Showing papers on "Lamb waves published in 2006"
TL;DR: A comprehensive review on the state of the art of Lamb wave-based damage identification approaches for composite structures, addressing the advances and achievements in these techniques in the past decades, is provided in this paper.
1,350 citations
TL;DR: In this paper, Lamb wave ultrasonic tomography is used to accurately map material loss on an exposed aircraft surface with sensors embedded on the structure's hidden surface, which is referred to as the surface that is not exposed to the atmosphere.
Abstract: Computerized tomography (CT) algorithms have been used mainly in the medical field but their powerful capabilities are being exploited more and more in industrial applications. This paper demonstrates that the technology is capable of detecting material loss on real aircraft components using embedded piezoelectric sensors on hidden surfaces. The work is novel in more than one respect. Firstly, it demonstrates that Lamb wave ultrasonic tomography can be used to accurately map material loss on an exposed aircraft surface with sensors embedded on the structure's hidden surface. Hidden, in this case, refers to the surface that is not exposed to the atmosphere—the underneath of an aircraft wing, for example. Secondly, it compares tomographic images generated by fan-beam back projection and the signal difference coefficient methods, showing clearly that the latter are more sensitive to material loss.
192 citations
TL;DR: In this article, the authors proposed a method based on Mindlin's plate theory and the plane wave expansion method to study the propagation of Lamb waves in two-dimensional phononic-crystal plates.
Abstract: Based on Mindlin's plate theory and the plane wave expansion method, a formulation is proposed to study the propagation of Lamb waves in two-dimensional phononic-crystal plates. The method is applied to calculate the frequency band structure of a square array of crystalline gold cylinders in an epoxy matrix with a finite thickness. It is found that complete frequency band gaps for Lamb waves between different pass bands are opened up by tuning thickness of the phononic-crystal plate. The influence of plate thickness on the width of complete frequency band gap is calculated and discussed as well; the existence of frequency stop bands is sensitive to the variation of the thickness of the plate. Finally, we note that the proposed method provides a concise and efficient way in analyzing the frequency band structures of phononic-crystal plates in lower bands.
186 citations
TL;DR: In this article, the results of experimental studies on piezoelectric lead-zirconate-titanate (PZT)-based active damage detection techniques for nondestructive evaluations (NDE) of steel bridge components are presented.
Abstract: This paper presents the results of experimental studies on piezoelectric lead-zirconate–titanate (PZT)-based active damage detection techniques for nondestructive evaluations (NDE) of steel bridge components. PZT patches offer special features suitable for real-time in situ health monitoring systems for large and complex steel structures, because they are small, light, cheap, and useful as built-in sensor systems. Both impedance and Lamb wave methods are considered for damage detection of lab-size steel bridge members. Several damage-sensitive features are extracted: root mean square deviations (RMSD) in the impedances and wavelet coefficients (WC) of Lamb waves, and the times of flight (TOF) of Lamb waves. Advanced signal processing and pattern recognition techniques such as continuous wavelet transform (CWT) and support vector machine (SVM) are used in the current system. Firstly, PZT patches were used in conjunction with the impedance and Lamb waves to detect the presence and growth of artificial cracks on a 1/8 scale model for a vertical truss member of Seongsu Bridge, Seoul, Korea, which collapsed in 1994. The RMSD in the impedances and WC of Lamb waves were found to be good damage indicators. Secondly, two PZT patches were used to detect damage on a bolt-jointed steel plate, which was simulated by removing bolts. The correlation of the Lamb wave transmission data with the damage classified by in and out of the wave path was investigated by using the TOF and WC obtained from the Lamb wave signals. The SVM was implemented to enhance the damage identification capability of the current system. The results from the experiments showed the validity of the proposed methods.
167 citations
TL;DR: The prototype of a guided ultrasonic wave array for the structural integrity monitoring of large, plate-like structures has been designed, built, and tested, and the sensitivity of the array device for defect detection has been established.
