Showing papers in "Ndt & E International in 1997"
TL;DR: In this article, a new scheme for bearing-localized defect detection based on wavelet transform is presented, which provides a variable resolution time-frequency distribution from which periodic structural ringing due to repetitive force impulses, generated upon the passing of each rolling element over the defect, are detected.
Abstract: This paper presents a new scheme for bearing-localized defect detection based on wavelet transform. Wavelet transform provides a variable resolution time-frequency distribution from which periodic structural ringing due to repetitive force impulses, generated upon the passing of each rolling element over the defect, are detected. In this paper, important properties of wavelet transform and its comparison with short-time Fourier transform are discussed. Then, a basic wavelet considered optimal for bearing-localized defect detection is constructed. Finally, the new scheme is described and its effectiveness is evaluated using actual vibration signals measured from bearings with defects at different locations and operating under different conditions.
187 citations
TL;DR: In this paper, typical defects found in adhesive joints are described together with their significance, and the limits and likely success of current physical nondestructive tests are described, and future trends outlined.
Abstract: One approach to testing the suitability of an adhesive joint for a particular application is to build and test to destruction a representative sample of the joint. In this way the best adhesive and surface treatment for a given application can be found. To reduce the costs of this approach, the designer will wish to call on previous experience with adhesives, surface treatments, and joint designs so as to reach a high probability of success before he builds and tests a structural prototype. If structures are expensive, it will be difficult to justify more than a very limited series of prototype tests before production begins. During the production phase, and also in service with critical structures, it is essential to use nondestructive tests to assess the quality and fitness for purpose of the product. The nondestructive test will not measure strength directly but will measure a parameter which can be correlated to strength. It is therefore, essential that a suitable nondestructive test is chosen and that its results are correctly interpreted. In this paper, typical defects found in adhesive joints are described together with their significance. The limits and likely success of current physical nondestructive tests are described, and future trends outlined.
159 citations
TL;DR: In this article, the authors used acoustic emission to detect debonding between the reinforcement and the matrix material, matrix cracking, failure of the reinforcement, and delamination in the case of layered composites.
Abstract: Applying a mechanical load to a composite material can result in many types of damage: debonding between the reinforcement and the matrix material, matrix cracking, failure of the reinforcement and delamination in the case of layered composites. To create a better understanding of the initiation, growth and interaction of the different types of damage, damage monitoring during mechanical loading is very important. Acoustic emission is the only nondestructive test technique capable of detecting all of the above mentioned damage types in composites. The possibilities of this technique to detect certain types of damage can only be proven by using other complementary techniques (replica technique, radiography, ultrasonics, optical and scanning electron microscopy). The research work on damage detection with the acoustic emission technique for different types of composite materials carried out at the Department of Metallurgy and Materials Engineering is presented.
157 citations
TL;DR: In this article, the interactive forces between concrete and the steel cross-sections of the reinforcement can be investigated using the acoustic emission (AE) technique, and a quantitative AE technique was developed for a comprehensive analysis of the AE waveforms including time domain and frequency.
Abstract: The interactive forces between concrete and the steel cross-sections of the reinforcement can be investigated using the acoustic emission (AE) technique. A quantitative AE technique was developed for a comprehensive analysis of the AE waveforms including time domain and frequency. A 3D-localization as well as a classification of the events was implemented. The characterization of the fracture mechanisms is obtained by the newly developed relative moment tensor inversion method using a cluster analysis technique. This potentially improves the conclusions of AE measurements and will lead to a better way of assessing the damage of concrete.
138 citations
TL;DR: In this article, the authors describe the physics of mapping CT density to mass density, development and scanning of density standards, post-reconstruction filtering to reduce the effects of quantum noise, and segmentation of the object data from the surrounding medium.
