Showing papers in "Ndt & E International in 2002"

Rolling element bearing fault diagnosis using wavelet packets

Abstract: A method is proposed for the analysis of vibration signals resulting from bearings with localized defects using the wavelet packet transform (WPT) as a systematic tool. A time–frequency decomposition of vibration signals is provided and the components carrying the important diagnostic information are selected for further processing. The proposed method is designed in such a way that it can exploit the underlying physical concepts of the modulation mechanism, present in the vibration response of faulty bearings. The flexibility of the WPT and the systematic parameter selection criteria, help in the minimization of interventions by the end user. The method is evaluated using simulated and experimental signals.

Automatic identification of different types of welding defects in radiographic images

Abstract: Radiographic testing is a well-established non-destructive testing method to detect subsurface welding defects. In this paper, an automatic computer-aided identification system was implemented to recognize different types of welding defects in radiographic images. Image-processing techniques such as background subtraction and histogram thresholding were implemented to separate defects from the background. Twelve numeric features were extracted to represent each defect instance. The extracted feature values are subsequently used to classify welding flaws into different types by using two well-known classifiers: fuzzy k-nearest neighbor and multi-layer perceptron neural networks classifiers. Their performances are tested and compared using the bootstrap method.

Advanced signal processing of magnetic flux leakage data obtained from seamless gas pipeline

Abstract: Natural gas is normally transported through a vast network of pipelines A major segment of this network employs seamless pipes The manufacturing processes associated with the production of seamless pipes contribute to a helical variation in the grain properties of the pipe This introduces an artifact, known as the seamless pipe noise (SPN), in the data obtained from magnetic flux leakage (MFL) inspection of these pipelines SPN can overwhelm the signals generated by defects and other elements in pipelines, and can therefore, mask their indications in the MFL data This paper presents a new technique for detecting signals in MFL data obtained from seamless pipes The overall approach employs an adaptive filter and a wavelet based de-noising technique The algorithm is computationally efficient and data independent Results from application of the approach to data from field tests are presented

Waveform based clustering and classification of AE transients in composite laminates using principal component analysis

Abstract: Acoustic emission (AE) transients in composite laminate tensile test specimens are analyzed by principal component analysis (PCA). Signals resulting from matrix cracking and local delaminations were acquired by broadband transducers. The aim was to study the clustering and classification ability of PCA based on the time history of the recorded AE events. An unsupervised clustering analysis showed that AE signatures from matrix cracking and local delaminations were separated in different clusters. Some differentiations within these two types of signals were also observed. From the classification based on a training set, a majority of the signals in the test set were given the correct class membership. The applied pattern recognition method is an objective method using the complete information in the AE transients and the investigation shows that it may be a useful complement in the field of non-destructive evaluation.

Fatigue crack monitoring of riveted aluminium strap joints by Lamb wave analysis and acoustic emission measurement techniques

Abstract: Statistics show that fatigue crack development comes first and foremost as a damage source in aerospace metallic structures. Currently, widespread methods are available to inspect these structures, but they are quite time-consuming, costly and require the structural system to be idle. Next, attempts to develop damage detection integrated systems are paramount for the safety and cost of such structures. This paper describes an investigation into the feasibility of using an integrated system based on Lamb waves in order to assess the integrity of riveted aluminium joints during cyclical loading. In this experimental analysis, Lamb waves are excited and received outside the joint area using piezoelectric transducers coupled onto the plates. The detected damage is cracks in joint resulting from fatigue loading. The collected signals on the piezoelectric transducers are analysed using Hilbert transform and time–frequency analysis. It is shown that the final interpretation of Lamb wave analysis may provide a means of sizing the defects and following the crack development. In addition to that, an acoustic emission system is used jointly with the Lamb wave analysis in order to discuss results and damage development. Finally, it is demonstrated that both methods can work together and the results obtained are in good agreement.

