Get smart

Dr Neil Calder discovers how the creation of smart structures using embedded optical sensors within composite materials could soon be a viable technology.


Smart structures have entered the mainstream of aerospace manufacturing. The need for reliable use and maintenance of polymer composite components and assemblies has driven a corresponding need to embed a degree of structural health monitoring functionality within these structures. The integration of this capability with other aircraft systems is now setting the development agenda as integrated system maturity, including reliability and credibility with operators and industry regulators, becomes the most significant factor. The technical difficulty in achieving reliable signals from Fibre Bragg Grating (FBG) sensors has always been in robustly coupling the optical environment with the structural one: mixing carbon and optical fibres. Early work had the optical fibres sticking out of the edge of structural panels with all the fragility that this would suggest. More reliable optical coupling has now been achieved and has brought these sensors into a more ‘plug and play' environment. Sensing the strain Fibre optic strain sensing of composites is a technology that goes wider than the aero sector. Suppliers of this technology are characterised by having product offerings in multiple sectors. The instrumenting of composite wind turbine blades has provided a recent market multiplier generating application experience, as well as revenue. FBG strain sensing has established itself as the preferred sensor technology for a number of reasons. They are rugged, passive components with a fundamental lifetime measured in decades, and forming an intrinsic part of a fibre optic cable that can transmit a measurement signal over many kilometres. Optical fibres are immune to interference from electromagnetic radiation and so can function in many hostile environments where conventional sensors would fail. They don't make use of electrical signals which makes them explosion safe. It is possible to multiplex many strain and temperature sensors in one optical fibre, driving down the cost of complex sensor networks. Belgian equipment supplier FOS&S provide health and usage monitoring systems (HUMS) equipment which is fitted to composite structures on the Airbus A380. This is the result of previous collaborative development work within the European IFSA project. This will allow continuous real-time in-flight structural health monitoring, resulting in improved safety, design optimisation and reduced maintenance costs. The system can monitor the occurrence and growth of damage in composite elements by monitoring the stress and strain inside the thermoplastic composite structures, which is not possible with measurement systems available today. Europe's flagship €1.6billion Clean Sky research programme has taken some time to emerge from red tape delays, but the green technology initiative has already produced some interesting hardware. Germany's Fraunhofer Institute for Structural Durability and System Reliability (LBF) used the Paris Airshow to display a 4m long mock-up of a light aircraft wing equipped with several sensor technologies for load monitoring in a realistic production scenario. Its starting premise for this work was that the complex damage behaviour compared with metallic materials means that the potential weight savings could not always be fully exploited. However, sensors in the wings and fuselage can detect early-stage structural damage that would otherwise not be externally visible, reducing inspection frequencies, durations and hence costs. Wider applications Insensys is currently working with Bell Helicopters on a US Army programme to fit sensors to helicopter main rotor blades for detecting impacts and to main rotor head structures to provide real-time load history data. In addition to developing techniques for deploying the optical fibres, this work has included the building of a new compact interrogator that is small enough to be mounted in the rotor head. This new compact, lightweight interrogator is capable of a sampling rate in excess of 8kHz. While the sensors for this particular programme are mostly surface bonded, Insensys is also working with United Technologies Research Laboratories (UTRC) on a second US Army programme that is working to embed the optical fibres within a structure representative of a typical rotor head component and is formed from prepreg carbon fibre laminates in a closed mould. A number of specimens incorporating an integral connector developed in collaboration with Deutsch UK have been successfully manufactured. This connector avoids the need for vulnerable flying leads, which is particularly important where closed moulds are used. In addition to these two programmes for the US Army, Insensys is working on a project using embedded fibre optic sensors for temperature measurement. The requirement for temperature measurement has arisen from the need to monitor temperature during manufacture of composite components where embedded sensors have a considerably longer life that surface mounted thermocouples and to monitor de-icing systems to warn of an over temperature that could impact on structural integrity. Technology developments in the structural integration of alternative strain sensors can be seen also. Loughborough University is investigating the manufacture of metallic smart structures through the process of ultrasonic consolidation, and the Fraunhofer Institut Fertigungstechnik Materialforschung in Bremen is depositing strain gauges and sensors using maskless non-contact printing technologies. Current Clean Sky calls for proposals specifically target work on the use of FBG sensing for stress corrosion in aluminium structures, and for advancement in the integration of sensing systems.

Interconnect technology, standardisation and maturity of the way in which certification of structures incorporating HUMS is handled are now receiving attention. The key principles of the sensing technology are well understood with a large body of experience in this area. The generation of field data on the maintainability benefits to aircraft operators is the focus of industry working groups operating under the umbrella of the SAE. The real impact of this work is expected in the timescale of the next generation short range airliners. BAE Systems, Rolls-Royce, Thales and Meggitt have each committed £1 million over five years to fund research at the Integrated Vehicle Health Monitoring (IVHM) centre of excellence jointly launched by Cranfield University and The Boeing Company in late 2007. The centre will conduct some work on the enabling technology aspects of sensor design and manufacturing, but more on whole vehicle system integration of loads, signals and data. This is consistent with the general thrust of work in smart structures moving up the systems hierarchy. www.insensys.com/aerospace
www.fraunhofer.de

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