Abstract: X-ray computed tomography (CT) can quantify variations in material density. However, little practical information concerning accurate translation of CT numbers (an approximation of the material's linear attenuation coefficient) into mass density exists. We describe our development of this methodology. Issues addressed include the physics of mapping CT density to mass density; development and scanning of density standards; post-reconstruction filtering to reduce the effects of quantum noise; and segmentation of the object data from the surrounding medium. Examples of our use of this procedure in studying density gradients during powder pressing and firing are given.
129 citations
TL;DR: In this article, the authors applied pulsed eddy current techniques to specimens containing cracks of different size and depth, and validated this approach by probing different samples with real flaws, using an original signal analysis was developed to extract the characteristics of well known defects.
Abstract: Eddy current tests in sinusoidal mode are of great interest to detect flaws in metallic structures. A limitation of this classical method concerns the detection of deep defects in very conductive materials. Such a drawback can be reduced by operating in the transient mode, using a pulsed eddy current technique. In fact, the pulsed character of the electric excitation allows a high peak value of the density of eddy currents in the material; also the broadband signal contains optimized frequencies to probe the sample over a more extended depth. The analysis of the resulting signals is then different from those issue of the sinusoidal mode. The purpose of this paper is to apply pulsed eddy current techniques to specimens containing cracks of different size and depth. First, an original signal analysis was developed to extract the characteristics of well known defects. Then, this approach has been validated by probing different samples with real flaws.
100 citations
TL;DR: In this article, the results of nondestructive testing of the same specimens by nine workgroups are described and compared, and the results show the remarkable progress achieved in the past few years: the thickness and location of a duct could be measured for one specimen with great precision.
Abstract: Pulse-echo methods (radar, impact-echo, ultrasonic impulse-echo) and simulation of wave propagation are applied for testing concrete speciments with metal ducts. The results of nondestructive testing of the same specimens by nine workgroups are described and compared. The research aims for the experiments are thickness measurement, location of a metal duct and of voided regions inside the duct. The results show the remarkable progress achieved in the past few years: the thickness and location of a duct could be measured for one specimen with great precision. In some cases voided regions in the duct could be located.
86 citations
TL;DR: In this article, a twin-arch masonry arch bridge is assessed using radar, sonic and conductivity data, and the results from the tomographic plots are discussed, concluding that these techniques are effective and can aid the interpretation of data.
Abstract: Masonry arch bridge assessment is overviewed strategically. It is proposed that where a bridge fails the load-carrying capacity assessment by a small margin, and it looks to be in good condition, that further investigation be undertaken using NDT techniques. The NDT techniques of radar, sonic and conductivity methods are reviewed and the results from sonic and conductivity NDT surveys are discussed. It is concluded that these techniques are effective and that tomographic plots aid the interpretation of data. It is shown on a full scale twin-arch masonry bridge that both sonics and conductivity could identify the masonry wall thickness. However with respect to sonics, great attention must be paid to recording the data. Meticulous work is also necessary during the preparation and analysis of the sonic data, but good results can be achieved when the correct grid density is set in relation to the goal of the test. The conductivity technique has demonstrated the ability to be a rapid, low cost, non-contacting technique from which tomographic cross-sections, inhomogeneity identification, moisture movement detection over time and layering within the masonry can be achieved. It should be noted that the conductivity method has given detailed information up to a certain depth into the structure, but no data could be collected at deeper locations. Nonetheless its extreme low cost, particularly in terms of time in the post data-collection phase, makes it convenient for repetitive surveys.
70 citations
TL;DR: In this paper, nine different iterative tomographic algorithms have been applied to the reconstruction of a two-dimensional object with internal defects from its projections, each projection of the solid object is interpreted as a path integral of the light-sensitive property of the object in the appropriate direction.
