| Nowadays, many composite materials, instead of metal matierials, have been applied in large aircrafts for decreasing weight, improving endurance ability and saving energy. It is a significiant work that FBG (Fiber Bragg Grating) sensors to be embedded in the composites for monitoring the structural health of large aircraft curing and service process. However, under the molding step of resin-based composites used in large aircraft, the optical fiber should be suffered to high temperature and suitable pressure. Since most of the existing high temperature fiber grating coatings were used for civil engineering, they are not competent for aircraft. Therefore, it is necessary to develop a new kind of high temperature coating for embedded in resin-based carbon fiber, which are generally used for aircrafts.Firstly, the heat-resistance performances of existing optical fiber coatings were evaluated in this thesis. The ordinary layers (PUA:polyurethane acrylate) coated optical fiber and uncoated optical fiber embedded in composite material were compared.The results show that after heated, the optical loss and flexible of fibes have no inficantly change, but the tensile strength decrease. Then FBG sensor coated with PUA(polyurethane acrylate) and FBG sensor without coating were embedded in the composites respectively.The accuracy of curing process and the mechine properties testing monitoring was studied. The results show that the FBG sensor coated with EA (Expoxy acrylate) has higher temperature sensitivity coefficient at normal atmospheric conditions, while it has lower stability and accuracy at high temperature. After the molding was complete, the coating was damaged, leading to the failure of sensors. FBG sensor without coating was damaged before the materials were failure because of the absence of protection buffers. As a result, the research of heat resistance FBG sensor coating is necessary.A new double layers FBG sensor coating was designed after evaluation of properties of the existing optical fiber coating and optical grating coating. The inner layer of the new coating was nickel plating, and the outer layer was nano-SiO2/PUA/EA hybrid material.This kind of coating could improve the sensitivity of FBG sensor and its performance during molding process at high temperature.First of all, the pretreatment methods of the fiber before nickel plating were determined. Then three kinds of electroless nickel plating method were compared. The best method was following:the fiber without coarsening was immersed in the colloid palladium.After that, it was treated with20%NaOH solution. Finally, it was chemical plated with nickel. It was found that the temperature sensitivity coefficient of nickel-plated FBG sensor was8.914pm/K from30℃to100℃, and10.167pm/K from100℃to200℃when the sensors were heat in the air. The strain sensitivity coefficient was0.58nm/N, and linear degree was100%.Secondly, preparation of the outer coating was studied. TDI (2,4-toluene diisocyanate), PPG (Poly propylene glycol) and HEA (Hydroxyethyl methacrylate) were used to synthesize PUA prepolymer. The performance of PUA prepolymer with different molecular weight of PPG was determined. The result indicated that PUA prepared with PPG2000showed best heat stability, which demonstrated starting weight loss at280℃under TG. The vitrification transition temperature (Tg) was104℃. Then the blend ratio of the polyurethane acrylate and polyurethane epoxy acrylate was studied. It was found that when the photoinitiator and reactive diluent were kept constant, and the quality of acrylic polyurethane and epoxy acrylate was2:1, blending system showed the most excellent thermal stability under hardness testing, curing degree test and TGA (Thermal gravimetric analysis).Silica particles were prepared via sol-gel method; nano silica epoxy acrylate/polyurethane acrylate hybrid materials were prepared via in-situ polymerization. Several catalysts, different silane coupling agents and their proportions on the properties of hybrid material were studied. The dispersion of silica in the hybrid system was determined by FESEM (field emission scanning electron microscopy) test, the thermal stability was determinded through DSC (Differential scanning calorimetry) and TGA. It showed that the hybrid materials (prepared with KH-570silane coupling agent as surface treatment agent, TEOS(Tetraethyl orthosilicate):(PUA+EA) was0.4:1) displayed best thermal stability, heat resistance was up to143.8℃when using hydrochloric acid as a catalyst. The UV curing kinetics of SiO2/EA/PUA hybrid materials were analyzed by model fitting, it showed that the UV curing of SiO2/EA/PUA hybrid materials was a first order reaction with a reaction constant of0.09486.Finally, the double layer coating with the nickel-plated inner layer and outer layer of SiO2/EA/PUA hybrid materials was coated on the grating portion of the FBG sensors, the basic properties, the temperature sensitivity coefficient and the stress sensitivity coefficient of the sensor were tested, which was then embedded in the composite so as to monitor the curing process of carbon fiber/epoxy composite during autoclave molding. The results showed that the double layer FBG sensor could monitor the whole molding process. The bend testing process was also monitored. The results showed the double layer coated grate could monitor the bending properties of composite more accurately than the uncoated or EA-coated FBG sensors. The FBG sensors without coating and coated EA were break down before failure of the materials. |
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