High-accuracy eddy current measurements of metals

Abstract

Eddy current measurements are widely used for non-destructive testing and identification of the electrically conductive materials (e.g. metals and alloys) and structures of such materials, also in aerospace industries. As well, new application fields are interesting, like monitoring of the composites, including electrically conductive carbon-fibres inside the structures. One challenge is the precise ”absolute” (”calibration-free”) measurement of the electrical and/or magnetic properties of the materials and their geometry, without using of the reference calibration pieces with known electromagnetic properties. Excluding of such calibrated and certified reference metal pieces from the metrology chain makes the measurements more accurate, covers wider frequency range, makes measurements faster and cheaper. The instrumentation and measurement approaches, models, simulations and algorithms for such measurements are explained, on practical examples, of precise measurement of electromagnetic properties of metals. For non-magnetic materials the electrical conductivity (inverse of the resistivity) can be measured with accuracy of better than 1 %, in the kHz or even MHz range. The measurement of the magnetic (e.g. ferrous) materials is more sophisticated and challenging. The additional problem to be solved is separating of the influence of the magnetic permeability and electrical conductivity of the material under test onto the eddy current measurement results. The solution is to estimate the magnetic permeability of the material by low-frequency measurements (in the 10 Hz or even lower range), but such measurements are very demanding for the needed resolution and accuracy of the used instrumentation. Currently the achievable accuracy of the magnetic permeability and electrical conductivity measurement could be estimated at the 10 % level. One problem to be considered is the frequency dependency of the magnetic permeability of the ferrous materials. Also, interesting future challenge is precise reconstructing of the geometry of the specimens under test, by using of the reverse eddy current models.