The detection of impact damage in fibre reinforced composites is of significant concern because such damage can reduce the load-bearing ability of the composite. A number of factors can influence the nature and extent of impact damage development in composites including: (a) the type of reinforcing fibre and resin system; (b) the magnitude of the residual (fabrication) stresses; (c) the lay-up sequence; and (d) other factors such as the nature of the impactor, impact velocity, impact energy, temperature, moisture content in the composites, etc. From a structural health monitoring point of view, it is necessary to investigate the distribution of damage through the thickness of the composite.This paper reports on a simple, partially multiplexed optical fibre strain sensor system for in-situ strain and residual strain measurements in a carbon fibre reinforced epoxy composite. An extrinsic Fabry-Perot interferometric (EFPI) sensor design was used along with single-mode fibres. The multiplexing scheme was based on wavelength division via the use of two super luminescent diodes (SLDs) at different wavelengths. A low-cost fibre optic CCD spectrometer was used as the detector. The multiplexing scheme was demonstrated using two EFPI sensors. In principle, a number of EFPI sensors can be multiplexed using the proposed scheme provided that each sensor is illuminated at a specified and different wavelength.The feasibility of detecting the residual strain in the composite was demonstrated successfully at two specified positions within a 16-ply carbon fibre reinforced composite panel. Preliminary results indicated that the sensor system was also capable of detecting the effects of a 3.2 J impact. Excellent correlation was obtained between the EFPI sensor output and that obtained using surface mounted strain gauges.
Optical fibre Bragg grating (FBG) sensors have significant potential for use as embedded devices to monitor the structural integrity of engineering materials. The main drawback of the FBG strain sensor is its cross-sensitivity to temperature. This paper reports a simple scheme for multiplexing a FBG and an extrinsic Fabry-Perot interferometric (EFPI) sensor to enable the decoupling of strain from temperature. The EFPI sensor was constructed using a precision bore quartz capillary tube which housed two cleaved optical fibres. The gap between the fibre end$aces served as the Fabry-Perot cavity. Since the coefficients of thermal expansion between the optical fibre and the capillary tube were similar, the EFPI sensor has a very low sensitivity towards temperature. Therefore, when both sensors are placed close together, the EFPI sensor can act as the strain sensor, and temperature can be determined from the FBG wavelength shift after taking out the strain effect. The signal processing for the EFPI sensor was based on a channelled spectrum method using a CCD spectrometer. The same CCD spectrometer was also used to determine the wavelength shift of the FBG. The cross-talk between the EFPI and FBG sensors was evaluated. The feasibility of conducting simultaneous strain and temperature measurements was demonstrated.
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