Impedance evaluation of rectangular coils for eddy current testing of planar media

Abstract: As for any other eddy current probe coil, the analysis of rectangular ones plays a vital role towards understanding their characteristics and performance. In this paper, closed-form expressions are provided both for the impedance of rectangular coils of rectangular cross-section located above a conducting half-space and for the eddy current density induced therein. The formulation is general and can be easily used for any coil shape.

Comparison between pulsed and modulated thermography in glass-epoxy laminates

Abstract: Infrared (IR) thermography is a two-dimensional, non-contact technique which can be usefully employed in non-destructive evaluation of materials Basically, two different approaches are possible: traditional pulse thermography (PT) and modulated (or lock-in) thermography (MT) The attention of the present work is focused on the peculiar aspects, which characterise the two different techniques Tests are carried out by considering glass–epoxy specimens and the results obtained, by employing either PT or MT, are compared The capability of each technique, to detect a defect and give precise information about size, depth and thermal resistance of the defect, is analysed The advantages and disadvantages of using these techniques are discussed in order to assess the fundamental requirements for the most appropriate choice in quality control processes

Comparison between thermographic techniques for frescoes NDT

Abstract: The use of infrared thermography in the architectural restoration field is examined. Three samples, made of a support of marble, brick, or tuff, covered with a layer of plaster with inclusions to simulate detachments or cracks in frescoes, are considered. Different techniques: pulse thermography, lateral heating thermography, lock-in or modulated thermography and pulse phase thermography are employed to detect the flaws artificially created; advantages and disadvantages of each technique are discussed. It is found that pulse thermography is easy and fast to use for information about the state of the art treasures, but data may be affected by non-uniform heating and local variation of thermal emission; the lateral heating can help to overcome interference effects due to non-uniform heating but it is more troublesome to use. When the evaluation regards rare art treasures the lock-in technique seems to be the only response since it is able to operate within very low increase of surface temperature; this technique is also able to give information about the material composition. The pulse phase thermography may be used to detect more in depth flaws but it needs higher temperature increase with respect to the ambient temperature and so it is recommended to control, before testing, the temperature sensitivity of the artefact.

Propagation of guided waves in adhesive bonded components

Abstract: The objective of this research is to use analytical and computational models to develop a quantitative understanding of the propagation of guided Lamb waves in multi-layered, adhesive bonded components. Key issues of this study include the effect of the adhesive bond layer, including its low stiffness (relative to the adherends) and viscoelastic behavior. The propagation of these guided waves are interpreted in terms of dispersion relationships, displacement profiles and attenuation curves (both as functions of frequency and wavenumber). The ultimate goal of this study is to determine the effectiveness and sensitivity of guided Lamb waves to determine the in situ properties of an adhesive bond. A combination of the analytical model, transient FEM simulation and experimental measurements provides a better understanding of the guided wave's behavior in this layered waveguide.

Bearing failure detection using matching pursuit

Abstract: In this paper, a new approach to the detection of localized defects of rolling element bearings is proposed. It employs matching pursuit with time-frequency atoms to analyze bearing vibration and extract vibration signatures. In particular, this approach utilizes not only the temporal and spectral but also the scale characteristics of the vibration generated due to the presence of a defect for the detection. This leads to a high signal-to-noise ratio and facilitates considerably the detection at the early stage of failure development. Experimental results show that the proposed approach is sensitive and reliable and works better than continuous wavelet transform and envelope detection.

