Showing papers in "Ndt & E International in 1998"

Imaging of concrete structures

Abstract: Demand for the development of non-destructive testing (NDT) techniques for concrete structures has increased with the growing concern about the deteriorating condition of the World's infrastructure. Efficient and accurate imaging techniques are needed for a reliable evaluation of safety and serviceability of concrete structures. Although, presently, imaging is routinely used in various fields, implementation of these technologies in NDT of civil engineering systems, especially of concrete structures, offers many challenges and requires additional development due to the composite nature of the concrete material and the complexities of reinforced or prestressed concrete systems. This paper presents the basic principles of various imaging techniques associated with several NDT methods applicable to concrete structures. The techniques considered are radiography, radioactive computerized tomography, infrared thermography, radar imaging and acoustic imaging. Special considerations regarding the applicability and accuracy of these techniques for the condition assessment of concrete structures are discussed, and examples of imaging applications are given.

Characterization of surface cracks with Rayleigh waves: a numerical model

Abstract: In the non-destructive testing of concrete structures, the use of Rayleigh waves shows some advantages to characterise surface cracks: easiness of excitation and recording, access to only one surface of the structure required, great spectral sensitivity to the propagation medium…. But the behaviour of Rayleigh waves on surface defects in concrete is difficult to perceive in the field, even if the dependence of the diffraction pattern on the crack's geometrical features is significant. A numerical model is adapted from earth physics in order to better understand the influence of the crack geometry on Rayleigh-wave propagation. This model, based on an indirect boundary element method, calculates the three-dimensional seismic response of two-dimensional structures. Synthetic seismograms are obtained for the propagation of a Rayleigh wave across various crack geometries. The variations of spectral ratios between the transmitted and incident waves are studied as a function of the crack depth. They are used to design an efficient procedure for the determination of crack depths.

Measurement of residual stress in steel using the magnetic Barkhausen noise technique

Abstract: Surface residual stresses on a structural beam steel sample were evaluated using a non-destructive technique based on the measurement of surface magnetic Barkhausen noise (MBN). MBN measurements were performed using a high resolution probe consisting of a small magnetic read head mounted between the poles of a ferrite U-core magnet. Applied stress magnitudes were correlated to MBN energy levels for calibration purposes. MBN energy measurements were done at various locations on the steel sample. The magnitude of the residual stress component along the sample beam axis was evaluated across the width of the beam using these calibration curves. The range of sensitivity of the MBN signal to stress as defined by the calibration curves was limited by the direction of easy magnetization. The effect of stress on the MBN signal was interpreted in terms of the active 180° domain wall population. The validity of the residual stress results was confirmed experimentally using conventional methods: cutting and sectioning and hole drilling. Possible sources of residual stresses and their distribution on the structural steel specimen are discussed.

Probabilistic models for condition assessment of oil and gas pipelines

Abstract: The paper is primarily concerned with the interpretation of in-line inspection data collected using magnetic flux leakage tools to characterize the actual condition of pipelines vulnerable to metal loss corrosion. The paper presents a probabilistic analysis framework to estimate the pipeline reliability incorporating the impact of inspection and repair activities planned over the service life. The framework is applied to determine the optimal inspection interval and the repair strategy that would satisfy a target reliability requirement. To update the pipeline failure probability after maintenance, a practical approximation is developed and validated using Monte Carlo simulation results.

An automated radiographic NDT system for weld inspection: Part II—Flaw detection

Abstract: An automated radiographic NDT system for weld inspection has been developed. The entire system consists of two major components: weld extraction and flaw detection. Part I of the system implemented a weld extraction methodology, which has been presented in a previous paper. This paper presents Part II of the system dealing with the detection of welding flaws. The flaw detection methodology is developed based on the fitted line profiles of a weld image and consists of four modules: preprocessing, curve fitting, profile-anomaly detection, and postprocessing. The successful detection rate and false alarm rate of the methodology are reported based on the test results of 24 representative welds.

