This research paper addresses the critical issue of identifying structural health conditions of bonded composites when subjected to varying bending modes. In response to heightened demands for eco-efficiency, the aviation sector has embraced lightweight designs, with composite materials supplanting traditional aluminum due to their remarkable tensile stiffness and reduced weight. However, the practical utilization of bonded composites introduces challenges, as non-uniform loads can lead to bending and deformation, potentially hindering failures detection capabilities within the bonded structures. This study explores the technology details for employing guided waves to assess health conditions. The damage extent is analyzed by capturing the reflected guided waves that propagate in the bending deflection of the bonded composite strip. The analytical results are also resolved aiming to compare with experiment and simulation results, which explains in more detail the propagation process of guided waves in bent materials. The Scanning Laser Doppler Vibrometry (SLDV) techniques are used to visualize the full wave fields. The research results show that the bending of the composite structure exerts a discernible impact on the guided wave's ability to accurately quantify cracks. Consequently, the indicators used to gauge faults undergo notable shifts in correspondence with these variations. Notably, employing a prognosis model trained via a sample with zero deflection, to predict the Remaining Useful Life (RUL) of a deflected sample introduces uncertainty error. This underscores the intricate relationship between deflection-induced changes and damage prediction using guided wave analysis. The deviations that occur in the process of predicting RUL serve as a pivotal reminder of the intricate nature of damage assessment in the presence of deflection, warranting comprehensive consideration in the development of predictive models based on bent specimens.
In this paper results of the damage assessment of composite panel using the guided wave propagation method are presented. Two approaches of elastic wave generation are investigated: (i) contact, piezoceramic transducer (PZT)-based and (ii) non-contact, air-coupled transducer (ACT)-based. Elastic wave sensing is based on scanning laser Doppler vibrometry (SLDV). Both methods of elastic wave generation are compared based on an analysis of elastic wave propagation and damage localization results. For this purpose wave irregularity mapping (WIM) algorithm was utilized. In this research square panels made of fibre reinforced polymer are investigated. Authors investigate artificial damage in the form of Teflon inserts. In this research low-cost and low-frequency (40 kHz) ACT is utilised. The use of the ACT-based wave generation together with the SLDV-based wave sensing give the possibility of realization of the full noncontact damage localization approach. Moreover, authors analysed the acoustic wave generation by the ACT and its propagation in the air using acousto-optic effect together with SLDV measurements.
In this paper results of damage assessment of composite panels using guided wave propagation phenomenon are presented. Elastic waves excitation is based on an air-coupled transducer (ACT) while the waves sensing is based on scanning laser Doppler vibrometry. It thus forms the full non-contact diagnostic approach. Thin panels made of fibre-reinforced polymer are investigated. The problem of optimal slope angle of ACT and the possibility of symmetric and antisymmetric elastic wave modes excitation is investigated. Research is related to the detection and localisation of artificial damage (additional mass, Teflon inserts). Moreover, the influence of single and multiple acoustic wave sources on artificial damage localization results and the problem of panel coverage area by elastic waves with large amplitudes to improve damage sensitivity are investigated. Different locations of ACTs and their influence on damage detection results are investigated. Two damage imaging algorithms based on elastic waves have been proposed, namely root mean square (RMS) energy maps and wavefield irregularity mapping (WIM). Moreover, results of simulations of elastic wave generation using ACT in CFRP plate based on a combination of FEM method and spectral element method (SEM) are presented. For this purpose COMSOL software and in-house MATLAB code are utilized, respectively
Evaluation of optimal slope angle of an air-coupled transducer (ACT) is important, to allow for effective generation of Lamb waves in solid structures. This effectiveness relies on both generation of desired wave modes, amplitude of measured signals and large coverage area for damage detection. Phenomenon of non-contact elastic waves generation in plate based on ACT is considered assuming few approaches. Numerical modeling in COMSOL is related to the analysis of different plate thicknesses and ACT excitation frequencies. Experimental research is based on wave excitation using piezoceramic ACT with a base excitation frequency of 40 kHz and SLDV measurements for a 1 mm-thick plate. A comprehensive investigation of optimal ACT slope angle for the generation of A0 mode is included. Propagation of S0 mode was not observed in numerical as well as experimental results. Optimal slope angle values were estimated in different ways numerically and experimentally with the use of dispersion curves and based on the amplitude of generated waves. Finally, the optimal angle value was determined and utilized for damage detection and localization problem in an aluminum plate.
