Method for measuring thermal expansion coefficient of composite material based on strain gauge method

Abstract

The invention discloses a method for measuring the thermal expansion coefficient of a composite material based on a strain gauge method, and the method comprises the steps: enabling a foil type strain gauge and a dynamic bridge strain gauge to form a full-bridge circuit, and controlling the test starting temperature, the test ending temperature and the test time in a test box through a PID temperature control module; a dynamic bridge strain gauge is used for measuring resistance change caused by the strain effect of a foil type strain gauge, finally, an upper computer is used for exporting data of strain of the composite material along with time and temperature changes, thermal expansion coefficients of all temperature intervals are measured through a fitting curve at intervals of 20 DEG C and serve as the thermal expansion coefficients of the temperature intervals, and the thermal expansion coefficients of the temperature intervals are calculated. And calculating an average value to characterize the thermal expansion coefficient of the material in the whole temperature interval. The method has the advantages of high test precision, simplicity in operation, low cost, no limitation of sample size, capability of simultaneously measuring multiple groups of samples and the like, and particularly provides reliable data support for research and application of thermal expansion performance of large-size and special-shaped structure composite materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of material science testing, and specifically relates to a method for measuring the thermal expansion coefficient of composite materials based on the strain gauge method. Technical Background With the development of modern industry, the performance requirements for materials are getting higher and higher. Composite materials are materials composed of two or more different materials combined by physical or chemical methods, and have superior properties than single materials, such as high strength, high stiffness, high temperature resistance, corrosion resistance, etc., and have been widely used in the fields of aerospace, automotive, electronics, architecture, etc. However, the thermal expansion coefficient of composite materials is usually different from that of each single material that composes it, which will cause thermal stress and deformation in the composite material when the temperature changes, thus affecting its performance and service life. Therefore, accurately measuring the thermal expansion coefficient of composite materials is of great significance for the design, manufacture and use of materials.

[0003] In the field of material science, accurately measuring the expansion coefficient of composite materials is crucial for material property evaluation and engineering applications. When measuring the thermal expansion coefficient of materials by the traditional push-rod method, the sample is first placed in a heating furnace, one end of which is in contact with a push-rod connected to a displacement measuring device. Heating causes the sample to expand, and the expansion amount is transmitted by the push-rod. Then, the expansion coefficient is calculated by measuring the expansion displacement and temperature change. However, it has strict requirements on the sample size, generally samples with a length of (50±2) mm or (25±2) mm and a cross-section that is circular or rectangular in the length direction. If not satisfied, the push-rod cannot be on the same horizontal line as the center position of the sample, resulting in inaccurate measurement results, restricting its application in the testing of composite materials with different size specifications, making it difficult to comprehensively understand the thermal expansion performance of materials, and this method can only measure one group of samples at a time, with low efficiency, increasing the time cost and workload in scenarios with a large amount of data requirements. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for measuring the thermal expansion coefficient of composite materials based on the strain gauge method.

[0005] To solve the above technical problems, the technical scheme adopted by the present invention is as follows: Based on the principle of the strain method and the flexible characteristics of the foil strain gauge, a full-bridge circuit is formed by combining the foil strain gauge and the dynamic bridge strain gauge, breaking through the limitations of the traditional push-rod method on the sample size and shape, broadening the application range, and improving the test accuracy of large samples; with the help of the PID temperature control module and the test chamber, precise control and measurement of the sample temperature are realized, and the thermal expansion coefficient of composite materials in the temperature range of (20~160) °C can be accurately and reliably measured, thus achieving the purpose of the present invention.

