Showing papers on "Guided wave testing published in 2009"

Evaluation of the damage detection capability of a sparse-array guided-wave SHM system applied to a complex structure under varying thermal conditions

Abstract: A sparse-array structural health monitoring (SHM) system based on guided waves was applied to the door of a commercial shipping container. The door comprised a corrugated steel panel approximately 2.4 m by 2.4 m surrounded by a box beam frame and testing was performed in a nonlaboratory environment. A 3-D finite element (FE) model of the corrugations was used to predict transmission coefficients for the A0 and S0 modes across the corrugations as a function of incidence angle. The S0 mode transmission across the corrugations was substantially stronger, and this mode was used in the main test series. A sparse array with 9 transducers was attached to the structure, and signals from the undamaged structure were recorded at periodic intervals over a 3-week period, and the resulting signal database was used for temperature compensation of subsequent signals. Defects in the form of holes whose diameter was increased incrementally from 1 to 10 mm were introduced at 2 different points of the structure, and signals were taken for each condition. Direct analysis of subtracted signals allowed understanding of the defect detection capability of the system. Comparison of signals transmitted between different transducer pairs before and after damage was used to give an initial indication of defect detectability. Signals from all combinations of transducers were then used in imaging algorithms, and good localization of holes with a 5-mm diameter or above was possible within the sparse array, which covered half of the area of the structure.

Interaction of the primary anti-symmetric Lamb mode (Ao) with symmetric delaminations: numerical and experimental studies

Abstract: The interaction of an ultrasonic guided Lamb wave mode with delamination type defects in a quasi-isotropic laminated composite plate has been studied, using both simulations and experiments. In a laminated composite plate with a symmetric delamination, when the primary anti-symmetric mode, Ao, is incident at the entrance and exit of a delamination, it generates a new mode, So, that is confined only to sub-laminates and undergoes multiple reflections in the delaminated region. It was observed that only the incident and mode-converted Ao modes propagate in the main laminate. The two modes reverberate between the two ends of the delaminations while undergoing multiple mode conversions, leading to a trail of signals that is captured by the finite element model. The numerical observations were validated using experiments conducted using air coupled ultrasonic transducers.

The application of synthetic focusing for imaging crack-like defects in pipelines using guided waves

Abstract: This paper deals with quantifying the performance of a technique for detection, location, and sizing of circumferential crack-like defects in pipelines using synthetically focused guided waves. The system employs a circumferential array of piezoelectric transducer elements. A torsional probing guided wave is excited using the array, which subsequently interacts with the reflecting features of the pipe, such as defects or weld caps. The recorded backscattered signals are synthetically focused to every point of interest in the pipe wall, to form an image of the reflecting features of the pipe. The defect image amplitude is used to estimate the defect depth, and the full width at half maximum of the defect image circumferential profile is used to estimate the circumferential extent of the defect. The imaging system is tested with data from finite element simulations and from laboratory experiments. It is found that reliable sizing of circumferential cracks in finite element simulations and experiments can be achieved if the circumferential extent of the defect is greater than 1.5 lambdaS, where lambdaS is the shear wavelength at the frequency of inspection. This result is theoretically valid for any pipe size, any axial defect location, and any inspection frequency. Amplitude gains of around 18 dB over an unfocused system have been observed experimentally in an 8-inch pipe with a 9 dB SNR improvement.

A nonlinear-guided wave technique for evaluating plasticity-driven material damage in a metal plate

Abstract: This research develops an experimental method for evaluating material damage due to plastic deformation in a metal plate using nonlinear-guided waves. An improved version of a previously proposed measurement technique is used. The material nonlinearities of aluminum specimens loaded to produce different levels of plastic strain are measured with Lamb waves. A pair of wedge transducers is used to generate and detect the fundamental (s1) and second harmonic (s2) Lamb waves. The weak amplitude of the second harmonic wave is extracted from the spectrogram of a received signal. The measured acoustic nonlinearity increases monotonically with the level of plasticity (plastic strain), which is similar to the behavior of longitudinal and Rayleigh waves. This result indicates that Lamb waves can be used to assess plasticity-driven material damage in the established framework of the second harmonic generation technique. The effects and the importance of signal processing in determining the nonlinearity parameter are also discussed.

