Piezoelectric Wafer Transducers Research Articles

The effect of temperature on guided wave signal characteristics in presence of disbond and delamination for health monitoring of a honeycomb composite sandwich structure with built-in PZT network

Guided wave (GW) based structural health monitoring (SHM) techniques being developed by researchers frequently use amplitude and group velocity variations between healthy and damage-affected GW modes to detect and localise damage. Nonetheless, external variables such as temperature and moisture influence these features, which were not considered in previous studies, particularly in the presence of damage in honeycomb composite sandwich structures (HCSSs). Therefore, a coordinated numerical and experimental study was carried out in an effort to examine the characteristics of GW propagation in an HCSS for two damages: a disbond between the face sheet and the core, and delamination between the face sheet layers for a temperature range of 0 ∘C–90 ∘C. Computationally efficient two-dimensional numerical models were developed using COMSOL Multiphysics that takes into account a variety of temperature-related phenomena, such as thermal stresses and changes in the material properties of honeycomb sandwich and piezoelectric wafer transducers (PZTs). The amplitude and group velocity of the fundamental anti-symmetric (A0) mode are found to increase in the presence of a disbond and decrease in the presence of face sheet delamination. However, it is observed that there is a linear decrease in the amplitude of A0 mode for both the healthy and damaged cases with an increase in temperature. Since the A0 mode is widely employed for interrogation due to its defect sensitivity, an amplitude and group velocity adjustment equation with temperature change is proposed. Finally, considering the amplitude difference of normalised A0 mode, the two damages are localised within a network of PZTs by using a probability-based signal difference coefficient method, which is found to be efficient and reliable for SHM of HCSS under variable temperature conditions.

Detection and Quantification of Asymmetrically Located Structural Damages by Mode Converted Guided Waves Using Piezo Electric Elements

Though damage identification using guided waves generated using ultrasonics is well proven, its usage for structural health monitoring poses difficulties. Piezo electric actuation and sensing overcomes this difficulty to some extent. In this work, usage of such guided waves for damage identification is investigated. Piezo electric wafer transducers are used for generating and sensing the guided waves. Presence of multiple modes and comparatively higher speeds of the guided waves throw up difficulties in damage identification. It is shown here that this problem can be addressed by considering different sensor location with respect to the damage with suitable interpretation of the results. Usage of fundamental antisymmetric (Ao) mode is found to be more suitable in localizing the damage compared to the fundamental symmetric (So) mode. Asymmetrically located damage causes mode conversion. It is demonstrated in this work that the mode converted guided wave (So) could be advantageously used for identification, localization, and quantification of the damage. Damage identification and localization schemes are evolved based on the location of the sensors with respect to the damage. It is shown that the reduction in the magnitude of the mode converted wave can be utilized for assessing the depth of the damage. 3D finite element based numerical models incorporating a PZT sensor are developed and validated with experimental results in terms of the characteristics of the waves, mode conversion due to damage and influence of the defect size on the received signals which are necessary for quantification of the damage.

Real-time monitoring of residual strength in corroding steel reinforcement using ultrasonic-guided waves and multi-physics modelling

Corrosion-induced material strength degradation is a major threat to the service life of reinforced concrete (RC) structures. Continued mass loss and pit formation after the initiation of corrosion in reinforcing steel consequently results in strength degradation. In this study, a non-destructive structural health monitoring (SHM) approach using ultrasonic guided waves with multi-physics modelling to monitor and forecast strength degradation of corroding steel is presented. Surface-bonded piezoelectric wafer transducers (PWTs) are used for actuating and sensing of guided waves in order to correlate the group velocity and amplitude of flexural wave modes with the strength reduction. The major findings of the work are presented in the form of empirical relationships between guided wave characteristics with reduced yield, ultimate, and buckling strength of corroded steel. It is observed that for a rebar of 12 mm diameter having slenderness ratio between 8 and 10, a 10% increase in amplitude and 8.44% increase in group velocity of flexural mode for corroding steel indicates a corresponding reduction of 10.278% in yield strength, 10.277% in ultimate strength, and 4.45% in buckling strength in progression phase of corrosion. A multi-physics numerical model of corrosion in RC structures that considers the effect of local exposure and environment is then elucidated with deviation less than 6.34% from experimental results in order to forecast the mass loss. The presented data fusion strategy, which combines guided wave-based monitoring and multi-physics modelling finds valuable application in assessing the current state of degradation in structural performance and strength-based service life prediction.

