Showing papers in "Ndt & E International in 1999"

Modeling elastic wave propagation in waveguides with the finite element method

Abstract: This paper reports on the application of guided waves techniques to nondestructively determine the structural integrity of engineering components. Specifically, this research uses a commercial finite element (FE) code to study the propagation characteristics of ultrasonic waves in annular structures. In order to demonstrate the accuracy of the proposed FE technique, the propagation of guided waves in a flat plate is examined first. Next, the propagation of guided waves in thick ring structures is investigated. Finally, these FE results are compared to analytical and experimental results. The results of this study clearly illustrate the effectiveness of using the FE method to model guided wave propagation problems and demonstrate the potential of the FE method for problems when an analytical solution is not possible because of “complicated” component geometry.

An SH-wave EMAT technique for gas pipeline inspection

Abstract: An EMAT technique has been developed to detect corrosion defects on the outer surfaces of steel pipelines. A periodic-permanent-magnet (PPM) EMAT is placed inside the pipe, which generates and receives the SH guided waves traveling in the circumferential direction with the axial polarization. Wheels installed on the PPM-EMAT facilitate the linear movement in the axial direction and are useful for a quick inspection. At each point in scanning, the amplitude and phase shift of the round-trip signals in the SH0 and SH1 modes are measured using a superheterodyne phase-sensitive detector. They respond uniquely to surface defects and provide relevant information for locating axial positions of defects and evaluating the depth. The amplitude and phase shift of the SH1 mode are more sensitive to the presence of the defects than those of the SH0 mode. This technique is useful even if the protective resin coating is tightly glued on the pipes and lowered the signal intensity.

Modal analysis of acoustic emission signals from CFRP laminates

Abstract: As a result of its continuous and in situ detection capabilities, the acoustic emission (AE) technique is the prime candidate for damage monitoring in loaded composite structures. None of the AE analysis techniques used in laboratory studies has, however, proven to be capable of consistently dealing with the difficulties encountered in larger structures: large amount of data, the elimination of noise sources and the influence of wave propagation effects (attenuation, dispersion). This work will use the modal acoustic emission (MAE) technique as a more intelligent and efficient way of analysing AE results. AE waveforms obtained during tensile and bending testing of CFRP laminates will be presented. It will be demonstrated how taking into account the modal nature of AE waves can in future lead to more quantitative and accurate results.

A reference standard for the development of acoustic emission pipeline leak detection techniques

Abstract: A reference standard was constructed for setting up and evaluating AE equipment to be used in pipeline leak detection. The reference standard comprises a short length of 2-inch diameter piping with facilities for introducing several kinds of controlled leaks. The reference standard proved very valuable not only for checking out equipment, but also for characterizing source mechanisms as part of an integrated approach to quantitative AE leak detection/location technology. The effects of pressure and air injection were measured for thread leaks on the order of 0.1 gal h −1 , a leakage rate that is important in the context of environmental protection regulations. Taking this knowledge to the field, a thread leak of only 0.014 gal h −1 was successfully detected and located by injecting nitrogen into the line at 25 psi. This leak was located with 1 foot accuracy, using two different location techniques and 25-foot sensor spacing. It is envisioned that in the future, AE inspectors in the field will make systematic use of several two-phase flow processes and soil enhancement mechanisms that are being characterized by means of this new reference standard.

NDT data fusion at pixel level

Abstract: No single non-destructive testing (NDT) technique or method is satisfactory to fully assess the structural integrity of a material. Indeed, each method presents some limitations in terms of defect detection and characterisation. Additionally, poor signal-to-noise ratio may make signal interpretation complex or unreliable. For these reasons, the concept of NDT data fusion — based on the synergistic use of information from multiple sources in order to facilitate signal interpretation and increase defect detection and characterisation — is expanding rapidly. A carbon fibre reinforced composite material previously subject to low energy impact damage has been non-destructively inspected using eddy current and infrared thermography. Following these non-destructive examinations, the information gathered from these two NDT methods has been fused at pixel level. Several well established and recognised data fusion processes based on statistical and probabilistic algorithms were used to combine eddy current and infrared thermographic data. The results obtained with these processes are presented in this paper and their efficiency discussed. It is shown that, in certain cases, NDT data fusion at pixel level may be adequate to increase knowledge about defect location and characterisation and to reduce ambiguity.

