Wave Structural Health Monitoring Research Articles

Special issue on ultrasonic guided waves for structural health monitoring

Special issue on ultrasonic guided waves for structural health monitoring

Guided Wave Phased Array Ultrasonic Imaging for Thin Plate Inspection

This study focuses on the application of phased array ultrasonic testing with guided waves for Structural Health Monitoring (SHM) in thin plates. Addressing the limitations of conventional ultrasonic testing efficiently by inspecting thin, long structures. This study employs phased array technology to thin isotropic plates at high frequency(2MHz), thereby elevating imaging and defect detection in structural components. Through the utilization of the full matrix capture (FMC) scanning approach in 3-D finite element (FE) simulations, A0 and S0 modes were generated on steel plates with through-hole geometries. The process involved capturing reflected signals to form an FMC matrix that includes all A-scan signals. Subsequently, the total focusing method (TFM) algorithm was applied to FMC data for the reconstruction of focused images of A0 and S0 modes separately. A key improvement in our approach is integrating a time-domain filtering method into the FMC data. This enhancement addresses the dispersive nature of guided waves, thus improving the accuracy in identifying defect locations and sizes within the images. Our finite element simulations on steel specimens, with varied through-hole locations, revealed the method's potential in accurately imaging defects located deeper within the structure. Additionally, the study expanded to evaluate the efficacy of phased array ultrasonic testing (PAUT) in SHM, particularly in conjunction with guided waves to study the flaw detection sensitivity of the defect through the TFM algorithm on isotropic steel plates up to wavelength(λ)/16. This whole analysis considered crucial factors such as ultrasonic wave mode, frequency, and group velocity for the structure under inspection. This study represents a significant contribution to the field of SHM, introducing a sophisticated, real-time monitoring methodology that promises to enhance structural integrity assessment. Keywords: SHM, PAUT, GW, FMC, TFM FE Simulations, Time Domain Filtering, Ultrasonic Testing.

A transfer learning approach for data-driven localization of damage areas in plate-like structures of CFRP materials

The application of machine learning techniques based on Lamb waves for structural health monitoring of composite material damage has been widely adopted. However, this method often struggles to acquire sufficient labeled historical Lamb wave signals for training machine learning models. This issue can be addressed through the implementation of transfer learning. This paper proposes a data-driven transfer learning model for locating coordinates of damage areas in plate-like structures with complex geometric features such as rivets and grooves. Based on the differences in conditional distributions in various structural health monitoring scenarios, a Cross-Component Regression for Damage Localization Model (CRDM) is introduced for damage localization across components. Due to the limited availability of labeled target data, CRDM aims to preserve global properties of the target data-dominated conditional distribution, besides considering the predictive accuracy of individual samples. Therefore, a hybrid loss function is constructed by combining Mean Squared Error (MSE) and Conditional Embedding Operator Difference (CEOD). Subsequently, the target model is fine-tuned based on this loss function. Experimental validations are conducted on finite element models and real datasets, confirming the effectiveness of the CRDM model in damage localization for plate-like structures.

Guided waves for structural health monitoring in composites: A review and implementation strategies

Structural Health Monitoring (SHM) systems provide a useful tool to diagnose any engineered structural system and eventual critical damage that may occur at any moment during the operational life. In the past two decades progress has been made in all the fields of SHM, from sensor technology to system integrated techniques. The common goal of any SHM system, whatever the specific application is, may be synthesized in three main points: identify structural deterioration stage, recognize its severity and evaluate the necessity to make a more detailed inspection or proceed to maintenance on condition, based on a potential hazard that may lead to a catastrophic failure. The implementation of such a system leads to making normal operations with the same safety levels but with more efficient maintenance procedures. In addition, it allows avoiding the oversizing of structural components extending the inspection and maintenance intervals. Both these results help to reduce the life-cycle cost of the specific engineered system as it is possible to perform maintenance when it is necessary, i.e. on-condition.The current state of the art about the guided waves (GW) based structural health monitoring of aerospace composite structures is reviewed in this paper, looking at the implementation of the methodologies proposed and assessed by the authors and giving an outlook on what has been done by the scientific community.

Application of interface guided waves for structural health monitoring of hybrid bonded joints

The application of ultrasonic interface guided waves for the inspection of adhesively bonded joints is studied. A thin adhesive layer forms a bond line between the surfaces of distinct structural elements. This work demonstrates the formation of specific ultrasonic guided waves which are propagating on the boundaries of bonded joints and can be used for health monitoring of adhesive bonds. Interface waves are extremely sensitive to changes in elastic and plastic characteristics such as density and viscosity of the adhesive layer. Using finite element simulations, the changes in propagation of wave form, attenuation of leaky interface wave and Time of Flight (TOF) were observed and recorded as a baseline. Then the pervasive effect of bonding failure on these parameters are used for damage detection in adhesive bonded joints. The cohesive damages such as dis-bond and delamination are slowing down interface wave speed depending on size and location, and the adhesive properties also change the Time of Flight of propagating wave. The results of this study show that interface waves can be used to inspect adhesively bonded joints and, possibly, to determine the strength of the bond line and predict the failure mechanisms of bonded structures.

