Among those involved in researching and developing a structural health monitoring (SHM) system for composites is a team at U.K.-based QinetiQ, led by Dr. Duncan Shepherd, capability leader of advanced diagnostics and prognostics.

"There are many hurdles to be overcome before a viable, reliable SHM system flies on a civil aircraft," he said. "Creating the sensing hardware and its detection capability is just the start, because it still has to be integrated with the aircraft infrastructure. Indeed, the 'Integrated Wing' program, in which we are currently engaged, is investigating the integration issues associated with the design and production of a civil aircraft wing, including a built-in SHM system. So, until this project is completed, we won't know how best to do this."

He also pointed out the continuing confusion within aerospace over what is termed structural health monitoring because it tends to cover a range of different systems. For example, the Eurofighter Typhoon has an SHM system to measure airframe strain cycles for fatigue monitoring, but it was not designed to detect damage in composite parts. There was no point in designing an SHM system to monitor the entire aircraft, so research has focused on the areas most important to the aircraft operators: namely those exposed to ground vehicle impacts and structures in the airframe that handle high loads. Installation, however, poses another problem.

"Embedding sensors in structures will ensure they're physically protected," said Shepherd, "but if they are, then they will not only need to be guaranteed to function for the aircraft's design life (say 30 years) but also guaranteed to be reliable throughout. Right now, we don't know how to do that."

A way also needs to be found to get the data from sensors embedded in the structure. And, while technologies such as wireless devices and fiber-optics could be used, there still is the problem of how to integrate these networks with the aircraft's other systems. Work also is ongoing with aircraft manufacturers on how to qualify an SHM system to a point where the OEMs, operators and regulatory authorities will let it fly in airline service.

"Currently, no aviation authority knows how to do this," said Shepherd. Today, the burden of ensuring the integrity of composite components lies with non-destructive testing (NDT), but QinetiQ is looking to develop an on-board SHM system that performs such testing as part of its normal function. However, Shepherd stressed that the installation of such a system would not mean the end of NDT. In his view, a transition would be necessary but, as monitoring technology matured, NDT would be used to assess and back-up the SHM findings.

"This is already happening," he said. "We have already been involved in trials where a QinetiQ-designed system detected damage previously missed by NDT. Following its indication, the damage was subsequently found by a second NDT inspection. So, results like this make us even more confident that this kind of technology has the long-term potential to deliver the hoped-for benefits."

Technologies

Fiber optic technology has the advantage of being unaffected by electromagnetic fields, making its integration with other electrical systems somewhat easier than wireless devices. However, if optical fibers are molded into composite materials, each fiber likely would have a number of sensors multiplexed onto it. The fibers would exit the structure via a surface-mounted connection device and be routed to a dedicated processor. This is because fiber-optic devices operate at Giga-hertz frequencies and, at these frequencies, information cannot be stored, Shepherd said. It must be processed immediately to identify relevant readings.