Showing papers on "Guided wave testing published in 2008"

Sensitivity enhancement of guided-wave surface-plasmon resonance sensors

Abstract: It is demonstrated theoretically and experimentally that, by using the guided-wave surface-plasmon sensor configuration with a top layer of dielectric thin film (10-15 nm) having a high value of the real part of the dielectric function, it is possible to improve the sensitivity of the sensor up to 1 order of magnitude. The stability is improved because the thin nanolayer acts as a protection layer for the metal. The enhancement is due to the increase in the interaction volume and the evanescent field enhancement near the top layer-analyte interface.

Theory of resonant acoustic transmission through subwavelength apertures.

Abstract: A complete landscape is presented of the acoustic transmission properties of subwavelength apertures (slits and holes). First, we study the emergence of Fabry-Perot resonances in single apertures. When these apertures are placed in a periodic fashion, a new type of transmission resonance appears in the spectrum. We demonstrate that this resonance stems from the excitation of an acoustic guided wave that runs along the plate, which hybridizes strongly with the Fabry-Perot resonances associated with waveguide modes in single apertures. A detailed discussion of the similarities and differences with the electromagnetic case is also given.

Ultrasonic guided waves in bone

Abstract: Recent progress in quantitative ultrasound (QUS) has shown increasing interest toward measuring long bones by ultrasonic guided waves. This technology is widely used in the field of nondestructive testing and evaluation of different waveguide structures. Cortical bone provides such an elastic waveguide and its ability to sustain loading and resist fractures is known to be related to its mechanical properties at different length scales. Because guided waves could yield diverse characterizations of the bone's mechanical properties at the macroscopic level, the method of guided waves has a strong potential over the standardized bone densitometry as a tool for bone assessment. Despite this, development of guided wave methods is challenging, e.g., due to interferences and rnultiparametric inversion problems. This paper discusses the promises and challenges related to bone characterization by ultrasonic guided waves.

Finite element model for waves guided along solid systems of arbitrary section coupled to infinite solid media.

Abstract: The Semi-Analytical Finite Element (SAFE) method is becoming established as a convenient method to calculate the properties of waves which may propagate in a waveguide which has arbitrary cross-sectional shape but which is invariant in the propagation direction. A number of researchers have reported work relating to lossless elastic waves, and recently the solutions for nonpropagating waves in elastic guides and for complex waves in viscoelastic guides have been presented. This paper presents a further development, addressing the problem of attenuating waves in which the attenuation is caused by leakage from the waveguide into a surrounding material. This has broad relevance to many practical problems in which a waveguide is immersed in a fluid or embedded in a solid. The paper presents the principles of a procedure and then validates and illustrates its use on some examples. The procedure makes use of absorbing regions of material at the exterior bounds of the discretized domain.

Planar plasmonic terahertz guided-wave devices.

Abstract: We describe the experimental realization of planar plasmonic THz guided-wave devices using periodically perforated metal films. These perforated films behave as effective media for which the dielectric function can be broadly engineered. We initially use transmission measurements to measure the complex dielectric constants of these effective media and show experimentally that the effective plasma frequency corresponds to the cutoff frequency of the rectangular apertures. Using these structures, we demonstrate not only straight planar THz waveguides, but also more complex devices such as Y-splitters and 3-dB couplers. In each of these embodiments, we demonstrate that the propagating THz radiation is well confined in both the in-plane and out-of-plane axes. This approach opens exciting new avenues for both passive and active THz guided-wave devices and circuits.

Ultrasonic guided-waves characterization with Warped Frequency Transforms

Abstract: Guided wave (GW) dispersion curves can be extracted from a time-transient measurement by means of time-frequency representations (TFRs). Unfortunately, any TFR is subject to the time-frequency uncertainty principle. This, in general, limits the capability of TFRs to characterize closely spaced guided modes over a wide frequency range. To overcome this limitation, we implemented a new warped frequency transform that presents enhanced mode extraction capabilities because of a more flexible tiling of the time-frequency domain. The tiling is designed to match the dispersive spectro-temporal structure of a GW by selecting an appropriate map of the time-frequency plane. The proposed transformation is fast, invertible, and covariant to group delay shifts. An application to Lamb waves propagating in an aluminum plate is presented. Time-transient GWs propagation events obtained both numerically and experimentally are considered. The results show that the proposed warped frequency transform limits the interference patterns which appear with other TFRs and produces a sparse representation of the Lamb wave pattern that can be suitable for identification and characterization purposes.

