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This paper concerns the inspection of steel plates, with particular emphasis on the assessment of increasing damage. Non-destructive tests were performed on four plates, one of which was undamaged, while the remaining three had defects in the form of circular holes with diameters of 2, 5 and 10 cm. Guided Lamb waves were used in the research, and the image reconstruction was performed using ultrasound computed tomography. The damage size was estimated by tracking the real course of rays and densifying the pixel grid into which the object was divided. The results showed the great potential of ultrasound tomography in detecting defects in steel elements, together with the possibility of estimating damage size.

A non-destructive testing system requires multiple system configuration parameters during the operation process. For a given transducer, scanning frequency and number of measurements for averaging are just among those parameters. This work tests the central limit theorem to optimally set these parameters. The authors have designed a compact ultrasound computer tomography scanner from scratch just to test this criterion. It is shown that optimal frequency value changes with respect to the scanning angle for an object with a heterogeneous inner profile. The proposed criterion helps to estimate the multi-spectral combination before the recovery stage minimum error. Possible industrial applications of such a system are 2D/3D multiphase flow profile recovery or crack detection in the composite matter, non-invasively.

Non-destructive testing of concrete for defects detection, using acoustic techniques, is currently performed mainly by human inspection of recorded images. The images consist of the inside of the examined elements obtained from testing devices such as the ultrasonic tomograph. However, such an automatic inspection is time-consuming, expensive, and prone to errors. To address some of these problems, this paper aims to evaluate a convolutional neural network (CNN) toward an automated detection of flaws in concrete elements using ultrasonic tomography. There are two main stages in the proposed methodology. In the first stage, an image of the inside of the examined structure is obtained and recorded by performing ultrasonic tomography-based testing. In the second stage, a convolutional neural network model is used for automatic detection of defects and flaws in the recorded image. In this work, a large and pre-trained CNN is used. It was fine-tuned on a small set of images collected during laboratory tests. Lastly, the prepared model was applied for detecting flaws. The obtained model has proven to be able to accurately detect defects in examined concrete elements. The presented approach for automatic detection of flaws is being developed with the potential to not only detect defects of one type but also to classify various types of defects in concrete elements.

Assessing internal decay in tree trunks can be of crucial importance for industrial, environmental and public safety reasons [1]. To this effect, non-destructive testing (NDT) methods can provide information on the structural condition of trees with minimum intrusion. In this work, authors have analysed the capabilities of ultrasonic tomography in evaluating the internal structure of living trees, with a special focus on the identification of internal decay areas and tree bark inclusions.The presented ultrasonic tomography provides an image of the distribution of the ultrasonic velocity of propagation within the investigated section of a mature horse chestnut (Aesculus hippocastanum). This technique has proven its viability to detect fungal decomposition [2]. However, there exist some open issues with regard to: a) the coupling of the transducers to the tree, b) the anisotropy of the wood, c) the signal attenuation and the resolution of the tomographic inversion. To overcome these challenges, research is underway to explore the integration and new data-fusion strategies with other NDT methods, such as ground penetrating radar (GPR), which have proven their effectiveness within this area of endeavour [3].Within this context, data have been obtained from a “diseased” horse chestnut tree located at the Kensington Gardens – The Royal Parks – in London, UK, using two different ultrasonic equipment, i.e., the PICUS Sonic Tomograph and the Arbotom Sonic Tomograph. After compilation of data, the tree was felled and cut at the two sections where ultrasonic tomography tests were performed. In more detail, 12 sensors were arranged around the perimeter of the tree in compliance with the manufacturer’s recommendations concerning the inspection methodology (sensors installed within the bark of the tree without any intrusion to the core of the tree). The adopted methodology takes to account the shape and size of the trunk [1]. The processed data were mapped against the cut sections of the tree for validity purposes. Results presented in this abstract are part of a major ongoing research project that the authors have undertaken for the last three years. AcknowledgementsThe authors would like to express their sincere thanks and gratitude to the following trusts, charities, organisations and individuals for their generosity in supporting this project: Lord Faringdon Charitable Trust,The Schroder Foundation,Cazenove Charitable Trust,Ernest Cook Trust,Sir Henry Keswick,Ian Bond, P.F. Charitable Trust,Prospect Investment Management Limited,The Adrian Swire Charitable Trust,The John Swire 1989 Charitable Trust,The Sackler Trust,The Tanlaw Foundation, and The Wyfold Charitable Trust. We would like to thank also Ian Rodger – Royal Parks Arboricultural Manager– for providing us with the tested tree. This paper is dedicated to the memory of our colleague and friend Jonathan West, one of the original supporters of this research project. References[1] Gilbert, G.S. et al. (2016). Use of sonic tomography to detect and quantify wood decay in living trees, Applications in Plant Sciences 4(12): 1600060.[2] Bucur, V. Acoustics of Wood. CRC Press Inc., Boca Raton,Argentina (1995).[3] Alani, A.M. et al. (2019). The Use of Ground Penetrating Radar and Microwave Tomography for the Detection of Decay and Cavities in Tree Trunks. Remote Sensing 11: 2073.

