New Publications are available for Nondestructive testing: eddy current testing and related techniques
http://dl-live.theiet.org
New Publications are available now online for this publication.
Please follow the links to view the publication.Analysis and research of eddy current detector based on virtual prototype
http://dl-live.theiet.org/content/conferences/10.1049/cp_20061152
The working principle and characteristics of eddy current detector is introduced in this paper, and in order to resolve the problem of contact collision existed in this system, the part simulation method is used by means of ADAMS & RecurDyn which based on the virtual technique about dynamic simulation model and the usage of interpolating spline to carry out data deliver between kinds of software, and the design of the virtual machine is completed. Then according to the simulation results, the simplified model of principal axis and bearings are set up, and the neutral phenomenon between the running main shaft and the probe to avoid the lift-off effect is analyzed. Meanwhile, the simulation of the whole equipment's working state is carried out, and the coordination of each system operation is observed. Currently, this detector has already put into the usage and obtained a good result.Shape identification of conductive anomalies by a new ECT data inversion algorithm
http://dl-live.theiet.org/content/conferences/10.1049/ic_20020193
Eddy current testing (ECT) is a technique used to reconstruct the unknown resistivity of conductive materials in a non-destructive way. In this technique a time-varying current flowing in an exciting coil placed near to the specimen induces eddy currents in the specimen under testing. The induced eddy currents, depending on the spatial values of the resistivity and magnetic permeability, affect the signal detected by the surrounding pick-up coils or magnetic sensors. Information concerning the spatial distribution of the resistivity is then retrieved by inversion of the measured data. In the present paper we tackle the non-linear inverse problem of retrieving the shape of interfaces separating different phases for a conductive materials made of two different conducting phases. Hence we assume that the resistivity at a given spatial coordinate assumes either the value η<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">i</sub> or η<sub xmlns="http://pub2web.metastore.ingenta.com/ns/">b</sub>. The measurement system consists of a set of coils and the measurements are the mutual impedances between coils. The main contribution of this paper is an extension to ECT measurements of a new inversion algorithm, initially proposed for electrical resistance tomography (ERT) by Rubinacci et al. The key feature of this algorithm is that its computational cost increases linearly with the number of unknown parameters. (2 pages)Eddy-current inversion for the determination of crack geometry
http://dl-live.theiet.org/content/conferences/10.1049/ic_19990113
In eddy-current inversion the aim is to find the shape and size of a flaw from a set of probe measurements. This may be difficult for an irregular crack simply because an accurate formal description is complicated but in most cases the gross features can be well characterized by making a number of simplifying assumptions. Assuming that the crack is impenetrable to eddy-current and lies in a known plane, the problem of predicting probe signals has been solved using a numerical model. The model is an integral part of a scheme to deal with the corresponding inverse problem in which the location of the edge of the crack is found using eddy-current probe impedance measurements. (4 pages)Multiple-technique NDT simulations of realistic configurations at the French Atomic Energy Commission (CEA)
http://dl-live.theiet.org/content/conferences/10.1049/ic_19990107
The ability of models to deal with accurate descriptions of the piece geometry and materials (by CAD) is a key-point for simulation work. Availability of semi-analytical models for simulating sensor and actuator behaviours and the interaction of the “field” with the piece allows both versatility and efficiency. Simulation tools developed at CEA for NDT of realistic configurations have been presented. Ultrasonic, radiographic and eddy-current techniques are considered. They are based on semi-analytical formulations for efficiency. Numerical integration is performed wherever necessary for generality. They are included in a general software CIVA allowing the processing, the imaging and the simulation of NDT data. Simulations are used as laboratory tools for conceiving and qualifying methods and demonstrating their performance, teaching and training operators. They are also used in field work context for in-service inspections to help experts' interpretation, by direct simulation of their guess for immediate comparison with actual data or by means of model-based inversion algorithms. (4 pages)Developments in NDT techniques for use in the offshore industry
http://dl-live.theiet.org/content/conferences/10.1049/ic_19990723