Ice detection and classification on an aircraft wing with ultrasonic shear horizontal guided waves

Abstract: Ice accumulation on airfoils has been identified as a primary cause of many accidents in commercial and military aircraft. To improve aviation safety as well as reduce cost and environmental threats related to aircraft icing, sensitive, reliable, and aerodynamically compatible ice detection techniques are in great demand. Ultrasonic guided-wave-based techniques have been proved reliable for "go" and "no go" types of ice detection in some systems including the HALO system, in which the second author of this paper is a primary contributor. In this paper, we propose a new model that takes the ice layer into guided-wave modeling. Using this model, the thickness and type of ice formation can be determined from guided-wave signals. Five experimental schemes are also proposed in this paper based on some unique features identified from the guided- wave dispersion curves. A sample experiment is also presented in this paper, where a 1 mm thick glaze ice on a 2 mm aluminum plate is clearly detected. Quantitative match of the experiment data to theoretical prediction serves as a strong support for future implementation of other testing schemes proposed in this paper.

Monitoring Corrosion of Rebar Embedded in Mortar Using High-Frequency Guided Ultrasonic Waves

Abstract: Corrosion of reinforced concrete structures creates serviceability and safety issues, costing millions of dollars for inspection, repair, and rehabilitation. Recent efforts have focused on monitoring corrosion in situ, providing accurate real-time information for decision-making. The goal of this research is the creation of an embeddable ultrasonic sensing network for assessment of reinforcement deterioration. Toward this effort, guided ultrasonic waves were used to monitor reinforced mortar specimens undergoing accelerated uniform and localized corrosion. Longitudinal waves were invoked at higher frequencies 2-9 MHz, where the attenuation is a local minimum. Using a through-transmission configuration, waveforms were sensitive to both forms of corrosion damage. Scattering, mode conversions, and reflections from irregularities at the bar surface from uniform corrosion and the severely tapered cross section from localized corrosion are thought to cause the increase in attenuation. Because localized corrosion did not yield a discontinuity that was nearly perpendicular to the bar axis, incident waves were severely scattered, mode converted, and rapidly attenuated. As evidence, this was the inability of pulse-echo testing to detect reflected waveforms for localized corrosion.

Velocity dispersion of guided waves propagating in a free gradient elastic plate: Application to cortical bone

Abstract: The classical linear theory of elasticity has been largely used for the ultrasonic characterization of bone. However, linear elasticity cannot adequately describe the mechanical behavior of materials with microstructure in which the stress state has to be defined in a non-local manner. In this study, the simplest form of gradient theory (Mindlin Form-II) is used to theoretically determine the velocity dispersion curves of guided modes propagating in isotropic bone-mimicking plates. Two additional terms are included in the constitutive equations representing the characteristic length in bone: (a) the gradient coefficient g, introduced in the strain energy, and (b) the micro-inertia term h, in the kinetic energy. The plate was assumed free of stresses and of double stresses. Two cases were studied for the characteristic length: h=10−4 m and h=10−5 m. For each case, three subcases for g were assumed, namely, g>h, g

Measurement of the Dispersion and Attenuation of Cylindrical Ultrasonic Guided Waves in Long Bone

Abstract: Osteoporotic bones are likely to have less cortical bone than healthy bones. The velocities of guided waves propagating in a long cylindrical bone are very sensitive to bone properties and cortical thickness (CTh). This work studies the dispersion and attenuation of ultrasonic guided waves propagating in long cylindrical bone. A hollow cylinder filled with a viscous liquid was used to model the long bone and then to calculate the theoretical phase and group velocities, as well as the attenuation of the waves. The generation and selection of guided wave modes were based on theoretical dispersive curves. The phase velocity and attenuation of cylindrical guided wave modes, such as L(0,1), L(0,2) and L(0,3), were measured in bovine tibia using angled beam transducers at various propagation distances ranging from 75 to 160 mm. The results showed that the phase velocity of the L(0,2) guided wave mode decreased with an increase in CTh. The attenuation of the low cylindrical guided wave modes was a nonlinear function that increased with propagation distance and mode order. The L(0,2) mode had a different attenuation for each CTh. The experimental results were in good agreement with the predicted values. Cylindrical guided waves of low-frequency and low-order have been shown to demonstrate more dispersion and less attenuation and should, therefore, be used to evaluate long bone.

