Showing papers on "Guided wave testing published in 1998"

The Reflection of Guided Waves From Circumferential Notches in Pipes

Abstract: The reflection of the L(0, 2), axially symmetric guidea elastic wave from notches in pipes is examined, using laboratory experiments and finite element simulation The result show that the reflection coefficient of this mode is very close to a linear function of the circumferential extent of the notch, and is a stronger function of the through thickne depth of the notch. The motivation for the work was the development of a technique for inspecting chemical plant pipework, but the study addresses the nature of the reflection function and has general applicability.

The Mode Conversion of a Guided Wave by a Part-Circumferential Notch in a Pipe

Abstract: A study of the reflection of mode-convertea guided waves from notches in pipes has been carried out. Measurements were made on a 76-mm bore diameter (nominal 3-inch ), 5.5-mm wall thickness pipe with circumferentially oriented through-thickness notches of various lengths. In parallel, a finite element model was used to simulate the experiments The axially symmetric L(0, 2) mode was incident on the notches and the L(0, 2), F( 1, 3), and F(2, 3) modes were received in reflection. The results showed excellent agreement between the measurements and the predictions for all three modes. They also showed that the F( 1, 3) mode reflects as strongly as the L(0, 2) mode when the notch length is short. Finally, it has been shown that a very simple analysis based on an assumed crack-opening profile may be used to make accurate predictions of the mode conversion.

Guided circumferential waves in a circular annulus

Abstract: A two-dimensional circular annulus is considered in this paper as a waveguide. The guided steady-state time-harmonic waves propagating in the circumferential direction are studied. It is found that the guided circumferential waves are dispersive. The dispersion equation is derived analytically and numerical examples are presented for the frequency dispersion curves. The displacement profiles across the wall thickness of the annulus are also obtained for the first five propagating modes. In addition, the analogy between a flat plate and an annulus in the asymptotic limit of infinite radius of curvature is discussed to reveal some interesting wave phenomena intrinsic to curved waveguides.

Amorphous silicon-based guided-wave passive and active devices for silicon integrated optoelectronics

Abstract: Waveguides and interferometric light amplitude modulators for application at the 1.3- and 1.55-/spl mu/m fiber communication wavelengths have been fabricated with thin-film hydrogenated amorphous silicon and its related alloys. The technique adopted for the thin-film growth is the plasma- enhanced chemical vapor deposition, which has been shown to give the lowest defect concentration in the film. Consequently the proposed waveguiding structures take advantage of the low optical absorption shown by a-Si:H at photon energies below the energy gap. In addition a good radiation confinement can be obtained thanks to the bandgap tailoring opportunity offered by this simple and inexpensive technology. In particular rib waveguides, based on a a-SiC:H/a-Si:H stack, have been realized on crystal silicon, showing low propagation losses. Recently, however, a new interest as low as 0.7 dB/cm. The same structure has been utilized for the fabrication of thermooptic Fabry-Perot modulators with switching times of 10 /spl mu/s. Modulators based on the alternative waveguiding configuration ZnO/a-Si:H, giving comparable results, are also presented.

Ultrasonic Guided Wave NDT for Hidden Corrosion Detection

Abstract: An experimental study of hidden corrosion detection by using ultrasonic guided waves is presented combined with a BEM numerical simulation. Both corrosion simulation specimens by machine cutting and real corrosion specimens by electrochemical processing were used in the investigation, with a range of corrosion depths from 0.02 to 0.4 mm (1.5 to 20% of the original plate thickness). Various wave modes were subsequently generated on these specimens to examine the implications of thinning on mode cutoff, group velocity, and transmission and reflection amplitudes. The transmission and reflection of guided waves upon entering the corrosion zone were simulated by a hybrid BEM calculation that combines a normal mode expansion technique of Lamb waves for far fields with the boundary element representation for the scattered near fields. A quantitative technique for hidden corrosion depth with guided waves is developed based on a frequency compensation concept. The estimated depth of the real corrosion by this method shows good agreement with that by an optical microscope.