Abstract: The prototype of a guided ultrasonic wave array for the structural integrity monitoring of large, plate-like structures has been designed, built, and tested. The development of suitably small transducers for the excitation and measurement of the first antisymmetric Lamb wave mode A/sub 0/ is described. The array design consists of a ring of 32 transducers, permanently bonded to the structure with a protective membrane, in a compact housing with the necessary multiplexing electronics. Using a phased addition algorithm with dispersion compensation and deconvolution in the wavenumber domain, a good dynamic range can be achieved with a limited number of transducers. Limitations in the transducer design and manufacture restricted the overall dynamic range achieved to 27 dB. Laboratory measurements for a steel plate containing various defects have been performed. The results for standard defects are compared to theoretical predictions and the sensitivity of the array device for defect detection has been established. Simulated corrosion pitting and a defect cut with an angle grinder simulating general corrosion were detected.
149 citations
TL;DR: The propagation of guided waves throughout the thickness of the cortical bone and their sensitivity to both the mechanical and structural changes during healing can supplement velocity measurements so as to enhance the monitoring capabilities of ultrasonic evaluation.
Abstract: Ultrasonic evaluation of bone fracture healing has been traditionally based on the measurement of the propagation velocity of the first arriving signal (FAS). However, the FAS in general corresponds to a lateral wave that propagates along the bone's subsurface. In this work, we study guided ultrasound propagation in intact and healing bones. We developed a 2-D model of a bone-mimicking plate in which the healing process was simulated as a 7-stage process, and we also carried out ex vivo experiments on an intact tibia. Guided waves were represented in the time-frequency (t-f) domain of the signal by incorporating the Lamb wave theory. Three t-f distribution functions were examined, namely the reassigned Spectrogram, the smoothed-pseudo Wigner-Ville, and the reassigned version of it. For the intact plate case, we found that the S2, A3 Lamb modes were the dominant waves for a broadband 1-MHz excitation, and the S2, S0 for a 500-kHz excitation. During the simulated healing process, the mechanical and geometrical callus properties affected the theoretically anticipated Lamb modes. The propagation of guided waves throughout the thickness of the cortical bone and their sensitivity to both the mechanical and structural changes during healing can supplement velocity measurements so as to enhance the monitoring capabilities of ultrasonic evaluation. Nevertheless, the applicability of the Lamb wave theory to real bones has several limitations mostly associated with neglecting the inhomogeneity, anisotropy and irregular geometry of bone.
142 citations
TL;DR: In this paper, the authors explore the use of flexible piezoelectric materials, e.g. piezoelastic polymers such as PVDF, for sending and receiving Lamb waves to be used in the structural health monitoring (SHM) applications.
Abstract: Piezoelectric wafer active sensors (PWAS) used in structural health monitoring (SHM) applications are able to detect structural damage using Lamb waves. PWAS are small, lightweight, unobtrusive and inexpensive. They achieve direct transduction between electric and elastic wave energies. PWAS are charge mode sensors and can be used as both transmitters and receivers. The focus of this paper is to find a suitable in situ piezoelectric active sensor for sending and receiving Lamb waves to be used in the SHM of structures with a curved surface. Current SHM technology uses brittle piezoceramic (PZT) wafer active sensors. Since piezoceramics are brittle, this approach could only be used on flat surfaces. The motivation of our research was to explore the use of flexible piezoelectric materials, e.g. piezoelastic polymers such as PVDF. However, PVDF stiffness is orders of magnitude lower than the PZT stiffness, and hence PVDF Lamb wave transmitters are much weaker than PZT transmitters. Thus, our research proceeded in two main directions: (a) to model and understand how piezoelectric material properties affect the behaviour of piezoelectric wafer active sensors; and (b) to perform experiments to test the capabilities of the flexible PVDF PWAS in comparison with those of stiffer but brittle PZT PWAS. We have shown that, with appropriate signal amplification, PVDF PWAS can perform the same Lamb wave transmission and reception functions currently performed by PZT PWAS. The experimental results of PZT-PWAS and PVDF-PWAS have been compared with a conventional strain gauge. The theoretical and experimental results in this study gave a basic demonstration of the piezoelectricity of PZT-PWAS and PVDF-PWAS.