High-Precision Probabilistic Imaging for Interface Debonding Monitoring Based on Electromechanical Impedance

Solid rocket motors (SRMs) are widely used in the aerospace and missile industries. The interface debonding between propellant/insulator/case determines the life of an SRM. Therefore, monitoring and further locating interface debonding are crucial to prevent catastrophic structural failures of an SRM. The electromechanical impedance (EMI) technique based on piezoelectric wafer transducers is a simple and sensitive structural health monitoring (SHM) method. In this study, a high-precision probabilistic imaging method of EMI for the debonding monitoring of the SRM case–insulator interface is proposed. In this method, the Gaussian window function is used to describe the variation of debonding damage index with the position. In addition, an experimental procedure based on simulated damage is presented to determine the parameter in the Gaussian window function. Finite element models of the steel case–insulator structure based on Rayleigh damping are established to analyze the effects of debonding and simulated damage on EMI signals. An experiment is conducted to verify the proposed probabilistic imaging method and the finite element analysis results. It is proved that the proposed probability method can achieve high-precision imaging of debonding of SRM case–insulator, even after several months of storage.

Lamb wave-based real-time damage location system with integrated internet of things

In this paper, an automated Structural Health Monitoring (SHM) system that uses Lamb waves is presented. A miniaturized prototype named ‘Real-Time Damage Location System’ (RTDLS) is designed and fabricated for thin plate structure, which can automatically locate single damage in an area of 100 mm × 100 mm by manually pressing a tactile button or by using an Internet of Things (IoT) platform. RTDLS is a lightweight, mobile, and plug-and-play device that uses 3 piezoelectric wafer transducers. It uses the pitch-catch technique with a novel damage location algorithm to locate damage in a plate. It displays the location of the identified damage on an LCD screen and on the webpage through live broadcasts using an IoT platform. RTDLS can store Lamb wave data corresponding to the pristine and damaged states of the structure in a FLASH memory so that no data is erased until reset action is performed.

Immunity of the second harmonic shear horizontal waves to adhesive nonlinearity for breathing crack detection

High-order harmonic guided waves are sensitive to micro-scale damage in thin-walled structures, thus, conducive to its early detection. In typical autonomous structural health monitoring (SHM) systems activated by surface-bonded piezoelectric wafer transducers, adhesive nonlinearity (AN) is a non-negligible adverse nonlinear source that can overwhelm the damage-induced nonlinear signals and jeopardize the diagnosis if not adequately mitigated. This paper first establishes that the second harmonic shear horizontal (second SH) waves are immune to AN while exhibiting strong sensitivity to cracks in a plate. Capitalizing on this feature, the feasibility of using second SH waves for crack detection is investigated. Finite element (FE) simulations are conducted to shed light on the physical mechanism governing the second SH wave generation and their interaction with the contact acoustic nonlinearity (CAN). Theoretical and numerical results are validated by experiments in which the level of the AN is tactically adjusted. Results show that the commonly used second harmonic S0 (second S0) mode Lamb waves are prone to AN variation. By contrast, the second SH0 waves show high robustness to the same degree of AN changes while preserving a reasonable sensitivity to breathing cracks, demonstrating their superiority for SHM applications.

A machine learning framework for guided wave-based damage detection of rail head using surface-bonded piezo-electric wafer transducers

Due to repeated heavy loads, environmental conditions and non-frequent monitoring, the rail is subjected to heavy damage resulting in sudden failure. Hence, a frequent, faster, and efficient monitoring strategy is required. This paper attempts to investigate the application of guided wave (GW) generated through surface-bonded piezo-electric wafer transducer (PWT) to detect damages in rail at high frequencies. Firstly, a combined experimental and simulation study is presented in an effort to understand the dispersion characteristics of guided wave and its interaction with head damages in a relatively small rail specimen. The numerical simulation results are validated with those obtained from the experiments showing a good agreement between them. Secondly, a framework based on the machine learning algorithm is proposed to efficiently detect damage in rail head. Numerous inseparable guided wave modes are observed at higher frequencies implying the inability to detect damage through specific mode. Therefore, a machine learning framework is trained using time, frequency, and time–frequency domain features of the signal. Total 672 numerical simulations of different types of damage with different severity and location in the rail head are carried out to train and validate the model. It is found that GW generated through surface bonded PWTs is able to detect minimum defect size of 5% of head area with 1 mm thickness. Finally, the proposed framework is tested using simulation and experiment results of arbitrary damage in the rail head. The error in estimating severity was found to be in the range from 2.00% to 16.67%.

