Luminescent Materials Show Structure Damage

Max Glaskin

Flashes of light given off by crystals as they fracture may provide clues to structural problems in composite materials used on aircraft and racing cars. When combined with optical fiber and silicon photodiodes, triboluminescent materials could be viable sensors to reveal damage that would otherwise go undetected.

Graphite and glass-fiber-reinforced polymers can suffer damage from impacts but show no signs of it on the surface. Research chemists at the Defence Evaluation and Research Agency have turned to triboluminescent materials to see if sensors could be embedded in composite materials to continuously monitor a component's integrity.

The most common triboluminescent material is granulated sugar, although other crystals such as diamond and quartz display the same property: Friction causes them to luminesce. The researchers have spent three years trying a combination of more efficient crystal materials and optical fibers that pick up the flash and transmit it to a photodiode.

Chemists at the Defence Evaluation and Research Agency in the UK are working with triboluminescent materials that they hope will someday serve as damage sensors.

The idea is that the crystal compound and optical fibers could be incorporated into the composite material during its manufacture. If the component is damaged, some crystals will be fractured. Grant Bourhill, project manager, said the flash of light that results from the fracture is as bright as a standard LED and visible to the human eye under normal lighting conditions. The light enters the fibers through their sides and is carried to a standard silicon photodiode array, an SD2000 from Ocean Optics Inc. of Dunedin, Fla.

A glass fiber-reinforced polymer panel incorporates triboluminescent damage sensors and an optical fiber network array to channel the light signal from the damage sensors to a remote detector.

"We estimate that one optical fiber is needed for every 2 in. across the width of a component," said Bourhill. "That network will reveal any damage caused by impacts, such as bird strikes, in real time as the damage propagates through the structure. Different triboluminescent materials produce different wavelengths. So, by distributing them carefully through a component, it may be possible to detect precisely where the damage is simply by monitoring the emitted wavelength."

Structures incorporating this technology are at least a decade away. The next step for researchers is to improve the efficiency by which the triboluminescent light is generated and coupled into the fibers, and to refine knowledge of sensor dependence on ambient conditions, such as temperature.