Showing papers in "Ndt & E International in 2006"

Ultrasonic arrays for non-destructive evaluation: A review

Abstract: An ultrasonic array is a single transducer that contains a number of individually connected elements. Recent years have seen a dramatic increase in the use of ultrasonic arrays for non-destructive evaluation. Arrays offer great potential to increase inspection quality and reduce inspection time. Their main advantages are their increased flexibility over traditional single element transducer methods, meaning that one array can be used to perform a number of different inspections, and their ability to produce immediate images of the test structure. These advantages have led to the rapid uptake of arrays by the engineering industry. These industrial applications are underpinned by a wide range of published research which describes new piezoelectric materials, array geometries, modelling methods and inspection modalities. The aim of this paper is to bring together the most relevant published work on arrays for non-destructive evaluation applications, comment on the state-of the art and discuss future directions. There is also a significant body of published literature referring to use of arrays in the medical and sonar fields and the most relevant papers from these related areas are also reviewed. However, although there is much common ground, the use of arrays in non-destructive evaluation offers some distinctly different challenges to these other disciplines.

A vision-based system for remote sensing of bridge displacement

Abstract: This study proposed the vision-based system which remotely measures dynamic displacement of bridges in real-time using digital image processing techniques. This system has a number of innovative features including a high resolution in dynamic measurement, remote sensing, cost-effectiveness, real-time measurement and visualization, ease of installation and operation and no electro-magnetic interference. The digital video camera combined with a telescopic device takes a motion picture of the target installed on a measurement location. Meanwhile, the displacement of the target is calculated using an image processing technique, which requires a target recognition algorithm, projection of the captured image, and calculation of the actual displacement using target geometry and number of pixels moved. For the purpose of verification, a laboratory test using shaking table test and field application on a bridge with open-box girders were carried out. The test results gave sufficient dynamic resolution in frequency as well as the amplitude.

Vibration monitoring for defect diagnosis of rolling element bearings as a predictive maintenance tool : Comprehensive case studies

Abstract: Vibration monitoring and analysis in rotating machineries offer very important information about anamolies formed internal structure of the machinery. The information gained by vibration analysis enable to plan a maintenance action. In this study, the vibration monitoring and analysis case studies were presented and examined in machineries that were running in real operating conditions. Failures formed on the machineries in the course of time were determined in its early stage by the spectral analysis. It was shown that the vibration analysis gets much advantage in factories as a predictive maintenance technique.

Continuous wavelet transform technique for fault signal diagnosis of internal combustion engines

Abstract: A fault signal diagnosis technique for internal combustion engines that uses a continuous wavelet transform algorithm is presented in this paper. The use of mechanical vibration and acoustic emission signals for fault diagnosis in rotating machinery has grown significantly due to advances in the progress of digital signal processing algorithms and implementation techniques. The conventional diagnosis technology using acoustic and vibration signals already exists in the form of techniques applying the time and frequency domain of signals, and analyzing the difference of signals in the spectrum. Unfortunately, in some applications the performance is limited, such as when a smearing problem arises at various rates of engine revolution, or when the signals caused by a damaged element are buried in broadband background noise. In the present study, a continuous wavelet transform technique for the fault signal diagnosis is proposed. In the experimental work, the proposed continuous wavelet algorithm was used for fault signal diagnosis in an internal combustion engine and its cooling system. The experimental results indicated that the proposed continuous wavelet transform technique is effective in fault signal diagnosis for both experimental cases. Furthermore, a characteristic analysis and experimental comparison of the vibration signal and acoustic emission signal analysis with the proposed algorithm are also presented in this report.

Numerical simulation on magnetic flux leakage evaluation at high speed

Abstract: High-speed non-destructive inspection (NDI) systems using magnetic flux leakage method (MFL) is in great demand in online metal inspection and defect characterisation, especially in pipeline and rail track inspection. For MFL systems at high speed measurement, in addition to magnetic flux, eddy currents exist in metal specimen because of the relative movement between the probe and specimen. These currents alter the profile of electromagnetic field, which increases difficulty in signal interpretation and defect characterization. In this paper, eddy currents generated by high speed movement and their characterisation in high-speed MFL inspection systems were investigated by using numerical simulations. Besides, the MFL signals from high speed MFL measurement against defect depth were examined.

