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Research on Delamination Damage Localization of Carbon Fiber-Reinforced Polymer Curved Plate Using Lamb Wave

Carbon fiber-reinforced polymers (CFRPs) are extensively employed in the aerospace industry due to their excellent properties. Delamination damage occurring at critical locations in CFRPs can seriously reduce the safety of in-service components. The detection and localization of delamination damage using Lamb waves hold significant potential for widespread application in non-destructive testing. However, the choice of damage localization algorithm may produce different delamination damage localization results. This research presented an IRAPID (improved reconstruction algorithm for probabilistic inspection of defects) method derived from the RAPID (reconstruction algorithm for probabilistic inspection of defects) method, aiming to improve the accuracy and reliability of delamination damage localization. Three CFRP curved plates, including a healthy curved plate and two curved plates with delamination damage sizes of Φ20 mm and Φ40 mm, were prepared in the experiment. The detection experiment of the CFRP curved plate using lead zirconate titanate (PZT) as a transducer to excite and receive Lamb waves was conducted, and the influence of excitation signal frequency on the performance of the proposed method was discussed. Under the condition of an excitation signal frequency of 220~320 kHz and a step size of 10 kHz, the accuracy of the delamination damage localization method proposed in this paper was compared with that of existing methods. The experimental results indicate that the IRAPID algorithm exhibits good stability in the localization of delamination damage across the range of frequency variations considered. The localization error of the IRAPID algorithm for delamination damage is significantly lower than that of the DaS (delay-and-sum) algorithm and the RAPID algorithm. As the size of the delamination damage increases, so does the localization error. The accuracy of delamination damage localization is lower in the X-axis direction than in the Y-axis direction. By averaging the localization results across various frequencies, we can mitigate the potential localization errors associated with single-frequency detection to a certain extent. For the localization of delamination damage, Lamb waves at multiple frequencies can be employed for detection, and the detection results at each frequency are averaged to enhance the reliability of localization.

A focusing MUSIC algorithm for baseline-free Lamb wave damage localization

The dispersion effect of Lamb waves can cause different degree of phase distortion of array signals, which will make the classical multiple signal classification (MUSIC) algorithm no longer valid in Lamb wave based damage localization. At the same time, baseline subtraction and estimation of the number of scattering sources are required in the classical MUSIC algorithm, which limits its application. A focusing MUSIC algorithm is proposed for baseline-free Lamb wave damage localization on isotropic materials. Based on the virtual time reversal technique, the dispersive wave packets adaptively back propagate according to their respective propagation distances so that the dispersion can be completely compensated and focusing signals are obtained. Wave packet truncation is implemented on the focusing signals and then let the truncated signals forward propagate with the group velocity at center frequency. The two operations, back propagation and forward propagation, can compensate the dispersion effect and avoid estimating of the number of scattering sources as well as baseline subtraction. Experiments are conducted on an aluminum plate to investigate the performance of the proposed algorithm. A uniform linear PZT array is glued on the aluminum plate to transmit and receive Lamb waves. Experimental results of the proposed algorithm as well as the classical MUSIC algorithm are given and compared. The comparison verifies the effectiveness of the proposed focusing MUSIC algorithm.

Lamb wave inspection using piezoelectric diaphragms: An initial feasibility study

Lamb wave-based inspection has been extensively investigated and reported in the literature as a non-destructive technique in structural health monitoring and other applications. The generation and reception of Lamb waves are typically performed with piezoelectric ceramics operating as actuators and sensors, respectively. This article presents a study of the feasibility of piezoelectric diaphragms as an alternative transducer, allowing to expand the research as well as to accelerate the implementation of Lamb wave-based SHM systems with lower installation costs. Piezoelectric diaphragms are sound components widely used in everyday electronic devices and, in addition to the low cost, they have the advantage of being mass-produced by different manufacturers in standard sizes and are readily available. To assess the feasibility of these components, basic analysis by finite element method (FEM) and experimental tests were carried out on aluminum plates and the results obtained with the diaphragms were compared with those obtained using conventional piezoelectric ceramics. Structural damage was simulated by making cracks and drilling holes in the structure and an imaging algorithm together with damage index based on the wavelet transform was used to detect and locate damage. In addition, tests were carried out to evaluate and compare the reproducibility of the results and the effects of temperature variation on the two types of transducers. FEM and experimental results show that piezoelectric diaphragms are feasible for generating and receiving Lamb waves, as well as detecting structural damage. Therefore, these commercial sound components can also be alternative transducers for applications involving Lamb waves.