131 citations
TL;DR: In this article, a Lamb wave-based crack identification technique for aluminium plates was developed with an integrated active piezoelectric sensor network, and a correlation function was further established, which helped identify the crack position based on a triangulation approach with the aid of a nonlinear least-squares optimization algorithm.
Abstract: With an integrated active piezoelectric sensor network, a Lamb wave-based crack identification technique for aluminium plates was developed. Experimental results showed that the propagation of Lamb waves in aluminium plate-like structures is considerably complicated due to wave dispersion, material attenuation, boundary reflection, etc. In order to eliminate the diverse interference, a wavelet transform technique was applied to purify the acquired Lamb wave signals, and the characteristics of Lamb wave signals were extracted from the wave energy spectrum. A correlation function was further established, which helped identify the crack position based on a triangulation approach with the aid of a nonlinear least-squares optimization algorithm. Such an approach provides satisfactory results in locating the crack position in aluminium plates with cracks of 5 and 20 mm in length.
123 citations
TL;DR: In this article, an effective model of surface-bounded flat PZT disks based on effective force, moment and displacement is developed for Lamb wave excitation, collection and propagation in isotropic plate and quasi-isotropic laminated composite.
120 citations
TL;DR: In this paper, an embedded sensor network technique was developed for improving the overall integrity of functionalized composite structures engaged in aircraft, where a set of miniaturized piezoelectric wafers was designed and circuited to configure a built-in active actuator/sensor network, which was immobilized into multi-layered composite laminates.
Abstract: An embedded sensor network technique was developed for improving the overall integrity of functionalized composite structures engaged in aircraft. A set of miniaturized piezoelectric wafers was designed and circuited to configure a built-in active actuator/sensor network, which was immobilized into multi-layered composite laminates. The propagation characteristics of Lamb waves generated and collected by this built-in sensor network in carbon fibre-reinforced composite laminates were investigated. The influence of a stiffener and of the excitation frequency on the propagation of the Lamb waves generated was evaluated. A study was carried out to assess delamination in CF/EP (carbon fibre/epoxy) woven laminates, by fusing information from multiple sensing paths of the embedded network on the basis of the Hilbert transform, signal correlation and probabilistic searching. An excellent identification capability indicates the considerable application potential of the proposed sensor network approach in providing high-fidelity data acquisition and condition monitoring for composite aircraft structures.
111 citations
TL;DR: The approximate analytical algorithms presented in this paper provides a convenient method enabling quick acoustic field analysis on large-diameter industrial pipes for NDE applications and predicts that the wave beam will converge to its original circumferential shape after the wave propagates for a certain distance.
TL;DR: A new method for detection and visualisation of inhomogeneities in composite materials using one-side access air-coupled ultrasonic measurement technique is described and enables detect delamination and impact type defects in honeycomb materials.
TL;DR: The existence of band gaps for low-order Lamb wave modes is demonstrated in the systems made of alternating strips of tungsten materials and silicon resin in this article, where the finite element method is employed to calculate the transmitted power spectra, in good agreement with the results by plane wave exposition.
Abstract: We study theoretically the propagation of lower-order Lamb waves in one-dimensional composite thin plates. The dispersion curves of Lamb modes propagating parallel to the surfaces of the thin plates in the periodic direction are calculated based on the plane wave expansion method. The existence of band gaps for low-order Lamb wave modes is demonstrated in the systems made of alternating strips of tungsten materials and silicon resin. The finite element method is employed to calculate the transmitted power spectra, which is in good agreement with the results by plane wave exposition. A crucial parameter, i.e., the ratio of the plate thickness $(L)$ to the lattice spacing $(D)$, is discussed in detail for the influence of formation of band gaps.
TL;DR: In this article, the authors reported a new type of phononic crystals manufactured by patterning periodical air-filled holes in thin plates, and confirmed the existence of SAW band gaps in the created phononic crystal through laser ultrasonics measurements.