NDE of degradation of thermal barrier coating by means of impedance spectroscopy

Abstract: Nondestructive evaluation of thermal barrier coating (TBC) degradation during service operation has received a wide attention for service life prediction of advanced gas turbines. In this work, TBC on nickel base superalloy degraded at various degrees by thermal aging is investigated. Particularly, TBC and Metal–Chromium–Aluminum–Yttrium alloy (McrAlY) (M indicates iron (Fe), cobalt (Co), nickel (Ni) or these combinations) bond coating interface where a detrimental reaction takes place and forms various reaction products during thermal aging, is studied in detail. Formation kinetics and physical properties of the reaction layer are evaluated by means of Impedance Spectroscopy (IS). Specimens aged at higher temperature and a longer aging time shows larger impedance. The impedance behavior of TBC and bond coating is found to be markedly changed by the formation of the reaction layer. Physical properties and thickness of the reaction layer are evaluated using the change of the impedance. A mechanistic interpretation of the cause of change in the physical properties and IS behavior are described.

Radar measurement of bridge scour

Abstract: Scour around bridge piers and abutments has resulted in many structural failures over the past decade and considerable research has been carried out to develop methods which can be used to evaluate the risk of scour affecting the integrity of these major structures. Traditional site investigation methods based on borehole core and samples of the riverbed sediments are expensive and time-consuming and may not always give a complete assessment of the lithological variation in the riverbed sediments. Geophysical methods can be used to determine the riverbed profile beneath the water in a river and may also be of value for obtaining the previous scour history below the riverbed level. Trials of ground penetrating radar (GPR) have indicated that this geophysical method is particularly effective in determining the sub-bottom geological structure in a shallow freshwater environment. In this presentation the results from a number of scour surveys using GPR are presented and discussed. It is concluded that GPR surveys can be effective in determining both the water depth and sub-bottom geological structure near bridge piers and abutments provided that the correct instrumentation and operational procedures are adopted.

Rapid prediction of eddy current testing signals using A−φ method and database

Abstract: This paper describes a new scheme for the fast evaluation of eddy current testing (ECT) signals by using databases and formulation of the A−φ method. As the calculation of flaw signals is localized around the flaw region, substantial computational work can be reduced comparing with the conventional FEM–BEM code even for a conductor with complex geometry. Unlike the approach proposed by Badics et al. (Rapid flaw reconstruction scheme for 3-d inverse in eddy current NDE. In Studies in Applied Electromagnetics and Mechanics, Vol. 12, eds T. Takagi et al., IOS Press, 1997, pp. 303–309), the new scheme can predict ECT signals due to a large and complex-shaped crack rapidly because of its higher order interpolation and the newly introduced special element, say, an element with different media. The efficiency of the new method is verified by comparing its numerical results with the measured impedance for several bench-mark problems. The fast and accurate features make this new forward approach especially feasible in the reconstruction of flaw shapes by combining with the conventional optimization method.

Non-contact estimation of thickness and elastic properties of metallic foils by laser-generated Lamb waves

Abstract: Non-contact estimation of the thickness and elastic properties of metallic foils was attempted by quantitative analysis of velocity dispersion of laser-generated Lamb waves. Lamb waves were generated in stainless steel (AISI304) foils with a thickness of less than 40 μm by a Q-switched Nd:YAG laser. Both the zeroth order symmetric S0 and anti-symmetric A0 waves were monitored using a heterodyne-type laser interferometer. Dispersion of group velocity of the A0 mode was obtained by the wavelet transformation, and was found to agree well with the numerical solution of the Rayleigh–Lamb equation. A modified method to estimate both the thickness and acoustic (or elastic) properties from the sheet wave velocity and the group velocity dispersion of the A0 mode was proposed. The modified method was found to provide a correct estimate for stainless steel foils thinner than 30 μm.

