Improved synthetic aperture focusing technique by Hilbert-Huang transform for imaging defects inside a concrete structure

A useful nondestructive testing tool for civil engineering should immediately reveal defects inside concrete structures at the construction sites. To date, there are few effective methods to image defects inside concrete structures. In this paper, a new nondestructive testing method using elastic waves for imaging possible defects inside concrete is developed. This method integrates the point-source/point receiver scheme with the synthetic aperture focusing technique (SAFT) to increase functioning depth and enhance received signals. To improve image quality, received signals are processed by Hilbert-Huang transform (HHT) to get time-frequency curves for the SAFT process. Compared with conventional SAFT method processing with time-amplitude signals, this new method is capable of providing a better image of defects not only in the numerical simulation but also in the experimental result. The image can reveal the number of defects and their locations and front-end profiles. The results shown in this paper indicate that this new elastic-wave-based method exhibits high capability in imaging the defects of in situ concrete structures.     

A new elastic-wave-based imaging method for scanning the defects inside the structure

In this paper, a new nondestructive testing method using elastic waves for imaging possible voids or defects in concrete structures is proposed. This method integrates the point-source/point receiver scheme with the synthetic aperture focusing technique (SAFT) process to achieve the effect like scanning with a phase array system. This method also is equipped with large functioning depth because of the high-energy feature that elastic waves usually possess over traditional ultrasound. Both numerical simulations and experimental tests were carried out to explore the capabilities of this method in revealing single or multiple defects implied in a matrix material. The results from numerical simulations indicate that this method can clearly reveal the number of the voids or defects, their locations, and front-end profiles. The influence of the accuracy of the wave velocity determination on the resultant image also was evaluated in this study. Furthermore, the effects of the types of the responses to be recorded and the wavelength of the introduced waves also were evaluated so that very good resultant images may be obtained. Both the results from the numerical simulations and the experimental tests indicate that this elastic-wave-based method exhibits high potential in inspecting the defects of in-situ concrete structures by imaging.     

Ultrasonic Synthetic Aperture Focusing Using the Root-Mean-Square Velocity

The synthetic aperture focusing technique (SAFT) can improve the image resolution of ultrasonic testing (UT) by applying a delay-and-sum (DAS) process to the received echoes. The drawback of using current SAFT to test a multi-layer medium is that the computation of delays is complicated and time-consuming. In this study, we propose a fast and simple method to calculate the approximated delays for using in SAFT to image flaws in multi-layer media without losing the resolution of the SAFT image. The approximated delays can be easily and quickly calculated by a hyperbolic time-distance relationship which is a function of the root-mean-square velocity (V _rms ) of layers under circumstances of short lateral distance and horizontal layers. The error of approximated delays is very small when incorporating the amplitudes radiated from the transducer into the processing of SAFT. Two extreme experiments of immersion testing were carried out to test and evaluate the proposed method. The experimental results show that the proposed method is feasible. Thus, the approximated delays calculated by the V _rms for using in SAFT to enhance the resolution of the scan image for testing the flaws in multi-layer medium are recommended.     

Progress in ultrasonic imaging of concrete

Among present non-destructive methods for concrete evaluation, ultrasonic testing uses relatively short wavelengths and therefore has particular potential for detailed assessment of concrete. Methods like SAFT (Synthetic Aperture Focusing Technique) and tomographic reconstruction are able to provide high-resolution images of concrete areas, which can be employed for tasks such as area imaging, duct localization, fault detection, and thickness measurement. This contribution is intended to give insight into some of the principles and possibilities of ultrasonic concrete imaging using SAFT and tomographic reconstruction. It thereby review progress that has been achieved at the author's institute during the last years. For SAFT reconstruction, the processing steps are explained that are necessary to obtain an image that is easy to interpret including the influence of transducers, their coupling, and image noise suppression. Quantitative evaluation of ultrasonic images enables the examination of tendon ducts for voids and the objective assessment of image quality. A field example demonstrates the possibilities of SAFT reconstruction. In a separate section, ultrasonic tomography is shown to have the capability to detect faults such as honeycombing in concrete pillars. Finally, the potential of ultrasonic imaging and remaining steps, necessary to open broad practical application are described.     

