Showing papers on "Guided wave testing published in 2001"

The effect of dispersion on long range inspection using ultrasonic guided waves

Abstract: The dispersion of ultrasonic guided waves causes wave-packets to spread out in space and time as they propagate through a structure. This limits the resolution that can be obtained in a long-range guided wave inspection system. A technique is presented for quickly predicting the rate of spreading of a dispersive wave-packet as it propagates. It is shown that the duration of a wave-packet increases linearly with propagation distance. It is also shown that the duration of a wave-packet after a given propagation distance can be minimised by optimising the input signal. A dimensionless parameter called minimum resolvable distance (MRD) is defined that enables a direct comparison to be made between the resolution attainable at different operating points. Some conclusions are made concerning the resolution of various operating points for the case of Lamb waves in an aluminium plate.

The Potential of Guided Waves for Monitoring Large Areas of Metallic Aircraft Fuselage Structure

Abstract: The potential for long-range propagation of ultrasonic guided waves through metallic aircraft fuselage structure has been investigated using dispersion analysis and numerical modelling, validated by experiment. In order to satisfy the pressing need for integrated structural health monitoring of ageing metallic aircraft, it is likely that an active guided wave system based on current technology must feature efficient propagation over distances of at least 1m with an attenuation of not more than about 40dB/m. Propagation was examined across free skin, tapering skin, skin loaded with sealant and paint, double skin jointed with either sealant or adhesive, and lap and stringer joints, which together adequately characterise metallic monocoque fuselage construction. Whilst the simple and tapering skins allow long range propagation of non-dispersive modes with little reflection at the transition to tapering skin, the attenuation caused by application of a sealant layer generally leaves no viable modes. Guided wave propagation through double skin features the inevitable generation of twin modes with similar phase velocity, which interact with each other during propagation. This interaction crucially determines the efficiency of propagation across narrow joints and effectively precludes propagation across a succession of joints. This work leads to the conclusion that an active aircraft system that relies on guided wave propagation of more than 1m is not feasible, whereas localised guided wave monitoring of structurally significant areas is a more practical approach.

Photonic crystal waveguides: Out-of-plane losses and adiabatic modal conversion

Abstract: An accurate model for the out-of-plane radiation losses occurring when a guided wave propagating in a conventional waveguide impinges on a photonic crystal waveguide is presented. The model makes clear that the losses originate from insertion losses resulting from a mode mismatch. A generic taper structure realizing an adiabatic modal conversion is proposed and validated through numerical computations for cavities with large Q’s and large peak transmission.

Implementing guided wave mode control by use of a phased transducer array

Abstract: A multichannel time-delay system has been built and applied to a transducer array for implementing guided wave mode control. The time-delay system has a capability of sending high energy controllable tone-burst signals from eight independent channels with arbitrary time delays from 0 to 30 /spl mu/s with resolution of 0.025 /spl mu/s. Software time delays are also provided for summing up received signals of each channel. Theoretical discussions indicate the impact of the time delay capability on the bandwidth and sensitivity improvement of a transducer array for guided wave generation. Determination of both physical and software time delay values is based on a knowledge of dispersion curves and element spacing. Based on reference signals, a non-knowledge-based automatic time-delay searching algorithm was introduced for guided wave mode selection. Experiments were conducted with a phased comb transducer array mounted on a carbon steel pipe. The experimental results show that signal to noise ratio has been greatly improved by use of the time-delay system. Some other benefits of the phased array, including unidirection generation and mode control flexibility, are discussed.

Guided-wave reflectometry with micrometer resolution

Abstract: We describe a new type of optical reflectometry which is useful in testing single-mode lightguide systems. This technique uses a scanning Michelson interferometer in conjunction with a broadband illuminating source and cross-correlation detection. High resolution is achieved through the limited coherence of the backscattered radiation. With this approach it is possible to distinguish scattering centers separated by only a few micrometers. In some cases loss may be estimated for components in the transmission path of a test lightguide. The basic principles of this diagnostic technique, along with some performance characteristics, are illustrated for an all-fiber reflectometer. We also discuss several laboratory applications which serve to demonstrate the resolution capabilities of this measurement concept.

