Lamb Wave-based Structural Health Monitoring Research Articles

Efficient zigzag theory-based spectral element model for guided waves in composite structures containing delaminations

Lamb wave-based structural health monitoring offers excellent potential for real-time delamination detection in laminated structures. It necessitates fast and accurate numerical simulation of guided wave interaction with delaminations. Towards this objective, we present an efficient layerwise zigzag theory (ZIGT) based time-domain spectral element for wave propagation analysis of composite beam- and panel-type structures (strips) containing delamination at arbitrary locations. The ZIGT delivers accuracy like layerwise theories by allowing slope discontinuities in the in-plane displacement field at layer interfaces while maintaining computational efficiency like equivalent single-layer (ESL) theories, making it ideal for such analyses. The high-order elemental nodes at Lobatto points have only four displacement variables, irrespective of the number of layers in the laminate. Delamination is modelled using the region approach, adapting the recently proposed hybrid point-least squares continuity method to satisfy the nonlinear displacement field continuity at the delamination fronts. The model’s efficacy is examined vis-à-vis elasticity-based elements, ESL theory-based elements, and the ZIGT-based standard finite element for free vibration and guided wave propagation responses of composite strips featuring multiple delaminations. The present model shows superior overall efficiency, accuracy, and convergence. An application of the model is also explored for identifying delamination locations from Lamb wave velocity fields.

Adhesively bonding a reusable PZT transducer for structural health monitoring (SHM) with an ethylene-acrylic acid copolymer adhesive

A novel approach to bonding a piezoelectric lead zirconate titanate (PZT) transducer on a structure for health monitoring was developed to replace the traditional method of bonding PZT permanently. Several experiments demonstrated the feasibility of a reusable PZT transducer based on the adhesive film made using ethylene-acrylic acid copolymer (EAA). Additionally, the adhesion performance of using EAA for bonding PZT at different temperatures and the effect of heating temperature on reusable PZT transducer removal from the test structure was investigated. The effect of adhesive thickness on the mechanical property and the performance of PZT has also been explained. Experimental results showed that the optimized heating temperature of the adhesive film was around 140 °C due to the stability of PZT being sensitive to high temperature and the ease of removing PZT from the test structure without damage. An analysis of the varying thickness of the adhesive layer on PZT performance indicated that the amplitude of Lamb wave signals changed with adhesive thickness, increasing with greater adhesive thickness. Therefore, the adhesive layer made using EAA provides a new approach to conducting Lamb wave-based structural health monitoring.

Insight into excitation and acquisition mechanism and mode control of Lamb waves with piezopolymer coating-based array transducers: Analytical and experimental analysis

In-situ fabricated piezopolymer coating-based transducers have been developed to build large-area, lightweight and flexible networks for wave-based structural health monitoring (SHM). Meanwhile, their tunability can be realized by array electrodes for mode control with proper tuning methods. However, conventional standard tuning by phase matching seems not always effective. In this study, the excitation and acquisition mechanism of Piezopolymer Coating-based Array Transducers (PCATs) were first studied for Lamb waves. Distinctly different coupling mechanisms of PCAT actuators and sensors were discussed by analytical models and experimental verification, respectively. Then comprehensive parameter studies were performed to understand the filtering effect with finite temporal pulse duration of PCAT actuators, and finite spatial electrode span of PCAT actuators and sensors. Corresponding bias tuning methods were proposed with analytical solutions to improve mode control in Lamb-wave excitation and acquisition. This new guideline of designing array electrodes for PCAT actuators and sensors has been proven effective by successfully tuning the poor mode-controlled wavefield originated by the standard tuning method. Such tunability has great potential to be applied for detecting various damages in Lamb wave-based SHM, where sensitivity, accuracy, and signal interpretation can be improved with good control of particular frequency-mode selections.

A High Reliability Damage Imaging Method Under Environmental Temperature Variations

Abstract In Lamb wave-based structural health monitoring (SHM), the environmental temperature variations can easily affect Lamb wave monitoring signals and seriously reduce the reliability of final damage detection results. To resolve the temperature effect problem, a temperature compensation method of improved baseline signal stretch (IBSS) is presented and applied for high reliability damage imaging under large environmental temperature variations in this paper. After the basic principle of IBSS is analyzed, the realization of IBSS is discussed. Then, a IBSS-based high reliability damage imaging method under temperature variation situation is developed. An experimental study is finally arranged.

