Showing papers on "Guided wave testing published in 2003"

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

Abstract: Guided acoustic and ultrasonic waves have been utilized in various manners for non-destructive evaluation and testing. If a guided wave mode is dispersive, a pulse of energy will spread out in space and time as it propagates. For a long-range guided wave inspection application, this constrains the choice of operating point to regions on the dispersion curves where dispersion effects are small. A signal processing technique is presented that enables this constraint on operating point to be relaxed. The technique makes use of a priori knowledge of the dispersion characteristics of a guided wave mode to map signals from the time to distance domains. In the mapping process, dispersed signals are compressed to their original shape. The theoretical basis of the technique is described and an efficient numerical implementation is presented. The robustness of the technique to inaccuracies in the dispersion data is also addressed. The application of the technique to experimental data is shown and the resulting improvement in spatial resolution is demonstrated. The implications of using dispersion compensation in practical systems are briefly discussed.

Omni-directional guided wave transducer arrays for the rapid inspection of large areas of plate structures

Abstract: Omni-directional guided wave array transducers contain a circular pattern of elements that individually behave as omni-directional point transmitters or receivers. The data set acquired from such an array contains time-domain signals from each permutation of transmitter and receiver. A phased addition algorithm is developed that allows an omni-directional, B-scan image of the surrounding plate to be synthesized from any geometry of array. Numerically simulated data from a single reflector is used to test the performance of the algorithm. The results from an array containing a fully populated circular area of elements (Type I array) are found to be good, but those from an array containing a single ring of elements (Type II array) contain many large side-lobes. An enhancement to the basic phased addition algorithm is presented that uses deconvolution to suppress these side-lobes. The deconvolution algorithm enables a Type II array to equal the performance of a Type I array of the same overall diameter. The effect of diameter on angular resolution is investigated. Experimental data obtained from a guided wave array containing electromagnetic acoustic transducers (EMAT) elements for exciting and detecting the S/sub 0/ Lamb wave mode in a 5-mm thick aluminium plate are processed with both algorithms and the results are discussed.

Finite-difference frequency-domain algorithm for modeling guided-wave properties of substrate integrated waveguide

Abstract: In multilayer microwave integrated circuits such as low-temperature co-fired ceramics or multilayered printed circuit boards, waveguide-like structures can be fabricated by using periodic metallic via-holes referred to as substrate integrated waveguide (SIW). Such SIW structures can largely preserve the advantages of conventional rectangular waveguides such as high-Q factor and high power capacity. However, they are subject to leakage due to periodic gaps, which potentially results in wave attenuation. Therefore, such a guided-wave modeling problem becomes a very complicated complex eigenvalue problem. Since the SIW are bilaterally unbounded, absorbing boundary conditions should be deployed in numerical algorithms. This often leads to a difficult complex root-extracting problem of a transcend equation. In this paper, we present a novel finite-difference frequency-domain algorithm with a perfectly matched layer and Floquet's theorem for the analysis of SIW guided-wave problems. In this scheme, the problem is converted into a generalized matrix eigenvalue problem and finally transformed to a standard matrix eigenvalue problem that can be solved with efficient subroutines available. This approach has been validated by experiment.

Practical long range guided wave testing: applications to pipes and rails

Abstract: The testing of large structures using conventional ultrasonic bulk wave techniques is slow because the test region is limited to the area immediately surrounding the transducer. Therefore, scanning is required if the whole structure is to be tested. Ultrasonic guided waves potentially provide an attractive solution to this problem because they can be excited at one location on the structure and will propagate many meters, returning echoes indicating the presence of corrosion or other discontinuities. However, guided wave testing is complicated by the presence of many possible wave modes, most of which are dispersive. These guided wave characteristics offer a wealth of opportunities for the extraction of information about the structure, but it is crucial to manage this complexity if the test is to be useable in industrial practice. Guided waves can be used in three regimes, which have been researched extensively by many authors: short range (« 1 m [39 in.], for example leaky lamb wave testing of composite materials and high frequency surface wave scanning), medium range (up to about 5 m [16.4 ft], for example shear horizontal and lamb waves in the 250 kHz to 1 MHz frequency range for plate and tube testing) and long range (up to 100 m [328 ft], for example the testing of pipelines). This paper concentrates on long range testing using frequencies below 100 kHz. The progress from research work to a robust, commercial pipe testing system is described, together with the more recent development of a test for rail. Future directions for guided wave testing are then discussed.