Lamb and SH waves generated and detected by air-coupled ultrasonic transducers in composite material plates

Abstract: Electrostatic, air-coupled, ultrasonic transducers are used to generate and detect guided waves in anisotropic solid plates. Waves considered in this study are Lamb-type and SH-type, guided modes. If the plane of propagation coincides with a plane of symmetry of the material, then Lamb modes only are launched and detected by the transducers. If the plane of propagation does not coincide with a plane of symmetry of the material, then Lamb modes are still generated and detected, but guided, SH-like modes are, too. The variation of phase velocity with frequency is measured for several modes propagating in different directions along a glass–epoxy composite plate. A numerical model that takes into account the anisotropy of composite materials is developed to predict the dispersion curves (phase velocity, group velocity or wave-number versus frequency) and the displacement fields of plate waves, the plane of propagation being either a plane of symmetry or not. The experimental phase velocities are in good agreement with the predicted dispersion curves, thus showing that the forward problem concerning the propagation of plate waves in anisotropic, homogeneous, composite material plates is properly solved. The dispersion curves associated with the predicted displacement fields show that guided modes in composite plates have different behaviors depending on their direction of propagation.

Underwater Pipeline Inspection Using Guided Waves

Abstract: Underwater pipeline inspections are conducted using ultrasonic cylindrical guided waves in the laboratory environment. Three different types of mechanical defects-gouge, removed metal, and dent-are fabricated in small-diameter, 22.22-mm, aluminum pipes and tested. To efficiently propagate the antisymmetric (flexural) cylindrical guided waves through the aluminum pipe in water, a new transducer holder device is designed. The device uses commercially available ultrasonic transducers that generate compressional ultrasonic waves in the water The device can change the striking angle of the incident beam from 0 to 51 deg. With the help of this device, the incident angle adjustment and frequency sweeping can be carried out. This is necessary for obtaining the time history of the received signals for various incident angles and signal frequencies; then these time histories are converted to V(f) curves, or received signal amplitude versus frequency curves. From the amplitude of these V(f) curves, the type and extent of the mechanical defects can be estimated. This investigation shows that the new coupler device can be effectively used for health monitoring of undenvater pipelines using guided waves.

Noncontact Air-Coupled Guided Wave Ultrasonics for Detection of Thinning Defects in Aluminum Plates

Abstract: Ultrasonic guided waves are gaining increasing attention for the inspection of platelike and rodlike structures. At the same time, inspection methods that do not require contact with the test piece are being developed for advanced applications. This paper capitalizes on recent advances in the areas of guided wave ultrasonics and noncontact ultrasonics to demonstrate a superior method for the nondestructive detection of thinning defects simulating hidden corrosion in thin aluminum plates. The proposed approach uses micromachined gas (air)-coupled capacitive transducers for the noncontact generation and detection of guided plate waves. Interesting features in the dispersive behavior of selected guided modes are used for the detection of plate thinning. It is shown that mode cutoff measurements provide a qualitative detection of thinning defects, while frequency shift measurements allow quantification of thinning depth. Measurement of the mode group velocity can be also used to quantify thinning depth. Similarly, thinning length can be determined by mode time-of-flight measurements.

A unified formalism using effective surface permittivity to study acoustic waves in various anisotropic and piezoelectric multilayers

Abstract: A unified formalism is presented that uses the effective surface permittivity (ESP) to study surface acoustic waves (SAW) in layered substrates and guided waves in layered plates. Based on known mathematical tools, such as ordinary differential equation and transfer matrix, a generalized surface impedance (GSI) concept is developed and exploited to investigate the acoustic propagation in various anisotropic and piezoelectric layered structures. The ESP function, originally defined for the surface of a homogeneous and semi-infinite piezoelectric substrate, is extended to both the top surface of and an interface in a layered half space, as well as to either surface of a finite-thickness plate. General ESP expressions for all mentioned configurations are derived in terms of an equivalent GSI matrix. It is shown that, when using the appropriate GSI matrices, the same form of the ESP expressions applies no matter whether the structure is a homogeneous half space alone or coated with a layered plate or a layered plate alone. GSI matrices are explicitly given in terms of the bulk partial mode solutions for a substrate and via the transfer matrix for a plate. Modified GSI matrices for structures consisting of both a plate and a substrate are also specified. Analytical development is fully detailed to suit program implementation. To illustrate its versatility, the formalism is also applied to two-substrate configurations, allowing one to analyze guided waves in a plate sandwiched between and interfacial waves existing along the boundary of two different media. Numerical examples are given to illustrate the spectrum features that the ESP shows for various structures. Deduced ESP expressions allow one to locate directly all piezoelectrically active waves in any structure including at feast one piezoelectric layer. Acoustic modes that are not piezoelectrically active and those in non-piezoelectric materials can be also obtained by using the intermediate results, such as derived GSI matrices.