Boundary element modelling of ultrasonic Lamb waves for structural health monitoring

In this paper, a novel boundary element plate formulation is proposed to model ultrasonic Lamb waves in both pristine and cracked plates for structural health monitoring (SHM) applications. Lamb waves are generated and sensed by piezoelectric discs. An equivalent pin-force model is newly proposed to represent the actuation effect of piezoelectric discs, which is more accurate than the classical pin-force model. The boundary element formulation is presented in the Laplace-transform domain based on plate theories, which allows three-dimensional analysis of Lamb wave behaviours, such as propagation and interaction with cracks, in thin-walled structures. A damage detection algorithm is used for crack localization alongside the BEM-simulated data. The BEM solutions show excellent agreement with both 3D finite element simulation and experimental results.

Long range guided waves for detecting holes in pipelines

ABSTRACT The use of low frequency (less than 10 kHz) guided waves for structural health monitoring (SHM) against third party intrusion into long pipelines is investigated. A guided wave (GW) is sent along a pipeline and its reflection is used to detect the presence of a drilled hole or a branch connected to the pipeline. Finite element (FE) analysis was conducted on 305 mm (12 inch) outside diameter, 12 mm wall thickness mild steel pipe. The effects of pipe diameter and thickness on the GW propagation characteristics are illustrated. It is shown that the use of low frequencies have a lower attenuation and therefore are suitable for long range propagation. It is shown that GWs have the potential to detect holes over 100s of meters of pipeline

Characterization of the Use of Low Frequency Ultrasonic Guided Waves to Detect Fouling Deposition in Pipelines.

The accumulation of fouling within a structure is a well-known and costly problem across many industries. The build-up is dependent on the environmental conditions surrounding the fouled structure. Many attempts have been made to detect fouling accumulation in critical engineering structures and to optimize the application of power ultrasonic fouling removal procedures, i.e., flow monitoring, ultrasonic guided waves and thermal imaging. In recent years, the use of ultrasonic guided waves has been identified as a promising technology to detect fouling deposition/growth. This technology also has the capability to assess structural health; an added value to the industry. The use of ultrasonic guided waves for structural health monitoring is established but fouling detection using ultrasonic guided waves is still in its infancy. The present study focuses on the characterization of fouling detection using ultrasonic guided waves. A 6.2-m long 6-inch schedule 40 carbon steel pipe has been used to study the effect of (Calcite) fouling on ultrasonic guided wave propagation within the structure. Parameters considered include frequency selection, number of cycles and dispersion at incremental fouling thickness. According to the studied conditions, a 0.5 dB/m drop in signal amplitude occurs for a fouling deposition of 1 mm. The findings demonstrate the potential to detect fouling build-up in lengthy pipes and to quantify its thickness by the reduction in amplitude found from further numerical investigation. This variable can be exploited to optimize the power ultrasonic fouling removal procedure.

Potassium sodium niobate (KNN)‐based lead‐free piezoelectric ceramic coatings on steel structure by thermal spray method

AbstractFor the first time, potassium sodium niobate (KNN)‐based lead‐free piezoelectric ceramic coating with strong piezoelectric response was fabricated on stainless steel substrates by thermal spray process, after introducing NiCrAlY and yttria‐stabilized zirconia (YSZ) intermediate layers. A large effective piezoelectric coefficient (d33) of 125 pm/V was obtained with the thermal‐sprayed KNN‐based ceramic coating on the steel substrates. The mechanisms of improving the structure and enhancing the properties of the KNN‐based piezoelectric ceramic coatings by introducing the intermediate layers were analyzed. Ultrasonic transducers were designed and fabricated from the KNN‐based coatings directly formed on a steel plate structure, and the feasibility for generation and detection of ultrasonic waves for structural health monitoring using the thermal‐sprayed lead‐free piezoelectric ceramic coating was demonstrated.

Shear-lag solution for excitation, sensing, and time reversal of Lamb waves for structural health monitoring

An analytical solution is presented for excitation, forward propagation, sensing, and time reversal of Lamb waves, considering the shear-lag effect of the bonding layer between the piezoelectric patch transducers and the plate. Consistent shear-lag solutions for actuator–plate and plate–sensor interactions are developed, considering plane strain condition. The Lamb wave solution is obtained by transforming the interfacial shear stress distribution and wave equations into the wavenumber domain by applying the spatial Fourier transform and converting the same to the physical domain using the inverse Fourier transform and residue theorem. The time-domain response for an arbitrary excitation is obtained by applying the temporal Fourier transform to the excitation and using convolution integral. The solution is compared with the experimental and numerical results for both forward and time-reversal responses and also for the best reconstruction frequency. The effect of transducer mass on the accuracy of the response is investigated. Finally, the shear-lag effect on the time reversibility of the reconstructed signal is studied for varying adhesive layer thickness, transducer thickness, host plate thickness, and the tone burst count. The solution will be useful for designing structural health monitoring systems using both baseline-based and baseline-free methods.