Extrapolation of AR models using cubic splines for damage progression evaluation in composite structures

This paper presents the potentiality of the use of extrapolation of a set of Auto-Regressive (AR) models to inspect a future damage sensitive indices based on changes in one-step-ahead prediction errors. The key idea is to use multiple AR models to assess a data-driven model to represent and predict the time-series outputs of the PZT sensors receiving Lamb waves in a composite coupon. Based on some simplified assumptions, after detecting initial damage using some previous classifier, its progression evaluation by interpolating the AR parameters is proposed and examined based on cubic spline functions. After, an extrapolated AR model using this information may verify the future state and to inspect how the damage could progress. An aeronautical composite panel with bonded piezoelectric elements that act both as sensors and actuators is utilized to examine the relationship between the variation of the identified model parameters with various levels of simulated damage. The results have shown a smooth and adequate correlation between the estimates obtained by the extrapolated model and the actual progress of the damage observed. The significant advantage of the proposed procedure is implementing this task without adopting a complicated and costly mathematical-physical model.

Laser ultrasonic visualization technique using a fiber-optic Bragg grating ultrasonic sensor with an improved adhesion configuration

In this research, we attempt to establish a reliable structural health monitoring technique for composite materials by combining phase-shifted fiber-optic Bragg grating sensing with the laser ultrasonic visualization technology. In the first part of this article, a novel cross-adhesion configuration is designed to resolve the directionality problem of the phase-shifted fiber-optic Bragg grating ultrasonic sensing. In the adhesion configuration, Lamb waves are guided by an orthogonally bonded optical fiber from the adhesion point to the phase-shifted fiber-optic Bragg grating sensor. The analysis of the ultrasonic measurement results reveals that the proposed adhesion method enables us to use one sensor to receive Lamb waves in all in-plane directions with similar magnitude because two wave components propagating along with the two orthogonal directions are guided to the phase-shifted fiber-optic Bragg grating sensor and exhibit a linear superposition in the sensor. This simplified configuration gives our method an advantage over the existing approaches, such as the rosette configuration in which three or more phase-shifted fiber-optic Bragg grating sensors are required to relieve the sensing directionality. The phase-shifted fiber-optic Bragg grating ultrasonic sensor with the proposed adhesion configuration is then applied to visualize the propagation of ultrasonic waves in aluminum plates and carbon fiber–reinforced plastic laminates. Those verification experiments also show us that the new adhesion configuration is effective at protecting the phase-shifted fiber-optic Bragg grating ultrasonic measurement from the sensing directionality. Meanwhile, the broad bandwidth of the phase-shifted fiber-optic Bragg grating sensor enables us to visualize the propagation behavior of various Lamb wave modes over a broad frequency range. Finally, we also validate that the ultrasonic visualization technique merged with the phase-shifted fiber-optic Bragg grating ultrasonic sensing can be used to identify the hidden damage in the carbon fiber–reinforced plastic composite.

Fatigue-Crack Detection and Monitoring through the Scattered-Wave Two-Dimensional Cross-Correlation Imaging Method Using Piezoelectric Transducers.

Piezoelectric transducers are convenient enablers for generating and receiving Lamb waves for damage detection. Fatigue cracks are one of the most common causes for the failure of metallic structures. Increasing emphasis on the integrity of critical structures creates an urgent need to monitor structures and to detect cracks at an early stage to prevent catastrophic failures. This paper presents a two-dimensional (2D) cross-correlation imaging technique that can not only detect a fatigue crack but can also precisely image the fatigue cracks in metallic structures. The imaging method was based on the cross-correlation algorithm that uses incident waves and the crack-scattered waves of all directions to generate the crack image. Fatigue testing for crack generation was then conducted in both an aluminum plate and a stainless-steel plate. Piezoelectric wafer transducer was used to actuate the interrogating Lamb wave. To obtain the scattered waves as well as the incident waves, a scanning laser Doppler vibrometer was adopted for acquiring time-space multidimensional wavefield, followed with frequency-wavenumber processing. The proof-of-concept study was conducted in an aluminum plate with a hairline fatigue crack. A frequency-wavenumber filtering method was used to obtain the incident wave and the scattered wave wavefields for the cross-correlation imaging. After this, the imaging method was applied to evaluate cracks on a stainless-steel plate generated during fatigue loading tests. The presented imaging method showed successful inspection and quantification results of the crack and its growth.

