Search

In any imaging modality the apparent size and shape of the object in the image plane is influenced by the camera parameters and experimental conditions. To extract an accurate measurement of object size from the image, the understanding of the influence of these parameters on the apparent size and shape is important. Lock-in Thermography is one of the advanced imaging Non Destructive Evaluation techniques which is widely used for quantitative defect evaluation and material characterization. The important experimental parameter in Lock-in Thermography is the excitation frequency, which greatly affects both the apparent shape (image definition) and size in resultant phase and amplitude images. Hence, in this study, an attempt is made towards understanding the effect of frequency on the image definition (apparent shape) and defect sizing in Lock-in Thermography through experimental study. Stainless Steel plate with flat bottom holes of various sizes and depths is considered for the experiment. Both phase and amplitude images are considered for analysis. The Canny edge detection technique is implemented for studying the effect of frequency on image definition by extracting the edges of the flat bottom hole. The binary image obtained after edge detection is used for sizing calculation. The apparent shape and size were greatly affected by excitation frequency and depth in both amplitude and phase images. A relation between excitation frequency and depth was established which can be used to predict the excitation frequency where the sizing measurement is accurate. [ABSTRACT FROM AUTHOR]

In the present study, two types of receiver coils, viz axial and radial type receivers, in the Remote field eddy current technique are considered to detect defects in ferromagnetic tubes. The appropriate distance between the exciter and the receiver coils and the excitation frequency are estimated using the finite element model and validated experimentally. Model-assisted probability of detection studies are performed to examine the capability of radial and axial type receiver coils. Experimental studies on various artificial defects reveal the excellent performance of the radial type receiver coil in terms of the higher number of defect detections, sensitivity and imaging. This paper provides a comprehensive study on radial and axial type receiver coils for developing array probes. Radial coil performs better and also preserves localisation of defect, which is more desirable for developing array probe. [ABSTRACT FROM AUTHOR]

Detection and criticality assessment of defects appearing in inaccessible locations in pipelines pose a great challenge for many industries. Inspection methods which allow for remote defect detection and accurate characterisation are needed. Guided wave testing (GWT) is capable of screening large lengths of pipes from a single device position, however it provides very limited individual feature characterisation. This paper adapts Plane Wave Imaging (PWI) to pipe GWT to improve defect characterization for inspection in nearby locations such as a few metres from the transducers. PWI performance is evaluated using finite element (FE) and experimental studies, and it is compared to other popular synthetic focusing imaging techniques. The study is concerned with part-circumferential part-depth planar cracks. It is shown that PWI achieves superior resolution compared to the common source method (CSM) and comparable resolution to the total focusing method (TFM). The techniques involving plane wave acquisition (PWI and CSM) are found to substantially outperform methods based on full matrix capture (FMC) in terms of signal-to-noise ratio (SNR). Therefore, it is concluded that PWI which achieves good resolution and high SNR is a more attractive choice for pipe GWT, compared to other considered techniques. Subsequently, a novel PWI transduction setup is proposed, and it is shown to suppresses the transmission of unwanted S0 mode, which further improves SNR of PWI. [ABSTRACT FROM AUTHOR]

The railroad rail support trains and contributes to their operation. Internal and surface defects occur on the rail due to various combinations of causes including fatigue loading and cyclic tension and compression among others from the deterioration of the rail along with the temperature differences of seasonal changes. Surface defects such as head check, shelling, and squats start out in the rail head and become internal defects due to poor maintenance, ultimately resulting in rail failure. In order to prevent rail failure, it is important that defects are identified through nondestructive evaluation (NDE) in advance and to carry out maintenance techniques including grinding. NDE methods include MFL, EMAT, and ECT, and among these, the ECT method is a representative method with excellent detection sensitivity that nondestructively inspects metal surfaces such as rails and pipes using an electromagnetic field. Also, since the defect signal is obtained as an electrical signal, the depth, length, and width of defects can be assessed using a defect evaluation algorithm. This study investigated the field applicability and future practical use of the 16 channel eddy current testing equipment and defect evaluation algorithm developed in this study. Therefore, the field applicability of the equipment and defect evaluation algorithm was investigated through the detection of artificial defects with varying size and depth. Afterwards, future practical use was evaluated by inspection of areas of rail that are in use and with naturally occurring surface defects and analysis of their size (length, width), depth, and phenomena. [ABSTRACT FROM AUTHOR]