Guided Wave Tuning Principles for Defect Detection in Tubing

Abstract: Dispersion diagrams for longitudinal modes and several flexural modes are given. A partial loading oblique incidence technique was introduced for non-axisymmetric guided wave generation. Acoustic fields for partially loaded generation of guided waves were obtained along a sample Inconel steam generator tube surface. The axial field and the circumferential fields were non uniform. Even though the acoustic field was much more complicated than in the case of axisymmetric modes, the study of non-axisymmetric mode cannot be avoided; excitation of a single symmetric mode is often difficult due to limited access and transducer efficiency and there is also mode conversion after scattering from defects. For 100% inspection coverage of tubing and piping, three dimensional tuning (distance, frequency, and incident angle tuning), was employed. A single combination of incident angle, position, and frequency may miss defects in “blind spots”. However, complete inspection coverage of a whole cross sectional area over a certain distance of tubing was successfully demonstrated through a multi crack detection experiment by using the three dimensional tuning concepts. Also, the use of non-axisymmetric guided waves for a large distance inspection capability was successfully demonstrated.

Acoustic scattering by an inhomogeneous layer on a rigid plate

Abstract: The problem of scattering of time-harmonic acoustic waves by an inhomogeneous fluid layer on a rigid plate in R 2 is considered. The density is assumed to be unity in the media: within the layer the sound speed is assumed to be an arbitrary bounded measurable function. The problem is modelled by the reduced wave equation with variable wavenumber in the layer and a Neumann condition on the plate. To formulate the problem and prove uniqueness of solution a radiation condition appropriate for scattering by infinite rough surfaces is introduced, a generalization of the Rayleigh expansion condition for diffraction gratings. With the help of the radiation condition the problem is reformulated as a system of two second kind integral equations over the layer and the plate. Under additional assumptions on the wavenumber in the layer, uniqueness of solution is proved and the nonexistence of guided wave solutions of the homogeneous problem established. General results on the solvability of systems of integral equations on unbounded domains are used to establish existence and continuous dependence in a weighted norm of the solution on the given data.

Analysis of a guided wave along a conducting structure in a borehole

Abstract: A guided wave along a borehole is often observed in borehole radar measurements. These guided waves deform the antenna pattern and can cause artefacts in radar measurements. A water-filled borehole or a conducting logging cable can function as a waveguide for electromagnetic waves under some conditions. We describe the theoretical characteristics of such a guided wave in a borehole and compare them with our experiments. The measured signal discussed was obtained with a directional borehole radar. This radar uses a cylindrical conformal array antenna as receiver and is a model of a conducting structure in a borehole. The induced field around the borehole was compared with the theory. The most fundamental symmetrical and asymmetrical modes were TM 01 and HE 11 , and they were identified in the measured signals using time-frequency distribution analysis and by observation of the azimuthal field distribution of the magnetic field.

Guided Waves in Multilayered Plates for Internal Defect Detection

Abstract: In this paper, stress and displacement fields inside a multilayered plate are analytically computed. For the theoretical modeling, individual layers of the multilayered plate are assumed to be elastic and homogeneous, but anisotropic. These theoretical computations are then experimentally verified in a qualitative manner. Experimental investigations are carried out on a ceramic composite plate that has 12 plies of fibers going in 0° and 90° directions, [0.90]\d3\i\ds. The incident angles and frequencies of the ultrasonic signal for generating different Lamb wave modes in this plate are obtained experimentally as well as from the theory. Displacement and stress fields inside the plate for some of these Lamb modes are then computed. Computation of stress and displacement fields inside the plate is useful for studying the internal damage. Internal stress distribution varies significantly from one Lamb mode to another. Hence, different Lamb modes should detect defects in different layers of the plate. The Lamb mode that produces maximum stress in a specific layer should be most sensitive for detecting defects in that layer. This is observed experimentally as well.

