| Abstract | Detection of non-surface fatigue cracking represents a challenge for most conventional non-destructive inspection (NDI) techniques. This report discusses a custom electromagnetic solution and demonstrates its capabilities and shortcomings. The laboratory experiments used aluminium plates with fastener holes and notches. The notch length and penetration depth varied from 1 to 6 mm and from 25 to 100% through-wall thickness, respectively.
This work embodies a two-fold novelty: (i) the through-transmission eddy current approach, with a stationary driving coil, attached to the far-side of the specimen, and (ii) the use of Hall effect solid-state sensors for the detection of the magnetic field generated by the eddy current flow in an aluminium plate containing crack-like discontinuities.
The experimental results confirm that the notch characteristics, particularly the depth and length, significantly affect the magnetic response as sensed by the Hall-effect probe. The field amplitude increased with the notch length, as well as with its degree of penetration through the wall of the aluminium plate. A direct correlation between the notch length and the signal strength was established; however, for notches penetrating less than 75% of the wall thickness, no useful signal was detected. This outcome was assumed to be due to the shielding effect of the eddy currents flowing in the conductive material on top of the partially penetrating notch. |
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