Guided wave excitation by a CLoVER transducer for structural health monitoring: theory and experiments

Abstract: The guided wave (GW) field excited by a wedge-shaped, anisotropic piezocomposite transducer, surface-bonded on an isotropic substrate is investigated with applications to large area structural health monitoring. This investigation supports the development of the composite long-range variable-direction emitting radar (CLoVER) transducer. The analysis is based on the three-dimensional equations of elasticity, and the solution yields expressions for the field variables that are able to capture the multimodal nature of GWs. The assumption of uncoupled dynamics between the actuator and substrate is used, and their interaction is modeled through shear tractions along the transducer's radial edges. A similar problem is modeled using three-dimensional finite element simulations to assess the spatial and transient accuracy of the solution. Experimental tests are also conducted on pristine structures to validate the accuracy of the theoretical approach. The experimental studies employ CLoVER transducers developed in-house, and their manufacturing procedure is briefly described. Frequency response experiments based on piezoelectric sensors are conducted to assess the performance of the solution in the frequency domain. These tests are complemented by laser vibrometer measurements that allow the spatial and temporal evolution of the solution to be evaluated. The numerical simulations and experimental tests show that the wave time of arrival, radial attenuation, and azimuthal distribution are well captured by the theoretical solution.

Elastic waves guided by a welded joint in a plate

Abstract: The inspection of large areas of complex structures is a growing interest for industry. An experimental observation on a large welded plate found that the weld can concentrate and guide the energy of a guided wave travelling along the direction of the weld. This is attractive for non-destructive evaluation (NDE) since it offers the potential to quickly inspect for defects such as cracking or corrosion along long lengths of welds. In this paper, a two-dimensional semi-analytical finite-element (SAFE) method is applied to provide a modal study of the elastic waves that are guided by the welded joint in a plate. This brings understanding to the compression wave that was previously observed in the experiment. However, during the study, a shear weld-guided mode, which is non-leaky and almost non-dispersive, has also been discovered. Its characteristics are particularly attractive for NDE, so this is a significant new finding. The properties for both the compression and the shear mode are discussed and compared, and the physical reason for the energy trapping phenomena is then explained. Experiments have been undertaken to validate the existence of the shear weld-guided mode and the accuracy of the FE model, showing very good results.

Planar plasmonic terahertz guided-wave devices

Abstract: We describe the realization of planar plasmonic THz guided-wave devices, including straight waveguides, Y-splitters and 3dB-couplers, using periodically perforated metal films. These perforated films behave as effective media whose waveguiding properties can be broadly engineered.

Guided-wave Health Monitoring of Aircraft Composite Panels under Changing Temperature

Abstract: This study deals with the health monitoring of fiber-reinforced composite panels using ultrasonic guided waves and flexible piezocomposite transducer patches in a changing temperature environment c...