Scattering of Guided Waves by Circumferential Cracks in Steel Pipes

Abstract: A novel numerical procedure is presented in this paper to study wave scattering problem by circumferential cracks in steel pipes. The study is motivated by the need to develop a quantitative ultrasonic technique to characterise properties of cracks in pipes. By employing wave function expansion in axial direction and decomposing the problem into a symmetry problem and an antisymmetry problem, a three-dimensional wave scattering problem is then reduced into two quasi-one-dimensional problems. This simplification greatly reduces the computational time. Numerical results for reflection and transmission coefficients of different incident wave modes are presented here for a steel pipe with cracks (may have arbitrary circumferential crack length and radial crack depth) and they are shown to agree quite closely with available but limited experimental data.

A signal processing technique to remove the effect of dispersion from guided wave signals

Abstract: A technique is described that compensates for the signal spreading caused by dispersion in long range guided wave testing. This is achieved using knowledge of the dispersion characteristics of the guided wave system. Experimental and numerical examples are presented that show the application of the technique and a discussion of the implications for long range testing is given.

Power scalable waveguide amplifier and laser devices

Abstract: The present invention is directed to guided wave systems, beam transport and waveguide techniques. The invention may comprise passive or active, hollow and dielectric core self-imaging mode wave guide systems, beam amplifiers (10, 40), laser resonators (70), beam transports, and waveguides. Embodiments may include rectangular cross-section waveguides, and preferably maintaining spatial profile of an input beam, such as a Gaussian beam, through the self-imaging period of the waveguide while unique new capabilities to mitigate non-linear distortions that corrupt spatial, spectral and temporal coherence and polarization. Additional aspects may include, for example, transport, amplification, phase/frequency control or modulation, deflection, conversion, synthetic aperture, distributed aperture, beam forming, beam steering, beam combining, power sampling, power combining and power splitting, among other features. Some embodiments may provide a self-imaging, multimode, waveguide (10) and self-imaging guided wave systems and beam transport. Embodiments of the present invention may further provide a method of self-imaging, multimode beam transport and other self-imaging wave guidance techniques.

Ultrasonic Guided Wave Inspection of Bonded Lap Joints: Noncontact Method and Photoelastic Visualization

Abstract: The main topic of this paper is the nondestructive inspection of adhesively bonded lap joints by using ultrasonic guided (plate) waves. A noncontact, couplant-free method that employs capacitive air-coupled transducers is demonstrated for the inspection of thin aluminum joints with dimensions typical of aircraft fuselage and wing panels. Two types of bond defects, disbonded regions and regions of poorly cured (low-cohesive-strength) adhesive, are successfully detected by measuring the amplitude decrease of selected plate waves leaking from one adherend to the other one through the bondline. It is shown that proper choice of the vibrating mode structure, in terms of cross-sectional displacement distributions, is needed in order to maximize the sensitivity of the inspection to the presence of the low-cohesive-strength bond. Results from a dynamic photoelasticity study are also presented to visualize fundamental behavior of propagating plate waves and study their interaction with bond defects in glass lap joints. The photoelastic results confirm in a clear, pictorial manner certain assumptions on plate wave leakage through the bond, including the influence of vibrating mode structure on the detection of low-strength bonds.

Long-Range Guided Wave Inspection of Structures Using the Magnetostrictive Sensor

Abstract: Long-range guided wave inspection is a new emerging technology for rapidly and globally inspecting a large area of a structure from a single test location. This paper describes a general overview of the guided wave properties and its application for long-range inspection of structures the principle and instrument system for a guided wave inspection technology called "magnetostrictive sensor (MsS)" that generates and detects guided waves electromagnetically in the material under testing, and examples of long-range guided wave inspection of structures that can be accomplished using the MsS.

