Showing papers in "Ndt & E International in 2008"
TL;DR: In this paper, the interaction of Lamb wave modes at varying frequencies with a through-thickness crack of different lengths in aluminium plates was analyzed in terms of finite element method and experimental study.
Abstract: The interaction of Lamb wave modes at varying frequencies with a through-thickness crack of different lengths in aluminium plates was analysed in terms of finite element method and experimental study. For oblique-wave incidence, both numerical and experimental results showed that the wave scattering from a crack leads to complicated transmission, reflection and diffraction accompanied by possible wave-mode conversion. A dual-PZT actuation scheme was therefore applied to generate the fundamental symmetrical mode (S0) with enhanced energy to facilitate the identification of crack-scattered wave components. The relationship between crack length and the reflection/transmission coefficient obtained with the aid of the Hilbert transform was established, through which the crack length was quantitatively evaluated. The effects of wavelength of Lamb waves and wave diffraction on the properties of the reflection and transmission coefficients were analysed.
163 citations
TL;DR: In this article, the defect detection capabilities of pulse transient thermography and lock-in thermography were compared using equal excitation energies, and a signal-to-noise ratio analysis was performed on defect images obtained by the two techniques.
Abstract: The defect detection capabilities of pulse transient thermography and lock-in thermography were compared using equal excitation energies. A signal-to-noise ratio analysis was performed on defect images obtained by the two techniques. The test piece imaged by both techniques was a carbon fibre composite plate containing back-drilled flat-bottomed hole artificial defects of 4, 6 and 12 mm diameters at depths ranging from 0.25 to 3.5 mm. Similar limits of defect detection were found for the two techniques. Lock-in thermography phase images were found to exhibit anomalous switches between positive and negative phase values for different modulation frequencies and for different combinations of defect diameter and depth. This effect resulted in values of defect phase response that differed substantially from the one-dimensional thermal wave interpretation of lock-in thermography imaging. The consequences of this anomalous effect and its physical origins are discussed.
147 citations
TL;DR: In this article, a normalization procedure for removing GPR signal losses resulting from depth-dependent two-way travel time variations in the data was developed and thresholds for predicting chain-drag and half-cell potential-based estimates of deterioration quantity and location were established.
Abstract: Ground-penetrating radar (GPR) surveys have tended to provide unpredictable accuracy in estimating areas of corrosion-induced damage in reinforced concrete bridge decks. Six in-service reinforced concrete bridge decks with exposed (unpaved) concrete wearing surfaces were surveyed using GPR to evaluate internal corrosion-induced damage for comparison against the chain drag and half-cell potential survey methods. A normalization procedure for removing GPR signal losses resulting from depth-dependent two-way travel time variations in the data was developed and thresholds for predicting chain-drag and half-cell potential-based estimates of deterioration quantity and location were established. Thresholds established using the amplitude–time correction provided significant improvements in the spatial and quantitative predictive capabilities of GPR for delineating corrosion-induced damage.
135 citations
TL;DR: A method is proposed which allows correcting beam hardening artifacts in two and multi-component objects and works without any estimate or knowledge of the incident X-ray spectrum nor material characteristics like mass density or absorption coefficients.
Abstract: In computed tomography (CT) several physical effects reduce the quality of reconstructed CT images. Beside scatter, one major deterministic artifact is the beam hardening effect which arises due to the polychromatic character of the used X-ray spectrum and causes cupping and streak artifacts. In this paper, a method is proposed which allows correcting beam hardening artifacts in two and multi-component objects. Neither is it necessary to have any estimate or knowledge of the incident X-ray spectrum nor material characteristics like mass density or absorption coefficients. The method works without any reference measurements and uses the original projection images. Several two-material specimens of miscellaneous material combinations were tested and corrected to show the efficiency of the proposed method.
132 citations
TL;DR: In this paper, the detection of multiple-layer reflections within the ground penetrating radar (GPR) return is solved by iteratively detecting the strong reflections present within the GPR signal using either a threshold or a matched filter detector.