An IoT based building health monitoring system supported by cloud

In this paper, we have proposed a smart building health monitoring system to avoid unwanted mishaps. Some accidents take place with no warning. The work in this paper presents an internet of things (IoT) based building health monitoring (BHM) system. The system comprises Piezoelectric sensors, Arduino and ESP8266 Wi-Fi module. Piezoelectric sensor (PZT) is used to generate and receive Lamb waves and the received wave is further analyzed to determine the health of the concrete structure. The approach is unique and uses cloud and mobile app-based monitoring. There are two important features of the suggested system; it monitors the damage in concrete structure and uploads the data to the cloud server. The exclusive technique takes the help of cloud sever system for its favorable position of data usability, ease of access and disaster recuperation. This proposed system generates all details associated with structural health of a specific building and can inform to the responsible authority for corrective measures. So that it can avoid mishaps and save lives, money, and properties.

Comparative Study of Coupling Techniques in Lamb Wave Testing of Metallic and Cementitious Plates.

Lamb waves have emerged as a valuable tool to examine long plate-like structures in a faster way compared to conventional bulk wave techniques, which make them attractive in non-destructive testing. However, they present a multimodal and dispersive nature, which hinders signal identification. Oblique incidence is one of the most known methods to generate and receive Lamb waves and it is applied in different experimental arrangements with different types of sensors. In this work, several setups were conducted and compared to determine the optimal ones to launch and detect ultrasonic Lamb waves, especially in non-homogeneous specimens. The chosen arrangements were contact with angle beam transducers, immersion in a water tank, localised water coupling using conical containers and air coupling. Plates of two different materials were used, stainless steel and Portland cement mortar. Theoretical and experimental dispersion curves were compared to verify the existence of Lamb modes and good correspondence was achieved.

Nonlinear guided wave technique for localized damage detection in plates with surface-bonded sensors to receive Lamb waves generated by shear-horizontal wave mixing

Mutual wave interactions provide a very promising technique for nondestructive testing and evaluation due to their exceptional sensitivity to micro-scale damage growth in metallic materials. This article describes detection of localized fatigue damage in aluminum plates using polyvinylidene difluorine (PVDF) sensors to simultaneously receive shear-horizontal waves and the secondary Lamb waves that they generate. Finite element simulations explore several aspects of wave interaction and mode identification. Then laboratory experiments confirm secondary wave generation at the sum frequency in an aluminum plate using the PVDF sensor by conducting frequency-wave number domain and supporting analyses. The PVDF sensor enables computation of amplitude ratios even though the primary and secondary waves have different polarities. Finally, a simple guided wave technique based on electrically scanning the wave mixing zone around a plate is shown to detect early localized fatigue damage in an aluminum plate.

Optimal Sensor Positioning for Damage Detection in Composite Sensorised Panels

In this work the optimal configuration of transducers for damage detection and localization has been investigated. A particular interest is given to three optimization methods: mini-max, average Probability of Non Detection (POND) and ray tracing approach, coupled with genetic algorithm. After optimal configurations have been computed for each technique, they are experimentally tested and compared on a composite panel with one or two damages by generating and receiving Lamb waves signals. Damage detection is carried out with the Probability Based Damage Index Method (PBDIM). It was found that, in most cases, the ray tracing method and the average POND technique give better results, with a good detection of damages in comparison to the minimax POND technique, even if the latter seems numerically better.

Measuring the disperse curves of air-coupled lamb waves in glass fibre reinforced aluminium laminate based on time–frequency analysis

The disperse curves of lamb waves in glass fibre reinforced aluminium laminate (GLARE laminate) is the basis of GLARE laminate's ultrasonic lamb waves’ detection. In this paper, air-coupled ultrasonic transmitter–receiver system and transducers were used to generate and receive lamb waves in the GLARE laminate. Continuous wavelet transform was applied to the lamb waves' signals to obtain the characteristic information in time–frequency domain. Select the points from the contour line and draw the disperse curves. Compare the experimental disperse curves with the theoretical ones. The experimental results of the A0 mode agree well with the theoretical dispersion curve.

Delamination damage detection of laminated composite beams using air-coupled ultrasonic transducers

Air-coupled ultrasonic transducers are used to generate and receive Lamb waves in quasi-isotropic laminated composite beams for delamination detection. The influence of incident angle on the excited mode is studied. Numerical calculation and experimental results show that a pure Lamb wave mode can be generated if the transmitting transducer is oriented at a specific angle, and the receiving transducer can either be oriented to detect the same mode as that generated by the transmitter or to detect another mode generated by mode conversion at a defect. A three-dimensional finite element model is created to predict the interaction of Lamb waves with delamination, and some unique mechanisms of interaction between A0 mode Lamb waves and delamination are revealed in detail. The experimental results obtained on laminated composite beam using air-coupled ultrasonic transducers are well in accordance with finite element simulation results. Research results show that air-coupled ultrasonic guided waves can be used for delamination damage detection effectively in laminated composite beams.