Ultrasonic NDT of titanium diffusion bonding with guided waves

Abstract: An ultrasonic guided wave technique is developed for the NDT of diffusion bonded titanium-to-titanium structures. A three-layer model based on the normal beam experimental results has been proposed. Dispersion curves and wave structure are analyzed to direct the experimental study. Two features related to Lamb waves propagating in diffusion bonded titanium plates, the spectral peak to peak ratios and the wave mode frequency shift, are extracted from the guided wave experimental results for both 2 mm (0.08 in.) and 4 mm (0.16 in.) diffusion bond panels. It is found for some specific modes and frequencies that these two features are sensitive to the diffusion bonding states and, therefore, could be used to distinguish good bond panels from poor bond ones.

Long-range guided wave inspection of pipe using magnetostrictive sensor technology: the feasibility of defect characterization

Abstract: The feasibility of characterizing a defect in pipe from the defect signal detected by using the magnetostrictive sensor (MsS) technique, which utilizes longitudinal guided waves for long-range inspection of pipe, was investigated. Signals from notches of various cross-sectional areas showed that the reflection coefficient of the wave from these planar defects is insensitive to the wave frequency and increases monotonically with the increasing cross-sectional area of the defect. Signals from simulated corrosion pits, on the other hand, showed that the reflection coefficient from these volumetric defects is dependent on both the cross-sectional area and the axial extent of the defect as well as on the wave frequency. The results indicated that it is feasible to differentiate planar from volumetric defects and to determine the defect size -- namely, its cross-sectional area and axial extent if it is volumetric.

Guided wave methods and apparatus for nonlinear frequency generation

Abstract: Methods and apparatus are disclosed for the nonlinear generation of sum and difference frequencies of electromagnetic radiation propagating in a nonlinear material. A waveguide having a waveguide cavity contains the nonlinear material. Phase matching of the nonlinear generation is obtained by adjusting a waveguide propagation constant, the refractive index of the nonlinear material, or the waveguide mode in which the radiation propagates. Phase matching can be achieved even in isotropic nonlinear materials. A short-wavelength radiation source uses phase-matched nonlinear generation in a waveguide to produce high harmonics of a pulsed laser.

Modal Expansions and Orthogonal Complements in the Theory of Complex Media Waveguide Excitation By External Sources for Isotropic, Anisotropic, and Bianisotropic Media

Abstract: A unified electrodynamic approach to the guided wave excitation by external sources in waveguiding structures with bianisotropic media is developed. Effect of electric, magnetic, and magneto-electr...

Optical dielectric waveguide analysis, based on the modified finite element and integral equation methods

Abstract: Efficient versions of the finite element and integral equation methods have been developed to analyse the guided (surface) modes of an arbitrary shaped optical fibre. The first approach is based on the replacement of the open waveguide by the screen guide structure with an artificial impedance wall. The integral equation is solved by the adaptive collocation technique. The methods are used to calculate the dispersion characteristics of the elliptical fibre modes and their cut-off frequencies. The results obtained are compared with those calculated by other methods.

Transducer misalignment effects in beam reflection from elastic structures

Abstract: In two-transducer, fluid-coupled ultrasonic reflection measurements phase matched to guided modes of elastic solid structures, the effects of misalignment (i.e., differences) between the receiver angle and the incident beam angle on the receiver voltage have been studied. The received voltage is typically due to contributions from the specular reflection and any of the several possible guided wave modes excited by the incident wave field. It is found that misalignment leads to changes in the relative amplitudes of the various contributions. Further, the more highly collimated the beam (or the contributor), the more pronounced are the effects. It is shown that the signal maximum is not a reliable indicator of receiver alignment. These conclusions are based on measurements and on calculations that have been performed at incident angles selected both close to, or far from, phase-matching angles to guided wave modes on plates, curved surfaces, and cylindrical shells. Receiver voltage coordinate scans have been performed with receiver angles misaligned from the incident beam axis by 1 to 4 degrees. The receiver voltage versus scan parameter in planar and curved structures is calculated by employing complex transducer points to synthesize two-dimensional, sheet-beam transducer fields with Gaussian apertures, spectral analysis to formulate the beam-structure interaction problem, and asymptotic methods to evaluate the resulting spectral integrals. The model predictions are generally in good agreement with the experiments.