Abstract: Iterative tomographic algorithms have been applied to the reconstruction of a two-dimensional object with internal defects from its projections. Nine distinct algorithms with varying numbers of projections and projection angles have been considered. Each projection of the solid object is interpreted as a path integral of the light-sensitive property of the object in the appropriate direction. The integrals are evaluated numerically and are assumed to represent exact data. Errors in reconstruction are defined as the statistics of difference between original and reconstructed objects and are used to compare one algorithm with respect to another. The algorithms used in this work can be classified broadly into three groups, namely the additive algebraic reconstruction technique (ART), the multiplicative algebraic reconstruction technique (MART) and the maximization reconstruction technique (MRT). Additive ART shows a systematic convergence with respect to the number of projections and the value of the relaxation parameter. MART algorithms produce less error at convergence compared to additive ART but converge only at small values of the relaxation parameter. The MRT algorithm shows an intermediate performance when compared to ART and MART. An increasing noise level in the projection data increases the error in the reconstructed field. The maximum and RMS errors are highest in ART and lowest in MART for given projection data. Increasing noise levels in the projection data decrease the convergence rates. For all algorithms, a 20% noise level is seen as an upper limit, beyond which the reconstructed field is barely recognizable.
64 citations
Journal Article•
52 citations
TL;DR: In this article, a mixed numerical/experimental technique (MNET) is proposed for the characterization of the stiffness properties of fiber reinforced composite (FRC) materials, which allows the simultaneous determination of all the anisotropic properties and provides an estimation of their statistical distribution.
Abstract: The characterization of the stiffness properties of fibre reinforced composite (FRC) materials presents more difficulties than the characterization of traditional isotropic materials. This paper first describes the difficulties that can arise and next presents a nondestructive method that offers a possible solution to the problems. The proposed method is a so-called ‘Mixed Numerical/Experimental Technique’ (MNET). From an experimental point of view, the method requires the measurement of resonant frequencies of freely suspended rectangular test plates. From the numerical side, an accurate model of the test plate must be available. The stress field associated with the different resonant frequencies is multi-axial. The measurement procedure however is very simple and requires little specimen preparation. The equipment is inexpensive and the measurement procedure can be controlled by a PC program. The proposed MNET allows the simultaneous determination of all the anisotropical properties and provides an estimation of their statistical distribution.
TL;DR: In this article, an artificial neural network was used as an expert system to obtain details of defect size and depth from transient thermography data, trained on a comprehensive range of finite difference modelling results and provided a simple means of interpreting practical experimental measurements.
Abstract: It is shown that an artificial neural network can be used as an expert system to obtain details of defect size and depth from transient thermography data. The neural network was trained on a comprehensive range of finite difference modelling results and is shown to provide a simple means of interpreting practical experimental measurements. An assessment is given of the sensitivity of the method to likely uncertainties in experimental measurement.
TL;DR: In this article, a remanent magnetism method was used to localize fractures of single prestressing steel bars in postensioned tendons, and the extent of damage (number of broken steel bars) was evaluated.
Abstract: Sudden collapses of structural components due to fractures of the prestressing steel have occurred. The paper describes the successful application of a nondestructive testing method to identify defective structures. The remanent magnetism method makes it possible to localize fractures of single prestressing steel bars in postensioned tendons. The tendons need not be accessible at any point, not even at the jacking ends. The ferromagnetic properties of the prestressing steel are used in this process. Characteristic magnetic leakage fields are caused by fractures. The magnetic field, measured at the concrete surface along the projection of the tendon profile, allows conclusions concerning existing fractures. Also the extent of damage (number of broken steel bars) can be evaluated.
TL;DR: In this paper, the impact acoustics and vibration at the same surface of the concrete was observed experimentally and some basic experiments were carried out to clarify the influence of cracks on amplitude and resonant frequency of the impact acoustic.
Abstract: A new nondestructive testing method for concrete using impact acoustics is investigated. Impact acoustics, which has a strong relation with the vibration of a concrete surface, can offer important information about the physical properties of concrete structures such as shapes, material properties and defects. In this paper, the relation between impact acoustics and vibration at the same surface of the concrete was observed experimentally. Then, some basic experiments were carried out to clarify the influence of cracks on amplitude and resonant frequency of the impact acoustics. These experimental results were compared with the results obtained from FEM analysis. It was clarified that degradation of stiffness of concrete due to crack progress can be evaluated by the impact acoustics method.