Focusing phase imaging for Lamb wave phased array

In Lamb wave-based structural health monitoring, amplitude damage imaging is commonly used because the defects feature can be easily amplified by summing all the response signals together. However, the grating and side lobes affect the imaging quality and blind areas further restrict the inspection area. Considering that the existing phase-based imaging algorithms are either unfit for dispersive Lamb wave or strict to many requirements to guarantee better performance, inspired by the absence of phase information in focusing phased array, a novel focusing phase imaging (FPI) method for Lamb wave phased array is developed. The main contribution of the paper is introducing the phase information to focusing phased array. By applying the inverse-dispersion effect to the excitation signals and the superposition operation, the energy can be focused at every inspection point. The phase damage index is constructed by directly measuring the degree of consistency and alignment of the instantaneous phases. The experiments for the circular and linear array under various excitation signals with multiple defects verify that the FPI is effective for both surface damage and through-hole damage. The proposed algorithm is superior for its ability in energy focusing for defects, the capability in suppression of grating and side lobes, strong anti-disturbance ability from boundary reflection, the nonexistence of imaging blind area, and its adaptability for various excitation parameters and array layout.

Estimation of Lamb wave propagation by means of Fourier spectral frequency response function

ABSTRACT Considering the frequency dispersion and the scale sensitivity in wave propagation, the central frequency and bandwidth should be carefully selected in Lamb wave-based structural health monitoring. These crucial parameters are hardly determined without the prior knowledge. Therefore, massive trials should be conducted before the formal test. A Fourier spectral Frequency Response Function (FRF) method is proposed to address this problem. As the FRF contains wide-band information on the relationship between excitation and output, the Lamb wave responses in a wide frequency range can be estimated by using an arbitrary virtual input within the frame of single FRF test, being free from the massive experimental works. The theoretical FRF can be obtained from the unit impulse response of a linear system, but this path is impracticable due to the difficulty of exciting the ideal impulse. Considering the integral relationship and Fourier differential properties between step excitation and the ideal impulse, a robust approximation of wave signal by FRF is developed. Synthetic data tests and experiments both verified that the present method can accurately extract the response in isotropic and anisotropic materials for the specifically designed excitation signal in a wide parameter range.

Development of Laboratory-scale Lamb Wave-based Health Monitoring System for Laminated Composites

This paper presents the development process of a laboratory-scale Lamb wave-based structural health monitoring (SHM) system for laminated composite plates. Piezoelectric patches are used in pairs as actuator/sensor to evaluate the time of flight (TOF), i.e. the time difference between the transmitted/received signals of a damaged plate and those of a healthy plate. The damage detection scheme is enabled by means of evaluating the TOF from at least three actuator/receiver pairs. In this work, experiments were performed on two GFRP plates, one healthy and the other one with artificial delamination. Nine piezoelectric transducers were mounted on each plate and the detection of the delamination location was demonstrated, using 4 pairs and 20 pairs of actuators/sensors. The combinations of fewer and more actuators/sensor pairs both provided a damage location that was in good agreement with the artificial damage location. The developed SHM system using simple and affordable equipment is suitable for supporting fundamental studies on damage detection, such as the development of an algorithm for location detection using the optimum number of actuator/sensor pairs.

Heteroepitaxy of flexible piezoelectric Pb(Zr0.53Ti0.47)O3 sensor on inorganic mica substrate for lamb wave-based structural health monitoring

Active Lamb wave detection is an effective and simple monitoring method for structural health monitoring (SHM). Piezoelectric lead zirconate titanate (PZT) ceramics are most commonly used as transducers to excite and receive Lamb wave. However, due to their hardness, thickness and brittleness, PZT ceramics are severely limited in detecting damage on curved surface structures. Herein, we report that flexible PZT film sensor is an excellent candidate for receiving Lamb wave used in Lamb wave-based SHM of aircraft. An improved piezoelectric coefficient d33 (~130 pm V−1) is obtained in epitaxial PZT films grown on flexible inorganic mica substrates via van der Waals heteroepitaxy. Superior stable ferroelectricity and piezoelectricity retain in flexible PZT film after bending 104 cycles under a radius of 8 mm. As proven by experiment and simulation, flexible PZT film is demonstrated as an ultrasonic sensor with high sensitivity to be bonded on the curved aluminum plate for Lamb wave-based damage monitoring. Our work demonstrates that flexible PZT films have enormous potential for real-time light-weight and high-sensitivity receiver sensors for the SHM of aging aircraft.

