Research On Residual Stress Measurement Based On Fiber Bragg Grating

With the continuous development of science and technology,more and more large,precision parts are put into use,there are trends that large tools,buildings and civil engineering structures are widely put into use.In the process of processing and forming,some residual stress will be introduced into the component,and the superposition of residual stress and working stress usually causes damage and deformation of the component.Because of its concealment and extensive existence,the research on residual stress testing has always been an important branch and hot topic in the field of structural health monitoring.Among which the drilling method is the most widely used residual stress testing method in engineering.However,there are still some shortcomings in this method at present,for example,the traditional electrical sensor can not be embedded in the material to measure in some harsh environment,and it is difficult to form a distributed measurement unit to increase the number of strain measurement points.Due to the superior performance of optical fiber sensors,researchers tend to focus on the research of residual stress measurement based on Optical Fiber Bragg Grating(FBG).In this thesis,based on the analysis and research of drilling test method,a method of residual stress test based on FBG is proposed.Based on the simulation,the relationship between wavelength drift and bandwidth is established to compensate the chirp effect of optical fiber grating,and the effectiveness of the compensation method is verified by experiments.By changing the traditional sensor arrangement,the fiber Bragg grating array is used as a distributed measurement unit,which increases the number of strain measurement points,and realizes the measurement of the strain field and residual stress of the material in the process of drilling test.The research content of this thesis mainly includes the following aspects:1.This thesis introduces and analyzes the basic principle and method of testing residual stress by drilling method.Combined with the sensing mechanism of FBG,a method of testing residual stress of material by drilling based on FBG sensor is proposed.In order to obtain the effective strain information,the temperature sensitivity of grating is calibrated.2.The finite element simulation software is used to calculate the strain gradientdistribution around the hole,and the spectral chirp of the grating under the strain gradient is calculated by using the transmission matrix method.The relationship between the spectral bandwidth of the grating and the central wavelength drift is obtained,which is used to compensate the wavelength drift of the chirped grating in the experiment.It is proved that the measurement caused by the chirp effect of the grating can be reduced by the compensation method quantity error.3.The strain distribution around the hole under the condition of non-uniform residual stress release is simulated.It is proved that the stress distribution of the hole wall can be retrieved by measuring the strain.Based on the tangential strain measurement,the original calculation formula of residual stress is modified and its validity is verified.The strain pattern of ring fiber grating is designed and arranged,and the strain distribution and its variation with time are studied.The residual stress is calculated by using multiple grating signals,and the calculation results and the causes of deviation are analyzed.The results of experiment and simulation show that the measurement error can be reduced and the strain distribution can be monitored well by using the spectral characteristics and distributed measurement characteristics of FBG sensor.