TL;DR: In this paper, a method for optimizing a system using a cost matrix approach is described. But it is difficult to compare and the specificity of each application is generally not taken into account.
Abstract: There is an urgent need for automatic systems for quality control of flat steel products to meet the ever increasing demand from customers, and for process control to prevent damage to production. To achieve these goals, those inspection systems should detect and recognize defects using image processing and pattern recognition methods. Such complex systems are difficult to compare and the specificity of each application is generally not taken into account. This paper describes a method for optimizing a system using a cost matrix approach. It produces an image which can be used to appreciate the quality of the classifier visually. We have applied the method in order to optimize the feature selection and to compare the classification methods used in a system designed for recognizing defects in flat steel products in a cold rolling mill. The results show significant improvements in the performance of the recognition system.
Journal Article•
TL;DR: In this paper, the authors used magnetic hysteresis measurements to evaluate creep damage in power plant weldments and found that during high temperature creep there is a significant change in microstructure such as the formation of voids, dislocation networks and grain boundary cavities.
Abstract: Magnetic hysteresis measurements have been used to evaluate creep damage in power plant weldments. This method relies on the sensitivity of the magnetic properties of steels, such as coercivity, remanence and hysteresis loss, to microstructural changes occurring during creep. During high temperature creep there is a significant change in microstructure such as the formation of voids, dislocation networks and grain boundary cavities. The evolution of these defects during creep affects the magnetic properties by changing the impedance to magnetic domain wall motion and also by introducing internal demagnetizing fields associated with cavities. The present paper discusses results obtained from on-site inspection of creep damaged Cr-Mo steel welds at two thermal power plants. One of the objectives of this research was to establish whether there were any trends in the magnetic properties as a result of creep damage which could be used later as part of a more comprehensive screening procedure for monitoring the progress of creep damage.
TL;DR: In this article, a network analyzer of the electrical response between 300 kHz and 200 MHz for a range of model waveguide structures was used to calculate the dielectric permittivity.
Abstract: Analysis using a network analyser of the electrical response between 300 kHz and 200 MHz for a range of model waveguide structures is discussed. The structures chosen illustrate the variations to be expected in real adhesive bonds and include changes in bond line thickness, width and electrical conductivity of the substrate. A simple theoretical model was used to simulate the time domain data and very good agreement between experiment and theory was observed. Deficiencies in the model appeared when it was used to simulate frequency domain data, and the limitations of these calculations are critically discussed. The accuracy to which this method can be used to calculate the dielectric permittivity was assessed from a study of a number of polymers with well-defined dielectric characteristics. Parallel plate structures were fabricated using low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), polycarbonate (PC) and polymethyl methacrylate (PMMA). Good agreement between literature and experiment was observed in all cases, validating the dielectric permittivity values obtained using this approach. This paper illustrates the potential of nondestructive electrical measurements to provide structural and quantitative permittivity information on adhesive joint structures.
TL;DR: In this article, the authors evaluate the conductivity profile of a layered planar structure after inverting the impedance of a circular air-cored probe coil, of rectangular cross-section, using multilayer perceptron neural networks, trained via the back propagation learning algorithm.
Abstract: The inversion of eddy current probe impedance measurements is widely recognized as a complex theoretical problem whose solution is likely to have a significant impact on the characterization of materials. In this paper the evaluation of the conductivity profile of a layered planar structure is performed after inverting the impedance of a circular air-cored probe coil, of rectangular cross-section, using multilayer perceptron neural networks, trained via the back propagation learning algorithm. The merits of the method are illustrated in the light of two engineering examples.