EMAT Generation of Horizontally Polarized Guided Shear Waves for Ultrasonic Pipe Inspection

Abstract: In this work, an ultrasonic guided wave inspection technique was evaluated to detect and locate defects in pipes using SH (Horizontally polarized Shear) plate waves. Electromagnetic Acoustic Transducers (EMATs) were designed and constructed for bi-mode SH waves applications. These probes were used to generate a single mode at a time. Advantages of SH waves, guided by the wall surfaces for circumferential propagation and full volume inspection, are demonstrated in the pulse-echo setup. Mode selection criteria were investigated to tune the SH waves to the geometry of the inspection specimen Examples of mode selection based on the interaction characteristics of SH wave modes with a defect are described. An application of multi-mode inspection for geometrical parameter evaluation (length, depth and orientation) of individual cracks was also demonstrated. The experimental work was performed on cylindrical steel pipe samples having several types of defects ranging from through wall cracks to shallow cracks (10% of wall thickness) both in the circumferential and longitudinal directions. Results from laboratory investigation of the influence of defect depth and orientation on reflectivity of various modes of SH waves are reported. Multi-mode SH-wave inspection is shown to be capable of detecting shallow discontinuities (microcracks) and of locating defects accurately. Inspection results on a pipe section are represented as 3-D images with B-Scan projections and their interpretation is discussed.Copyright © 1998 by ASME

Computer optimization of electromagnetic acoustic transducers

Abstract: Electromagnetic acoustic transducers (EMATs) are well known devices that are commercially available. Their advantage is that they do not require a coupling medium and/or contact to the part being inspected; the disadvantage is their low transduction efficiency. In order to optimize their efficiency, the EMATs are best customized to their particular application with special attention to the material properties and geometry of the part to be inspected. Here we report-apparently for the first time-on numerical simulation of electromagnetic acoustic transduction for both the transmitting and receiving case. The emphasis in this paper is on guided wave generation and reception in thin plates. We have adapted the finite element/boundary element program entitled CAPA to computer simulate the electromagnetic acoustic transduction process.

Analysis and design of a novel leaky YIG film guided wave optical isolator

Abstract: A guided wave optical isolator using a four layered leaky yttrium iron garnet (YIG) waveguide with a YIG guiding layer is proposed and analyzed through a transfer matrix formulation. The dependence of isolation characteristics on different waveguide parameters are discussed. It is shown that for four layered structures an isolation ratio of about 24 dB can be achieved with YIG films having large gyrotropy. It is also shown that by replacing the single guiding YIG film with multilayers of YIG with compensated walls and proper permittivity, high isolation ratio can be achieved even with currently available YIG films having low gyrotropy.

Guided Waves Along Hydraulic Fractures

Abstract: Hydraulic fracturing as an oil and gas reservoir stimulation technique aims at creating large fractures around a borehole, which improves the inflow of hydrocarbons at production wells, as well as the injectivity of water at injection wells. For hydraulic fracturing experiments it is desirable to find techniques to estimate fracture dimensions to aid the reservoir engineer in the fracture treatment or design. In this abstract we numerically model the propagation of guided waves along hydraulic fractures. These guided waves could be used in wellbore measurements to estimate fracture length or volume. It is well-known that guided waves can propagate along dry or fluid-filled fractures. In the geophysical community fractures are often described by the linear-slip model and the guided waves that are associated with this model are referred to as generalized Rayleigh waves. Previous studies show that both pressure symmetric as well as pressure anti-symmetric generalized Rayleigh waves exist. In this abstract we focus on hydraulic fractures which are mechanically open, i.e. the fractures faces are disconnected and do not show substantial mechanical contact. As a result the fluid in the fracture is connected and we describe the fracture by a thin fluid-filled layer model. Numerically modeled seismograms show that for these kind of fractures not only generalized Rayleigh waves exist, but also a slow and strongly dispersive guided wave. We will refer to this wave as the slow channel wave, because it appears to be the two-dimensional equivalent of the cylindrical tube wave. The slow channel wave is absent in the linear-slip model because this model does not incorporate the explicit boundary condition of zero shear stresses at fluid-solid interfaces. During hydraulic impedance testing (HIT) a pressure pulse is send down into the wellbore by opening the fluid pump during the hydraulic fracturing treatment. Some studies claim the observation of the reflection of a slow wave in the fluid at the tip of the fracture. In the low-frequency limit the channel wave corresponds to the slow fluid wave related to the waves that are exited in the fracture during HIT-experiments. This also resolves a possible ambiguity when both geophysicists and reservoir engineers talk about fracture waves.