Slow waves in fractures filled with viscous fluid

Abstract: Stoneley guided waves in a fluid-filled fracture generally have larger amplitudes than other waves, and therefore, their properties need to be incorporated in more realistic models. In this study, a fracture is modeled as an infinite layer of viscous fluid bounded by two elastic half-spaces with identical parameters. For small fracture thickness, I obtain a simple dispersion equation for wave-propagation velocity. This velocity is much smaller than the velocity of a fluid wave in a Biot-type solution, in which fracture walls are assumed to be rigid. At seismic prospecting frequencies and realistic fracture thicknesses, the Stoneley guided wave has wavelengths on the order of several meters and an attenuation Q factor exceeding 10, which indicates the possibility of resonance excitation in fluid-bearing rocks. The velocity and attenuation of Stoneley guided waves are distinctly different at low frequencies for water and oil. The predominant role of fractures in fluid flow at field scales is supported by permeability data showing an increase of several orders of magnitude when compared to values obtained at laboratory scales. These data suggest that Stoneley guided waves should be taken into account in theories describing seismic wave propagation in fluid-saturated rocks.

Guided wave propagation and mode differentiation in hollow cylinders with viscoelastic coatings.

Abstract: Guided wave propagation theories have been widely explored for about one century. Earlier theories on single-layer elastic hollow cylinders have been very beneficial for practical nondestructive testing on piping and tubing systems. Guided wave flexural (nonaxisymmetric) modes in cylinders can be generated by a partial source loading or any nonaxisymmetric discontinuity. They are especially important for guided wave mode control and defect analysis. Previous investigations on guided wave propagation in multilayered hollow cylindrical structures mostly concentrate on the axisymmetric wave mode characteristics. In this paper, the problem of guided wave propagation in free hollow cylinders with viscoelastic coatings is solved by a semianalytical finite element (SAFE) method. Guided wave dispersion curves and attenuation characteristics for both axisymmetric and flexural modes are presented. Due to the fact that dispersion curve modes obtained from SAFE calculations are difficult to differentiate from each other, a mode sorting method is established to distinguish modes by their orthogonality. Theoretical proof of the orthogonality between guided wave modes in a viscoelastic coated hollow cylinder is provided. Wave structures are also calculated and discussed in view of wave mechanics in multilayered cylindrical structures containing viscoelastic materials.

Scattering of the fundamental shear horizontal guided wave by a part-thickness crack in an isotropic plate.

Abstract: The interaction of the fundamental shear horizontal (SH0) guided mode with part-thickness cracks in an isotropic plate is studied as an extension within the context and general framework of previous work [“Short range scattering of the fundamental shear horizontal guided wave mode normally incident at a through thickness crack in an isotropic plate,” J. Acoust Soc. Am. 122, 1527–1538 (2007); “Angular influence on scattering when the fundamental shear horizontal guided wave mode is incident at a through-thickness crack in an isotropic plate,” J. Acoust. Soc. Am. 124, 2021–2030 (2008)] by the authors with through-cracks. The symmetric incidence case where the principal direction of the incident beam bisects the crack face at 90° is studied using finite element simulations validated by experiments and analysis, and conclusions are inferred for general incidence angles using insights obtained with the through-thickness studies. The influence of the crack length and the monitoring distance on the specular refl...