Abstract: One of the problems encountered in the nondestructive testing of pavements with ground penetrating radar (GPR) is the detection of multiple-layer reflections within the GPR return. Detecting reflections is especially problematic when the pavement layers are thin with respect to the probing pulse width, in which case overlapping between the reflected pulses occurs, causing the weak reflections to be masked by the stronger reflections in their vicinity. In this study, the problem is solved by iteratively detecting the strong reflections present within the GPR signal using either a threshold or a matched filter detector. The detected pulses are then used in a reflection model to synthesize a signal “similar” to the measured GPR signal in the least-squares sense. The synthesized signal is then subtracted from the measured signal to reveal the masked weak reflections, which are later detected iteratively using the same method. This technique was successfully applied to field GPR data collected from an experimental pavement site: the Virginia Smart Road.
129 citations
TL;DR: In this article, two non-destructive techniques, namely ground penetrating radar (GPR) and a more original approach based on capacitive measurement, have been considered in this research project that focuses on the evaluation of cover concrete moisture content.
Abstract: Two non-destructive techniques, namely ground penetrating radar (GPR) and a more original approach based on capacitive measurement, have been considered in this research project that focuses on the evaluation of cover concrete moisture content. Following a numerical modeling step for the capacitive technique, the two methods were successfully compared during an experimental campaign conducted in the laboratory against several control test slabs.
127 citations
TL;DR: In this paper, the representativity of electrical resistivity values is assessed by analysis of measurements on eight slabs and the complete analysis process is detailed on two slabs which have been casted according to two different fibres distributions.
Abstract: Steel fibre reinforced concrete (SFRC) is a new material allowing innovative projects for concrete structures. Such structures are designed using assumptions on the material fabric. Checking these assumptions requires, according to recommendations, coring and mechanical testing of samples. Non-destructive assessment, if validated, would provide an interesting alternative, making the checking process easier, quicker and less expensive. Blind tests in laboratory compare electrical resistivity measurements obtained with a four-probes square device, to visual analysis during pouring. After having defined a measurement process, the representativity of electrical resistivity values is assessed by analysis of measurements on eight slabs. Electrical resistivity allows the identification of a high or low resistivity axis, which gives the local orientation of steel fibres. The calculation of electrical anisotropy gives an indication of the “intensity of fibres orientation” in each area. In this article, the complete analysis process is detailed on two slabs which have been casted according to two different fibres distributions. These tests confirm the ability of electrical resistivity method to provide data on steel fibres within concrete, via a non-destructive way.
126 citations
TL;DR: In this paper, a non-contact vibrometer was designed for dynamic testing and monitoring of large structures, which consists of a radar sensor apt to simultaneously measure the displacement of several points of a structure with high sensitivity.
Abstract: This paper addresses the application of a non-contact vibrometer, designed for dynamic testing and monitoring of large structures. The new system consists of a radar sensor apt to simultaneously measure the (static or dynamic) displacement of several points of a structure with high sensitivity. In this paper, the sensor and its major characteristics are first described; subsequently, application to the measurement of ambient vibration response of a concrete bridge is summarized. The correspondence between the signals simultaneously acquired from radar and accelerometers was extensively investigated; furthermore, resonant frequencies and mode shapes identified with the radar system are compared to the corresponding quantities measured with conventional sensors.
113 citations
TL;DR: In this paper, the authors introduced the application of Hilbert transform to extract a new descending feature point and use the point as a cutoff point of sampling data for detection and feature estimation.
Abstract: Pulsed eddy current (PEC) is a new emerging nondestructive testing (NDT) technique using a broadband pulse excitation with rich frequency information and has wide application potentials. This technique mainly uses feature points and response signal shapes for defect detection and characterization, including peak point, frequency analysis, and statistical methods such as principal component analysis (PCA). This paper introduces the application of Hilbert transform to extract a new descending feature point and use the point as a cutoff point of sampling data for detection and feature estimation. The response signal is then divided by the conventional rising, peak, and the new descending points. Some shape features of the rising part and descending part are extracted. The characters of shape features are also discussed and compared. Various feature selection and integrations are proposed for defect classification. Experimental studies, including blind tests, show the validation of the new features and combination of selected features in defect classification. The robustness of the features and further work are also discussed.
