Hindrances associated with examination of two-layer structures with use of the eddy current method

Purpose: Determination and description of essential phenomena that occur during examination of two-layer structures with use of the eddy-current method. Development of guidelines to establish procedures that make it possible to find out optimum frequencies of eddy currents, for which measurements of the outer film covering the examined structure exhibit best performance. Estimation of errors that occur when measurements of both geometrical and electrical parameters of the upper film within the two-layer structures are carried out simultaneously. Design/methodology/approach: During the design and scalling of the devices based on phenomenon of eddy currents implying changes of contact coil impedance components, significant influence of the electromagnetic field frequency on measurement results was observed. Additionally, some other phenomena can be observed which causes invalid interpretation of devices indications. Basing on a mathematical model of a contact coil located above a conductive non-ferromagnetic two-layer plate, the sensitivities to the measured parameters are determined. The developed mathematical model serves and the basis to calculate theoretical values of errors when all parameters of the outer film on the examined structure are measured simultaneously. Findings: Depending on specific applications, recommendations enabling proper choice of the electromagnetic field frequency are formulated. Practical implications: Describred phenomena and calculations are useful for the designers of the devices utilising the phenomenon of eddy currents, and also for the users of flaw detectors and conductometers. Remarks included in this paper can be useful for proper interpretation of the observed results and phenomena. Originality/value: Determination of impact coefficients that define how parameters of the two-layer structure are vulnerable to impedance components of the measuring contact coil.