The monitoring of bridges for scour by sonar and sedimetri

Abstract: Scour is one of the major causes for bridge failure. This problem represents a heavy burden for the Italian railroads in terms of operating disruption and expense for the restoration of the damaged bridges. Scour failures tend to occur suddenly and without prior warning or sign of distress to the structure. The nature of the failure is often the complete collapse of the entire part of a bridge. Furthermore the pits of erosion tend to fill as soon as the flood begins to decrease, therefore following inspections and measures, in periods of dry weather, cannot furnish indications on the maximum depth reached by erosion during the event of flood. Until recently, there has been little awareness among bridge engineers of the danger of scour, compared to the effort concentrated on structural assessment. Examination into scour on existing structures has been very limited and of doubtful effectiveness. This is reflected in the total absence or at most a passing mention of scour in most bridge inspection and assessment documents. In order to resolve these problems of scour, or to limit the effects, the Italian Railways (F.S.) has provided, up until the recent past, a series of preventive interventions consisting in the consolidation and protection of the bridge foundations. These interventions are expensive and cannot be carried out for all bridges. In recent years the consolidation works have been defined only in the cases of real necessity or where the risk of erosion is high, opting for monitoring systems in other cases. These systems, on the other hand, are able to monitor the levels of scour during a flood and therefore the danger level. Owing to the need to have systems to measure continuously the Italian Railways (F.S.) have installed two experimental monitoring systems on the River Po. The following paper intends to outline the first results of this experiment, which began in 1995: in particular, the bed movement near the pier foundation during a flood.

A new non-contacting non-destructive testing method for defect detection in concrete

Abstract: A new non-destructive testing (NDT) method for defect detection in concrete structures is presented. The method is based on the dynamic response of flawed concrete structures subjected to impact loading. Conversely to similar NDT techniques, such as the impact-echo method, the present method uses non-contacting devices for both impact generation (a shock tube producing shock waves) and response monitoring (laser vibrometers measuring concrete surface velocity). Experimental and numerical (finite element) studies have been carried out for concrete specimens containing artificial defects (penny-shaped cracks parallel to the free surface) with varying length and depth. According to the experimental and numerical results, it appears that the present method enables an effective detection of defects, particularly in the range of shallow defects.

On-line optical monitoring system for arc welding

Abstract: This paper describes the technical features of four electro-optic sensors for the monitoring of arc welding. The energy released in the process determines the formation of a strongly radiative plasma in the interaction zone. The working principle of the prototype is based on the detection of visible, infrared and ultraviolet emission of the plasma. A discrimination algorithm has been found that allows the characteristics of the weld to be evaluated and to distinguish bad weldings from good ones.

Neural network based defect detection and depth estimation in TNDE

Abstract: For many years, applications of the TNDE (Thermographic NonDestructive Evaluation) technique has been limited due to the complex non-linearity nature of related inversion problems such as defect depth estimation. Artificial neural networks have recently obtained success in revealing and providing quantitative information concerning defects in TNDE. In this paper, a three dimensional thermal model for non-homogenous materials such as carbon fiber reinforced plastic (CFRP) is first given. The modeling results are compared with the analytical solution based on Duhamel's theorem. Two back propagation neural networks (NN) as defect detector and depth estimator are then presented. Finally, simulated and experimental results are presented and discussed.

Air-coupled ultrasonic NDE: experiments in the frequency range 750 kHz–2 MHz

Abstract: Possibilities of air-coupled ultrasonic testing and imaging are examined by means of a measurement system designed for both through-transmission and reflection technique. For that purpose, B- and C-scans have been performed with respect to a wide variety of materials of different acoustic impedance. The observations reported in the paper deal with coating on textile, flaws in an aluminum plate, spot welds on metallic plates, tiny air inclusions in thin castings and ultrasonic reflection on an epoxy plate with a copper layer. The measurements have been performed in either continuous or pulse mode. In the former case, frequency swept sinusoidal signals are applied, in the latter modulated chirps. All results have been obtained at sound frequencies between 0.75 and 2 MHz, produced by air-coupled piezo-based transducers with matching layer.

Time frequency and wavelet transform applied to selected problems in ultrasonics NDE

Abstract: In this paper, we contribute by the development of some signal processing in order to enhance the sensibility of flaw detection, to measure thin materials thickness and to characterize defects in nature (planar or volumetric). Features for discrimination of detected echos are extracted in time domain, spectral domain and discrete wavelet representation. Compact feature vector obtained is then classified by different methods: K nearest neighbour algorithm, statistical Bayesian algorithm and artificial neural network. Mallat decomposition algorithm is also developed in order to enhance flaw detectability. Finally, time frequency algorithms based on STFT, Wigner–Ville, Gabor transform are developed and applied to thickness measurements of materials with small thickness.

