The reflection of the fundamental torsional mode from cracks and notches in pipes

Experimental validation of an inverse method for defect reconstruction in a two-dimensional waveguide model

Defect reconstruction is essential in non-destructive testing and structural health monitoring with guided ultrasonic waves. This paper presents an algorithm for reconstructing notches in steel plates, which can be seen as artificial defects representing cracks by comparing measured results with those from a simulation model. The model contains a parameterized notch, and its geometrical parameters are to be reconstructed. While the algorithm is formulated and presented in a general notation, a special case of guided wave propagation is used to investigate one of the simplest possible simulation models that discretizes only the cross section of the steel plate. An efficient simulation model of the plate cross section is obtained by the semi-analytical scaled boundary finite element method. The reconstruction algorithm applied is gradient-based, and algorithmic differentiation calculates the gradient. The dedicated experimental setup excites nearly plane wave fronts propagating orthogonal to the notch. A scanning laser Doppler vibrometer records the velocity field at certain points on the plate surface as input to the reconstruction algorithm. Using two plates with notches of different depths, it is demonstrated that accurate geometry reconstruction is possible.

Systematic Evaluation of Ultrasonic In-Line Inspection Techniques for Oil and Gas Pipeline Defects Based on Bibliometric Analysis

The global reliance on oil and gas pipelines for energy transportation is increasing. As the pioneering review in the field of ultrasonic defect detection for oil and gas pipelines based on bibliometric methods, this study employs visual analysis to identify the most influential countries, academic institutions, and journals in this domain. Through cluster analysis, it determines the primary trends, research hotspots, and future directions in this critical field. Starting from the current global industrial ultrasonic in-line inspection (ILI) detection level, this paper provides a flowchart for selecting detection methods and a table for defect comparison, detailing the comparative performance limits of different detection devices. It offers a comprehensive perspective on the latest ultrasonic pipeline detection technology from laboratory experiments to industrial practice.

Damage Monitoring of Steel Bars Based on Torsional Guided Waves

Ultrasonic guided waves represent a new development in the field of non-destructive testing. Longitudinal guided waves are mostly used to monitor the damage of steel bars, but the received signal is usually degraded and noisy owing to its dispersive propagation and multimodal behavior, making its implementation and location challenging. The torsional mode of T (0, 1) is not dispersive in the propagation of a steel bar and only produces circumferential displacement. It was chosen, in this study, to conduct guided wave-based damage monitoring on steel bars to reduce the signal processing complexity. The defects of steel bars, including circular surface defects, internal defects, and uniform damage defects, were thoroughly investigated, respectively, using numerical simulation. The waves were excited and received using the pitch-and-catch technique and the collected monitoring signals were processed using Hilbert transformation to highlight the amplitude and time-of-flight values of the wave signals, which were used for defect identification. In this paper, the reflectivity of guided waves is compared between torsional waves and longitudinal waves, in each case. The impact of defect size changes on damage monitoring is studied and the sensitivity of both the wave frequency and the wave mode (L and T) is also discussed. The results show that the monitoring method based on the torsional wave T (0, 1) is more sensitive to surface defects than the conventional method based on longitudinal waves. The reflectivity of the torsional wave T (0, 1) can be twice that of the longitudinal wave L (0, 1) when the depth of the defect in the circumferential grooves is less than 50% of the diameter of the steel bar. It is more sensitive to shallow surface defects within half of the bar's radius, and it can also effectively identify defects under the conditions of the uniform damage defects of steel bars, even when the measurements are heavily noise-polluted. This proves the superiority of the torsional guided wave T (0, 1) in defect monitoring and provides a theoretical basis for the application of the torsional guided wave T (0, 1) in actual monitoring.

Numerical and experimental investigations on quasi-static component generation of longitudinal wave propagation in isotropic pipes

In this paper, the quasi-static component (QSC) generation of longitudinal waves propagating in an isotropic pipe is investigated numerically and experimentally. The three-dimensional (3D) finite element (FE) simulations are first carried out to gain physical insights into the characteristics of QSC generation from longitudinal wave travelling in an isotropic pipe with weak material nonlinearity. By applying the axial displacement excitation in the FE model, L(0, 1) mode and L(0, 2) mode are excited simultaneously. Then, the generated QSC pulses are extracted using the phase reversal approach for analysis. It is found that the QSC pulses generated by L(0, 2) mode and L(0, 1) mode are L(0, 1) mode. Meanwhile, the shapes of QSC pulses at different locations are extracted and compared. In this study, a data pre-processing method is proposed to handle numerically calculated and subsequent experimentally measured displacement signals, and a nonlinear acoustic parameter is defined to evaluate the incipient damages. After that, an experiment is conducted to measure the QSCs induced by the propagation of longitudinal waves in an aluminum pipe. The experimental results indicate that the propagation of longitudinal waves in the aluminum pipe can induce the QSCs. Different levels of corrosion are created on the surface of the aluminum pipe and are assessed by the generated QSCs. The results show that the nonlinear acoustic parameter has a monotonically increasing trend with the growing severity of corrosion. The QSCs generated by longitudinal wave can be used to detect and evaluate the early-stage surface corrosion in the aluminum pipe.

Circumferential Damage Monitoring of Steel Pipe Using a Radar Map Based on Torsional Guided Waves

Ultrasonic guided wave technology has been successfully applied to detect multiple types of defects in pipes. However, the circumferential location and coverage of a defect are less studied because it is difficult to determine. In this study, the fundamental torsional mode T (0, 1) is selected to conduct monitoring of the circumferential defect in pipelines because of its almost non-dispersive property. A radar map of the peak wave signals at 30 circumferential positions is proposed to detect the damage. The circumferential defect of a steel pipe is thoroughly investigated using numerical simulation. First, the circumferential positioning of defects in various areas of the pipe is studied. Second, the results are compared to those based on longitudinal guide waves. Finally, the circumferential coverage of a defect in the pipeline is determined. The waves are excited and received using the pitch-catch approach, and the collected monitoring signals are processed using the Hilbert transformation. According to the findings, the circumferential defect in the pipe can be effectively identified from a 'T' shape in the radar image, and the monitoring method by the torsional guided wave is superior to the longitudinal wave method. The results clearly demonstrate the advantages of torsional guided waves in defect monitoring. The proposed method is expected to provide a promising solution to circumferential damage identification in pipelines.