107 citations
TL;DR: In this paper, the authors address the problem of automatic inspection of composite materials using an ultrasonic technique and consider two main steps for interpreting ultrasonic data: the pre-processing technique necessary to normalize the signals of composite structures with different thicknesses and the classification techniques used to compare the ultrasonic signals and detect classes of similar points.
Abstract: The detection of internal defects in composite materials with non-destructive techniques is an important requirement both for quality checks during the production phase and in-service inspection during maintenance operations. Visual inspection allows only the analysis of surface characteristics of materials and, then, if internal faults occur inside composite structures, a deeper analysis is required. A comparison between the reactions of different materials to ultrasonic signals can be used to highlight the difference in the internal structures and also to detect the depth position of these anomalies. However, ultrasonic data are difficult to interpret since they require the analysis of a continuous signal for each point of the material under consideration. An automatic procedure is necessary to manage large data sets and to extract significant differences between them. In this paper, we address the problem of automatic inspection of composite materials using an ultrasonic technique. We consider two main steps for interpreting ultrasonic data: the pre-processing technique necessary to normalize the signals of composite structures with different thicknesses and the classification techniques used to compare the ultrasonic signals and detect classes of similar points.
106 citations
TL;DR: In this article, a lock-in infrared thermography (LIT) was used to estimate the sizes and locations of subsurface defects using a fixed number of pixels, where the inspected image is shifted to obtain a shifted image while subtraction of one image from the other gives the shearing-phase distribution.
Abstract: This paper describes the quantitative determination of the sizes and locations of subsurface defects using lock-in infrared thermography. A phase (or temperature) difference between the defect area and the healthy area indicates the qualitative location and size of the defect. To accurately estimate these parameters, the shearing-phase technique has been employed, where the inspected image is shifted by a certain number of pixels to obtain a shifted image while subtraction of one image from the other gives the shearing-phase distribution. The shearing-phase distribution has maximum, minimum, and zero points that help determine quantitatively the size and location of the subsurface defect. Experimental results for a steel plate with artificial subsurface defects show good agreement with actual values.
TL;DR: In this paper, the application of ground-penetrating radar (GPR) as a non-destructive technique for the monitoring of ring separation in masonry arch bridges was studied.
Abstract: The application of ground-penetrating radar (GPR) as a non-destructive technique for the monitoring of ring separation in masonry arch bridges was studied. Numerical modelling techniques were used to simulate tests using GPR—these numerical experiments were backed up and calibrated using laboratory experiments. Due to the heterogeneity of these structures, the signals coming from the interaction between the GPR system and the bridge are often complex, and hence hard to interpret. This defined the need to create a GPR numerical model that will allow the study of the attributes of reflected signals from various targets within the structure of the bridge. The GPR numerical analysis was undertaken using the finite-difference time-domain (FDTD) method. Since “micro regions” in the structure need to be modelled, subgrids were introduced into the standard FDTD method, in order to economize on the required memory and the calculation time. Good correlations were obtained between the numerical experiments and actual GPR experiments. It was shown both numerically and experimentally that significant mortar loss between the masonry arch rings can be detected. However, hairline delaminations between the mortar and the brick masonry cannot be detected using GPR.
TL;DR: The experimental results show that the proposed approach is effective and feasible to segment and locate defects in noisy and low contrasted X-ray images of weld.
Abstract: Because X-ray images of weld contain uncertain noise and also the defects inside them have low contrast to their background, it is difficult to detect weld defects of X-ray images. The goal of this paper is to locate and segment the line defects in X-ray images. Firstly, we present an approach to extract features of X-ray images with multiple thresholds. Then, use the support vector machine (SVM) technique to classify the defect and non-defect features to obtain a coarse defect region. Furthermore, perform the Hough transform to remove the noisy pixels in the coarse defect region. Then the defect is located and segmented. The experimental results show that the proposed approach is effective and feasible to segment and locate defects in noisy and low contrasted X-ray images of weld.
