Eddy current sensor networks for aircraft fatigue monitoring

Abstract

Networks of surface mounted and embedded eddy current sensor arrays permit fatigue damage monitoring at numerous critical locations in an aircraft structure. Fatigue characterization using eddy current sensors has evolved over the years from line scans with a single sensing element meandering winding magnetometer (MWM) to area scanning with high resolution imaging MWM arrays and fatigue monitoring with permanently mounted arrays. Scanning arrays have provided images of widespread fatigue damage in a four point bending fatigue specimen and in aircraft structural components such as an F-18 bulkhead and 727 lap joint. Arrays permanently mounted in high stress regions of fatigue specimens detected small cracks, for example, less than 50 μm (2 x 10 -3 in.) long cracks. Crack initiation and growth monitoring was demonstrated with arrays mounted at fastener holes on exposed surfaces and between aluminum layers. Also, magnetic permeability monitoring and imaging provide the capability to characterize the evolution of precrack fatigue damage in steels, including 304 stainless steel and 4340 low alloy steel. This paper describes two architectures for onboard fatigue monitoring and a demonstration of capability to independently monitor multiple sensor arrays simultaneously.