This paper presents a natural frequency-based method for concentrated mass localization in beam-like structures. The localization of a concentrated mass is achieved by developing a novel index using relative natural frequency change (RNFC) curves and measured RNFCs. The index shows the exact location of the concentrated mass for asymmetrical beams and shows general locations of the concentrated mass including the actual location and its fake counterpart at the symmetrical position for symmetrical beams. With the general locations known, some local inspection methods can be used to identify the specific location. The effectiveness of the proposed method is confirmed by experimental investigation of a simply-supported beam and a clamped-free beam carrying a concentrated mass. In the experiment, the natural frequencies of the beam before and after adding a concentrated mass are measured by non-contact measurement using a scanning laser Doppler vibrometer.
Typically, material properties originate from destructive tensile tests and are used in computational models in the design and analysis process of structures. This approach is well-established in relation to isotropic homogeneous materials. However, if this approach is used for composite laminates, inaccuracies can arise that lead to vastly di erent stress distributions, strain rates, natural frequencies, and velocities of propagating elastic waves. In order to account for this problem, the alternative method is proposed, which utilizes Lamb wave propagation phenomenon and optimization techniques. Propagating Lamb waves are highly sensitive to changes in material parameters and are often used for structural health monitoring. In the proposed approach, the elastic constants, which are utilized to determine dispersion curves of Lamb waves, are optimized to achieve a good correlation between model predictions and experimental observations. In the first step of this concept, parametric studies have been carried out in which the influence of mass density, Young's modulus, Poisson's ratio of reinforcing fibers as well as a matrix of composite laminate and volume fraction of reinforcing fibers on dispersion curves of Lamb waves was investigated. The dispersion curves of Lamb waves were calculated by using the semi- analytical spectral element method considering the variability of properties of composite constituents. The resulting dispersion curves were also compared with experimental measurements of full wavefield data conducted by scanning laser Doppler vibrometer and processed by 3D Fourier transform. It allowed formulating fitness function which will be next used in a genetic algorithm for optimization and identification of elastic constants.
Guided waves have been extensively explored in the field of Structural Health Monitoring (SHM) and Non-Destructive Testing (NDT) over many years. Guided waves are usually excited and registered by using an array of piezoelectric transducers attached to the surface of the inspected structure. However, in some cases, piezoelectric transducers cannot be used directly on the structure and noncontact wave excitation methods are preferable. This paper is a continuation of authors’ previous research on complete non-contact NDT methods in which low-cost resonant-based ultrasonic transmitters together with scanning laser Doppler vibrometer are used. The aim of the paper is the comparison of guided wave actuation ability in a composite plate by using the single ultrasonic transmitter, flat ultrasonic transmitter array and spherical ultrasonic transmitter array. Full wavefield of propagating guided waves was registered on a fine grid of points by using laser vibrometer and processed by using the wavenumber filtering method. Finally, the accuracy of delamination localization and size was estimated for each actuation setup.
Identifying delaminations at the early stage is crucial to ensure the integrity and safety of carbon fiber reinforced polymer (CFRP) laminated plates. To precisely localize incipient delaminations relying on mode shapes, very small spatial sampling intervals matching the size of a delamination are required; however, noise components inevitably involved in densely-sampled mode shapes can cause intense noise interference, masking actual delamination-caused changes. Under this situation, developing noise-robust methods for identifying incipient small-sized delaminations in CFRP laminated plates is one of the current research interests. To this end, a noise-robust damage index is formulated in this study for damage identification in plates under noisy environments. The damage index relies on the 2D multiresolution modal Teager-Kaiser energy of measured and reconstructed mode shapes. The capability of the damage index for identifying incipient delamination is experimentally validated on a CFRP laminated plate with an incipient delamination, whose mode shapes are acquired via the non-contact measurement using a scanning laser vibrometer. The experimental result show that the damage index can effectively designate the presence and location of the incipient delamination in the CFRP laminated plate under noisy environments.
