Increased field depth penetration with pulsed eddy current

Non Destructive Evaluation (NDE) using conventional eddy current testing has become quite prevalent as it can readily identify both surface and near-surface defects in non-ferromagnetic materials. However, there exist inspection requirements for thick multi-layer aluminum structures in the Canadian Forces that are beyond the depth at which conventional eddy current can identify discontinuities with a high degree of probability. The advantage offered by pulsed eddy current (PEC) is the increased depth at which a diffused magnetic field can be sensed. Time domain analysis of the signal response from a far surface discontinuity in a multi-layer structure demonstrates a linear time response with depth, while the signal amplitude decreases exponentially with depth. As such, time gating techniques can be used to obtain information pertaining to specific depths within the test piece by considering the characteristic relaxation time associated with the coupled circuit of the driver and pickup coils. An improvement to the field depth penetration in thick multi-layer structures was sought by way of a time domain analysis. Obtaining information through greater depths is tied to increasing the signal strength later in time, such that variations sensed as a result of indications can be separated from that of noise. Experimental observations are made on plates manufactured from 2024 T3 Aluminum.