MFL signals and artificial neural networks applied to detection and classification of pipe weld defects

Abstract: This work evaluates the use of artificial neural networks (ANNs) for pattern recognition of magnetic flux leakage (MFL) signals in weld joints of pipelines obtained by intelligent pig. Initially the ANNs were used to distinguish the pattern signals with non-defect (ND) and signals with defects (D) along of the weld bead. In the next step the ANNs were applied to classify signal patterns with three types of defects in the weld joint: external corrosion (EC), internal corrosion (IC) and lack of penetration (LP). The defects were intentionally inserted in the weld bead of a pipeline of API 5L-X65 steel with an outer diameter of 304.8 mm. In this way, the MFL signal itself, digitized with 1025 points, was used as the ANN input. Initially the signals were used as inputs for the neural network without any type of pre-processing, later four types of pre-processing were applied to the signals: Fourier analysis, Moving-average filter, Wavelet analysis and Savitzky–Golay filter. Signal processing techniques were employed to improve the performance of the neural networks in distinguishing between the defect classes. The results showed that it is possible to classify signals of classes D and ND using ANN with very efficient results (94.2%), as well as for corrosion (CO) and LP signals (92.5%). Also it is possible to classify the defect pattern signals: EC, IC and LP using neural networks with an average rate of success of 71.7% for the validation set.

Ultrasonic C-Scan and shearography NDI techniques evaluation of impact defects identification

Abstract: The new all-composite wing for semi-aerobatic sport and training aircraft is being developed in the Czech aircraft industry. The typical outer composite structure of this wing is sandwich with carbon skins in combination with honeycomb. This new composite structure is designed in accordance with CS-23 airworthiness requirements. The NDI techniques for reliable detecting of manufacture or in-service defects (disbonding, defects from impact, etc.) are therefore, necessary. This paper compares the results of visual, ultrasonic C-Scan and laser shearography impact damage assessment and determination of sandwich panels, which were cut out from sandwich skins of this developmental wing. Considering the reliability, simplicity and rapidity of each technique and with the digital indicator measurement as a base, the laser shearography is evaluated as the most suitable method for that purpose.

Characterisation of defects in the railhead using ultrasonic surface waves

Abstract: The ability to detect defects in rail, and in particular surface defects such as gauge corner cracking, is essential. Current ultrasonic testing of the rail uses contacting transducers in a send-receive mode which are at present limited in testing speed to around 20-30 mph, and signals from serious surface defects may be masked by nearby shallow surface defects. Using Rayleigh-like surface waves generated and detected in a pitch-catch manner we can overcome the effect of this masking, and by using electro-magnetic acoustic transducers (EMATs) working at a small standoff we may be able to lift many of the current rail testing speed limitations with a high accuracy for detecting and gauging surface defects. We present experiments on rail samples with real and machined defects, using EMATs to generate and detect a low-frequency wide-band surface wave. The depths of the defects can be characterised through analysis of both the time domain signal amplitude and frequency dependent behaviour. The design of EMAT used is discussed, with consideration of signal, frequency content, directionality of the signal and the effect of standoff above the sample.

Flaw detection in radiographic weld images using morphological approach

Abstract: It is necessary to detect suspected defect regions in the radiographic weld images to find the type of flaw and its causative factors. This requires processing of radiographic images by a suitable approach. This paper presents an approach to process these radiographic weld images of the weld specimens considering morphological aspects of the image. The proposed approach first determines the flaw boundaries by applying the Canny operator after choosing an appropriate threshold value. The boundaries are then fixed using a morphological image processing approach i.e. dilating few similar boundaries and eroding some irrelevant boundaries decided on the basis of pixel characteristics. The flaws detected by this approach are categorized according to their properties.