Stack imaging of spectral amplitudes based on impact-echo for flaw detection

Abstract: The impact-echo method recently has drawn a remarkable amount of attention for nondestructive evaluation of defects in concrete structures Extracting resonance frequencies responsible for the locations of reflectors, the depth and the presence of defects are estimated So far, however, the technique has some limitations for practical applications This is because there exist unresolved problems in the application of the impact-echo to concrete structures in service Consequently, the method is studied theoretically on the basis of the elastodynamics and the signal analysis In order to circumvent the difficulty to identify peak frequencies in the conventional procedure, a new procedure to evaluate defects in concrete is investigated, applying an imaging procedure Thus, stack imaging of spectral amplitudes based on the impact-echo is developed This procedure is applied to a prestressed concrete beam to classify a grouted duct and an ungrouted duct The location and the presence of the ungrouted duct can be visually identified

Use of acoustic emission technique for the early detection of aluminum alloys exfoliation corrosion

Abstract: Exfoliation corrosion of aluminum alloys is a form of localized corrosion which affects many industries, specially aeronautics. The study of this corrosion mode using only electrochemical techniques is not fully efficient for the detection and control on line of this phenomenon. Therefore, we developed a non-destructive testing technique based on the acoustic emission recordings in order to follow-up this form of corrosion on aluminum alloys. Indeed, recent works have shown the interest of the acoustic emission for the detection, the monitoring and the localization of pitting corrosion on aluminum alloys. This pitting corrosion phenomenon is currently well understood and the experimental methodology acquired during that study is transposed to the study of exfoliation corrosion of aluminum alloys. The present study is conducted on two aluminum alloys: (Al 2024 T3, and Al 7449 T6 and T7). Samples are immersed 4 days in the modified ASTM STP 1134 saline solution. Observations of the structures after tests show that the exfoliation corrosion sensitivity of alloy 7449 T6 is more important than for alloy 7449 T7 which exhibits only the presence of small and non-occluded pits. Very severe exfoliation corrosion was also observed on Al 2024 T3, but after a longer immersion time or in a more acid solution. The recording of the acoustic emission activity shows evident links between this activity and the exfoliation corrosion rate. The analysis of the signal's characteristics reveals a population corresponding to the release of hydrogen bubbles. A few more energetic signals have also been observed. Their source can be either, the cracking resulting from the separation of sheets of metal, or the development and evolution of hydrogen bubbles formed inside blisters during exfoliation corrosion.

Monitoring extremely slow rolling element bearings: part II

Abstract: The most established technique for monitoring the integrity of rolling element bearings is vibration analysis. However, at extremely slow rotational speeds monitoring the health of rolling element bearings is fraught with difficulty. Part I of this work details difficulties with monitoring rolling element bearings rotating at speeds of less than 2 rpm. Furthermore, it presents a study of the high frequency acoustic emission (AE) technique as a means of monitoring low-speed rolling element bearings. The application of AEs to monitor operational slow-speed bearings with bore diameters of 120 mm is presented. It is concluded that the mechanism of AE generation was the relative movement between bearing elements experiencing loss of mechanical integrity, for example, the rolling/sliding action of a roller on a defective outer race.