Damage Quantification and Identification in Structural Joints through Ultrasonic Guided Wave-Based Features and an Inverse Bayesian Scheme

In this paper, defect detection and identification in aluminium joints is investigated based on guided wave monitoring. Guided wave testing is first performed on the selected damage feature from experiments, namely, the scattering coefficient, to prove the feasibility of damage identification. A Bayesian framework based on the selected damage feature for damage identification of three-dimensional joints of arbitrary shape and finite size is then presented. This framework accounts for both modelling and experimental uncertainties. A hybrid wave and finite element approach (WFE) is adopted to predict the scattering coefficients numerically corresponding to different size defects in joints. Moreover, the proposed approach leverages a kriging surrogate model in combination with WFE to formulate a prediction equation that links scattering coefficients to defect size. This equation replaces WFE as the forward model in probabilistic inference, resulting in a significant enhancement in computational efficiency. Finally, numerical and experimental case studies are used to validate the damage identification scheme. An investigation into how the location of sensors can impact the identified results is provided as well.

Guided waves-based damage identification in plates through an inverse Bayesian process

The use of guided waves to identify damage has become a popular method due to its robustness and fast execution, as well as the advantage of being able to inspect large areas and detect minor structural defects. When a travelling wave on a plate interacts with a defect, it generates a scattered field that will depend on the defects geometry. By analysing the scattered field, one can thus characterize the type and size of the plate damage. A Bayesian framework based on a guided waves interaction model for damage identification of infinite plate for the first time is presented here. A semi-analytical approach based on the lowest order plate theories is adopted to obtain the scattering features for damage geometries with circular symmetry, resulting in an efficient inversion procedure. Subsequently, ultrasound experiments are performed on a large aluminium plate with a circular indentation to generate wave reflection and transmission coefficients. With the aid of signal processing techniques, the effectiveness and efficiency of the proposed approach are verified. A full finite element model is used to test the damage identification scheme. Finally, the scattering coefficients are reconstructed, reliably matching the experimental results. The framework supports digital twin technology of structural health monitoring.

A Review of Signal Processing Techniques for Ultrasonic Guided Wave Testing

Ultrasonic guided wave testing (UGWT) is a non-destructive testing (NDT) technique commonly used in structural health monitoring to perform wide-range inspection from a single point, thus reducing the time and effort required for NDT. However, the multi-modal and dispersive nature of guided waves makes the extraction of essential information that leads to defect detection an extremely challenging task. The purpose of this article is to give an overview of signal processing techniques used for filtering signals, isolating modes and identifying and localising defects in UGWT. The techniques are summarised and grouped according to the geometry of the studied structures. Although the reviewed techniques have led to satisfactory results, the identification of defects through signal processing remains challenging with space for improvement, particularly by combining signal processing techniques and integrating machine learning algorithms.

Periodical Focusing Phenomenon of Ultrasonic Guided Waves in Pipes

We put forward a focusing formula to describe the guided waves periodical focusing phenomenon, which, apart from being a fundamental problem in the guided waves' propagation in the pipes, has essential applications in the field of nondestructive evaluation. Due to the partial circumferential loads or nonaxisymmetric defects, the guided waves are not only in the zeroth-order axisymmetric forms but also in the higher order nonaxisymmetric forms. When multiple orders of the same mode exist simultaneously, the angular profile is adopted to describe the circumferential energy distribution of the superposed wave field. However, the angular profile varies in the propagation process. In our finding, this variation is periodic, meaning that the circumferential energy will repeat the process of dispersing and focusing. Thus, we put forward a focusing formula to describe the phenomenon. The proposed formula indicates that the angular profile varies periodically with the ratio of propagation distance to wavenumber, and this period is only related to the pipe radius. Thus, three factors, including propagation distance, excitation frequency, and pipe radius, will affect the angular profile. Moreover, we established an experimental system to verify this phenomenon, based on which we have designed three groups of experiments to investigate these three factors. The experimental results are in good agreement with the theoretical predictions.

Optimizing the placement of piezoelectric wafers on closed sections using a genetic algorithm - Towards application in structural health monitoring

Sensor network design is essential to efficiently integrate Structural Health Monitoring (SHM) systems in aerospace, automotive, and civil structures. This study describes an optimization model for piezoelectric (PZT) wafer placement on curved structures and closed sections. The proposed approach relied on the transformation of any complex/closed surface regardless of the shape of its cross-section into a flat plate and imposed a set of boundary conditions to account for the wave propagation characteristics. Because the structure was continuous and the wave could propagate in every direction, for simplicity and without sacrificing accuracy, our model assumed that a pair of PZT elements communicated information in the two shortest directions. Thus, the concept of having two paths for each PZT couple was introduced to tackle this multidirectional behavior. The plate was then discretized into a set of control points that represented the structure geometry. The PZT couples covered the control points along the line of sight and in the neighborhood of their direct and indirect paths. The objective function was to maximize the number of covered points while minimizing the number of PZT wafers. The proposed model was solved using a genetic algorithm and was validated on circular and square sections. Sensors were spread on the circumference of the structure rather than mounting them in the form of rings or axial lines. The optimized PZT networks had high coverage that reached 99% in simulations. Notably, the optimized model improved the preliminary solution coverage by 14%. Experimental validation was performed on the circular section (pipe). The results demonstrated the proficiency of the developed model in distributing the PZT wafers on closed sections. The coverage was further evaluated by assessing if damaged areas on the pipe surface could be identified. Artificial damage was accurately located within 18 mm from the actual location. These results demonstrate that our model efficiently distributes PZT wafers on closed structures.

A micro-transducer matrix design for the detection of flexural guided waves

In this paper, a new approach is proposed for the detection of ultrasonic guided waves using a LiNbO₃ single crystal-based micro-transducer matrix. This matrix was designed, manufactured, and then used to detect Lamb and Pochhammer-Chree guided waves in plate- and cylinder-like structures. This study highlights the identification of the first flexural mode F(1,1) in cylinders at low frequencies. A network analyser and a laser Doppler vibrometer (LDV) were used to characterise and study the behaviour of the micro-transducer matrix. An experimental device was designed and used to acquire electrical measurements of the micro-transducer vibrations. Then, an original experimental device was developed to generate a selected flexural guided mode in a solid aluminium cylinder. The emitter comprised two semicircular piezoelectric transducers excited with only one phased signal thanks to the inverse position of polarisation. Finally, the results prove that the flexural mode F(1,1) is selected and generated by the emitter, then detected and identified by the micro-transducer matrix.

