Showing papers in "Ndt & E International in 2010"

Defect detection using ultrasonic arrays: The multi-mode total focusing method

Abstract: Ultrasonic arrays allow a given scatterer to be illuminated from a wide range of angles and hence are capable of extracting significant information about the scatterer. In this paper a general imaging methodology, termed multi-mode total focusing method, is proposed in which any combination of modes and reflections can be used to produce an image of the test structure. Like the total focusing method, this approach is implemented by post-processing the full matrix of array data to achieve a synthetic focus at every pixel in the image. A hybrid model is used to predict the array data and demonstrate the performance of the multi-mode imaging concept. This hybrid model combines far field scattering coefficient matrices with a ray-based wave propagation model. This allows the inclusion of longitudinal waves, shear waves and wave mode conversions. It is shown that, with prior knowledge of likely scatterer location and orientation, the mode combination and array location can be optimised to maximise the performance of array inspections. A practically relevant weld inspection application is then described and its optimisation is discussed.

A review of non-destructive techniques for the detection of creep damage in power plant steels

Abstract: The assessment of creep damage in steels employed in the power generation industry is usually carried out by means of replica metallography, but the several shortcomings of this method have prompted a search for alternative or complementary non-destructive techniques, ranging from ultrasonic to electromagnetic methods, hardness measurements and nuclear techniques. A critical review of the main results obtained to date in the secondary and tertiary stages of creep is presented in this paper, and the advantages and disadvantages of each method are discussed. Ultrasonic and potential drop techniques appear to be the most promising, but further research is needed before they are fully mature for deployment in the field.

Vision-based displacement measurement method for high-rise building structures using partitioning approach

Abstract: The horizontal displacement of a high-rise building structure is usually considered as one of the major indicators to assess the structural safety. It is, however, difficult to directly measure the displacement of such a flexible structure due to the inaccessibility to a reference point usually needed for conventional displacement sensors and the huge size of the structure. In order to resolve this issue, a novel vision-based displacement measurement technique is proposed by employing a partitioning approach (i.e., successive estimation of relative displacements and rotational angles throughout a large flexible structure). A series of the experimental tests using two webcams installed on a flexible steel column structure have been conducted to validate the feasibility of the proposed method. The test results showed that the difference between the displacement measured from the proposed method and the exact value is less than 0.5%. Therefore, the proposed method could be considered as one promising candidate for measuring the displacement of high-rise building structures.

Pulsed eddy current technique for defect detection in aircraft riveted structures

Abstract: The Pulsed Eddy Current (PEC) technique is an effective method of quantifying defects in multi-layer structures. It is difficult to detect defects in riveted structures of aging aircraft. Based on theoretical analysis of PEC technique, three different probes, including a differential hall probe, a differential coil probe, and a two-stage differential coil probe are designed to detect this kind of defects. The averaging method and wavelet analysis method are used to de-noise the hall response signals. By selecting peak amplitude and zero-crossing time of response signal in time domain as key features, defects in riveted structures can be detected effectively. The experimental results indicated that the differential coil probe acted as effectively as the differential hall probe. The defects between third layer and fourth layer in riveted structures can be detected by utilizing the two-stage differential coil probe. The PEC technique has a promising application foreground in the field of aeronautical nondestructive testing.

Quantitative evaluation of angular defects by pulsed eddy current thermography

Abstract: Pulsed eddy current (PEC) thermography employs a combination of PEC and thermographic non-destructive testing (NDT) techniques. This study considers the capabilities of PEC thermography for obtaining quantitative information about cracks set at an angle to the surface. The investigation is implemented by simulating the transient thermal distribution for angular slots, via time-stepping 3D finite element analysis (FEA), with the experimental work undertaken for verification. A slope inclination feature of the transient temperature distribution has been extracted and presented for estimating the angle of slots that is independent of slot depth and length inside the sample. With the identification of the slot angle, quantification of the length/depth of the slot inside the sample can then be made through a maximum temperature amplitude feature. Experimental studies have been undertaken for evaluation of the numerical simulation and transient feature extraction methods.