Modelling of Lamb waves for damage detection in metallic structures: Part I. Wave propagation

Abstract: Lamb waves are the most widely used acousto-ultrasonic guided waves for damage detection. The method is generally complicated by the coexistence of at least two highly dispersive modes at any given frequency. Furthermore pure Lamb wave modes may generate a variety of other modes by interacting with defects and/or by crossing different boundaries. Knowledge and understanding of Lamb wave propagation is important for reliable damage detection. However, the theoretical analysis of guided wave scattering forms an extremely difficult problem. This paper reports an application of the local interaction simulation approach for wave propagation in metallic structures. The focus of the analysis is on damage detection applications. The study also involves wave propagation in a piezoceramic actuator/sensor diffusion bond model in which one of the piezoceramics generates the thickness mode vibration. The simulated results are validated experimentally. The results show the potential of the method for wave propagation analysis in damage detection applications.

Ultrasonic guided waves for inspection of grouted tendons and bolts

Abstract: There is currently a need to improve the nondestructive testing techniques that are used to inspect grouted steel reinforcing tendons, anchors, and rock bolts for corrosion and fracture. A method of inspection using guided ultrasonic waves has been proposed, which uses a pulse-echo technique carried out from the free end of the structure. The maximum inspection range is determined by the amount of attenuation that the wave experiences as a result of leakage into the embedding material and material losses. However, previous work has identified high frequency modes that have low attenuation and so increase the inspection range. Research has been carried out with a focus on the inspection of the posttensioning tendons used to reinforce concrete. The research presented in this paper uses experimental techniques to measure the attenuation in short lengths of grouted tendons, to evaluate the reflection coefficient of the modes from different geometry breaks, and to assess the impact of tendon curvature. The outcome of this research shows that the inspection range for tendons is limited, but the outlook for the inspection of the larger diameter grouted bolts and rebars that are used in the construction industry is promising. Considerable success has already been achieved in the testing of epoxy bonded rock bolts using this method.

Stress measurement and defect detection in steel strands by guided stress waves

Abstract: Health monitoring of steel strands is the subject of much research in the nondestructive evaluation and civil engineering communities. This paper deals with a guided stress wave method for stress monitoring and defect detection in seven-wire strands. A simplified acoustoelastic formulation of the Pochhammer-Chree vibrations in cylindrical waveguides is derived in the framework of the partial wave representation for guided waves. Magnetostrictive transducers are used to excite and detect the waves in the experiments. Results from acoustoelastic measurements on single wires and on strands are presented, showing the feasibility of the method for stress measurement, although an anomalous behavior of the strands at low stress levels remains the subject of current investigation. Improvements to the inherently low sensitivity of acoustoelastic stress measurements are suggested by adding the effect of strand elongation. The role of the strand anchorages is also examined in the context of wave attenuation. Finally, the suitability of the guided wave method for the detection of indentations and broken wires in the strands is demonstrated, including the possibility of inspecting the critical anchored regions.

Guided wave helical ultrasonic tomography of pipes

Abstract: Ultrasonic guided waves have been used for a wide variety of ultrasonic inspection techniques. We describe here a new variation called helical ultrasound tomography (HUT) that uses guided ultrasonic waves along with tomographic reconstruction algorithms that have been developed by seismologists for what they call "cross borehole" tomography. In HUT, the Lamb-like guided waves travel the various helical criss-cross paths between two parallel circumferential transducer arrays instead of the planar criss-cross seismic paths between two boreholes. Although the measurement itself is fairly complicated, the output of the tomographic reconstruction is a readily interpretable map of a quantity of interest such as pipe wall thickness. In this paper we demonstrate HUT via laboratory scans on steel pipe segments into which controlled thinnings have been introduced.

