-free delamination inspection technique of composite plates. According to VTR algorithm, time reversal process is virtually performed through signal operations and the hardware manipulation for time reversal is not required. Baseline-free damage inspection can be achieved by comparing the first input actuation signal with the reconstructed final signal obtained by VTR algorithm. An air-coupled Lamb wave scan method combined with VTR-based probabilistic imaging algorithm is developed for delamination inspection of composite plates. Carbon fiber-reinforced composite plates with the delaminations of different shapes and sizes were experimentally tested. The testing results are well in accordance with the actual delamination locations and sizes as well as the results obtained with the commercial point-to-point immersion C-scan system.

A new multichannel time reversal focusing method for circumferential Lamb waves and its applications for defect detection in thick-walled pipe with large-diameter

This paper proposes a new multichannel time reversal focusing (MTRF) method for circumferential Lamb waves which is based on modified time reversal algorithm and applies this method for detecting different kinds of defects in thick-walled pipe with large-diameter. The principle of time reversal of circumferential Lamb waves in pipe is presented along with the influence from multiple guided wave modes and propagation paths. Experimental study is carried out in a thick-walled and large-diameter pipe with three artificial defects, namely two axial notches on its inner and outer surface respectively, and a corrosion-like defect on its outer surface. By using the proposed MTRF method, the multichannel signals focus at the defects, leading to the amplitude improvement of the defect scattered signal. Besides, another energy focus arises in the direct signal due to the partial compensation of dispersion and multimode of circumferential Lamb waves, alongside the multichannel focusing, during MTRF process. By taking the direct focus as a time base, accurate defect localization is implemented. Secondly, a new phenomenon is exhibited in this paper that defect scattered wave packet appears just before the right boundary of truncation window after time reversal, and to which two feasible explanations are given. Moreover, this phenomenon can be used as the theoretical basis in the determination of defect scattered waves in time reversal response signal. At last, in order to detect defects without prior knowing their exact position, a large-range truncation window is used in the proposed method. As a result, the experimental operation of MTRF method is simplified and defect detection and localization are well accomplished.

Simulation of an Electromagnetic Acoustic Transducer Array by Using Analytical Method and FDTD

Previously, we developed a method based on FEM and FDTD for the study of an Electromagnetic Acoustic Transducer Array (EMAT). This paper presents a new analytical solution to the eddy current problem for the meander coil used in an EMAT, which is adapted from the classic Deeds and Dodd solution originally intended for circular coils. The analytical solution resulting from this novel adaptation exploits the large radius extrapolation and shows several advantages over the finite element method (FEM), especially in the higher frequency regime. The calculated Lorentz force density from the analytical EM solver is then coupled to the ultrasonic simulations, which exploit the finite-difference time-domain (FDTD) method to describe the propagation of ultrasound waves, in particular for Rayleigh waves. Radiation pattern obtained with Hilbert transform on time-domain waveforms is proposed to characterise the sensor in terms of its beam directivity and field distribution along the steering angle, which can produce performance parameters for an EMAT array, facilitating the optimum design of such sensors.

Directivity analysis of meander-line-coil EMATs with a wholly analytical method.

HIGHLIGHTSWe propose a wholly analytical model for meander‐line‐coil EMATs.The Dodd and Deeds solution adapted for the straight wire.Radiation pattern of Rayleigh waves obtained with the proposed model.Wire lengths effect on the directivity of Rayleigh waves analysed quantitatively. ABSTRACT This paper presents the simulation and experimental study of the radiation pattern of a meander‐line‐coil EMAT. A wholly analytical method, which involves the coupling of two models: an analytical EM model and an analytical UT model, has been developed to build EMAT models and analyse the Rayleigh waves’ beam directivity. For a specific sensor configuration, Lorentz forces are calculated using the EM analytical method, which is adapted from the classic Deeds and Dodd solution. The calculated Lorentz force density are imported to an analytical ultrasonic model as driven point sources, which produce the Rayleigh waves within a layered medium. The effect of the length of the meander‐line‐coil on the Rayleigh waves’ beam directivity is analysed quantitatively and verified experimentally.

A Wholly Analytical Method for the Simulation of an Electromagnetic Acoustic Transducer Array

This paper presents a wholly analytical method which combines electromagnetic and ultrasonic simulations for the study of an Electromagnetic Acoustic Transducer Array (EMAT). Analytical solutions to the eddy current problem for the meander coil used in an EMAT are adapted from the classic Deeds and Dodd solution, which was originally intended for circular coils. The analytical solution resulting from this novel adaptation exploits the large radius extrapolation and shows several advantages over the finite element method (FEM), especially in the higher frequency regime. The calculated Lorentz force density from the EM solver is then coupled to the ultrasonic simulations, which exploit an analytical solution for generating the distribution of surface waves. Beam features are studied, which can produce performance parameters for an EMAT array, facilitating the optimum design of such sensors. This total analytical approach to an EMAT simulation has not been reported previously to our knowledge.