Damage characterization of carbon/carbon laminates using neural network techniques on AE signals

Abstract: Carbon/Carbon (C/C) composites have a large number of microcracks in matrix-matrix or fiber-matrix regions resulting from the thermal processes during manufacturing. Although not harmful to the overall structural integrity, such a network of microcracks creates a noisy background from early load application which ‘covers’ the onset of critical failure mechanisms. Conventional AE analysis based on a sudden activity increase observed in the cumulative events versus load plot, or amplitude distribution histograms, provides limited information on this type of AE activity. Instead, multivariate techniques of unsupervised pattern recognition, taking into account a large number of AE signal descriptors, are proven useful for the clustering of similar AE events. AE results from a systematic fracture mechanics study of 2D woven C/C laminates are analyzed in this paper. Artificial Neural System (ANS) methods are employed for the clustering of similar AE signals, enabling a phenomenological correlation with the actual failure modes. The numerical procedure introduces a modified Learning Vector Quantization (LVQ) technique which was proved fast and suitable for the type of AE data emitted by composites. Fractographic evidence from failed tensile coupons corroborates the predictions of the numerical method in recognizing different failure mechanisms. Cumulative event charts of the various classes versus load demonstrate the criticality of each class on final coupon failure, and may lead to the definition of reliable criteria for the evaluation of remaining strength or life.

NDE of masonry structures: use of radar tests for the characterisation of stone masonries

Abstract: Ground penetrating radar is being used as a survey tool to study stone and brick masonries. The aim of the research is to produce guidelines for a technical procedure for the acquisition and interpretation of radar traces to detect inclusions, voids and other defects, the effectiveness of repair, the characteristics of leaves in multiple-leaf walls and to find the presence of moisture in a stone masonry wall. This paper shows first results of in situ tests carried out on historic buildings.

Estimation of impact-induced damage in CFRP laminates through ultrasonic imaging

Abstract: Ultrasonic C-scans are used frequently to detect defects in composite components caused during fabrication and damage resulting from service conditions. Evaluation and characterization of defects and damage require experience and good understanding of the composites. Under low-velocity impact loading delamination is observed to be a major failure mode. In this paper, the results of extensive experimental work on damage of carbon fiber reinforced plastic (CFRP) laminates due to low-velocity impact are presented. The study was carried out on 10 different layups over four different thickness. Both drop-weight and projectile impacts simulating practical conditions like tool-drop and runway debris, respectively, were carried out at energy levels varying from 3 to 30 J. The ensuing delamination damage was determined by ultrasonic C-scans using the pulse-echo immersion method for both projected and layerwise distribution. While projected delamination was obtained by placing a gate over the backwall echo, layerwise distribution was obtained by successive time delay from the frontwall to the backwall echo covering each interface. Delamination areas were quantified accurately by processing the raw image data using a digital image processing technique. Based on the data obtained an empirical relation was established between the delamination area and the impact energy.

The detection and measurement of impact damage in thick carbon fibre reinforced laminates by transient thermography

Abstract: Transient thermography (TT) images of impact damaged 3.44, 8.66 and 13.76 mm thick composites, from the impact face and rear face, are compared with the corresponding ultrasonic c-scan (UCS) images. Comparisons of apparent delamination area and size were made using novel image analysis techniques. The defect image sizes were also compared with crack lengths obtained from a selected number of specimens that were sectioned. For all composite thicknesses, the UCS damage area was proportional to the impact energy. The damage area obtained by TT from the front faces of the specimens did not correlate to the UCS damage areas although there was a correlation for the rear face damage on 3.44 and 6.88 mm thick specimens. Crack lengths obtained from sectioning showed that UCS damage length was equal to the maximum crack length and back and front sub surface cracks corresponded to back and front TT images.

Failure mechanisms analysis of 2D carbon/carbon using acoustic emission monitoring

Abstract: The present work aims toward the application of an innovative methodology for the analysis of acoustic emission (AE) activity monitored during the quasistatic tensile loading of centre-hole carbon/carbon. An in-house developed algorithm is proposed, which utilises the results of unsupervised pattern recognition classification of AE data. Correlation between clusters and specific material failure modes was achieved, using algorithm results and cluster activation parameters. During the analysis a dependence of AE activity on the hole diameter was observed.