Ultrasonic evaluation of stresses in orthotropic materials using Rayleigh waves

Abstract: The characterization of stress states in materials is often necessary in mechanical construction. The realization of aeronautical structures increasingly needs knowledge concerning the initial states of the internal stresses, because they induce bending or twisting deformations on the pieces during the tooling. Several non-destructive methods for the determination of stresses have been developed. As the material being characterized here is an aluminum alloy in which the size of grains is very important (order of 150 μm) and, moreover, that the control must be performed on-line, the ultrasonic techniques seem to be more convenient. In this paper, we present a method based on the measurement of ultrasonic Rayleigh wave velocity variations versus the stress state in the thickness of sheets. We show some residual stress profiles obtained by this method. These are then compared with other profiles determined using a destructive technique that makes it possible to check our results. Finally, we discuss various factors that could affect the acoustic measurement of stress profiles in sheets with our method.

Ultrasonic testing of adhesive bonds of thin metal sheets

Abstract: This article discusses the use of pulse-echo ultrasonic testing for the inspection of adhesive bonds between thin metal sheets (0.8 mm). The method is based on the measurement of the reflection coefficient at the metal/adhesive interface. After describing briefly the physical aspects of the phenomenon, an index is defined to detect defective zones of the joint (both for the lack of adhesive and for insufficient adhesion); the influence of the experimental variables (transducer frequency, coupling medium and contact force) on the measurement is discussed. The analysis shows that coupling medium and transducer frequency do not influence the results and that the method is robust with respect to the variation of contact force. By means of a control experiment it is shown that the statistical distribution of the index corresponding to good and defective adhesion zones are sufficiently separate to be distinguished. Finally, a procedure based on the statistical theory of decisions is proposed to evaluate the integrity of the joint under test.

On the NDT and E for the diagnosis of defects using infrared thermography

Abstract: The increasingly recognized applicability of infrared thermography has caused developments of remote-sensing diagnoses for various engineering applications. A significant advantage of this technique is that we can diagnose invisible defects nondestructively and safely. For maintaining and managing various structures satisfactorily, it is very important to detect many kinds of invisible defects such as separation, cavity, inclusion and so on. Although the infrared thermography for NDT and E has therefore been examined extensively, few fundamental investigations have addressed the numerical computation to evaluate the detection mechanism and the quantitative limit under various conditions. It is important to discuss them theoretically with the aid of the appropriate numerical computation. In this paper, therefore, we certify them using a series of fundamental numerical computation with the aid of the concerned experimental investigation using the infrared thermography. From the numerical and experimental investigations, the effects of defect's depth and size on the detection are elucidated. In addition to the fact, it is obvious that the applicability of the present NDT and E depends on a heating condition and a relative difference of thermophysical property between the defect and its surrounding.

Imaging attributes of railway track formation and ballast using ground probing radar

Abstract: Trial pitting and portable ballast samplers currently provide one means of assessing and monitoring the composition of railway track formations. A technique has been sought in which the lateral continuity of properties observed in these pits and cores can be assessed quickly with a view to characterising sections of the formation or ballast. The technique that is being developed by IMC Geophysics Ltd., in association with Scott Wilson Pavement Engineering, uses Ground Probing Radar (GPR) technology to image attributes of the formation and ballast. Trials have demonstrated that significant horizons can be imaged and that character changes along those horizons can be correlated with quality or structural variations. At sites where the ballast is relatively clean the interface between ballast and subgrade has been clearly imaged. In particular, changes in the quality of the image have correlated well with marked changes observed in trial pits, and with significant changes in track geometry records. From a combination of trial pits, cores, track geometry records and GPR it should be possible to improve the economics of track maintenance.

