Improved eddy current testing sensitivity using phase information

This paper describes a two-coil eddy current sensor being used in a transmit-receive arrangement at 1 MHz, where the drive and amplification electronics are miniaturised and built directly behind the coil to reduce noise and the effects from cable length. Small, simulated defects are detected, less than 500 microns in length, on titanium and titanium aluminide, which is an increasingly important alloy for aerospace applications. Data is analysed quantitatively in a parametric approach. This experiment uses a transmitting coil driven by a constant current source and a separate receiving coil, where the magnitude and phase of the induced voltage signals on both coils are measured independently. Experimental measurements are validated using finite element modelling and the phase of the signal on the receiving coil in particular is less susceptible to variations caused by changes in lift-off. A combination of experimental and simulation data of 2D surface scans and lift-off measurements show the variation in the magnitude and phase of the eddy current signal with lift-off on Ti, TiAl (Ti-45Al-2Mn-2Nb-1B) and 316L stainless steel. It is also shown that the high-frequency lower noise approach can reliably detect defects of less than 500 microns in length in both Ti and TiAl.