STRAIN CAPABILITY OF OPTICAL FIBRE BRAGG GRATING SENSING IN COMPOSITE SMART STRUCTURES C Y Wei 1 , S W James 2 , C C Ye 2 , N D Dykes 1 R P Tatam 2 , and P E Irving 1 1 Damage Tolerance Group, School of Industrial and Manufacturing Science 2 Optical Sensors Group, Centre for Photonics and Optical Eng., School of Mech. Eng. Cranfield University, Cranfield, Bedford MK43 0AL, UK SUMMARY: This paper determines the performance of Fibre Bragg Grating (FBG) sensors for strain sensing applications in carbon fibre composite materials. Carbon fibre laminates in either cross-plied or quasi-isotropic stacking sequences were fabricated using T300/Hexcel 914 prepregs. The FBG optical sensors were either surface attached, or embedded within the laminates. The sensor orientation was aligned either parallel or transverse to the adjacent carbon fibre layers. The sensor was also bonded to the surface of an aluminium plate using a high failure strain adhesive to allow the maximum failure strain of the sensor to be determined. The composite structures with integrated FBG sensors were subjected to static tensile loading. A scanning fibre Fabry-Pérot filter was used to monitor the reflected Bragg wavelengths. The optical sensor embedded between two 90° carbon fibre plies shows a high sensitivity to multi-site cracking formed in the transverse plies. The embedding in 90° plies seems to change the local stress distributions and to become a source of crack initiation. Efficient stress transfer from the host materials to the sensors is dependent upon incorporation methods, the thickness of the adhesive layers, and the location of the sensors. KEYWORDS: Fibre Bragg grating sensor, Strain sensor, Smart materials INTRODUCTION The potential for the use of optical fibres as sensing elements in smart materials structures has resulted in considerable interest from the civil [1] and aerospace engineering communities [2] over the past decade. Fibre Bragg grating (FBG) sensors appear to offer a real prospect for widespread implementation, as they have wavelength-based sensing, offer single ended connection to control systems and exhibit a linear relation between applied strain and reflected wavelength [3]. A FBG consists of a periodic modulation of the refractive index of the core of an optical fibre, produced by exposure of the fibre to a structured UV laser beam. The FBG is a distributed Bragg reflector, acting as a narrow-band spectral channel-dropping filter in transmission, and as a narrow band reflection filter. The centre wavelength of the reflection band is dependent upon the period of the index modulation, which is sensitive to the strain and temperature experienced by the local region of the fibre. All other wavelengths outside the reflected band are transmitted. The physical features of optical fibres, (size, mass, and thermal coefficient etc), allow them to be integrated into fibre composites during processing. The embedding has been shown to have minimal effects upon the stiffness, tensile and shear strength of the materials, although the compressive strength appears to be compromised [4-6]. Surface attachment of the optical fibre