RResearch and development of algorithm for determination of absolute conductivity value without calibration by eddy current techniques

• Applicants: Professor Dr.-Ing. Henning Heuer; Professor Dr. Jörg Wensch
• Subject Area: Measurement Systems
• Term: from 2015 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 271629349

Non-destructive testing methods are essential for safe and reliable operation of technical components by determination of load limits of the material. A widely-used procedure is eddy current testing of conductive materials. Besides detection of material defects it is possible to perform complex material characterization non-destructively. Material properties are determined by their influence on electrical properties. The electromagnetic field induces eddy currents in the material whose values are influenced by the material parameters (conductivity, permeability, permittivity, geometry). These influences can be measured by determining the impedance of the coil. These values of the impedance depend on the frequency and the lift off, e.g. the distance between the coil and the material to be tested. Until now the measurement of conductivity by eddy current is based on a empirical approach. Using some reference samples with different conductivity a calibration curve is determined. This calibration is valid only for one device, one frequency and mostly only for a very limited range of lift off. At the same time there is the problem there are often not enough exactly defined and well-graduated reference samples for determining those curves. A new theoretical approach was found by simulation results. It is another way that makes possible direct determining of conductivity. In this case it is derived directly from the physical laws and no longer from the empirical calibration curve. Thereby it is not limited to a specific measurement frequency. Additionally the lift off value can be assigned directly to the measured impedance value. This method enables carrying out a conductivity tomography as sliced depth profiling. Integration over the field distribution inside the material should result the conductivity value. The relationship between the value of the integral and the conductivity has to been found. This is mathematically very demanding because it leads to an inverse problem. The electrical properties of the propagation volume have to be determined from the measured values on a point of the surface. It is assumed that the conductivity changes only in the depth (z direction, direction of wave propagation), whereas it has in plane the same value. Changing the penetration depth by frequency change the material in a certain depth has a different influence on the calculated integral. In this way an algorithm for determining conductivity as a function of depth has to be developed. In case of thin layers it seems to be possible to determine the thickness and the conductivity of the layer at the same time and additionally the approximate conductivity of the substrate. This new approach has to be examined in detail and prepared for the practical use. A modified hardware is used measuring not only the voltage in the receiving coil but additionally the current in the exciting coil making possible the calculation of the impedance of the measurement set.

• DFG Programme: Research Grants