Guided wave testing of rail

Abstract: Guided wave inspection has a number of advantages over conventional ultrasonic inspection. For instance, guided waves can propagate over many tens of metres in rail, they can fully penetrate alumino-thermic welds and theyare very sensitive to cracks in the transverse-vertical plane that may cause catastrophic failure. With financial and technical support from Network Rail, the authors are developing a guided wave inspection system for rail, which is described in this paper. Experimental and finite element results showing the interaction of various guided wave modes with a wide variety of defect geometries are presented. This study has enabled characteristic mode conversion signatures for various defects and features to be obtained and these signatures will provide the basis for an automatic feature extraction algorithm. Results from laboratory tests and site trials are presented that demonstrate the ability of the system to detect a range of defects in free rail, aluminio-thermic welds and in rail at a level crossing.

Light propagation in semi-infinite photonic crystals and related waveguide structures

Abstract: A transfer-matrix method (TMM) that employs the plane-wave expansion of electromagnetic (EM) fields has been developed to handle EM wave propagation in semi-infinite photonic crystal and related waveguide structures. The major aim is to account for wave scattering only at the concerned boundary and to completely remove multiple reflections in the presence of other structural boundaries. It turns out that the scattering problem is closely connected to the eigenmodes of the transfer matrix for the unit cell of the crystal. A natural boundary condition is imposed to describe the asymptotic propagation behavior of scattered EM waves in a region far away from the interface. Theories for a variety of important structures have been systematically set up. These include wave propagation in a semi-infinite photonic crystal, a coated semi-infinite photonic crystal, a heterostructure formed by two different semi-infinite photonic crystals face to face, and a complex sandwiched structure formed by two semi-infinite photonic crystals separated by a general grating slab. In combination with a supercell technique, the developed formulations can also be used to handle photonic crystal waveguide structures. We have applied the developed TMM to two-dimensional photonic crystal and related waveguide structures. The first is the coupling of an external wave into a photonic crystal (photonic crystal waveguide) and the related inverse problems of coupling of a Bloch's wave (guided wave) out of the photonic crystal (photonic crystal waveguide). The second is scattering of a guided wave by a cavity introduced into a photonic crystal waveguide. The developed TMM can help to understand optical properties and design optimal structures of individual functional elements in an optical integrated circuit built in a photonic crystal environment.

Laser-air, hybrid, ultrasonic testing of railroad wheels

Abstract: A remote, non-contact system for detecting a defect in a railroad wheel as the wheel is stationary or moving along a railroad track includes; (1) a pulsed, laser light source for generating an ultrasonic wave in the wheel, the ultrasonic wave having a direct portion and reflected and transmitted portions if the direct portion encounters a defect in the wheel, (2) an optical component in the path of the light from the light source for forming the light into a specified illumination pattern so that the generated ultrasonic wave has a specified wavefront, (3) an air-coupled transducer or a group of transducers for sensing the acoustic signal emanating from the wheel that results from the ultrasonic wave traveling through the wheel, and (4) a signal processor, responsive to the sensed acoustic signal, capable of distinguishing whether the sensed signal has a component that indicates the existence of a reflected portion in the ultrasonic wave, wherein the presence of such a component in the acoustic signal indicates the existence of a defect in the railroad wheel.

The magnetostrictive sensor technology for long range guided wave testing and monitoring of structures

Abstract: A technical background on and applications of a guided wave technology called the magnetostrictive sensor are described. The magnetostrictive sensor technology is for long range global testing and condition monitoring of structures such as piping, plates and steel cables. For long range testing of piping in processing plants, such as refineries and chemical plants, the magnetostrictive sensor system, including the probe, instrument and data analysis software, is matured for use in commercial testing services. Capabilities of the present magnetostrictive sensor system for pipe testing are presented together with an example of testing data and their analysis.

Lamb Waves for Detecting Delamination between Steel Bars and Concrete

Abstract: Traditional ultrasonic methods for inspecting defects in concrete are good for detecting large voids in concrete, but are not very efficient for detecting delamination at the interface between concrete and steel bars. This paper investigates the feasibility of detecting interface degradation and separation of steel bars in concrete beams using generalized Rayleigh-Lamb waves and cylindrical guided Lamb waves. The Lamb wave can propagate a long distance along the reinforcing steel bars as the guided wave and is sensitive to the interface bonding condition between the steel bar and the concrete. A special coupler between the steel bar and ultrasonic transducers has been used to launch nonaxisymmetric guided waves in the steel bar. The results of this investigation show that the proposed transducer-receiver arrangements can successfully inspect reinforced concrete beams for delamination at the steel-concrete interface. Directions for further research are suggested.