TL;DR: In this paper, a nonlinear Rayleigh surface wave was generated without having to drive the transmitting piezoelectric transducer at high voltages; driving at low voltages limits the excitation of the intrinsic nonlinearity of the transducerer element, and enables an efficient measurement procedure to isolate inherent material non-linearity.
Abstract: This note presents a procedure to generate nonlinear Rayleigh surface waves without having to drive the transmitting piezoelectric transducer at high voltages; driving at low voltages limits the excitation of the intrinsic nonlinearity of the piezoelectric transducer element, and enables an efficient measurement procedure to isolate inherent material nonlinearity. The capabilities of this proposed technique are demonstrated by measuring the material nonlinearity of aluminum alloy 2024 and 6061 plates with Rayleigh surface waves.
TL;DR: In this article, the authors proposed a novel technique to visualize ultrasonic waves propagated in a general solid medium, where a sample was scanned by a pulsed laser for generation of ultrasound, and the propagated waves were received by a fixed transducer.
Abstract: This study proposes a novel technique to visualize ultrasonic waves propagated in a general solid medium. A specimen was scanned by a pulsed laser for generation of ultrasound, and the propagated waves were received by a fixed transducer. This technique then provided a snapshot of traveling waves based on the reversibility for the wave propagation, which is confirmed in this paper. We experimentally and numerically investigated and visualized the wave propagation in a steel plate with a slit. Furthermore, we demonstrated inspection of an elbow pipe with an artificial defect, as an application of the proposed technique to a structural component. Non-contact laser scanning for ultrasound excitation enabled us to inspect a wide area of an arbitrarily shaped object. The distinguishing advantages of this technique were determined to be well-suited to the quick and easy wide-range inspection of real structures.
TL;DR: The established model can not only offer an effective solution in terms of faster simulation time and higher computational accuracy, but also be used for PEC evaluation in industry and in the inverse process for exploring the structural and electrical information of stratified conductive specimens during real-time monitoring.
Abstract: Numerical simulations of electromagnetic non-destructive evaluation (ENDE) can be time-consuming in comparison to analytical methods which provide fast closed-form solutions to the ENDE problems. In this paper, the Truncated Region Eigenfunction Expansion (TREE) modelling is extended to solve problems of pulsed eddy current (PEC) evaluation from the traditional multifrequency eddy current. The Fourier transform is employed to make the TREE feasible for solutions to PEC problems in both time and frequency domains. Moreover, because PEC employs magnetic field sensors/arrays to quantify magnetic field, the magnetic field signals from solid-state magnetic field sensors have been simulated using the extended TREE. It has been found that the predicted signals using the extended TREE has good agreement with the experimental results. Consequently, the established model can not only offer an effective solution in terms of faster simulation time and higher computational accuracy, but also be used for PEC evaluation in industry and in the inverse process for exploring the structural and electrical information of stratified conductive specimens during real-time monitoring.
TL;DR: In this article, nonlinear wave modulation spectroscopy (NWMS) was used to quantitatively track the evolution of microcracks in Portland cement mortar samples and quantitatively assess the deterioration of cement-based materials.
Abstract: This paper presents the successful application of a nonlinear ultrasonic technique, nonlinear wave modulation spectroscopy (NWMS) to quantitatively track the evolution of microcracks in Portland cement mortar samples. The damage type considered in this study is microcracking due to alkali–silica reaction (ASR), a deleterious reaction occurring in concrete structures around the world. Nonlinear ultrasonic measurements are conducted on six different mortar specimens that are monitored from their initial, intact state up to their fully damaged state. The objective of this research is to determine the sensitivity and suitability of NWMS to quantitatively track this damage state throughout an entire life-cycle and to nondestructively identify the initiation time and the extent of microcracking in these mortar specimens. The nonlinear ultrasonic measurements are made with standard laboratory equipment, and the inherent high attenuation of cement-based materials is overcome with a procedure that uses the sideband energy instead of measuring peak amplitudes. The results show that the NWMS method can track the progressive damage in mortar, demonstrating the feasibility of using this nonlinear ultrasonic technique to quantitatively assess the deterioration of cement-based materials.