C-scan and L-scan generated images of the concrete/GFRP composite interface

Abstract: Glass Fiber Reinforced Plastics (GFRP) are fast becoming a viable new construction material. They are being used for rehabilitating old concrete structures—columns, beams, slabs and walls—by gluing the composite plate to the concrete surface. Proper attachment between the GFRP plate and the concrete surface is necessary for efficient use of GFRP composites to increase the strength of the rehabilitated structures. Delamination between the rehabilitated structure and the GFRP plate significantly reduces the strength of the reinforced structure. Hence, delamination defects, if present at the interface, should be detected as soon as possible. In this paper, two ultrasonic techniques to detect delamination defects between the GFRP plate and the concrete surface are presented. One technique uses longitudinal waves or P-waves and the second technique uses Lamb waves. It is found that both techniques can detect the defect when used properly. However, Lamb waves give a better image quality of the defect than longitudinal waves.

The application of time domain ground penetrating radar to evaluate railway track ballast

Abstract: This paper will evaluate Ground Penetrating Radar (GPR) as a Non Destructive Test (NDT) technique for the assessment of railway track bed ballast. Ballast and tracked deterioration is responsible for the majority of differential settlement of tracks, resulting in poor track geometry. Traditionally ballast is only replaced when it has visually deteriorated, resulting in the planning and programming of maintenance and renewals currently being condition driven. There is a significant potential cost saving if ballast deterioration can be detected earlier. GPR can survey large lengths of track ballast in a relatively short space of time. This paper will show how GPR can be used to quickly identify the degree of track bed ballast deterioration and detect the ballast/formation interface.

Determination of the group and phase velocities from time–frequency representation of Wigner–Ville

Abstract: The experimental measurement of the group and phase velocities of some circumferential waves propagating around a thin elastic tube is a still complex operation. In this study, we show that the dispersion velocity can be determined from a time–frequency representation. We use the Wigner–Ville method by virtue of its interesting properties. On some time–frequency images, the symmetric (S0) and antisymmetric (A1) circumferential waves are identified. The group velocity dispersion estimated from these images is compared with that computed by the proper mode theory method. A good agreement is obtained. The phase velocity is also determined from the group velocity.

Acousto-ultrasonic evaluation of adhesively bonded CFRP-aluminum joints

Abstract: Correlation between the amount of artificial defects in bonded region and the acousto-ultrasonic parameters (AUPs) including the signal amplitude in adhesively bonded joints of carbon-fiber reinforced plastic (CFRP) laminates and Al6061 plates was investigated The effect of the frequency content and the bandwidth of input signals were studied using 200 kHz, 650 kHz, 1 MHz, 2 MHz pulses and 1 MHz tone-burst signals With increasing fraction of defects, the signal amplitude and AUP1 were decreased but AUP2 was increased This result was attributed to the energy transfer characteristics of bonded joints with delamination-type defects and the change of spectral content as a result of the defects Considering the highly attenuative nature of composite laminates, a pulse signal with major frequency content at or around the third harmonic of thickness mode resonance, 600∽700 kHz for the dimension of specimens used in this study, was found optimal for acousto-ultrasonic testing of CFRP-aluminum joints

3D ray tracing in austenite materials

Abstract: In general, ultrasonic sound wave propagation through austerlitic (stainless steel) weld material cannot be predicted because of the anisotropic and inhomogeneous structure of the weld. To support NDT inspections, a ray tracing algorithm was developed to follow longitudinal, horizontal and vertical polarized shear wave propagation from the base material through the cladding in three dimensions. The algorithm was implemented in the `3D-Ray-SAFT' software package that allows experimental modelling, to visualize the rays in three-dimensions and to include reflections from arbitrarily placed flaws. The presented algorithm results that were experimentally validated are compared to similar, existing algorithms.