Ability of the direct wave of radar ground-coupled antenna for NDT of concrete structures

Abstract: The research work presented in this paper aims at evaluating the potential of the direct wave of radar ground-coupled antennas for the on site physical characterization of concrete. According to studies highlighting the ability of radar reflected waves to characterize concrete, the potential of the direct wave is studied by systematic comparison of direct and reflected wave attenuations. Among the various features of electromagnetic waves propagating through concrete, attenuation is the most sensitive to moisture and chloride contents. In order to study the sensitivity of the direct wave to the physical condition of the concrete, laboratory experiments were conducted on concrete slabs with various porosities, water contents and chloride contamination levels. The results of this study show very satisfactory correlations between attenuations of direct and reflected waves, indicating that both waves provide similar information regarding the physical state of concrete. This correlation was confirmed on two bridges by comparison of direct wave attenuation maps with reflection attenuation maps. Therefore, radar interpretation method based on the direct wave attenuation should be able to detect, in a very fast way, physical contrasts in concrete structures and, consequently, identify probable pathological areas.

Gammadensimetry: A method to determine drying and carbonation profiles in concrete

Abstract: The aim of this paper is to describe a new and efficient method to determine drying and carbonation profiles in concrete by gammadensimetry. First, the paper focuses on density and density variation measurements. A direct calculation of porosity and moisture saturation rate is proposed. Then, the uncertainties of these measurements are assessed. The results obtained on several materials are finally analyzed and validated by comparison with classical test results. The porosity profiles agree with mercury intrusion porosimetry and carbonation results with depths obtained by pH-indicator spraying. Thus, it is shown that gammadensimetry enables to monitor the evolution of density according to depth and time. Moreover, the auscultation by gamma-ray makes it also possible to determine the water content during drying as well as the quantity of fixed carbon dioxide in concrete during the carbonation process.

Eddy currents and hardness testing for evaluation of steel decarburizing

Abstract: Usual heat treatments of steels like austenitization are generally conducted in air. In such atmosphere, a part of the atoms of carbon could be removed from the superficial zone of steel. Indeed, those atoms of carbon, combined with oxygen present in atmosphere, can take gaseous form of carbon monoxide due to the great attraction between atoms of carbon and oxygen. This well-known phenomenon is called decarburizing. It can change microstructure to a large extent and, as a result, bring dramatic modifications of mechanical properties of steel, like decrease of fatigue lifetime. To characterize the extent of decarburizing phenomenon, observations by optical microscopy and/or hardness profiles measurements must be performed in a cross section, as it is advised by international standard. Until today, the eddy current technique is used to detect superficial defects. In practice, the control consists to create two groups of available samples or not, from a large sampling. In this study, we propose to estimate the decarburizing extent. In the present work, different durations of austenitizing at 920 °C temperature before oil quenching (50 °C) were performed on the SAE 92V45 steel in order to obtain various morphologies and different total depths of the decarburized zone. We will show, in the continuation, how eddy currents control is used to assess the level of decarburizing after a Fourier transformation performed on the output signal. This analysis allows us to link the harmonic decomposition of the signal to the duration of the heat treatment and/or to the total decarburizing depth and, consequently, to the mechanical properties.

Transient and harmonic eddy currents: Lift-off point of intersection

Abstract: It has been previously determined experimentally that the transient responses of pulsed eddy current excitation of a conductive semi-infinite plane pass through a common point, regardless of the lift-off value. The origin of this intersection point is investigated and related to the harmonic components of the Fourier series representation of the pulse. Discrete sinusoidal signals, when individually sent through the same inspection system, also exhibit this common point feature when only the lift-off is varied. Moreover, the transient responses can be reconstructed from the harmonic outputs by using truncated Fourier series.

3D FEM analysis in magnetic flux leakage method

Abstract: The magnetic flux leakage (MFL) method is currently the most commonly used pipeline inspection technique. In this paper, 3D FEM is used to analyze the MFL signals, a generalized potential formulation to the magnetostatic field MFL problem is discussed, typical 3D defects are accurately modeled and detail MFL signal in test surface are calculated by the method. The relation between defect parameters and MFL signals are also analyzed.

