Study On Sensing Performances Of Fiber Bragg Grating Embedded In Metal Structures

Metal structures have played an extensive and irreplaceable role in industry. Optical fiber smart metal structure is one of most hot topics of researches on engineering fields in the world.Fiber Bragg grating(FBG) sensors embedded into metal matrix can detect many parameters of the internal metal structures such as temperature,stresses and the safety and reliability of the metal structures can be improved largely.Embedding FBG sensors into metal materials is very difficult and challenging,and it is the primary problem to realize the optical fiber smart metal structures.After being embedded into the metal structures,the FBG's sensing performance needs to be further explored.Financially supported by the National Basic Research Priorities Program of China,the National Natural Science Foundation and Natural Science Foundation of Jiangxi Province,eletroless composite coating and duplex metal coating on the surface of optical fiber,the effects of metal coating upon the fiber Bragg grating's temperature sensitivity coefficient and the method to embed the metal coated FBG into metal matrix have been studied in the thesis.Firstly,the technologies of electroless Ni-P-ZrO₂ coating and electroless copper coating on the surface of optical fiber were studied.The optimized technology of Ni-P-ZrO₂ coating is as the following:NiSO₄·6H₂O---25g/L,NaH₂PO₂·H₂O---20g/l, H₃BO₃---20g/l,C₃H₆O₂---20ml/L,ZrO₂---1-3g/L,T---86℃,PH---4～6,stirring method---discontinuous shake,stirring intensity---20-30 times/h.The optimized bath for electroless copper coating is as the following:CuSO₄·5H₂O---10g/L, NaKC₄H₄O₆·4H₂O---40g/L,NaOH---8g/L,Na₂CO₃---2g/L,NiCl₂·6H₂O---1g/L, HCHO(37%)---20ml/L,PH---12,T---30℃.Secondly,how to embed the metal-coated FBG into metal matrix was discussed. After a comparative analysis,the brazing method was selected as the appropriate method.The metallographic specimen of embedded FBG was prepared to observe the effect.And the thermal tests of embedded FBG verified the feasibility of brazing.Next,this thesis developed a temperature model to analyze the effects of thermal stresses(including radial stressσ_r and axial stressσ_z) upon Ni-Cu-coated FBG and put forward the concept of the temperature sensitivity enhancing efficiency of metal coating.The temperature model can be used to explain the temperature sensitivity enhancing mechanism of metal coating.Three thermal testes were conducted:the first one was to verify the copper coating's role on enhancing the temperature sensitivity of FBG;the second one was to explore the different thickness of nickel coating's effects on the temperature sensitivity of Ni-Cu coated FBG;the third one was to observe the temperature sensing performance of Ni-Cu cated FBG after being embedded into 45^# steel.The obtained data have verified the soundness of the model for duplex metal coating as well as one layer metal coating.Although the model is based on the analysis of the Ni-Cu-coated FBG,results show that the model can be extended to Cu-coated FBG,Ni-coated FBG,Cu-Ni-coated FBG as well as Ni-Cu-coated FBG embedded into 45^# steel.After nickel coated FBG sensors being embedded into metal structure,they were tested by three-point bending tests.This thesis studied the FBG's central wavelength changing mechanism under the bending stresses.A model for predicting the relation between the FBG's central wavelength and the external force was developed.The factors involving protective metal layer,brazing filler metal layer,embedding position and errors of embedding position was analyzed to explore the FBG's sensing performance of bending stresses.Analyses show that the protective metal layer should be larger Young's modulus and thinner,the brazing filler metal layer should be smaller Young's modulus and the metal coted FBG should be embedded near the point of concentrated force and far away the neutral layer as much as possible.The bending tests data indicated the model is feasible.