Semi-Elliptical Surface Flaw EC Interaction and Inversion: Experiment

Abstract

During recent years, rapid progress has been made in modeling the interaction of nonuniform eddy-current (EC) probe fields with surface flaws and verifying these models by comparisons with experiment. Following the development of the basic analysis for nonuniform probe fields [1], the first comparison of measured eddy current signals with predictions of the theory was made in 1983 [2]. The following year brought development of a full inversion procedure for rectangular flaws [3], which was successfully demonstrated using flaw signals measured for a series of rectangular electrical-discharge machined (EDM) notches in aluminum [4]. This year, in a companion paper [5], the model for interaction of nonuniform probe fields with flaws has been extended to semi-elliptical flaws. The present paper describes an extensive series of measurements of flaw signals for both fatigue cracks and semi-elliptical EDM notches in aluminum and titanium alloys. The measured flaw signals are compared with theoretical predictions and are also used to test the inversion procedure based on the theory [5]. EXPERIMENTThe objective of this study was to make a serjes of carefully controlled measurements of eddy current flaw signals for well characterized, semielliptical flaws in aluminum and titanium alloys to compare with flaw signals calculated using theoretical models described in a companion paper [5]. Both real fatigue cracks and EDM simulations were studied. Most measurements emphasized air-core probes, since the magnetic field distributions for these probes could be calculated using the theory of Dodd and Deeds [6]. One * Contribution of the National Bureau of Standards, not subject to copyright.

Semi-Elliptical Surface Flaw EC Interaction and Inversion: Theory

“…Figures 5 and 6 show comparisons of Stanford calculations with NBS measurements for an EDM notch and a fatigue crack. More detailed discussion of these and other results is given in [3]. One point should, however, be emphasized.…”

“…The results, used in [6], are listed in Table 1. [3] and Martin Marietta [7], using the same air-core probe as in 1984 [1]. Figure 3 shows computed magnitude and phase profiles for rectangular and semi-elliptical fatigue crack models.…”

“…This section constitutes a brief review of the experimental data discussed in [3] and their inversion. The flaw shapes are identified in Table 2 (SE =semi-elliptical EDM, R =rectangular EDM, F =fatigue).…”

“…During the past year the goal has been to develop a probe-flaw interaction theory for semi-elliptical flaws in a nonuniform interrogating field. This is again a collaborative effort between Stanford and the National Bureau of Standards at Boulder, The theory, and some comparison with experiment, is given in this paper and details of the experiments are described in a companion paper [3]. Figure 1 illustrates the measurement of the flaw profile for a flaw whose surface length is much greater than the mean diameter 2r of the eddy current probe.…”

Pickup coil spacing effects on eddy current reflection probe sensitivity

“…The probes commonly used in a production shop for nondestructive testing (NDT) are invariably wound on ferrite cores because of the increased sensitivity that results from the use of high magnetic permeability core materials. However, recent advances in the theory of flawfield interactions have stimulated interest in the use of air core probes [4,5]. The use of air core coils in the detector helps to minimize the complexity of the calculations and leaves the experimenter with very adequate tools for verification studies.…”