Deflection measurement on vibrating stay cables by non-contact microwave interferometer

Abstract: The microwave interferometry has recently emerged as an innovative technology, suitable to the non-contact vibration monitoring of large structures. The paper addresses the application of microwave remote sensing to the measurement of the vibration response in the longer cables of two cable-stayed bridges. In order to highlight the reliability and accuracy of the radar technique, the natural frequencies (and the cable tensions predicted from natural frequencies) identified from radar data were compared to the corresponding quantities obtained by using more conventional techniques. The results of the investigation highlight the accuracy and the simplicity of use provided by the microwave remote sensing, as well as its effectiveness to simultaneously measuring the dynamic response of all the stay cables of an array.

Quantitative study of metal magnetic memory signal versus local stress concentration

Abstract: The technique of metal magnetic memory (MMM) has attracted great attentions in the field of non-destructive tests due to its unique advantages of easy-operation, low cost, and high efficiency. However, a thorough understanding of the physical mechanism of MMM phenomenon has not been clearly addressed. Generally, MMM tests can only find the possible locations of defects without quantitative descriptions about the defect characteristics. To promote study in this area, a linear magnetic-charge model is employed to analyze the self-magnetic flux leakage (SMFL) distribution in the local stress-concentration zone. Theoretical results based on this model can capture some basic characteristics of SMFL signals of ferromagnets observed in experiments. Specially, the model provides some quantitative results about the effects of defect depth and location (surface- or inner-defects) on SMFL signals.

Infrared thermography processing based on higher-order statistics

Abstract: Active thermography has reached a high status as a non-destructive evaluation method due to both ease and speed of inspection. Nevertheless, automatic processing of an infrared (IR) sequence is essential in order to reduce human intervention. Unfortunately, this target is difficult to achieve given the amount of data recorded by the IR camera during a typical inspection process and human participation is absolutely necessary. In this paper, higher-order statistics (HOS) analysis is employed to process IR sequences and to compress the most useful information into a unique image for each inspection. Pulsed infrared thermographic temporal response is well-known with a statistical behaviour. This statistical behaviour is analyzed and the results of its application to carbon fibres reinforced plastic (CFRP) samples are reported.

Non-destructive determination of residual stress state in steel weldments by Magnetic Barkhausen Noise technique

Abstract: The purpose of this study is non-destructive determination of residual stresses in the welded steel plates by Magnetic Barkhausen Noise (MBN) technique. A MBN-stress calibration set-up and a residual stress measurement system with scanning ability were developed. To control the accuracy and the effectiveness of the developed system and procedure, various MBN measurements were carried out. The MBN results were verified by the hole-drilling method. Microstructural investigation and hardness measurements were also conducted. It was concluded that if the calibration procedure including the effect of microstructure is appropriate then MBN is a very promising method for non-destructive, fast and accurate prediction of residual stresses in the welded plates.

Autofocusing ultrasonic imagery for non-destructive testing and evaluation of specimens with complicated geometries

Abstract: This paper introduces the autofocus problem for non-destructive testing and evaluation of specimens with complicated geometries using ultrasonic arrays. We present an algorithm for autofocusing imagery of point-like defects obtained from a flexible array with an unknown profile. The relative positions of the array elements are parameterised using a cubic spline function and the spline control points are estimated by iterative maximisation of an image contrast metric via simulated annealing. The resultant spline gives an estimate of the array profile and the profile of the surface that it has conformed to, allowing the generation of a well-focused image. The algorithm is demonstrated on experimental data obtained using a flexible array prototype.