Explainable 1-D convolutional neural network for damage detection using Lamb wave

Recently researchers are exploring the data-driven machine learning techniques to identify damage from Lamb wave response. This is primarily because the physics based model are often difficult to implement for complicated aerospace structures. In this study, one-dimensional convolutional neural network (1D CNN) is used for automated damage detection using Lamb wave response of a thin aluminium plate. An attempt is made to interpret the 1D CNN model in terms of damage feature contributions using Local Interpretable Model-Agnostic Explanations (LIME) and find that the interpretation corresponds to insightful damage signature. The database consists of finite element (FE) simulated data (180 cases) along with experimental data (48 cases) which is divided into Training, Testing, and Validation datasets. Each of the training and validation datasets contains FE simulated data with the major part used for training, whereas, experimental data are used for testing. Here, FE simulation responses are only used as a surrogate of experimental results. FE model is not used in the damage detection process rendering the technique to be purely data-driven. Finite element simulations of Lamb wave response are obtained using commercial package (ABAQUS) for different frequencies (100 KHz, 125 KHz, 150 KHz). The experimental data was generated for a 1.6 mm thick aluminium plate using piezoelectric wafer transducers. The 1D CNN architecture gave around 100% accuracy. Using the significant features affecting classification which is obtained from LIME, damage localization was achieved with around 7% deviation from actual damage localization.

Role of transducer inertia in generation, sensing, and time-reversal process of Lamb waves in thin plates with surface-bonded piezoelectric transducers

An analytical model is presented for the generation, sensing, and time-reversible process of Lamb waves in thin isotropic plates with surface-bonded piezoelectric wafer transducers, incorporating the shear-lag effect of the bonding layer and inertia effects of the system in transducer modeling. A one-dimensional dynamic shear-lag model for the actuator-plate interaction is used to obtain the shear stress distribution at the actuator-plate interface. The Lamb wave solution for the plate under this shear traction excitation is obtained using the two-dimensional (2D) elasticity equations. A consistent sensor-plate interaction model incorporating the shear-lag and inertia effects is developed to determine the induced sensor voltage from the Lamb strain at the plate surface. The model is validated by comparing it with the 2D coupled piezoelasticity-based finite element simulation and experimental data. Detailed parametric studies are conducted to illustrate the effect of inclusion of inertia of actuator, sensor, adhesive, and plate in the transducer modeling on the Lamb wave generation, sensing, time reversibility, and the system’s best reconstruction frequency, and to ascertain how various geometrical and material parameters of the system influence the same. The developed closed-form solution will be immensely useful for the design of Lamb wave based structural health monitoring systems.

Time reversibility of Lamb waves in thin plates with surface-bonded piezoelectric transducers is temperature invariant at the best reconstruction frequency

The Lamb wave time-reversal method has been widely proposed as a baseline-free method for damage detection in thin-walled structures. Under varying thermal environments, it would require that the time reversibility of Lamb waves is temperature invariant. In this study, we examine the temperature dependence of Lamb waves and its time reversibility using experiments and finite element simulations on isotropic plates with surface-bonded piezoelectric wafer transducers for actuation and sensing. The study is conducted at three different temperatures of the system from 25°C to 75°C for a wide range of excitation frequency. The results indicate that the time reversibility can undergo significant changes due to temperature variations depending on the excitation frequency. However, at the best reconstruction frequency corresponding to the maximum similarity of the reconstructed signal with the original input signal (proposed recently as the probing frequency), the change in the percent similarity with temperature is insignificant. The results also demonstrate that changes in the physical properties of both adhesive layers and piezoelectric transducers with temperature play a dominant role in influencing Lamb wave amplitudes. However, only the change in the characteristics of the adhesive layers is responsible for the temperature dependence of the time reversibility of Lamb waves.

