Evaluation Of Pulsed Eddy Current Generation Through Aerospace Bolts Using An Internally Mounted Sensor

Modern inspection techniques of bolt holes in multi-layer aluminium wing skins involve hand-held single coil Eddy Current (EC) inspections, causing extended grounding periods and only providing discrete data points at inspection intervals. We investigate the suitability of a permanently installed Pulsed Eddy Current (PEC) Structural Health Monitoring (SHM) approach, to reduce grounding periods and hence costs, and enhance understanding of defect formation over the wing’s load cycles. The paper investigates the applicability of a PEC sensor layout proposed in literature, adapted to suit the needs of an SHM system. A driving coil, located around the bolt inside the wing, is used to drive magnetic flux through the fastener, to achieve greatest penetration depth. Finite Element (FE) models were developed to simulate the sensor configuration and evaluate the impact of nut and bolt material properties on current generation in defective regions. These studies demonstrate the shielding effect of a ferrous nut, reducing current generation (by 96.1% and 40.9% for ferrous and non-ferrous bolts respectively) compared against a non-ferrous nut; and the flux-carrying potential of ferrous bolts, increasing current generation by 30.2% over a non-ferrous bolt. Future work will experimentally validate these results and evaluate defect detection ability using various pickup coil configurations.