Characterization of entrained air voids in cement paste with scattered ultrasound

Abstract: This research develops a technique that uses the attenuation of ultrasonic waves to characterize the average size and volume fraction of entrained air voids in hardened cement paste. Quantitative knowledge of entrained air void size and distribution helps ensure that an adequate design strength is developed, while maintaining resistance to freeze-thaw damage in cement-based materials. Ultrasonic attenuation coefficients obtained from pulse-burst signals are measured in the frequency range of 500 kHz–5 MHz. From these parameters, the average size and the volume fraction of the entrained air voids are determined using a combination of an ultrasonic scattering model and an inversion algorithm. Experiments are performed on specimens produced with and without entrained air voids. There is a good agreement between the model prediction and the experiments in these systems that contained o10% by volume of entrained air voids. r 2006 Elsevier Ltd. All rights reserved.

A noncontact NDE method using a laser generated focused-Lamb wave with enhanced defect-detection ability and spatial resolution

Abstract: The laser generation method of a focused Lamb wave is expected to have high defect-detection abilities and spatial resolution, with the added advantage of noncontact testing. In this method, the laser beam is illuminated on the surface of an object through an arrayed arc slit. The energy of the generated Lamb wave is then concentrated at a focus of an arc. This focusing effect improves the NDE (nondestructive evaluation) performance, which is dependent on the geometries of an arrayed arc slit. In this paper, the relationships between the parameters determining the slit geometry and the focusing effect of the generated Lamb wave was investigated using a parametric study, and appropriate values of the parameters were obtained to maximize the focusing ability. In order to validate the performance of this method, an NDE system was constructed and experiments were performed to detect through-hole defects in a plate. The results showed that the proposed system could clearly detect defects as small as 0.3 mm in diameter, while the conventional line array method failed to detect defects smaller than 1 mm in diameter. Moreover, this method showed possible detection of defects much smaller than 0.3 mm, and great improvements in the spatial resolution as compared with the line array method.

2D Eddy current sensor array

Abstract: This paper presents the design and development of a two-dimensional eddy current sensor array and the improved response of the array using a super-resolution algorithm. A computational model of the array is derived using dyadic Green's function and volume integral method. This application was realized for detecting fatigue cracks, introduced in AISI 304 samples by subjecting them to cyclical loading.

Detection and measurement of surface cracks in ferromagnetic materials using eddy current testing

Abstract: Eddy current testing is used to detect and quantify defects in electric conductive materials. Defects investigated are specifically mechanical, chemical and microstructural heterogeneities. In this scope, artificial defects simulating superficial cracks have been achieved in order to establish an eventual relationship between the orientation, the shape and the size of the defect in one hand, and the signature that it generates in the impedance plane on the other hand. We determine in the first stage the operating conditions for the minimization of all signals that may perturb the crack signal such as the background one. We have been able to detect easily a surface crack on ferromagnetic steel in a large range of the frequency. Curves allowing the discrimination between signals corresponding to the presence of cracks and those corresponding to a lift-off have been established. The limit depth has as well been determined and we found that this limit has no relationship with the depth of penetration of the eddy current. The operative parameters such as the diameter and the working mode of the probe, the excitation frequency and the setting of the measurement equipment have been optimized for the testing of the quality and the reliability of critical ferromagnetic steel parts.

A calibration method for misaligned scanner geometry in cone-beam computed tomography

Abstract: Computed tomography (CT) images often suffer from artifacts caused by misaligned scanner geometry of CT system. Calibration and correction must be done before image reconstruction. A method for calibration of misaligned scanner geometry in cone-beam CT with single-circle orbit is proposed. In this method, a four-point phantom is used to estimate a set of parameters that describe the geometry of a cone-beam CT system. It requires only one projection of the phantom, instead of several projections at multi-angles. An analytical formula is derived, which avoids falling into local minimum. In addition, there is no restriction on any one of the geometric parameters. The simulated results are illustrated. It is proved that this method is applicable and efficient for misaligned scanner.