• A novel non-linear Log frequency modulated thermal wave imaging modality is introduced. • Experimental results validate the deeper defect detection capability of the proposed modality. • A quantitative analysis has been performed to assess defect detectability and sizing etc. Thermographic analysis for subsurface non-destructive evaluation often encounters challenges in achieving better depth scanning and enhanced depth resolution. Usually, deeper depth analysis will be facilitated by the strength of the low-frequency components and the depth resolution on the frequency sweep of the stimulation, along with material properties in this active thermographic modality. Non-linear frequency modulation became prominent for subsurface analysis due to its ability to provide a band of frequencies with increased energy at low frequencies. This manuscript introduced novel Logarithmic frequency modulated stimulation for subsurface analysis and compared it with its well-established linear frequency counterpart. A quantitative analysis was performed using detectability and sizing of the defects. Experimental results demonstrate that the proposed modality offered an edge over the state of the art, as shown by the experimentation carried out over a carbon fibre-reinforced plastic specimen with embedded flat bottom holes. [ABSTRACT FROM AUTHOR]

Detection and criticality assessment of defects appearing in inaccessible locations in pipelines pose a great challenge for many industries. Inspection methods which allow for remote defect detection and accurate characterisation are needed. Guided wave testing (GWT) is capable of screening large lengths of pipes from a single device position, however it provides very limited individual feature characterisation. This paper adapts Plane Wave Imaging (PWI) to pipe GWT to improve defect characterization for inspection in nearby locations such as a few metres from the transducers. PWI performance is evaluated using finite element (FE) and experimental studies, and it is compared to other popular synthetic focusing imaging techniques. The study is concerned with part-circumferential part-depth planar cracks. It is shown that PWI achieves superior resolution compared to the common source method (CSM) and comparable resolution to the total focusing method (TFM). The techniques involving plane wave acquisition (PWI and CSM) are found to substantially outperform methods based on full matrix capture (FMC) in terms of signal-to-noise ratio (SNR). Therefore, it is concluded that PWI which achieves good resolution and high SNR is a more attractive choice for pipe GWT, compared to other considered techniques. Subsequently, a novel PWI transduction setup is proposed, and it is shown to suppresses the transmission of unwanted S0 mode, which further improves SNR of PWI.

A metal magnetic memory (MMM) testing technique is an extensively used qualitative method for locating the position of possible damages in ferromagnetic metal structures. The mechanism of the MMM technique is described by the magnetomechanical effect under the constant magnetizing field of the Earth. Discontinuous defect remarkably affects MMM signals. For the exploration of the influences of defect on MMM signals and the determination of a method to size the defect, experimentation and reconstruction calculation were performed in this paper. Stress was imported into a specimen fabricated with a circular defect to generate the stress-induced MMM signal. The defect size and the residual magnetic field of the specimen were measured after loading. The stress-induced magnetization inside the specimen was reconstructed with measured magnetic field signals using a reconstruction algorithm. The influences of the discontinuous defect on the stress-induced magnetization were quantitatively analyzed. A method for defect imaging and sizing using the MMM technique was proposed. Results corroborate that the reconstructed defect width is congruent with the actual defect width, thereby confirming the effectiveness and applicability of the proposed method.

The objective of this study is to compare the performance of different ultrasonic non-destructive testing (NDT) techniques for bonding quality evaluation. Aluminium-epoxy-aluminium single lap joints containing debonding in the form of release film inclusions have been investigated using three types of ultrasonic NDT methods: contact testing, immersion testing, and air-coupled testing. Apart from the traditional bulk wave ultrasound, guided wave testing was also performed using air coupled and contact transducers for the excitation of guided waves. Guided wave propagation within adhesive bond was numerically simulated. A wide range of inspection frequencies causing different ultrasonic wavelengths has been investigated. Average errors in defect sizing per ultrasonic wavelength have been used as a feature to determine the performance of each ultrasonic NDT technique. The best performance is observed with bulk wave investigations. Particularly, the higher frequencies (10–50 MHz) in the immersion testing performed significantly better than air-coupled testing (300 kHz); however, air coupled investigations have other advantages as contactless inspection. Whereas guided wave inspections show relatively lower accuracy in defect sizing, they are good enough to detect the presence of the debonding and enable to inspect long range. Even though each technique has its advantages and limitations, guided wave techniques can be practical for the preliminary in-situ inspection of adhesively bonded specimens. [ABSTRACT FROM AUTHOR]