The accuracy of transmission ultrasonic tomography for the detection of brittle damage in concrete beams can be effectively supported by the graph theory and, in particular, by Dijkstra’s algorithm. It allows determining real paths of the fastest ultrasonic wave propagation in concrete containing localized elastically degraded zones at any stage of their evolution. This work confronts this type of approach with results that can be obtained from non-local isotropic damage mechanics. On this basis, the authors developed a method of reducing errors in tomographic reconstruction of longitudinal wave velocity maps which are caused by using the simplifying assumptions of straightness of the fastest wave propagation paths. The method is based on the appropriate elongation of measured propagation times of the wave transmitted between opposite sending-receiving transducers if the actual propagation paths deviate from straight lines. Thanks to this, the mathematical apparatus used typically in the tomography, in which the straightness of the fastest paths is assumed, can be still used. The work considers also the aspect of using fictitious wave sending-receiving points in ultrasonic tomography for which wave propagation times are calculated by interpolation of measured ones. The considerations are supported by experimental research conducted on laboratory reinforced concrete (RC) beams in the test of three-point bending and a prefabricated damaged RC beam.

Due to the nature of their use, concrete structures are typically subjected to various aggressive actions derived from the loads and the environmental agents, which make the concrete condition change over time. Constant monitoring of structures is a necessary procedure since the preventive detection of problems allows the use of more straightforward and cost-effective methods for evaluation and repair. In order to carry out continuous monitoring of the state condition of concrete elements, there are many tools available. Nondestructive Testing (NDT) applications are a new strategy to monitor the state of concrete structures continually. Recent advancements in NDT technologies are increasingly turning them into attractive and viable strategies to support this type of monitoring and visual inspection. In particular, besides been viable and attractive, ultrasonic tests are a relatively cheap alternative and represent a range of different methods, each one with its benefits, to analyze concrete structures. The Research Group of LEME-UFRGS has developed research and application of NDT methods in general, and ultrasonic based methods in particular, for the last 15 years. The research done by the group demonstrates that these methods can contribute to monitoring deterioration and the quality of concrete structures. The present study demonstrates particular cases involving ultrasonic pulse testing and ultrasonic tomography. The aim was to check for these technique improvements for the analysis of concrete elements. The results highlight the potentialities and shortcomings of each technique.

X-ray tomography and magnetic resonance imaging (MRI) are excellent techniques for non-destructive or non-invasive inspections, however, they have shotcomings including the expensive cost in both the devices themselves and their protection facilities, the harmful side effects of the X-rays to human bodies and to the environment. In view of this argument, it is necessary to develop new, inexpensive, safe and reliable tomographic techniques, especially in medical imaging and non-destructive inspections. There are new tomographic techniques under development such as optical tomography, photo-acoustic tomography, ultrasonic tomography and so on, from which we take ultrasonic tomography as the topic in this paper. We introduce a review of the known ultrasonic tomographic techniques and discuss their future development.

Non-destructive tests play a crucial role for rehabilitation, restoration and strengthening of historical masonry constructions: this justifies increasing research interests on this issue. In particular, sonic and ultrasonic tomography are among the most used NDT techniques for applications to historical constructions since they are relatively fast, reliable and inexpensive. Anyway, the resolution and the capability of those techniques in assessing hidden defects, voids, damage, the masonry layout and the mechanical properties of masonry can be further substantially improved. To this aim, several issues about the experimental setup, the modeling of propagation phenomena of acoustic waves, and the inversion algorithms should be addressed. In this paper some improvements of the ultrasonic tomography for masonry constructions are proposed, based on the implementation of advanced inversion algorithms, on the use of the information about wave attenuation (attenuation tomography) in addition to those related to the time of flight of acoustic waves, and on the enhancement of the experimental setup, in particular for what concerns the coupling between probes and masonry. With reference to tuff samples having known internal defects like holes, cuts and inclusions, the capability of the proposed approach is compared with the standard ultrasonic tomography. The experimental results show the effectiveness of the proposed approach, and suggest some directions for further improvements.