Parallel implementation of the time domain spectral element method on GPU (Graphics Processing Unit) is presented. The proposed spectral element method implementation is based on sparse matrix storage of local shape function derivatives calculated at Gauss–Lobatto–Legendre points. The algorithm utilizes two basic operations: multiplication of sparse matrix by vector and element-by-element vectors multiplication. Parallel processing is performed on the degree of freedom level. The assembly of resultant force is done by the aid of a mesh coloring algorithm. The implementation enables considerable computation speedup as well as a simulation of complex structural health monitoring systems based on anomalies of propagating Lamb waves. Hence, the complexity of various models can be tested and compared in order to be as close to reality as possible by using modern computers. A comparative example of a composite laminate modeling by using homogenization of material properties in one layer of 3D brick spectral elements with composite in which each ply is simulated by separate layer of 3D brick spectral elements is described. Consequences of application of each technique are explained. Further analysis is performed for composite laminate with delamination. In each case piezoelectric transducer as well as glue layer between actuator and host structure is modeled.
Various non-destructive testing (NDT) methods have been developed to extract information about state of a structure. Two of them: vibration-based and guided wave-based techniques are one of the most commonly used and well developed. Both approaches can be implemented using Scanning Laser Doppler Vibrometer measurements and excitation by means of piezoelectric transducer. In this paper authors present a combined approached for NDT using successive and simultaneous measurement of both mode shapes and guided waves. Vibration-based damage detection is focused on detection of mode shape singularity, created by material discontinuity. This method utilizes wavelet transform and Teager energy operator for damage indication. Guided wave-based damage detection uses propagating elastic wave energy variation on the specimen surface as well as any changes in wave propagation pattern due to its interaction with material discontinuity as a tool for structural health assessment. Combining this two different techniques can give higher accuracy in defect detection. At the same time any additional specimen preparation are necessary, any set-up changes are required and the all the data can be registered in the same amount of time (simultaneous excitation). To confirm proposed technique a honeycomb core sandwich aluminum plate with debonding is tested. A results obtained with both techniques and combined approach are presented.
The aim of this paper is to present aspects of Lamb wave propagation in randomly oriented short fiber reinforce composites with delamination. Prediction of elastic constants is based on mechanics of composites, rule of mixture and total mass balance tailored to the spectral element mesh composed of 3D brick elements. Piezoelectric excitation as well as glue layer are taken into account. Complex full wave field includes multiple reflections at short fibers. This wave pattern is also obtained by the use of laser vibrometry confirming good quality of the model. Further studies are related to symmetrical and non-symmetrical delamination in respect to the thickness of the composite plate. Square delamination of the side length 10 mm is investigated. It has been found that reflections from delamination are mostly superimposed with reflections coming from short fibers. Hence, delamination detection by direct analysis of wave propagation pattern on the surface of the plate is ineffective. However, adaptive wavenumber filtering method overcome these difficulties and enables not only to detect the delamination but also is helpful for delamination size estimation. Moreover, the method is more effective if the full wavefield measurements are acquired on the surface of the plate which is closer to the delamination.
KEYWORDS: Sensors, Damage detection, Fiber Bragg gratings, Modal analysis, Data fusion, Structural health monitoring, Data acquisition, Fiber optics sensors, Beam shaping, Laser Doppler velocimetry, Ferroelectric materials, Microsoft Foundation Class Library, Data processing
This work addresses the severe lack of literature in the area of modal analysis for multi-metric sensing. The paper aims at providing a step by step tutorial for performance of modal analysis using Fiber Bragg Grating (FBG) strain sensors and Laser Doppler Vibrometer (LDV) for displacement measurements. The paper discusses in detail the different parameters which affect the accuracy of the experimental results. It highlights the often implied, and un-mentioned problems, that researchers face while performing experiments. The paper tries to bridge the gap between the theoretical idea of the experiment and its actual execution by discussing each aspect including the choice of specimen, boundary conditions, sensors, sensor position, excitation mechanism and its location as well as the post processing of the data. The paper may be viewed as a checklist for performing modal analysis in order to ensure high quality measurements by avoiding the systematic errors to creep in.
In recent years electromagnetic Terahertz (THz) radiation or T-ray has been increasingly used for nondestructive evaluation of various materials such as polymer composites and porous foam tiles in which ultrasonic waves cannot penetrate but T-ray can. Most of these investigations have been limited to mechanical damage detection like inclusions, cracks, delaminations etc. So far only a few investigations have been reported on heat induced damage detection. Unlike mechanical damage the heat induced damage does not have a clear interface between the damaged part and the surrounding intact material from which electromagnetic waves can be reflected back. Difficulties associated with the heat induced damage detection in composite materials using T-ray are discussed in detail in this paper. T-ray measurements are compared for different levels of heat exposure of composite specimens.