Neural network solution of the inverse vibration problem

Abstract: The feasibility of using general regression neural networks (GRNN) to solve the inverse vibration problem of cracked structures was investigated. The case study used in the investigation was a steel cantilever beam with a single edge crack. The first six natural frequencies were used as network inputs, and crack size and crack location were the outputs. The effect of the number of frequency inputs to the network on prediction accuracy was quantitatively evaluated. The results show that GRNN is a powerful instrument for predicting crack size and location over a wide range, and that the prediction accuracy increases with larger number of vibration modes.

Geophysical imaging inside masonry structures

Abstract: Geophysical techniques offer the capability to non-invasively investigate engineering structures, both in terms of their spatial variability, and the prediction of properties of interest to the engineer from the ‘pixel’ values of the geophysical tomograms. The combined use of different geophysical methods provides a means of identifying the signature of salient engineering conditions. The response of seismic, radar and electrical resistivity tomography to simple targets within masonry structures is demonstrated. The benefits of combining rapid standard radar surveys with more detailed tomography is discussed and strategies for investigating typical structures developed. The response of the three methods, acoustic, electrical and radar to changes in the condition of masonry, such as reduction in strength and ingress of water, are discussed, with particular reference to a case study utilising the Ribblehead viaduct as an example. The corresponding responses to changes in ground conditions are also presented and shown to be important in understanding possible further applications of these methods, such as investigating behind retaining walls.

Hydrogen damage detection by ultrasonic spectral analysis

Abstract: The non-destructive testing technique known as ultrasonic spectral analysis was applied to steel samples for the detection of minute cracks produced by hydrogen in a H2S environment. The cracks were characterized and quantified by metallographic analysis and compared with the ultrasonic results. The backwall echoes and the backscattered signals were analyzed in the frequency domain and the characteristics of these spectra were evaluated by second order moments, which emphasizes the higher frequency components. The second order moments showed a greater variability for hydrogen attacked materials than for non-attacked ones, denoting good sensitivity to this type of material degradation.

Power spectrum analysis—modern tool in the study of rail surface corrugations

Abstract: A method adopting the unsprung-mass acceleration signals is one of the essential techniques for the investigation of rail surface irregularities. This paper presents results of the spectral analysis of unsprung-mass acceleration signals for selected tracks of the Polish State Railways network. In particular, the following spectral characteristics of the rail surface irregularities have been determined: variance, spectral power in selected bands (in wave number or in wavelength domains), spectral moments of various orders and spectral width parameters. These characteristics have been determined for the track sections with various rolling surface conditions. The resulting standard deviations of vertical accelerations varied from 7.5 m/s 2 , for the track without corrugations, to 122.8 m/s 2 , for track section with corrugated rails, while the average wavelengths were in the range of 0.055–0.080 m.

A projection method for reconstructing X-ray images of arbitrary cross-section

Abstract: This paper describes a new method for reconstructing cross-sectional images of an object at arbitrary height and arbitrary angled planes. This method, called the geometric projection method, can produce cross-sectional images for arbitrary planes using only those images acquired from the focal plane of an object. This paper develops the mathematical formulae for the image reconstruction of arbitrary height and arbitrary angled planes. Then, by means of the derived formulae, the cross-sectional images of arbitrary planes are realized via computer simulations. Finally, by using experimental results conducted on the solder joints of a ball grid array package, it is shown that cross-sectional images for arbitrary planes of the object can be reconstructed correctly by the geometric projection method.

Towards automated interpretation of ultrasonic NDT data

Abstract: Expert systems have demonstrated potential for automated interpretation of ultrasonic NDT data, but rules do not provide the best solutions for all interpretation problems. In this work, 3D images are reconstructed from raw A-Scans, using CAD models of test specimen geometry to correct positions and remove geometric echoes. A knowledge model for ultrasonic NDT has been implemented in a working expert system, with uncertainty managed using fuzzy logic. A suite of data processing tools is available both to the expert system, and also through an interactive 3D visualization system, which therefore provides a development environment for automated interpretation, as well as being powerful in its own right.