Multi-stage temperature compensation method for Lamb wave measurements

One of the important issues related to the applications of Lamb waves for structural health monitoring is their undesired sensitivity to variation of environmental conditions. Temperature is the main factor that can affect wave propagation and hence significantly reduce performance of a SHM system. Therefore, there is a need for development of robust monitoring methods with low sensitivity to temperature variations. This paper is aimed at verification of efficiency of four methods designed for damage detection using Lamb wave measurements performed in variable environmental conditions. The methods investigated in the comparison are the following: optimal baseline selection approach, the damage index based on a signal alignment with respect to instantaneous phase, and a group measurement approach capable of distinguishing local damage-related changes from temperature-induced global ones. The fourth method relies on fusion all these solutions simultaneously. The methods’ ability to damage detection is compared using a specimen that is subjected to large temperature changes. It is found that although all the methods have their strengths and weaknesses, a cooperation of all solutions allows for significant increase of the damage detection efficiency.

Application of soft-thresholding on the decomposed Lamb wave signals for damage detection of plate-like structures

Effective application of the Lamb waves for structural health monitoring and damage identification intensively relies on the accurate damage-related feature extraction in the received signals. Most of existing signal processing methods extract the damage-related features from the time–frequency joint spectrum which requires a quite amount of effort. In this paper, the soft-thresholding process, based on different signal decomposition methods, is introduced to damage identification so that the damage-related signal features can be manifested more distinctively. By applying two popular signal decomposition methods (i.e., the discrete wavelet transform (DWT) and the empirical mode decomposition (EMD)), the signal of interest can be represented by a series of components with different frequencies. Since most noises exist in the high frequency range, it is feasible to alleviate noise by restricting the energy of high-frequency components. Finally, a denoised signal is synthesized using the corresponding reconstruction method. As an application, the soft-thresholding process is performed to detect a small crack on an isotropic aluminum plate under the white Gaussian noise contamination. The results, from both the numerical finite element simulation and experimental test, indicate that the soft-thresholding process is capable of effectively reducing the effect of noise, convincingly improving the sensitivity of damage identification, and discriminating relatively small damage.

Scattering Phenomena of Edge Guided Waves at and around Notches

This work presents a computational investigation into the scattering of edge guided waves travelling by a notch. To establish a good understanding of this scattering phenomenon, the analysis was conductedon a range of length scales. The finite element analysis indicate that the edge guided surface waves are scattered by the presence of a notch which resulted in a SH0-like appearance wave radiating into the medium. This can be mistaken as a mode conversion of the fundamental lamb modes or even a source at notch tips. The phenomenon becomes harder to notice at higher frequency as increasing the frequency decreases the speed and both the bulk and surface waves travel at identical speeds. A clear understanding of this interaction furthers our knowledge in one of the most prominent interaction in the study of acoustic waves for structural health monitoring.

Predictive model of fatigue crack detection in thick bridge steel structures with piezoelectric wafer active sensors

This paper presents numerical and experimental results on the use of guided waves for structural health monitoring (SHM) of crack growth during a fatigue test in a thick steel plate used for civil engineering application. Numerical simulation, analytical modeling, and experimental tests are used to prove that piezoelectric wafer active sensor (PWAS) can perform active SHM using guided wave pitch-catch method and passive SHM using acoustic emission (AE). AE simulation was performed with the multi-physic FEM (MP-FEM) approach. The MP-FEM approach permits that the output variables to be expressed directly in electric terms while the two-ways electromechanical conversion is done internally in the MP-FEM formulation. The AE event was simulated as a pulse of defined duration and amplitude. The electrical signal measured at a PWAS receiver was simulated. Experimental tests were performed with PWAS transducers acting as passive receivers of AE signals. An AE source was simulated using 0.5-mm pencil lead breaks. The PWAS transducers were able to pick up AE signal with good strength. Subsequently, PWAS transducers and traditional AE transducer were applied to a 12.7-mm CT specimen subjected to accelerated fatigue testing. Active sensing in pitch catch mode on the CT specimen was applied between the PWAS transducers pairs. Damage indexes were calculated and correlated with actual crack growth. The paper finishes with conclusions and suggestions for further work.

