Plastics can provide their own feedback

Measurement and feedback of physical forces experienced by composites can be achieved with the use of modified fibre optics. Mark Fletcher reports

Researchers at DaimlerChrysler have created a technology which will allow composites to exhibit a basic nervous system.

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By creating Bragg grating sensors in long runs of glass fibres, the researchers have made it possible to measure many of physical phenomena that composite structures may experience.

Stress, strain, elongation, deformation and vibration are just a few of the experiences a composites may have to face. By integrating the fibres into the material's matrix these can be measured at critical and inaccessible locations which maybe difficult to measure by other means. It is anticipated that this ability will help extend a component's service life and reduce maintenance costs.

The primary application areas for the technology are envisioned in the automotive and aerospace industries. However, structural composites are not unique to these two industries. Other applications will exist where the use of composite structures are widespread.

Optical fibres are, in effect, wave guides which can transmit light over long distances with negligible loss. The researchers have been able to modify these fibres by incorporating Bragg gratings at specific locations along the fibres length. They are created by exposing the fibre to superimposed rays of ultraviolet light.

Infra red light, at a specific wavelength, will be reflected by these gratings. The wavelength of the light is a function of the elongation of the fibre which, in turn, is a indicator of the distortion at the specific location. Spectrographic methods are then used to evaluate these changes and from the information the physical experiences at particular points can be ascertained. By crating Bragg gratings in specific places it is possible to determine the load patterns and deformation throughout the entire component.

The advantage of the Bragg grating approach is its multiplex capacity. One single fibre can be used to address several hundred sensors, since a number of channels can be transmitted through one fibre without interference. A capability used in the optical transmission of data.

The fibre optic approach offers many advantages over a more traditional strain gauge system. It is more sturdy and offers greater resistance to wear leading to longer service life. The lack of cabling is also a major boon.

With just light as a data source EMI is not a problem and the architecture is unaffected by operation in areas exhibiting high voltages or radiation.

The fibre optic system is currently being used in load sensors for aircraft components. Tests which simulate the entire load acting on an aircraft throughout its service life will be come a lot more cost effective. Automotive applications will include pressure sensing in the engine compartment and assessment of impact force for the deployment of airbags and other safety features. In heavy goods vehicles they can be used to monitor loading in real time to determine whether the permissible gross vehicle or combination weights have been exceeded.

Design Pointers

The sensors are unaffected by EMI and high radiation environments

Multiplex capability allows one fibre to address a number of sensors

They can be moulded into a composite material's matrix

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