Showing papers on "Eddy-current testing published in 1987"

The application of finite-element calculations to the remote-field inspection technique

Abstract: Calcul par elements finis applique a la technique de controle par courants de Foucault a champ lointain

Electromagnetic field calculations for the low frequency eddy current testing of tubular products

Abstract: Low frequency eddy current technique, known as remote field eddy current (RFEC) technique, is being widely used for the nondestructive testing (NDT) of metallic tubes. Despite the early recognition, advantages and successful application in the in-situ inspection of oil well casings this technique lacked the development of an adequate theoretical base to help understand its complex phenomenon. Theoretical simulation of a practical RFEC geometry (moving probe inside a flawed tube) is described in this paper through the application of finite element numerical analysis technique. The predicted RFEC output traces show a higher and constant detection sensitivity irrespective of flaw depth, flaw location (ID or OD) and location of the sensor within the remote field zone.

Eddy current test method for rods, wires or tubes

Abstract: An eddy current test device is used for testing tubes, rods, strips or the like. Any material defects present affect the voltage induced in a receiver coil (10) and can thus be recognised. So as to be able to recognise faults even close to the central axis (2) of the test specimen (1), a coil system (5 to 8) is proposed which generates a magnetic field perpendicular to the central axis (2).

Theoretical prediction of remote‐field eddy current response for the nondestructive evaluation of metallic tubes

Abstract: The remote‐field eddy current effect refers to low‐frequency eddy current nondestructive testing (NDT) phenomenon in tubular conductors in which the behavior of both amplitude and phase of induced magnetic field are in apparent contradiction to the well‐known ‘‘skin‐effect’’ theory. Near‐equal detection sensitivity across the wall thickness, the ability to measure wall thickness, and the absence of lift‐off problems are some of the attractive features of this technique. Despite its early recognition and useful application in down‐hole inspection of oil‐well casing, no development of adequate scientific basis that could explain this phenomenon has been reported in the open literature. Modeling of the remote‐field eddy current phenomenon using the axisymmetric finite element computer code is described in this paper. The results presented show that the finite element numerical technique originally developed for the computation of fields in electrical and magnetic devices can be used as well for the modeling ...

Three-dimensional finite-element modelling in eddy-current nondestructive testing

Abstract: Eddy-current nondestructive testing is increasingly used in industry as a means of noninvasive detection of flaws in metals that are electrically conducting. Various computer modelling techniques have been used in the past for simulating different NDT processes. This contribution is concerned with the prediction of electromagnetic fields in the vicinity of a surface flaw using a general three-dimensional finite-element eddy-current program. Fields are modelled in terms of either the magnetic vector potential A or a magnetic scalar potential. Some experimental and calculated results are presented that suggest that the program could be used in its present form for a certain range of NDT problems. Some areas where further improvement would be desirable are highlighted

Effects of Shielding on Properties of Eddy Current Probes with Ferrite Cup Cores

Abstract: In eddy current inspection the ability to detect small defects depends on the sensitivity of the system and on the relative sizes of the probe and the defect. To detect defects on the opposite surface the probe radius should be at least as great as the thickness of the material. This limits the sensitivity to small defects that can be achieved by decreasing the probe size. Assuming the instrumentation is a given, further sensitivity can be achieved by improving the sensitivity of the probe itself.

Eddy Current Induction by a Coil near a Conducting Edge in 2D

Abstract: Difficulties often occur in eddy current inspection when a defect is situated close to an edge. This is due to the large signal arising from the edge (‘edge-effect’) which tends to obscure any signals coming from the defect [1,2]. To understand the edge-effect in eddy current NDI in more detail, the induction of eddy currents in a conducting 90° edge by a two-dimensional coil is considered in this paper. Firstly, the impedance of a two-dimensional coil in the vicinity of a conducting quarter-space is calculated numerically using a dual boundary-integral-equation (BIE) method and the results compared with experiment. The behaviour of the induced currents (vector potential) in the vicinity of the edge is then examined using the numerical (BIE) results and the results of an analytical approximation [3] valid in the limit of small skin-depth.

Method for testing materials or determining the properties of materials using an eddy-current test apparatus

Abstract: In the case of a method for testing materials or determining the properties of materials using an eddy-current test apparatus, in particular for the purpose of distinguishing or redetecting metals, metal alloys and metal joints after heat treatment or strain-hardening, a transmission coil is operated at a frequency for generating eddy currents in the workpiece to be tested and the reaction of the eddy currents generated is measured via an evaluation device and the measured-value vectors are represented in the complex plane, it being possible to adjust variable measurement parameters such as frequency, coil current, premagnetisation, the form of the exciting field, etc. It is provided for the purpose of optimum selection of the setting of the measurement parameters to determine the direction of the anisotropy of the scatter of the measured values of a plurality of specimens for a specific setting of measurement parameters and a specific type of specimen.

Geometrically anisotropic probes: an improved eddy current technique

Abstract: Geometrically anisotropic eddy current probes are a type of separate function probes especially fit to the detection of defects showing a preferential direction. This kind of flaw induces a coupling between the transmitter and the receiver by guiding eddy currents from the one to the other. On the other hand, this coupling will be almost non-existent in the presence of defects or spurious effects not displaying this geometrical particularity. Basic studies on an elementary two-coil set-up allow the acknowledgment of the intrinsic qualities of such probes: good signal-to-noise ratio, influence field practically constant on the whole defect length, ability to detect “bridged” defects, insensitivity to lift off. These results can be improved by achieving multicoils probes adapted to different kinds of problems. An application to continuous casting slabs testing yields very interesting results in the detection of cracks, while getting rid of the effect of oscillation marks with no need of any signal processing. These achievements are promising for the improvement of eddy current testing, in terms of detection sensitivity as well as of rapidity to obtain information.

