Showing papers on "Guided wave testing published in 2007"

Active health monitoring of an aircraft wing with embedded piezoelectric sensor/actuator network: I. Defect detection, localization and growth monitoring

Abstract: This work focuses on an ultrasonic guided wave structural health monitoring (SHM) system development for aircraft wing inspection. In part I of the study, a detailed description of a real aluminum wing specimen and some preliminary wave propagation tests on the wing panel are presented. Unfortunately, strong attenuation and scattering impede guided waves for large-area inspection. Nevertheless, small, low-cost and light-weight piezoelectric (PZT) discs were bonded to various parts of the aircraft wing, in a form of relatively sparse arrays, for simulated cracks and corrosion monitoring. The PZT discs take turns generating and receiving ultrasonic guided waves. Pair-wise through-transmission waveforms collected at normal conditions served as baselines, and subsequent signals collected at defected conditions such as rivet cracks or corrosion detected the presence of a defect and its location with a novel correlation analysis based technique called RAPID (reconstruction algorithm for probabilistic inspection of defects). The effectiveness of the algorithm was tested with several case studies in a laboratory environment. It showed good performance for defect detection, size estimation and localization in complex aircraft wing structures.

Macro-fiber composite piezoelectric rosettes for acoustic source location in complex structures

Abstract: An approach based upon the employment of piezoelectric transducer rosettes is proposed for passive damage or impact location in anisotropic or geometrically complex structures. The rosettes are comprised of rectangular macro-fiber composite (MFC) transducers which exhibit a highly directive response to ultrasonic guided waves. The MFC response to flexural (A0) motion is decomposed into axial and transverse sensitivity factors, which allow extraction of the direction of an incoming wave using rosette principles. The wave source location in a plane is then simply determined by intersecting the wave directions detected by two rosettes. The rosette approach is applicable to anisotropic or geometrically complex structures where the conventional time-of-flight source location is challenging due to the direction-dependent wave velocity. The performance of the rosettes for source location is validated through pencil-lead breaks performed on an aluminum plate, an anisotropic CFRP laminate and a complex CFRP-honeycomb sandwich panel.

An Investigation Into the Temperature Stability of a Guided Wave Structural Health Monitoring System Using Permanently Attached Sensors

Abstract: When testing complex structures using a deployable guided wave system, the measured signals are often far too complex to directly interpret without a priori knowledge of the geometry of the structure. However, in a structural health monitoring (SHM) scenario, the sensors are permanently attached to the structure and so changes in the system can be monitored by comparison to an earlier baseline measurement, taken when the structure was undamaged. This paper investigates such a system for the SHM of plate-like structures using guided waves. The basis of the system is a distributed array of permanently attached piezoelectric guided wave transducers where pairs of transducers are used in pitch-catch configuration allowing the detection and localization of damage in the plate. It has been observed that the use of a single baseline measurement suffers in the medium to long-term due to the variation of environmental conditions. A key parameter in the instability is the change in the temperature of the test structure. A method of using a database of baselines, termed optimal baseline subtraction (OBS), is described and applied to an experimental SHM system. The objective of this paper is to investigate the suitability and robustness of OBS under varying rates of temperature change and its use in the localization of damage. Results have shown that through the OBS an improvement of up to 20 dB in signal-to-coherent noise ratio may be achieved compared with single baseline subtraction

Health Monitoring of UAV Wing Skin-to-spar Joints using Guided Waves and Macro Fiber Composite Transducers:

Abstract: This article deals with the monitoring of the composite wing skin-to-spar joint in unmanned aerial vehicles using ultrasonic guided waves The study investigates simulated wing skin-to-spar joints with two different types of bond defects, namely poorly cured adhesive and disbonded interfaces The bond-sensitive feature considered is the ultrasonic strength of transmission through the joints The dispersive wave propagation problem is studied numerically by a semi-analytical finite element method that accounts for viscoelastic damping, and experimentally by ultrasonic testing that uses highly durable, flexible macro fiber composite transducers The discrete wavelet transform is also employed to de-noise and compress the ultrasonic measurements Both numerical and experimental tests confirm that the ultrasonic strength of transmission increases across the defected bonds

Wave propagation along transversely periodic structures

Abstract: The dispersion curves for guided waves have been of constant interest in the last decades, because they constitute the starting point for NDE ultrasonic applications. This paper presents an evolution of the semianalytical finite element method, and gives examples that illustrate new improvements and their importance for studying the propagation of waves along periodic structures of infinite width. Periodic boundary conditions are in fact used to model the infinite periodicity of the geometry in the direction normal to the direction of propagation. This method allows a complete investigation of the dispersion curves and of displacement ∕ stress fields for guided modes in anisotropic and absorbing periodic structures. Among other examples, that of a grooved aluminum plate is theoretically and experimentally investigated, indicating the presence of specific and original guided modes.

