ABSTRACT
Takeda and colleagues (e.g. Okabe et al., 2004) appear to have been the first to use chirped FBG (CFBG) sensors to detect damage in composite materials. For CFBGs, a uniform strain applied parallel to the length of the sensor gives rise to a shift of the entire spectrum (as all the grating spacings change in response to the strain), but a non-uniform local strain caused by damage alters the local density of the grating spacings, and the resulting perturbation in the reflected spectrum identifies the damage location. By embedding a CFBG sensor in the 0° ply of a cross-ply CFRP laminate close to the 0/90 interface, Okabe and colleagues were able to identify the position of individual matrix cracks in the 90° ply which developed when the coupons were loaded. Load-shedding around the 90° ply cracks into the 0° ply caused local perturbations to the strain field which modified the local grating
1 INTRODUCTION
Fibre Bragg grating (FBG) sensors have been in use for many years to monitor strain or temperature changes. The basic principle of operation is simple, in that a periodic variation of the refractive index introduced within the core of an optical fibre reflects light with a wavelength, the Bragg wavelength, λB (Figure 1), where λB is related to the grating spacing, Λ, and the effective refractive index, neff, by
λB = 2neffΛ
In essence, then, when the grating spacing changes, as a consequence of strain or temperature, the reflected wavelength, λB, from an input broadband light source (i.e. a light source having an approximately uniform intensity over a range of wavelengths) changes. The strain sensitivity is typically approximately 1.2 × 10−3 nm/με. For a chirped FBG sensor, on the other hand, the grating spacing changes uniformly along the sensor
spacing, giving rise to changes in the reflected intensity corresponding to the crack locations.