Assessment of Eddy Current Probe Interactions with Defect Geometry and Operating Parameter Variations

Abstract: Many current generation aerospace designs are based on the application of linear elastic fracture mechanics to establish critical flaw sizes and inspection intervals based on the expected service loads and life cycles. These fracture critical designs require the application of highly reliable nondestructive inspection procedures during production and maintenance to detect defects which would result in system failure if allowed to enter or remain in service.

The touch method of eddy-current testing

Abstract: The eddy-current touch method of crack detection can size cracks from the changes in the characteristics of the lift-off curves. Results obtained for mild steel samples show accuracies better than those obtained with the ACPD method for crack depths of 5 mm and less. For materials, such as different types of steels, having not very dissimilar properties, a calibration block for one type can be used for another. This is advantageous with the testing of welds

An introduction to impedance plane phase analysis eddy current equipment and its use on aircraft structures

Abstract: Utilisation d'appareils a courants de Foucault bases sur l'analyse de phase dans le plan d'impedance. Application aux structures d'avions

Reliability Assessment of High-Risk Steel Bridges by Nondestructive Test Methods

Abstract: Six conventional NDT techniques were tested in the laboratory using butt weld configured specimens having intentionally embedded cracks. Those NDT methods were 1) visual inspection, 2) magnetic particle testing, 3) dye penetrant testing, 4) fluorescent enhanced surface testing, 5) eddy current testing, and 6) ultrasonic testing. The other NDT methods provided better crack detection capabilities than visual inspection. Visual inspections by different personnel produced low detection rates. Tests using the fluorescent magnetic particle method missed several indications because of marginal test conditions. The other NDT methods detected all cracks. Laboratory tests revealed that the visually enhanced NDT methods could play useful roles in bridge inspection. Magnetic particle and dye penetrant testing were slow, but they would be beneficial for spot bridge inspections. Eddy current testing was found to be the fastest NDT method and would be best for large scale bridge inspections. Ultrasonic testing was useful for sizing. Modern digital ultrasonic flaw detectors offer advantages in calibration and data recording that make them more advantageous than conventional units for large scale bridge tests. A stand alone, microcomputer based data acquisition and data processing system was designed and assembled to assist highway personnel in planning and prioritizing bridge inspections. It may be used to acquire live load strain gage readings from selected bridge members and to store the data in digital form. The data may be processed by seven sequential computer programs to yield 1) digitized stresses, 2) stress range cycle histograms, 3) minor equivalent stress ranges, 4) root mean square equivalent stress ranges, 5) weighted equivalent stress ranges from several tests, and 6) predictions of fatigue lives of structural members based on the AASHTO fatigue design curve for specific structural members. The system has been tested and is capable of providing crack initiation data useful for scheduling or prioritizing nondestructive inspections of bridges.

Eddy Current Probe Performance Variability

Abstract: Eddy current testing is used extensively by the Air Force for nondestructive inspection of many aircraft structural components. Although the reliability and consistency of inspections depends to a large extent on the characteristics of the eddy current probes used, no adequate specifications or certification methods presently exist for assuring probe performance. Because of the variability in probe performance, a need exists for establishing a means to control probe performance characteristics which will eventually lead to improved test results.

An automatic approach to optimize multifrequency eddy current testing

Abstract: This paper presents a novel microcomputer-controlled multifrequency eddy current tester permitting the optimal frequency combination to be automatically determined for a given testing application. Measuring signals are digitized at the earliest possible time which is an essential criterion for flexible evaluation algorithms.

Eddy-current surface-crack sizing in steel with high lift-off

Abstract: Comparisons are made of the performances of single- and differential-coil probes for the eddy-current testing for surface defects in mild steel with high lift-off With a differential-coil probe it was found possible to detect cracks for lift-off values of more than 12 mm and to measure them for values of up to between 10 and 12 mm

3-D Eddy Current Nondestructive Testing Modeling for Surface Flaws

Abstract: In eddy current non-destructive testing, numerical models have been used for a long time to optimize the method. Those models solve Maxwell’s equations and compute the coil impedance. Many 2-D programs have been realized: for axisymmetrical geometries, with axisymmetrical or very small flaws, to model tube sheets of a steam generator or wires [1,2]. for an E-shape sensor which generates a one dimensional field to scan a plane surface [3].

Feasibility study on non-destructive testing for fatigue crack detection of corner weld

Abstract: This study reviews the non-destructive testing methods and evaluates their reliability and adaptability for the detection of fatigue cracks which develop from a blowhole at the root of the corner weld. X-ray, ultrasonic, magnetic particle, liquid penetrant, and eddy current testing methods are provided. For the internal crack, ultrasonic testing appears to be desirable. Minimum crack sizes detectable are about 8mm∅ with a high accuracy. For the crack opened to the surface, eddy current and magnetic particle testing give a fair estimation. But it must be understood that fatigue cracks will be missed by liquid penetrant testing.