Pulsed eddy current testing of thermally aged and cold rolled Fe-Cu alloys

In this work Fe-1wt%Cu alloy samples exposed to thermal aging and cold-rolling were studied as an attempt to emulate material degradation in neutron irradiated nuclear reactor pressure vessel. The cold rolling was tested by pulsed eddy currents (PEC) using a directional probe which was rotated in the sample plane. Normalised PEC signals represented by typical “figures of eight” on the polar plot, showed very strong anisotropy which has a monotonic dependence on the cold work amount. Since the normalised PEC signals represent response due to electrical conductivity and do discard the ferromagnetic effect, it is concluded that the change is due to anisotropic magnetoresistivy of the rolled steel. The normalised difference PEC responses have been found to be described by quadratic dependence of cosine of in-plane angle shifted by an angle which is believed to correspond to slip planes of dislocations induced by rolling. The technique advantageously combines effects of rolling on electrical conductivity and magnetic permeability and does not require referencing to an untreated standard, which is important for future material and device health management.