Modeling the excitation of guided waves in generally anisotropic multilayered media

Abstract: The design of transducers to excite and detect guided waves is a fundamental part of a nondestructive evaluation or structural health monitoring system and requires the ability to predict the radiated guided wave field of a transmitting transducer. For most transducers, this can be performed by making the assumption that the transducer is weakly coupled and then integrating the Green’s function of the structure over the area of the transducer. The majority of guided wave modeling is based on two-dimensional (2D) formulations where plane, straight-crested waves are modeled. Several techniques can be readily applied to obtain the solution to the forced 2D problem in terms of modal amplitudes. However, for transducer modeling it is desirable to obtain the complete three-dimensional (3D) field, which is particularly challenging in anisotropic materials. In this paper, a technique for obtaining a far-field asymptotic solution to the 3D Green’s function in terms of the modal solutions to the forced 2D problem i...

Three-dimensional finite element modeling of guided ultrasound wave propagation in intact and healing long bones.

Abstract: The use of guided waves has recently drawn significant interest in the ultrasonic characterization of bone aiming at supplementing the information provided by traditional velocity measurements. This work presents a three-dimensional finite element study of guided wave propagation in intact and healing bones. A model of the fracture callus was constructed and the healing course was simulated as a three-stage process. The dispersion of guided modes generated by a broadband 1-MHz excitation was represented in the time-frequency domain. Wave propagation in the intact bone model was first investigated and comparisons were then made with a simplified geometry using analytical dispersion curves of the tube modes. Then, the effect of callus consolidation on the propagation characteristics was examined. It was shown that the dispersion of guided waves was significantly influenced by the irregularity and anisotropy of the bone. Also, guided waves were sensitive to material and geometrical changes that take place during healing. Conversely, when the first-arriving signal at the receiver corresponded to a nondispersive lateral wave, its propagation velocity was almost unaffected by the elastic symmetry and geometry of the bone and also could not characterize the callus tissue throughout its thickness. In conclusion, guided waves can enhance the capabilities of ultrasonic evaluation.

Physics of negative refraction and negative index materials : optical and electronic aspects and deversified approaches

Abstract: Negative Refraction of Electromagnetic and Electronic Waves in Uniform Media.- Anisotropic Field Distributions in Left-Handed Guided Wave Electronic Structures and Negative Refractive Bicrystal Heterostructures.- "Left-Handed" Magnetic Granular Composites.- Spatial Dispersion, Polaritons, and Negative Refraction.- Negative Refraction in Photonic Crystals.- Negative Refraction and Subwavelength Focusing in Two-Dimensional Photonic Crystals.- Negative Refraction and Imaging with Quasicrystals.- Generalizing the Concept of Negative Medium to Acoustic Waves.- Experiments and Simulations of Microwave Negative Refraction in Split Ring and Wire Array Negative Index Materials, 2D Split-Ring Resonator and 2D Metallic Disk Photonic Crystals.- Super Low Loss Guided Wave Bands Using Split Ring Resonator-Rod Assemblies as Left-Handed Materials.- Development of Negative Index of Refraction Metamaterials with Split Ring Resonators and Wires for RF Lens Applications.- Nonlinear Effects in Left-Handed Metamaterials.