Guided waves on a planar tensor impedance surface

Abstract: Guided wave mode propagation constants are determined for an infinite planar surface characterized by an anisotropic impedance tensor. Restrictions on the tensor elements implied by the imposition of a passivity requirement are presented. The conditions under which E and H modes can exist are determined. The special case of a lossless surface is considered.

Rapid inspection of composite skin-honeycomb core structures with ultrasonic guided waves

Abstract: Guided wave inspection of composite skin-honeycomb core structures is an efficient and sensitive alternative to other common inspection methods. This paper shows that sweeping experimentally through the dispersion curves is an effective way to experimentally locate guided wave modes sensitive to skin-core delamination. Composite skin-Nomex honeycomb core specimens were developed with simulated delaminated areas. The delaminated areas were detected with guided waves and confirmed with conventional ultrasonic testing methods. Calculated phase velocity dispersion curves are given to define the practical phase velocity and frequency ranges. Example wave structures in this range are given to illustrate the change in sensitivity as frequency is swept for a given mode.

Guided Wave Propagation Mechanics Across a Pipe Elbow

Abstract: Wave propagation across a pipe elbow region is complex. Subsequent reflected and transmitted waves are largely deformed due to mode conversions at the elbow. This prevents us to date from applying guided waves to the nondestructive evaluation of meandering pipeworks. Since theoretical development of guided wave propagation in a pipe is difficult, numerical modeling techniques are used. We have introduced a semi-analytical finite element method, a special modeling technique for guided wave propagation, because ordinary finite element methods require extremely long computational times and memory for such a long-range guided wave calculation. In this study, the semi-analytical finite element method for curved pipes is developed. A curved cylindrical coordinate system is used for the curved pipe region, where a curved center axis of the pipe elbow region is an axis (z′ axis) of the coordinate system, instead of the straight axis (z axis) of the cylindrical coordinate system. Guided waves in the z′ direction are described as a superposition of orthogonal functions. The calculation region is divided only in the thickness and circumferential directions. Using this calculation technique, echoes from the back wall beyond up to four elbows are discussed.Copyright © 2003 by ASME

Practical Long Range Guided Wave Inspection — Managing Complexity

Abstract: Guided waves are very attractive for the rapid inspection of large structures. However, until recently the number of practical applications has been limited. This is largely due to the presence of many possible wave modes, most of which are dispersive, which can lead to high levels of coherent noise. Techniques to manage this complexity are discussed and practical applications to pipes, railroad rail, plates and rock bolts are described. Areas for future research are then suggested.

Long range inspection of rail using guided waves - field experience

Abstract: Ultrasonic techniques have been used for many years for the inspection of rail. These measurements can detect the presence of a wide variety of defects but there are practical difficulties with the technology. While large transverse cracks of the type likely to cause catastrophic failure can be detected, the large, critical defects can be masked by large numbers of small, surface defects along the length of the rail. It would be very useful to be able to determine reliably the largest defect size in a length of rail. Also alumino-thermic welds are difficult to inspect due to the typical defect orientation and the attenuation of the weld material. Guided waves provide a very attractive solution to these problems; they travel along the rail, for tens or hundreds of metres, and are partially reflected by any defects which are present. They are particularly sensitive to vertical defects and they are used at relatively low frequency so they are not significantly attenuated by weld material. With financial support from Network Rail, the authors have developed a practical inspection tool based on guided wave measurements. This paper describes the design of a pre-production prototype guided wave instrument suitable for site trials. Results obtained at a level crossing are presented that demonstrate the use of the system as a practical screening tool. For the covering abstract see ITRD E123761.

Computerized time-reversal method for structural health monitoring

Abstract: In this paper an experimental and theoretical investigation of the applicability of the time-reversal concept to guided waves in plates, where the waves are dispersive and of multi-modes. It is shown that although temmporal and spatial focusing can be achieved through time reversal, the dispersive behavior of the flexural waves renders it impossible to exactly reconstruct the waveform of the original excitation. Nevertheless, the temporal and spatial focusing allows the development of a synthetic time-reversal array method for a distributed network of sensors and actuators. This new method, which overcomes the limitation of the conventional phased array method that operates under pulse-echo mode, can considerably enhance the signal strength, thus reducing the number of sensors and actuators required to achieve a given signal-to-noise ratio.