A New Nomogram Proposal to Determine Concrete Compressive Strength by Combined Nondestructive Testing Methods

Concrete is the most widely used construction material in contemporary construction technology. In particular, it holds an important place in the building industry in the world. Since concrete is a nonhomogeneous material, determining the strength of concrete accurately is quite difficult. In this respect, to check the mechanical properties of concrete in-situ, nondestructive test (NDT) methods can be used. They are useful as they do not damage concrete; however, these test results are sometimes deceptive. To reduce these deceptive results, destructive test methods were proposed to increase the accuracy of nondestructive methods.

The objective of this study is to determine the strength of concrete with different characteristic strengths using destructive and NDT methods, and to establish new relationships between the compressive strength of concrete and Schmidt rebound values, and ultrasonic wave velocities. For this purpose, 101 concrete cube samples were prepared. After 7 and 28 days of curing, Schmidt rebound and ultrasonic wave velocity tests were applied as NDT methods. New formulations and graphs were established by carrying out a multiple regression analysis between the ultrasonic wave velocities, Schmidt rebound values, and compressive strengths. Thus, a new combined NDT method was developed with a nomogram. Furthermore, the applicability and accuracy of the formula and graph obtained were investigated by comparing the results of core samples from existing structures.

The most important findings obtained from the study are summarized below. The concrete strength can be determined with nondestructive formulas obtained with an accuracy of 85%. High strength concrete cube samples are obtained in-house with 1.6% deviation, and core samples in-situ can be obtained with 7.3% deviation with the newly developed NDT nomogram. These test results show that the newly developed NDT formulas in this study are a very good alternative for determining concrete strength in-situ.     

层状无砟轨道结构脱层缺陷超声成像

无砟轨道是典型的层状混凝土结构,脱层缺陷是其最常见的损伤形式,影响着高速列车的安全运行。传统的合成孔径聚焦成像方法是基于恒定声速的超声检测方法,忽略层间的声阻抗差异以及声波在层间界面处的折射,导致声束难以在缺陷处聚焦,声波在层状结构中传播的时间误差较大。鉴于此,提出了一种多层结构合成孔径聚焦成像方法,充分考虑多层结构中的层间声速差异,采用射线追踪方法准确获取声波在多层结构中的传播时间。在此基础上,分析了不同入射波模式以及不同激发频率对多层结构合成孔径聚焦成像结果的影响。结果表明：采用多层结构合成孔径聚焦成像方法,使用频率为50 kHz的横波入射成像分辨率更高,对无砟轨道脱层缺陷检测效果更好。该研究为该类缺陷检测提供了理论支撑。     

Time of flight diffraction imaging for double-probe technique

Due to rapid progress in microelectronics and computer technologies, the system evolving from analog to digital, and a programmable and flexible synthetic aperture focusing technique (SAFT) for the single-probe pulse-echo imaging technique of ultrasonic nondestructive testing (NDT) becomes feasible. The double-probe reflection technique usually is used to detect the nonhorizontal flaws in the ultrasonic NDT. Because there is an offset between the transmitter and receiver, the position and size of the flaw cannot be directly read from the image. Therefore, a digital signal processing (DSP) imaging method is proposed to process the ultrasonic image obtained by double-probe reflection technique. In the imaging, the signal is redistributed on an ellipsoid with the transmitter and receiver positions as focuses, and the traveltime sum for the echo from the ellipsoid to the focuses as the traveltime of signal. After redistributing all the signals, the useful signals can be constructively added in some point in which the reflected point is; otherwise, the signals will be destructively added. Therefore, the image resolution of the flaw can be improved and the position and size of the flaw can be estimated directly from the processed image. Based on the experimental results, the steep flaw (45°) cannot be detected by the pulse echo technique but can be detected by the double-probe method, and the double-probe B-scan image of 30° tilted crack is clearer than the pulse echo B-scan image     

