Experimental Study on Axial Thermal Expansion of Composite Thin-walled Tube

Carbon fiber reinforced composite thin-walled tubes have the characteristics of light weight, high strength and small deformation, and are widely used in aviation, aerospace and other fields. Under the working conditions of high and low temperature, the thermal deformation of tube will directly determine the service reliability of their constituent structures. It is necessary to accurately measure the thermal expansion performance of composite thin-walled tube and reasonably evaluate the stability of their constituent structures. When the existing thermal expansion performance measurement system measures the thermal expansion of tube, there are problems of sampling damage to the structural state and failure to effectively contact the end face of tube in real time. For the above situations, A method for measuring the axial thermal expansion performance of composite thin-walled tube is proposed, and the measurement system is built. The thermal expansion performance of thin-walled tube is successfully and effectively evaluated, and the error of the measurement system is reasonably evaluated using the standard parts with known thermal expansion coefficient.


Introduction
Carbon fiber reinforced composite thin-walled tubes are commonly used as typical component units of composite structures.They are widely used in aviation, aerospace and other fields because of their light weight, high strength, good rigidity, small thermal deformation and other characteristics [1].For example, key components such as large antenna supports, spacecraft deployment mechanism trusses are mostly composed of thin-walled tube [2,3].The deformation of tube under the influence of high and low temperature environment under service conditions will directly affect the accuracy of the antenna surface and the directivity of the deployment mechanism.Accurate evaluation of the thermal expansion performance of tube is the key to the development of spacecraft structure technology [4,5].Due to the differences in the type of carbon fiber, the type of adhesive resin, the laying method of carbon fiber, the number of layers, etc. of various tube, the thermal expansion performance of tube is not a fixed value, and cannot be directly evaluated using the material properties [6][7][8].The traditional measurement method of thermal dilatometer is to take samples of a certain size from the tube, place them in the thermal expansion performance tester for temperature control, use the direct measurement method or comparison method to obtain the deformation of the sample, and evaluate its thermal expansion performance [9,10].However, due to the thin-walled characteristics of composite thinwalled tube, it is very easy to change the stress state of the tube interface during the sampling process, and due to the uneven distribution of materials in various areas of the thin-walled tube, the deformation of the anisotropic axis is different, and there is a large error in the traditional contact measurement, so the traditional measurement method cannot accurately evaluate the performance of the tube.In this paper, a method for measuring the thermal expansion performance of carbon fiber tube is proposed, and an experimental system is built according to this method, and the thermal expansion performance of tube is successfully and accurately obtained.

Measurement Method and Test System
In order to accurately measure the deformation of carbon fiber tube in high and low temperature environment, this paper uses the Digital Image Correlation (DIC) measurement technology to measure the axial deformation of carbon fiber composite thin-walled tube.In consideration of the different characteristics of the deformation of each axis caused by the uneven distribution of the tube material, the tube wall axis of the tube to be measured shall be measured uniformly, and the deformation of the different axes of the tube shall be obtained respectively, and the thermal expansion performance shall be reasonably and comprehensively evaluated by further average processing.
This method is used to design and build the measuring system as shown in figure 1, which is mainly composed of high and low temperature test chamber, optical observation window, DIC measuring device and liquid nitrogen refrigeration system.The high and low temperature test chamber is mainly used to load the test piece in the high and low temperature environment.The temperature sensor inside the chamber can control the temperature environment inside the chamber.The liquid nitrogen refrigeration system mainly uses liquid nitrogen to provide a cold source for the environment inside the chamber.The optical observation window is used to expose the measuring area of the measured tube fitting in the measuring system.The DIC measuring device obtains the tube fitting image through the binocular camera to analyse and measure the deformation.

Test Verification
This measurement method is used to test the thermal expansion performance of a certain type of carbon fiber composite thin-walled tube fitting.The tested piece is a composite thin-walled tube fitting with a total length of 60mm.Before the test, the test piece is subject to target treatment.Ten evenly distributed axes are selected on the cylinder surface of the test piece, and five axes on one side are exposed under the observation window respectively.As shown in figure 2, the sample line numbers are A1~A5 from top to bottom, ensuring that the DIC measuring camera can obtain images of five axes.During the experiment, the sample is heated from room temperature (20℃) to 100℃ at intervals of 20℃, and the thermal deformation of the sample is recorded by three-dimensional DIC technology.After completing the measurement of five axes, carry out the same experimental measurement on the five axes on the other side of the test piece: B1~B5, to ensure that the temperature gradient of the two experiments is consistent, and average the measurement results of the ten axes, so as to obtain the axial thermal expansion deformation of the sample.
At the same time, in order to reasonably evaluate the accuracy of the measurement results, a test standard made of aluminum alloy with known thermal expansion coefficient is placed in the test chamber during the test process, and the target characteristic line on the standard part is monitored to achieve real-time error evaluation of the system.In the process of experimental measurement, the sampling frequency of the camera is set to 1/2 Hz, that is, the thermal strain of the test piece is recorded in real time by shooting multiple digital images at 2.0 s intervals.Considering the hysteresis of the thermal deformation of the test piece and ensuring that the temperature distribution of the whole sample is uniform, an interval of more than 45 min is set between each temperature section in this experiment, and the deformation measurement is carried out in the whole process.At the same time, take the average value of not less than 100 groups of measured data as the effective data at this temperature, and homogenize the data to reduce the impact of external environmental factors and equipment deformation factors on the results.