Abstract: Phononic structures and acoustic band gaps based on bulk materials have been researched in length in the past decades. However, few investigations have been performed on phononic structures in thin plates to form surface acoustic wave (SAW) band gaps. In this letter, we report a new type of phononic crystals manufactured by patterning periodical air-filled holes in thin plates. We confirmed the existence of SAW band gaps in the created phononic crystals through laser ultrasonics measurements. Wide multiple SAW band gaps and special structures, such as narrow pass bands within a band gap were observed experimentally.
TL;DR: In this paper, the problem of Lamb wave propagation in waveguides with varying height is treated by a multimodal approach based on a rearrangement of the equations of elasticity that provides a new system of coupled mode equations preserving energy conservation.
Abstract: The problem of Lamb wave propagation in waveguides with varying height is treated by a multimodal approach. The technique is based on a rearrangement of the equations of elasticity that provides a new system of coupled mode equations preserving energy conservation. These coupled mode equations avoid the usual problem at the cut-offs with zero wavenumber. Thereafter, we define an impedance matrix that is governed by a Riccati equation yielding a stable numerical computation of the solution. Incidentally, the versatility of the multimodal method is exemplified by treating analytically the case of slowly varying guide and by showing how to get easily the Green tensor in any geometry. The method is applied for a waveguide whose height is described by a Gaussian function and the energy conservation in verified numerically. We determine the Green tensor in this geometry.
TL;DR: In this paper, the propagation of plane waves in a fiber-reinforced, anisotropic, generalized thermoelastic media is discussed, and the governing equations in x-y plane are solved to obtain a cubic equation in phase velocity.
Abstract: The propagation of plane waves in a fibre-reinforced, anisotropic, generalized thermoelastic media is discussed. The governing equations in x–y plane are solved to obtain a cubic equation in phase velocity. Three coupled waves, namely quasi-P, quasi-SV and quasi-thermal waves are shown to exist. The propagation of Rayleigh waves in stress free thermally insulated and transversely isotropic fibre-reinforced thermoelastic solid half-space is also investigated. The frequency equation is obtained for these waves. The velocities of the plane waves are shown graphically with the angle of propagation. The numerical results are also compared to those without thermal disturbances and anisotropy parameters.
TL;DR: In this article, the authors present the results of testing a system for the identification of structural damage based on fibre Bragg grating sensors, which use Bragg gratings as acoustic receivers of ultrasonic Lamb waves.
Abstract: Structural health monitoring has become a respected and established discipline in engineering. Health monitoring involves the development of autonomous systems for continuous monitoring, inspection and damage detection of structures with minimum involvement of labour. The ultimate goal of structural health monitoring is to increase reliability, improve safety, enable light-weight design and reduce maintenance costs for all kinds of structures. The identification of structural damage is therefore a key issue in structural health monitoring. The scope of this paper is to present the results of testing a system for the identification of structural damage based on fibre Bragg grating sensors. The basic idea is to use fibre Bragg gratings as acoustic receivers of ultrasonic Lamb waves. The layout of such a damage identification system is introduced and its theoretical limits are studied numerically and experimentally. The set-up for damage identification experiments is described and the results of initial experiments introducing damage detection based on the analysis of Lamb wave signals are presented. The results for the Bragg grating sensors are then compared to the results of established technology for Lamb wave detection using piezoceramic transducers.
TL;DR: It is shown that, for any guided elastic wave, Lamb or generalised Lamb modes, stop bands appear in the dispersion curves, displaying a phononic band structure in both cases.
TL;DR: In this article, a new compact sensor configuration comprising a single transmitter and multi-receivers (STMR) is presented for the in situ structural health monitoring (SHM) of large plate-like isotropic structures.