Many studies have been published in recent years on Lamb wave propagation in isotropic and (multi-layered) anisotropic structures. In this paper, adiabatic wave propagation phenomenon in a tapered composite panel made out of glass fiber reinforced polymers (GFRP) will be considered. Such structural elements are often used e.g. in wind turbine blades and aerospace structures. Here, the wave velocity of each wave mode does not only change with frequency and the direction of wave propagation. It further changes locally due to the varying cross-section of the GFRP panel.
Elastic waves were excited using a piezoelectric transducer. Full wave-field measurements using scanning Laser Doppler vibrometry have been performed. This approach allows the detailed analysis of elastic wave propagation in composite specimen with linearly changing thickness. It will be demonstrated here experimentally, that the wave velocity changes significantly due to the tapered geometry of the structure. Hence, this work motivates the theoretical and experimental analysis of adiabatic mode propagation for the purpose of Non-Destructive Testing and Structural Health Monitoring.
Curvature mode shape is an effective feature for damage detection in beams. However, it is susceptible to measurement
noise, easily impairing its advantage of sensitivity to damage. To deal with this deficiency, this study formulates an
improved curvature mode shape for multiple damage detection in beams based on integrating a wavelet transform (WT)
and a Teager energy operator (TEO). The improved curvature mode shape, termed the WT - TEO curvature mode shape,
has inherent capabilities of immunity to noise and sensitivity to damage. The proposed method is experimentally
validated by identifying multiple cracks in cantilever steel beams with the mode shapes acquired using a scanning laser
vibrometer. The results demonstrate that the improved curvature mode shape can identify multiple damage accurately
and reliably, and it is fairly robust to measurement noise.
The aim of this paper is to present a method for visualization thermally induced delamination in composite material based on guided wave propagation phenomenon. Tested specimen was submitted to short time period high temperature source, which generated thermal degradation. In particular, delamination in material occurred. This procedure simulates some real case scenarios damage like one cased by atmospheric discharge striking wind turbine blade. Proposed method utilizes processing of full wavefield data acquired by the Scanning Doppler Laser Vibrometer. Registered wavefield images are transformed to wavenumber domain where the wave propagation pattern is removed. In this way after transformation signal back to space domain it contains only information about changes in wave propagation and may be used for damage visualization. However, attenuation of waves cause that visualized anomalies has lower amplitudes with increased distance from the actuator. The proposed enhancement of signal processing algorithm enables quantification of the size of the damage. The enhancement is a technique for compensation of the wave attenuation so that the effects of structural damages have the same influence regardless of the location.
The main objective of this study is to present a novel method for damage detection in plate-type structures using twodimensional (2D) continuous wavelet transforms. For this purpose, the 2D Mexican wavelet is employed to remold the equation of motion for transverse vibration of a plate. The remolded vibration equation of a plate can serve as a multiscale damage detection scheme that characterizes damage using an indicator of multiscale pseudo-load. Effects of multiscale pseudo-load can pinpoint the location of the damage as well as revealing its configuration; moreover, the strong solid mechanics foundation of the method results in the identified damage with an explicit physical implication. The performance of the proposed technique is validated through an experimental program of using a scanning laser vibrometer (SLV) to measure the transverse deflection of an aluminum plate bearing a cross-like notch and an added small mass. The results confirm the robustness and superior capability of the proposed method in detecting damage in plate-type structures.
The aim of this paper is to present methods for enhancing damage visualization in structures based on wave propagation
phenomenon. The method utilizes filtering and processing of full wavefield acquired by the laser vibrometer. Laser
vibrometer allows to register full wavefield in elements of a structure instead of single point measurements acquired by
e.g. piezoelectric sensor. In this way new possibilities for Nondestructive Evaluation arise enabling visualization of
elastic waves interacting with various types of damages. Measurements obtained with a scanning laser vibrometer can be
combined with effective signal and imaging processing algorithms to support damage identification. In this paper new
method for wave filtering of propagating waves is tested on both numerical results and experimental data obtained from
laser vibrometry measurements of composite plates. Processing of signals registered at a rectangular grid of
measurement points covering inspected area of the plate involve 2D DFFT (Discrete Fast Fourier Transform),
wavenumber filtering and inverse DFFT. As a result new damage index is proposed and compared with other methods
like RMS and frequency-wavenumber filtering.
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