Electromagnetic wave propagation in an almost circular bundle of closely packed metallic carbon nanotubes

Abstract: An equivalent-multishell approach for the approximate calculation of the characteristics of electromagnetic waves propagating in almost circular (azimuthally symmetric), closely packed bundles of parallel, identical, and metallic carbon nanotubes (CNTs) yields results in reasonably good agreement with a many-body technique, for infinitely long bundles when the number of CNTs is moderately high. The slow-wave coefficients for azimuthally symmetric guided waves increase with the number of metallic CNTs in the bundle, tending for thick bundles to unity, which is characteristic of macroscopic metallic wires. The existence of an azimuthally nonsymmetric guided wave at low frequencies in a bundle of a large number of finite-length CNTs stands in contrast to the characteristics of guided-wave propagation in a single CNT. The equivalent-multishell approach yields the polarizability scalar and the antenna efficiency of a bundle of finite-length CNTs in the long-wavelength regime over a wide frequency range spanning the terahertz and the near-infrared regimes. Edge effects give rise to geometric resonances in such bundles. The antenna efficiency of a CNT bundle at the first resonance can exceed that of a single CNT by 4 orders of magnitude, which is promising for the design and development of CNT-bundle antennas and composite materials containing CNT bundles as inclusions.

Finite element modeling and experimental characterization of crosstalk in 1-D CMUT arrays

Abstract: Crosstalk is the coupling of energy between the elements of an ultrasonic transducer array. This coupling degrades the performance of transducers in applications such as medical imaging and therapeutics. In this paper, we present an experimental demonstration of guided interface waves in capacitive micromachined ultrasonic transducers (CMUTs). We compare the experimental results to finite element calculations using a commercial package (LS-DYNA) for a 1-D CMUT array operating in the conventional and collapsed modes. An element in the middle of the array was excited with a unipolar voltage pulse, and the displacements were measured using a laser interferometer along the center line of the array elements immersed in soybean oil. We repeated the measurements for an identical CMUT array covered with a 4.5-mum polydimethyl-siloxane (PDMS) layer. The main crosstalk mechanism is the dispersive guided modes propagating in the fluid-solid interface. Although the transmitter element had a center frequency of 5.8 MHz with a 130% fractional bandwidth in the conventional operation, the dispersive guided mode was observed with the maximum amplitude at a frequency of 2.1 MHz, and had a cut-off frequency of 4 MHz. In the collapsed operation, the dispersive guided mode was observed with the maximum amplitude at a frequency of 4.0 MHz, and had a cut-off frequency of 10 MHz. Crosstalk level was lower in the collapsed operation (-39 dB) than in the conventional operation (-24.4 dB). The coverage of the PDMS did not significantly affect the crosstalk level, but reduced the phase velocity for both operation modes. Lamb wave modes, A0 and S0, were also observed with crosstalk levels of -40 dB and -65 dB, respectively. We observed excellent agreement between the finite element and the experimental results

Numerical investigation of elastic modes of propagation in helical waveguides

Abstract: Steel multi-wire cables are widely employed in civil engineering. They are usually made of a straight core and one layer of helical wires. In order to detect material degradation, nondestructive evaluation methods based on ultrasonics are one of the most promising techniques. However, their use is complicated by the lack of accurate cable models. As a first step, the goal of this paper is to propose a numerical method for the study of elastic guided waves inside a single helical wire. A finite element (FE) technique is used based on the theory of wave propagation inside periodic structures. This method avoids the tedious writing of equilibrium equations in a curvilinear coordinate system yielding translational invariance along the helix centerline. Besides, no specific programming is needed inside a conventional FE code because it can be implemented as a postprocessing step of stiffness, mass and damping matrices. The convergence and accuracy of the proposed method are assessed by comparing FE results with Pochhammer-Chree solutions for the infinite isotropic cylinder. Dispersion curves for a typical helical waveguide are then obtained. In the low-frequency range, results are validated with a helical Timoshenko beam model. Some significant differences with the cylinder are observed.

A single probe spatial averaging technique for guided waves and its application to surface wave rail inspection

Abstract: The nondestructive testing of structures using guided waves requires systems with high mode selectivity. Usually this is achieved with relatively complex probes comprising multiple transducer rings or arrays. For the rapid inspection of very long structures with only partial access to the waveguide, this may not be a viable solution. In this paper we present a very flexible alternative whereby a simple robust probe is scanned along the wave guide, and the acquired scan data is used for customizing the mode selectivity at the postprocessing stage. The characteristics of this spatial averaging method are discussed using a simple analytical model and compared to an existing linear array technique. The mode selectivity is found to be mainly limited by the uncertainty of the phase velocity assumed for the mode of interest. The method was successfully applied to surface wave rail inspection and was found to suppress unwanted modes very efficiently.