A Method of Dispersion Compensation Based on Warped Frequency Transform

The use of Lamb waves for structural health monitoring (SHM) has complicated by its multi-mode character and dispersion effect, which impacts the damage positioning and high-resolution imaging. The group velocity dispersion curves of Lamb waves can be employed to warp the frequency axis, and then to establish warped frequency transform (WFT) to process Lamb waves. In this paper, received signals are directly compensated with warped frequency transform to suppress dispersion, and a new imaging method is proposed based on warped frequency transform. The propagation of Lamb waves in damaged aluminum plate is simulated by finite element software ABAQUS, results show that warped frequency transform can effectively compensate dispersive wave-packets, and high-resolution damage imaging can be obtained by the proposed method.

Investigation on the Design of Piezoelectric Actuator/Sensor for Damage Detection in Beam with Lamb Waves

Piezoelectric wafer type actuator/sensor is widely used to generate and sense Lamb waves for Structural Health Monitoring (SHM). However, multiple Lamb waves modes are generally excited with this type of transducer. As a result, there is some difficulty in using Lamb waves for damage detection. To selectively generate a single A0/S0 Lamb mode, the tuned excitation of Lamb waves has been studied by some researchers. This paper investigates the design of the PZT actuator/sensor bonded to beam-like structure for generating single A0/S0 Lamb mode. In the study it is found that some factors, including the bonding layer, the unknown material properties and dynamical characteristics of the beam, will influence the design of PZT actuator/sensor. Piezoelectric impedance technique is introduced to facilitate the design of PZT actuator/sensor. Crack detection in beam using the tuned A0/S0 Lamb waves is performed.

Development of Lamb Waves-Based SHM Systems

Guided waves (GW) based methods are a promising tool for structural health monitoring (SHM) of plate-like metallic and composite structures in which high safety standards are required. In this paper we present research with the aim to design and manufacture a prototype of Lamb waves (LW) SHM system. Two approaches can be applied for SHM of plate-like structures. One of them can be based on a sparse array and damage imaging involving incoherent summation of signals envelope. The second approach involves phased arrays with transducers spaced at a distance lower than half wavelength of the excited Lamb-mode. The influence of an arrays parameters on beamforming of Lamb waves is discussed in the case of linear array. It appears that an unequivocal localization of damage on a plate requires a 2D arrays topology; therefore a star-shaped active array was designed and manufactured for the developed SHM system. Two signal processing approaches were applied for that array, the standard one, based on the delay and sum (DAS) synthetic aperture focusing scheme, and the second one, using a self-focusing technique to obtain the separate images for each scatterer existing in the plate.

Fractal dimension–based Lamb wave tomography algorithm for damage detection in plate-like structures

This study presents a new signal-processing technique that utilizes the fractal proprieties of guided ultrasonic waves for structural health monitoring in complex structures. In particular, we use the fractal dimension of Lamb waves as a damage-sensitive feature. In the last decade, a fractal analysis has been extensively used in image analysis problems, especially in the medical field. In this article, a damage index based on the fractal dimension of Lamb waves is proposed using a modified box-counting algorithm. The proposed damage index has been also combined with a tomographic imaging algorithm to identify damages in plate-like structures. The performance of the proposed approach is validated through experimental tests performed on an aluminum plate instrumented with eight circular piezoelectric (lead zirconate titanate) transducers.

Redirection of Lamb Waves for Structural Health Monitoring

Currently, structures are designed without structural health monitoring (SHM) in mind. It is proposed that SHM should be addressed at the design stage of new structures. This paper explores the benefit which can be gained from such considerations. The scope encompasses Lamb-wave-based SHM and a given fatigue critical location (FCL). Optimization is performed using specialised ray tracing. A case study is carried out using a specimen that simulates a hard-to-inspect region in a fuel vent hole in wings structures of aircraft. This work will report on the potential use of the focussing of stress wave to improve detectability of defect in this hard-to-inspect location. Following optimization, results are produced numerically and experimentally. The results revealed sensitivity to damage is nearly doubled while minimum detectable damage size is significantly decreased. As a result, this study brings together an assortment of specialised tools to form a workflow ready for implementation.

Ultrasonic defect localization in pipes using time reversal.

Time reversal mirrors have been shown to effectively focus acoustic waves for structural health monitoring in thin plates. This approach achieves a high degree of accuracy, despite the complex propagation environment inherent to thin plates. We consider ultrasonic defect localization via time reversal in thin‐walled pipes. The different boundary conditions in pipes give rise to new wave modes and dispersive effects. We use an array of six transducers arranged around the circumference of a 10‐cm‐diameter steel pipe to excite guided ultrasonic wave modes, and apply iterative time reversal to focus these waves on a defect. We apply maximum likelihood to detect the presence of a defect, and then correlate the response from the defect with a library of known signatures to locate the defect. Results are provided from a simulation based on experimental data collected from a stainless steel laboratory pipe specimen 11.5 cm in diameter and 1.5 m in length. We simulated our defect with a grease‐coupled commercial t...