Smart structures approaches for health monitoring of aircraft structures

Due to economic pressures both commercial and military aircraft fleets are operating a greater number of ageing aircraft. It can therefore be expected that the occurrence of structurally significant defects will significantly increase in the future. In the Australian context, a significant portion of the Royal Australian Air Force fleet are being operated well past the designed life. For example the F-111C fleet will be in service till the year 2015 which is about 20 years more than the original design life of the aircraft. As fleets get older a greater share of the operator's resources need to be used on through-life-support of the airframe. One way of reducing costs and increasing aircraft availability is through the use of smart materials technology. Smart materials are materials with the ability to respond to changes in the operating environment or to other stimuli in an intelligent way. This ability may be achieved fi om sensors and actuators embedded in or attached to the structure or, more simply, from an inherent response mechanism in the material. Tn the context of ageing airframes, smart materials/structures technology has excellent potential to provide improvements in through-life support, including health and usage monitoring (HUMS), with the eventual, aim of allowing condition based maintenance procedures to be adopted rather then relying on current expensive time-based maintenance procedures. This paper discusses the development and evaluation in DSTO of smart structure technologies to be applied to structural health monitoring of aircraft structures. Systems are being developed by DSTO with the specific aim of retro-fitting to existing airframe structures (e.g. smart repairs and reinforcements with the ability to self-monitor patch system integrity), where the systems need to be autonomous, distributed, robust and reliable. The paper expands on the health monitoring techniques being developed and evaluated, including the use of electrical-resistance foil strain gauges, piezoelectric elements and optical fibre sensors. Since the emphasis here is on retrofitting systems to existing structures the paper also touches on fully-autonomous systems which incorporate self-powering and wireless access techniques.