In this paper, we present challenges and achievements in development and use of a compact ultrasonic Phased Array (PA) module with signal processing and imaging technology for autonomous non-destructive evaluation of composite aerospace structures. We analyse two different sets of ultrasonic scan data, acquired from 5 MHz and 10 MHz PA transducers. Although higher frequency transducers promise higher axial (depth) resolution in PA imaging, we face several signal processing challenges to detect defects in composite specimens at 10 MHz. One of the challenges is the presence of multiple echoes at the boundary of the composite layers called structural noise. Here, we propose a wavelet transform-based algorithm that is able to detect and characterize defects (depth, size, and shape in 3D plots). This algorithm uses a smart thresholding technique based on the extracted statistical mean and standard deviation of the structural noise. Finally, we use the proposed algorithm to detect and characterize defects in a standard calibration specimen and validate the results by comparing to the designed depth information. [ABSTRACT FROM AUTHOR]

In this paper, we present challenges and achievements in development and use of a compact ultrasonic Phased Array (PA) module with signal processing and imaging technology for autonomous non-destructive evaluation of composite aerospace structures. We analyse two different sets of ultrasonic scan data, acquired from 5 MHz and 10 MHz PA transducers. Although higher frequency transducers promise higher axial (depth) resolution in PA imaging, we face several signal processing challenges to detect defects in composite specimens at 10 MHz. One of the challenges is the presence of multiple echoes at the boundary of the composite layers called structural noise. Here, we propose a wavelet transform-based algorithm that is able to detect and characterize defects (depth, size, and shape in 3D plots). This algorithm uses a smart thresholding technique based on the extracted statistical mean and standard deviation of the structural noise. Finally, we use the proposed algorithm to detect and characterize defects in a standard calibration specimen and validate the results by comparing to the designed depth information.

Abstract The long pulse thermography is a suitable technique for detection and quantification of defects in low thermal conducting materials, especially for composites. Basalt fiber reinforced epoxy composite is studied for determining defect depths using long pulse thermography. Flat bottom holes with specified depths and sizes are made in the composite for simulating delamination defects. Thermal images of the specimen are studied to obtain thermal contrast and characteristic peak time. A relation between the slope peak time and defect depth by peak derivative method is obtained analytically for long-pulsed thermography. Defect sizing is determined from the full- width- half- maximum approach. Experimental and numerical results are presented and analyzed. [ABSTRACT FROM AUTHOR]

Using ultrasonic arrays to detect and characterise surface breaking cracks is important in the non-destructive evaluation (NDE) field. It can provide early warning of failure and useful information for component integrity assessment. Typically, cracks are approximated by machined slots and used to examine and assess defect detection and characterisation methods. In this paper, real surface breaking cracks are fabricated in 3-point bending specimens following ASTM standard E1820 and used to examine the performance of two array defect characterisation methods: image-based and scattering matrix sizing. In both cases, an array is used to record the full matrix capture (FMC). In image-based sizing, the total focusing method (TFM) is used to form an image from which the defect size is measured directly. This approach is shown to work well for cracks greater than two wavelengths in size. The FMC is also used to extract the defect scattering matrix which is then compared to a pre-computed smooth-crack scattering matrix database. The best match between experiment and this database is found by cross-correlation and used to characterise the defect. This approach is shown to work well for defects in the range of 0.78–1.84 wavelengths. Within these ranges of applicability, both methods show excellent agreement between the known crack length and that measured ultrasonically, with errors less than 19% in all cases. [ABSTRACT FROM AUTHOR]