Eddy current measurements of electrical conductivity and magnetic permeability of porous metals

Abstract: This paper presents a method, which simultaneously estimates the electrical conductivity and magnetic permeability of porous metals. Porous Cu and Fe manufactured by the lost carbonate sintering process have been tested. An air-cored solenoid coil was designed for the measurements of rod-shaped samples when inserted coaxially with the coil. It was theoretically found that the phase-frequency response of the normalised eddy current signal of the coil is virtually independent of the radius, electrical conductivity and magnetic permeability of the test samples. For non-magnetic, conductive porous Cu, the electrical conductivity was measured with a calibration curve of the coil relating the impedance change and the electrical conductivity of the sample. For magnetic porous Fe, the imaginary part of the signal at the lowest frequencies can be used to estimate the permeability. The measured conductivity values of the porous Cu are shown in the paper in comparison with that of bulk materials with known conductivity. The measured permeability values of the porous Fe are given and the sample-length effect on the measurements is also discussed.

Dual EMAT and PEC non-contact probe: applications to defect testing

Abstract: For many non-destructive testing (NDT) applications, more information and greater reliability can be gained by using different techniques for defect detection, especially when the methods are particularly sensitive to different types of defects. However, this will often lead to a much longer and more expensive test and is not always practical due to time and cost constraints. We have previously discussed initial experiments using a new dual-probe combining electromagnetic acoustic transducers (EMATs) generating and detecting ultrasonic surface waves, and a pulsed eddy current (PEC) sensor [1]. This enables more reliable detection and sizing of surface and near-surface defects, with a reduced testing time compared to using two NDT techniques separately. In this paper, we present experiments using the dual-probe on samples which are more representative of real defects, for example testing for surface defects in rails. Several aluminium calibration samples containing closely spaced and angled slots have been measured, in addition to rail samples containing manufactured and real defects. The benefits of using the dual-probe are discussed.

Real-time arc welding defect detection technique by means of plasma spectrum optical analysis

Abstract: An optimized technique for real-time spectral analysis of thermal plasmas, with application in the monitoring and defect detection of industrial welding processes, particularly arc-welding, is presented in this paper. The calculation of the plasma electronic temperature by means of a sub-pixel algorithm permits on-line quality assessment of the welds, allowing the detection of common defects to be found in the welding seam, such as oxidation due to insufficient shielding gas flux or lack of penetration caused by current fluctuations of the welding power source. The proposed technique has been successfully checked in a real-time arc-welding monitoring system, and experimental results of stainless-steel welds are also reported.

Temperature control in laser brazing of a steel/aluminium assembly using thermographic measurements

Abstract: One way of making car bodies lighter is to introduce some aluminium parts in place of steel. Steel and aluminium can be joined by laser braze welding. As in other types of thermal joining, inter-metallic phases may weaken the joint. In laser braze welding, these appear as a thin layer of brittle compounds at the steel/seam interface. Their formation is related to temperature. It has been shown that, if the layer is less than 10 μm thick, the joint is not compromised [Kreimeyer M., Sepold G. Laser steel joined aluminium-Hybrid structures, Proceedings of ICALEO'02, Jacksonville, USA; 2002]. Not only can temperature gradient be calculated by numerical simulation, but it is also possible to measure the surface temperature by thermography. We show here how thermography may be used to control temperature during laser braze welding.

Microwave NDT of cement-based materials using far-field reflection coefficients

Abstract: Periodic assessment of ageing civil structures is important for structural integrity and public safety. Numerous destructive and non-destructive techniques employing different energy sources have been proposed for structural health monitoring. Amongst the non-destructive evaluation techniques proposed for monitoring material strength and contents of concrete structures, techniques employing microwaves offer distinct advantages in that they are non-radioactive and provide good penetration, excellent contrast between steel reinforcement bar and concrete, and insensitivity to ambient temperature. In this paper, the feasibility of a far-field microwave NDE technique for concrete imaging is investigated via numerical simulations and experiments on cement based samples using far-field microwave reflection coefficients.