TL;DR: In this article, a region growing approach is used to segment the image and identify the voids without human interference or prior knowledge of the conditions using infrared thermography, which was tested on images collected from concrete bridge deck specimens containing various man-made defects.
Abstract: The objective of the project presented in this paper is to automate the detection of subsurface defects in concrete bridge decks using infrared thermography. The algorithm developed for this purpose is based on the region growing approach, which segments the image and identifies the voids without human interference or prior knowledge of the conditions. The segmentation algorithm starts with the hottest pixels in the image as seed points, and then regions are grown around them based on a neighborhood selection criterion. The algorithm was tested on images collected from concrete bridge deck specimens containing various man-made defects and also on a defect-free control model. The experimental work successfully identified defects in concrete bridge decks up to 3 in. below surface using thermograph imaging.
TL;DR: In this paper, the results of the residual magnetic field measurements performed on mechanically notched samples exposed to tensile loads were presented, indicating a high sensitivity of the RMF to any changes in stress.
Abstract: The residual magnetic field (RMF) of a ferromagnetic element is a value that can be affected by several physical effects. These are: the magneto-mechanical effect, the effect of magnetic field leakage caused by a discontinuity or a structural inhomogeneity of the material, and the processes of mutual interacting of magnetic fields with dislocations and their accumulation. The following paper presents the results of the RMF measurements performed on mechanically notched samples exposed to tensile loads. The distribution of the RMF was mapped for subsequent loads applied. There were substantial changes in the pattern of isolines and in all of the components of the RMF of the samples in comparison to the initial state. That indicates a high sensitivity of the RMF to any changes in stress. The results obtained were compared to the calculated maps of stresses and deformation. A correlation was found out between these values. Measurements of the RMF make it possible to determine the notch effect and a qualitative determination of the distribution of inner stresses.
TL;DR: In this article, a ground-penetrating radar (GPR) survey with multiple sets of 1 and 2 GHz air-horn antennae was conducted at the Transportation Technology Center, Inc. (TTCI) in Pueblo, Colorado.
Abstract: This paper will evaluate ground-penetrating radar (GPR) as a non-destructive method to rapidly, effectively, and continually assess the conditions of railroad ballast. Compared to uniformly graded, clean ballast, fouled ballast has a finer, well-graded particle size with fewer air voids. Ballast under different conditions generates various GPR electromagnetic scattering patterns. A field GPR survey with multiple sets of 1 and 2 GHz air-horn antennae was conducted in summer 2005 at the Transportation Technology Center, Inc. (TTCI) in Pueblo, Colorado. The 2 GHz antenna was found to be more sensitive to the change in scattering pattern. Appropriate data processing was used to remove the effects of the rails to obtain clear GPR images of the subsurface layers. From the image analysis, ballast thickness, ballast fouling condition, and trapped water can be assessed.
TL;DR: In this paper, the authors report a direct comparison between the two measurement techniques (radar interferometer vs. accelerometers) both employed during a field test on a bridge.
Abstract: Interferometric radar has been recently proposed as a measurement instrument for dynamic testing/monitoring of large structures, such as bridges, towers, buildings, and dams, which is currently performed by networks of accelerometers. In this paper, the authors report a direct comparison between the two measurement techniques (radar interferometer vs. accelerometers) both employed during a field test on a bridge. As different quantities, displacement and acceleration are measured by the two techniques, a preliminary discussion about signals and noise has been necessary. Finally, the experimental results are critically discussed.
TL;DR: In this article, the normal component of stress-induced magnetic field, H p ( y ), was measured during tensile tests on the surfaces of sheet specimens of three ferromagnetic materials.