Experimental investigation of reflection in guided wave-based inspection for the characterization of pipeline defects

Abstract: In guided wave-based pipeline inspection, the reflection from a defect usually includes sufficient defect-relevant information. It has been found that the reflection of guided waves at defect is the joint result of interference between reflections from both its front and back edges. However, the majority of published work has only studied and considered the overall resulting reflection signal for the inspection-related tasks. This paper reports the findings of extensive experimental investigation on the effects of a defect’s geometric parameters on the two reflection signals from the respective edge of defect. We show that the two edge reflections present different signal features, which further results in the complexity of the overall reflection signal from the defect. We accordingly propose a new strategy that considers the extraction of two edge reflection signals embedded in overall reflection signal and the use of the identified edge signals so as to enable an accurate and quantitative pipeline defect characterization.

Theoretical studies of metal magnetic memory technique on magnetic flux leakage signals

Abstract: Compared to traditional nondestructive testing methods, the technique of metal magnetic memory (MMM) is a more effective way in evaluating early damages of ferromagnets due to the existence of material stresses. In practical engineering, this technique has been extensively applied in different fields. However, very limited quantitative research has been carried out on quantitatively studying the relations between the stress state and self-magnetic flux leakage (SMFL) signal. In this paper, the distribution of SMFL is investigated based on the theory of magnetic charges. A linear description of the magnetic charge and the stress state is given. The theory can capture some basic characteristics of the SMFL distribution in the stress concentration of ferromagnets, e.g. the tangential SMFL component H P(x ) reaching a maximum value and the normal component H P(y ) changing positive–negative sign in the maximum stress concentration zone (SCZ). Moreover, the effects of the stress-concentration range and lift-off value on SMFL signals are discussed as well.

Classification of pulsed eddy current GMR data on aircraft structures

Abstract: This paper presents a technique to automatically detect third-layer cracks at rivet sites in aircraft structures using the response signals collected by giant magneto-resistive (GMR) sensors. The inspection system uses pulsed waveform as the excitation source of a multi-line coil and captures the transient fields associated with the induced eddy currents via a GMR sensor, which was developed to detect cracking and corrosion in multi-layer aircraft structures. An automatic scan of the region around the rivet generates C-scan image data that can be processed to detect cracks under the rivet head. Using a 2-D image of each rivet head, feature extraction and classification schemes based on principal component analysis and the k-means algorithm have been successfully developed to detect cracks of varying size located in the third layers at a depth of up to 10 mm below the surface.

Acoustic emission activities and damage evaluation of reinforced concrete beams strengthened with CFRP sheets

Abstract: In this study, the applicability of acoustic emission (AE) techniques to monitor damage evolution in reinforced concrete (RC) beams strengthened in flexure with carbon fiber reinforced polymer (CFRP) sheets is investigated. The objective is to initiate the creation of a user-friendly health monitoring system for RC structures strengthened with CFRP sheets using AE techniques. Five beams, 200 mm×300 mm in cross-section, were tested under three-point bending over a span of 1700 mm. One of the beams was tested in its virgin condition to serve as reference; the remaining four beams were tested after being strengthened with CFRP sheets bonded on the tension face. The parameters investigated in this study include both the amount of CFRP sheets and construction imperfections (the CFRP sheets were intentionally bonded without adhesive in the centermost 10% and 20% bonding area). The AE signals were collected and analyzed for all specimens. The AE parameters were analyzed for four levels of damage based on initial crack, propagation, yielding of main bars, and fracture or rip-off of the CFRP sheets. The frequency-peak magnitude distribution of the AE parameters was used to determine the b-value, defined by the Gutenberg–Richter relationship, for evaluating the damage evolution and fracture process of RC beams strengthened in flexure with CFRP sheets. From the results of this study, the signal characteristics – event, amplitude versus frequency, and amplitude versus duration – show clear differences in the different loading stages, depending upon the active damage mechanism. The b-value is correlated to the fracture process of the RC beams bonded with CFRP sheets and the degree of localization of damage. The AE technique is a useful nondestructive technique for monitoring the behavior of RC beams that are externally reinforced in flexure with CFRP sheets.