Non-destructive evaluation of smart materials by using extrinsic Fabry–Perot interferometric and fiber Bragg grating sensors

Abstract: The real-time non-destructive evaluation (NDE) technologies of engineering structures are very important to assess the performance of in-service structures. It is very difficult to carry out the on-line structural integrity monitoring by using classical NDE methods. Fiber optic sensors in smart structures provide a unique opportunity to real-time monitor the structural health status by using embedded sensors. This paper examines the possibility of extrinsic Fabry–Perot interferometric (EFPI) and fiber Bragg grating (FBG) sensors for NDE of composite and aluminum structures. The experiments of 3-point bending measurement of composite laminates and aluminum plates with and without damages by using the EFPI and FBG sensors are performed, respectively. The experimental results show that the flexural strain of damaged structures is much higher when compared with relevant undamaged structures under same bending load by using both EFPI and FBG sensors. Furthermore, an excellent accordance was found between the results of the 3-point bending tests by using surface-mounted EFPI and FBG sensors

An artificial neural network for eddy current testing of austenitic stainless steel welds

Abstract: An artificial neural network is presented for on-line eddy current testing of austenitic stainless steel welds. Time-domain parameters that are functions of digitized in-phase and quadrature components of probe impedance are used as input to the neural network and the network output, in depth units, is evaluated and displayed continuously. The neural network is trained to recognize disturbing variables such as variations in weld microstructure, lift-off and edge-effect as well as notches of different depth. The neural network is able to detect and characterize longitudinal and transverse surface-breaking notches, despite the presence of disturbing variables.

Comparison of NDT techniques on a post-tensioned beam before its autopsy

Abstract: Following the complete demolition of a prestressed concrete bridge in southern France, a suspected weak post-tensioned beam was retained for non-destructive testing (NDT). Ground penetrating radar, Ferroscan ® (covermeter), gamma-ray radiography and impact-echo methods have all been tested and their results then discussed after the autopsy of the beam by means of hydro-demolition. This paper describes the extent to which NDT surveys are able to respond to the needs of structural engineers, through the use of complementary NDT approaches. The introduction of a second technique should provide a more pertinent response while eliminating certain ambiguities either by improving measurement reliability or by focusing on questionable zones to obtain more precise local measurements. Afterwards, the problems still not adequately resolved by these techniques are pinpointed. This step concludes with the set of needs heretofore unmet by such techniques.

Detection and classification of flaws in concrete structure using bispectra and neural networks

Abstract: The problem addressed in this paper is the detection and classification of flaws in concrete structure. It is known that higher-order spectra contain information not present in the power spectrum and can suppress Gaussian noise. Thus estimates of higher-order spectra have been shown to be useful in certain signal processing problems. This paper is concerned with the feature extraction from bispectra for concrete flaw detection. Impact-echo experiments are carried out for three different types of flaw in concrete structure. For each monitoring signal, after bispectral estimation, features are selected from the modules of bispectra in the primary region. For automatic interpretation, a multilayer back-propagation neural network is used as a classifier. Both clean data and data with additive white Gaussian noise are used for training and testing. The classification results obtained experimentally demonstrate that this method has good detection rates in varying environments.

Combination of NDT techniques for site investigation of non-ballasted railway tracks

Abstract: Today's civil engineering NDT needs are for quantitative, fast to apply and easy to interpret methods. They are required on site both for evaluation of existing structures/infrastructure and for quality control in new construction. With this purpose, BAM has carried out in-situ feasibility studies, using radar, impact-echo and ultrasonic-echo methods. Two examples are given of their combined application on different railway slab–track constructions, using high frequency radar, scanning impact-echo and ultrasonic array methods. The aim was to evaluate limitations and advantages, comparability and complementarity of these methods. The on-site measurements demonstrated the possibility to investigate, with good accuracy via NDT, the bonding conditions between sleepers and slab, to locate voids and measure layer thickness. The acoustic and electromagnetic methods used, were found to be complementary.

