Research On Performances & Technics Of 3D Braided Composites Based On Optical Fiber Measurement & Testing Technology

As one kind of advanced materials, 3D braided composites are used widely in many fields, such as aeronautics & astronautics, because of their high strength, stiffness and good resistance to impact. However, due to their complicated braiding and cure processes, knowledge about them is far from enough. Research of health monitoring and cure monitoring about braided composite are quite little.In this dissertation, internal strain and cure process of braided composites are studied using optical fiber sensing technology. The contents are as follows:1. Based on study of 3D braiding process, two methods for optical fiber braiding and protecting are presented. Optical fibers are successfully braided into composite testpieces by four-step method. Cure process of braided composites is also studied. Resin Transfer Molding (RTM) cure system is designed. Testpieces with braided optical fibers are successfully fabricated.2. Compatibility of optical fiber and braided composites is studied. Cut-off wavelength, mode field diameter, numerical aperture and cross talk are chosen as test parameters of optical fibers. Research of those parameters shows that they change just a little before, after optical fiber's being braided into the structures, and after the molding process. Contrast tests are also done to compare performances of testpiece with optical fiber with that of one without optical fiber. Tensional and compressive elastic moduli are researched. Results show that co-braided optical fibers have just a small quantity of effect on the elastic modulus of the braided composites. If volume ratio of optical fibers and their jackets to the braided testpiece is about one per cent, the tensional and compressive elastic moduli will decrease respectively by six and six point seven per cent. The fixing and lead-out design of optical fiber sensor are also studied.3. Optical fiber sensing technology is applied in internal strain measurement of braided composites. First, transmission mechanism of light wave in graded index fiber and stepindex fiber is analyzed using ray method. Scalar analysis and vector analysis are discussed using mode method. Mechanism of polarized, interferential and micobend optical fiber sensor is analyzed. Applications of these sensors in internal strain measurement of braided composites are experimentally studied. Results show that these sensors are effective for internal strain measurement. Method of internal parameter measurement based on co-braided optic fiber sensor is very valuable for further research to study structural features and monitor structural health condition.4. The RTM cure process of braided composites is monitored using microbend fiber optic sensor. Experimental results show the sensor is able to monitor variation of resin viscosity and to indicate time that resin viscosity is lowest and cure ends.5. Applications of optical fiber with carbon coating are also studied. This kind of optical fiber has advantages of high strength and antifatigue performance. Experimental results show that the optical fiber has good sensing features.The original researches done in this dissertation are as follows:1. The braiding and cure process for braided composites with monomode optical fiber are presented. Testpieces are successfully fabricated.2. The compatibility between optical fibers and braided composites is studied3. Internal strain of braided composites is measured by devising three kinds of optical fiber sensors. The sensors are polarimetric sensor, interferential sensor and microbend sensor.4. The RTM process is monitored using co-braided microbend fiber optic sensor.5. Applications of carbon hermetically coated optical fiber in braided composites are studied.