In this paper, a condition assessment of masonry pillars is presented. Non-destructive tests were performed on an intact pillar as well as three pillars with internal inclusions in the form of a hole, a steel bar grouted by gypsum mortar, and a steel bar grouted by cement mortar. The inspection utilized ultrasonic stress waves and the reconstruction of the velocity distribution was performed by means of computed tomography. The results showed the possibilities of tomographic imaging in characterizing the internal structure of pillars. Particular attention was paid to the assessment of the adhesive connection between a steel reinforcing bar, embedded inside pillars, and the surrounding pillar body.

The authors are trying to develop an ultrasonic CT technique for concrete structures. As the first step of its development, we study how the ultrasonic waves propagate in concrete structures and pose a problem to develop ultrasonic CT, applying which we give a non-destructive inspection technique for concrete cover by application of the idea of the ultrasonic CT. The authors claim that this study gives the foundation of the theory of ultrasonic CT for concrete structures.

An investigation of tunnel linings is performed at two tunnels in the US using complimentary noncontact techniques: air-coupled ground penetrating radar (GPR), and a vehicle-mounted scanning system (SPACETEC) that combines laser, visual, and infrared thermography scanning methods. This paper shows that a combination of such techniques can maximize inspection coverage in a comprehensive and efficient manner. Since ground-truth is typically not available in public tunnel field evaluations, the noncontact techniques used are compared with two reliable in-depth contact nondestructive testing methods: ground-coupled GPR and ultrasonic tomography. The noncontact techniques are used to identify and locate the reinforcement mesh, structural steel ribs, internal layer interfaces, shallow delamination, and tile debonding. It is shown that this combination of methods can be used synergistically to provide tunnel owners with a comprehensive and efficient approach for monitoring tunnel lining conditions.

A technology for ultrasonic tomography aimed at the detection of flawed zones in unilaterally accessible concrete structures and the determination of their thickness and coordinates was presented. The construction of tomograms was described step by step. The results obtained by the current and previous tomograph variants were given. The tomograms of concrete specimens with specified model flaws and without them were obtained. Results with a lower noise level were obtained using the current tomograph variant. The need for the processing of a set of signals for the diagnostics and monitoring of concrete objects was shown.

Ultrasonic tomography can be an important tool in the nondestructive evaluation of structural elements of wood and trees. Although wood is an anisotropic material, it is common to use an isotropic formulation in the tomographic processing. This paper presents a method that considers wood anisotropy in ultrasonic tomography. Christoffel and Hankinson equations were used to represent the orthotropic behavior of wood. In addition, the growth rings shape was also considered. The proposed method was validated through samples of wood with simulated non-homogeneities. The resulting tomograms were able to identify mortises when the specimen anisotropy was considered.

Tunnel inspection is a challenging problem because of high-volume traffic and routine operations in naturally aggressive environments. The need to keep tunnels open during inspection and minimize tunnel closures and user delays must be carefully balanced with the need to conduct in-depth lining inspections to ensure the safety of drivers. This paper describes the laboratory validation and field performance of a recently developed in-depth nondestructive testing technology for the detection of impairments in tunnel lining: linear array shear wave tomography, typically referred to as ultrasonic tomography (UST). Before this equipment is used in the field, the system is first evaluated through the use of laboratory specimens with artificial defects that mock common structural problems, such as air- and water-filled voids, delaminations, and other potential abnormalities. The device is also used to determine concrete thickness and reinforcement depth and spacing. The test results are discussed to determine the device's capabilities and limitations in locating defects in concrete structures. After the system is evaluated on the basis of its ability to detect these simulated defects, the system is taken to the field to inspect a public tunnel for natural structural defects. Potential regions of interest are first identified through high-speed air-coupled ground-penetrating radar (GPR) and visual inspection and subsequently inspected with UST. This paper shows that the combination of preliminary inspection procedures (GPR surveys and visual inspection) and an in-depth technique like UST is powerful for the assessment of the condition of tunnel linings and can detect potential anomalies, such as delamination, reinforcement depth and layout, and lining thickness.