Shear horizontal wave transducers for structural health monitoring and nondestructive testing: A review

Shear horizontal (SH) waves are of great importance in structural health monitoring (SHM) and nondestructive testing (NDT), since the lowest order SH wave in isotropic plates is non-dispersive. The SH waves in plates, circumferential SH waves and torsional waves in pipes have remarkable resemblances in dispersion characteristics and wave structures, so the latter two can also be called as SH waves in pipes. This paper reviews the state-of-the-art research on SH wave transducers for SHM and NDT. These transducers are grouped into the following categories: Lorentz-force-based electromagnetic acoustic transducers (EMATs), magnetostrictive EMATs, shear wave piezoelectric wedge transducers, thickness-shear piezoelectric transducers and face-shear piezoelectric transducers. The working principles, applications, merits and limitations of different kinds of SH wave transducers are summarized, with a focus on discussing the various configurations for exciting and receiving directional, omnidirectional SH waves in plates and torsional waves in pipes. This paper is expected to greatly promote the applications of SH waves in SHM, NDT and the related areas such as elastic metamaterials.

Location Specific Temperature Compensation of Guided Wave Signals in Structural Health Monitoring

In guided wave structural health monitoring, defects are typically detected by identifying high residuals obtained through the baseline subtraction method, where an earlier measurement is subtracted from the "current" signal. Unfortunately, varying environmental and operational conditions (EOCs), such as temperature, also produce signal changes and hence, potentially, high residuals. While the majority of the temperature compensation methods that have been developed target the changed wave speed induced by varying temperature, a number of other effects are not addressed, such as the changes in attenuation, the relative amplitudes of different modes excited by the transducer, and the transducer frequency response. A temperature compensation procedure is developed, whose goal is to correct any spatially dependent signal change that is a systematic function of temperature. At each structural position, a calibration function that models the signal variation with temperature is computed and is used to correct the measurements, so that in the absence of a defect the residual is reduced to close to zero. This new method was applied to a set of guided wave signals collected in a blind trial of a guided wave pipe monitoring system using the T(0, 1) mode, yielding residuals de-coupled from temperature and reduced by at least 50% as compared with those obtained using the standard approach at positions away from structural features, and by more than 90% at features such as the pipe end. The method, therefore, promises a substantial improvement in the detectability of small defects, particularly at the existing pipe features.

Scattering of the Fundamental Shear Guided Wave From a Surface-Breaking Crack in Plate-Like Structures

Cracks in critical sections of steel structures pose a major safety concern in many industries. Existing high-frequency ultrasonic techniques offer high detection sensitivity to cracks but have poor inspection volume coverage, limiting their practical use for monitoring large areas of structures. Low-frequency guided waves have relatively high inspection area coverage and are currently used in pipeline monitoring for corrosion defects but face challenges in detecting critical cracks that often cause over an order of magnitude lower cross-sectional area loss. A study of scattering from small cracks in a thin-walled (<12 mm) section with an incident plane SH0 guided wave at higher frequencies but remaining below the SH1 cutoff is presented here using quasi-static approximations, the aim being to explore the possibility of using this regime for crack growth monitoring applications. A 3-D solution was developed using dimensional analysis, which showed that the SH0 reflection ratio is proportional to frequency to the power 1.5, to the effective crack size cubed, and is inversely proportional to the plate thickness and to the square root of the distance from the crack to the receiving sensor. Finite element analysis was used to validate these power coefficients and to calculate the proportionality constant. The results show that a higher inspection frequency offers improved sensitivity, but the validity of the results here is limited to the SH1 cutoff frequency. The predicted 3-D solution was validated by measurements on a pipe with a progressively grown notch.

Excitation of moderate-frequency Love wave in a Plexiglas plate on aluminum semi-space

Love waves are of great importance in geophysics, electronics, and engineering. Despite intensive studies on high-frequency Love waves for delay lines and chemical/biomedical sensors, moderate-frequency and low-frequency Love waves have seldom been investigated. Here, Love waves in a 2-mm-thick Plexiglas plate bonded on a 50-mm-thick aluminum block were successfully excited and received by using d₂₄ and d₁₅ shear mode PZT wafers. A d₃₁ mode PZT wafer was also employed as the sensor, and results show that no Rayleigh-Lamb type waves were generated. The group velocity curve of the excited Love wave from 120 to 525 kHz matches the theoretical curve well. This work could promote applications of Love waves in many engineering fields.

Magnetostrictive Guided Wave Technique Verification for Detection and Monitoring Defects in the Pipe Weld

During inspection of piping in nuclear power plants or other industries, it is difficult to implement conventional nondestructive techniques due to limited accessibility or obstacles such as pipes with insulation, pipes buried underground, structural complexity, or radiation environments. In addition, since the defects mainly occur in the weld region or support area, it is not easy to separate defect signals from those of structural components. To solve these problems, we developed a technique to detect and monitor the formation and growth of defects, using a magnetostrictive guided wave sensor. This sensor has advantages (such as sharp and clear signal patterns and ability to easily eliminate the signal from the geometric structure) over the conventional piezoelectric transducer. To verify our technique, signals from actual pipe welds with defects were acquired and processed with our phase matching/subtraction program. The proposed technique shows a superior capability for detection and monitoring of defects, compared to the conventional guided wave methods.

Development of ultrasonic guided wave inspection methodology for steam generator tubes of prototype fast breeder reactor

An ultrasonic guided wave based methodology is developed for inspection of steam generator tubes of the prototype fast breeder reactor. To this aim, axisymmetric longitudinal mode (L(0,2)) at the frequency of 250 kHz is optimized using 3D-finite element simulation and experiments. The group velocity of mode L(0,2) at 250 kHz is found to be 5387 m/s. First, the long range propagation of the L(0,2) mode at 250 kHz is examined and the mode is found to propagate over a distance of 45.6 m with a sufficiently good SNR. Secondly, the detection of multiple defects such as circumferential, axial, partial-pinholes and tapered defects lying in the same line of sight is investigated using 3D-finite element simulation and the results obtained are validated experimentally for the first three cases. The sensitivities achieved are 0.23 mm depth (10%WT) for circumferential, axial and tapered defects and for partial-pinholes: 1 mm diameter and 1.38 mm depth (60%WT). Thirdly, 3D-FE simulations with ID and OD pinhole defects are performed which show that the ID and OD defects are detected by L(0,2) with a fairly similar sensitivity. Finally, study on the thermal expansion bend (with three successive bends) shows that the bend does not have much influence on the mode and the multiple circumferential defects considered in the bend are detected with good sensitivity.