Shear-wave time of flight diffraction (S-TOFD) technique

Abstract: Ultrasonic time of flight diffraction (TOFD) for sizing defects is based on the time of flight of the diffracted echo that is generated when a longitudinal wave is incident on a crack tip. This technique has the limitation during near-surface inspection due to signal superposition. Here, this limitation is overcome by using the shear wave-diffracted signal (instead of longitudinal wave) and hence called S-TOFD. Experiments were conducted on samples with defect tip closer to the surface of a flat plate sample to illustrate the utility of the S-TOFD technique. An increase in the flaw sizing accuracy, by using the shear wave-diffracted echoes from the tip and through the application of a signal processing technique (ESIT), was demonstrated.

Improved ultrasonic wave reflection technique to monitor the setting of cement-based materials

Abstract: A test setup using the ultrasonic wave reflection technique is used to monitor the hydration process of early-age cement-based materials throughout their setting and hardening. The focus of the investigation presented here is the monitoring of the stiffening behavior of cement pastes. Specimens are produced and kept under constant curing conditions. Immediately after placing the cement paste the data collection starts and continues throughout the setting process. The obtained ultrasonic data are correlated to mechanical and chemical properties of the specimens. The results illustrate that the applied experimental wave reflection technique is suitable to monitor the stiffening of cement pastes accurately.

Infrared thermography for inspecting the adhesion integrity of plastic welded joints

Abstract: This work aim at developing a non-destructive tool for the evaluation of bonded plastic joints. The paper examines infrared thermographic transmission and reflection mode imaging and validates the feasibility of the thermal NDT approach for this application. Results demonstrate good estimation performance for adhesion integrity, uniformity and bond strength using a transmission mode application of infrared thermography. In addition, results from a pulsed infrared thermographic application using a modified dynamic infrared tomography scheme show good performance for estimating adhesion layer thickness mapping and detecting delaminations.

Calculation and simulation of impedance diagrams of planar rectangular spiral coils for eddy current testing

Abstract: The knowledge of analytical expressions for the electromagnetic (EM) fields produced by the coils used in eddy current testing is an important point in the development and application of these devices. In the present work, the second order vector potential formulation is used for the calculation of the fields produced by planar rectangular spiral coils of arbitrary number of turns and finite rectangular cross-section placed on a conducting half-space. Impedance plane diagrams are calculated for different frequencies, lift-off and half-space conductivity. Tests for conductivity assessment are also simulated. The theoretical results are compared with experimental measurements.

Matrix factorization methods: Application to thermal NDT/E

Abstract: A typical problem in thermal nondestructive testing/evaluation (TNDT/E) is that of unsupervised feature extraction from the experimental data. Matrix factorization methods (MFMs) are mathematical techniques well suited for this task. In this paper we present the application of three MFMs: principal component analysis (PCA), non-negative matrix factorization (NMF), and archetypal analysis (AA). To better understand the peculiarities of each method the results are first compared on simulated data. It will be shown that the shape of the data set strongly affects the performance. A good understanding of the actual shape of the thermal NDT data is required to properly choose the most suitable MFM, as it is shown in the application to experimental data.

IE-SASW method for nondestructive evaluation of concrete structure

Abstract: The Impact–Echo (IE) method has been widely used to evaluate the integrity of concrete structures. In this method, the P-wave velocity of concrete is a crucial parameter in determining the thickness of concrete slab and the location of cracks or other defects. To determine P-wave velocity of concrete, in this paper, Spectral Analysis of Surface Wave (SASW) method was employed, and IE–SASW method was suggested by combining two nondestructive testing methods. IE method was used for the detailed nondestructive evaluation of concrete whereas SASW method was employed for the measurement of the average P-wave velocity and for the status evaluation of concrete. The feasibility study of SASW method was performed by using finite element method. Experimental studies were also performed in the slab type concrete model specimens in which various types of defects or boundaries were included at known locations. SASW tests showed the potential of determining the P-wave velocity of concrete accurately and IE tests were able to determine the thickness of structures and locations of defects. Based on both experimental and numerical studies, the feasibility of the proposed method was verified.