Acoustoelastic effect in concrete material under uni-axial compressive loading

Abstract: This study deals with the general matter of non-destructive evaluation of pre-stressed structures in civil engineering. Usually such structures are composed of concrete and are steel reinforced. Proposed idea is the evaluation of mechanical stress state of a concrete body (instead of steel cables) via ultrasonic non-destructive evaluation (NDE), by using the link between ultrasonic velocities and mechanical stresses provided by the acoustoelasticity theory. Velocities of the ultrasonic waves (longitudinal and transversal with different polarizations) are observed during propagation through a concrete body submitted to uni-axial loading (compressive testing). Obvious variations in velocity are found depending on the mechanical stress state (e.g. Δ c =92 m/s at σ =16 MPa for longitudinal waves). Thus acoustoelastic behavior of concrete is demonstrated. Further analyses provide acoustoelastic coefficients of concrete about ten times higher than the common ones of steel. The feasibility of stress evaluation using ultrasounds in concrete structures is proved under laboratory conditions.

An adaptive-network-based fuzzy inference system for classification of welding defects

Abstract: In this paper, we describe an adaptive-network-based fuzzy inference system to recognise welding defects in radiographic images In a first stage, image processing techniques, including noise reduction, contrast enhancement, thresholding and labelling, were implemented to help in the recognition of weld regions and the detection of weld defects In a second stage, a set of 12 geometrical features which characterise the defect shape and orientation was proposed and extracted between defect candidates In a third stage, an adaptive-network-based fuzzy inference system (ANFIS) for weld defect classification was used With the aim of obtaining the best performance to automate the process of the classification of defects, of all possible combinations without repetition of the 12 features chosen, four were used as input for the ANFIS The results were compared with the aim to know the features that allow the best classification The correlation coefficients were determined obtaining a minimum value of 084 The accuracy or the proportion of the total number of predictions that were correct was determined obtaining a value of 826%

Ultrasonic and structural characterization of anisotropic austenitic stainless steel welds: Towards a higher reliability in ultrasonic non-destructive testing

Abstract: The non-destructive testing of austenitic stainless steel welds of the primary coolant piping system is a significant problem for the nuclear industry. Ultrasonic techniques would be very helpful to detect, locate and size potential defects. Unfortunately, austenitic welds are coarse-grained, heterogeneous and anisotropic. This leads to aberration and scattering of the ultrasonic waves. In this paper, we present several experimental results of ultrasonic testing of two austenitic welds exhibiting high anisotropy. In order to explain the observed display of wave propagation phenomena such as beam deviation, we use finite element modeling. The modeling is associated with a complete characterization of the inspected welds. Two essential characteristics of the welds are determined: the average elastic constants of the weld and the grain orientations. The capability of the model is illustrated in different testing configurations. This work associating structural characterization and modeling shows that a better understanding of the phenomena of ultrasonic propagation should allow the interpretation and reliability of the industrial inspections of heterogeneous anisotropic welds to be improved.

Dynamic modulus of asphalt mixture by ultrasonic direct test

Abstract: This paper describes the experimental procedure followed for direct determination of dynamic modulus of asphalt mixtures by ultrasonic direct test at a specified temperature. Tests were performed on ten cylindrical samples of dense and porous asphalt mixtures manufactured with dolerite and limestone aggregates. Dynamic moduli obtained by ultrasonic transmission, calculated at a frequency of 65 kHz, were compared with values directly determined by standard dynamic tests applied in Spain at frequencies of 2, 5, 8 and 10 Hz. The obtained results demonstrate that the magnitudes for moduli calculated by ultrasonics are higher than those obtained by standard dynamic tests. It is concluded that for asphalt mixtures tested ultrasonically the increase of moduli magnitude can be associated with an increase in the frequency used but may also be due to the different testing methods. Nevertheless, these values can be used as a reference value for dynamic modulus of asphalt pavements at low strain, being necessary to apply a correction factor to replace the low frequency standard dynamic test, which is more expensive, difficult and time consuming.