The use of ultrasonic guided waves for the inspection of square tube structures: Dispersion analysis and numerical and experimental studies

Square steel tubes have been widely used in buildings and machines in civil engineering. The inspection of square tubes is becoming increasingly urgent and important to ensure the safety of these buildings and machines. However, the current most frequently used traditional ultrasonic inspection method is time-consuming and inefficient when dealing with long square tubes. There is an urgent need to develop an efficient approach to inspect square tubes. In this article, the use of ultrasonic guided waves is proposed. Phase and group velocity dispersion curves of square tube structures are first derived using the semi-analytical finite element method. An appropriate guided wave mode used for inspecting square tubes is selected. Ultrasonic guided waves propagating in normal, in-plane surface-damaged, and edge-damaged square tubes are numerically studied. It is illustrated that the monitoring points are able to receive reflected wave signals from both the in-plane surface and the edge damages. Experimental studies are also conducted to study ultrasonic guided waves interacting with circular through-hole damages located in surfaces and slot damages at edges. It is shown that both the circular through-hole damages located in different surfaces and slot damages at different edges can be clearly detected by reflected guided wave packets. It is found that the signal-to-noise ratios have been significantly improved after applying impedance matching to piezoelectric wafer transducers. The results have shown that ultrasonic guided waves are a promising and effective method for the inspection of square tubes.

Some new algorithms for locating a damage in thin plates using lamb waves

The aeroplane structures are aerodynamic complex shapes with multiple stiffeners attached to them. The presence of stiffeners poses complexity in the nondestructive evaluation of the aeroplane structures using guided (Lamb) waves. Thus, in the present study, this issue is addressed and resolved by carrying out experiments on a thin Aluminium plate with a stiffener using Lamb waves. In the present work, there are four piezoelectric wafer transducers, which are bonded onto the plate. These transducers have functions of generating and sensing Lamb waves in the plate. Authors have developed, two different algorithms for finding location of a damage in the plate by processing the data obtained from the experiments. The algorithms are named as Curve-Intersection and Sub-Quadrant methods based on their working principles. Thus, presented algorithms are capable of finding location of a damage in thin plate structures in the presence of stiffener using a minimal number of transducers.

Non-Destructive Damage Assessment of Five Layers Fiber Glass / Polyester Composite Materials Laminated Plate by Using Lamb Waves Simulation

Composite materials are widely used in the engineered assets as aerospace structures, marine and air navigation owing to their high strength/weight ratios. Detection and identification of damage in the composite structures are considered as an important part of monitoring and repairing of structural systems during the service to avoid instantaneous failure. Effective cost and reliability are essential during the process of detecting. The Lamb wave method is an effective and sensitive technique to tiny damage and can be applied for structural health monitoring using low energy sensors; it can provide good information about the condition of the structure during its operation by analyzing the propagation of the wave in the plate. This paper presented the results and conclusions of a theoretical, numerical and experimental study of this method which was used for damage detection in the five layers fiberglass/polyester composite laminated plate, the fiber layer type is plain woven (0o/90o). The numerical analysis has been carried out by FEM using ABAQUS software for intact and cracked test specimen with the various boundary conditions and excitation frequencies, the results obtained have been confirmed experimentally by using piezoelectric wafer transducers (PZT) as actuator and sensor for both cases of experiments, it has been noted as a good agreement between experimental and numerical results.

Localization of Damages in Plain And Riveted Aluminium Specimens using Lamb Waves

The Lamb wave-based localization of damage is presented here separately for the plain and riveted aluminium (Al) specimens. The first part of this paper deals with the experimental damage localization of the plain Al specimen using Lamb waves and four piezoelectric wafer (PW) transducers. The PW transducers mounted onto the specimen in a collocated way are used to actuate and sense Lamb waves. The responses are obtained for both the pristine and damaged states of the Al specimen. The signal processing is carried out on the residual response using the continuous wavelet transform (CWT), and time of arrival (TOA) data is obtained for each collocated actuator-sensor pair. The TOA data of the wave reflected from the damage is used in the two arrival time difference and astroid algorithms to locate the damage in an enclosed area. The genetic optimization (GO) method is used to further refine the location of damage within the enclosed area obtained using astroid algorithm. The second part of the paper deals with the localization of a faulty rivet in a riveted specimen. The responses are obtained in the cases of both healthy and faulty riveted specimens. The presence of a faulty rivet is indicated by the inflation in amplitude of the second harmonic. A new algorithm is therefore proposed by the authors to localize the faulty rivet, using the spectral content information. The results obtained through both the studies manifest the ability of the proposed methods for locating different types of defects and faulty rivets using an array of PW transducers.

