<title>Multiplexed extrinsic Fabry-Perot sensor system for in-situ strain and impact damage detection in composites</title>

Abstract

The detection of impact damage in fiber 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: (1) the type of reinforcing fiber and resin system; (2) the magnitude of the residual (fabrication) stresses; (3) the lay-up sequence; and (4) 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 fiber strain sensor system for in-situ strain and residual strain measurements in a carbon fiber reinforced epoxy composite. An extrinsic Fabry-Perot interferometric (EFPI) sensor design was used along with single mode fibers. The multiplexing scheme was based on wavelength division via the use of two super luminescent diodes at different wavelengths. A low-cost fiber 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 fiber reinforced composite. 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.