Processing strategies for high-resolution GPR concrete inspections

Abstract: A high-resolution multi-sensor and multi-polarization Ground Penetrating Radar (GPR) dataset was acquired on a concrete retaining wall. This dataset was characterised as a low pass filter with the help of a moving window spectral analysis. In order to examine the benefits and limits of innovative processing strategies, the dataset was processed with three different methods: classical 2-D processing, full 3-D processing followed by data fusion and inverse scattering followed by data fusion. A comparison of the results for two layers of rebar present in the wall shows that the innovative approaches improve the results for near surface structures when compared to classical 2-D processing. For deeper structures, the benefits of the innovative approaches are limited because of the low pass properties of the concrete.

Stress concentration degree affects spontaneous magnetic signals of ferromagnetic steel under dynamic tension load

Abstract: Under the excitation of geomagnetic field, stress can induce spontaneous magnetic signals in ferromagnetic materials, which can be potentially applied to estimate the damage degree. In this research, the normal component of magnetic field, Hp(y), was measured during dynamic tension test on the surfaces of ferromagnetic specimens having different stress concentration factors, α constant amplitude sinusoidal tensile load cycles were applied on 18CrNi4A steel plate specimens whose α were respectively 1, 3 and 5. Then correlation of Hp(y) values’ changes with loading cycles reported. The results indicate that stress concentration factor, α extremely affects the magnetic signal, the absolute value of the gradient, |K|, increases with the increase of α. Meanwhile, |K| also goes up when loading cycle increases in the initial state. However, after a few cycles, those Hp(y) curves corresponding to different cycles become similar before cracking.

A comparative study of high resolution cone beam X-ray tomography and synchrotron tomography applied to Fe- and Al-alloys

Abstract: X-ray computed tomography (XCT) has become a very important method for non-destructive 3D-characterization and evaluation of materials. Due to measurement speed and quality, XCT systems with cone beam geometry and matrix detectors have gained general acceptance. Continuous improvements in the quality and performance of X-ray tubes and XCT devices have led to cone beam CT systems that can now achieve spatial resolutions down to 1 μm and even below. However, the polychromatic nature of the source, limited photon flux and cone beam artefacts mean that there are limits to the quality of the CT-data achievable; these limits are particularly pronounced with materials of higher density like metals. Synchrotron radiation offers significant advantages by its monochromatic and parallel beam of high brilliance. These advantages usually cause fewer artefacts, improved contrast and resolution. Tomography data of a steel sample and of two multi-phase Al-samples (AlSi12Ni1, AlMg5Si7) are recorded by advanced cone beam XCT-systems with a μ-focus (μXCT) and a sub-μm (nano-focus, sub-μXCT) X-ray source with voxel dimensions between 0.4 and 3.5 μm and are compared with synchrotron computed tomography (sXCT) with 0.3 μm/voxel. CT data features like beam hardening and ring artefacts, detection of details, sharpness, contrast, signal-to-noise ratio and the grey value histogram are systematically compared. In all cases μXCT displayed the lowest performance. Sub-μXCT gives excellent results in the detection of details, spatial and contrast resolution, which are comparable to synchrotron-XCT recordings. The signal-to-noise ratio is usually significantly lower for sub-μXCT compared with the two other methods. With regard to measurement costs “for industrial users”, scanning volume, accessibility and user-friendliness sub-μXCT has significant advantages in comparison to synchrotron-XCT.