Non-invasive moisture monitoring within an earth embankment — a precursor to failure

Abstract: An experimental monitoring system is described using electrical resistivity as a proxy for imaging changing moisture content distribution in engineered earthworks. The approach is illustrated using a case history concerning a road embankment constructed of tropical red soil in western Kenya. Tropical red soils have highly variable properties, governed by their soil fabric and mineralogy. As earthworks materials, their geotechnical behaviour is extremely sensitive to changes in moisture content and compaction. The relationship between moisture content and electrical resistivity was established in the laboratory on core obtained using a monitored drilling and sampling technique. The non-invasive nature of electrical resistivity surveys was exploited in the design of a monitoring system placed below the pavement in the topmost layers of compacted soil. Monitoring over a period of 18 months is reported, starting prior to the construction of the pavement following the completion of soil compaction. Initially substantial variability in moisture content was inferred from surface monitoring, and even larger changes were seen in corresponding downhole measurements. The moisture content within the body of the embankment stabilised after six months, while a moist layer ‘trapped’ beneath the pavement dissipated over the following 10 months. Two surveys were undertaken during the ‘December rains’; they showed large changes in moisture content had occurred quickly in the surface layers on one side of the embankment. This area subsequently failed as a small landslip and was remediated by additional drainage.

Field pattern characteristics of GPR antennas

Abstract: Sub-surface radar has become increasingly popular for carrying out completely non-invasive integrity tests on concrete structures. Commercial systems are available with a range of antenna frequencies that may be selected for an investigation. The choice of a particular antenna frequency is often a compromise between the physical size of the antenna and the penetration and resolution capabilities provided by the antenna. However, there is an interaction between a surface contact antenna and the substrate under investigation that will alter the field pattern being transmitted that may have significant implications for interpretation of results. An experimental programme is described in which the characteristics of commercially available 900 MHz and 1 GHz antennas are compared in air and when in contact with concrete and water surfaces, together with an emulsion used in previous studies to simulate the radar properties of concrete. Results are presented showing the influence of the material being measured upon the signal divergence. These effects are considered in terms of the practical implications for field-testing using radar.

The transmission of acoustic emission across large-scale turbine rotors

Abstract: Seal and blade-tip rubbing in turbo machinery can often result in costly rotor damage. Acoustic emission analysis at the bearings has previously been cited as a viable non-destructive monitoring technique by which the onset of such rotor–stator contacts can be detected in large-scale turbines [Aerospace Conf., IEEE Proc. 6 (2000) 79; Electr. Engng Jpn 110 (2000)]. These studies suggested that the stress waves or acoustic emissions produced during the frictional rub process can propagate across the turbine surface and across the bearing interfaces to be detectable by the sensor attached to the bearing housing. However, results from the actual large-scale turbine units have been scarce. This paper presents a fundamental/pre-requisite study on the transmissibility of acoustic emission (AE) signatures across very large-scale turbine rotors. It is concluded that the actual AE attenuation incurred does not inherently exclude the detection of seal and blade rubbing in turbines. However, its success is highly dependent on advanced signal processing.

Advanced monitoring of cracked structures using video microscope and automated image analysis

Abstract: Traditionally, the monitoring of cracks in buildings is performed by very simple techniques yielding only limited information. By means of a video microscope in combination with automated image analysis, an advanced and more precise analysis method of cracks in structures is obtained. The system was evaluated during laboratory tests on concrete beams and slabs. Furthermore, on site monitoring has been evaluated by measurements in a historical building in Ghent. The results were verified by means of traditional measuring techniques. For in situ monitoring of cracked structures, the combination of video microscope and automated image analysis seems to be a user-friendly and very accurate technique.

Wavelet analysis based deconvolution to improve the resolution of scanning acoustic microscope images for the inspection of thin die layer in semiconductor

Abstract: Scanning acoustic microscopy (SAM) is used as an important non-destructive test tool in semiconductor reliability evaluation and failure analysis. However, inspection of thin die layer has proven difficult as the reflected signals from the die top and bottom are superimposed. Conventional deconvolution techniques have been used for the improvement of time resolution in A-scan signal, however, they are not effective for SAM signal because the waveform of the reflected echo is quite different from the incident waveform due to the frequency dependent attenuation and the focal effect. In this paper, in order to overcome this difficulty, a new signal processing method, wavelet analysis based deconvolution technique is proposed. Its validity is approved by computer simulations and practical performances are demonstrated by experiments for the fabricated semiconductor sample.