NDT of polymer materials using lock-in thermography with water-coupled ultrasonic excitation

Abstract: We present an NDT method that uses lock-in thermography with ultrasonic heating. The acoustical damping causes the mechanical energy to be converted to thermal energy. In the defective regions the damping is stronger resulting in higher temperature generation. Thus the heating is selective to defects and gives good defect detectability. Because defects also affect the heat conduction, the method gives combined information of the change of the mechanical and thermal properties of the sample due to defects. The ultrasonic excitation is amplitude modulated and the magnitude and the phase of the temperature are measured by infrared camera and software lock-in detection. The applicability of the method was proved with several polymer and composite samples having delaminations, impact damages, voids, and inclusions.

LIBS for non-destructive testing of element distributions on surfaces

Abstract: A laser-based technique for the quasi non-destructive evaluation of surfaces of building materials is presented. Laser-induced breakdown spectroscopy (LIBS) is an analytical tool to determine the chemical composition of laser evaporated material. After the material has been vaporized by the high energy laser pulse the elements emit their specific fluorescence. This radiation is used to identify the elements in the evaporated material by spectroscopic methods. The mass of the evaporated material is of the order of micrograms; this means LIBS is a quasi non-destructive method for building applications. The main advantages of LIBS are the direct measurement without any preparation and the possibility to measure on site and to obtain on-line results. LIBS is a well-known technique which has been successfully applied in the areas of material testing, medicine and industrial process control. This article reports on the progress of LIBS applications for building materials by the use of some examples: identification and analysis of the main components of concrete; determination of salt concentrations in building materials; and measuring the relative element composition to a depth of up to 10 mm with high geometrical resolution.

Investigation of dielectric properties of brick materials as a function of moisture and salt content using a microwave impulse technique at very high frequencies

Abstract: A pulsed microwave method using a frequency range of about 7 GHz has been developed based on the principle of the pulsed radar method. This method is employed for the non-destructive quantitative determination of the moisture content and distribution in brickwork. The measurements are more precise than previous measurements that have used frequencies below 1 GHz. Owing to the shorter wavelength and the higher absorption coefficient of water in this frequency region, both real and imaginary parts of the complex dielectric properties are measured with a higher accuracy. The expected non-linear relationship between the dielectric properties of several building materials and their moisture content and salt concentration (NaCl) requires calibration measurements. Therefore, systematic measurements on brick material, like sand-lime brick, solid brick, hollow brick and porous concrete, have been carried out. The results are demonstrated and compared with actual theoretical models.

Developments for the non-destructive evaluation of highway bridges in the USA

Abstract: The collapse of the Silver Bridge in Point Pleasant, West Virginia, in 1967 initiated the formal requirements for the inspection of highway bridges in the USA. Almost 30 years later, visual inspection remains the primary method of bridge inspection. However, many new and promising techniques for the nondestructive evaluation (NDE) of highway bridges have emerged in recent years through the NDE research program of the Federal Highway Administration (FHWA). These include infrared and radar imaging systems, portable ultrasonic systems, telemetry systems, laser deflection measurement systems, and many others. This paper discusses the recent development of these instruments, and the potential impact it may have on bridge inspection and bridge management systems. The paper also discusses the need for validation of system performance, and the FHWA plans to construct a national center for NDE validation at the Turner Fairbank Highway Research Center.

Assessing Bridge Pier Scour By Radar

Abstract: Detection of river-bed scour around bridge piers represents a major current problem for engineers both in the UK and internationally. Techniques using boat-mounted impulse radar survey equipment have been suggested as a potentially viable solution to the problem. A programme of systematic laboratory model tests has been undertaken to examine the influences of pier and hole geometry upon measured radar response. These tests have been supplemented by field trials on real scour holes. Results from both laboratory and field studies have been used to validate numerical models developed using ray-tracing and finite difference techniques.