Pipeline inspection is a topic of particular interest to the companies. Especially important is the defect sizing, which allows them to avoid subsequent costly repairs in their equipment. A solution for this issue is using ultrasonic waves sensed through Electro-Magnetic Acoustic Transducer (EMAT) actuators. The main advantage of this technology is the absence of the need to have direct contact with the surface of the material under investigation, which must be a conductive one. Specifically interesting is the meander-line-coil based Lamb wave generation, since the directivity of the waves allows a study based in the circumferential wrap-around received signal. However, the variety of defect sizes changes the behavior of the signal when it passes through the pipeline. Because of that, it is necessary to apply advanced techniques based on Smart Sound Processing (SSP). These methods involve extracting useful information from the signals sensed with EMAT at different frequencies to obtain nonlinear estimations of the depth of the defect, and to select the features that better estimate the profile of the pipeline. The proposed technique has been tested using both simulated and real signals in steel pipelines, obtaining good results in terms of Root Mean Square Error (RMSE). [ABSTRACT FROM AUTHOR]

Nondestructive evaluation (NDE) techniques of phased array ultrasonic testing (PAUT) and digital X-ray radiography were employed on friction stir (FS)-welded Aluminum Alloy (AA)-2219-T87 specimens. PAUT intricacies required for scanning of FS-welded specimens with a 10-MHz 32-element transducer are discussed. The time corrected gain (TCG) calibration is required for scanning with an increase in index offset to compensate for decrease in A-Scan signal peak amplitude. Calibration techniques to find small defects with appropriate size tolerances are also established. The NDE technique of digital X-ray radiography is compared to PAUT, where it was found that a calibrated PAUT system is able to discover defects less than 0.2 mm where X-ray radiography could not. Incomplete penetration (IP), wormhole (WH), surface cavity (SC), and internal void (IV) defects are analyzed. Furthermore, an online PAUT system for FSW has been developed and successfully tested. The work provided herein will provide a gateway for an ultimate goal of an automated PAUT online sensing system. [ABSTRACT FROM AUTHOR]

Signals collected from the magnetic scans of metal-loss defects have distinct patterns. Experienced pipeline engineers are able to recognize those patterns in magnetic flux leakage (MFL) scans of pipelines, and use them to characterize defect types (e.g., corrosion, cracks, dents, etc.) and estimate their lengths and depths. This task, however, can be highly cumbersome to a human operator, because of the large amount of data to be analyzed. This paper proposes a solution to automate the analysis of MFL signals. The proposed solution uses pattern-adapted wavelets to detect and estimate the length of metal-loss defects. Once the parts of MFL signals corresponding to metal-loss defects are isolated, artificial neural networks are used to predict their depth. The proposed technique is computationally efficient, achieves high levels of accuracy, and works for a wide range of defect shapes. [ABSTRACT FROM AUTHOR]

The paper illustrates the results of an experimental test which intends to check the efficacy of ultrasonic testing (UT) in detecting anomalies inside concrete elements. For this purpose, UT has been carried out on a small concrete wall having different defects deliberately settled inside the wall during casting. A grid of several measurements points has been arranged on the wall surfaces and for each point the ultrasonic signal has been acquired after passing through the thickness of the wall and the propagation velocity V has been extracted and analyzed. A graphic representation of V distribution has been implemented by a map where each pixel identifies one measurement point and is representative of its neighborhood. This map highlights areas with different velocity values, and allows to visually detect areas having particularly low velocity. The matching between the low-velocity areas and the artificial defects has been analyzed, and the level of accuracy of the V map in detecting and sizing the concrete inner defects has been discussed with reference to different spacing of the grid points. Finally, some considerations regarding the choice of the most suitable measurements grid have been addressed.

Pipeline inspection is a topic of particular interest to the companies. Especially important is the defect sizing, which allows them to avoid subsequent costly repairs in their equipment. A solution for this issue is using ultrasonic waves sensed through Electro-Magnetic Acoustic Transducer (EMAT) actuators. The main advantage of this technology is the absence of the need to have direct contact with the surface of the material under investigation, which must be a conductive one. Specifically interesting is the meander-line-coil based Lamb wave generation, since the directivity of the waves allows a study based in the circumferential wrap-around received signal. However, the variety of defect sizes changes the behavior of the signal when it passes through the pipeline. Because of that, it is necessary to apply advanced techniques based on Smart Sound Processing (SSP). These methods involve extracting useful information from the signals sensed with EMAT at different frequencies to obtain nonlinear estimations of the depth of the defect, and to select the features that better estimate the profile of the pipeline. The proposed technique has been tested using both simulated and real signals in steel pipelines, obtaining good results in terms of Root Mean Square Error (RMSE).

