Abstract : In the welding process of material structure construction, it is usually necessary to study the control of strain deformation and temperature of metallic materials. This paper uses fiber Bragg gratings to conduct distributed measurement experiments on the strain and temperature changes of aluminum alloy materials during welding, overcoming the limitations of traditional welding high temperatures and the inability to simultaneously measure strain and temperature. Experimental results show that fiber Bragg gratings can effectively measure the strain and temperature distribution of aluminum alloy materials during welding. Keywords: Fiber Bragg grating; strain; temperature; distributed measurement 1. Introduction Measurement systems based on fiber Bragg gratings have advantages such as high accuracy, good stability, strong anti-interference ability, and easy multiplexing, enabling distributed measurement. Furthermore, because they employ wavelength encoding and digital measurement technology, they effectively reduce measurement errors caused by light source jitter and power fluctuations in the optical path. Therefore, they are now widely used in many fields. Welding is a process of localized rapid heating to high temperatures followed by cooling. As the heat source moves, the temperature of the weldment changes drastically with time and space, and the thermophysical properties of the material also change drastically with temperature [1-2]. Moreover, the strain and instability deformation of the metal material during welding is another major reason for the instability of product quality [3]. In order to control the influence of strain deformation and temperature on the physical properties of the material structure during welding, we first need to understand the distribution of strain and temperature changes in the metal material during welding. This paper introduces a method for simultaneous measurement of strain and temperature based on fiber Bragg grating according to experimental requirements. Based on the characteristic that FBG is both a sensitive element and a light transmission element [4-5], the structural design of the sensing probe is greatly simplified. By placing one channel grating in a thin steel tube to sense the temperature distribution at the measurement point, and then fixing another channel grating on the substrate to sense the common changes of strain and temperature at the corresponding measurement point, the distribution of strain at the measurement point is indirectly obtained, and the distributed measurement of strain and temperature of the material structure during welding is completed. This solves the problem of cross-sensitivity of strain and temperature in fiber grating and completes the distributed measurement of strain and temperature of aluminum alloy material structure during welding. 2. Measurement Principle A fiber Bragg grating is an all-fiber passive device. When a beam of light enters the FBG, it reflects the incident light whose wavelength satisfies the Bragg reflection condition. This reflection is a narrowband reflection, and its reflection spectrum has a peak at the Bragg wavelength, essentially a narrowband filter centered on the resonant wave. According to coupled-mode theory, the central reflection wavelength of the fiber Bragg grating is: where n[sub]eff[/sub] is the effective refractive index of the guided wave mode, and Λ is the period of the grating. Differentiating equation (1) yields equation (2). It can be seen from equation (2) that the central wavelength of the Bragg grating changes with the effective refractive index n[sub]eff[/sub] and the grating period Λ, and the changes in refractive index and grating period are related to strain and temperature. Fiber Bragg gratings are sensitive to external strain and temperature. Under stress, the central wavelength changes due to the stretching and contraction of the grating period and the elasto-optic effect, while the effect of temperature is due to thermal expansion and thermo-optic effects. Studies have shown that the effects of force and heat on the wavelength of fiber Bragg gratings are independent and linearly variable. The relative change in the central reflection wavelength of the fiber Bragg grating is related to strain and temperature as follows: Where p is the effective elastic-optical coefficient of the fiber grating, p = 0.22; ε is the axial deformation, which can generally be considered as ; ζ is the thermo-optical coefficient, which is generally taken as 8.3 × 10⁻⁶ for typical silica fiber; α is the coefficient of thermal expansion, which is generally taken as 0.55 × 10⁻⁶; ΔT is the temperature change in °C. 3. Sensing System Design We used a 600mm × 600mm × 6mm aluminum alloy plate, with the weld seam as the centerline of the cross-section. The experiment required measuring the strain and temperature distribution of the aluminum alloy plate in the directions parallel and perpendicular to the weld seam during the welding process along the weld seam depth. In the experiment, we arranged four fiber optic channels. The fiber Bragg grating (FBG) layout and connection of