Stress-strain monitoring in textile composites by means of optical fibers

This work investigated the possibility of using fiber optic sensors to monitor the loading state of a structural element made of textile composite material. This objective is important because of the potential use in health monitoring of structures, intelligent curing systems and the characterization of mechanical behavior for the development of new textile configurations. The method proposed is based on measurement of the optical loss in the transmission of a signal through the fiber optics; such loss is caused by stress and strain of the fiber sensor. As a first step, a procedure to insert fiber optics in the textile structure was developed. Normally, an optical fiber could be introduced during the manufacturing process as an element of the yam; in this activity, the insertion was carried out by hand in the laboratory. The procedure adopted allows the introduction of fiber optics into any yarn, assuming similar waviness, with terminals coming in areas suitable for connecting the fiber to the light source and signal processing system. A numerical activity was developed before testing to forecast the ability of the fiber optics to detect the textile loading state. In fact, the difference in stiffness values of optical and carbon fibers creates the risk that the sensor could be masked; the numerical simulation confirmed. within the limits of a linear analysis, the existence of this problem, but also that the stress in the sensor is scaled of a constant factor along the considered yarn. Moreover, an important contribution to loss level is caused by deformation and, in this case, the sensor shape follows the waviness of the yam. As confirmation of the numerical forecasting, experimental testing demonstrated that the sensor system had a good level of sensitivity, although with substantial behavior differences for different ways of insertion.