Circumferential higher order guided wave modes for the detection and sizing of cracks and pinholes in pipe support regions

Abstract: The non-dispersive propagation of ultrasonic guided wave higher order modes cluster (HOMC) traveling along the circumferential direction in a hollow cylinder and its interaction with defects in pipe support regions is reported. These circumferential guided waves were generated in mild steel (MS) pipe specimens containing artificially created axial notches (simulating axial cracks) and pinholes (simulating pinhole-like defects) of different sizes in order to simulate conditions such as cracking and corrosion under pipe supports. The characteristics of these guided waves were also studied as a function of parameters related to how they were generated; namely, using: (a) 2.25 MHz linear phased array transducer, (b) 2.25 MHz conventional circular transducer and (c) 1 MHz conventional circular transducer. These higher frequency modes were explored for their ability to detect and size defects. Because of access limitations to the pipe support region in actual field testing, the transducer was always placed at a fixed circumferential position and moved axially along the length of the pipe. The defect position along the circumference was ascertained from the time of flight while the defect size was estimated using the amplitude data. The signals obtained for all three transducer configurations are compared for their ability to locate, detect and size the above-mentioned defects. It was shown that at these relatively higher frequencies, the guided wave modes exhibit small dispersion and have the ability to provide improved imaging of small size defects throughout the cross-section of the pipe.

Ultrasonic guided waves for health monitoring of high-pressure composite tanks

Abstract: Ultrasonic guided wave modes are proposed to control the integrity of high-pressure composite tanks produced by EADS-ASTRIUM, France. The purpose is to demonstrate the potentiality of air-coupled transducers to set up a contact-less, single-sided technique for testing the moisture content and/or the micro-cracking of carbon-epoxy composite wound around a Titanium liner. Although guided waves have been experimentally propagated on a real tank, it was not allowed to damage this specimen. Therefore, plates made of similar composite materials than that constituting the tank winding were submitted to water intake or to thermal stresses. After immersing some plates in a humid chamber, it was demonstrated that the attenuation of the A0 guided wave mode is sensitive to the moisture content. Other plate samples were submitted to immersion in liquid nitrogen that induces transversal cracks shown to cause significant drops in the celerity of several guided waves. Inverse problems have been used for quantifying the effects of these damages on the material properties, and they showed that water intake increases the imaginary part of the Coulomb moduli, while micro-cracking decreases all the material stiffness moduli. Such changes in the material properties have then been used as input data for simulating waveforms corresponding to the propagation of circumferential or longitudinal wave modes in the tank. Changes in these waveforms, caused by simulated damages of the composite winding of the tank, have been shown to be quite significant. To conclude the study, an experimental sep-up using air-coupled transducers was employed to generate–detect guided wave modes over large distances in the real tank, with very good signal-to-noise ratios, thus demonstrating the possibility of using such elements for the non-destructive testing of high-pressure composite tanks during their lives.

Guided wave arrays for high resolution inspection

Abstract: The paper describes a general approach for processing data from a guided wave transducer array on a plate-like structure. The raw data set from such an array contains time-domain signals from each transmitter–receiver combination. The technique is based on linear superposition of signals in the frequency domain with some amplitude and phase factors and can be applied to any array geometry and any types of array elements. The problem of finding optimal coefficients, which allow the best resolution to be achieved with the minimum number of array elements, is investigated. It is shown that improvements in resolution are obtained at the expense of sensitivity to noise. A method of quantifying this sensitivity is presented. Results are shown that illustrate the application of the technique to a linear array and an array of circular geometry (containing a single ring of elements). Experimental data obtained from a guided wave array containing electromagnetic acoustic transducer elements for exciting and detecti...

Modeling the impact of soft tissue on axial transmission measurements of ultrasonic guided waves in human radius.

Abstract: Recent in vitro and simulation studies have shown that guided waves measured at low ultrasound frequencies (f=200 kHz) can characterize both material properties and geometry of the cortical bone wall. In particular, a method for an accurate cortical thickness estimation from ultrasound velocity data has been presented. The clinical application remains, however, a challenge as the impact of a layer of soft tissue on top of the bone is not yet well established, and this layer is expected to affect the dispersion and relative intensities of guided modes. The present study is focused on the theoretical modeling of the impact of an overlying soft tissue. A semianalytical method and finite-difference time domain simulations were used. The models developed were shown to predict consistently real in vivo data on human radii. As a conclusion, clinical guided wave data are not consistent with in vitro data or related in vitro models, but use of an adequate in vivo model, such as the one introduced here, is necessary. A theoretical model that accounts for the impact of an overlying soft tissue could thus be used in clinical applications.