[0006] The present invention relates to a method for measuring the thermal expansion coefficient of composite materials based on the strain gauge method, including the following steps: 1) Sample treatment: Polish the surface of the sample, remove impurities, and clean the area where the foil strain gauge is to be pasted. 2) Device connection: The device for testing the coefficient of thermal expansion of composite materials based on the strain method includes a foil strain gauge, a test chamber, a PID temperature control module, a dynamic bridge strain gauge, and a host computer. A temperature sensor is installed inside the test chamber. The PID temperature control module is connected to the temperature sensor in the test chamber to control the initial temperature, final temperature, and test time in the test chamber. The foil strain gauge and the dynamic bridge strain gauge form a full-bridge circuit, and the dynamic bridge strain gauge is connected to the host computer. 3) Test preparation: Paste the foil strain gauge onto the sample to be tested, and squeeze out the excess glue and air bubbles between the foil strain gauge and the sample to be tested. Place the sample to be tested with the glued foil strain gauge into the test chamber. 4) Test: Start the device for testing the coefficient of thermal expansion of composite materials based on the strain method, set the PID temperature control module to control the initial temperature, final temperature, and heating rate of the test chamber. Set the dynamic bridge strain gauge to automatically measure the length of the sample to be tested as needed. The host computer collects and records the length data of the sample to be tested measured by the dynamic bridge strain gauge. 5) Data processing and analysis: Use the least squares method to perform fitting processing and analysis on the data collected by the host computer, and calculate the coefficient of thermal expansion of the sample to be tested.

[0007] Preferably, in step 3), the excess glue and air bubbles between the foil strain gauge and the sample to be tested are squeezed out by placing a flexible insulator on the front surface of the foil strain gauge and pressing the flexible insulator.

[0008] Preferably, the flexible insulator includes a polyethylene resin sheet and a polyimide film.

[0009] Preferably, in step 4), the length data of the sample to be tested includes the initial length and the length at the highest temperature.

[0010] More preferably, the length data of the sample to be tested further includes the lengths at the same temperature intervals.

[0011] Even more preferably, the temperature interval is 20 °C.

[0012] Preferably, in step 5), the coefficient of thermal expansion of the sample to be tested is the average value of multiple results.

[0013] The method for testing the coefficient of thermal expansion of composite materials based on the strain gauge method in the present invention uses a foil strain gauge and a dynamic bridge strain gauge to form a full-bridge circuit. The 20-channel dynamic bridge strain gauge can measure 5 groups of small samples simultaneously by the single-point testing method, avoiding the cumbersome process of measuring each sample one by one, greatly shortening the measurement cycle, effectively improving the measurement efficiency, and providing convenience for large-scale measurement. It can also adopt multi-point distributed measurement for large plates, breaking through the limitation of the traditional push-rod method on the sample size, and improving the measurement accuracy of the coefficient of thermal expansion of large plates. The foil strain gauge adopted in the present invention has the characteristic of flexibility, can also be adapted to measure samples with non-planar structures, effectively expands the application range, and enhances the versatility of the method. In addition, the method of the present invention also has the advantages of simple operation, high reliability, and low cost, and can be widely applied to the testing of the coefficient of thermal expansion of different materials. The test results are stable and accurate, and the equipment and consumable costs are low, reducing the test difficulty and cost. Description of the Drawings

[0014] Figure 1 It is a position relationship diagram of each functional component of the device for testing the coefficient of thermal expansion of composite materials in the present invention; Figure 2 It is a diagram showing the pasting positions of the foil strain gauges of the samples to be tested in Example 2; Figure 3 It is a strain-temperature-time signal diagram of the 5# foil strain gauge of the sample to be tested in Example 2 at a heating rate of 2 K / min; Figure 4 It is a diagram showing the pasting positions of the foil strain gauges of the samples to be tested in Example 3; Figure 5 It is a strain-temperature-time signal diagram of the sample to be tested in Example 3 at a heating rate of 2 K / min; Figure 6 It is a strain-temperature-time signal diagram of the sample in the comparative example at a heating rate of 2 K / min.

