Fiber Bragg Grating Capillary Spot Welding Packaging And Its Temperature Sensing Characteristics

Fiber Bragg Grating(FBG)is mainly used in the field of sensing to monitor the internal strain,pressure,temperature,vibration,load fatigue,structural damage and other parameters within the structure.FBG is widely used in the sensing field due to its strong anti-interference ability,absolute measurement,small size,simple structure,easy to form sensing network,good measurement repeatability,corrosion resistance and other advantages.However,the main material of FBG is SiO2,which is easy to be damaged due to its small and brittleness in nature,and the sensing sensitivity is low,so it can not be directly used in the measurement of physical parameters.Therefore,in view of the above-mentioned defects in the application of FBG in the field of sensing,this paper proposes to encapsulate FBG in a capillary steel tube by resistance spot welding,and to detect its temperature sensing characteristics.The main research contents are as follows:1)Metallization protection of FBG.In order to weld and fuse FBG with the capillary steel tube,three different types of metallization methods of electroless nickel film combined with electroplating nickel,zinc,copper layer were proposed in this paper.The results showed that the thickness of the electroless nickel plating film by the water bath heating method was about 2?m,and the surface of the film was smooth and flat without obvious defects;The electroplated nickel,zinc and copper layer were all closely combined with electroless nickel plating film,without obvious dividing line.The surface quality of electroplated nickel and zinc layer was good,without defects such as aggregation of particles,while the surface of electroplated copper layer was granular and not dense enough.2)Internal stress monitoring of electroplated metal layer.In this paper,FBG was used to monitor the internal stress in the electroplated metal layer in-situ and real time,and the monitoring results of the internal stress in electroplated nickel layer was verified by the nanoindentation method.At the current density of 24 mA/cm2,the thickness of the nickel,zinc and copper layer on the FBG surface averaged 79.5?m,80?m and 85 ?m in 420 min respectively,and the average internal stress generated in the layers was 551 MPa,69 MPa and 10 MPa.The results indicated that the internal stress in the nickel electroplating layer was larger,and the internal stress in the electroplated zinc and copper metal layers was smaller.The internal stress of the nickel electroplating layer was measured by nanoindentation method,and the results showed that the results were consistent with the FBG detection results.3)The metallized FBG sensor was encapsulated by capillary resistance spot welding.A encapsulating platform that can apply adjustable prestress to the FBG was designed,and the capillary steel tube and metallized FBG sensor can be fixed by adjusting the positions of the X/Z axis of the platform in both directions to keep the centers of the two in alignment.A small resistance spot welding machine was used to spot weld the two ends of the capillary steel tube.The results showed that the FBG sensor with a diameter of 390?395 ?m can be encapsulated in the capillary steel tube by resistance spot welding,and the sensitivity of the FBG sensor can be changed by adjusting the thickness of the electroplating metal layer and the diameter of the capillary steel tube.4)Sensing characteristics and welding reliability test of capillary resistance spot welding encapsulated FBG sensor.Encapsulated FBG sensor was tested by FBG network analyzer and spectrum analyzer,and welded joint reliability was tested by a tensile machine.The results showed that the fitting linearity between the central wavelength and the time of the encapsulated FBG sensor was more than 0.999 at the temperature of 30?80?,and the temperature sensitivity was 2.7 times higher than that of the bare grating.The center wavelength was stable in a long time,and the reflection spectral power loss of encapsulated FBG sensor after welding was less than 2%.The tensile strength of the welding joint was greater than the tensile strength of grating of electroless nickel plating,indicating that the spot welding method was reliable.