Erol Uzal

Impedance of coils over layered metals with continuously variable conductivity and permeability: Theory and experiment

The frequency‐dependent impedance of right‐cylindrical air‐core eddy‐current probes over thick metal plates whose conductivity and permeability vary as a function of depth in the near‐surface region have been studied both experimentally and theoretically. Measurements of probe impedance were made from 1 kHz to 1 MHz using an impedance analyzer. Precision‐wound air‐core coils were used for testing the theory, and commercial eddy‐current probes were used to connect with industrial practice. The samples were of two types. First, to model a continuous profile, otherwise uniform plates of metal covered with many thin, discrete layers of other metals were considered. Second, as a practical example, case‐hardened titanium plates, whose near‐surface conductivity varies smoothly and continuously as a function of depth, were considered. Two theoretical results are presented for continuously varying profiles. First, an exact closed‐form solution (within the quasistatic approximation) is reported for the impedance of...

The impedance of eddy current probes above layered metals whose conductivity and permeability vary continuously

A numerical method for calculating the impedance of a cylindrical air-core probe over a layered metallic half-space is presented. The method permits the conductivity and permeability to vary as arbitrary functions of the depth into the material. The utility and accuracy of the method are illustrated by comparison to the available experimental data, and to the exact results for a conductivity profile that varies as a hyperbolic tangent. The shape of the conductivity profile has an effect on the impedance response; the steeper the profile, the larger the impedance. Preliminary experimental studies show some agreement with theory. >

Magnetic Permeability and Eddy-Current Measurements

The calculation of the impedance of a right-cylindrical air-core coil placed next to a flat plate of a uniform polycrystalline metal alloy is a canonical problem in quantitative eddy-current inspection. Cheng [1] and Dodd and Deeds [2] proposed a widely-used solution for a coil next to a semi-infinite half-space of metal, which agrees quantitatively with impedance measurements made for a wide variety of thick but nonmagnetic metal plates. Said plates were assumed to have a uniform and frequency-independent conductivity, σ. The same authors proposed a similar solution for ferromagnetic metals (such as iron, steel and nickel), characterized by both a uniform σ and a nontrivial uniform permeability ¼.

Pulsed Eddy-Current Measurements of Corrosion-Induced Metal Loss: Theory and Experiment

Corrosion is one of the most important factors limiting life-extension of aircraft in both commercial and military fleets. Non-destructive methods for characterizing damage caused by hidden corrosion in layered structures such as aircraft lap-splices are, consequently, a high priority for commercial airlines and the Department of Defense. A considerable effort is underway to develop new eddy-current techniques for detecting and characterizing hidden corrosion. Eddy-currents have the advantage of penetrating into subsurface layers and therefore being sensitive to their condition, whether or not the layers are mechanically bonded. In contrast, ultrasonic techniques require a mechanical bond between layers for the ultrasonic energy to penetrate to the second or third layers. Both time-domain [1,2] (pulsed) and frequency-domain [3,4] (continuous wave) methods are under development.

Eddy-Current Measurements of Corrosion-Related Thinning in Aluminium Lap Splices

The aging commercial aviation fleet requires new methods for detecting and characterizing corrosion. In particular, the need exists for a rapid and reliable method of nondestructively detecting and characterizing corrosion in layered aircraft skins. Aircraft skin consists of thin aluminum sheet. There are many joints at which these sheets are overlapped and attached to the airframe by fasteners that extend through the sheets and into the substructure. The overlapping aluminum sheets are generally separated by a thin gap, which may be filled with a sealant. Because of the overlapping nature of such joints, the second sheet and the substructure are not directly accessible from the surface.

Determining Conductivity and Thickness of Continuously Varying Layers on Metals Using Eddy Currents

Modifications to metal surfaces are important for many products; they can improve the interaction of the product with its environment, while retaining the structural properties of the bulk metal. Surface modifications provide properties such as good electrical contact as well as resistance to wear, corrosion and high temperatures. Consequently, it is desirable to develop nondestructive methods for characterizing near-surface properties, such as the electrical conductivity and magnetic permeability. In this paper we present an eddy current method to determine the structure of continuously changing surface layers.

Measuring Thickness and Conductivity of Metallic Layers with Eddy Currents

Coated metals are used increasingly for a variety of technological purposes; the coatings provide wear resistance, good electrical contact, corrosion protection, and thermal isolation. Consequently the ability to determine the thickness, conductivity, and structural integrity of such coatings 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. Current inspection practices using these methods are often limited in their ability to provide quantitative estimates of the important parameters. In this paper we present a robust method that uses eddy current measurements to determine the thickness and conductivity of uniform conductive layers.