Feasibility of low frequency straight-ray guided wave tomography

Abstract: Corrosion is a major problem in the petrochemical industry. Corrosion patches are often inaccessible, and the majority of the conventional nondestructive evaluation techniques are compromised. There is therefore a need for a rapid, accurate, long range inspection technique to measure the remaining thickness in corrosion patches. Low frequency Lamb wave tomography is a potentially attractive technique to rapidly evaluate the thickness of large sections of partially accessible structures. This approach has been used in the past to detect the shape of defects in a range of applications. Time-of-flight straight-ray tomography relies on the dispersive nature of a guided wave mode to reconstruct the depth profile of a corrosion patch. If the frequency is limited to below the cut-off of the higher order modes the interpretation of the signals and the time-of-flight measurement are easier. In order to reconstruct a thickness map with time-of-flight straight-ray tomography the ray theory needs to be valid. There are two validity criteria: the characteristic size of the defect must be larger than the wavelength and larger than the width of the Fresnel zone. For realistic defect sizes, the likely points of operation in the low frequency regime respect the wavelength condition but not the more stringent Fresnel zone condition. The paper demonstrates, with finite element simulations and experiments, that the ray theory criteria cannot be relaxed for low frequency Lamb wave tomography to evaluate the maximum depth of corrosion patch in typical pipe inspection problems.

Application of laser-generated guided wave for evaluation of corrosion in carbon steel pipe

Abstract: Local wall thinning of pipes, resulting from corrosion, is the major cause of accidents in nuclear power plants. In order to assure the integrity of pipes, a variety of NDE techniques have been proposed. It is well recognized that the utilization of guided waves to detect flaws in pipes has been a very effective tool for long-range inspection in NDE. Since most conventional research on guided waves has focused on long-range inspection, it is difficult to evaluate a defect in a local area. An objective of this study is to develop a non-contact inspection technique for the quantitative evaluation of defects on a local area in a cylindrical tube. Therefore, the purpose of this study is to introduce an advanced inspection system and a new signal processing method for the evaluation of defects such as corrosion in a pipe. In this study, an advanced non-contact method for pipe inspection is proposed, which generates the ultrasonic guided wave by laser and receives it by a dual air-coupled transducer. Information on each directional defect location and length is obtained by using a line scan along a circumferential and longitudinal direction. Received signals are analyzed by using the peak-to-peak amplitude of waveform and the maximum center frequency magnitude of the frequency spectrum. The optimal mode selection of guided waves based on a dispersion curve is also discussed. The experimental results using the proposed method show that the location and the size of the defect could be evaluated successfully in the 2-dimensional scanning images.

Matched Representations and Filters for Guided Waves

Abstract: We propose time-frequency methods to filter dispersive guided wave signals. Guided waves occur in acoustical propagation (oceanic waveguides), geophysics (layered medium), or optics (dielectric optical waveguides). In waveguides, signals can be decomposed into normal modes which contain information on environmental parameters and source localization. As modes present nonlinear time-frequency evolution, modal filtering is not possible with conventional tools. To overcome the difficulty presented by these nonlinearties, we have developed matched tools: matched frequency and time-frequency representations and the modal filterings associated with these representations. The tools developed are based on unitary equivalence principle. Performance and robustness of different proposed modal filters are evaluated and compared. All of these tools can be used for both source localization and environmental inversion.

Guided Wave and Damage Detection in Composite Laminates Using Different Fiber Optic Sensors

Abstract: Guided wave detection using different fiber optic sensors and their applications in damage detection for composite laminates were systematically investigated and compared in this paper. Two types of fiber optic sensors, namely fiber Bragg gratings (FBG) and Doppler effect-based fiber optic (FOD) sensors, were addressed and guided wave detection systems were constructed for both types. Guided waves generated by a piezoelectric transducer were propagated through a quasi-isotropic carbon fiber reinforced plastic (CFRP) laminate and acquired by these fiber optic sensors. Characteristics of these fiber optic sensors in ultrasonic guided wave detection were systematically compared. Results demonstrated that both the FBG and FOD sensors can be applied in guided wave and damage detection for the CFRP laminates. The signal-to-noise ratio (SNR) of guided wave signal captured by an FOD sensor is relatively high in comparison with that of the FBG sensor because of their different physical principles in ultrasonic detection. Further, the FOD sensor is sensitive to the damage-induced fundamental shear horizontal (SH0) guided wave that, however, cannot be detected by using the FBG sensor, because the FOD sensor is omnidirectional in ultrasound detection and, in contrast, the FBG sensor is severely direction dependent.

Excitation and scattering of guided waves: relationships between solutions for plates and pipes.

