Damage Monitoring Of Composities Using FBG Sensors

The composite materials possessing excellent specific strength and specific stiffness, fatigue resistance and durability, have been used for building materials, automobiles, high-speed rail, aerospace and other fields. However, along the thickness direction, there is no reinforcement existing in the composite materials, and therefore the composite materials are very sensitive to injury. Efficient nondestructive detecting technology is very important for composite structure damage inspection and continuous monitoring. The optical fiber grating sensors (FBGs) embedded in the composite material structures can achieve real-time health monitoring of composite structures. FBG sensors with advantages including low cost, no electromagnetic interference and easy internal monitoring, are promising in application of composite structure nondestructive detecting. The study of FBG sensors embedded in the composite material structures is very important for health monitoring in composite material.In this paper, autoclave molding was used to study the eduction and protection of optical fibers. In view of autoclave molding technology, two kinds of eduction mode, end face eduction and surface eduction, were studied. After optical fibers were embedded in composites, the section morphology was observed through optical microscope and scanning electron microscope. Static mechanic performance of composite materials was tested respectively after the optical fibers were embedded in the composite materials. The results showed that the embedding has effects on the ensile and bending performance. Optical fiber diameter also has some influence on the mechanical properties of the composite. With the decrease of the optical fiber diameter, the effect on the mechanical properties of composite become smaller. When the optical fibers were embedded in different thickness locations of the omposite materials, it has different effects on the bending property of the composite materials. When the optical fiber was embedded closer to the center layer, it has the greater influence on composite bending strength.After FBG sensors were embedded in the carbon fiber/epoxy composite materials, tensile properties were tested, and the results show that the FBG sensors could respon the changes of the material internal stress. The FBG sensors were pasted on the surfaces or embedded into the composite materials, the tensile were tested and the relations of strain-wavelength was observed. The results show that the wavelength shift of the FBG sensors pasted on the surfaces is larger than that of the embedded FBG sensors. When the FBG sensors embedded in the carbon fiber composite materials in different layer positions, it can be found that it is more sensitive when the the FBG sensors embedded in the90℃layers.Tensile experiments were carried out on the carbon fiber/epoxy composite material test specimens with hole defects and with embedded FBG sensors. It is found that the reflective spectra of the FBG sensors in the specimens with hole defects have waveform changes and more than one reflective peak appeared, but only wavelength shifts were observed on the test specimens with no hole defects. Through the experiment, it can be found that the FBG sensors can respon to the internal delamination or fractures near the sensors.