Strain Sensing Experiment And Application Of Fiber Bragg Grating In Low Temperature Environment

Extreme environment test technology like Low temperature has a wide and important application in aerospace,power engineering,energy technology,modern medical and other fields.When the material is in extremely complex environment such as low temperature,strong electromagnetic field and high vacuum,its failure is usually manifested as brittle fracture at low temperature and structural failure in multiple fields,which is difficult to find in time.Therefore,in order to prevent the safety accidents and economic losses caused by the lack of detection and prediction,it is necessary to effectively monitor the temperature,magnetic field,stress and strain of the structure in the low temperature extreme environment.As a new type of sensor,the fiber Bragg grating(FBG)has the advantages of fast response,high precision,low cost,corrosion resistance,anti-electromagnetic interference,quasi distributed and so on,showing the high applicability and reliability.In this thesis,the theoretical and experimental study of low temperature strain sensing characteristics of the fiber Bragg grating(FBG)is formed for the strain monitoring requirements of structures in low temperature environment.Based on the temperature and strain sensing of the optical fiber FBG,the temperature sensitivity coefficient and force sensitivity coefficient at low temperature are measured in experiments,and the application of low temperature strain measurement and magnetic field measurement based on optical fiber FBGs is carried out.Firstly,the environmental response characteristics of the FBG are introduced,and the main components of FBG wavelength signal change are given.The sensing characteristics,measurement principle and main influencing factors of temperature and strain in low temperature environment are defined,and the corresponding solutions are given.Then,the temperature sensitivity coefficient and the force sensitivity coefficient of the fiber-optic FBG are calibrated in the room temperature(295 K)and liquid nitrogen(77 K)environment through the self-built test platform,and the wavelength temperature and wavelength strain relationships of the fiber-optic FBG in different temperature regions are detectd.The corresponding relationships are given,and the temperature sensitivity coefficient and the force sensitivity coefficient of the fiber-optic FBG sensor at different temperatures are obtained.The results show that the wavelength of FBG is nonlinear with temperature,but linear with strain in the low temperature.The temperature sensitivity coefficient of FBG changes linearly with temperature,and its value at 77 K(2.5 pm/K)is about 1 / 4 of that at room temperature(10.0 pm/K),and the temperature sensitivity coefficient is greatly affected by the fiber FBG coating material.The FBG force sensitivity coefficient is basically consistent with that at room temperature,and the temperature effect can be ignored in a certain range.Based on the above results,in practical application of FBG,different batch of optical fiber needs to calibrate temperature sensitivity coefficient in the low temperature,and the temperature measurement in extremely low temperature range should also consider the setting of sensitization;The force sensitivity coefficient can be used to reduce the influence of low temperature,and the calibration value at room temperature can be extended to low temperature environment.Furthermore,based on the calibrated fiber Bragg grating(FBG),the surface mounted and internal embedded strain measurements are carried out,and the elastic strain and thermal strain of the structure at low temperature are measured;The results show that: the embedded fiber FBG has good stability,and the thermal strain of resin material changes little with temperature at 77-100 k.It can be applied to the structure curing bonding and magnet filling at low temperature,which provides an effective way for the development of optical fiber embedded measurement technology.Finally,combined with the strain measurement method of fiber-optic FBG at low temperature and the magnetostrictive properties of giant magnetostrictive material(GMM),we design the magnetic field sensing structure of FBG-GMM and study the application of magnetic field detection at low temperature based on the research on the magnetomechanical properties of GMM smart material at low temperature.The results show that the low temperature environment has a significant effect on the magneto mechanical properties of GMM smart materials,and the magneto induced strain of GMM at 77 K is about an order of magnitude lower than that at room temperature.The effect of low temperature preloading on the magneto mechanical properties of GMM smart materials is almost completely degraded,and the magneto induced strain magnetic field linear region of GMM is broadened.Furthermore,in order to overcome the deficiency of low magnetic field sensitivity of FBG-GMM at low temperature,we adopt the structure design of fiber Bragg grating partial mount to realize the effective sensitization of magnetic field sensor.Compared with the direct paste type magnetic field measurement,the sensitivity coefficient is increased by about 7 times,which is suitable for high sensitivity magnetic field monitoring at low temperature.The present research can be attempt and provide a feasible method for the mechanical exploration and testing of the structure at low temperature and extreme environment.Based on the advantages of fiber Bragg grating sensing measurement technology,it is expected to further expand to the extreme multi field environment,as well as the continuous and multi signal measurement field.