ABSTRACT
Non-destructive mcasurement techniques for the determination of conductive coating properties and thickness have many uses including production process control, quality assurance and inspection. This paper considers the theoretical development of a method for determining the thickness of a conductive coating on a bar or wire togethcr with material properties using four wire impedance measurement data. The method requires complex impedance data taken over several decades of frequency as input, and provides five parameters: coating thickness; coating electrical conductivity; coating magnetic permeability; core conductivity; and core permeability; as output. Thc method relies on the well known phenomena of eddy currents which push the bulk current flowing in a conductor towards surface as frequency is increased. This is known as the skin effect has been widely used for non-destructive tcsting (NDT) employing inductive coils and magnetic/eddy current techniques for sub-surface inspection [I] and crack detection [2,3]. More recently the use of spectral methods has enabled the accurate measurement of coating thickness with coatings of similar electrical conductivity [4], indicating the power and potential of employing spectroscopic tcchniqucs. Thc skin effect means that at higher frequencies the electromagnetic propcrtics of the matcrial ncar the centre of the bar or wire will have Icss effcct on the overall impedance than at lower frequencies. Scanning the frequency in this way thereby allows enough independent data to be obtained to determine the material electromagnetic properties over the cross-section of the bar or wire. In this paper, the scope of the problem has been limited to a solid bar consisting of a uniform material and a single coating of another uniform material having a uniform thickness. In this way a high degree of information redundancy can be guaranteed which is necessary to minimise the effects of random signal noise. Inductive methods have the significant advantage of requiring no electrical contact with the object under test, however, the measurement coils are subject to parasitic impedances, which limits their physical size and frequency range while requiring expert users to perform the measurement and calibration procedures. Direct current injection and resistance measurements can be used where access to a conducting surface is permitted. Unfortunately, possibly as a consequence of the perceived difficulties in obtaining reliable electrical contacts, this approach has possibly received less attention. Nevertheless, single frequency methods are successfully
used to measure crack depth in industry [5]. This paper now discusses the development of a forward model, an inversion algorithm and presents inversion results for a wide range of material scenarios.
