Conventional non-destructive inspection approaches can be costly, require physical access to the subject and some of the established inspection methods are more difficult to implement on polymer composite materials. This has driven a growing interest in the use of embedded sensors. The physical form of optical fibres means they are well suited to embedment in fibre reinforced composites however there are technical challenges associated with their use.<p> </p>The non-uniform geometry of woven fabric composite materials can induce localised macro bending in embedded optical fibre Bragg grating (FBG) sensors when they are compacted between layers during the lay-up process. This leads to a non-uniform strain profile along the optical fibres which can limit the efficacy of conventional peak tracking algorithms for demodulating strain.<p> </p>This paper investigates the effect of gauge length on sensor response for FBGs of different length embedded in a woven glass fibre reinforced composite coupon. The experimentally measured FBG reflection spectra were compared to model predictions for the unloaded state assuming an FBG bend radius of similar dimensions to the weft of the fabric. Through thickness fibre optic strains under four point loading conditions were compared to side-imaged thermoelastic response measurements. The results show that the ratio of the gauge length to the curvature radius of the macro bending is critical with the optimal gauge length being a compromise between FBG reflectivity and sensor response.
Lamb-wave based structural health monitoring (SHM) approaches are typically constrained to operate below the first cut-off frequency to simplify the interpretation of the wave field in the time-domain. However from a diagnostic perspective, it is desirable to unlock the additional information encoded in the higher-order Lamb wave spectrum. Wave-mode decomposition is necessary for the extraction of useful information from multi-modal acoustic wave fields, which requires spatially dense sampling over the field. The instrument of choice for this task is the laser Doppler vibrometer, which is capable of producing detailed spectral decompositions. However vibrometry is not suited to in-situ measurement for SHM. Fibre Bragg gratings (FBGs) are capable of sensing Lamb waves and detection of higher order modes using FBGs has been previously demonstrated. The ability to multiplex multiple short-length gratings along a single fibre to create an FBG array gives rise to an in-situ sensor with sufficiently dense spatial sampling of an acoustic wave field to perform useful wave-mode decomposition. This paper explores some of the fundamental limits to modal decomposition resolution and bandwidth that exist for such sensors. Potential sources of noise and distortion encountered due to limitations of the sensor fabrication and interrogation methods are also discussed. In addition, modal decomposition of Lamb waves with frequencies up to 1.25 MHz is demonstrated in a laboratory experiment using an array of sixteen ~1 mm long gratings bonded to an aluminium plate. At least four modes are distinguishable in the resulting spectral decomposition.