In the field of non-destructive evaluation of structures, 2D and 3D imaging of internal flaws is a critical task. Defect imaging allows making informed follow-up decisions based on the morphology of the flaw. This paper will present advances in ultrasonic tomography for the 2D and 3D visualization of internal flaws in solids. In particular, improvements to the conventional tomographic imaging algorithms have been made by utilizing a mode-selective image reconstruction scheme that exploits the specific displacement field, respectively, of the longitudinal wave modes and the shear wave modes, both propagating simultaneously in the test volume. The specific mode structure is exploited by an adaptive weight assignment to the ultrasonic tomographic array. Such adaptive weighting forces the imaging array to look at a specific scan direction and better focus the imaging onto the actual flaw (ultrasound reflector). Moreover, the introduction of a global matched coefficient, computed through the matching of measured and expected times of flight for each pixel, is illustrated. The benefits deriving from the application of this coefficient to conventional imaging frameworks are shown. This study shows that the adaptive weighing based on wave structure and the integration of the global matched coefficient improve image contrast and resolution compared to a conventional ultrasonic imaging technique based on a delay-and-sum or minimum variance distortionless method. Results will be shown from experimental tests of simulated flaws in solids.

Ultrasonic tomography is a non-destructive method that enables the mapping of an internal section of a structure from multiple projections for the identification of discontinuities and/or damages. This study addresses the application of the ultrasonic tomography method for the qualitative assessment of concrete. Several experimental tests with variations in concrete composition, presence of voids, arrangement and frequency of transducers were conducted on concrete prisms. The results show the ultrasonic tomography was more efficient than conventional ultrasonic tests. Analyses of several transducers arrangements revealed a large number of measurements not necessarily returns better images. The great potential of the ultrasonic tomography technique for the evaluation of homogeneity and detection of discontinuities and damages of concrete structures has been confirmed.

A series of standard concrete beams with plastic P-T tendon ducts were tested at the University of Edinburgh using a range of NDT techniques including impact–echo (I–E), stack imaging of spectral amplitudes based on the impact–echo (SIBIE) and ultrasonic tomography. The findings were confirmed by comparison with earlier experiments undertaken using GPR. It has been argued that I–E cannot detect voids in plastic post-tensioned ducts readily. However it has been shown that whilst I–E is less effective on plastic ducts, the SIBIE analysis is more certain. This technique is easier to use than ultrasonic tomography and gives a credible evaluation. The paper goes on to analyse the above tests on a number of beams with confirmation for ultrasonic tomography and GPR.

Conventional X-ray and ultrasonic NDT methods are very popular in Indian industry and their scope is growing at a steady pace. Tomographic extensions of these methods, however, have not made a mark yet. There is considerable activity in the country to develop CT scanners, X-ray as well as ultrasonic, for a variety of NDT applications.

Some concern exists over the safety and durability of the 600 post-tensioned bridges in the UK, and the much larger number worldwide. The objective of the work reported herein was to identify voiding in the metallic tendon ducts in these bridges. Voiding can give rise to two sets of problems: (a) possible ingress of chlorides, which would cause corrosion; and (b) a lack of redistribution of stress within the beam. It was against this background that it was important to first of all identify the extent of voiding in post-tensioned bridges. The new technique of ultrasonic tomography was used for the trials reported in this paper. Two test beams were examined: a 10 m long beam at the Transport Research Laboratory (TRL), Crowthorne, UK and a short test beam constructed at Stanger Science and Environment, Elstree, UK. The ducts in the TRL beam were 40 mm in diameter. This is smaller than would normally be encountered in a post-tensioned bridge beam. A more usual duct diameter would be 100–110 mm with a cover of around 125 mm. The second test beam at Stanger Science and Environment, Elstree contained 100-mm diameter ducts. The time-of-flight tomography data obtained demonstrated that it is a potentially highly successful method of investigating post-tensioned concrete beams. The method is somewhat time consuming and so should be used in conjunction with a simpler testing method, e.g. sonic impact-echo, which identifies areas of interest. The smaller the ducts to be investigated, the smaller the required distances between testing stations. This therefore significantly increases the testing time.

In order to improve ultrasonic inspection, in terms of detection, sizing and characterization of defects, the French Atomic Energy Commission (CEA) has developed a dynamic adaptive focusing system called FAUST: Focusing Adaptive UltraSonic Tomography. This sytem is based on ultrasonic phased array transducers connected to a multi-channel acquisition system, which provides the delay and amplitude laws, allowing one to control dynamically the ultrasonic beam. This paper presents the general architecture of the system and some applications of it for defect characterization and improvement of the ultrasonic inspections.