Incremental scattering of the A0 Lamb wave mode from a notch emanating from a through-hole

Lamb wave scattering from a crack originating at a through-hole is of practical importance because of the abundance of fastener holes used in engineering structures. Notches are often used to simulate cracks so that Lamb wave methods can be more conveniently investigated in the laboratory. A linear, three-dimensional finite element model is employed in this paper to study incremental scattering of the fundamental anti-symmetric (A₀) Lamb wave mode from notches emanating from through-holes. The term "incremental scattering" refers to the change in scattering caused by introduction of the notch and is motivated by structural health monitoring for which transducers are fixed and signal changes are interpreted to detect damage. Far-field angular scattering patterns are generated for multiple incident angles and frequencies, and such patterns are experimentally validated at one frequency by laser vibrometry measurements. Comparisons are made between a vertical notch alone (no hole) and notches located above and below the through-hole. Additionally, holes of different sizes are considered to investigate the effect of hole diameter on incremental scattering patterns. Results show that the presence, location and size of the through-hole affect both the shape and strength of notch incremental scattering patterns.

Detection, Localisation and Assessment of Defects in Pipes Using Guided Wave Techniques: A Review

This paper aims to provide an overview of the experimental and simulation works focused on the detection, localisation and assessment of various defects in pipes by applying fast-screening guided ultrasonic wave techniques that have been used in the oil and gas industries over the past 20 years. Major emphasis is placed on limitations, capabilities, defect detection in coated buried pipes under pressure and corrosion monitoring using different commercial guided wave (GW) systems, approaches to simulation techniques such as the finite element method (FEM), wave mode selection, excitation and collection, GW attenuation, signal processing and different types of GW transducers. The effects of defect parameters on reflection coefficients are also discussed in terms of different simulation studies and experimental verifications.

New Magnetostrictive Transducer Designs for Emerging Application Areas of NDE

Magnetostrictive transduction has been widely utilized in nondestructive evaluation (NDE) applications, specifically for the generation and reception of guided waves for the long-range inspection of components such as pipes, vessels, and small tubes. Transverse-motion guided wave modes (e.g., torsional vibrations in pipes) are the most common choice for long-range inspection applications, because the wave motion is in the plane of the structure surface, and therefore does not couple well to the surrounding material. Magnetostrictive-based sensors for these wave modes using the Wiedemann effect have been available for several years. An alternative configuration of a sensor for generating and receiving these transverse-motion guided waves swaps the biasing and time-varying magnetic field directions. This alternative design is a reversed Wiedemann effect magnetostrictive transducer. These transducers exhibit a number of unique features compared with the more conventional Wiedemann sensor, including: (1) the use of smaller rare earth permanent magnets to achieve large, uniform, and self-sustained bias field strengths; (2) the use of more efficient electric coil arrangements to induce a stronger time-varying magnetic field for a given coil impedance; (3) beneficial non-linear operating characteristics, given the efficiency improvements in both magnetic fields; and (4) the ability to generate unidirectional guided waves when the field arrangement is combined with a magnetically soft ferromagnetic strip (patch). Reversed Wiedemann effect magnetostrictive transducers will be presented that are suitable for different inspection applications, one using electromagnetic generation and reception directly in a ferromagnetic material, and another design that integrates a magnetostrictive patch to improve its efficiency and enable special operating characteristics.

Use of the reciprocity theorem for a closed form solution of scattering of the lowest axially symmetric torsional wave mode by a defect in a pipe

Guided waves can effectively be used for inspection of large scale structures. Surface corrosion is often found as major defect type in large scale structures such as pipelines. Guided wave interaction with surface corrosion can provide useful information for sizing and classification. In this paper, the elastodynamic reciprocity theorem is used to formulate and solve complicated scattering problems in a simple manner. The approach has already been applied to scattering of Rayleigh and Lamb waves by defects to produce closed form solutions of amplitude of scattered waves. In this paper, the scattering of the lowest axially symmetric torsional mode, which is widely used in commercial applications, is analyzed by the reciprocity theorem. In the present paper, the theorem is used to determine the scattering of the lowest torsional mode by a tapered defect that was earlier considered experimentally and numerically by the finite element method. It is shown that by the presented method it is simple to obtain the ratio of amplitudes of scattered torsional modes for a tapered notch. The results show a good agreement with earlier numerical results. The wave field superposition technique in conjunction with the reciprocity theorem simplifies the solution of the scattering problem to yield a closed form solution which can play a significant role in quantitative signal interpretation.

Damage detection in bent plates using shear horizontal guided waves

Study of the interaction of shear horizontal guided mode with defects in the bend region of an isotropic top hat stiffener is presented. Compared with the SH0 wave in a plate, the shear mode in the bend is dispersive and its wavefield characteristics are affected by the curvature of the bend. The scattering studies showed that the sensitivity of the wave to outer surface cracks in the bend increases with increasing frequency compared to inner surface cracks. Further numerical simulations demonstrated that the shear mode is sensitive to the delamination in the bend due to non-zero transverse shear stress. Results of finite element modeling were validated by experiments and reasonably good agreements were obtained.

Deconvolution imaging of weak reflective pipe defects using guided-wave signals captured by a scanning receiver

Guided-wave echoes from weak reflective pipe defects are usually interfered by coherent noise and difficult to interpret. In this paper, a deconvolution imaging method is proposed to reconstruct defect images from synthetically focused guided-wave signals, with enhanced axial resolution. A compact transducer, circumferentially scanning around the pipe, is used to receive guided-wave echoes from discontinuities at a distance. This method achieves a higher circumferential sampling density than arrayed transducers-up to 72 sampling spots per lap for a pipe with a diameter of 180 mm. A noise suppression technique is used to enhance the signal-to-noise ratio. The enhancement in both signal-to-noise ratio and axial resolution of the method is experimentally validated by the detection of two kinds of artificial defects: a pitting defect of 5 mm in diameter and 0.9 mm in maximum depth, and iron pieces attached to the pipe surface. A reconstructed image of the pitting defect is obtained with a 5.87 dB signal-to-noise ratio. It is revealed that a high circumferential sampling density is important for the enhancement of the inspection sensitivity, by comparing the images reconstructed with different down-sampling ratios. A modified full width at half maximum is used as the criterion to evaluate the circumferential extent of the region where iron pieces are attached, which is applicable for defects with inhomogeneous reflection intensity.