Abstract: A new compact sensor configuration comprising a single transmitter and multi-receivers (STMR) is presented for the in situ structural health monitoring (SHM) of large plate-like isotropic structures. The STMR exploits the long-range propagation characteristics of ultrasonic guided Lamb waves and a phase reconstruction algorithm to provide defect detection and location capability under non-dispersive as well as dispersive regimes of guided waves. Simulations are performed on defect-free and defective finite plates of aluminum to demonstrate the various features of the STMR system. Experiments were carried out on 1 mm thick aluminum plates initially using a pair of individual sensors and subsequently using a prototype STMR array. The simulated results of the STMR performance were validated well through these experiments. Features of the STMR system such as its small footprint, the relatively simple data acquisition and processing discussed here have applications in the SHM of plate-like structures, and particularly of aerospace structures.
TL;DR: In this article, the relationship between the parameters determining the slit geometry and the focusing effect of the generated Lamb wave was investigated using a parametric study, and appropriate values of the parameters were obtained to maximize the focusing ability.
Abstract: The laser generation method of a focused Lamb wave is expected to have high defect-detection abilities and spatial resolution, with the added advantage of noncontact testing. In this method, the laser beam is illuminated on the surface of an object through an arrayed arc slit. The energy of the generated Lamb wave is then concentrated at a focus of an arc. This focusing effect improves the NDE (nondestructive evaluation) performance, which is dependent on the geometries of an arrayed arc slit. In this paper, the relationships between the parameters determining the slit geometry and the focusing effect of the generated Lamb wave was investigated using a parametric study, and appropriate values of the parameters were obtained to maximize the focusing ability. In order to validate the performance of this method, an NDE system was constructed and experiments were performed to detect through-hole defects in a plate. The results showed that the proposed system could clearly detect defects as small as 0.3 mm in diameter, while the conventional line array method failed to detect defects smaller than 1 mm in diameter. Moreover, this method showed possible detection of defects much smaller than 0.3 mm, and great improvements in the spatial resolution as compared with the line array method.
TL;DR: In this paper, an in situ and nondestructive method for the impact damage monitoring of carbon/epoxy laminated composites is defined, which aims to improve the operational safety of the structure using integrated piezoelectric sensors.
Abstract: The growth of the use of composite materials to replace traditional materials as primary structures remains limited by the significant inspection costs that are required to detect the appearance of internal defects. Indeed, many different damage processes can be observed in such kinds of heterogeneous media. The present article defines an in situ and nondestructive method for the impact damage monitoring of carbon/epoxy laminated composites. This structural health monitoring (SHM) method aims to improve the operational safety of the structure using integrated piezoelectric sensors. We describe the propagation of Lamb waves inside a stiffened aeronautical structure, and the definition of associated data processing systems. The detection of realistic defects thanks to the definition of a damaging parameter, based on the real time analysis of the ultrasonic signatures transmitted inside the structure is outlined.
TL;DR: In this paper, the use of a Lamb wave based structural health monitoring (SHM) system was proposed to identify and locate defects in large multilayered composite plates. But the proposed algorithm was based on phased addition in the frequency domain to compensate for the dispersion of Lamb waves.
Abstract: Platelike structures, made of composites, are being increasingly used for fabricating aircraft wings and other aircraft substructures. Continuous monitoring of the health of these structures would aid the reliable operation of aircrafts. This paper considers the use of a Lamb wave based structural health monitoring (SHM) system to identify and locate defects in large multilayered composite plates. The SHM system comprises of a single transmitter and multiple receivers, coupled to one side of the plate that send and receive Lamb waves. The proposed algorithm processes the data collected from the receivers and generates a reconstructed image of the material state of the composite plate. The algorithm is based on phased addition in the frequency domain to compensate for the dispersion of Lamb waves. In addition, small deviations from circularity of the slowness curves of Lamb wave modes, due to anisotropy, are corrected for by assuming that the phase and group velocity directions coincide locally. Experiment...
TL;DR: In this article, the authors describe experiments on the generation of the first order symmetric (S1) Lamb mode by a pulsed yttrium aluminum garnet laser, where the vibration of the plate is detected at the same point by a heterodyne interferometer.