Step heating thermography is a non-destructive testing (NDT) technique that inspects interior defects by observing surface temperature rises during a long pulse heat stimulation. Due to the heat diffusion induced blurring of defect shapes, it is a challenging task to accurately determine the sizes of deep and small defects. This work focuses on estimating the size of a subsurface defect by an inversion of temperature images for the step heating thermography, to overcome the information loss during the propagation of thermal waves. We assume a space- and time-dependent virtual heat flux on the defect surface, which is reconstructed from surface temperature measurements by solving a two-dimensional inverse heat conduction problem. The local future time concept is combined with the Tikhonov's regularization to stabilize the inverse solution. A prudent choice of the regularization parameter is carried out through the L-curve. The feasibility of the proposed approach is assessed by numerical simulations and experiments. It is shown that the inversion brings an improvement in defect sizing accuracy for defects with low width-to-depth ratios and an efficient suppression of measurement noises. [ABSTRACT FROM AUTHOR]

We developed a new approach for sizing subsurface defects in the square pulse thermography of metallic plates by employing the oriented gradient of histograms. To size defects with high accuracies is still a challenge in infrared (IR) thermography today. Especially for blurry defects, accurate sizing of them is difficult with existing methods. The oriented gradient of histograms, which is used in the successful probability of boundary (Pb) contour detector in natural image processing literature, is employed in this work to improve the sizing accuracy in square pulse thermography. Experiments on a corroded steel plate with flat blind holes have verified the effectiveness of the proposed approach to size defects. Experimental results show that the proposed approach can size distinct and blurry defects with high accuracies. Comparison research is also implemented between the proposed approach and other sizing methods. The comparison results show that the proposed approach is superior to existing methods. [ABSTRACT FROM AUTHOR]

• The Total focusing method (TFM) image-based approach presented for sizing non-sharp defects. • Parametric numerical and experimental full matrix capture datasets generated for imaging. • Measurement limitation and accuracy of notch parameters presented. • Impact of sizing inaccuracy on structural integrity assessment described. • Demonstrated the ability to distinguish between non-sharp defects. Existing structural integrity assessment procedures typically assume flaws to be infinitely sharp when they cannot be considered as local thinned areas. This assumption is often over-conservative, resulting in a pessimistic assessment of structural components and a significant underestimation of their margins of safety against fracture. One of the main challenges while adopting non-destructive evaluation techniques is distinguishing between sharp cracks (e.g., fatigue) and non-sharp defects and identifying the more severe ones. Towards this broader challenge, the present work aims to examine the sizing limitation and accuracy of ultrasonic array image-based techniques for non-sharp defects (surface breaking u-notches) and investigate how these measurements would affect the structural integrity assessment of components. Parametric numerical simulations and experimental measurements are performed to generate full-matrix capture datasets, which are then processed using the total focusing method to form an image. The image-based sizing approach is shown to perform efficiently for notch depths higher than the inspection wavelength (λ L), i.e. as small as 0.2 mm, and semi-notch widths as small as 0.1 mm. The influence of ultrasonic measurements on structural integrity assessments is highlighted using different case studies in the context of non-sharp defects of fatigue and fracture strength estimates. For the cases under analysis, resolving non-sharp defects led up to 5× and 3× values of effective fracture toughness and fatigue strength, respectively. We have also seen that a 30 % uncertainty in semi-notch width sizing would result in a 30 % and 20 % error in fatigue strength and fracture toughness estimations, respectively. [ABSTRACT FROM AUTHOR]

In this paper, SH (shear horizontal) guided waves propagating in the circumferential direction of pipeline are modeled in 3 dimensions, with the aim for axial cracking detection implementation in ILI (in-line inspection) tools in mind. A theoretical formulation is given first, followed by an explanation about the 3D numerical modeling work. Displacement wave structures from the simulation and dispersion equation are compared to verify the effectiveness of the FEM package. Transverse slots along the axial direction are modeled to simulate axial cracking. Reflection and transmission coefficients curves are obtained to provide insight in using circumferential SH guided waves for quantitative testing of axial pipeline cracking. [ABSTRACT FROM AUTHOR]

