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United Kingdom

Improving black box data

Some of the drawbacks of using conventional strain gauges for monitoring the structural integrity of rotor blades can be avoided by the use of an optical gauge. Such a device has recently been put on the market by a firm of British yacht builders. All strain gauges can pinpoint strains at critical points, enhancing the designer's knowledge of safety margins and thus material requirements. An optical gauge increases accuracy and avoids the "drift" problem of traditional gauges, which can make them difficult to calibrate.

The improved knowledge provided by an optical gauge could lead to lighter blades, if safety margins are shown to have been too generous, or it could reveal the need for more material if strains are shown to be higher than expected. Furthermore, early detection of fatigue cracks could prevent blade failures.

Smart Fibres Ltd of Southampton has developed this load monitoring technology and associated analysis hardware and software to act as a sensitive nervous system, capable of accurately displaying real time and recording life time load data within any critically loaded composite structure. The Smart Fibre system builds on well-developed technology, which allows a load sensing array to be incorporated into a length of optical fibre cable. This array can be designed to ensure load sensors are located at load bearing points within the structure in a non-intrusive manner.

The system can be used for long term structural health monitoring, real time structural health monitoring, or event recording. With long term monitoring, peak load data at 0.5Hz intervals is recorded on the on-board computer display enabling access to accurate historical load data. This reveals information on significant load events to assess possible over stressing. For real time structural health monitoring, continuous load data are visually displayed by means of a colour coded computer display. For event recording-"Black Box" data acquisition-all data are recorded in five minute cycles to provide detailed information in the event of catastrophic failure.

The Smart Fibre System has special qualities when compared to conventional strain gauges, including automatic temperature compensation, complete immunity from electrical interference and the ability to provide multiples of sensors along a single optical fibre. Significantly, the fibre optic cables are embedded within the laminate structure during manufacture.

The development work was carried out in conjunction with British Aerospace, a shipyard, and Birmingham's Aston University in a government supported program. It has been confirmed that the optical fibres do not affect the integrity of the yacht masts which were used for test purposes. Although the technique was developed for carbon fibre yacht spars, it can be used with glass fibre or wood epoxy blades. Embedding the optical fibres in such materials would, if anything, be simpler as these are less strong than carbon fibre and therefore have more generous dimensions. The interface units consist of a small electronic devise and lap-top display unit which can plug into any Smart Fibre System network.

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