Structural health monitoring by extraction of coherent guided waves from diffuse fields.

Abstract: Recent theoretical and experimental studies in a wide range of applications have demonstrated that Green's functions (impulse responses) can be extracted from cross-correlation of diffuse fields using only passive sensors. This letter demonstrates the passive-only reconstruction of coherent Lamb waves (dc-500 kHz) in an aluminum plate of thickness comparable to aircraft fuselage and wing panels. It is further shown that the passively reconstructed waves are sensitive to the presence of damage in the plate as it would be expected in a typical "active" guided wave test. This proof-of-principle study suggests the potential for a structural health monitoring method for aircraft panels based on passive ultrasound imaging reconstructed from diffuse fields.

Simulation of piezoelectric excitation of guided waves using waveguide finite elements

Abstract: A numerical method for computing the time response of infinite constant cross-section elastic waveguides excited by piezoelectric transducers was developed. The method combined waveguide finite elements (semi-analytical finite elements) for modeling the waveguide with conventional 3-D piezoelectric finite elements for modeling the transducer. The frequency response of the coupled system was computed and then used to simulate the time response to tone-burst electrical excitation. A technique for identifying and separating the propagating modes was devised, which enabled the computation of the response of a selected reduced number of modes. The method was applied to a rail excited by a piezoelectric patch transducer, and excellent agreement with measured responses was obtained. It was found that it is necessary to include damping in the waveguide model if the response near a ldquocut-onrdquo frequency is to be simulated in the near-field.

Scattering of the fundamental shear horizontal mode in a plate when incident at a through crack aligned in the propagation direction of the mode.

Abstract: A study of the scattering of the fundamental guided wave SH(0) at a through-thickness narrow notch directed along the wave's propagation in a plate is presented. The results are obtained from Finite Element simulations and experimental measurements. Good agreement is found between the simulations and the measurements. The results are shown for a range of crack lengths and shapes. The scattered wave field consists of the reflected and diffracted SH(0) mode and also contributions from mode conversions to the S(0) mode. It is found that the coefficient of direct reflection of the SH(0) mode has an undulating nature depending on the length of the crack. This is caused by interference phenomena that are related to the interaction of different surface wave types generated on the crack surfaces and their diffractions at both tips of the crack. It is shown that the dominating part of this reflection is generated by the delayed "Rayleigh type" surface waves reflected from the far tip of the crack.

Improved Full-Vector Finite-Difference Complex Mode Solver for Optical Waveguides of Circular Symmetry

Abstract: An improved full-vector finite-difference (FD) complex mode solver for circularly symmetric optical waveguides is developed and validated. The formulations are derived from Taylor expansion of transverse electric and magnetic fields and match of boundary conditions at radial index discontinuities. Calculation of the guided and leaky modes of step index fibers, Bragg fibers, and surface plasmon polaritons structures shows significant improvement in terms of accuracy and rate of convergence without increase of computational efforts, in comparison with the conventional FD scheme based on the average-index approximation. It is demonstrated that the performance of the new scheme is robust for high-order modes and waveguides with high index contrast. Contrary to the conventional FD solvers that claim the magnetic field formulation is more rigorous than the electric field formulation, the improved solver yields practically identical results for both formulations.