Weighted Structured Sparse Reconstruction-Based Lamb Wave Imaging Exploiting Multipath Edge Reflections in an Isotropic Plate.

Lamb wave-based structural health monitoring techniques have the ability to scan a large area with relatively few sensors. Lamb wave imaging is a signal processing strategy that generates an image for locating scatterers according to the received Lamb waves. This paper presents a Lamb wave imaging method, which is formulated as a weighted structured sparse reconstruction problem. A dictionary is constructed by an analytical Lamb wave scattering model and an edge reflection prediction technique, which is used to decompose the experimental scattering signals under the constraint of weighted structured sparsity. The weights are generated from the correlation coefficients between the scattering signals and the predicted ones. Simulation and experimental results from an aluminum plate verify the effectiveness of the present method, which can generate images with sparse pixel values even with very limited number of sensors.

A flexible laser ultrasound transducer for Lamb wave-based structural health monitoring

Lamb waves are widely used in structural health monitoring (SHM) for plate-like structures. In this paper, a new and flexible ultrasonic transducer with a high photo-acoustic conversion efficiency was proposed by using candle soot nanoparticles (CSNPs) and polydimethylsiloxane (PDMS). Experimental results demonstrate that the developed transducer can generate a longitudinal wave with a short duration of 0.28 μs under the illumination of a nanosecond laser pulse. The amplitude of the excited longitudinal wave is 10 times that of the signal generated by the traditional laser ultrasound technique. Further, wedge-shape transducers were developed to excite Lamb waves in a 1.5-mm thick aluminum substrate by the oblique incidence method. The specific dA0 and S0 modes of the Lamb wave with the central frequency of 647 kHz were successfully excited in the aluminum plate. Based on the synthetic aperture focusing imaging technique, a delay-and-sum signal processing method was adopted for damage location in the plate by using the A0 mode Lamb. A 3.5-mm defect was well imaged and the results demonstrate that the developed flexible photo-acoustic transducer can be a good alternative method for SHM.

Simulation Method of an Expandable Lamb Wave Sensor Network for Aircraft Smart Skin

In order to realize a large-scale and lightweight Lamb wave sensor network, which is important to develop aircraft smart skin for Structural Health Monitoring (SHM), the expandable sensor network based on the island-interconnect design is a feasible way. However, the analysis of mechanical properties and expandability is the key point to design such sensor network. In this paper, the simulation method of thin island-interconnect structures with arbitrary shape is proposed based on ABAQUS. It includes two key parts. The first is buckling mode analysis to extract buckling modes and corresponding eigenvalues of the structure. The second is postbuckling behavior analysis to analyze mechanical properties of the structure under large tensile deformation, based on a certain buckling mode. The simulation method is not limited by the manufacturing process and can be combined with micro- or nano-manufacturing process or conventional Flexible Printed Circuit (FPC) process. Three simulation cases are given and analyzed, including serpentine, serpentine-based fractal and irregular island-interconnect structure. Based on the fractal island-interconnect structure, an expandable Lamb wave sensor network is designed and manufactured by the FPC process for validation of the simulation method. Results of the tensile test show good agreements with the simulation results. And the sensor network can be applied to Lamb wave-based SHM. The simulation method can provide an effective analysis tool and guidance for further design of expandable sensor networks.

Identification and Compensation Technique of Non-Uniform Temperature Field for Lamb Wave-and Multiple Sensors-Based Damage Detection.

