Research On Strength Of Optical Smart Structure Based On Interface Analysis

Optical fiber sensors have the advantages of their insensitivity to electromagnetic interference, high sensitivity, and convenience for network. They have the predominance of small physical size, capability of sensing at high temperature and in environmentally unfavorable conditions. Therefore, individual fibers or fiber networks are often embedded in the composites as sensors for real time structural health monitoring. Optical smart composites have extensive potential applications in monitoring fatigue and damage inside the structures. Accordingly, the life of the structure can be prognosticated. However, despite of the small physical size of the optical fiber compared with the host composite structure, the diameter of the optical fiber was much larger than that of reinforcing fibers. This mismatch in dimensions would inevitably lead to incontinuity of the composites. On the other hand, the embedded optical fibers not only apperceive surrounding changes, but also are subjected to load. The interrelationship of the embedded optical fibers and the composite were deserved study.The interface between the embedded optical fibers and the resin and the reinforced fibers were studied. So did the mechanical influence of interface on the smart composite. The methods to improve the interface structure were proposed. Small diameter optical fiber was accordingly used to compose a smart structure. Therefore, new optical fiber sensing systems were developed and applied in load signal monitoring of composite.The main achievements are described as follows:(1) The mechiancal performance of smart composite with embedded optical fibers was studied. Experimental tests on tensile and bending performance of the composite were carried out. It was proposed that angle difference between the optical fiber and the reinforced fiber should be as small as possible. Thus the structure property was optimized.(2) Experimental program and set-up were designed to study the interfacial shear strength between the embedded optical fiber and the matrix.Coupling was proposed to enhance the optical smart composite from the point of interfacial treatment. Interfacial strength study showed that the shear strength was very low. The interface between the optical fiber and the polymer could be easily destroyed. Thus the mechanical performance of the structure would be weakened. With the help of coupling agent, the optical fiber was chemically bonded with organic material. Therefore, the two materials were glued well and the mechanical character of the smart composite could be enhanced. Experiment on compressive strength showed that the composite with coupled optical fiber embedded inside had better mechanical facility.(3) The interfacial structure was studied experimentally. The relation between the mechanical characteristic of smart composite and the size of interface was studied. Another way to improve the mechanical performance was to decrease the size of the used optical fiber. Hence, the size of the interface between the optical fiber and the composite was reduced. The stress singularity at the pointed end would be depressed.(4)Fiber Bragg grating sensors in small diameter were developed. The strain and temperature sensitivities of the fabricated fiber Bragg grating were studied experimentally. Based on coupled-mode theory, optical properties of the designed FBG were studied. The reflection and transmission spectrums of the designed FBG with small diameter were studied as the grating length and the refractive index were changed. The parameters for manufactiong FBG from a single mode fiber were studied. The connector of the optical fiber sensor in small diameter and the demodulator was designed. The central wavelength of the FBG in small size changed linearly with strain. The relation between the wavelength shift and temperature change is linear too. Therefore, the deformation of the structures can be captured by monitoring the central wavelength changes of FBG. In order to overcome the disadvantage of cross sensitivity, and to measure both parameters at the same time and location, a novel scheme for simultaneous strain and temperature sensing system was presented with the help of a uniform strength cantilever.(5)Dynamic monitor capacity of a small size optical fiber sensor system was studied. The sensor system was experimentally applied in aluminum plate frequency detection. It was applied in monitoring impact signal of a carbon fiber reinforced plastic too. Support vector machine was applied in discrimination load locations on composite.