Pipe Attrition Acoustic Locater (PAAL) from multi-mode dispersion analysis

Multi-mode dispersion imaging shows that pure dispersion-free torsional waves are reflected at a pipe end and flexural wave modes are suppressed. This effect can be used to locate and assess internal damage. The end reflection coefficient of this single propagating mode decreases with increasing wear. The pipe damage is located from the travel time of the torsional wave component reflected from the damage point.

Excitation Waveform Design for Lamb Wave Pulse Compression

Most ultrasonic guided wave methods focus on tone burst excitation to reduce the effect of dispersion so as to facilitate signal interpretation. However, the resolution of the output cannot attain a very high value because time duration of the excitation waveform cannot be very small. To overcome this limitation, a pulse compression technique is introduced to Lamb wave propagation to achieve a δ-like correlation so as to obtain a high resolution for inspection. Ideal δ-like correlation is impossible as only a finite frequency bandwidth can propagate. The primary purpose of this paper is to design a proper excitation waveform for Lamb wave pulse compression, which shortens the correlation as close as possible to a δ function. To achieve this purpose, the performance of some typical signals is discussed in pulse compression, which include linear chirp (L-Chirp) signal, nonlinear chirp (NL-Chirp) signal, Barker code (BC), and Golay complementary code (GCC). In addition, how the excitation frequency range influences inspection resolution is investigated. A strategy for the frequency range determination is established subsequently. Finally, an experiment is carried out on an aluminum plate where these typical signals are used as excitations at different frequency ranges. The quantitative comparisons of the pulse compression responses validate the theoretical findings. By utilizing the experimental data, the improvement of pulse compression in resolution compared with tone burst excitation is also validated, and the robustness of the waveform design method to inaccuracies in the dispersion compensation is discussed as well.

High-frequency lowest torsional wave mode ultrasonic inspection using a necked pipe waveguide unit

We propose an effective method to transmit only the non-dispersive lowest torsional wave mode at a high frequency range even above the cutoff frequency of the third torsional mode. Unlike existing methods that tune the wavelength or phase of the target wave mode, the proposed method is based on the thickness change and the cutoff phenomenon. A specially configured necked waveguide, consisting of three regions of which the middle region is thinner than the so-called cutoff thickness, is put in end-to-end contact with a test pipe to transmit only the first torsional wave mode to a test pipe. After explaining the underlying role of the proposed necked waveguide, we propose a technique to mainly transmit the lowest torsional wave mode at a frequency where higher modes can also propagate. Numerical simulations and damage detection experiments were carried out to show the effectiveness of the proposed method.

Enhancement of the excitation efficiency of a torsional wave PPM EMAT array for pipe inspection by optimizing the element number of the array based on 3-D FEM

Electromagnetic acoustic transducers (EMATs) can generate non-dispersive T(0,1) mode guided waves in a metallic pipe for nondestructive testing (NDT) by using a periodic permanent magnet (PPM) EMAT circular array. In order to enhance the excitation efficiency of the sensor, the effects of varying the number of elements of the array on the excitation efficiency is studied in this paper. The transduction process of the PPM EMAT array is studied based on 3-D finite element method (FEM). The passing signal amplitude of the torsional wave is obtained to represent the excitation efficiency of the sensor. Models with different numbers of elements are established and the results are compared to obtain an optimal element number. The simulation result is verified by experiments. It is shown that after optimization, the amplitudes of both the passing signal and defect signal with the optimal element number are increased by 29%, which verifies the feasibility of this optimal method. The essence of the optimization is to find the best match between the static magnetic field and the eddy current field in a limited circumferential space to obtain the maximum circumferential Lorentz force.

Reciprocity principle for scattered fields from discontinuities in waveguides

This study investigates the scattering of guided waves from a discontinuity exploiting the principle of reciprocity in elastodynamics, written in a form that applies to waveguides. The coefficients of reflection and transmission for an arbitrary mode can be derived as long as the principle of reciprocity is satisfied at the discontinuity. Two elastodynamic states are related by the reciprocity. One is the response of the waveguide in the presence of the discontinuity, with the scattered fields expressed as a superposition of wave modes. The other state is the response of the waveguide in the absence of the discontinuity oscillating according to an arbitrary mode. The semi-analytical finite element method is applied to derive the needed dispersion relation and wave mode shapes. An application to a solid cylinder with a symmetric double change of cross-section is presented. This model is assumed to be representative of a damaged rod. The coefficients of reflection and transmission of longitudinal waves are investigated for selected values of notch length and varying depth.

Frequency-sweep examination for wave mode identification in multimodal ultrasonic guided wave signal

Ultrasonic guided waves can be used to assess and monitor long elements of a structure from a single position. The greatest challenges for any guided wave system are the plethora of wave modes arising from the geometry of the structural element which propagate with a range of frequency-dependent velocities and the interpretation of these combined signals reflected by discontinuities in the structural element. In this paper, a novel signal processing technique is presented using a combination of frequency-sweep measurement, sampling rate conversion, and Fourier transform. The technique is applied to synthesized and experimental data to identify different modes in complex ultrasonic guided wave signals. It is demonstrated throughout the paper that the technique also has the capability to derive the time of flight and group velocity dispersion curve of different wave modes in field inspections.