Abstract: In this letter, we describe experiments on the generation of the first order symmetric (S1) Lamb mode by a pulsed yttrium aluminum garnet laser. The vibration of the plate is detected at the same point by a heterodyne interferometer. The acoustic signal is dominated by the resonance at the point of the dispersion curve where the group velocity vanishes. The time decay of the signal leads to the local attenuation coefficient of the material. The spectrum exhibits a very sharp peak, the frequency of which is sensitive to the plate thickness. For a 0.49-mm-thick Duralumin plate, thickness variations as small as 0.1μm have been detected. Moving the detection point away from the source allows us to record the standing wave pattern resulting from the interference between the S1 and S2b Lamb waves having opposite wave vectors at the zero group velocity point.
TL;DR: In this article, the authors reported a new structure of a shear horizontal (SH) type boundary acoustic wave for cellular phone applications, which is composed of electrodes with a low shear wave velocity between two materials, namely, the SiO2 film/Au-electrode/LiNbO3 substrate.
Abstract: In this paper, we report a new structure of a shear horizontal (SH) type boundary acoustic wave for cellular phone applications. Such a structure composed of electrodes with a low shear wave velocity between two materials, namely, the SiO2 film/Au-electrode/LiNbO3 substrate, is proposed. The ladder filter used in this paper had this structure. By changing the propagation angle of the acoustic wave, the electromechanical coupling factor k2 range from 0 to 16% was obtained, as well as a normalized bandwidth range 0.67 to 1.85 fold as large as that of a 36–46°Y–X LiTaO3 leaky surface acoustic wave (LT-LSAW) filter. In addition, an excellent temperature coefficient of delay time (TCD=25 ppm/°C) and a large mutual coupling coefficient κ12 (=0.15) were also obtained.
TL;DR: In this paper, the elastic plate with a free edge was studied from the point of view of complex resonance and the variations of the real part and of the imaginary part of the complex resonance frequency as a function of the Poisson ratio ν were determined numerically.
Abstract: The resonance for the elastic plate with a free edge is studied from the point of view of complex resonance. The variations of the real part and of the imaginary part of the complex resonance frequency as a function of the Poisson ratio ν are determined numerically. The results confirm the real resonance frequency theoretically predicted in I. Roitberg et al., Q. J. Mech. Appl. Math. 51, 1–13 (1998) for a zero Poisson ratio ν1=0, and a real resonance frequency that corresponds to a Lame mode is discovered for a Poisson ratio ν2=0.2248. It is shown that both real resonance frequencies may exist, at these two particular values of ν, because of the decoupling between the propagating Lamb mode and the set of evanescent Lamb modes.
TL;DR: In this paper, a signal processing approach, taking advantage of correlation filtering-based matching pursuit (CF-MP), was developed, which calibrated correlation among wave signals captured from the same structure under different damage statuses, served as an indicator for the occurrence and severity of structural damage.
Abstract: Time of flight (ToF) plays a key role in positioning structural damage, but the exact determination of ToF in complicated Lamb wave signals is somewhat challenging. A signal processing approach, taking advantage of correlation filtering-based matching pursuit (CF-MP), was developed. In this approach, correlation among wave signals captured from the same structure under different damage statuses was calibrated, which served as an indicator for the occurrence and severity of structural damage. The ToF of a damage-scattered Lamb wave was pinpointed with high precision. With it, the approach was then applied to Lamb wave signals acquired from delaminated carbon-fibre/epoxy (CF/EP) composite beams, and the location and size of the delamination were exactly predicted. Experimental validation indicated that such an approach is able to filter boundary-reflected signal components involved in a complex wave signal, making the wave-based damage identification technique practical for small structures.
TL;DR: In this paper, the amplitude and energy ratios of reflected and refracted waves are derived in closed form for a particular model and the results obtained are depicted graphically, and it is verified that during transmission there is no dissipation of energy at the interface.
TL;DR: In this article, the amplitude of ultrasonic Lamb waves reflected from a single 20 mm-diameter flat-bottomed hole located adjacent to the weld of a 6 mm-thick butt welded steel plate was measured using a prototype omnidirectional multi-element transducer.