Structural Health Monitoring with Piezoelectric Wafer Active Sensors for Space Applications

Abstract: Ultrasonic guided waves inspection using Lamb waves is suitable for damage detection in metallic structures. This paper will present experimental results obtained using guided Lamb waves to detect flaws in aluminum specimens with design features applicable to space applications. Two aluminum panels were fabricated from a variable-thickness aluminum top plate, with two bolted I-beams edge stiffeners and four bonded angle stiffeners. Artificial damages were introduced in the two panels: cracks, corrosions, and disbonds. The proposed investigation methods used bonded piezoelectric wafer active sensors to excite and receive Lamb waves. Three wave propagation methods were used: pitch-catch, pulse-echo, and the embedded ultrasonic structural radar. In addition, we also used a standing-wave damage detection technique, the electromechanical impedance method. The paper will present in detail the salient results from using these methods for damage detection and structural health monitoring. Where appropriate, comparison between different methods in detecting the same damage will be performed. The results have demonstrated the ability of piezoelectric wafer active sensors working in conjunction with guided Lamb waves to detect various types of damages present in complex geometry structures typical of space applications.

Short range scattering of the fundamental shear horizontal guided wave mode normally incident at a through-thickness crack in an isotropic plate

Abstract: Interaction of the fundamental shear horizontal mode with through-thickness cracks in an isotropic plate is studied in the context of low frequency array imaging for ultrasonic guided wave nondestructive evaluation with improved resolution. Circular wave fronts are used and the symmetric case where a line from the wave source bisects the crack face normally is considered. Finite element simulations are employed to obtain trends subject to analytical and experimental validation. The influence of the crack length and of the location of source and measurement positions on the specular reflection from the crack face is first examined. These studies show that low frequency short range scattering is strongly affected by diffraction phenomena, leading to focusing of energy by the crack in the backscatter direction. Study of the diffraction from the crack edges reveals contributions due to a direct diffraction at the edges and multiple reverberations across the crack length. A simple diffraction model is shown to adequately represent cracks up to moderate lengths, providing an easy means of estimating the far field of the waves. The presence of multiple diffraction components is quantitatively established and surface waves on the crack face are identified as equivalent to low frequency symmetric modes of rectangular ridge waveguides.

Guided-wave tomographic imaging of defects in pipe using a probabilistic reconstruction algorithm

Abstract: There has been much interest recently in monitoring the structural integrity of predetermined critical zones in pipeline using leave-in-place sensors. Ultrasonic guided waves have significant advantages over vibration and impedance methods for structural health monitoring. With guided waves it is possible to locate and size defect regions. The work presented here introduces a Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID), which is used to construct tomographic images of multiple defects in a 16"-diameter schedule 30 steel pipe. Imaging is completed using an array of 16 low-cost transducers. It is shown that defect location and severity are accurately predicted by employing the RAPID algorithm with multifrequency data sets. A sparse array study is completed in which images are reconstructed using only eight of the 16 available transducers. It is found that damage can still be detected using the sparse array, though defect sizing and location accuracy suffer. It is shown that multiple defects can still be resolved using 13 of the original 16 transducers chosen at random.

Phased array focusing with guided waves in a viscoelastic coated hollow cylinder

Abstract: Guided wave phased array focusing has shown many advantages in long-range pipeline inspection, such as, longer inspection distance, greater wave penetration power and higher detection resolution. Viscoelastic coatings applied to a large percentage of pipes for protection purposes created some challenges in terms of focusing feasibility and inspection ability. Previous studies were all based on bare pipe models. In this work, guided wave phased array focusing in viscoelastic coated pipes is studied for the first time. Work was carried out with both numerical and experimental methods. A three-dimensional finite element model was developed for quantitatively and systematically modeling guided waves in pipes with different viscoelastic materials. A method of transforming measured coating properties to finite element method inputs was created in order to create a physically based model of guided waves in coated pipes. Guided wave focusing possibilities in viscoelastic coated pipes and the effects from coatings were comprehensively studied afterwards. A comparison of focusing and nonfocusing inspections was also studied quantitatively in coated pipe showing that focusing increased the wave energy and consequently the inspection ability tremendously. This study provides an important base line and guidance for guided wave propagation and focusing in a real field pipeline under various coating and environmental conditions.