Abstract: The detection of localized defects such as cracks and corrosion in pipes using guided elastic waves is now an established non-destructive testing procedure. However, the prediction of guided wave excitation and scattering in pipes is a complex three-dimensional (3D) problem with many parameters that can generally only be solved using numerical methods. In many important industrial applications, the diameter of a pipe is much larger than wall thickness. In this case an approximate theory is applicable, when a pipe is considered as an unwrapped isotropic plate. In this paper, a technique for obtaining pipe mode amplitudes in terms of the solution to the forced 3D problem on a plate is presented. The same principle is extended to relate guided wave scattering from defects in plates to scattered circumferential modal amplitudes from defects in pipe. This is of practical benefit as the scattering of guided waves by defects in a plate is a much simpler problem than that in a pipe, and one that, in some cases, can be solved using analytical methods. Results are shown that illustrate the application of the method to reflection from through-thickness circumferential cracks in pipes.

A guided wave approach to defect detection under shelling in rail

Abstract: Rail defects are responsible for many railroad accidents. Trains are derailed and serious consequences often occur. Traditional bulk wave testing wheel probes are often inadequate for finding all defects in a rail, especially under shelling. Shelling takes place as a result of wheel to rail Hertzian contact stresses that lead to surface and subsurface defects, as a result of high stresses below the surface of the rail. Guided waves can detect shelling, if so desired, by employing the proper mode and frequency. Guided waves can also detect transverse cracking under shelling by selecting a mode and frequency insensitive to the shelling, but sensitive to transverse cracking under the shelling. Special modes and frequencies can also be used to detect defects in the web or base of a rail. The guided wave methodology of using a hybrid analytical-FEM technique to accomplish the sensor design and subsequent scattering analysis is presented in this paper. Sensor design to generate rail boundary conditions via dispersion curve and wave structure analysis is illustrated along with a few static shots of animations of wave propagation and reflection from defects in a rail.

Analytic modal solution to transmission and collimation of light by one- dimensional nanostructured subwavelength metallic slits

Abstract: Light transmitting through a subwavelength slit on an ordinary metal plate is diffracted to all directions but if the exit plane of the slit is patterned with periodical nanostructures, the diffracted light may be compressed into a collimated beam within a small angle. In this paper, we develop a rigorous theoretical method for solving the surface wave induced beam collimation in nanostructured subwavelength metallic slits. The method combines the analytical modal expansion method, the supercell technique, the transfer-matrix method, and the conventional Kirchhoff’s diffraction theory. It allows for quantitative investigation of coupling of the incident light into the guided wave of the slit and coupling of the guided wave out of the nanostructured exit plane. We have used the method to examine light transmission through the nanostructured metallic slit and the corresponding diffraction and beam collimation behaviors. We have extensively analyzed the angular transmission spectrum as a function of the nano...

Lamb wave characterization of corrosion-thinning in aircraft stringers: Experiment and three-dimensional simulation

Abstract: The development of automatic guided wave interpretation for detecting corrosion in aluminum aircraft structural stringers is described. The dynamic wavelet fingerprint technique (DWFT) is used to render the guided wave mode information in two-dimensional binary images. Automatic algorithms then extract DWFT features that correspond to the distorted arrival times of the guided wave modes of interest, which give insight into changes of the structure in the propagation path. To better understand how the guided wave modes propagate through real structures, parallel-processing elastic wave simulations using the finite integration technique (EFIT) has been performed. Three-dimensional (3D) simulations are used to examine models too complex for analytical solutions. They produce informative visualizations of the guided wave modes in the structures and mimic the output from sensors placed in the simulation space. Using the previously developed mode extraction algorithms, the 3D EFIT results are compared directly to their experimental counterparts.

Modal, Spectral, and Polarization Entanglement in Guided-Wave Parametric Down-Conversion

Abstract: We offer a comprehensive theoretical study of the properties and applications of modal, spectral, and polarization entanglement of biphotons generated via spontaneous parametric down-conversion in 1-D planar and 2-D circular waveguides using continuous pumping.

