Fiber optic sensing for composite smart structures

Abstract

Structurally integrated fiber optic sensors (SIFORS) will form a necessary part of future Smart Structures and strain sensing will provide valuable information on the loading and response of such structures. Strain measurements may be used to check structural integrity and in the case of Smart Adaptive Structure they will provide the deformation information required by the actuation control system. Continuous monitoring of applied loads could be used to determine the fatigue life of the structure at any point in its operation. In the case of composite materials improved quality control during fabrication may be possible from such a built-in sensing system and this could lead to a more consistent and reliable product. Overall the introduction of an embedded fiber optic strain sensing system to composite structures may enhance confidence in their use and lead to a wider range of application, especially in the aerospace field. In this paper the types of fiber optic sensor are reviewed and it is shown that the intracore Bragg grating and the intrinsic Fabry-Perot interferometer appear to best qualify for continued consideration for a broad range of Smart Structures. The key elements of strain sensing with both types of structurally integrated fiber optic sensor are reviewed and issues such as thermally induced apparent strain are discussed. It is shown that although both sensors now have good methods of demodulation, the Bragg grating sensor has the greatest potential to satisfy the requirements demanded of the ideal sensor for Smart Structures due to the recently developed ratiometric wavelength demodulation system. This system promises the fabrication of a robust, compact, absolute measurement, low cost multisensor system that could be integrated on to an optoelectronic chip that can form part of the interface to any structure and in essence solve the critical interconnect problem. Indeed, an extremely user-friendly, noncontact form of communication could be possible with the developments of this optoelectronic Smart Structure interface—termed an “optical synapse”.