[0015] Among them, 1 - foil strain gauge, 2 - sample to be tested, 3 - test chamber, 4 - PID temperature control module, 5 - dynamic bridge strain gauge, 6 - upper computer. Detailed Embodiments

[0016] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0017] The foil strain gauge of the present invention is composed of a base, a sensitive grid, an adhesive, a cover layer, etc. The CEA-06 series of foil strain gauges produced by Vishay are selected, with a minimum reading of 10 -6 microstrain and a maximum range of 2×10 -4Microstrain, with a sensitivity coefficient of 2.0 ± 1%.

[0018] Example 1 In this example, the sample 2 to be measured is a cylindrical carbon fiber composite product with a length of 50 mm and a radius of 3 mm. The steps for measuring its coefficient of thermal expansion are as follows: (1) Treatment of the sample 2 to be measured First, use sandpaper with a grit size of 200# - 300# to polish the surface of the sample 2 to be measured, and then use acetone to remove impurities such as surface paint and plating.

[0019] Dip industrial thin paper in acetone solution and clean the side wall of the sample 2 to be measured where the foil strain gauge 1 is pasted.

[0020] (2) Device connection As Figure 1 shown, the device for testing the coefficient of expansion of a composite material based on the strain method of the present invention includes a foil strain gauge 1, a test chamber 3, a PID temperature control module 4, a dynamic bridge strain gauge 5, and a host computer. A temperature sensor is provided inside the test chamber 3. The PID temperature control module 4 includes a signal input terminal, a solid-state relay, and a display screen; the signal input terminal is used to receive the signal from the temperature sensor; the signal output terminal of the solid-state relay is connected to an external controlled device to control the operation of the device; the display screen is connected to the relevant circuit in the PID temperature control module 4 to display temperature information.

[0021] Connect the appropriate power supplies to the PID temperature control module 4 and the test chamber 3 respectively. The signal input terminal of the PID temperature control module 4 is connected to the temperature sensor of the test chamber 3, so that the starting temperature, ending temperature, and test time of the test chamber 3 can be controlled. The connecting wire of the foil strain gauge 1 and the dynamic bridge strain gauge 5 form a full-bridge circuit by short-circuiting, and the dynamic bridge strain gauge 5 is connected to the host computer 6 through a USB cable.

[0022] (3) Test preparation Use 502 glue to bond the back of the four foil strain gauges 1 for measuring one position point to the side wall of the cleaned sample 2 to be measured. Due to the unique flexible structure of the foil strain gauge 1, it can also fit well with the cylindrical wall surface of the sample 2 to be measured. To avoid the influence of external environmental factors such as dust and conductivity, place an insulating flexible polyethylene resin sheet on the front of the foil strain gauge 1, press it with your finger to squeeze out the excess glue and air bubbles between the foil strain gauge 1 and the sample 2 to be measured. After pressing for 1 minute, remove the polyethylene resin sheet and confirm whether it is firmly pasted.

[0023] Put the sample 2 to be measured with the foil strain gauge 1 bonded into the test chamber 3.

[0024] (4) Test Start the device for testing the coefficient of thermal expansion of composite materials based on the strain method. Set the PID temperature control module 4 to raise the temperature in the test chamber 3 from 20 °C to 160 °C at a heating rate of 2 K / min. Set the dynamic bridge strain gauge 5 to automatically measure the initial length of the sample to be tested 2 and its length at 160 °C respectively. The upper computer 6 collects and records the length of the sample to be tested 2 measured by the dynamic bridge strain gauge 5.

[0025] (5)Data processing and analysis Use the least squares method to fit the data collected by the upper computer 6, and calculate the coefficient of thermal expansion of this embodiment using formula (1).

[0026] (1) Among them, α is the average coefficient of thermal expansion, with the unit of 1 / °C, generally in units of 10 -6 / °C; L 1 is the length of the sample to be tested at temperature T 1, with the unit of mm; L 2 is the length of the sample to be tested at temperature T 2, with the unit of mm; T 1, T 2 are two temperature points selected during the measurement process respectively.

[0027] Repeat the above operations 3 times and take the average. The three test results of the coefficient of thermal expansion of the sample to be tested 2 are shown in Table 1.