Guided wave simulation and visualization by a semianalytical finite element method

Abstract: Simulation and visualization of guided wave propagation can be very useful in both educational and research studies. However, even with the tremendous computational power available today, calculation time and memory have become major issues due to large calculations corresponding to the guided wave testing area. To overcome these problems, we employ a semianalytical finite element method for simulation of guided waves in a plate and a pipe. Two major characteristics of a guided wave, dispersion and multimode existence, are demonstrated in visualization results. Moreover, guided wave propagation in a pipe is discussed for three cases: axisymmetric guided modes in a straight pipe; focusing effect due to tuning time delays and amplitudes; and axisymmetric input for a pipe with an elbow.

A guided wave technique for the characterization of highly attenuative viscoelastic materials

Abstract: The measurement of the acoustic properties of highly attenuative materials such as bitumen is very difficult. One possibility is to use measurements of the extent to which filling a cylindrical waveguide with the material affects the dispersion relationship of the cylinder. Torsional modes have been excited using piezoelectric transducers placed at one end of the cylinder, while the phase velocity and attenuation spectra have been measured by means of laser scanning. At each frequency, under the hypothesis of linear viscoelasticity, the phase velocity and attenuation of the fundamental torsional mode have been calculated as a function of the bulk shear velocity and the bulk shear attenuation of the inner core at that frequency. The resulting phase velocity and guided wave attenuation contour plots have been employed for deriving the unknown shear properties from the measured velocity and attenuation of the guided wave. The monochromaticity of the approach has not required a particular frequency dependence of the material properties to be assumed. Results for bitumen are given.

Cure monitoring using ultrasonic guided waves in wires

Abstract: The possibility of using ultrasonic guided waves for monitoring the cure process of epoxy resins is investigated. The two techniques presented use a wire waveguide which is partly embedded in the resin. The first technique is based on the measurement of attenuation due to leakage of bulk waves into the resin surrounding the waveguide. The second technique measures the reflection of the guided wave that occurs at the point where the waveguide enters the resin. Both the attenuation and the reflection coefficient change significantly during cure, and the numerical methods to relate these to the material properties of the curing resin are presented in this paper. The results from the modeling are experimentally verified and show good agreement. The applicability of each testing method is discussed, and typical cure-monitoring curves are presented.

Long Range Inspection of Rail Using Guided Waves

Abstract: Low frequency guided acoustic waves will propagate many tens of metres along a rail The guided wave modes that can exist in a rail are found using a 2‐dimensional finite element (FE) technique and their interaction with a variety of features and defects is investigated with 3‐dimensional time‐marching FE models Results obtained from a prototype testing system are presented and excellent agreement is obtained with FE predictions Good sensitivity to transverse defects and defects at alumino‐thermic welds is demonstrated

Guided wave thickness measurement with EMATs

Abstract: Guided waves can be used for the thickness measurement of a stainless steel vessel. Commonly used piezoelectric transducers for guided wave generation are often not suitable because of potential couplant contamination. Therefore, a non-contact electromagnetic acoustic transducer (EMAT) was used in this study for non-contact measurement. A sample stainless plate with four different thicknesses was made for the study. A direct and an indirect measurement method based on the relationship between the peak frequency shift and the thickness change were developed. Great accuracy for the thickness measurement was realised.

An improved compact 2-D finite-difference frequency-domain method for guided wave structures

Abstract: An improved compact two-dimensional (2-D) finite-difference frequency-domain method is presented to determine the dispersion characteristics of guided wave structures. Eigenvalue equations that contain only two transverse electric field components are derived from Maxwell's differential equations. Compared to the traditional 2-D FDFD containing four or six field components, both the order and number of nonzero elements of the coefficient matrix are reduced simultaneously. The method is verified by two application examples.

Guided wave flexural mode tuning and focusing for pipe testing

Abstract: Beam focusing potential for ultrasonic guided wave testing of pipes is discussed. Natural focusing with nonaxisymmetric wave impingement is studied along with circumferential phased array focusing. A frequency tuning exercise with a new signal display is presented, as are a few sample experiments.