Evolution-based genetic algorithms for analysis of non-destructive surface wave tests on pavements

Abstract: Spectral analysis of surface waves (SASW) is an in situ non-destructive method for evaluation of near-surface stiffness profiles of pavement systems. This method is based on the dispersion phenomenon of Rayleigh surface waves in layered media. In the SASW method, the stiffness profile of a pavement is determined by fitting measured dispersion data with an adjustable theoretical model of the pavement system that depends on layer thicknesses and elastic moduli. The best-fit stiffness profile is usually found with search or optimization techniques. Unfortunately, the parameter space for this problem is highly complex and multi-modal and, consequently, traditional calculus-based search techniques often fail. In this study, a robust search technique employing genetic algorithms is applied. These algorithms are based on the principles of natural evolution, i.e. survival-of-the-fittest. A basic description of this search technique and its implementation for optimization in the SASW method are presented. Its effectiveness is also demonstrated on a difficult, three-layer pavement site.

Development of a computer model for an ultrasonic polymer film transducer system

Abstract: A piezoelectric polymer film such as polyvinylidene fluoride (PVDF) offers the possibility of making cheap ultrasonic transducers which can be permanently bonded to structures for NDE applications. However, before this can be achieved it is necessary to have a full understanding of the behaviour of PVDF in the context of a transducer application. This paper describes the development of an interactive computer program to implement a one-dimensional model of a thin film transducer system mounted on a solid substrate. Starting from a typical transducer system mounted on a solid substrate. Starting from a typical tranducer equivalent circuit, this paper explains how that circuit can be expanded into a complete model representing a PVDF transducer with one or more backing layers bonded to an arbitrary solid substrate. The computer program uses this model to provide immediate predictions of electrical input impedence, acoustic response and pulse-echo response. A series of experiments has been performed using PVDF transducers bonded to a variety of materials in order to validate the model. This has enabled the degree of accuracy required for various parameters within the model, such as mechanical and dielectric loss values, to be assessed. Once validated, the model has been used as a tool to predict the effect of physical parameters, such as bond and electrode thickness, on the performance of a transducer.

The effect of stresses approaching and exceeding the yield point on the magnetic properties of high strength pearlitic steels

Abstract: The results of a study of the effects of stresses that approach and exceed the yield point on the magnetic properties of a sample of 50D pearlitic steel are reported. Unlike previous work which has only examined residual stress behaviour, measurements were made in-situ while sample remained under stress. Hysteresis loops, permeability curves and magnetostriction loops are presented and a variety of magnetic parameters analysed. The implications of this analysis for the application of non-destructive evaluation techniques such as magnetic flux leakage, magnetic Barkhausen noise and magnetoacoustic emission are discussed. Key changes in magnetic behaviour occur well before yield and this raises the possibility of developing magnetic NDE methods of predicting when a sample is approaching the yield point.

The assessment of the value of GPR imaging of flexible pavements

Abstract: A survey of the different applications of ground penetrating radar (GPR) for highway pavement investigation is presented with a critical appraisal of the success with which it was applied. The problems associated with this technique are also discussed. These include discussions of the accuracy of the technique and the difficulties in trying to interpret the recorded signals. Suggestions for the future direction of the research in this field to enhance the data collection and interpretation are made. The remainder of this paper investigates the advances that can be made in the use of existing GPR technology in the field of thin pavements investigation.

Non-destructive investigation of sluices using radar and ultrasonic impulse echo

Abstract: Radar and ultrasonic impulse echo have been successfully applied for the assessment of two almost 100 year old sluices. The interior condition of the side walls has been investigated with radar enabling the detection of detachments of the faced brickwork as well as the determination of moisture content. With ultrasonic impulse echo working joints were found in the concrete slab of the sluice heads. The results of these investigations gathered comparative data to reduce the number of cores which had to be taken.