Thermal Expansion Test Results of Thin-walled Tube
Figure 3 shows the real-time test results of the thermal strain of 10 sampling lines on both sides of the tested carbon fiber composite thin-walled tube, where A1~A5 are the thermal strain of 5 sampling lines on one side of the first measurement, and B1~B5 are the thermal strain of 5 sampling lines on the other side of the second measurement.
According to the measurement results, the thermal strain results of all the sampling lines in the range of 20℃ to 100℃ are below 0.02%, and the deviation of the measurement results of ten sampling lines is small, among which the five sampling lines measured for the first time are relatively large, and the maximum change of A3 thermal strain is close to 0.02%.A total of five temperature sections are set at 20℃ as a gradient during the whole experiment process, in order to ensure sufficient deformation of the results, each temperature lasts for 100 minutes for insulation.Figure 4 shows the processing results of the thermal deformation test data of the samples at 8 different temperatures by selecting the data interval of the stable strain section for the test data of each sample line and taking the average value as the processing results of the thermal deformation test data of the samples at 5 different temperatures, and performing a linear curve fitting on the processed results of each sample line to obtain the average thermal strain measurement results of each sample line.According to the calculation principle of thermal expansion of test piece, the thermal expansion coefficient is equal to the thermal strain under unit temperature change, so the slope of each linear fitting curve is the equivalent thermal expansion coefficient measured by the corresponding sampling line.Therefore, the test results of the axial equivalent thermal expansion coefficient measured by five sampling lines for the first time in the sample are 1.77×10 -6 /℃, 1.98×10 -6 /℃, 1.96×10 -6 /℃, 1.83×10 -6 /℃ and 1.95×10 -6 /℃.The average thermal expansion result of A1 to A5 is 1.898×10 -6 /℃.The test result of the equivalent thermal expansion coefficient of five sample lines measured for the second time are 1.94×10 -6 /℃, 1.98×10 -6 /℃, 1.87×10 -6 /℃, 1.87×10 -6 /℃ and 1.79×10 -6 /℃.The average thermal expansion result of B1 to B5 is 1.890×10 -6 /℃, the average result of two measurements is 1.894×10 -

Evaluation of Measurement Error of Standard Test Piece
Figure 5 shows the thermal strain measurement results (the first test: C1, C2, the second test: D1, D2) obtained by setting sampling lines for the test calibration block made of aluminum alloy during the two experiments, which are consistent with the treatment method of carbon fiber composite thinwalled tube.The average thermal strain results at each temperature section of each sampling line are selected to form a linear fitting curve.According to the fitting curve results of the average thermal strain of the sample line, the test results of the equivalent thermal expansion coefficient measured by each sample line in the test calibration block are respectively 22.4×10 -6 /℃, 21.6×10 -6 /℃, 20.9×10 -6 /℃ and 21.0×10 -6 /℃.The average of the measured results of all sampling lines in the test calibration block is 21.48×10 -6 /℃.In contrast, the thermal expansion coefficient of aluminum alloy sample measured by special thermal expansion instrument (DIL) is 21.475×10 -6 /℃.Therefore, the measurement error of thermal expansion coefficient of the test system is less than 0.1×10 -6 /℃.

Conclusion
In this paper, a method for measuring the axial thermal expansion performance of composite thinwalled tube is proposed.By using DIC measurement technology and special optical measurement and temperature control method, the problem that the traditional thermal expansion measurement system destroys the structural state of the test piece and the measuring end face cannot effectively maintain the surface contact is effectively solved.In this paper, the measurement system is built using this measurement method, and the axial thermal expansion coefficient of carbon fiber composite tube is measured and developed.The thermal expansion of 10 sampling lines of tube is effectively measured, and the overall thermal expansion performance of the test piece is reasonably evaluated.The measurement error is evaluated using standard test pieces, and the measurement results show that the system measurement error is within 0.1×10 -6 /℃.

Figure 1 .
Figure 1.Thermal expansion performance measurement system for thin-walled tube.

Figure 2 .
Figure 2. Distribution of sampling lines on thin-walled tube and standard parts.

Figure 3 .
Figure 3. Thermal strain caused by temperature change of 5 groups of test sample lines equispaced along the axial direction of the sample.

Figure 4 .
Figure 4. Average thermal strain of five groups of test sample lines with equal spacing along the axial direction of the sample at different temperatures.

Figure 5 .
Figure 5.The thermal strain measurement results :(a) Thermal strain of the test sample line of the test calibration block with temperature change; (b) Test the average thermal strain of the calibration block test sample line at different temperatures.