Interaction of low-frequency axisymmetric ultrasonic guided waves with bends in pipes of arbitrary bend angle and general bend radius

The use of ultrasonic guided waves for the inspection of pipes with elbow and U-type bends has received much attention in recent years, but studies for more general bend angles which may also occur commonly, for example in cross-country pipes, are limited. Here, we address this topic considering a general bend angle φ, a more general mean bend radius R in terms of the wavelength of the mode studied and pipe thickness b. We use 3D Finite Element (FE) simulation to understand the propagation of fundamental axisymmetric L(0,2) mode across bends of different angles φ. The effect of the ratio of the mean bend radius to the wavelength of the mode studied, on the transmission and reflection of incident wave is also considered. The studies show that as the bend angle is reduced, a progressively larger extent of mode-conversion affects the transmission and velocity characteristics of the L(0,2) mode. However the overall message on the potential of guided waves for inspection and monitoring of bent pipes remains positive, as bends seem to impact mode transmission only to the extent of 20% even at low bend angles. The conclusions seem to be valid for different typical pipe thicknesses b and bend radii. The modeling approach is validated by experiments and discussed in light of physics of guided waves.

Data communications using guided elastic waves by time reversal pulse position modulation: experimental study

In this paper, we present and demonstrate a low complexity elastic wave signaling and reception method to achieve high data rate communication on dispersive solid elastic media, such as metal pipes, using piezoelectric transducers of PZT (lead zirconate titanate). Data communication is realized using pulse position modulation (PPM) as the signaling method and the elastic medium as the communication channel. The communication system first transmits a small number of training pulses to probe the dispersive medium. The time-reversed probe signals are then utilized as the information carrying waveforms. Rapid timing acquisition of transmitted waveforms for demodulation over elastic medium is made possible by exploring the reciprocity property of guided elastic waves. The experimental tests were conducted using a National Instrument PXI system for waveform excitation and data acquisition. Data telemetry bit rates of 10 kbps, 20 kbps, 50 kbps and 100 kbps with the average bit error rates of 0, 5.75 x 10-⁴, 1.09 x 10-² and 5.01 x 10-², respectively, out of a total of 40, 000 transmitted bits were obtained when transmitting at the center frequency of 250 kHz and a 500 kHz bandwidth on steel pipe specimens. To emphasize the influence of time reversal, no complex processing techniques, such as adaptive channel equalization or error correction coding, were employed.

Higher torsional mode suppression in a pipe for enhancing the first torsional mode by using magnetostrictive patch transducers

Small-sized defects in pipes can be better detected if the first nondispersive torsional mode is used in a higher frequency range. However, dispersive higher torsional modes accompany the first mode if the actuation frequency is above the first cutoff frequency, thereby making the detection difficult. This study proposes a new technique that is particularly useful for guided torsional waves in a pipe; it enhances the desired first nondispersive mode and suppresses the undesired second mode. The technique uses two transmitting transducers separated by an optimized distance and actuated with an optimized delay time. Unlike previous methods, such as a method tuning the delay time for desired mode enhancement and tuning the distance for undesired mode suppression, the proposed approach determines both the distance and delay time mainly to suppress the undesired second mode. With the selected values, the desired first mode is substantially enhanced. This phenomenon is unique in torsional waves, not longitudinal waves, for which delay time and distance controlling methods have been developed. After wave simulations were carried out to show why the proposed method is more effective for the case of torsional waves, several experiments using magnetostrictive transducers were performed to demonstrate the effectiveness of the proposed method.

Circumferential phased array of shear-horizontal wave magnetostrictive patch transducers for pipe inspection

Several investigations report effective uses of magnetostrictive patch transducers to generate and measure longitudinal and torsional guided waves in a pipe. They can be used to form a phased array for the circumferential inspection of pipes. Although there are circumferential phased arrays employing piezoelectric transducers or EMAT's, no magnetostrictive patch transducer based array system has been attempted. In this investigation, we aim to develop a circumferential phased magnetostrictive patch transducer (PMPT) array that can focus shear-horizontal waves at any target point on a cylindrical surface of a pipe. For the development, a specific configuration of a PMPT array employing six magnetostrictive patch transducers is proposed. A wave simulation model is also developed to determine time delays and amplitudes of signals generated by the transducers of the array. This model should be able to predict accurately the angular profiles of shear-horizontal waves generated by the transducers. For wave focusing, the time reversal idea will be utilized. The wave focusing ability of the developed PMPT array is tested with multiple-crack detection experiments. Imaging of localized surface inspection regions is also attempted by using wave signals measured by the developed PMPT array system.

Study and comparison of different EMAT configurations for SH wave inspection

Guided wave inspection has proven to be a very effective method for the rapid inspection of large structures. The fundamental shear horizontal (SH) wave mode in plates and the torsional mode in pipe-like structures are especially useful because of their non-dispersive character. Guided waves can be generated by either piezoelectric transducers or electro- magnetic acoustic transducers (EMATs), and EMATs can be based on either the Lorentz force or magnetostriction. Several EMAT configurations can be used to produce SH waves, the most common being Lorentz-force periodic permanent magnet and magnetostrictive EMATs, the latter being directly applied on the sample or with a bonded strip of highly magnetostrictive material on the plate. This paper compares the performance of these solutions on steel structures. To quantitatively assess the wave amplitude produced by different probes, a finite element model of the elementary transducers has been developed. The results of the model are experimentally validated and the simulations are further used to study the dependence of ultrasonic wave amplitude on key design parameters. The analysis shows that magnetostrictive EMATs directly applied on mild steel plates have comparatively poor performance that is dependent on the precise magneto-mechanical properties of the test object. Periodic permanent magnet EMATs generate intermediate wave amplitudes and are noncontact and insensitive to the variations in properties seen across typical steels. Large signal amplitudes can be achieved with magnetostrictive EMATs with a layer of highly magnetostrictive material attached between the transducer and the plate, but this compromises the noncontact nature of the transducer.

Shear-horizontal wave-based pipe damage inspection by arrays of segmented magnetostrictive patches

The lowest-branch torsional guided wave is very effective in pipe damage inspection because of its non-dispersive characteristics, but it cannot be used for the simultaneous identification of axial and circumferential locations of a defect in a pipe. Motivated by recent developments in magnetostrictive transducer technology, which is especially efficient in torsional and shear wave generation, the goal of this investigation is to extend this technology for simultaneous identification of the axial and circumferential locations of cracks by using shear horizontal (SH) waves. Unlike the conventional magnetostrictive patch method using a single complete patch wound around the pipe's circumference, the proposed method segments the patch into several pieces to generate SH waves propagating over the pipe surface. Accordingly, SH waves in a pipe are generated and sensed individually by a meander coil placed separately on each segment. By using two sets of segmented-patch arrays separated by some distance, the cylindrical surface of a pipe can be inspected both axially and circumferentially. After the underlying angular profile of the patch segment is investigated, experiments identifying the axial and circumferential locations of multiple cracks in a pipe are carried out to demonstrate the potential of the proposed methodology.