Method for sizing of 3-D surface breaking flaws by leakage flux

Abstract: A method is proposed for sizing of 3-D surface breaking flaws with complex profiles. The method can be used for either magnetic materials or conductive materials when a magnetic field or electric current are distributed homogeneously inside the sample. The distribution of the intensity of the leakage magnetic field or electric field has to be measured at a constant distance above the surface of the sample in a direction perpendicular to the long axis of the flaw. The method is based on a dipole model of the flaw which predicts the intensity of the leakage field and on minimisation of the error between the measured distribution of the intensity of the leakage field and the corresponding theoretical distribution which changes with the size of the flaw. Computations are made for sizing of flaws with different shapes and sizes while it is assumed that the density of the charge at the walls of the flaw can vary. It is shown that the method allows fast distinguishing of the shape of a particular flaw where the sample contains several flaws with different shapes and the sizes of these flaws are known. Fast determination of the size of a particular flaw is achieved when the sample contains flaws with the same shape and different but known sizes. Also, the size of the flaw and the density of the charge at the walls of the flaw could be computed precisely, independently of the initial approximation of the minimisation procedure, when the shape of the flaw is known. The speed of the minimisation process increases with the decrease of the measurement distance from the surface of the flaw. Experimental requirements for precise flaw sizing are discussed.

A beam hardening correction for X-ray microtomography

Abstract: We propose a correction of the transmitted intensities in X-ray microscopy allowing improved quantification for polychromatic sources, particularly in tomographic reconstruction. The correction is based on the application of Beer's law for each energy present in the source spectrum. Knowledge of the source spectrum and the average specimen composition are used. The correction is demonstrated on a chemically homogeneous specimen (a glass capillary filled with glass beads) and on a polyphasic oil reservoir rock sample. In the former, detection of 20 μm defects is demonstrated. In the latter, different minerals, quartz, feldspath and kaolinite, can be discriminated and their distributions mapped.

Improvement of ultrasonic testing of concrete by combining signal conditioning methods, scanning laser vibrometer and space averaging techniques

Abstract: This paper deals with the combination of several techniques to improve the ultrasonic pulse echo testing of concrete elements when a laser doppler interferometer is used as the ultrasonic receiver. This techniques involves specially designed ultrasonic probes, a pulse compression technique, random speckle modulation and space time signal processing methods. The pulse echo technique is carried out by sending frequency modulated chirp signals and performing a cross-correlation between the received and the transmitted signals. In combination with the application of recently available ultrasonic concrete probes as transmitter, this leads to an improvement in the signal-to-noise ratio. A laser doppler interferometer, equipped with a random speckle modulator, is used as detector of the ultrasound. Finally, the data sets are processed with various methods, involving time signals of several space points. Examples are the space averaging and the synthetic aperture focusing technique (SAFT). The advantage of the suggested technique is demonstrated by practical measurements on a test specimen. The improvement in results as compared to standard laser interferometric measurements will increase the feasibility of laser interferometric detection for non-destructive testing in civil engineering.

Detection of debonding in composite-aluminum joints using gamma-ray Compton scattering

Abstract: Gamma-ray Compton scattering is used for the detection of debonding in adhesively bonded composite-aluminum joints. A collimated narrow beam of monochromatic photons, generated by a 137 CS source, is directed towards the joint and scattered photons are recorded, using a detector located on the same side as the source. The energy of the scattered radiation is measured and related to the angle of scattering. The occurrence of debonding is indicated by a change in the count rate at an energy corresponding to its location. The performance of the technique is successfully demonstrated experimentally for joints of different adhesive-bonding thicknesses and for artificially induced debonds.