Energy concentration at the center of large aspect ratio rectangular waveguides at high frequencies

Abstract: Waveguides in non-destructive evaluation (NDE) applications are commonly of a regular geometry (e.g., circular and ring cross section) for which analytical solutions exist. In this paper, wave propagation in infinitely long strips of large rectangular aspect ratio is discussed. Due to the finite width of strips, a large number of modes exist within the structure. This complicates the analysis and usually discourages the use of strip waveguides in NDE sensors. However, it is shown that among the many modes of a strip, there are some with very desirable properties. This is highlighted by the example of two guided wave modes of a large aspect ratio rectangular strip whose dispersion characteristics approach those of the fundamental modes of an infinitely wide plate at high frequencies. The energy of these modes concentrates in the central region of the strip and decays toward the edges so that the strip waveguide can easily be mechanically attached to other components without influencing the wave propagation. Dispersion curves and mode shapes were derived by using a semianalytical finite element technique and are presented over a range of frequencies. It is shown that selective excitation of both modes is possible in practice and the experimental setup is described.

Guided wave SHM with a distributed sensor network

Abstract: It has been shown by many researchers that guided wave structural health monitoring is capable of detecting the presence of damage in a structure. The requirements for grid spacing and sensitivity to temperature change have been established and can be used to specify an array with a given signal to noise ratio. What is not clear at this point is how, given that damage is detectable, its location should be found. This paper discusses two different imaging algorithms and investigates the relative merits of each. This is initially done on the smallest possible array of three transducers. This is then carried forward to larger sparse arrays to show how a larger structure with a distributed sensor network can be imaged with several "units" of transducers working together. It is shown that in general using more transducers is beneficial to the quality of imaging achieved. However it is still necessary to perform imaging using smaller arrays to ensure that in the event of multiple damage sites occurring simultaneously each can be detected.

Fdtd study on wave propagation in layered structures with biaxial anisotropic metamaterials

Abstract: The Gaussian beam propagation in multi-layered structures that include indefinite anisotropic metamaterial (AMM) are simulated with shift operator method in Finite-Difference Time-Domain method (FDTD). The excitations of backward and forward surface affected by the types of biaxial AMM are investigated. Numerical results show that the directions of the guided wave excited are influenced by the sign of z component of relative permeability tensor of AMM that determines the energy flow is positively refracted or negatively refracted. Positive or negative Goos-Hanchen shift associated with Total Cutoff media are also shown.

Piezoelectric wafer active sensors for in situ ultrasonic‐guided wave SHM

Abstract: In situ structural health monitoring aims to perform on-demand interrogation of the structure to determine the presence of service-induced damage and defects using non- destructive evaluation ultrasonic wave methods. Recently emerged piezoelectric wafer active sensors (PWAS) have the potential to significantly improve damage detection and health monitoring. PWAS are low-profile transducers that can be permanently attached onto the structure or inserted in between composite laminates, and can perform structural damage detection in thin-wall structures using guided wave methods (Lamb, Rayleigh, SH, etc.). This paper describes the analytical and experimental work of using PWAS-guided waves for in situ structural damage detection on thin-wall structures. We begin with reviewing the guided wave theory in plate structures and PWAS principles. The mechanisms of Lamb wave excitation and detection using PWAS is presented. Subsequently, we address in turn the use of PWAS to generate Lamb waves for damage (cracks and corrosion) detection in metallic structures. Pulse-echo, pitch-catch, phased array and time reversal methods are illustrated demonstrating that PWAS Lamb-waves techniques are suitable for damage detection and structural health monitoring. The last part of the paper treats analytically and experimentally PWAS excitation and tuning in composite materials. The research results presented in this paper show that in situ SHM methodologies using PWAS transducers hold the promise for more efficient, effective and timely damage detection in thin-wall structures.

Ultrasonic Circumferential Guided Wave for Pitting-Type Corrosion Imaging at Inaccessible Pipe-Support Locations

