Showing papers on "Eddy-current testing published in 1981"
TL;DR: In this paper, a general theory of eddy current flaw response, appropriate to both standard low frequency and ferromagnetic resonance (FMR) probes, was developed for simple two-dimensional and three-dimensional open and closed surface flaw geometries.
Abstract: A general theory of eddy current flaw response, appropriate to both standard low frequency and ferromagnetic resonance (FMR) probes, is developed for simple two-dimensional and three-dimensional open and closed surface flaw geometries. This analysis, based on the assumption of a uniform interrogating field applied to the flaw by the probe, shows that flaw opening responses increase with the operating frequency of the probe. Experimental results using both types of probe confirm this result for realistic practical geometries, where variations of crack mouth opening displacement under load provide useful information about crack dimensions.
66 citations
TL;DR: In this article, a general theory, based on the Lorentz reciprocity relation, is used as a vehicle for comparing microwave with low-frequency eddy-current techniques, in the case of the FMR resonator, but other probe geometries are also considered.
Abstract: During the past two years small ferromagnetic resonators (FMR) operating at frequencies above 1000 MHz have been shown to offer promise as a new type of eddy-current probe for surface-breaking cracks in metals. Strong signals have been obtained from both electrical discharge machining (EDM) notches and actual fatigue cracks, and the FMR probes have been shown to have lift-off discrimination capability. Since the skin depth at these microwave frequencies is of the order of microns, the surface currents must penetrate the crack and, in order to detect its presence, the flaw detection mechanism must be necessarily somewhat different in detail than for conventional low-frequency eddy-current methods, where the flaw has dimensions typically of the order of the skin depth. A general theory, based on the Lorentz reciprocity relation and applicable to all types of detection systems, is reviewed and used as a vehicle for comparing microwave with low-frequency eddy-current techniques. In the microwave case, the FMR resonator is emphasized but other probe geometries are also considered.
49 citations
Patent•
06 Oct 1981
TL;DR: In this article, a test probe has two mutually concentric sensing coils, connected in series and wound in opposite directions, the inner coil on a ferrite core and the outer coil on the inner.
Abstract: The test probe has two mutually concentric sensing coils, 11, 13 connected in series and wound in opposite directions, the inner coil on a ferrite core 10 and the outer coil on the inner The sensing coils each have nominally the same number of turns, but are 'balanced' in a calibrating magnetic field to constitute a differential configuration by the addition to or substraction from one coil of a turn or part of a turn A field coil 14 is wound on the outer sensing coil The probe has the advantage of being omnidirectional in effective operation in fault location using eddy current techniques
16 citations
TL;DR: In this article, it is shown how the output of an eddy-current instrument based on the inductance bridge is related to these impedance components and can thus be used for testing without the need for calibration against a test piece.
Abstract: In many cases, known relationships exist between the components of impedance of an eddy-current detecting coil and the characteristics of metal samples, as obtained experimentally, theoretically, and with the use of models. It is shown here how the output of an eddy-current instrument based on the inductance bridge is related to these impedance components and can thus be used for testing without the need for calibration against a test piece.
8 citations
TL;DR: In this paper, a systematic, automated process has been developed at ORNL to allow rapid development of accurate multiple property eddy-current tests, which can be done using pulsed techniques or multiple frequencies.
Abstract: A rapid, accurate, and easy-to-use inspection method for steam generator tubes is clearly needed and not entirely provided by our present eddy-current tests. The signals from eddy-current tests are affected by changes in tube wall thickness, the location and size of defects, the electrical conductivity, the magnetic permeability, and the tube-to-tube support spacing and denting (which changes the probe to tube spacing). Simultaneous variations in these test properties produce signals that are ambiguous in present commercial eddy-current instruments. However, this type of multiple property eddy-current testing can be done using pulsed techniques or multiple frequencies. A systematic, automated process has been developed at ORNL to allow rapid development of accurate multiple property eddy-current tests.
7 citations
TL;DR: In this paper, two approaches to solving the NDE inversion problem are discussed for geometries with two components of eddy current, one using the finite element method of numerical analysis and the other using an analytical scattering technique.
Abstract: The eddy-current nondestructive evaluation (NDE) inversion problem is to determine flaw parameters from eddy-current sensor impedance changes. Two approaches to solving this problem are discussed for geometries with two components of eddy current. The first is to use the finite-element method of numerical analysis to compute the sensor impedance change for each flaw parameter value. The second approach is to combine the finite-element method with an analytical scattering technique. These two approaches are applied to the problem of an infinitely long coil surrounding an infinitely long conducting bar with an infinitely long surface crack. The calculated impedance changes show good agreement with known analytical and experimental results.
