8 research outputs found
Computation of Eddy Current Signals and Quantitative Inversion with Realistic Probe Models
No full textWe have performed experiments to test both the physical correctness of the theory presented in an earlier paper (Muennemann et al., 1983), referred to below as MAFP, and the usefulness which it and its extension (Auld et al., 1984a) can achieve in practice. This paper describes the experiments performed and modifications to the earlier inversion procedure. These modifications were motivated by our “hands-on” experience applying the principles presented in the papers referred to above, particularly the extension to nonuniform fields.</p
Experimental Methods for Eddy Current Probe Design and Testing
No full textThe purpose of this paper is to briefly review the influence of EC probe parameters on the performance of the complete NDE system and to describe experimental methods for measuring these parameters. Combined theory and experiment is required to quantify probe response to design optimum probes for specific applications, to verify the reproducibility of probe performance during manufacture, and to verify the stability and precision of probe calibration. For these purposes it is necessary to consider, at least, the following probe parameters (1) input impedance, for design of adjacent circuitry; (2) self-resonant frequency, for upper frequency limits of operation; (3) the ratio of probe field intensity to input current, for sensitivity; and (4) the distribution (or shape) of the flaw interrogating field generated by the probe — for control of flaw response, liftoff response and spatial resolution (i.e., separation of closely spaced flaws and discrimination against edges and corners).</p
Surface Flaw Detection with Ferromagnetic Resonance Probes
No full textEddy current methods of flaw detection have been in use for many years. Frequencies used in this type of flaw detector normally range from tens of kilohertz to a few megahertz. We report on recent progress using a resonant probe which operates in the gigahertz frequency range, and compare its performance with classical eddy current methods.</p
Eddy Current Signal Calculations for Surface Breaking Cracks
No full textThis paper contains a brief status report on analytical modeling of the probe-flaw interactions for surface breaking cracks and some data on comparisons of theory and experiment for EDM notches and true fatigue cracks. The goal of the work reported here and in companion papers by Rummel and Rathke (1984), Auld, et al. (1984), and Martinez and Bahr (1984) is to improve the quantitative character of eddy current testing. In this joint effort, the role of probe-flaw interaction modeling is to provide engineering tools not previously available for: (1) setting design guidelines to optimize sensitivity and spatial resolution, (2) permitting analytic extrapolation of measured flaw response data, (3) defining a test basis for monitoring probe calibration, and (4) establishing a rational inversion procedure based on multifrequency measurements and the shape signature of a scanned flaw signal as a function of position.</p
Measurement of Surface Crack Opening Displacements Using Microwave Frequency Eddy Currents
No full textAn electromagnetic NDE technique for measuring the crack opening displacement of surface fatigue cracks is described. A ferromagnetic resonance probe utilizing yttrium-iron-garnet was used to induce eddy currents in an aluminum plate. The crack opening disolacement of a semi-elliptical fatigue crack evaluated at the surface was measured optically at several values of bending stress on the specimen. A technique is presented which allows the crack depth to be calculated from the measured COD at a given stress. The relative phase and magnitude of input vs. output signal to and from the resonating YIG sphere was recorded during the interaction of the FMR probe eddy currents and the fatigue crack. A method is shown to extract quantitative information from these signals and to correlate this information with the crack opening displacement.</p
Inversion of Eddy Current Signals in a Nonuniform Probe Field
No full textWe present a simple analytical method for predicting the eddy current signal (ΔZ) produced by a surface flaw of known dimensions, when interrogated by a probe with spatially varying magnetic field. The model is easily parameterized, and we use it to construct inversion schemes which can extract overall flaw dimensions from multiposition, multifrequency measurements. Our method is a type of Born approximation, in which we assume that the probe’s magnetic field at the mouth of the flaw can be used as a boundary condition on the electromagnetic field solutions inside the flaw. To simplify the calculation we have chosen a “rectangular” 3-dimensional flaw geometry for our model. We describe experimental measurements made with a new broadband probe on a variety of flaws. This probe operates in a frequency range of 200 kHz to 20 MHz and was designed to make the multifrequency measurements necessary for inversion purposes. Since inversion requires knowledge of the probe’s magnetic field shape, we describe experimental methods which determine the interrogating field geometry for any eddy current probe.</p