Lamb wave-based damage detection for large-scale composites is one of the most prosperous structural health monitoring technologies for aircraft structures. However, the temperature has a significant effect on the amplitude and phase of the Lamb wave signal so that temperature compensation is always the focus problem. Especially, it is difficult to identify the damage in the aircraft structures when the temperature is not uniform. In this paper, a compensation method for Lamb wave-based damage detection within a non-uniform temperature field is proposed. Hilbert transform and Levenberg-Marquardt optimization algorithm are developed to extract the amplitude and phase variation caused by the change of temperature, which is used to establish a data-driven model for reconstructing the reference signal at a certain temperature. In the temperature compensation process, the current Lamb wave signal of each exciting-sensing path under the estimated structural condition is substituted into the data-driven model to identify an interpolated initial temperature field, which is further processed by an outlier removing algorithm to eliminate the effect of damage and get the actual non-uniform temperature field. Temperature compensation can be achieved by reconstructing the reference signals within the identified non-uniform temperature field, which are used to compare with the current acquired signals for damage imaging. Both simulation and experiment were conducted to verify the feasibility and effectiveness of the proposed non-uniform temperature field identification and compensation technique for Lamb wave-based structural health monitoring.

Higher-order structural theories for transient analysis of multi-mode Lamb waves with applications to damage detection

Addressing the need for more computationally efficient and accurate simulation tools for Lamb wave-based structural health monitoring (SHM) systems, this paper proposes the use of refined theories for the study of the wave propagation phenomena in thin-walled structures. When dealing with complex structures, classical plate models can be insufficiently accurate whereas 3D finite elements soon reach their limits due to the high computational requirements in ultrasonic wave analysis. As a solution to these issues, a finite element framework based on the Carrera's unified formulation (CUF) is presented for the simulation of Lamb waves in continua. Different structural theories are discussed and tested, showing that the Lamb modes can be well represented using beam elements with enriched kinematics. The validity and convergence rates of the proposed models are assessed through benchmark solutions. An example of damage detection in a typical reinforced panel is included to show the potential of the method.

A Lamb wave signal reconstruction method for high-resolution damage imaging

In Lamb wave-based Structural Health Monitoring (SHM), a high-enough spatial resolution is highly required for Lamb wave signals to ensure the resolution and accuracy of damage detection. However, besides the dispersion characteristic, the signal spatial resolution is also largely restricted by the space duration of excitation waveforms, i.e., the Initial Spatial Resolution (ISR) for the signals before travelling. To resolve the problem of inferior signal spatial resolution of Lamb waves, a Lamb Wave Signal Reconstruction (LWSR) method is presented and applied for high-resolution damage imaging in this paper. In LWSR, not only a new linearly-dispersive signal is reconstructed from an original Lamb wave signal, but also the group velocity at the central frequency is sufficiently decreased. Then, both dispersion compensation and ISR improvement can be realized to achieve a satisfying signal spatial resolution. After the frequency domain sensing model and spatial resolution of Lamb wave signals are firstly analyzed, the basic idea and numerical realization of LWSR are discussed. Numerical simulations are also implemented to preliminarily validate LWSR. Subsequently, LWSR-based high-resolution damage imaging is developed. An experiment of adjacent multiple damage identification is finally conducted to demonstrate the efficiency of LWSR and LWSR-based imaging methods.

Design of nonlinear-Lamb-wave-based structural health monitoring systems with mitigated adhesive nonlinearity

Structural health monitoring (SHM) techniques with nonlinear Lamb waves offer the possibility of detecting incipient damages through the second harmonic generation. Particularly, the primary S0-secondary S0 Lamb wave mode pair in a weakly nonlinear plate is promising for SHM due to its appealing characteristics such as the cumulative second harmonic generation and flexible frequency selection. In a practical PZT-actuated SHM system, apart from the damage-related material nonlinearity of the plate (MNP), the adhesive nonlinearity (AN) is also proven to be a non-negligible nonlinear source, which may jeopardize damage diagnosis if not properly apprehended. By combining a nonlinear shear-lag model and the normal mode expansion method, a theoretical model is proposed in this work, which allows the investigation and the comparison of these two types of important nonlinearities in the system (AN and MNP). With the developed model, optimized nonlinear Lamb wave-based SHM systems can be designed with the mitigated influence of the undesired AN on the system. Finite element (FE) simulations are carried out to validate the proposed model and the effectiveness of the optimized systems through a tactical adjustment of the different nonlinear sources. The designed optimized system is verified by experiments, in comparison with an arbitrarily chosen one. The dominance level of the nonlinear sources in the two systems is evaluated through changing the MNP, which is realized by a thermal aging treatment. Both FE and experimental results demonstrate that the proposed model can effectively guide the design of SHM systems to mitigate AN so that the MNP can be well measured and further used as an indicator of the incipient structural damage.