7 citations
TL;DR: In this article, a brief report on some early work on the testing of steel sheets using pulsed eddy currents is given. But this work is limited to the case where the input pulse shape is known.
Abstract: This is a brief report on some early work on the testing of steel sheets using pulsed eddy currents. Penetration up to about 10 mm has been achieved and some artificial defects have been observed. Longer pulses are to be employed for deeper penetration, and more work done on the observation of defects. Some theoretical work is also reported. An indication is given of further work and the preparation of computer programs to produce a calculated output pulse where the input pulse shape is known.
6 citations
TL;DR: The ferromagnetic resonance probe described in the companion paper by B. A. Auld as mentioned in this paper has been used for defect detection for a variety of real and fabricated flaws. But it has not been shown to be suitable for the detection of liftoff signals.
Abstract: Experimental results obtained with the ferromagnetic resonance probe described in the companion paper by B. A. Auld are presented for a variety of real and fabricated flaws. Details of the apparatus are given with a briefexplanation of the measurement technique used. Particular reference is made to the important property of discrimination between flaw and liftoff signals. An illustration of the edge-discrimination characteristics of the ferromagnetic resonance probe is shown.
6 citations
TL;DR: In this paper, a multifrequency algorithm is applied to suppress signals caused by disturbing parameters in a multidimensional vector space (MVSP) and the electronics employed in prototype form are described.
Abstract: A multifrequency algorithm is applied to suppress signals caused by disturbing parameters. This procedure (based on the projection principle in a multidimensional vector space) and the electronics employed in prototype form are described. The prototype operates with up to four frequencies in a wide range (500 Hz to 1 MHz). The frequencies are continuously tunable and are fed into the coil one after the other by high-speed multiplexing. The multifrequency approach yields a read-out value affected only by defects. The defect signals then can be assessed by means of reference defects of known depths giving equivalent indication peaks. The elimination of the disturbing signals chiefly increases defect detection sensitivity. In general, however, the suppression is not total, and, additionally, the original defect signals are reduced to a greater or lesser degree by the application of the projection principle. The testing frequencies used have a strong influence on the projection losses and therefore on the defect detection sensitivity. A computer program system that allows the reaction of the impedance of the coil to be calculated as a function of the electric, magnetic, and geometrical properties of the specimens and the dimensions of the coil itself is used to determine an optimum frequency combination for the given test situation. This procedure is applied to the testing of heat-exchanger tubes, claddings, and welded joints. The results obtained with the optimized test system are presented.
5 citations
TL;DR: In this paper, an eddy-current test technique for the detection of material flaws in graphite-reinforced aluminum panels is discussed, which involves scanning the specimen and recording both real and imaginary impedance components of the test coil using a format similar to that of an ultrasonic C-scan.
Abstract: The application of an eddy-current test technique for the detection of material flaws in graphite-reinforced aluminum panels is discussed. This particular technique involves scanning the specimen and recording both real and imaginary impedance components of the test coil using a format similar to that of an ultrasonic C-scan. The benefits of this approach are discussed and the results are compared with those of X-radiography and ultrasonics.
1 citations
TL;DR: In this paper, a device for eliminating the effects of variation of the impedance of an eddy-current test coil, due to temperature fluctuations, is described, which is found to be highly effective at temperatures of up to 30°C, with tests made on simulated defects at a frequency of 5 kHz.
Abstract: A device for eliminating the effects of variation of the impedance of an eddy-current test coil, due to temperature fluctuations, is described. It was found to be highly effective at temperatures of up to 30°C, with tests made on simulated defects at a frequency of 5 kHz.
01 Jan 1981
TL;DR: In this article, the specific problem examined involves the development, storage, and utilization of calibration masters for industrial applications of eddy current technology, technical assistance may be generated through the theoretical community.
Abstract: It is anticipated, that through exposing some of the problems encountered in industrial applications of eddy current technology, technical assistance may be generated through the theoretical community. The specific problem examined involves the development, storage'and utilization of calibration masters. Although the use of probes versus coils greatly reduces the number of calibration masters in that probes are not sensitive to overall geometry and mass variations, the eddy current response is influenced by near probe geometry variations. At present, an empirically derived algorithm is relied upon to characterize this phenomenon although it is hoped that an appropriate theoretical model will be developed. In addition to geometry, the non-linear permeability present in ferromagnetics, and the microstructural and compositional variations greatly increase the complexity of constructing such a model.