Damage imaging in composite plate by using double-turn coil omnidirectional shear-horizontal wave magnetostrictive patch transducer array

A double-turn coil omnidirectional shear-horizontal wave magnetostrictive patch transducer (DC-OSH-MPT) array for imaging detection in composite plates is presented. DC-OSH-MPT parameters are optimized in order to make the transducer more suitable for imaging detection. Besides, the characteristic of the lowest shear-horizontal mode (SH0), which can be generated and received in all directions by DC-OSH-MPT bonded on composite plate, is beneficial for imaging. Due to anisotropic property of composite plates, the group velocity of SH0 mode in different azimuthal directions was experimentally measured in damage imaging. The location of simulated damage, which was a bonded circular cylinder, was estimated by DC-OSH-MPT array and ellipse imaging algorithm. The imaging results have demonstrated that DC-OSH-MPT can be considered as a spatially distributed array suitable for imaging detection in composite plates.

Damage Localization and Quantification of Truss Structure Based on Electromechanical Impedance Technique and Neural Network

Truss structure is widely used in civil engineering. However, it is difficult to quantitatively monitor the state of truss structures because of the connection diversity and complexity of truss structures. In this paper, electromechanical impedance (EMI) technique was proposed to measure impedance spectra by using PZT elements and backpropagation (BP) neural network was used as an effective nonlinear conversion tool to quantify the health state of truss structures. Firstly, frequency band of the spectrum was experimentally determined by the trial-and-error approach. Then four connection rods of this truss structure were selected for experimental research. These connection rods were loosened gradually with a small angle increment and the impedance spectra were recorded. Then, the measured data were compressed through dividing the frequency range into multiple subbands. And RMSD values of these bands showed that data points were reduced while damage features remained. Finally, one four-layered BP neural network model was constructed based on these compressed data. The research results showed that compressed impedance data could retain their damage features. After the training, the developed neural network model could not only determine the location of loosened rod, but also quantify the loosening levels.

Defect detection in helical and central wires of steel strands using advanced ultrasonic guided wave technique with new type magnetostrictive transducers

Health conditions of steel strands influence directly the durability and security of whole prestressed structures. Advanced ultrasonic guided wave technique is used for defect detection of multi-wire steel stands with help of new type magnetostrictive transducers. A kind of special magnetostrictive transducer is developed for the excitation and reception of the lowest longitudinal mode L(0,1) at 160 kHz which is suitable for defect detection of steel strands. Artificial notches in helical and central wires of steel strands which nominal diameter is 17.80 mm are detected using L(0,1) mode excited and received by a pair of magnetostrictive transducers. For analyzing the sensitivity to the notches with variable depth in different wires, the experimental relation curves between the amplitudes of notch echoes and the notch depths are obtained. Experimental results show that ultrasonic guided waves can be used for the inspection of whole steel strands. Furthermore, from the amplitudes of defect echoes in steel strands, it is obviously found that L(0,1) modes is much more sensitive to the defects in helical wires than ones in central wires although the sizes of these defects in different wires are almost same.

Full non-contact laser-based Lamb waves phased array inspection of aluminum plate

Abstract Lamb waves defect detection technique is suitable to detect long-range defects and multi-defects in plates because of its attractive properties such as multi-modes, low attenuation, and multi-defect sensitivity. As the laser beams have the tiny size and noncontact property, in this paper a full non-contact laser-based Lamb waves phased array inspection technique was proposed for defect detection in an aluminum plate. A Q-switched Nd: YAG pulse laser generating Lamb waves and a continuous laser inspection system was used to inspect the out-of-plane displacements. The laser-generated Lamb waves have the broadband property and low temporal resolution that are not suitable for defects detection directly. So the continuous wavelet transform was used to extract narrowband signals from laser-generated Lamb wave and a linear mapping algorithm was adopted to compensate for the dispersion of the extracted narrowband signals. Two experiments were carried out to verify the applicability and effectiveness of the developed full non-contact laser-based Lamb waves phased array inspection technique. Two compact phased array imaging algorithms, the total focusing method and the sign coherence factor algorithm, were adopted to image the defects with processed signals. The experimental results accurately located the positions of the defects.Graphical Abstract