Abstract: Stress alone applied to ferromagnetic materials can induce the generation of weak magnetic signals on their surfaces, which can be potentially used to estimate the degree of damage of ferromagnetic components. In this paper, the normal component of stress-induced magnetic field, H p ( y ), was measured during tensile tests on the surfaces of sheet specimens of three ferromagnetic materials. It has been concluded that H p ( y ) depends on the applied stress and will present different characteristics on the elastic and plastic deformation stages, respectively. The phenomenon of sharp changes in magnetic signals occurring at the instant of fracture was also discussed from the view of the interaction energy in a ferromagnet.
TL;DR: In this article, the authors present the methodology and results of a study of the reinforced concrete base of a large block of flats, where flow water and flooding are common in the wet and dry seasons.
Abstract: Ground-penetrating radar (GPR) is a non-destructive geophysical technique that is used to analyse soils and structures by providing continuous images of the interior of the media being analysed. In this paper, we present the methodology and results of a study of the reinforced concrete base of a large block of flats, where flow water and flooding are common in the wet and dry seasons. There are also a large number of cracks and fissures in the soil under the base. The damaged building is located in Valencia (Spain). The measurements were taken during two different seasons to include a period when flooding occurred and a period without flooding. The aim of the study was to determine whether the base would remain watertight in future and to analyse the depth of the damage observed on the surface. The results provided us with a detailed map of the damage and the zones that require more detailed analysis. Such damage affects the whole concrete slab. In addition to this analysis, several measurements were carried out to determine the possible causes of the inundations by locating potential wet areas and zones with high water content. A water table and a well were detected, which could be the cause of the humidity damage.
TL;DR: In this paper, 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.
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) 2.25 MHz linear phased array transducer, (b) 2.25 MHz conventional circular transducer and (c) 1 MHz 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.
TL;DR: In this paper, the authors demonstrate the potentiality of air-coupled transducers to set up a contactless, single-sided technique for testing the moisture content and/or the micro-cracking of carbon-epoxy composite wound around a Titanium liner.
Abstract: Ultrasonic guided wave modes are proposed to control the integrity of high-pressure composite tanks produced by EADS-ASTRIUM, France. The purpose is to demonstrate the potentiality of air-coupled transducers to set up a contact-less, single-sided technique for testing the moisture content and/or the micro-cracking of carbon-epoxy composite wound around a Titanium liner. Although guided waves have been experimentally propagated on a real tank, it was not allowed to damage this specimen. Therefore, plates made of similar composite materials than that constituting the tank winding were submitted to water intake or to thermal stresses. After immersing some plates in a humid chamber, it was demonstrated that the attenuation of the A0 guided wave mode is sensitive to the moisture content. Other plate samples were submitted to immersion in liquid nitrogen that induces transversal cracks shown to cause significant drops in the celerity of several guided waves. Inverse problems have been used for quantifying the effects of these damages on the material properties, and they showed that water intake increases the imaginary part of the Coulomb moduli, while micro-cracking decreases all the material stiffness moduli. Such changes in the material properties have then been used as input data for simulating waveforms corresponding to the propagation of circumferential or longitudinal wave modes in the tank. Changes in these waveforms, caused by simulated damages of the composite winding of the tank, have been shown to be quite significant. To conclude the study, an experimental sep-up using air-coupled transducers was employed to generate–detect guided wave modes over large distances in the real tank, with very good signal-to-noise ratios, thus demonstrating the possibility of using such elements for the non-destructive testing of high-pressure composite tanks during their lives.
TL;DR: In this paper, a contact-free method such as infrared thermography is used for surface temperature measurement, and the fine agreement obtained between the experimentally and calculated data, validate the infrared thermograph as a non-destructive method for real-time monitoring of the welding process.
Abstract: In laser welding technique, a real-time control of temperature distribution inside the irradiated materials is essential when attempting to optimize the process. For all laser welding methods that operate by the transmission principle, the difficulty of recording the developed temperature at the interface derives from the fact that materials to be welded are in contact throughout the entire process. In the present study, in order to overcome this issue, a contact-free method such the infrared thermography is used for surface temperature measurement. Corroborating this data with a numerical simulation of the temperature field evolution inside the components, an assessment of optimal process parameters is possible. The experimental investigations are made on amorphous polymers, in a typical configuration for through-transmission laser welding. The fine agreement obtained between the experimentally and calculated data, validate the infrared thermography as a non-destructive method for real-time monitoring of the welding process.
