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Embedded fiber Bragg grating as local damage sensors for composite materials

Airbus Espana, INTA and UPM are collaborating in a joint research directed to increase their experience and the knowledge in the use of fiber Bragg gratings as strain and damage sensors in advanced composite structures for aerospace applications. In certain conditions, fiber Bragg gratings are able to detect damage by measuring sudden changes in the strain distribution of composite monolithic structures due to delaminations or debonding of stiffening elements. However, strain measurement of FBGs can be compromised by spurious phenomena like birefringence promoted by manufacturing induced residual stresses, with the subsequent spectral peak splitting, and spectral chirping induced by local non uniform strain fields. These effects add a new issue to the difficult task of demodulating the spectral response of fiber Bragg gratings when employed as strain sensors in composite structures. However, photoelastic models already developed allow obtaining considerable information about this phenomena, and relate them with local changes in the stress field promoted by structural damage. Thus, it is possible to think in FBGs not only as load monitoring sensors, but as a part of SHMS for composite structures designed under damage tolerance criteria.