Synthetic phase tuning of guided waves

Abstract: A novel method has been developed to generate and manipulate multi-mode guided waves. This technique uses a linear phased array whose elements are activated according to a prescribed time delay profile obtained from the dispersion curves. It is shown that a desired guided wave mode can be tuned by synthetically constructing a virtual wave from individually acquired waveform data. In addition to the development of such a synthetic phase tuning (SPT) technique, a pseudo pulse-echo (PPE) operation scheme is also developed for nondestructive testing. Experimental results are compared with those obtained by more traditional techniques using variable angle wedges and array transducers. It was shown that the new technique is convenient, robust, and flexible in utilizing multi-mode guided waves for nondestructive evaluation (NDE). It is a dynamic method that can produce desired guided wave modes propagating in the desired direction without any mechanical alignment. The advantages and limitations of the technique are addressed.

Method for inspecting electric resistance welds using magnetostrictive sensors

Abstract: A method and apparatus is shown for implementing magnetostrictive sensor techniques for the nondestructive evaluation of plate type structures such as walls, vessels, enclosures, and the like. The system includes magnetostrictive sensors specifically designed for application in conjunction with plate typ structures or pipes that generate guided waves in the plates or pipes which travel therethrough in a direction parallel to the surface of the plate or pipe. Similarly structured sensors are positioned to detect the guided waves (both incident and reflected) and generate signals representative of the characteristics of the guided waves detected that are reflected from anomalies in the structure such as corrosion pits and cracks. The sensor structure is longitudinal in nature and generates a guided wave having a wavefront parallel to the longitudinal axis of the sensor, and which propagates in a direction perpendicular to the longitudinal axis of the sensor. The generated guided waves propagate in the plate within the path of the propagating wave. The reflected waves from these abnormalities are detected using a magnetostrictive sensor. Shear horizontal waves may also be created by rotating the magnetic bias 90° and used for similar inspection techniques. Pipes, which act as curved plates, may also be inspected as well as electric resistance welds therein. In addition, steel sheet butt welds may be inspected with this technique.

Guided waves propagating in a piezoelectric plate immersed in a conductive fluid

Abstract: In this paper, a theoretical and experimental study on a leaky Lamb wave (LLW) propagating in a piezoelectric plate loaded by a dielectric/conductive fluid is presented. The analysis is based on a partial wave analysis for the piezoelectric plate, taking into account the mass and dielectric/conductive loading of the fluid. Based on theory, dispersion curves, presented in the form of reflection coefficient minima, can be obtained for an X-cut LiNbO 3 plate immersed in a fluid with various levels of conductivity. Commonly used as a measure in chemical micro-sensors, the frequency shift of LLW modes induced by the change of fluid conductivity was also calculated to see if it matched theoretical predictions. Our results show that our theoretical calculations verify our experimental study, which applied bulk wave ultrasound transducers to measure the frequency shift for various levels of fluid conductivity.

Behavior of first guided wave on finite cylindrical shells of various lengths: experimental investigation.

Abstract: Acoustic backscattering from elastic cylindrical shells of finite lengths, immersed in water, is investigated. These objects, characterized by the ratio of length over diameter (L/2a=9.76, 4.88, 2.44, a: outer radius), are excited by an obliquely incident plane acoustic wave. In the three cases studied here, the radii ratio b/a (b: inner radius) is fixed at 0.97. The investigated dimensionless frequency range extends over 10⩽k1a⩽50 (k1: wave number in water). The first guided wave, T0, is of particular interest here. The influence of the shell’s length on the backscattered pressure is experimentally observed in the time-angle and frequency-angle representations. In support of this experimental study, a time-domain representation is used by extending a theoretical model that provides a geometrical description of the helical propagation of the surface waves around the shell [Bao, J. Acoust. Soc. Am. 94, 1461–1466 (1993)]. Theoretical results on cylindrical shells considered as infinitely long, with identica...

Apparatus and method for analysis of guided ultrasonic waves

Abstract: Method and apparatus for using a guided wave to determine the location of one or more flaws in an inspected object are disclosed. An ultrasonic guided wave is launched into the object using conventional ultrasound methods, and the reflected/received guided wave is sampled to capture a series of individual reflected waveforms. The individual reflected waveforms are then partitioned according to the sampling time. Each of the partitioned acquired waveforms is compared with a selected time-varying dispersion-modeled reference waveform associated with the unique geometry of the inspected object, a multiplicity of 'theoretical' flaw locations, and the characteristics of the original ultrasonic guided wave. To make a comparison, the reference waveform is also generated as a series of partitioned waveforms which model the shape of a wave that may be expected to be reflected from a multiplicity of theoretical flaws located in the object. The shape of the reflected waveforms is correlated with the shape of the reference waveforms, and a high level of correlation indicates the presence of a real flaw at the theoretical flaw location.