Abstract: A higher order cylindrically guided ultrasonic wave was used for the detection and sizing of hidden pitting-type corrosion in the hidden crevice regions (between the pipe and the pipe supports) without lifting or disturbing the structural layout arrangement of the pipelines. The higher order circumferential guided waves were generated using a piezoelectric crystal based transducer, located at the accessible top region of the pipes, in a pulse-echo mode. By studying the experimental parameters such as dispersion, particle displacement, and wavelength of the ultrasonic guided wave modes, an appropriate higher order mode was selected for excitation using an appropriately designed acrylic angle wedge that conforms to the pipe's outer curvature. A manual pipe crawler was designed with a provision for holding the wedge, and the essential hardware such as data acquisition card, encoder, etc., was integrated with the system so that the corrosion was mapped in real time during the scanning of the pipes. The system was validated on pipes ranging from 6 in. to 24 in. outer diameters of wall thicknesses up to 12 mm, by mapping defects as small as 1.5 mm diameter and 25% penetration wall thickness. A 2D finite element model using ABAQUS® was used to understand the wave propagation in pipe wall and its interaction with pinhole-type defects.

Design of resonance grating coupler

Abstract: A new integrated-optic coupler was proposed for coupling a guided wave to a free-space wave propagating vertically from the waveguide surface. The coupler consists of a grating coupler in a cavity formed by two distributed Bragg reflectors, and has a small aperture. The cavity was designed to eliminate both transmission and reflection of the incident guided wave, resulting in 100 % radiation by a several-micron aperture. Design consideration was theoretically discussed based on the coupled mode analysis. Predicted performance was simulated and confirmed by the finite difference time domain method.

Finite element simulation of the generation and detection by air-coupled transducers of guided waves in viscoelastic and anisotropic materials

Abstract: The measured characteristics (efficiency and sensitivity) of two air-coupled transducers allow for the prediction of the absolute values of the pressure of the bulk waves generated in air and for the measurement of the pressure of the field radiated in air by guided waves propagating in a structure. With finite element software, the pressure field generated by an air-coupled transducer is simulated by introducing a right-hand side member in the Helmholtz equation, which is used for computing the propagation from the transducer to a plate. The simulated source is rotated in order to impose an angle of incidence with respect to the normal of the plate and generate the corresponding guided mode. Inside the plate, the propagation is simulated with the dynamic equations of equilibrium and a complex stiffness tensor to take into account the viscoelastic anisotropy of the material. For modeling the three-dimensional fields of the guided modes propagating in a two-dimensional non-symmetry plane, a 2.5 dimensional model is introduced. The model computes the value of the pressure field radiated in air by the plates for any guided modes and can predict the detectability of the system for a known defect in a structure. A test bed incorporating two air-coupled transducers is used to generate and receive various guided modes. Two plates made of Perspex and carbon-epoxy composite are tested. The pressure measured by the receiver at various positions is compared to the results of the model to validate it.

Detection of Disbonds in Multi-layer Structures by Laser-Based Ultrasonic Technique

Abstract: Adhesively bonded multi-layer structures are frequently used, mostly in the aerospace industry, for their structural efficiency. Nondestructive evaluation of bond integrity in these types of structures, both after manufacturing and for periodic inspection during service, is extremely important. A laser-based ultrasonic technique has been evaluated for non-contact detection of disbonds in aluminum multi-layer structures. Two configurations have been used to detect disbonded areas: pitch-catch with unidirectional guided wave scan and through-transmission with bidirectional scan. Guided wave scanning was done with a laser line source and air-coupled transducer sensing at 500 kHz, 1 MHz, and 2 MHz. Signals showed attenuation of the main frequency component and frequency shift on disbonded areas, whereas, a regular and standard waveform is seen outside disbonds. In through-transmission the longitudinal wave at normal incidence was monitored with a 1 MHz probe. One sample showed, besides the introduced inserts,...

Guided wave pipeline inspection system and method with enhanced natural focusing techniques

Abstract: A method for the nondestructive inspecting of coated or uncoated pipeline, using ultrasonic guided waves excited on the outer or inner pipe surface, wherein at least one or more transducers are individually or simultaneously excited to generate ultrasound, wherein multiple received signals with different focal spot positions are processed and combined to produce a reduced number of final waveforms that show defect axial positions in the pipe, wherein a data calibration scheme is utilized to adjust velocity variability for all the guided wave modes at different frequencies, and wherein the hardware arrangement has at least one pulser channel and one receiver channel for the collection and storage of signals.