Microwave nondestructive detection of delamination in IC packages utilizing open-ended coaxial line sensor

Abstract: A new method of microwave nondestructive testing which utilizes an open-ended coaxial line sensor is developed in an attempt to increase the spatial resolution. With the aid of this technique, the delamination in IC packages is inspected. An open-ended coaxial line sensor with inner and outer conductors in smaller dimension than the wavelength is used to incident and receive the test signal that interacts with the detected objects. The magnitude of effective reflection coefficient, which is proportional to the total reflection from different interfaces, is measured as a characteristic signal to distinguish the delamination. A phenomenon of magnitude coherent resonance is observed in detail, by which the measurement sensitivity is enhanced significantly. Four IC packages were used as samples, and the measurement results indicate that the microwave technique using an open-ended coaxial line sensor has a bright prospect to evaluate the delamination in IC packages nondestructively.

Diagnostic method for vehicle brakes

Abstract: The heat generation process during the work of brake mechanism, and also the change of its temperature in the early period of work is considered. Theoretical and experimental validation is provided for thermal diagnosis of vehicle brakes in a wheeled tractor as an example. The diagnostic parameter is adopted as the maximum rate of temperature increases in the drum brake under steady-state conditions. The braking moment is used as a structural parameter.

Laser infrared photothermal radiometric depth profilometry of steels and its potential in rail track evaluation

Abstract: Laser Infrared Photothermal Radiometry has been utilized for several thermal-wave inverse-problem NDE applications. These include depth profilometry of steels and rails and scanning tomography of sub-surface defects in steels. Further, a computational algorithm has been refined to address the ill-posedness of the thermal-wave inverse problem. As a result, depth profiles of case-hardened steels, railway track heads from the field, and machined sub-surface hole thermal-diffusivity images have been reconstructed successfully using this emerging NDT technology.

Application of the acoustic resonance method to evaluate the grain size of low carbon steels

Abstract: The velocities and attenuations of ultrasonic waves in low carbon steels were measured precisely and automatically by the acoustic resonance method using a Lorentz-type EMAT. The attenuations were measured from the decay of a signal from the resonant vibration after stopping the external excitation at the predetermined resonant frequency, while the velocities were calculated directly from the resonant frequencies and material thicknesses. It was not possible to predict the yield strength from the velocity measurements. There was a close relationship between grain size and attenuation in the specimens which consist of ferrite and pearlite. The attenuations measured at a frequency of about 5 MHz showed a good correlation with the average grain size and yield strength. The yield strength could be evaluated within the accuracy of ±50 MPa by the acoustic resonance method. The results would be used for on-line evaluation of the grain size and the mechanical strength of steels in industry.

Adapting an ultrasonic image threshold method to eddy current images and defining a validation domain of the thresholding method

Abstract: This paper presents a generalization to eddy current images of a thresholding method developed for ultrasonic imaging. It is also shown how to improve the thresholding strength through the definition of a cooperation rule between two threshold assessment measures. This rule is also used to reduce time of calculus. These measures are calculated from the image cooccurrence matrix. The aim is to build a thresholding method that gives a threshold value but also an indicator which allows the operator to know if the methodology applies to the processed image. Examples of results are presented.

Evaluation of the structural integrity of timber bridges

Abstract: Timber bridges are used on all types of rail lines including important main transcontinental rail lines. A combined structural dynamic and ultrasonic inspection approach is proposed for testing these bridges. This paper presents results of preliminary field tests. Dynamic excitation of the system is used to identify suspect elements in the bridge. Ultrasonic inspection is used to evaluate the suspect elements identified from global testing. The potential for on-site evaluation based on two different dynamic excitations is investigated. Testing of a bridge member using dry coupled ultrasonics is shown to demonstrate this technique on aged timber.