Damage Localization in Composite Laminates by Building in PZT Wafer Transducers: A Comparative Study with Surface‐Bonded PZT Strategy

Lamb waves provide sensitive and effective approach for nondestructive inspection of large‐scale composite structures. Traditionally in Lamb wave‐based damage detection systems, the piezoelectric (PZT) wafer transducers are attached on the surface of composite components. However, surface‐bonded PZT wafers are fragile, and they may lose serviceability when external load happens. In this paper, numerical and theoretical investigations are firstly carried out to analyze the Lamb wave features generated by PZT wafers that are located in the mid‐thickness of a thin‐walled plate. Subsequently, the authors integrate the PZT wafer arrays in woven glass fiber reinforced epoxy (WGF/epoxy) laminates with stacking sequence of [90°/0°]4. It is found that the approximate single S0 Lamb wave mode is generated by the built‐in PZT wafers. This makes the received signals very simple and easy to be interpreted. Utilizing the signals obtained by the built‐in and surface‐bonded PZT wafer transmitter‐receiver arrays, damage localization algorithms that considered the slowness profiles are compiled to help locating the defects. The compared results demonstrate that the built‐in PZT strategy has great potential for quantitative non‐destructive evaluation of composite structures.

Identification of incipient pitting corrosion in reinforced concrete structures using guided waves and piezoelectric wafer transducers

Corrosion poses a great threat to ageing civil infrastructure in the world, and researchers are seeking methods to monitor the corrosion in reinforced concrete structures. Detection of corrosion at its incipient stage has been an impending task in the non-destructive testing of materials. Several non-destructive testing methods to assess the presence of corrosion exist. The limitation of the current methods is that either they require measurement at several points or they require a large network of sensors. Guided wave-based monitoring overcomes these limitations because a large area can be scanned using fewer sensors. The process of corrosion is complex, and it leads to a simultaneous reduction in the diameter and the debonding between concrete and reinforcing steel bars in reinforced concrete structures. However, some of the recent studies that explore the use of guided waves focus only on the detection of the individual effect of diameter reduction and debonding of the rebars in reinforced concrete by artificially inducing the damage. In this study, an accelerated corrosion setup is deployed to induce pitting corrosion in reinforced concrete beams using the impressed current method. These beams are continuously monitored using ultrasonic guided waves that are generated and received by piezoelectric wafer transducers that are attached to the rebars. It is shown that the incipient stage of pitting corrosion can be detected successfully, and the mechanism of corrosion process, which involves the corrosion initiation, progression, and diameter reduction-and-cracking phases, can be established from the signal characteristics of the longitudinal and flexural-guided wave modes. The impressed current flow in the corrosion cell also confirms the various phases of corrosion.

Identification of breathing type disbonds in stiffened panels using non-linear lamb waves and built-in circular PWT array

