Abstract

Fiber optic sensors, both FBG point sensors and distributed sensors based on OFDR, are accepted and validated as temperature and strain sensors, enjoy a high TRL and are already in use in many industrial applications. The determination of occurrence of damage from strain measures is a technique still not mature, its TRL can be 2-3. A local crack can greatly weaken a structure, it will be the point of initiation of the catastrophic failure, but until the failure has occurred, the crack only causes very small changes in the overall behavior of the structure (stiffness, natural frequencies, strain field, etc.). Therefore, and unless the position of the crack coincides with the predetermined position of the sensors, it is not easy to detect the existence of damage from strain measurements. In this work two parallel techniques, experimentally validated, are presented. The first one is to use distributed measurement, with an optical fiber placed in the area where the damage is expected. It is verified that a delamination causes residual strains in the delaminated area, of  significant value, and that will be detected by the fiber, if it crosses the delamination zone. The second technique is to compare the strains measurements obtained at different points, when the structure subjected to external loads. Although the changes are going to be very small, local damage causes a redistribution of the rigidity of the structure, and of the load paths, and therefore the strains readings, may be slightly different. Results of the application of the model are presented to actual composite elements, such as an isogrid structure, and a small wind turbine blade.