Identification of damage in composite structures using Gaussian mixture model-processed Lamb waves

Composite materials have comprehensively better properties than traditional materials, and therefore have been more and more widely used, especially because of its higher strength–weight ratio. However, the damage of composite structures is usually varied and complicated. In order to ensure the security of these structures, it is necessary to monitor and distinguish the structural damage in a timely manner. Lamb wave-based structural health monitoring (SHM) has been proved to be effective in online structural damage detection and evaluation; furthermore, the characteristic parameters of the multi-mode Lamb wave varies in response to different types of damage in the composite material. This paper studies the damage identification approach for composite structures using the Lamb wave and the Gaussian mixture model (GMM). The algorithm and principle of the GMM, and the parameter estimation, is introduced. Multi-statistical characteristic parameters of the excited Lamb waves are extracted, and the parameter space with reduced dimensions is adopted by principal component analysis (PCA). The damage identification system using the GMM is then established through training. Experiments on a glass fiber-reinforced epoxy composite laminate plate are conducted to verify the feasibility of the proposed approach in terms of damage classification. The experimental results show that different types of damage can be identified according to the value of the likelihood function of the GMM.

Spectral element method for modeling Lamb wave interaction with open and closed crack

Lamb wave-based structural health monitoring is one of the most widely used damage detection techniques. For quantitatively identifying the damage, damage features that Lamb waves carry may need to be carefully studied by numerical simulation. In this paper, spectral element method (SEM) is used to simulate Lamb wave interaction with open and closed crack. Cracked spectral element models are established for open and closed cracks, respectively. Results calculated by SEM are compared with the conventional finite element method to verify the proposed model. Some simulations are conducted to study different damage features between open and closed crack models. Wave reflection and transmission ratios with different crack depths are also quantitatively analyzed. Damage features obtained are used to conduct a simple experiment to identify the location and size of the crack.

Lamb Wave-Based Structural Health Monitoring on Composite Bolted Joints under Tensile Load.

Online and offline monitoring of composite bolted joints under tensile load were investigated using piezoelectric transducers. The relationships between Lamb wave signals, pre-tightening force, the applied tensile load, as well as the failure modes were investigated. Results indicated that S0/A0 wave amplitudes decrease with the increasing of load. Relationships between damage features and S0/A0 mode were built based on the finite element (FE) simulation and experimental results. The possibility of application of Lamb wave-based structure health monitoring in bolted joint-like composite structures was thus achieved.

Multimodal sparse reconstruction in guided wave imaging of defects in plates

A multimodal sparse reconstruction approach is proposed for localizing defects in thin plates in Lamb wave-based structural health monitoring. The proposed approach exploits both the sparsity of the defects and the multimodal nature of Lamb wave propagation in plates. It takes into account the variation of the defects’ aspect angles across the various transducer pairs. At low operating frequencies, only the fundamental symmetric and antisymmetric Lamb modes emanate from a transmitting transducer. Asymmetric defects scatter these modes and spawn additional converted fundamental modes. Propagation models are developed for each of these scattered and spawned modes arriving at the various receiving transducers. This enables the construction of modal dictionary matrices spanning a two-dimensional array of pixels representing potential defect locations in the region of interest. Reconstruction of the region of interest is achieved by inverting the resulting linear model using the group sparsity constraint, where the groups extend across the various transducer pairs and the different modes. The effectiveness of the proposed approach is established with finite-element scattering simulations of the fundamental Lamb wave modes by crack-like defects in a plate. The approach is subsequently validated with experimental results obtained from an aluminum plate with asymmetric defects.

Deconvolution processing for improved acoustic wavefield imaging

Sparse sensor networks for Lamb wave-based structural health monitoring (SHM) can detect defects in plate-like structures. However, the limited number of sensor positions provides little information to characterize the unknown scatterer. This can be achieved by full wavefield analysis e.g. using Laser Doppler vibrometry measurements.This paper proposes deconvolution processing that enhances the acoustic wavefield interpretation by increasing the temporal resolution of the underlying ultrasound signals. Applying this preprocessor to the whole wavefield allows improved non-destructive assessment of the defect. This approach is verified experimentally through a case study on an isotropic aluminum plate with four cracks.