Analysis of internal wave reflection within a magnetostrictive patch transducer for high-frequency guided torsional waves

Recently, megahertz-range torsional waves have been successfully generated and measured by a magnetostrictive patch transducer employing a meander coil. But the waveform of a high-frequency torsional wave generated by magnetostrictive patch transducers becomes greatly distorted with multiple trailing pulses. The hypothesis explaining the cause of the waveform distortion is that the distortion results mainly from the internal wave reflection within the magnetostrictive patch, which is in turn caused by the impedance mismatch between the bare and patch-bonded parts of the pipe. Based on the hypothesis, we developed an analytic model for internal reflection simulation and conducted several experiments using a patch transducer to verify the hypothesis. The comparison of the analytical and experimental results showed that the internal reflection at the edge of the patch was responsible for the distortion of the measured waveform. The present study also confirmed that the standard acoustic impedance matching to avoid sudden discontinuities at the patch edges can effectively reduce the internal reflection and alleviate the waveform distortion problem.

Guided wave propagation in single and double layer hollow cylinders embedded in infinite media

Millions of miles of pipes are being used for the transportation, distribution, and local use of petroleum products, gas, water, and chemicals. Most of the pipes are buried in soil, leading to the significance of the study on the subject of guided wave propagation in pipes with soil influence. Previous investigations of ultrasonic guided wave propagation in an elastic hollow cylinder and in an elastic hollow cylinder coated with a viscoelastic material have led to the development of inspection techniques for bare and coated pipes. However, the lack of investigation on guided wave propagation in hollow cylinders embedded in infinite media like soil has hindered the development of pipe inspection methods. Therefore the influence of infinite media on wave propagation is explored in this paper. Dispersion curves and wave structures of both axisymmetric and nonaxisymmetric wave modes are developed. Due to the importance of the convergence of numerical calculations, the requirements of thickness and element number of the finite soil layer between hollow cylinder and infinite element layer are discussed, and an optimal combination is obtained in this paper. Wave structures are used for the mode identification in the non-monotonic region caused by the viscoelastic properties of coating and infinite media.

Scattering of plane guided waves obliquely incident on a straight feature with uniform cross-section

A frequency-domain finite element (FE) method is presented for modeling the scattering of plane guided waves incident on an infinitely-long, straight feature with uniform cross-section in a planar host waveguide. The method utilizes a mesh of 2-dimensional finite elements with harmonic shape functions in the perpendicular direction. The model domain comprises a cross-section through the feature and short lengths of the adjoining host waveguide. A spatial frequency equal to the wavenumber of the desired incident mode multiplied by the sine of the desired incidence angle is prescribed for the element shape functions. An integral representation of the incident mode is used to determine a suitable system of harmonic forces to uniquely excite that mode. These are applied at nodes through the thickness of the host waveguide on one side of the feature. The displacement field is measured at nodes through the thickness of the host waveguide on either side of the feature and decomposed into reflected and transmitted modes. The cases of guided wave transmission in a featureless waveguide and the reflection of guided waves from a free-edge are examined as validation cases. Finally, the results for transmission at an adhesively-bonded stiffener are presented and compared with experimental measurements.

Low-frequency pulse echo reflection of the fundamental shear horizontal mode from part-thickness elliptical defects in plates

Defect characterization using guided ultrasonic waves remains a challenging subject and requires a full understanding of the interaction of guided waves with a realistic representation of the defect. The characteristics of pulse echo reflection of the SH0 mode from part-thickness elliptical defects in plates is studied via finite element analysis and experimental measurements. The study shows that the reflection ratio spectrum of the SH0 mode from an elliptical defect exhibits periodic pattern due to interference between reflections from the two edges of the defect. The pattern of the reflection ratio spectrum is determined by the ratio of defect length in the incidence direction to wavelength, while the magnitude is affected by the maximum depth and the effective aspect ratio of the defect. Both the pattern and magnitude of the reflection ratio spectrum are found to be highly sensitive to the incidence angle, and the form of the variation of the reflection with angle is a strong function of the defect shape. In addition, a study of circular defects with tapered depth profiles reveals that the reflection is a function of average length of the tapered defect to wavelength ratio, and the magnitude of the reflection diminishes as the ratio increases.

The scattering of the fundamental torsional mode from axi-symmetric defects with varying depth profile in pipes

Guided ultrasonic waves have been successfully applied to detect defects and corrosion in plates and pipes; however, defect sizing remains challenging due to the complex profiles of the defects encountered in practice. A study of the reflection of the fundamental torsional mode T(0,1) from an axi-symmetric defect with varying depth profile has been carried out via finite element modeling and experimental validation. Defects with gradually varying depth are represented by a series of tapered steps. It is revealed that, for both tapered up- and down-steps, the wave packet is reflected only at the start and end of the steps. The reflections from the start and end of the step have been predicted, and the frequency dependence of the overall reflection from the step has been studied. The study shows that the reflection coefficient varies as a function of the ratio of the average length of the tapered defect to the wavelength due to the interference between the waves reflected from the two ends of the defect. The reflection coefficient maxima decrease as the slope of the taper becomes more gradual, this effect being more pronounced when the ratio of the average defect length to the wavelength increases.

Megahertz-range guided pure torsional wave transduction and experiments using a magnetostrictive transducer

This work is concerned with high-frequency guided torsional wave experiments in the range of 1 to 2 MHz in a cylindrical waveguide. A specially configured meander coil-magnetostrictive patch transducer was developed and successful experimental results of generation and measurement of guided pure torsional waves of up to 2 MHz were achieved. The usefulness of the developed method was demonstrated through a case study to detect a small-sized crack which would be otherwise difficult to identify with a lower-frequency torsional wave.