Finite-Element Method Simulations of Guided Wave Phenomena at Terahertz Frequencies

Abstract: As the science and engineering associated with terahertz time-domain spectroscopy and imaging evolves past the use of conventional free-space optics, the continued development of waveguides for terahertz pulses is increasingly relevant. The ability to model and simulate terahertz wave propagation aids in the development, visualization, and understanding of novel terahertz devices and phenomena. We discuss the use of the finite-element method, a powerful computational tool for the modeling of guided wave phenomena and devices at terahertz frequencies.

Genetic algorithm based reconstruction of the elastic moduli of orthotropic plates using an ultrasonic guided wave single-transmitter-multiple-receiver SHM array

Abstract: The reconstruction of all nine unknown elastic moduli of orthotropic plate structures has been achieved using a single-transmitter-multiple-receiver (STMR) compact structural health monitoring (SHM) array. This method uses the velocity measurement of the fundamental guided Lamb wave modes (S0 and A0), generated from a central transmitter, and received by a sparse array of receivers that encircle the transmitter. The measured velocities are then used in an inversion algorithm based on genetic algorithms. A prototype compact STMR array was developed and used in the measurement. Simulated data were used to demonstrate the feasibility of the technique. Experiments were conducted on 3.15 mm graphite–epoxy composite plate using a PZT based STMR array as well as laser vibrometer based displacement measurement. Experimental Lamb wave velocity data were used to validate the present technique. This technique finds application in the areas of material characterization and SHM of anisotropic plate-like structures used in aerospace and automobile components made using fiber reinforced composites.

Demonstration of Negative Refraction in a Cutoff Parallel-Plate Waveguide Loaded With 2-D Square Lattice of Dielectric Resonators

Abstract: A 2D negative-refractive index metamaterial is proposed and investigated, which is composed of a parallel-plate waveguide loaded with a square lattice of disc-type dielectric resonators. Collective and macroscopic behavior of the dielectric resonator lattice under the fundamental TE resonance gives negative effective permeability, whereas the parallel-plate waveguide below the cutoff for TE modes provides negative effective permittivity. Thus, the double-negative condition for the propagated waves with the appropriate polarizations can establish the left-handedness. Equivalent-circuit models are shown to give a good insight into the physical mechanism of the guided waves. Numerical simulation of the 2D dispersion diagram verifies the existence of the left-handed (LH) guided modes along with the isotropic characteristics. A triangular prism of the proposed LH structure that is sandwiched by the right-handed parallel-plate waveguides is designed and fabricated to directly observe the negative refractive index of the LH waveguide. The numerical and experimental results validate the negative refraction for the proposed structure.

3-D Elasticity-based Modeling of Anisotropic Piezocomposite Transducers for Guided Wave Structural Health Monitoring

Abstract: Anisotropic piezocomposite transducers (APTs), such as macro fiber composites and active fiber composites, have great potential to be used as structurally integrated transducers for guided-wave (GW) structural health monitoring. Their main advantages over conventional monolithic piezoceramic wafer transducers are mechanical flexibility, curved surface conformability, power efficiency, their ability to excite focused GW fields, and their unidirectional sensing capability as a GW sensor. In this paper, models are developed to describe excitation of GW fields by APTs in isotropic structures. The configurations explored are plane Lamb-wave fields in beams with rectangular cross-section, axisymmetric GW fields in cylinders, and 3-D GW fields in plates. The dynamics of the substrate and transducer are assumed uncoupled. The actuator is modeled as causing shear traction at the edges of the actuator's active area along the fiber direction. The sensor is modeled as sensing the average extensional strain over the active area along the fiber direction. The work is unique in that the formulation is based on 3-D elasticity, and no reduced-order structural assumptions are used. This is crucial to model multimodal GW propagation, especially at high frequencies. A formulation is also proposed to model the behavior of APTs as GW sensors. Finally, results from experimental tests to examine the validity of the models are discussed and the possible sources of error are examined in detail.

