Eddy Current Signal Calculations for Surface Breaking Cracks

This 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

Improved Probe-Flaw Interaction Modeling, Inversion Processing, and Surface Roughness Clutter

In Reference 1 a first comparison was made of measured eddy current signals with calculations based on nonuniform probe-field interaction theory. These calculations followed the basic analysis developed in Reference 2. They used interrogating field distributions calculated by Dodd and Deeds theory for the air core coils of Reference 3. (Note that Fig. 6 in Reference 1 and Fig. 7 in Reference 3 should be interchanged). In Reference 1 theoretical and experimental plots of the flaw profile curve (a plot of △Z versus distance along the mouth of a surface breaking flaw) were found to be in good agreement, with regard to shape, for several selected EDM notch samples in aluminum. An iterative procedure was also developed for systematically varying the length, depth, and opening width to obtain a best fit to the experimental data.4 In the present paper a full inversion procedure is developed and illustrated for approximately rectangular-shaped EDM notches. The mathematical structure of the inversion problem is first examined and a solution is proposed. Physical reasoning, based on the form of the flaw profile curves, is then used to simplify the approach and to provide guidance in selection of the most suitable probe geometry. Other topics briefly addressed include, possible improvements in the theory for the region with a/§ close to unity and for more realistic flaw shapes (i.e., semi-elliptical, rather than rectangular), inaccuracies due to errors in the probe scan path, and background clutter due to surface roughness, machining marks, and micro-structure

Experimental Methods for Eddy Current Probe Design and Testing

The 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