the four channels are shown in Figure 1. Two channels are located above the weld centerline, perpendicular to the weld direction, with a total of four measurement points. These correspond to four FBGs with different center wavelengths. Channel 1's FBG is suspended in a thin steel tube with a diameter of 0.08 mm to sense temperature changes at each measurement point. Channel 2's FBG is entirely glued and fixed to an aluminum alloy substrate to sense wavelength changes caused by the combined effects of strain and temperature at the corresponding measurement points. One fiber senses only temperature changes as Channel 3, and another fiber senses both strain and temperature changes at the corresponding measurement point as Channel 4. The fiber Bragg grating structure design at each measurement point is shown in Figure 2 below: In Figure 2, the sensing segment at a measurement point consists of two FBG fiber gratings. FBG1 is firmly attached to the substrate at the measurement point and senses the changes caused by the combined effects of strain and temperature. FBG2 is suspended in a thin steel tube with a diameter of 0.8 mm and only senses the temperature change at the measurement point. In this way, by substituting the wavelength change caused by temperature obtained by FBG2 into equation (3), the wavelength change caused by strain at the corresponding point can be calculated. Through the wavelength changes and their correlation coefficients, the distribution of strain and temperature changes at the welding point during the welding process can be obtained. Figure 3 below is a schematic diagram of the experimental principle of the fiber Bragg grating strain and temperature measurement system. Connect the four channels of the fiber Bragg grating to the corresponding channels of the fiber grating demodulator, and then connect the fiber grating demodulator to the computer to monitor the drift of the center wavelength. 4. Experimental Data and Analysis The system's acquisition frequency is 50Hz. We selected a data interval of 1, i.e., 50 sets of data were acquired per second to obtain the center reflection wavelength values ​​of each FBG, monitoring the wavelength shift of each FBG during welding. The temperature coefficient of the fiber Bragg grating we typically used is 10.3275 pm/ °C, and the laboratory room temperature was 15 °C. The temperature changes at four measurement points perpendicular to the weld seam above the weld centerline are shown in Figure 4. The distribution of micro-strain in the direction perpendicular to the weld seam is shown in Figure 5. When processing the temperature data, we selected six well-preserved sets of data. The offset data of the FBG center wavelength at five measurement points 15 mm away from the weld seam were processed to obtain the temperature distribution in the direction parallel to the weld seam, as shown in Figure 6. The temperature distribution curve obtained from processing the six sets of data is shown in Figure 7. Selecting measurement points 10-12, the transverse strain distribution curves along the centerline perpendicular to the weld direction are shown in Figure 8. Figure 8 shows that the strain along the longitudinal centerline perpendicular to the weld exhibits the same overall trend, with the amplitude varying with distance from the weld. The maximum strain occurs when the weld point is on the centerline perpendicular to the weld direction. The maximum micro-strain is 5000 at a distance of 25mm from the weld centerline. The micro-strain amplitude decreases sequentially with increasing distance from the weld centerline. Based on the above data analysis and graphical display, we can obtain the strain and temperature distribution in various directions of the aluminum alloy structure during welding, meeting the requirements of this experiment. 5. Conclusion This paper introduces a method for simultaneous strain and temperature measurement based on a fiber Bragg grating. According to the fiber Bragg grating sensing principle, the strain and temperature variations can be separated from the changes in the center wavelength of the Bragg grating at different center wavelengths. This effectively solves the problem of cross-sensitivity between strain and temperature in fiber Bragg gratings, and completes the distributed measurement of strain and temperature in the welding process of aluminum alloy structures. References [1] Li Zhiyuan, Qian Yiyu et al. Advanced connection method. Machinery Industry Press, 2000:1-6 [2] Xu Jijin, Chen Ligong et al. Prediction of the highest welding temperature of multi-pass welding of thick plate butt joint. Welding. 2000:6-8 [3] Tang Muyao. Welding test technology [M]. Beijing: Machinery Industry Press, 1988. [4] Wang Muguang. Research on fiber optic grating sensor for simultaneous measurement of strain and temperature [J]. Sensing Technology, 2001, 20 (9). [5] Guan Bai-ou. Sensing research on dual parameters of temperature and strain of single fiber optic grating [J]. Chinese Journal of Lasers, 2001, 28 (4). [6] Guan Bai-ou, Tam Hwa-yaw, Tao Xiao-ming, et al. 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