Study Of Structural States Monitoring Method For Typical Structures Based On Distributed Optical Fiber Sensing

The rapid development of engineering science and technology has made the structure and materials of aerospace aircraft more complex and more advanced,which may not only bring new safety problems,but also make the traditional safety problems more prominent.The disintegration of the American Helios UAV showed that the online deformation monitoring of the aircraft with flexible structures of high chord ratio is of great significance.The Columbia space shuttle accident in the United States showed the serious consequences of debonding of insulating materials.With the development of reusable aircraft of China,the online monitoring of debonding of insulating materials is becoming much more important.Structural damage has always been one of the main threats to the structural safety of aircraft.Nowadays,with the extensive application of carbon fiber composite materials with failure mechanism quite different from traditional metal materials,online monitoring of structural damage has more practical significance.In recent years,researchers have carried out a series of studies on these types of structural safety problems,and developed many identification methods for deformation,debonded insulation layer and damages of structures,but there is still a lack of reliable online sensing methods for structural state.The distributed optical fiber sensor based on backward Rayleigh scattering demodulated using optical frequency domain reflection principle is a product of new type of optical fiber sensing technology in the nearly 20 years.It not only has the traditional characteristics of optical fiber sensor,it also measures strain in high spatial resolution and high sensitivity.The distributed optical fiber sensor is an ideal tool that can be used on state sensing of aerospace aircraft structures.The purpose of this article is developing methods and strategies of structural state sensing of aerospace aircraft structure based on the distributed optical fiber sensor network.It mainly focuses on methods of online deformation estimation,debonding monitoring and damage identification based on distributed optical fiber sensor network.The research contents and results of this paper mainly include:(1)The Hamiltonian system of cantilever plate bending problem is established.In Hamiltonian system,all kinds of working conditions are attributed to the external loads of the structure,various boundary conditions and various external load distribution on the boundary.Furthermore,the problems of all kinds of working conditions are transformed into the problems of non-homogeneous term of Hamiltonian regular equations,so the general problem is obtained.By solving the eigenvalues and eigensolutions of the basic problems,the analytic solutions expressed in series are obtained.For the finite truncation of series solution,the specific problem is reduced to the problem of determining the coefficient of the eigensolution,that is,the problem of solving algebraic equations.Thus a symplectic numerical simulation method is formed.In the Hamiltonian system,the expressions of the relationship between the surface strain,the middle surface strain,curvature and displacement are established.(2)Based on the two coupling modes of surface sticking and internal embedding,a multi-layer coupling structure model of distributed optical fiber sensor,bonding layer,coating layer and tested structure is established.According to the principle of mechanics,the relationship sand law between the structural strain and the strain information from distributed optical fiber sensor are determined,thus the expression of the strain transfer efficiency function between the structure and the distributed optical fiber sensor is obtained.The expression of strain transfer efficiency not only sums up the influencing factors of strain transfer rate of the sensor,but also reveals its variation and spatial distribution characteristics.(3)The strain monitoring method based on distributed optical fiber sensor was studied.According to the strain data of geometrically nonlinear structure,the influence of different gauge length and density of sensing points on strain measurement was analyzed,too.This paper discussed the technology of embedding the distributed optical fiber sensor into the carbon fiber composite laminate and the whole-life state monitoring of the composite structure,including the curing process monitoring and the strain monitoring under the load condition.The imaging method of high-density strain information was discussed,and the strain field of structure was also reconstructed considering time or space factors.(4)Based on the high-density strain information,the method of structural deformation estimation is developed,and the finite-difference form of the formula for obtaining the lateral deformation of the bending structure from the strain information of the distributed optical fiber sensor is given.Based on the results of bending test of cantilever structure,this paper analyzes and discusses the causes of measuring error of distributed optical fiber sensor in bending problem.And it puts forward the data correction strategy of strain measurement of distributed optical fiber sensor combined with the results of finite element analysis and dialgauge measurement,and proves that this strategy can significantly improve the accuracy of structural deformation measured by distributed optical fiber sensor.(5)An on-line identification method for debonding of insulation structure based on distributed optical fiber sensor was developed.According to the influence between debonding and bending section coefficient of the structur,an on-line debonding identification method based on the change of strain was proposed.A reasonable on-line identification strategy for debonding was originally established based on the results of finite element analysis and principle test.The high-density strain collected by applying shear-bending coupling to a cantilever plate with insulation structure was used to verify the effectiveness of the method.The positioning accuracy of debonding boundary reached sub-centimeter level.(6)Research on damage identification based on distributed optical fiber sensor was carried out.Considering the disturbance range of strain field caused by damage,the applicability of damage identification based on distributed optical fiber sensor network was obtained.Based on the damage indexes,an online identification method of damage with or without structural health information are studied.It is found that the distributed optical fiber sensor network can accurately locate the crack tip with errors in millimeter level online.The research results of this paper are helpful to the application of structural state sensing applications of aerospace vehicle structure and are of great significance for improving the safety of aerospace vehicle in service.