The use of small electrochemical perturbations to assess the corrosion of steel in concrete

Abstract: In this work an experimental and theoretical comparison has been made between the use of the potentiostatic and coulostatic methods to assess the corrosion rate of steel in concrete. Numerical Laplace transformation was used for preliminary impedance analysis of the coulostatic transients to aid the understanding of the experimental data. The corrosion tests were carried out on a range of reinforced concrete specimens made with various types of cements (plain, flyash and slag blended cements), different water to cement ratios (0.4 and 0.5) and various admixed chloride concentrations varying from 0 to 3% by weight of cement. Despite the inherent variability of concrete, which usually results in a large scatter of results, similar trends in the performance of the different concrete mixes were predicted by the two methods. Curve fitting analysis of the transients obtained from both methods was found to result in a lower estimate of the polarisation resistance parameter than other conventional methods. However, this analysis may represent an improvement over the other estimates of its value as it provides a consistent methodology of excluding the very fast or slow transient processes which may not result from the corroding interface. The differences in the results obtained from the various methods of analysis of the two methods, together with their advantages and disadvantages, are discussed.

Portable linear accelerators for X-ray and electron-beam applications in civil engineering

Abstract: Portable linear accelerator systems are being used to provide high-intensity X-rays for structural engineering studies and non-destructive testing in the USA and Europe. These devices greatly enhance the ability of engineers to examine structural members for hidden defects, as well as to study internal design and construction details of large buildings, bridges, parking garages and other structures, including steel memebers up to 16" thick and concrete over 62" thick. The resulting radiographic images are of excellent quality, enabling quick, reliable analysis of hidden conditions in the field. Three representative case studies are discussed in detail, and new developemnts are highlighted, including use of robotics for positioning and the use of accelerators for curing composite materials in-situ.

Propagation characteristics of SH ultrasonic waves through the surface depth of an isotropic medium

Abstract: The propagation characteristics of SH waves passing from the surface through an isotropic medium were investigated by adjusting the incident and receiving angles of two SH wave transducers. Approximately 70% of the total SH wave energy reached a depth of 2 mm when the incident adn receiving angles were 24 and 25°, respectively. This phenomenon can be explained easily if the wave diffraction pattern between the transmitter and the receiver is assumed to take an angular path. SH wave attenuation provides a sensitive probe for determining variations in the hardness of surfaces arising from the accumulation of elastic strain and transformation products.

Comparison of laboratory and simulated data for radar image interpretation

Abstract: It has been shown that the output from in-house ray-tracing simulation software provides a means by which the impulse radar response of a structural system can readily be simulated. Radar simulations were undertaken of re-bar targets in a tank. A 900 MHz commercial antenna was simulated. The simulations undertaken have shown the effects that target spacing and depth have on the radar signature shapes and the results compare well with real-case survey plots. Simulation data have been used to assess the output specification from a commercial antenna, with interesting findings. It is expected that simulation will allow the interpretation of actual surveys to be more readily understood, which will in turn allow a better engineering assessment to be made of a structure under investigation.

NDE of a 3-D surface crack using magnetic field induced by DC current flow

Abstract: A method is proposed for nondestructive evaluation of a 3-D surface crack by means of measurement of a magnetic field induced in the air by DC current flow in a material. A semielliptical crack is considered. The crack length is assumed to be known in advance and the crack depth is to be found. The current is applied between two close points on both sides of the crack and the change in magnetic: flux density in the air due to the crack is measured in non-contact with the surface of the material. An approximate equation relating the magnetic flux density to the depth and length of the crack is derived numerically and some experimental results show that the evaluated crack depth is in good agreement with the actual one.

Application of a two-layer semi-analytical model for the improvement of laser-ultrasonic generation

Abstract: The new laser-based ultrasound model presented in this paper takes into account the layered structures generally encountered in aeronautic materials (painted metals, composites, lapped joints…) which are subjected to impacts, fatigue and corrosion. The structure is supposed to be a plate made of two layers of different materials. The sample presents a cylindrical orthotropy. Validations of the model are performed by comparing the results it provides with experimental measurements using a Nd:YAG pump laser and an interferometric detection. The symmetry of the model allows fast calculation and prediction of the displacements over a long time period. These advantages are used to study the improvement of the laser-ultrasonic generation, by varying the thickness of the first layer in the case of a NDT strippable paint layer covering a metal layer.