Objectives: This study sought to determine whether transthoracic echocardiography (TTE) can provide safety and efficacy equivalent to transesophageal echocardiography (TEE) for assessment and guidance of transcatheter atrial septal defect (ASD) closure in pediatric patients. Methods: We performed a prospective randomized trial of ASD closure using the Amplatzer septal occluder (ASO) from March 2008 to April 2012. Inclusion criteria were isolated secundum ASD, age 2 to 18 years, and adequate TTE windows. Forty patients were enrolled and randomized to either TEE or TTE. In the TEE group, we used “stop flow” balloon sizing to determine device size. In the TTE group, we used the average ASD diameter times 1.2 (scaled). Patients were followed up to 1 year. Results: Patient general and hemodynamic characteristics were similar in both groups. Procedural success was 100% in both groups. The average TEE stop flow diameter was similar to the scaled TTE diameter (15.35 ± 4.62 mm vs. 16.57 ± 5.47 mm; p = 0.46). Device size (16.0 ± 4.94 mm vs. 16.37 ± 5.05 mm, p = 0.82) and ratio of device to defect size (1.0 ± 0.06 vs. 0.99 ± 0.03, p = 0.52) were also similar. Total procedure (70.6 ± 22.98 min vs. 51.1 ± 17.61 min, p = 0.005), room (126.8 ± 28.41 min vs. 95.7 ± 20.53 min, p = 0.0004), and fluoroscopy (13.6 ± 6.17 min vs. 8.9 ± 8.45 min, p = 0.007) times were all significantly shorter in the TTE group. Neither group had significant complications during the procedure nor in follow-up. Rates of shunt resolution were similar between groups. Conclusions: This study suggests that in selected pediatric patients, use of TTE is as efficacious and safe as TEE for assessment and guidance of ASD occlusion using the ASO. TTE also may offer the additional safety benefit of reduced fluoroscopy exposure. [Copyright &y& Elsevier]

The study aims at investigating the imaging and sizing capabilities of ultrasonic phased arrays and the synthetic aperture focusing technique (SAFT) of surface-breaking defects in a carbon steel plate. The defects were imaged using a phased array transducer and the relative arrival time technique (RATT) was used to size the defects. Studies were carried out to size near-through-wall defects so as to estimate the largest size of defect that can be accurately sized in accordance with the leak before break (LBB) criterion in critical applications. Additionally, the study also aims at sizing the smallest defect that can be sized accurately using the two techniques. The study also shows a direction for improved sizing accuracy of certain defects by means of the hybrid-based genetic algorithm (GA) approach which otherwise was limited by the two imaging techniques. [ABSTRACT FROM AUTHOR]

Ultrasonic guided waves have been used for the rapid screening of pipes and pipelines for corrosion over the past 10 years. The hardware used in guided wave testing has improved since the early equipment, and now has up to 24 transmit-receive channels. This added capability has facilitated step changes in the way these tests are conducted, and has led to the introduction of better phased array focusing of guided waves and greatly enhanced post-processing techniques. The focusing technique provides increased sensitivity to defects and information about their circumferential position and extent. However, it relies on defects being detected in the initial screening test to identify the locations at which to apply the focused excitation. This paper presents an enhanced signal display technique that presents the operator with more information from the initial screening test, helping to prevent false negatives and to choose effective focal positions. A defect characterisation method is then presented that convolves information from this focused inspection with the information from the initial screening test to give defects a severity rating. Defect severity and position can be assessed with a higher degree of confidence than was previously achievable. The process has been tested under field conditions and has been shown to give good results. [ABSTRACT FROM AUTHOR]

In this article a method to evaluate defect dimensions in adhesively bonded lap joints based on the measurement of scattering effects of ultrasonic guided waves is presented. A simplified theoretical model is proposed which was initially tested in plates with through holes. The experimental results obtained using both pitch-and-catch and pulse-echo techniques for 500 kHz and 1 MHz frequencies confirm the validity of this model. To evaluate the lap joint defects, a set of samples with artificial defects were manufactured and the form and dimensions were confirmed using C-scan ultrasonic images. With the same methodology used in through-hole analysis, scattering effects of defects were measured. The results obtained with the pitch-and-catch technique with 1 MHz transducers allow us to say that an estimate of defect dimensions could be done by using the proposed model with reasonable accuracy and according with the predictions. [ABSTRACT FROM AUTHOR]

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