TL;DR: In this article, the authors used infrared thermography in near ambient conditions to check the leakage point and its applicability is estimated by visualizing the temperature field generated around the point, which is not usually consistent with the surrounding temperature.
Abstract: It is important to check for leakage points of fluids on a surface to maintain and manage various structures satisfactorily. In particular, it is necessary that the leaking walls of a bank, dam, tunnel, pipeline and so on are diagnosed appropriately using some remote sensing technique and that they should be repaired immediately. Recently, infrared thermography for measuring temperature has become attractive in a wide variety of engineering applications. Infrared thermography in near ambient conditions is introduced to check the leakage point and its applicability is estimated by visualizing the temperature field generated around the point. The generated temperature around the leakage point is not usually consistent with the surrounding temperature. When using infrared thermography, we can detect the leakage point at the location where a marked temperature difference occurs.
Journal Article•
TL;DR: In this article, a technique that uses wavelet basis functions to provide selective invariant features and eliminate image intensity variations from undesirable changes in operational variables is described. But the performance of the invariance transformation is demonstrated by applying the method to magnetic flux leakage images obtained using a finite element simulation of in-line inspection of natural gas transmission pipelines.
Abstract: The magnetic flux leakage method, used for nondestructive evaluation of ferromagnetic objects, generates greyscale images that are representative of the integrity of the specimen. Defective areas typically appear as bright regions in the image. Unfortunately, the task of defect characterization becomes more challenging due to the effects of variations in the test parameters associated with the experiment. One such test parameter is the permeability of the test object. Conventional invariant pattern recognition algorithms are not capable of performing invariance transformations to compensate for such variations. This paper describes a novel technique that uses wavelet basis functions to provide selective invariant features and eliminate image intensity variations from undesirable changes in operational variables. The performance of the invariance transformation is demonstrated by applying the method to magnetic flux leakage images obtained using a finite element simulation of in-line inspection of natural gas transmission pipelines.
TL;DR: In this article, a method based on acoustic emission (AE) generated by fibre fractures during tensile test has been developed for the determination of the strength distribution of E-glass fibre strands containing as many as 4000 fibres (filaments).
Abstract: A method based on acoustic emission (AE) generated by fibre fractures during tensile test has been developed for the determination of the strength distribution of E-glass fibre strands containing as many as 4000 fibres (filaments). This technique is novel in that it allows fast data acquisition and provides accurate data for the load, location and time of fracture of each filament. A range of statistical data including the fracture strength and AE signal parameters can be extracted to characterise the strand of fibres under investigation. Software has been developed for the evaluation of the large number of collected data and for the graphical presentation of the AE signal parameters. The final test package thus allows automatic testing and provides the analyst with an overall picture of the real strength distribution of the fibres in a strand. The test and analysis for 4000 fibres is completed within 75 min. Complete strand tests of 4000 filaments using this method have revealed greater detail of the characteristic flaw distribution in a strand that was not possible with tests conducted on single filaments or bundles containing small numbers of filaments (see Ref. [1] ). The new study has provided concrete evidence of the existence of a bimodal flaw population rather than the unimodal population that has been previously reported.
TL;DR: In this article, an electromagnetic inversion algorithm for ground probing radar data to include information about the dipole moment of the antenna was proposed, and a new elastodynamic inversion scheme was even capable of processing the complete pressure and shear wave information as contained in the components of the displacement vector simultaneously, where they compute the necessary synthetic data with the numerical EFIT code (Elastodynamic Finite Integration Technique) as extended to statistically inhomogeneous media in order to validate the algorithm.
Abstract: Electromagnetic and elastodynamic scattering and inverse scattering techniques are combined to provide a better understanding of wave propagation and obstacle imaging in concrete. In particular we propose an electromagnetic inversion algorithm for ground probing radar data to include information about the dipole moment of the antenna; a new elastodynamic inversion scheme is even capable of processing the complete pressure and shear wave information as contained in the components of the displacement vector simultaneously, where we compute the necessary synthetic data with the numerical EFIT code (Elastodynamic Finite Integration Technique) as extended to statistically inhomogeneous media in order to validate the inversion algorithm.