[0028] Table 1 Three test results of the coefficient of thermal expansion of the sample to be tested 2 in this embodiment

[0029] As can be seen from Table 1, the deviation of the average coefficient of thermal expansion of the sample to be tested 2 in this embodiment at (20 - 160) °C is only 0.39, indicating that the device for testing the coefficient of thermal expansion of composite materials in the present invention has high stability and reliability.

[0030] Example Two The sample to be tested 2 in this embodiment is a carbon fiber composite product with a length of 300 mm, a width of 300 mm, and a thickness of 10 mm.

[0031] Adopt the measurement method of multi-point distributed pasting. The pasting positions are as Figure 2 shown. Four foil strain gauges 1 are pasted at the center position of the upper surface and the four corners of the sample to be tested 2 respectively.

[0032] The test steps of the sample to be tested 2 in this embodiment are the same as those in Example One.

[0033] The signal diagram of strain-temperature-time of the four foil strain gauges 1 at the 5# measurement point in the central position is shown in Figure 3 . The single thermal expansion coefficient test results of the five measurement points are shown in Table 2.

[0034] Table 2 Single thermal expansion coefficient test results of the sample 2 to be tested in this embodiment

[0035] As can be seen from Table 2, the standard deviation of the thermal expansion coefficients of the sample 2 to be tested in this embodiment at five different positions in the range of (20 - 160) °C is 0.86. The deviation increases compared with that of a single measurement point because the area of the sample 2 to be tested is large and the temperature distribution is not uniform enough. However, it still meets the test requirements of the thermal expansion coefficient, indicating that the device of the present invention has good stability and is suitable for testing the thermal expansion coefficient of this type of sample.

[0036] Example 3 The sample 2 to be tested in this embodiment is a special-shaped part of a carbon fiber composite material product. The cross-sectional area of this special-shaped part is a square with a side length of 10 mm, and the whole is arc-shaped with an arc length of 30 mm. Its structure and the pasting positions of the four foil strain gauges 1 for measuring a single position point are as Figure 4 shown.

[0037] The test steps of the sample 2 to be tested in this embodiment are the same as those in Example 1. The signal diagram of strain-temperature-time is shown in Figure 5 shown.

[0038] Repeat the above operation 3 times and take the average value. The three test results of the thermal expansion coefficient are shown in Table 3.

[0039] Table 3 Three test results of the thermal expansion coefficient of the sample 2 to be tested in this embodiment

[0040] As can be seen from Table 3, the standard deviation of the thermal expansion coefficients of the sample 2 to be tested in this embodiment at three different positions in the range of (20 - 160) °C is 0.77, indicating that the device has good stability and is suitable for testing the thermal expansion coefficient of this type of sample.

[0041] Example 4 The difference between the test steps of the sample 2 to be tested in this embodiment and those in Example 1 lies in: (3) Test preparation To avoid the influence of external environmental factors such as dust and conductivity, an insulating flexible polyimide film is placed on the front side of the foil strain gauge 1.

[0042] (4) Test A dynamic bridge strain gauge 5 is set to automatically measure the initial length of the sample 2 to be measured respectively, and record the length of the sample 2 to be measured every 10 minutes.

[0043] In this embodiment, the method of measuring the coefficient of thermal expansion in each interval at intervals of 20 °C and taking it as the value of the coefficient of thermal expansion in this temperature range is adopted. Then, the average value is obtained to characterize the thermal expansion characteristics of the material. The processed data is shown in Table 4.

[0044] Table 4 Test results of the coefficient of thermal expansion of the sample 2 to be measured in this embodiment

[0045] As can be seen from Table 5, the standard deviation of the test results of the coefficient of thermal expansion every 20 °C increases compared with that in the first embodiment. However, this method can finely reflect the expansion change process of the material during the heating process, effectively improve the test accuracy, accurately capture the differences in thermal expansion behavior corresponding to the possible microstructural evolution of the material at different temperature stages, and provide more detailed and reliable data support for in-depth exploration of the thermophysical properties of the material.