Ray model for a planar anisotropic dielectric waveguide

Abstract: A ray model for the treatment of a planar anisotropic dielectric waveguide is presented. The attenuation coefficient of the guided wave predicted by this model is consistent with the energy-conservation requirement.

Boundary element modeling for guided wave reflection and transmission factor analyses in defect classification

Abstract: The guided wave scattering process by different defects in plate structures is studied with a boundary element method. The purpose is to separate sharp crack-like defects from smooth volumetric-like ones by analyzing the transmission and reflection factors. The traditional normal mode expansion technique has been combined with the boundary element discretization to form a hybrid BEM scheme. With this method, the scattered near-fields and the transmission and reflection coefficients of the far fields on individual modes can be obtained simultaneously. Several features related to the Lamb mode reflection and transmission process in a plate with these defect types are extracted and compared. The theoretical analysis provides a guideline for data acquisition and feature selection for use in the decision algorithm development program via neural nets.

Detection of Defects in a Thin Steel Plate Using Ultrasonic Guided Wave

Abstract: In order to establish a technical concept for the detection of defects in weldments in thin steel plate, an experimental and theoretical investigation was carried out for artificial defects in a steel plate having a thickness of 2.4mm by using the guided wave technique. In particular the goal was to find the most effective testing parameters paying attention to the relationship between the excitation frequency by a tone burst system and various incident angles. It was found that the test conditions that worked best was for a frequency of 840kHz and an incident angle of 30 or 85 degrees, most of the defects were detected with these conditions. Also, it was clear that a guided wave mode generated under an incident angle of 30 degrees was a symmetric mode, So, and that of 85 degrees corresponded to an antisymmetric mode, Ao. By using the two modes, most of all of the defects could be detected. Furthermore, it was shown that the antisymmetric mode was more sensitive to defects near the surface than the symmetric mode. Theoretical predictions confirmed this sensitivity improvement with Ao for surface defects because of wave structure variation and energy concentration near the surface.

Study of energy distribution of guided waves in multilayered media

Abstract: The energy distributions of guided waves in multilayered elastic solid media are investigated in three dimensions. A guided wave is the result of the interaction of the acoustic source and the interfaces in the material structure, and does not lose energy in the course of propagation along the horizontal direction. It should be pointed out that the guided wave cannot be excited alone by a practical acoustic source in this paper. The mean energy flux density of the guided waves (excited by a nonaxisymmetric acoustic source) has the tangential component except the radial component, but the effective part of the mean energy flux density has only the radial component. Only in the case that the propagation distance is greater than the wavelength, is the propagation velocity of the mean value of the total energy equal to the group velocity of the guided wave. It is found that the propagation velocity of the mean energy density is equal to the phase velocity of the guided wave in the lowest layer medium in the m...

Simulation of Lamb Wave Propagation Using Pure Mode Excitation

Abstract: A combination of studies of dispersion behavior (wave mode properties) with time domain Finite Element modelling constitutes a very useful approach to developing guided wave NDE techniques. The dispersion curves and the mode shapes reveal a lot of information about how a guided wave will behave in a plate and can be used to judge modes and frequencies that are likely to be sensitive to defects. However, because of their modal nature, the dispersion curves cannot predict how a guided wave will react with a local geometric change in a plate such as a defect, which is of great importance to Lamb wave testing. Finite element modelling can fill in this information.