Damage monitoring of carbon-fibre-reinforced plastics using Michelson interferometric fibre-optic sensors

Abstract

Structures having the ability both to inspect their integrity with sensors and to control damage propagation with actuators are referred to as ` smart structures''. Fibre-optic sensors have the advantage that they are immune to electromagnetic interference and they can measure various physical parameters using one probe. Thus ®bre-optic sensors have been promising sensors for smart structures [1, 2]. Fibrereinforced plastics have been considered prospective smart structural materials because ®bre-optic sensors can be easily embedded in them. Recently, ®brereinforced plastics have been widely used as structural materials for aerospace applications. A smart airplane that would be developed in the future would have the function to inspect its own structural integrity, i.e., to measure strain measurement and to detect damage. The use of ®bre-optic sensors for damage detection is well known. Hofer [3] and other researchers [4±6] have demonstrated that optical ®bre systems are capable of detecting damage in composite materials. They indicated that damage in composite materials could be detected by the fracture of embedded optical ®bres. The intensity of the light propagating through an optical ®bre drops substantially whenever a ®bre is broken. Fukuda et al. [7] have measured the strain of composite materials with an embedded optical interferometer by utilizing the optical interferometric phenomenon. In the present study, the possibility of damage monitoring of composite materials using optical interferometers was investigated. A sudden occurrence of an interference signal with a high amplitude and frequency during tensile loading was found to result from damage of the composite. Optical interferometric sensors proved to be effective not only to measure strain but also to monitor damage of composite materials. Optical interferometric strain sensors utilize the change in optical path length caused by deformation in an optical ®bre. Consider the case where coherent light of wavelength, e, is launched into two single-mode optical ®bres. One ®bre is strained, and the other is maintained in a strain-free state. The strained and strain-free ®bres are called the sensing and the reference ®bres, respectively. The change in optical path length is associated with the phase shift in the interference signal obtained by combining the light in a sensing ®bre with that in a reference ®bre. When the sensing ®bre with a gauge length, L, is strained along the ®bre axis by az, the induced phase shift, AO, in the interference signal can be expressed by Equation 1 [8]