Optical fibres with improved sensitivity to damage are to be installed in a test composite wing spar for strain detection trials involving lightning and bird strikes later this year.
The 0.25mm (0.01in) diameter fibre could have 100 gauges along its length to help measure damage compared with the normal 10. For strain measurement, pulses of light are transmitted down the fibre using a system called time-division multiplexing. This uses the difference in time that it takes for the reflected light from each gauge to return to the sensor to differentiate between gauges. Strain at a gauge also causes the reflected light to change wavelength. By embedding the fibre in a composite structure, the differences in returned light wavelength and signal return time provides a detailed strain pattern and the damage can be measured.
"We will have only three [gauges] in the front spar, not 100, because we simply want to show that the technology works. The fibre will be a loop [within the structure] to cover three areas of the spar [during the impact tests]. We are not doing fine health monitoring with the fibre," says GKN Aerospace engineering services' head of research and development Giovanni Marengo.
The composite spar will also use embedded surface- and edge-connectors. Embedded connectors overcome the problems of composites having connector tails on their surfaces that can be damaged during handling.
Source: Flight International
