A model of bolt hole inspection via eddy current

In this paper we report on the development of an eddy-current measurement model, which is a generalization of the one reported earlier [1-2]. Our objective is to establish a computer model that is capable of simulating eddy current NDE in generic inspection geometries. To achieve this goal, we started with an earlier version of the model applicable to a flat-plate geometry and a tightly closed crack [3]. The model has been generalized so that the current version can handle more general specimen geometries, including curved surfaces and corners. Other features of the original model were kept unchanged; for instance, it uses the boundary integral equation method, can handle tight cracks, and works for an air-core coil driven at arbitrary frequencies

Thickness and Conductivity of Metallic Layers from Pulsed Eddy Current Measurements

Coatings and surface treatments find a wide range of technological applications; they can provide wear resistance, oxidation and corrosion protection, electrical contact or isolation and thermal insulation. Consequently, the ability to determine the thickness of coated metals is important for both process control and in-service inspection of parts. Presently ultrasonic, thermal, and eddy current inspection methods are used, depending on the circumstances. A number of commercial instruments for determining the thickness of nonconducting coatings on metal substrates are based on the fact that the impedance change of the coil decreases exponentially with the distance of the coil from the metal (the lift-off effect). However, these instruments are not suitable for determining the thickness of metal layers on conducting substrates

Impedance of a Coil in the Vicinity of a Crack

In the design of electromagnetic NDE systems for the detection and examination of cracks and other defects in conducting materials, it is desirable to have a quantitative description of the fields in the vicinity of the defect. In previous work by this author and co-workers [1,2], the fields in the vicinity of a crack were calculated for models based on excitation by a spatially uniform applied field, as in the interior of a solenoid. The present work reports on an improved model which includes non-uniformity of the field of the exciting coil and the effects of coil size and position relative to the crack

Registration Issues in the Fusion of Eddy Current and Ultrasound NDE Data Using Q-Transforms

Data fusion methods are finding increasing application in nondestructive evaluation (NDE) [2, 3, 4] for enhancing the reliability of inspection. These techniques typically combine information from two or more NDE modalities to improve the probability of detecting flaws and enhance specimen characterization results [1]. Eddy current methods rely on diffusion for propagating energy. Ultrasonic methods, in contrast, rely on wave propagation. Consequently, the two tests rely on different material/energy interaction processes and can potentially provide complementary perspectives of the flaw in a specimen. This paper proposes a novel phenomenological approach using Q-transforms for addressing the registration issue in the fusion of eddy current and ultrasonic data. Specifically, ultrasonic signals are Q-transformed to the diffusion domain. The transformation allows the superposition of the transformed field on the eddy current field as shown in Figure 1. It is anticipated that the resulting field will have a lower signal-to-noise ratio

Frequency Dependence of Electric Current Perturbation Probe Response

The electric current perturbation (ECP) probe1–3 is similar to a conventional eddy current probe in that a coil, typically a cylindrical winding, is used to induce current in the test piece. The ECP probe differs in the use of a separate differential sensor coil, with axis parallel to the surface of the piece, and usually located just outside the induction coil winding. We have found that this sensor orientation tends to minimize probe-to-surface coupling and therefore minimizes liftoff noise

Eddy Current Corner Crack Inspection

The purpose of this paper is to report on the development of an eddy current (EC) measurement model applicable to corner crack inspections. Naturally, corner cracks are more difficult to detect than those on flat surfaces, because the specimen edge itself gives a large response to the EC probe. The flaw signal, if any, tends to be obscured by the large edge signal. Thus, probe impedance should be determined more accurately than usual in order to extract flaw signals out of the background. Experimentally, this requires high-accuracy impedance measurements with rigid control over probe motion. In modeling point of view, this means that predictions should be made from an exact model, or at least from a model which can achieve the required level of accuracy [1–3]

Experimental Measurements of the Eddy Current Signal Due to a Flawed, Conducting Half Space

The eddy current method of nondestructive evaluation involves the induction of eddy currents in a conductive test object by a time-varying field produced by a suitable distribution of impressed currents and the detection of the resultant field. The method is ordinarily used at frequencies sufficiently low to neglect effects due to displacement current; hence a theoretical analysis entails calculating the self-impedance of the coil in the presence of the test object. In practice, one often needs only the change in impedance produced by the test object or by changes in the nominal properties of the test object (e.g., changes in its geometry or position with respect to the test coil or coils, or distributed or localized changes in the resistivity of the test object)

Eddy Current Response to Three-Dimensional Flaws by the Boundary Element Method

In planning an inspection procedure, or in designing parts with flaw detectability as a design goal, it is essential that the engineer have available some form of model for estimating the probability of flaw detection. In the past this need has been met, with varying degrees of success, by relying on experience in the inspection of similar parts, sometimes supplemented by experimental testing. With the rapid advances in computer technology in recent years, it is now feasible to consider replacing, or at least enhancing, such practices with predictions based on numerical simulation of the flaw detection process [1]

Calibration Methods for Eddy Current Measurement Systems

Calibration of eddy current measurement systems is an important factor for attaining the accuracy and precision of measurement that quantitative nondestructive evaluation requires. The quantity of interest in most forms of eddy current inspection is △Z, the change in probe impedance induced by a flaw. Flaw signals produced by surface-breaking cracks are small; typical flaw signals for an air core probe amount to a few tenths of one percent of the probe’s impedance in air. Such small signals are easily obscured by the impedance changes caused by small variations in the height of the probe above the workpiece (lift-off). To discriminate against lift-off, conventional eddy current instruments determine the phase of △Z relative to lift-off and the magnitude of the component of △Z in quadrature with lift-off. But this information is not sufficient to perform flaw signal inversion; rather, the absolute magnitude and phase of △Z are necessary. Thus, quantitative inversion of eddy current signals to obtain flaw sizes requires methods for calibrating eddy current measurement system

Eddy-Current Probe Design

This paper describes theoretical and experimental work directed toward finding the optimum probe dimensions and operating frequency for eddy current detection of half-penny surface cracks in nonmagnetic conducting materials. The study applies to probes which excite an approximately uniform spatial field over the length of the crack at the surface of the material. In practical terms, this means that the probe is not smaller than the crack length in any of its critical dimensions