Guided wave phased array beam steering in composite plates

Abstract: Guided wave phased array systems have great potential in structural health monitoring (SHM), especially for aircraft applications due to its capability of steering the emitted guided wave beam to inspect a large area with the sensor array at just one accessible position. However, when the guided wave phased array is applied to composite panels of an aircraft component, the anisotropic behavior of the composite materials leads to a significant influence on the beam steering performance of the phased array. In this study, mode neighborhoods in dispersion curves where guided waves have quasi-isotropic behavior (i.e. constant or similar phase velocities in different wave propagation directions) are explored for unidirectional, cross-ply, and quasi-isotropic composite plates. It was demonstrated that the energy skewness of guided waves was well suppressed in these mode neighborhoods. Furthermore, by utilizing a modified delay-and-sum beam forming algorithm, the guided wave beam of a linear phased array can be steered quite well to the desired directions in a composite plate.

Optical range microcavities and filters using multiple dielectric layers in metal-insulator-metal structures.

Abstract: We present a technique to adjust omnidirectional resonances in planar metallic microcavity structures. It is demonstrated numerically with realistic material parameters that by changing the thickness of different dielectric layers in metal-dielectric-metal structures, the omnidirectional resonance can be chosen to lie at an arbitrary frequency in the optical range. These ideas can also be applied to design a high-pass filter in the optical frequency range.

Enhanced transmission through subwavelength metallic coaxial apertures by excitation of the TEM mode

Abstract: We present here the first theoretical demonstration of enhanced transmission (up to 90%) through annular aperture arrays engraved into opaque metallic plates thanks to the excitation of the TEM guided mode inside each coaxial cavity. The generation of this peculiar mode is obtained, first, by illuminating the structure under oblique incidence and, second, by considering a TM polarization. Analytical demonstration is performed to confirm the involvement of these two conditions for the emergence of this guided mode. The originality of this configuration comes, first, from the fact that the TEM mode has no cut-off wavelength and, second, from the fact that the transmission peak position is independent of the angle of incidence.

Finite-Difference Time-Domain Study of Guided Modes in Nano-Plasmonic Waveguides

Abstract: A conformal dispersive finite-difference time-domain (FDTD) method is developed for the study of one-dimensional (1-D) plasmonic waveguides formed by an array of periodic infinite-long silver cylinders at optical frequencies. The curved surfaces of circular and elliptical inclusions are modelled in orthogonal FDTD grid using effective permittivities (EPs) and the material frequency dispersion is taken into account using an auxiliary differential equation (ADE) method. The proposed FDTD method does not introduce numerical instability but it requires a fourth-order discretisation procedure. To the authors' knowledge, it is the first time that the modelling of curved structures using a conformal scheme is combined with the dispersive FDTD method. The dispersion diagrams obtained using EPs and staircase approximations are compared with those from the frequency domain embedding method. It is shown that the dispersion diagram can be modified by adding additional elements or changing geometry of inclusions. Numerical simulations of plasmonic waveguides formed by seven elements show that row(s) of silver nanoscale cylinders can guide the propagation of light due to the coupling of surface plasmons.

Analysis of Piezoelectric Ultrasonic Transducers Attached to Waveguides Using Waveguide Finite Elements

Abstract: A finite-element modeling procedure for computing the frequency response of piezoelectric transducers attached to infinite constant cross-section waveguides, as encountered in guided wave ultrasonic inspection, is presented. Two-dimensional waveguide finite elements are used to model the waveguide. Conventional three-dimensional finite elements are used to model the piezoelectric transducer. The harmonic forced response of the waveguide is used to obtain a dynamic stiffness matrix (complex and frequency dependent), which represents the waveguide in the transducer model. The electrical and mechanical frequency response of the transducer, attached to the waveguide, can then be computed. The forces applied to the waveguide are calculated and are used to determine the amplitude of each mode excited in the waveguide. The method is highly efficient compared to time integration of a conventional finite- element model of a length of waveguide. In addition, the method provides information about each mode that is excited in the waveguide. The method is demonstrated by modeling a sandwich piezoelectric transducer exciting a waveguide of rectangular cross section, although it could be applied to more complex situations. It is expected that the modeling method will be useful during the optimization of piezoelectric transducers for exciting specific wave propagation modes in waveguides.