Abstract: Measurements are reported on the amplitude of ultrasonic Lamb waves reflected from a single 20 mm-diameter flat-bottomed hole located adjacent to the weld of a 6 mm-thick butt welded steel plate. The hole was introduced to simulate the presence of corrosion defects in the heat-affected zone, and served to assess the suitability of using the fundamental So Lamb waves as a means for remote detection of corrosion in fluid-filled storage tanks. Measurements were made using a prototype, omnidirectional multi-element transducer developed at Imperial College, and also using a single edge-mounted transducer. Detection of a hole was only possible when using the plate tester at oblique incidence angles, because of the inability to discriminate weld echoes from those due to the hole at normal or near-normal incidence. Detection of holes was also limited to those with a minimum depth of approximately 3 mm by the presence of coherent background noise. Measurements using a single edge-mounted transducer demonstrated that preferential waveguide propagation of the So mode occurred along the weld, in a manner coined 'weld-guided waves'. This permitted easy detection of a 2 mm deep-hole located adjacent to the weld at a distance of 1 m from the transducer. It is likely that a practical surface-mounted transducer based on the S 0 waveguide mode of operation could be developed for the inspection of corrosion defects within the heat-affected zone of fluid-filled tanks. It is probable that such a device would have a range of many metres, be simple in construction, and that the method would be relatively unaffected by the presence of fluid and inherently sensitive because of the suppression of other propagation modes.
TL;DR: This theoretical study revealed that both the S0 and S1 mode velocity curves are significantly changed in acrylic, however, in human cortical bone plates, only the S1 modes curve is significantly altered by water loading, with the S 0 mode exhibiting a small deviation from the unloaded curve.
Abstract: Understanding the velocity and attenuation characteristics of ultrasonic waves in cortical bone and bone mimics is important for studies of osteoporosis and fractures. Three complementary approaches have been used to help understand the ultrasound propagation in cortical bone and bone mimics immersed in water, which is used to simulate the surrounding tissue in vivo. The approaches used were Lamb wave propagation analysis, experimental measurement and two-dimensional (2D) finite difference modelling. First, the water loading effects on the free plate Lamb modes in acrylic and human cortical bone plates were examined. This theoretical study revealed that both the S0 and S1 mode velocity curves are significantly changed in acrylic: mode jumping occurs between the S0 and S1 dispersion curves. However, in human cortical bone plates, only the S1 mode curve is significantly altered by water loading, with the S0 mode exhibiting a small deviation from the unloaded curve. The Lamb wave theory predictions for velocity and attenuation were then tested experimentally on acrylic plates using an axial transmission technique. Finally, 2D finite difference numerical simulations of the experimental measurements were performed. The predictions from Lamb wave theory do not correspond to the measured and simulated first arrival signal (FAS) velocity and attenuation results for acrylic and human cortical bone plates obtained using the axial transmission technique, except in very thin plates.
TL;DR: In this article, the results of experiments carried out to study the identification of damage using Bragg grating sensors as ultrasonic receivers of Lamb waves are given, where the experiments involve a rectangular aluminium plate and damage was introduced into the plate by drilling a hole into the center of the plate.
Abstract: Damage detection is an important issue in structural health monitoring. Lamb waves are the most widely used acousto-ultrasonic guided waves for damage detection. This paper gives the results of experiments carried out to study the identification of damage using Bragg grating sensors as ultrasonic receivers of Lamb waves. The experiments involve a rectangular aluminium plate. Damage was introduced into the plate by drilling a hole into the centre of the plate. In order to obtain different severity of damage, the hole diameter was increased step by step. Several signal processing tools are presented and then applied to the Lamb wave signals in order to find a parameter that corresponds to the severity of damage. The parameter that serves as the damage index has to have small cross-sensitivity to other physical parameters, e.g. temperature. Therefore, additional experiments have been carried out to study the temperature dependence of the Lamb wave signals. In order to determine the influence of the temperature on the damage detection results, the cross-sensitivity is studied within this paper.