Abstract
The structural health monitoring of ferromagnetic materials is essential for controlling product quality. This paper presents the first known characterisation, utilising magnetic induction spectroscopy, of the magnetic permeability of ferromagnetic materials, a property that can facilitate such monitoring. Magnetic induction coils are interrogated by a sweep of excitation frequencies in the range of 1–100 kHz, with amplitudes of induced voltages collected from both non-magnetic and ferromagnetic specimens, and their associated spectral permeability imaged. The reconstructed images show that magnetic permeability can be reconstructed using frequency difference imaging in the investigated frequency range. As such, the relative magnetic permeability of a ferromagnetic specimen can be reconstructed without needing a time difference measurement. This provides a robust imaging approach for the material characterisation of ferromagnetic specimens.
Highlights
Ferromagnetic materials such as steel are an integral component of modern infrastructure
This paper presents the evidential basis of frequency-dependent permeability from magnetic induction measurements and uses a frequency difference technique to reconstruct the images of the amplitude of complex magnetic permeability
Within the frequency range of 1–100 kHz, and in the presence of both non-magnetic and ferromagnetic specimens, it is shown that only ferromagnetic specimens have a frequency difference response with regard to their reconstructed image
Summary
Ferromagnetic materials such as steel are an integral component of modern infrastructure. Frequency-dependent magnetic permeability has been observed using other non-imaging techniques, such as impedance measurements picked up by coils placed closely to the ferromagnetic specimen[7], a four point alternating current potential drop measurement technique[8], and a direct measurement of the hysteresis curve of the material of interest by H-coils[9]. Within the frequency range of 1–100 kHz, and in the presence of both non-magnetic and ferromagnetic specimens, it is shown that only ferromagnetic specimens have a frequency difference response with regard to their reconstructed image. This is, to the best of our knowledge, the first report of this nature employing both magnetic induction measurements and a tomographic approach. Given that the frequency sweep interval is, at minimum, 5 s, it is safe to assume that any material response due to frequency differences is settled before the interrogation within the ferromagnetic specimen under investigation
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