TL;DR: In this article, a new method is proposed as an alternative to analysing the waveform: it is based on the analysis of locus of the vector consisting of tangential and normal components of the magnetic flux leakage signals.
Abstract: This paper clarifies the relation between the magnetic flux leakage signals caused by two adjacent parallel slots on a metal surface and the distance between the two. The depth of smaller slot can be evaluated by considering an effect that the amplitude of leakage flux decreases as the depth ratio of two slots and the slot-to-slot distance increase. A new method is proposed as an alternative to analysing the waveform: it is based on the analysis of locus of the vector consisting of tangential and normal components of the magnetic flux leakage signals.
TL;DR: In this paper, it was shown that methyl chloroform is still more suitable than HCFC-123 especially for application to shallow discontinuities for liquid penetrant tests, where the rise capability factor is important.
Abstract: Methyl chloroform is an important part of the special penetrant process with many desirable characteristics, but it is also a ‘severe ozone depletor’. It is still used for some advanced and special purposes which are scheduled for elimination in 2002 under the Montreal Protocol. For the replacement of methyl chloroform, the first potentially ozone-friendly chemical is HCFC-123. Here, rise capability factor is important for the applications of liquid penetrant tests, but it seems that methyl chloroform is still more suitable than HCFC-123 especially for application to shallow discontinuities.
Journal Article•
TL;DR: In this paper, an automatic moisture measuring and monitoring system has been employed to investigate different parts of road structures using the time domain reflectometry (TDR) method, and comprises a pulse generator/analyser, a coaxial transmission line, and specially designed built-in probes.
Abstract: An automatic moisture measuring and monitoring system has been employed to investigate different parts of road structures. The system uses the time domain reflectometry (TDR) method, and comprises a pulse generator/analyser, a coaxial transmission line, and specially designed built-in probes. It is able to monitor volumetric moisture content and bulk electrical conductivity of various road building materials in situ. In this paper the TDR system and the probes are described, as well as measurements with the system in different parts of the road structure. A base layer of crushed concrete and a sub-base layer of course sand in a primary road were subject to in situ experiments, of monitoring the moisture content at several spots.
Journal Article•
TL;DR: In this article, composite piezoelectric sensors were used to monitor strain and acoustic emission levels during tensile testing, and the piezelectric coefficient and dielectric constant as well as the electromechanical coupling factor of the composite films were determined.
Abstract: Polymer/ceramic composite piezoelectric materials with 0-3 connectivity have been produced by dispersing ceramic powder of calcium modified lead titanate into the matrix of three different host polymers. The piezoelectric coefficient and the dielectric constant as well as the electromechanical coupling factor of the composite films are determined. Two piezoelectric transducers have been constructed using these films. The composite piezoelectric sensors were used to monitor strain and acoustic emission levels during tensile testing.
TL;DR: The quantitative measurement of the positions and diameters of steel reinforcements in buildings is based on the concept of non-linear backprojection (NLBP) and the special advantage is the reconstruction of direction-selective structures.
Abstract: The quantitave measurement of the positions and diameters of steel reinforcements in buildings is based on the concept of non-linear backprojection (NLBP). Advantages are 3D-image reconstructions from a small number of views, fast numeric algorithms and the consideration of the statistical, spectral and other characteristics of the object. For the radiographic inspection a 60Co source was used and both X-ray films and imaging plates for detection. The imaging plates were combined with special filter screens for contrast optimization. The digitized images from the plates have a high dynamic range and can be read into a computer without a photographic developing process. A numerical preprocessing procedure was developed as space filtering in different directions followed by a combination of the image angle components. A generalized Hough or Radon transformation and a statistical evaluation of the resulting image was employed. The special advantage of this method is the reconstruction of direction-selective structures.