Input coupling and guided-wave distribution schemes for board-level intra-chip guided-wave optical clock distribution network using volume grating coupler technology

Abstract: The input coupling and guided-wave distribution schemes for a novel board-level guided wave optical clock distribution network using volume grating coupler technology are described. Volume grating coupler technology is employed to couple light into the optical waveguide distribution with high efficiency, allowing for compact packaging of the entire optical system and reduced optical input power requirements. A novel single-split-region waveguide distribution network is proposed to allow for larger radii of curvature in the final, smallest-radii levels for reduced bending loss in comparison to a conventional multiple-split-region topology.

Off-axis propagation of ultrasonic guided waves in thin orthotropic layers: theoretical analysis and dynamic holographic imaging measurement

Abstract: The elastic properties of many materials in sheet or plate form can be approximated with orthotropic symmetry. In many sheet material manufacturing industries (e.g., the paper industry), manufacturers desire knowledge of certain anisotropic elastic properties in the sheet for handling and quality issues. Ultrasonic wave propagation in plate materials forms a method to determine the anisotropic elastic properties in a nondestructive manner. This work explores exact and approximate analysis methods of ultrasonic guided wave propagation in thin layers, explicitly dealing with orthotropic symmetry and propagation off-axis with respect to the manufacturing direction. Recent advances in full-field ultrasonic imaging methods, based on dynamic holography, allow simultaneous measurement of the plate wave motion in all planar directions within a single image. Results from this laser ultrasonic imaging approach are presented that record the lowest anti-symmetric (flexural) mode wavefront in a single image without scanning. Specific numerical predictions for flexural wave propagation in two distinctly different types of paper are presented and compared with direct imaging measurements. Very good agreement is obtained for the lowest anti-symmetric plate mode using paper properties independently determined by a third party. Complete determination of the elastic modulus tensor for orthotropic layers requires measurement of other modes in addition to the lowest anti-symmetric. Theoretical predictions are presented for other guided wave modes [extensional (S), flexural (A), and shear-horizontal (SH)] in orthotropic plates with emphasis on propagation in all planar directions. It is shown that there are significant changes in the dispersion characterization of these modes at certain frequencies (including off-axis mode coupling) that can be exploited to measure additional in-plane elastic moduli of thin layers. At present, the sensitivity of the imaging measurement approach limits experimental investigation to relatively large amplitudes easily produced by flexural wave motion (>0.1 nm). Extension of the measurement range and application to other plate wave modes are in progress and shall be reported in future work.

A Method for Computing Guided Waves in Integrated Optics. Part I: Mathematical Analysis

Abstract: Electromagnetic waveguides in integrated optics are propagation structures which are invariant under translation in one space direction and whose cross section is a local perturbation of a stratified medium In this paper, we propose a new method for computing the guided modes of such devices under the weak guiding assumption The method results from a combination of analytical techniques which take into account the unbounded and stratified character of the propagation medium so that numerical computations can be reduced to a neighborhood of the perturbation

Guided Waves in Thin-Walled Structural Members

Abstract: A semi-analytical finite element (SAFE) formulation is proposed to study the wave propagation characteristics of thin-walled members with an infinite length in the longitudinal (axial) direction Common structural members are considered as an assemblage of thin plates The ratio of the thickness of the plate to the wavelength in the axial direction is assumed to be small so that the plane-stress assumption is valid Employing a finite element modeling in the transverse direction circumvents difficulties associated with the cross-sectional profile of the member The dynamic behavior is approximated by dividing the plates into several line (one-dimensional) segments and representing the generalized displacement distribution through the segment by polynomial interpolation functions By applying Hamilton's principle, the dispersion equation is obtained as a standard algebraic eigenvalue problem The reasonably good accuracy of the method is demonstrated for the lowest modes by comparing, where feasible, the results with analytical solutions To demonstrate the method's versatility, frequency spectra are also presented for I and L shaped cross sections

A method of calculating the propagation of electromagnetic fields both in waveguides and in free space, using the “Fast Fourier Transform”

