Experimental Methods for Eddy Current Probe Design and Testing

Abstract

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) selfresonant 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).Measurements of probe impedance and self-resonant frequency are standard, and will not be discussed here. For items (3) and (4) there are, in general, two classes of experiment that may be used: direct methods, where one measures probe characteristics with instruments which are independent of the probe (such as measuring the magnetic field distribution of a probe with a small Hall-effect probe), and perturbation methods, where one examines the field 477

Calibration Methods for Eddy Current Measurement Systems

“…Its physical dimensions were: r = 0.81 mm, ar = 1.22 mm, t = 3.56 mm, and ho = 0.51 mm. This same probe was used in several earlier studies by other groups (Auld et al, 1984c;Muennemann et al, 1984;and Rummel and Rathke, 1984).…”

“…The assumptions of presently available models include a high-conductivity workpiece, flaw depth greater than skin depth (a/o > 2), and flaw length greater than mean coil radius (2c > r). An additional requirement is that the field distribution of the probe be known, either by calculation or measurement (Auld et al, 1984c). The availability of such models points to the possibility of using artifact standards ("standard flaws") to calibrate eddy current measurement systems, a possibility we consider here.…”

“…Calibration methods based on comparing measurements of probe response to theoretical predictions require knowledge of the probe's magnetic field intensity and spatial distribution. This information can be calculated for air core probes using existing theory, but for ferrite core probes it would have to be obtained by additional measurements (Auld et al, 1984c). Second, the electrical calibration technique offers the possibility of in situ calibration: calibrating resistances could be switched into the circuit during the measurement process to provide fiducial marks on the response obtained for an unknown flaw.…”

Assessment of the Effects of Scanning Variations and Eddy Current Probe Type on Crack Detection

“…In addition, plan view projections of the images were plotted and quantified with respect to the physical dimensions of the probes. This method supplements the direct measurement of probe fields [6] and provides a direct correlation of probe response to a flaw. and a response at a greater distance from the crack than that obtained with the air core probe.…”

Computation of Eddy Current Signals and Quantitative Inversion with Realistic Probe Models

“…Our techniques for the measurement of probe magnetic fields are described in detail elsewhere (Auld et al, 1984b;Beissner et al., 1980). We have made preliminary comparisons to determine what makes an optimum perturber for magnetic field studies.…”