Application of ultrasonic guided waves for non-destructive testing of defective CFRP rods with multiple delaminations

Abstract: One of the most important parts of gliders is a lightweight longeron reinforcement made of carbon fibre reinforced plastic (CFRP) rods. During manufacturing, in order to build the arbitrary spar profile, these small diameter (few millimetres) rods are glued together into epoxy filled matrix. Still, defects present in the rods, such as break of fibres, multiple delaminations due to lack of bonding and reduction in density affect construction strength markedly and are extremely complicated to eliminate. Therefore, appropriate non-destructive testing techniques intended for carbon fibre rods should be applied prior to gluing them together. The aim of the present paper is to analyse development possibilities of NDT technique based on application of ultrasonic guided waves and intended for CFRP rods that are used for aerospace applications and are defective with multiple delaminations. The regularities of ultrasonic guided wave propagation in a defective CFRP circular-shape rod with multiple delaminations have been investigated using 3D numerical simulations, finite difference and finite element models. The corresponding experiments have been conducted as well. Based on leaky wave suppression over a defective region due to the weak vertical component of particle displacements, the mechanism of guided wave interaction with the region of multiple delaminations is explained from the ultrasonic NDT viewpoint.

A hybrid finite element model for simulation of electromagnetic acoustic transducer (EMAT) based plate waves

Abstract: This paper reports simulations of the generation of Lamb wave modes in thin plates using a meander coil (meander line) EMAT, which works under the principle of Lorentz force mechanism in non-magnetic materials. The numerical simulations have been compared with measurements. The numerical work presented in this paper is divided into two parts. First, a 2D electromagnetic model is developed for calculating the Lorentz force density, which is the driving force for elastic wave generation within the plate. Second, the calculated force at each point in the metal is used as the driving force for generating elastic wave modes in the plate. These calculated Lamb wave modes are compared with those generated experimentally in a thin plate. Additionally, the measured wave modes are analyzed with the help of dispersion curves and a time–frequency analysis. Our numerical work is also extended to analyze the interaction of Lamb wave modes with defects. The simulations compare favorably with the measurements presented here.

Shape based reconstruction of experimental data in 3D electrical capacitance tomography

Abstract: Electrical capacitance tomography (ECT) is a non-invasive and non-destructive imaging technique that uses electrical capacitance measurement at the periphery of an object. The image reconstruction problem in ECT is an ill-posed inverse problem. This paper presents a level set based shape reconstruction method applied to 3D ECT using experimental data. The finite element models have been implemented based on a 32 electrode ECT system to formulate the forward problem. Development of the level set technique enables detection of smaller inclusions and improves the accuracy of boundary shapes of inclusions. The paper uses a shape based method rather than traditional image based methods. The shape-based approach offers several advantages compared with more traditional voxel-based approaches. The incorporation of an intrinsic regularization in the form of a-priori assumptions, regarding the general anatomical structures present in the medium, reduces the dimensionality of the inverse problem and thereby stabilizes the reconstruction. The level set strategy (which is an implicit representation of the shapes) can handle the topological during this reconstruction process. Additionally in this paper a new two-stage level set method has been developed, which shows significant improvement compared with the traditional level set reconstruction algorithm.

Study of wave propagation in a multiwire cable to determine structural damage

Abstract: Multiwire cables are widely used in engineering structures, even though there is a growing concern about the presence of environmental degradation, random overloads, and corrosion affecting their structural integrity. In this work, propagation of guided waves in a multiwire aluminium conductor steel reinforced cable (ACSR) for structural damage detection is studied using time-frequency analysis based on short time Fourier transform (STFT). Identification of propagation vibrational modes and measurements of mode energy amplitude are used to correlate the spectral energy change to the extension of an artificial damage introduced to the cable sample by a notch transversal to the pulse propagation. The results showed that mechanical contact among the wires generates mode conversion of longitudinal to flexural modes.

Physics-based image enhancement for infrared thermography

Abstract: Thermal imaging with an infrared camera can be used to view the location and intensity of heat sources in space and time. In a thermal conductor, thermal diffusion blurs out those heat sources. Knowledge of the physics of thermal diffusion can be used to enhance the spatial and temporal resolution of thermal images. In two dimensions, quantitative reconstruction of the heat source intensity is possible. The same algorithm applied to three-dimensional heat flows provides dramatic improvements in temporal and spatial resolution of the thermal images. Performance is illustrated both in theory and by experiment. An application example demonstrates utility to nondestructive evaluation.