Scattering of the fundamental torsional mode at an axial crack in a pipe

A quantitative study of the interaction of the T(0,1) torsional mode with an axial defect in a pipe is presented. The results are obtained from finite element simulations and experiments. The influence of the crack axial extent, depth, excitation frequency, and pipe circumference on the scattering is examined. It is found that the reflection from a defect consists of a series of the wave pulses with gradually decaying amplitudes. Such behavior is caused by the shear waves diffracting from the crack and then repeatedly interacting with the crack due to circumferential propagation. Time-domain reflection coefficient analysis demonstrates that the trend of the reflection strength for different crack lengths, pipe diameters, and frequencies from a through-thickness crack satisfies a simple normalization. The results show that the reflection coefficient initially increases with the crack length at all frequencies but finally reaches an oscillating regime. Also, at a given frequency and crack length the reflection decreases with the increase in pipe circumference. An additional scattering study of the shear wave SH(0) mode at a part-thickness notch in a plate shows that the reflection coefficient, when plotted against depth of the notch, increases with both frequency and notch depth.

Rapid thickness measurements using guided waves from a scanning laser source

Guided waves have been effectively used for rapid inspections of plates and pipes. However, the guided-wave technique is not generally used for measuring the remaining thickness in a plate and a pipe due to the difficulties in guided-wave motion. Instead, time-consuming and costly direct contact thickness measurements are still used in practice. This study describes a thickness measurement technique using the A0 mode of a Lamb wave generated by a laser source. A finite element analysis of Lamb wave revealed that this mode propagates with small reflections and mode conversions at a rounded shallow defect and has larger amplitude at thinner regions. Using these characteristics, it is experimentally demonstrated that the distributions of plate thickness were obtained from the amplitude of A0 mode generated by a scanning laser source and received by an angle-beam transducer. The resulting distribution images were obtained at extremely high speed compared to the conventional thickness measurements.

Semi-Quantitative Analysis of Defect in Pipelines through the Use of Technique of Ultrasonic Guided Waves

Modernized cities must have adequate infrastructures to support the daily needs from her citizens. The continuity in pipeline services that supply water, gas and oil to citizens and deliver wastes to designated collectors are in prime concern to any modernized city. However, in-service pipeline is prone to defects due to aging, external impacts, or hazardous operating environment. It is of prime importance to apply an efficient inspection method to characterize the potential defect in pipeline so that the information of damage caused can be determined prior to the fatal rupture of pipeline. An early warning generated from an accurate characterization of defect can encourage the performance of proper remedy and maintenance for minimizing the scope of damage to pipelines. In this paper, a presentation is given to an advanced inspection technique based on ultrasonic guided waves. This technique has already shown great potentials in non-destructive testing of material and structures in many fields. The advantages and difficulties involved in the pipeline inspection using ultrasonic guided waves have been identified. For the quantitative characterization of defect in pipeline inspection based on advanced guided waves, we propose the method through considering the reflected signal since it provides useful information related to defect. The method analyzes the captured signals reflected from the potential defect, decomposes the embedded dimensional information of defect and then accordingly identifies its severity. Although the experiments were conducted on artificial defects, the results proved that qualitative characterization of defect is feasible. Combined with guided waves, our method can provide comprehensive information related to the existence, location, severity of defect etc., through the analysis of reflected signal from the interactions of excited guided waves with pipeline defect.

The application of synthetic focusing for imaging crack-like defects in pipelines using guided waves

This paper deals with quantifying the performance of a technique for detection, location, and sizing of circumferential crack-like defects in pipelines using synthetically focused guided waves. The system employs a circumferential array of piezoelectric transducer elements. A torsional probing guided wave is excited using the array, which subsequently interacts with the reflecting features of the pipe, such as defects or weld caps. The recorded backscattered signals are synthetically focused to every point of interest in the pipe wall, to form an image of the reflecting features of the pipe. The defect image amplitude is used to estimate the defect depth, and the full width at half maximum of the defect image circumferential profile is used to estimate the circumferential extent of the defect. The imaging system is tested with data from finite element simulations and from laboratory experiments. It is found that reliable sizing of circumferential cracks in finite element simulations and experiments can be achieved if the circumferential extent of the defect is greater than 1.5 lambda(S), where lambda(S) is the shear wavelength at the frequency of inspection. This result is theoretically valid for any pipe size, any axial defect location, and any inspection frequency. Amplitude gains of around 18 dB over an unfocused system have been observed experimentally in an 8-inch pipe with a 9 dB SNR improvement.

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

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.

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

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 degrees 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 reflection is first examined, followed by a study of the angular profile of the reflected field. With each crack length considered, the crack depth and operating frequencies are varied. For all crack depths studied, the trend of the results is identical to that for the corresponding through-thickness case and the values differ only by a frequency dependent scale factor. Theoretical analysis is used to understand the physical basis for such behavior and estimates are suggested for the scale factor--exact for the high-frequency scattering regime and empirical for the medium- and low-frequency regimes.

Attenuation and scattering of axisymmetrical modes in a fluid-filled round pipe with internally rough walls

The attenuation of axisymmetric eigenmodes in a cylindrical, elastic, fluid-filled waveguide with a statistically rough elastic wall is studied. It is shown that small perturbation theory can be used to relate explicitly the statistical characteristics of the internal wall surface roughness of an elastic pipe to the attenuation and scattering coefficients of the acoustic modes in the filling fluid. Analytical expressions for modal attenuation coefficients are obtained. The analysis of the frequency dependent attenuation coefficients and the ratio between the roughness correlation length and the inner radius of the pipe is made for different correlation functions of the roughness. It is shown that two scale parameters control the overall behavior of the modal attenuation coefficients. These are the ratios of the roughness correlation length and the inner pipe radius to the acoustic wavelength. The numerical results for sound propagation in a pipe and in a borehole with statistically rough, elastic walls are obtained and discussed.

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

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

Circumferential and longitudinal defect detection using T(0,1) mode excited by thickness shear mode piezoelectric elements

Different kinds of defects, such as corrosions, notches and cracks etc, exist in pipes. Mode choice is important since unfortunately not all ultrasonic guided wave modes are suitable for these kinds of defect detection. T(0,1) mode which is non-dispersive is the lowest and fastest torsional mode and most suitable for defect detection in pipes. Two completely different artificial defects including longitudinal and circumferential defects are processed successively in a 4-m-long, 60-mm-OD, 3.5-mm-wall steel pipe. T(0,1) mode at 45 kHz is excited to detect these defects using thickness shear mode piezoelectric elements. Experimental results show that two kinds of defects are detectable using T(0,1) mode. Comparing with longitudinal modes, torsional modes are dominant in pipe inspection for their sensitivities to different kinds of defects.