Experimental analysis of the response of fiber Bragg grating sensors under non-uniform strain field in a twill woven composite

Fiber Bragg grating optical sensors are nowadays widely employed for strain measurement for structural health monitoring and in experimental mechanics. Compared to other techniques, i.e. electrical strain gauges, fiber Bragg grating offer immunity to electromagnetic interference and allow for long transmission lead lines. Moreover, thanks to multiplexing interrogation, several sensors can be photo-imprinted into a single fiber core allowing for strain evaluation at multiple locations simultaneously. They have high adaptability to composite materials, particularly because it is possible to be embedded into laminates without affecting their strength and stiffness. Fiber Bragg grating strain measurements are based on the detection of the wavelength shift of their peak reflected spectrum. However, subjected to strain gradients, the spectral response of fiber Bragg grating sensors may be distorted and the sharp peak may not be retained. In this work, the response of fiber Bragg grating sensors having different grating lengths and bonded to the surface of a carbon fiber-reinforced twill woven laminate was analyzed. The analysis combined transfer matrix (T-matrix) with digital image correlation methods. Digital image correlation technique was used to capture the non-uniform strain fields in the woven composites and measured strains were employed in T-Matrix algorithm to simulate fiber Bragg grating response. Using this approach, the effect of the length of the fiber Bragg grating on the strain measurement is assessed and results discussed. Moreover, it is shown that T-matrix formulation combined with a non-contact strain field measurement technique, as DIC, is an appropriate technique to simulate the behavior of fiber Bragg grating bonded to composite materials of complex microstructure.