An in-situ sensor design for monitoring fatigue damage in bolted joints

One of the most common forms of damage for metallic airframe structures is fatigue cracks [1] that originate at holes in bolted joints [2]. Structural health monitoring (SHM) systems that can be permanently installed within a vehicle to monitor this sort of damage bring with them the promise of a reduction in cost for vehicle inspections and improved safety when compared to traditional, hand-operated, non-destructive inspection techniques. In this paper the concept of an in-situ sensor system for monitoring fatigue cracks at hole locations in layered metallic joints is presented. Non-contact, eddy current sensors designed for this application are presented along with results for detection of fatigue cracks in laboratory specimens. These sensors have been optimized to increase sensitivity with an increasing lift-off, or distance between sensor and joint layer surfaces. A damage index is formulated from sensor signals that shows good correlation to crack growth in excess of 1/2 '' from a thru-hole in aluminum.