Detecting Low Velocity Impact Damage in Composite Plate Using Nonlinear Acoustic/Ultrasound Methods

The objective of this work was to image the presence of impact damage by monitoring the nonlinear response of damaged carbon/epoxy composite samples. The presence of microcracks, debonding, delamination, etc… induce the material to behave in a nonlinear elastic fashion highlighted by the presence and amplitude of harmonics in the spectrum of the received signal when the sample is periodically excited at one of its resonance frequencies. The sensitivity of a second harmonic imaging technique (SEHIT) based on material nonlinear elastic effect known as second harmonic generation (SHG) was investigated. The proposed imaging process was used to detect barely visible impact damage (BVID) due to low velocity impact (<12 J). The results showed that the SEHIT methods appear to be highly accurate in assessing the presence and magnitude of damage with a very promising future for both NDT and possibly structural health monitoring (SHM) applications. Moreover the technique was validated with two conventional NDT techniques: pulse thermography and thermosonics. The first NDT method failed in detecting the damage on the impacted face. The second technique was capable of localising and quantifying the damage on the impacted surface agreeing well with the results obtained using the proposed nonlinear imaging method.     

混凝土分层缺陷的超声检测方法研究

为解决混凝土结构中分层缺陷的在线非接触检测难题,论文提出了利用空气耦合(简称:空耦)超声导波定量检测混凝土结构中分层缺陷的新方法。首先研究了空耦超声导波在混凝土结构中的传播特性,理论分析和实验表明,利用空耦超声波以入射角8.7°入射厚度为50 mm的混凝土板时,可以激发以A0模态为主的导波。然后构建了空耦超声导波扫查实验系统,在混凝土结构单侧利用一对倾斜8.7°的空耦探头激励和接收导波信号,通过分析发现A0模态对分层缺陷敏感,且其幅度与扫查路径中的分层缺陷尺寸存在单调变化关系;在此基础上,对检测区域进行扫查,利用不同位置处的导波信号幅度实现分层缺陷的二维成像。实验结果表明,该方法不仅可以避免耦合剂对检测结果的影响,同时可实现对服役状态下混凝土结构中分层位置及尺寸的定量检测。     

Imaging of the Elastic Wave Propagation in Concrete Using Scanning Techniques: Application for Impact-Echo and Ultrasonic Echo Methods

Acoustic NDT methods like ultrasonic echo and impact-echo are successfully used for NDT of concrete structures. This paper describes useful techniques for a detailed experimental study of the elastic wave propagation, which is highly relevant for the interpretation of the results obtained from practical measurement applications. By using a scanning laser vibrometer it becomes possible to obtain a 2D visualization of the elastic waves propagating along the surfaces of concrete specimens. Time slices are prepared so that the wave field becomes apparent. In order to visualize especially the surface wave propagation a similar technique using a scanning system with a piezoelectric sensor is applied. The results obtained provide an appropriate basis for the comparison with numerical results from 3-D Elastodynamic finite integration technique (EFIT) calculations, which is demonstrated here. Examples are presented for the application with phased array ultrasonic echo, air-coupled ultrasonic echo and impact-echo.     

界面剥离钢-混凝土组合结构应力波传播谱元法模拟研究

建立钢-混凝土组合结构截面二维谱元法模型,针对钢-混凝土界面完好以及存在界面剥离时模型在单点激励下弹性应力波场进行模拟分析,探究弹性应力波的传播规律。采用去除混凝土单元的方法反映组合结构中钢板与混凝土之间的界面剥离。通过对比分析表明界面剥离的存在对弹性应力波场以及模型上不同位置的位移响应产生影响。通过改变界面剥离程度,得到测点位移的变化规律。通过模拟,谱元法可以有效模拟钢-混凝土组合结构中弹性波的传播规律,为研究基于应力波测量的钢-混凝土组合结构界面缺陷检测机理提供帮助。     