Abstract: The following calculation method permits the evaluation of the transverse distribution of the field amplitude Ψ(x,y,z) propagating in a waveguide. This method is based on a standard method for propagation in free space, which is using the Fast Fourier Transform (FFT). It is very simple in practice, and it does not require a mode decomposition for the waveguide propagation. It uses the properties of periodicity and symmetry of the FFT in the transverse plane in order to take into account the influence of the waveguide. The advantage of this calculation method is the full compatibility with the case of a guided wave (waveguide propagation), an unguided wave (free space propagation) and any intermediate regime where the guiding occurs softly or only in a limited part of the waveguide; for example when an input wave is not guided at the entrance of the waveguide, and becomes guided at the end. In practice, these conditions are observed experimentally for example in the infrared “Free Electron Lasers”, which are broadband tunable lasers, and some of them are using a waveguide, in the center of the optical cavity, coupled with free space areas. The calculation method presented here works in this special configuration. On the other hand, although not using a mode decomposition, this method allows also a rapid decomposition in eigenmodes TE pq and TM pq . Using only one symmetry operator and one FFT, it gives the whole set of mode amplitudes.

A finite element analysis of the time-delay periodic ring arrays for guided wave generation and reception in hollow cylinders

Abstract: A new guided wave transducer model, time-delay periodic ring arrays (TDPRAs), is proposed and investigated in this paper for guided cylindrical wave generation and reception in hollow cylinders with application interests focusing on non-destructive testing (NDT) of piping/tubing. A finite element simulation has been performed for axisymmetric guided-mode excitation and reception with TDPRAs. By arranging a proper configuration of the time-delay profile and the electric-connection pattern of a ring array, unidirectional excitation and reception of guided waves can be achieved. The numerical results are obtained for the first three axisymmetrical modes and are compared with respect to generation efficiency and mode selectivity. Parametric influences on the performance of TDPRAs are discussed, combining a 2-D phase velocity-frequency spectrum approach with the mode dispersion and displacement structure analyses. The identification of converted modes in guided cylindrical wave reflections with a flexible TDPRA receiver has also been studied through sample notch reflection.

Wavelength-selective coupling between vertically integrated thin-film waveguides via supermode by a pair of grating couplers

Abstract: Grating coupling between vertically integrated waveguides via supermode was discussed and demonstrated. Two thin-film waveguides with grating couplers were stacked on a substrate. A guided wave in one waveguide is converted by a grating coupler to a supermode propagating contradirectionally, and then converted by another grating coupler to a guided wave in the other waveguide. The coupling shows high wavelength selectivity, and the wavelength-division add/drop multiplexing function can be combined. A demonstrator was designed and fabricated. Theoretically predicted coupling efficiency was almost 100% with wavelength selectivity of 2 nm, while the experimentally obtained value was 40% in the efficiency and 1 nm in the selectivity.

Scattering of guided SH-wave by a partly debonded circular cylinder in a traction free plate

Abstract: A solution of the scattering problem of guided SH-wave by a partly debonded circular cylinder centered in a traction free plate has been set up. The plate is divided up into three regions with two imaginary planes perpendicular to the plate walls. In the central region where the partly debonded cylindrical obstacle is posted, the wave field is expanded into the cylindrical wave modes and Chebyshev polynomials. In the other two exterior regions the fields are expanded into the plate wave modes. A system of fundamental equations to solve the problem is obtained according to the traction free boundary condition on the plate walls and the continuity condition of the traction and the displacement across the imaginary planes. The approximate numerical method termed mode-matching technique is used to construct a matrix equation to obtain curves showing the coefficient of reflection and transmission versus the ratio of the cylinder's radius to the plate's half-thickness and the angular width of the debonded region. A comparison of the numerical results between the welded interface condition and the debonded interface condition is made, and the results are discussed.

Optical interconnects realizable with thin–film helicoidal bianisotropic mediums

Abstract: Parallel–plate waveguides consisting of a thin–film helicoidal bianisotropic medium (TFHBM) layer bounded by dielectric half–spaces are shown to support guided wave propagation with guide wavenumbers dependent on the direction of signal propagation. Thus, the TFHBM interconnect behaves as a space–guide . The modal fields and power transmission distributions, as determined by the time-averaged Poynting vector, are investigated and a scheme classifying each guided wave mode as either hybrid electric or hybrid magnetic is presented.