Structural Health Monitoring of Thin Aluminum Plate Using Acoustic Sensors

SHM is defined as “an emerging technology that can be defined as continuous, autonomous, real time, in-service monitoring of the physical condition of a structure by means of embedded or attached sensors with minimum manual intervention” .SHM provides the ability of a system to detect adverse changes within a system’s structure to enhance reliability and reduce maintenance costs. There are different Non-Destructive techniques like acoustic emission, ultrasonic, acousto-ultrasonic, guided ultrasonic waves or Lamb waves which are nowadays investigated for the development of an efficient and user-friendly damage identification system. This paper deals with the latter which is based on Lamb wave propagation. It has been developed especially for distinguishing different kinds of damages. The Lamb wave-based active SHM method uses piezoelectric (PZT) sensors to transmit and receive Lamb waves in a thin Aluminum plate. The Lamb wave modes (AO &SO) travel into the structure and are reflected by the structural boundaries, discontinuities, and damage. By studying their propagation and reflection, the presence of defect in the structure is determined. Laboratory level experiments have been carried out on thin Aluminum plates with angular, horizontal and vertical defect. The obtained waveform is filtered to avoid unwanted noise & disturbances using Savitzky-Golay filtering. The filtered waveforms are compared to differentiate the defects. Short Time Fourier Transform has been carried out on the acquired waveform. This study provides significant insight into the problem of identifying localized damages in the structure using PZT and dispersion of signal after they interact with different types of damage. Those small defects like the horizontal one that may be nearly missed in time domain analysis can also be clearly identified in the STFT analysis. Moreover the occurrence of So mode is also clearly seen. Thus, Lamb waves generated by PZT sensors and time-frequency analysis techniques could be used effectively for damage detection in aluminum plate. This study has given a complete idea of the working and the basic requirements of SHM system.

Phase Transformation Temperature of NiTi Alloy Sheet Examined by Lamb Wave

In this study, the ultrasonic PZT transducers were used for exciting and receiving Lamb waves on NiTi alloy sheet. Lamb waves were measured when the temperature of the NiTi alloy changed. Analysis on frequency spectrums of the Lamb waves was also done. Some marked changes were observed in the dependence of the waveforms and the frequency spectrums of the Lamb waves versus temperature during phase transformation of NiTi alloy. The results show that phase transformation temperature of NiTi alloy sheet may be examined by Lamb wave method.

Damage Identification Using Lamb Waves

The concept of damage detection using sensors in composite panels is of growing interest, due to the reduction in maintenance activity and increasing of the aircraft safety. Piezoelectric materials have been widely used to build a variety of transducers for activating and receiving Lamb waves which are used for delamination/damage detection. The aim of this paper is to use smart structures for detecting and locating damage in composite panels, by using piezoelectric patches as both sensors and actuators.

Elastic constants measurement of anisotropic Olivier wood plates using air-coupled transducers generated Lamb wave and ultrasonic bulk wave

A hybrid elastic wave method is applied to determine the anisotropic constants of Olive wood specimen considered as an orthotropic solid. The method is based on the measurements of the Lamb wave velocities as well as the bulk ultrasonic wave velocities. Electrostatic, air-coupled, ultrasonic transducers are used to generate and receive Lamb waves which are sensitive to material properties. The variation of phase velocity with frequency is measured for several modes propagating parallel and normal to the fiber direction along a thin Olivier wood plates. A numerical model based mainly on an optimization method is developed; it permits to recover seven out of nine elastic constants with an uncertainty of about 15%. The remaining two elastic constants are then obtained from bulk wave measurements. The experimental Lamb phase velocities are in good agreement with the calculated dispersion curves. The evaluation of Olive wood elastic properties has been performed in the low frequency range where the Lamb length wave is large in comparison with the heterogeneity extent. Within the interval errors, the obtained elastic tensor doesn’t reveal a large deviation from a uniaxial symmetry.