The paper considers the problems of ultrasonic nondestructive testing of products intended for mechanical engineering. The functional and electronic circuits of an ultrasonic tomograph are presented. The function of signal radiation from the clocked multielement apparatus is described, the cross-functional flowchart of the prototype of a US tomograph is considered. The development trends of ultrasonic tomography for near-term outlook are demonstrated.

During the past two decades there have been significant developments in many aspects of acoustical imaging techniques for various applications. One aspect of this development has been the application of ultrasonic tomography to medical imaging and diagnosis instrumentation, and also to non-destructive evaluation of materials, to oceanographic systems for measurements of such factors as temperature and currents etc, and to seismic exploration. This paper reviews the theoretical and practical work in acoustical tomography to the present time.

Ultrasonic tomography is a powerful tool for identifying defects within an object or structure. Ultrasonic tomography is limited by time consuming transducer coupling. Air‐coupled UPV may eliminate this problem and allow for more rapid data collection. This research aims to integrate recent developments in air‐coupled ultrasonic measurements with the current tomography technology to image inclusions within solids. Air‐coupled ultrasonic signals are collected though large PVC samples. Volumetric and planar inclusions within the samples are identified in the constructed velocity tomographs.

To obtain the high performances requested by the safety authorities for ultrasonic inspections, in term of sizing, characterization and on-line monitoring, it is needed to vary the refracted angles, focusing depths and the size of the ultrasonic beams rapidly and over a wide range. This is not possible with conventional techniques which allow only fixed focusing. The F.A.U.S.T. (Focusing Adaptive UltraSonic Tomography) system, which is described in this paper, is an open and powerful tool designed to overcome the difficult problems in flaw assessment and monitoring.

Ultrasonic tomography is an emerging method of nondestructive concrete pavement diagnostics which can be used for improved quality assurance/quality control during concrete pavement construction and assist in rehabilitation decision making. Detection of flaws using ultrasonic tomography requires significant effort and user expertise. To address these limitations, a quantitative method for determining the presence of defects in concrete pavements was developed. The proposed method is an adaptation of the recently developed impact-echo signature analysis method (IESA), which is used for comparison of impact-echo signals. The proposed two-dimensional ultrasonic tomography signature analysis (2D-UTSA) method was used to compare two-dimensional B-scans obtained using a commercial test system in field trials at the Minnesota Rd. Research Facility and the Federal Aviation Administration’s National Airport Pavement Test Facility. Analysis of the results showed that the 2D-UTSA method is capable of identifying subsurface defects.

Ultrasonic inspection of manufactured parts has been widely used in industry for over 30 years (1, 2). Conventional methods, however, are largely limited to determining the presence or absence of defective regions in these parts without characterizing the detailed nature of the defect. As manufacturing methods have progressed and new, high-strength, high­ performance alloys have found widespread use, more detailed information about material parameters is required to assess the failure probability of parts in critical applications. One of the most promising new techniques that has been developed in recent years to deal with this need for quantitative materials information is ultrasonic tomography. This article reviews the current status of ultrasonic tomography in the context of the broader area of ultrasonic nondestructive evaluation (NDE). Tn particular, the role of ultrasonic tomography as one part of a large arsenal of NDE methods is examined in order to determine the area of application in which ultrasonic tomography can make the greatest contribution. Based on an assessment of available results (which are relatively few owing to the earJy stage of development of this field), some likely extensions of current techniques are suggested. Application of ultrasonic tomography to medical imaging, which is much more extensive because of its greater similarity to x-ray computerized tomography (CT), is not discussed here except as it directly impacts the NDE field.

Both laboratory and field ultrasonic measurements were carried out to assess the feasibility of ultrasonic tomography for non-invasive testing of living trees. Trees with internal decay were investigated and later cut down. The laboratory measurements, performed on wood disks and specimens, led to the quantitative assessment of the reliability of the tomographic result. Statistical analysis of data allowed considerations to be made on the main aspects connected to the tomographic imaging, such as uncertainty, spatial resolution and anisotropy. The obtained results show that the tomographic image of the ultrasonic velocity distribution is in good agreement with the distribution of the wood decay in the investigated section and the proposed methodology therefore seems to be an improvement on currently used surveying methods.