Several novel, rapid inspection techniques have become available for the noninvasive inspection of plant components, thus helping effective inspection to be performed with minimum interruption to production, These techniques are being used to help the plant owners operate more cost effectively in the current offshore economic climate. This paper describes some of the techniques including: INCOTEST, a pulsed eddy current system; real time radiography; LORUS, long range ultrasonics; magnetic flux leakage tools; and MAPSCAN, ultrasonic C-scan mapping. (5 pages)Effects of probe and inducer on saturation of crack signal in high-sensitivity AC field measurement technique
http://dl-live.theiet.org/content/journals/10.1049/ip-smt_20010517
As in the eddy-current technique, a major problem in separating and sizing deep surface cracks in the AC field measurement (ACFM) technique is the saturation of the crack signal. It is essential that the operator ensures the crack signal does not reach saturation for the deepest possible crack. The paper explains how the parameters of the probe and inducer in the high-sensitivity ACFM technique with a rhombic inducer control the sensitivity and saturation level. Theoretical results are presented on the effects of the inducer size, probe position and probe length on the crack signal. It is shown that for an inspection task where there is an indication of the maximum crack depth, the saturation problem can be avoided by choosing the right size inducer, the right position for the probe and the correct probe length.Characterisation of cylindrical eddy-current probes in terms of their spatial frequency spectra
http://dl-live.theiet.org/content/journals/10.1049/ip-smt_20010461
The change in impedance of a cylindrical eddy-current coil, when placed on a layered conducting structure, is determined by the material properties and geometry of the structure and the spatial frequency spectrum of the coil. It is shown how the spatial frequency spectrum can be derived from the impedance spectrum of a coil for a characterised specimen. Once this is known, the performance of the coil can be predicted for any specimen. This process is illustrated using impedance measurements of both a large air-cored coil and a small ferrite-cored coil.Analysis of a fork-shaped rectangular coil facing moving sheet conductors
http://dl-live.theiet.org/content/journals/10.1049/iet-smt.2008.0087
The electromagnetic induction method utilising eddy current plays an important role in a non-destructive material test. In testing slab-type material by electromotive force method, there is the fork-shaped coil method, which has two coils placed on both sides of the test piece. In most studies, circular coils have been analysed. However, it has been pointed out quantitatively that a rectangular coil is more useful than a circular coil for a non-destructive test. The authors derive a rigid theoretical formula with a test theory experiment.Electromagnetic field distributions around conducting slabs, produced by eddy-current probes with arbitrary shape current-carrying excitation loops
http://dl-live.theiet.org/content/journals/10.1049/ip-smt_20010483
The theoretical prediction of eddy-current probe output signals for various nondestructive testing (NDT) applications usually involves solution of the electromagnetic field due to a current-carrying excitation loop in the vicinity of a flawless conductor. The paper presents a general theory for the time-harmonic magnetic field distribution produced by an arbitrary shape current-carrying excitation loop around a linear, isotropic, homogeneous conducting slab. In this theory the authors develop a Fourier-integral-based model for computing the magnetic field distributions, which greatly simplifies the computation procedure. The main feature of the model is its ability to analyse two- and three-dimensional excitation geometry, with a similar degree of computation burden. This feature stems from the fact that in this model, knowledge of the field distribution at the place of the conductor surface in free space suffices to compute the field in the presence of the conductor. To demonstrate the accuracy of the model, the authors consider two special cases of an infinite straight wire and an elliptical loop exciter. The comparison of the results with those obtained using the conventional algorithms in the literature validates the model introduced in the paper. To show the generality of the model, the authors also present results associated with a solenoid exciter with a three-dimensional geometry for which no analytical solution is available in the literature.Solving 3-D eddy current problem containing thin cracks using dual formulations and shell elements
http://dl-live.theiet.org/content/journals/10.1049/ip-smt_19990005
The thin cracks in three-dimensional eddy current problems, frequently encountered in nondestructive testing applications, are modelled by surface domains of zero thickness. The double layer nodal and edge shell elements are employed in the dual eddy current formulations for the interpolation of scalar and vector potentials in the cracks. With the help of shell elements, the volume integrals corresponding to the magnetic energy and joule losses in the cracks are transformed to surface integrals. This modelling allows consideration of the field jump across the thin cracks provided that the field continuity on the edge of the crack is correctly taken into account. An eddy current problem in presence of thin cracks is solved using the model and results are compared with those of conventional finite element modelling.