Characterization of Reflector Types by Phase-Sensitive Ultrasonic Data Processing and Imaging

Ultrasonic and radar imaging techniques are limited in resolution by the wavelength in the material, yet information beyond those limits is hidden in complex frequency dependent reflection coefficients. The analysis of the phase of complex reflection coefficients together with the properties of imaging algorithms can help to characterize and to classify indications of defects in concrete buildings. This paper describes a method to extract phase information from measurements and SAFT reconstructed images. The influencing factors like material properties, transducer characteristics, and imaging algorithms based on Born or Physical Optics approximations are elaborated. Simulated and experimental results are briefly discussed.     

ICARUS: imaging pulse compression algorithm through remapping of ultrasound

In this work we tackle the problem of applying to echographic imaging those synthetic aperture focusing techniques (SAFT) in the frequency domain commonly used in the field of synthetic aperture radars (SAR). The aim of this research is to improve echographic image resolution by using chirp transmit signals, and by performing pulse compression in both dimensions (depth and lateral). The curved geometry present in the unfocused radio-frequency (RF) ultrasonic image is the main cause of inaccuracy in the direct application of frequency domain SAFT algorithms to echographic imaging. The focusing method proposed in this work, after pulse compression in the depth dimension, performs lateral focusing in the mixed depth-lateral spatial frequency domain by means of a depth variant remapping followed by lateral pulse compression. This technique has the advantage of providing a resolution that is uniform in nonfrequency selective attenuation media, and improved with respect to conventional time domain SAFT, without requiring the acquisition and processing of channel data necessary for the most advanced synthetic transmit aperture techniques. Therefore, the presented method is suitable for easy real-time implementation with current generation hardware.     

A nondestructive ultrasonic imaging system for detection of flawsin metal blocks

An ultrasonic imaging system is presented for the nondestructive evaluation of internal defects in metal blocks. The surfaces of the blocks are coarsely scanned by a single ultrasonic transducer to record multiple A-scans. The recorded A-scans are processed by a synthetic-aperture focusing technique (SAFT) algorithm to search for defects in a metal specimen. The processed information is then sent to a graphics package to be displayed on a CRT screen of a personal computer. The current linear imaging technique is presented and is compared to the SAFT algorithm. The theory and limitations of the SAFT algorithm as well as the hardware implementation are discussed. Images of defects in aluminum blocks are illustrated with varying operating parameters showing the performance of the imaging system     

Synthetic aperture focusing technique signal processing

The synthetic aperture focusing technique (SAFT) is briefly reviewed and addressed as a heuristic digital ultrasonic imaging scheme which exploits the idea of back-propagating a set of measured and digitally stored A-scans. It is shown that for a far-field experimental set-up, ie for small, isolated defects remote to the transducer, SAFT reduces to the filtered back-projection imaging scheme which is well known within the framework of conventional X-ray computer tomography. Therefore, alternative data processing via Fourier transforms only, similar to the Fourier slice theorem of tomography, is possible, which sheds considerable light upon the heuristic SAFT pixel-space envelope-detection scheme. The resulting imaging identity has been termed POFFIS (physical optics far-field inverse scattering). The far-field assumption is then omitted yielding a Fourier-transform-SAFT algorithm (FT-SAFT) whose results are identical to back-propagation imaging with the definite advantage of fast processing capabilities based upon standard hardware and allowing immediate implementation of high resolution procedures as well as inclusion of mode-conversion effects; the theoretical background is pulse-echo diffraction tomography. The above results are supported and illustrated by application of all three algorithms — SAFT, POFFIS, FT-SAFT — to experimental data obtained from scanning a line aperture for several test specimens.     