A Monolithic Interdigitated PVDF Transducer for Lamb Wave Inspection

A monolithic interdigitated PVDF transducer (PVDF IDT) was built in this study. This PVDF IDT is capable of generating and receiving Lamb waves for structural health monitoring. Unlike most existing laminate-structured interdigitated PVDF transducers, this PVDF IDT features a monolithic structure, which eliminates sensor delamination as a potential problem. The flexible nature of PVDF promises the ability for the sensor to be attached to different structural surfaces. Other advantages of this PVDF IDT include low unit cost, easy installation, low power consumption, and the ability for wireless operation or remote interrogation. The fabrication of this sensor was accomplished by using etch-back photolithography technology. The interpretation of the complex Lamb wave signals was realized by using the continuous wavelet transform (CWT).

Nonlinear sensitivity of capacitive micromachined ultrasonic Lamb wave transducers.

A capacitive micromachined Lamb wave transducer using the second‐order bending mode of high‐aspect rectangular diaphragms to excite and receive Lamb waves in a silicon plate was presented [L. Ge, Proc. SPIE 6358, 1 (2006)]. Thus, the width of the diaphragm is made equal to the wavelength of the A0 wave, and all the transducer parameters can be determined by the TDK model for CMUTs with high‐aspect rectangular diaphragms. The sensitivity, i.e., a ratio of the relative frequency shift to the change in mass loading of the plate, is derived and is a nonlinear function, which is the same as that for capacitive ultrasonic mass resonators [L. Ge, IEEE IUS, 1034 (2008)]. Also, it is a general nonlinear sensitivity expression available for both capacitive and piezoelectric gravimetric sensors. The area density increment, caused by the adsorbed or bound molecules or the fluid loading, is usually comparable with the area density of the silicon plate. Therefore, it is significant to use the nonlinear expression for determining the sensitivity accurately. The capacitive Lamb wave transducers are very attractive as gravimetric sensors for chemical and biological sensing since they have much simple structure and high sensitivity. [Work supported by NSFC (60774053).]

Structural Health Monitoring with Piezoelectric Wafer Active Sensors for Space Applications

Ultrasonic guided waves inspection using Lamb waves is suitable for damage detection in metallic structures. This paper will present experimental results obtained using guided Lamb waves to detect flaws in aluminum specimens with design features applicable to space applications. Two aluminum panels were fabricated from a variable-thickness aluminum top plate, with two bolted I-beams edge stiffeners and four bonded angle stiffeners. Artificial damages were introduced in the two panels: cracks, corrosions, and disbonds. The proposed investigation methods used bonded piezoelectric wafer active sensors to excite and receive Lamb waves. Three wave propagation methods were used: pitch-catch, pulse-echo, and the embedded ultrasonic structural radar. In addition, we also used a standing-wave damage detection technique, the electromechanical impedance method. The paper will present in detail the salient results from using these methods for damage detection and structural health monitoring. Where appropriate, comparison between different methods in detecting the same damage will be performed. The results have demonstrated the ability of piezoelectric wafer active sensors working in conjunction with guided Lamb waves to detect various types of damages present in complex geometry structures typical of space applications.

OS17-2-2 Experimental characterization of Active Fiber Composites used as piezoelectric transducers for emitting and receiving Lamb waves in plate-like structures

Active Fiber Composites (AFC) consist of one layer of piezoelectric ceramic fibers embedded in an epoxy matrix and sandwiched between two sets of interdigitated electrodes. They show orthotropic mechanical properties due to their design and manufacturing. The small thickness and the conformability to curved surfaces make them suitable for applications in structural health monitoring (SHM) with acoustic nondestructive testing methods. The piezoelectric properties of the AFC allow for both actuation and sensing capabilities with the same element which enables emitting and receiving of structural waves (e.g., Lamb waves) for monitoring of critical structural elements. Before realizing specific applications for AFC in SHM systems, it is necessary to characterize the transfer behavior of AFC bonded on a structure for excitation and sensing of transient waves which propagate in the structure. To characterize this behavior experimentally, the acoustic wave field in a thin, planar structure excited from surface bonded AFC is explored with a laser interferometer. Surface velocities at different positions around an AFC bonded on an aluminum plate during transient excitation are recorded. The transfer function is calculated for different angles from the fiber direction of the AFC for both the first symmetric and the first antisymmetric Lamb wave mode. The sensing process is also investigated: structural waves generated by surface bonded piezoelectric ceramic discs are measured likewise with laser interferometry in the vicinity of the AFC and with the AFC itself. Results show, that the emitting and receiving capabilities of the AFC decrease with increasing angle from the fiber direction of the AFC. Typical maxima and minima in the transfer function depend on the length of the AFC.