Force-thermal Coupling Test of Fiber Grating Sensor Based on Clamp-adhesive Type
To solve the problems of inaccurate results in the theoretical calculation of Fiber Bragg Grating(FBG)sensors for strain testing at high temperatures,and the complexity of the preparation process of special fiber gratings,moreover,to enhance the operability of strain testing in practical engineering by using FBG,this paper proposes a bare FBG sensors testing method based on the clamp-adhesive type,and combines the temperature compensation method,to achieve the accurate measurement of 3 000 με within 250℃.Firstly,for the problem of temperature-strain cross-sensitivity,the temperature compensation method is proposed to remove the effect of temperature,and the formula is derived.Based on this formula,the temperature compensation based FBG sensor temperature decoupling method is by using another FBG sensor that only responds to temperature.Then,after the two FBG sensors are integrated with the substrate,a suitable paste method can be selected so that one sensor can only sense temperature,and another sensor can sense temperature and strain,and the real strain value can be obtained after the temperature value brought into the formula.Secondly,the strain transfer rates of two FBG integration methods(clamp-adhesive and surface-adhesive)are compared by finite element analysis,and the simulation results indicate that the strain curve of the clamp-adhesive method fits the substrate strain curve better,and the strain transfer rate is 99.47%.In contrast,the gap between the strain of the surface pasted method and the substrate strain is larger,and the strain transfer efficiency is 95.8%.Therefore,the strain transfer efficiency of the clamp-adhesive sensor is higher than that of the surface-adhesive FBG sensor.In addition,the surface-adhesive type needs to cover the grating of the Bragg grating sensor with glue,and the change of the material properties of the adhesive layer in the high temperature environment has a large impact on the reflection of the grating,so the penetration of the glue into the fiber grating area will make the grating fail,which has a large impact on the strain measurement.Thirdly,with reference to the strain transfer theory of clamp-adhesive FBG sensor,the influence factors of the scale ratio on the strain transfer rate are analyzed.It can be concluded through simulation that when the scale ratio is greater than 1,the strain transfer rate decreases at a faster rate.When the scale ratio is less than 1,the strain transfer rate only differs by 0.1%.Considering the influence of the coverage area of the adhesive layer on the adhesive firmness and the fluidity of the adhesive,the chosen gauge scale is 1.Finally,in the experiment,the scale ratio of the clamp-adhesive FBG sensor is 0.87 due to the glue fluidity and manual operation.Meanwhile,a fiber grating temperature sensor is connected as temperature compensation.Then,the temperature/strain calibration tests are carried out,resulting in the fiber temperature sensitivity coefficient of FBG1(12.393),the strain sensitivity coefficient(1.596)and the fiber temperature sensitivity coefficient FBG2(12.293).The high-temperature tensile test of 3 000 με within four temperature gradients from 100 ℃ to 250 ℃(100 ℃,150 ℃,200 ℃ and 250 ℃)is carried out in a tensile testing machine.The results show that the strain measured by the fiber grating sensors based on the temperature compensation decoupling is similar as data from the standard high-temperature strain gauges,in which maximum relative average error of 2.26%.The results of the research present the reference significance on the engineering application of the strain measurement and integration method of FBG sensors according to a high temperature environment.
Fiber grating sensorStrain transferClamp-adhesive typeFinite element analysisStrain testing under high temperatureTemperature compensation decoupling
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