基于频域合成孔径聚焦的输电线路钢管杆超声导波成像检测

输电线路钢管杆结构是一种带有拔梢斜度的大口径薄壁管,超声导波周向B扫描检测传感器激发的超声导波在钢管杆传播过程中存在波束扩散现象。为了解决钢管杆超声导波检测过程中波束扩散导致周向检测分辨率降低的问题,提出了一种钢管杆超声导波频域合成孔径聚焦方法(Synthetic Aperture Focusing Technique, SAFT)。采用文中构建的超声导波合成孔径成像检测装置对直径为608 mm 的钢管杆进行超声导波扫查,可获得周B扫描图像和频域合成孔径聚焦图像。对比钢管杆周向B扫描图像和频域合成孔径聚焦图像,频域合成孔径聚焦图像中散射体和通孔的周向检测分辨率明显优于B扫描图像,文章提出的钢管杆超声导波频域合成孔径聚焦方法能够显著改善超声导波周向检测分辨率。     

Apport des techniques de mesures fluxmétriques dans les méthodes de maturométrie des bétons

Résumé La réaction d'hhdration des bétons, très sensible aux conditions météorologiques, doit faire l'objet d'un suivi afin d'optimiser la gestion d'un chantier. Les procédés classiques utilisés sont les techniques de maturométrie, basées sur l'évolution de la température du béton. Seulement, dans le cas de structures peu épaisses, la prépondérance des sollicitations naturelles peut rendre difficile toute interprétation. Le but de cet article est de proposer une méthode complémentaire au suivi de la prise des bétons, à partir de mesures fluxmétriques réaliséesin situ.

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Contribution of fluxmeter measurements to concrete maturity methods

The hydration reaction of concrete, highly dependent on weather conditions, has to be carefully monitored in order to optimize the management of a worksite. The classical technique used is the maturity concept, based on the evolution of concrete temperature. However, in the case of not very thick structures, the preponderance of natural solicitation can make any interpretation difficult. The aim of this article is to suggest a complementary method of harding concrete monitoring, based on fluxmeter measurements effectedin situ.

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Methodology for Mapping the Residual Stress Field in Serviced Rails Using LCR Waves

Thermal barrier coatings (TBCs), as effective thermal protection separating the substrate from high-temperature combustion gases and reducing the substrate temperature, are widely used in aerospace and other fields. During the service cycle of life, surface crack defects, interface disbond defects, and coating thickness changes are the main non-destructive testing (NDT) objects of TBCs. In this paper, the main active infrared thermography NDT techniques including the optical infrared thermography testing, the ultrasonic infrared thermography testing, and the microwave thermography testing techniques are reviewed. Through the summary and highlight of the detection principle and application status of these state-of-the-art techniques, the development of the active infrared thermography DNT technique in TBCs is presented. By comparing the sensitivity, advantages, and disadvantages of the techniques in TBC NDT, can provide a significant reference for researchers to choose an appropriate method. It is noteworthy that fabrication techniques of artificial defects for calibration of the active infrared thermography NDT technique inspection of TBC systems are also reviewed. Moreover, future trends in NDT for the TBC system based on the active infrared thermography NDT technique are also discussed and analyzed.

     

Image Fusion for Improved Detection of Near-Surface Defects in NDT-CE Using Unsupervised Clustering Methods

The capabilities of non-destructive testing (NDT) methods for defect detection in civil engineering are characterized by their different penetration depth, resolution and sensitivity to material properties. Therefore, in many cases multi-sensor NDT has to be performed, producing large data sets that require an efficient data evaluation framework. In this work an image fusion methodology is proposed based on unsupervised clustering methods. Their performance is evaluated on ground penetrating radar and infrared thermography data from laboratory concrete specimens with different simulated near-surface defects. It is shown that clustering could effectively partition the data for further feature level-based data fusion by improving the detectability of defects simulating delamination, voids and localized water. A comparison with supervised symbol level fusion shows that clustering-based fusion outperforms this, especially in situations with very limited knowledge about the material properties and depths of the defects. Additionally, clustering is successfully applied in a case study where a multi-sensor NDT data set was automatically collected by a self-navigating mobile robot system.