NDT measurements for the prediction of 28-day compressive strength

Abstract: A procedure is proposed for the prediction of the compressive strength of concrete based on the use of NDT measurements. Prior to formwork removal, electrical resistivity can be assessed and based on an electrical resistivity versus compressive strength correlation curve, the strength of concrete can be estimated. This procedure ensures, for example, that the formwork is only removed when the concrete reaches a predefined compressive strength, dispensing with the calculation of concrete maturity. On the other hand, the whole surface of the concrete structure can be systematically measured so as to estimate 28-day compressive strengths and detect areas that could potentially not conform to the specified requirements. In this manner, a realistic overview of the in situ compressive strength of the whole structure can be obtained. The procedure proposed predicts the 28-day strength of concrete based on two models. The first model is based on an empirical equation used for the maturity assessment of concrete. The second model is based on a theoretical equation for the nucleation process of cement hydration. The effect of temperature and different curing conditions (concrete maturity) can be effectively taken into account by the conversion of all electrical resistivity measurement to the same referential. The models are used to estimate 28-day compressive strength based on the 7-day electrical resistivity measurements. The results show that the errors in estimation of the 28-day compressive strength are less than 22% and 10% for these models, respectively. If 28-day electrical resistivity measurements are used then the error in estimation is lower, circa 9% and 5% for these models, respectively.

Configuration optimization of magnetostrictive transducers for longitudinal guided wave inspection in seven-wire steel strands

Abstract: The configuration of magnetostrictive transducers for both transmitter and receiver was optimized for the generation and reception of ultrasonic longitudinal guided waves in seven-wire steel strands in a pitch catch arrangement. Three axisymmetric permanent magnets significantly improved the capability of magnetostrictive transducers compared to two permanent magnets, and effectively increased the amplitude of the longitudinal guided wave mode, L(0, 1) at 160 kHz. Experimental results show that the maximum amplitude of a received guided wave signal could be obtained by using a receiver with a three-layer coil in parallel and a transmitter with a three-layer coil in series. The amplitudes of the defect-reflected signal increased by as much as 50% or more as compared with those when both transducers used a single layer coil. As a result, magnetostrictive transducers with an optimized configuration, including permanent magnet distribution and multilayer coil connection, could be efficiently used for the inspection of seven-wire steel strands by using ultrasonic guided waves in a pitch catch arrangement.

Rapid computation of eddy current signals from narrow cracks

Abstract: An existing boundary element method model for eddy current inspection of ideal or narrow cracks is revised. Using some lately devised concepts on efficiently and rapidly simulating canonical eddy current geometries, we show how this model can be made easier to implement and faster to calculate. The approach is largely based on a novel method for rapidly calculating the Green's function and reducing the matrix fill-time, and also on a similar method for rapidly computing the incident electric field. As a result, the model has been made an order of magnitude faster than the existing one without sacrificing accuracy. We pay attention to numerical analysis details and analyze issues that so far have not been clarified. The validity of our approach is also verified by the experiment. Although we have tested the model against published data, we have also produced our own precision measurements for surface and through-the-thickness slots in plates with the coils performing scans along and across the slots at various frequencies.

The pulsed eddy current response to applied loading of various aluminium alloys

Abstract: In this paper the pulsed eddy current (PEC) response of a range of heat treatable and non-heat treatable aluminium alloys (AA-1050, 2024, 5083 and 7075) to applied stress well below the elastic limit is studied. The effect of prior heat treatment and prior plastic deformation on the stress dependency of the pulsed eddy current response is quantified using the peak value of the PEC difference signal. In certain cases non-linear stress responses are measured. Because it is well known that the effect of stress on conductivity is anisotropic both isotropic (circular) probes and directional (rectangular) probes are compared. It is found that both prior plastic work and heat treatment condition can have a strong effect on the stress coefficients, even changing the sign of the stress dependence. The implications of these effects for the future characterisation of residual stress state by PEC are discussed.