Abstract A non-linear Lamb wave based active sensing technique for identification of breathing type disbonds in stiffened metallic plates has been presented. In this technique, a built-in dual transmitter and multi receivers (DTMR) clock-like array of piezoelectric wafer transducers (PWT) is used. The central part of the array consists of two transmitters, which are placed concentrically on either side of the plate. A Hanning window modulated sine pulse has been used for excitation. Opposite polling is applied to the transmitters to generate a predominant anti-symmetric (A0) Lamb mode to encourage breathing in the disbond. It is observed that the contact acoustic nonlinearity (CAN) of breathing disbond generates higher harmonics when a primary Lamb wave interacts with the disbond. Since these harmonics carry damage information, a bandpass filter is used to extract the second harmonic component from the received signals followed by wavelet transformation. Finally, the transformed signals are used to device a baseline free damage detection algorithm for locating the disbond. Experimental investigations are carried out to interrogate a full width disbond that is located either at the middle of the T-stiffener or at an offset from its center. The damage index (DI) map obtained for the entire plate show higher values at the disbond location. Once the disbond is localized, a damage quantity (DQ) curve is devised to determine the extent of disbond with a high degree of accuracy. Extensive numerical simulations are carried out using commercially available finite element package, ANSYS 14.0, in an effort to examine the effectiveness of the damage detection algorithm and to further establish the DQ curves. The numerical results are found to be in good agreement with the experimental findings that affirm the robustness of the nonlinear active sensing technique for detection and assessment of breathing type disbond in stiffened aluminium panels without requiring the baseline signals.

Optimizing location of damage within an enclosed area defined by an algorithm based on the Lamb wave response data

In the present study, experiments are carried out on an Aluminium (Al) plate with and without damage, using Lamb waves and an array of four Piezoelectric Wafer (PW) transducers bonded onto it. The PW transducers are used to actuate and sense Lamb waves. These transducers are very compact, cheap, and can be easily bonded onto the structure. The responses recorded from the experiments, are further analyzed using a new algorithm developed by the authors, in order to localize the damage. The damage location is obtained as a small enclosed area formed by the intersection of various curves generated by an algorithm proposed by the authors. The damage location is further refined within this enclosed area using the optimization methods. Three different types of damages are considered in the present work. The case of multiple damage localization is also reported here. The results obtained by the algorithm are in excellent agreement with the actual location of damage. Thus, the present method has the capability of locating single as well as multiple damages in plate structures using minimal number of transducers.

Identification of Zero Effect State in Corroded RCC Structures Using Guided Waves and Embedded Piezoelectric Wafer Transducers (PWT)

Reinforced cement concrete (RCC) structures are prone to damages such as corrosion of reinforcement, surface cracking, debonding and the like. The corrosion process results in two effects, namely, reduction in the diameter of rebar and debonding at the steel/concrete interface. Several researchers have worked to establish a correlation between intrinsic damage features and guide wave signal characteristics. However, the work till date is focused mainly on identifying the individual damage phenomenon, and few researchers have attempted to correlate the combined effect of these damage parameters. Therefore, it is an impending task to study the combined influence of corrosion and debonding on wave characteristics and to establish methods to distinguish and correctly quantify the damage level. A three-dimensional finite element model of a RCC beam is developed in the Abaqus software by using an explicit method in order to study wave propagation in an RCC beam after including the two effects of corrosion. The implicit and explicit models are co-executed using standard-explicit co-simulation. It is observed that the reduction in diameter would result in the diminishing of the guided wave amplitude, whereas debonding results in an increase in the guided wave amplitude. It is found that these two complimentary effects on wave features result in a signal that has the same wave features as undamaged rebar that is surrounded by concrete, which could be termed as the zero effect state.

Ultrasonic guided wave propagation and disbond identification in a honeycomb composite sandwich structure using bonded piezoelectric wafer transducers

A coordinated theoretical, numerical, and experimental study is carried out in an effort to understand ultrasonic guided wave propagation and interaction with disbond, as well as, to identify disbond in a honeycomb composite sandwich structure using surface-bonded piezoelectric wafer transducers. In contrast to most of the work done previously, a fast and efficient two-dimensional semi-analytical model based on global matrix method is used to study dispersion characteristics as well as transient response of the healthy honeycomb composite sandwich structure subjected to relatively high-frequency piezoelectric wafer transducer excitations. Numerical simulations are then conducted using commercially available finite element package, ABAQUS, in order to explore guided wave propagation mechanisms due to the presence of disbond. Numerical simulations are further broadened to investigate the effect of disbond size on the amplitudes and group velocities of propagating guided wave modes. A good agreement is observed between the theoretical, numerical and experimental results in all cases studied. It is noticed that the presence of disbond, in particular, amplifies the first anti-symmetric (A0) mode and increases its group velocity. Finally, based on these modal behaviors, the location of an unknown disbond, within the piezoelectric wafer transducer array is experimentally determined by applying a probability-based damage detection algorithm.