Acousto-Ultrasonic In Situ Monitoring of Fatigue Cracking in an Aircraft Wing Skin Specimen

The concept of in situ Structural Health Monitoring (iSHM) involves incorporating a diagnostic capability to detect and monitor damage in structural 'hot spots'. These systems allow the introduction of Condition Based Monitoring (CBM) approaches for ageing aircraft fleets and thus could help significantly reduce through-life support costs and increase aircraft availability. The Australian Defence Science and Technology Organisation (DSTO) has a program of work aimed at developing autonomous, robust, reliable iSHM systems with the specific aim of retro-fitment to existing aircraft. A current laboratory demonstrator of the technology is focused on the application of an acousto-ultrasonic (AU)-based iSHM system for monitoring fatigue cracks on generic structurally-detailed lower wing skin (LWS) panel specimens. This particular scenario, which simulates the LWS on an F-111C aircraft, is considered an ideal developmental platform and springboard for future iSHM applications, since it captures a number of key issues that AU-based iSHM systems need to address to ensure robust reliable diagnostics, viz., complex structural geometry, crack closure and high operational loads. This paper describes the design, application and assessment of an AU-based iSHM system to detect and monitor growth of fatigue cracking in the generic complex-geometry LWS panel specimens subject to representative spectrum loading. Its performance is discussed and compared to measurements furnished by thermoelastic stress analysis and visual observation. Testing associated with this complex realistic panel specimen revealed several deficiencies in the design of the iSHM system.