SIW-Like Guided Wave Structures and Applications

Abstract: In this paper, the research advances in SIW-like (Substrate Integrated Waveguide-like) guided wave structures and their applications in the State Key Laboratory of Millimeter Waves of China is reviewed. Our work is concerned with the investigations on the propagation characteristics of SIW, half-mode SIW (HMSIW) and the folded HMSIW (FHMSIW) as well as their applications in microwave and millimeter wave filters, diplexers, directional couplers, power dividers, antennas, power combiners, phase shifters and mixers etc. Selected results are presented to show the interesting features and advantages of those new techniques.

Next generation guided wave health monitoring for long range inspection of pipes

Abstract: Shown in this paper are recent accomplishments of robust leading edge technologies in the guided wave technique and the technology transfer to the Gas and Petrochemical Industries. Focusing of guided wave beams is a key for success, compared to conventional axisymmetric excitation. Guided wave scanning and significant sensitivity enhancement are established via the newly implemented focusing scheme. In addition, using both longitudinal and torsional modes is essential to improve sensitivity and reliability in data analysis over the conventional technique based on only longitudinal or torsional. The related theoretical studies and experimental data are also presented along with field test results for confirmation.

Low-frequency vibration modulation of guided waves to image nonlinear scatterers for structural health monitoring

Abstract: Guided wave structural health monitoring offers the prospect of continuous interrogation of large plate-like structures with a sparse network of permanently attached sensors. Currently, the most common approach is to monitor changes in the received signals by subtraction from a reference signal obtained when the structure was known to be defect-free. In this paper a comparison is made between this defect-free subtraction approach and a technique in which low-frequency vibration modulation of guided wave signals is used to detect nonlinear scatterers. The modulation technique potentially overcomes the need for the defect-free reference measurement as the subtraction is now made between different parts of an externally applied low-frequency vibration. Linear defects were simulated by masses bonded onto a plate and nonlinear scatterers were simulated by loading a similar mass against the plate. The experimental results show that the defect-free subtraction technique performs well in detecting the bonded mass whereas the modulation technique is able to discriminate between the bonded and loaded masses. Furthermore, because the modulation technique does not require a defect-free reference, it is shown to be relatively independent of temperature effects, a significant problem for reference based subtraction techniques.

Feasibility of low‐frequency straight‐ray guided wave tomography

Abstract: Many aging pipelines and aircraft are suffering from corrosion and the corrosion patches are often inaccessible There is therefore a need for a rapid, accurate, long range inspection technique to measure the remaining thickness in corrosion patches Low‐frequency guided wave tomography is a potentially attractive technique to rapidly evaluate the thickness of large sections of partially accessible structures This paper demonstrates that in the low‐frequency regime the ray theory may not be valid which compromises the use of any straight‐ray tomography algorithm This paper also shows, in simulations and experimentally, that the same frequency regime can be used to successfully reconstruct thickness reduction in plates with diffraction tomography

A Single Polarized Triangular Grid Tapered-Slot Array Antenna

Abstract: A triangular grid single polarized tapered-slot array antenna for radar applications is studied. Compared with a rectangular grid an equilateral triangular grid allows a larger unit cell without any onset of grating lobes. Since single polarized tapered-slots in triangular grids support guided modes, which cause scan blindness, the increase in unit cell size is smaller than the optimal 15%. The design presented in the paper is capable of scan angles out to 60° from broadside in the E and H planes. To improve the match over the radar band a local minimum in the active reflection coefficient is positioned at the most critical scan direction, resulting in a reflection coefficient that is less than -12 dB in the X-band. To reduce the radar cross section for the cross-polarization an absorbing layer is positioned above the ground plane, which affects some of the guided modes that lead to scan blindnesses. An experimental antenna with 16 × 16 elements was built, and it was found that the H-plane performance for large scan angles for the finite antenna deviates more than expected from the infinite array approximation. Otherwise both mutual coupling measurements and embedded element patterns agrees well with the numerical results.