TL;DR: In this article, the interaction of the fundamental Lamb modes with asymmetrical discontinuities in isotropic plates is studied numerically and experimentally, and the power reflection and transmission coefficients are computed with the well-known average power flow equation described in a previous work.
Abstract: The interaction of the fundamental Lamb modes with asymmetrical discontinuities in isotropic plates is studied numerically and experimentally. Three kinds of discontinuities that enable mode conversions are considered: steps down, steps up and asymmetrical notches. The anti-symmetrical and symmetrical Lamb modes contributions are separated by means of the simple arithmetic operations of addition and subtraction. The power reflection and transmission coefficients are then computed with the well-known average power flow equation described in a previous work. Furthermore, the asymmetrical notch is reconstructed from the superposition of a step down and a step up. It is shown that it allows directly the determination of all power coefficients of the reflected, the transmitted and the multiple converted Lamb modes contrarily to the finite element model. Finally, an experimental device is realized to test aluminum plates with different notch depths. Good agreement is found between experimental and numerical results.
TL;DR: In this paper, the authors investigated the impact response of concrete blocks and studied the feasibility of using the impact-echo method for detection of flaws in concrete blocks with and without flaws.
Abstract: The objectives of this paper are to investigate the impact response of concrete blocks and to study the feasibility of using the impact-echo method for detection of flaws in concrete blocks. Numerical studies were carried out to acquire the transient responses of intact concrete blocks subjected to impact. In addition, the impact responses of concrete blocks containing flaws were explored. Numerical results were verified by experimental studies on concrete blocks with/without flaws. It is shown that the impact response of a concrete block is composed of frequencies corresponding to the modes of vibration of the block. Among these frequencies, there is a predominant frequency and its value depends on the geometry and dimensions of the block for a given P-wave speed in concrete. It is also shown that the presence of a flaw disrupts the modes of vibration. A shift of the predominant frequency to a lower value is a key indication of the presence of the flaw. In addition, multiple wave reflections between the impact surface and the surface of the flaw produce a large amplitude peak in the spectrum at the frequency corresponding to the depth of the flaw.
TL;DR: In this article, the authors present three-dimensional (3D) measurements by digital volume correlation (DVC) with images from X-ray micro-computed tomography (XμCT) and Optical Scanning Tomography (OST).
Abstract: In this study, we present three-dimensional (3D) measurements by digital volume correlation (DVC) with images from X-ray micro-computed tomography (XμCT) and optical scanning tomography (OST). In this article, we compare both techniques and we define their field of application by evaluating their measurement uncertainty in the case of rigid body translations, homogeneous strain tests and localised compression tests. For displacement, measurement uncertainty is around 0.037 voxel for OST and 0.049 voxel for XμCT which correspond to a measured strain about 0.1%. For larger strain, uncertainty increases with strain intensity but relative error remains constant about 10%. To resume, measurement uncertainties given by DVC are similar to the ones which we obtain generally for a two-dimensional (2D) study with DIC.
TL;DR: In this paper, the power reflection and transmission coefficients for a symmetrical notch were computed using finite element and the well-known average power flow equation, with the help of a finite element finite element model.
Abstract: The aim of this work is to predict the propagation of the fundamental Lamb modes in an isotropic structure containing discontinuities in a simple and a fast way. The key point is to decompose the symmetrical notch into two elementary abrupt changes in the plate section. The power reflection and transmission coefficients are computed, using two techniques, with the help of the finite element and the well-known average power flow equation. In the first technique, the through-thickness displacements and stresses are used while in the second technique only the normal or tangential displacement at a given location on the plate surface is used. An equality relation between the direct and the inverse abrupt changes of the plate section is given, which allows computing the power reflection and transmission coefficients for a symmetrical notch from those obtained from one elementary damage. Finally, aluminum plates with different notches depths are tested. Experimental and numerical results are in good agreement.