[0046] Comparative example Using the same sample as in the first embodiment, in accordance with GB / T 2572-2005 "Test Method for Average Coefficient of Thermal Expansion of Fiber Reinforced Plastics", the coefficient of thermal expansion of the carbon fiber composite material at (20~160) °C is measured by DIL 402C (push rod method) produced by Netzsch Company. The signal diagram of its strain-temperature-time is shown in Figure 6 , and the results are shown in Table 5.

[0047] Table 5 Test results of the coefficient of thermal expansion of the sample 2 to be measured by the DIL 402C method

[0048] As can be seen from Table 5, the standard deviation of the average coefficient of thermal expansion measured by Netzsch DIL 402C is 0.05, and the relative deviation between the average coefficient of thermal expansion measured by the present invention and that measured by Netzsch DIL 402C is only 0.72%, indicating that the device of the present invention already has good test accuracy.

Claims

1. A method for measuring the coefficient of thermal expansion of a composite material based on the strain gauge method, characterized in that, It includes the following steps: 1) Sample treatment: Polish the surface of the sample, remove impurities, and clean the area where the foil strain gauge (1) is to be pasted; 2) Device connection: The device for testing the coefficient of thermal expansion of composite materials based on the strain method includes a foil strain gauge (1), a test chamber (3), a PID temperature control module (4), a dynamic bridge strain gauge (5) and a host computer; A temperature sensor is arranged in the test chamber (3); The PID temperature control module (4) is connected to the temperature sensor of the test chamber (3) and is used to control the initial temperature, the final temperature and the test time in the test chamber (3); The foil strain gauge (1) and the dynamic bridge strain gauge (5) form a full-bridge circuit, and the dynamic bridge strain gauge (5) is connected to the host computer (6); 3) Test preparation: Paste the foil strain gauge (1) onto the sample to be tested (2), and squeeze out the excess glue and air bubbles between the foil strain gauge (1) and the sample to be tested (2); Place the sample to be tested (2) with the foil strain gauge (1) pasted thereon into the test chamber (3); 4) Test: Start the device for testing the coefficient of thermal expansion of composite materials based on the strain method, set the PID temperature control module (4) to control the initial temperature, the final temperature and the heating rate of the test chamber (3); Set the dynamic bridge strain gauge (5) to automatically measure the length of the sample to be tested (2) as required; The host computer (6) collects and records the length data of the sample to be tested (2) measured by the dynamic bridge strain gauge (5); 5) Data processing and analysis: Use the least squares method to perform fitting processing and analysis on the data collected by the host computer (6), and calculate the coefficient of thermal expansion of the sample to be tested (2).

2. The method for measuring the coefficient of thermal expansion of a composite material based on the strain gauge method according to claim 1, characterized in that: In step 3), the excess glue and air bubbles between the foil strain gauge (1) and the sample to be tested (2) are squeezed out by placing a flexible insulator on the front surface of the foil strain gauge (1) and pressing the flexible insulator.

3. The method for measuring the coefficient of thermal expansion of a composite material based on the strain gauge method according to claim 1, wherein: The flexible insulator includes a polyethylene resin sheet and a polyimide film.

4. The method for measuring the coefficient of thermal expansion of a composite material based on the strain gauge method according to claim 1, characterized in that: In step 4), the length data of the sample to be tested (2) includes the initial length and the length at the highest temperature.

5. The method for measuring the coefficient of thermal expansion of a composite material based on the strain gauge method according to claim 4, characterized in that: The length data of the sample to be tested (2) also includes the lengths at the same temperature difference intervals.

6. The method for measuring the coefficient of thermal expansion of a composite material based on the strain gauge method according to claim 5, characterized in that: The temperature difference is 20 °C.

7. The method for measuring the coefficient of thermal expansion of a composite material based on the strain gauge method according to claim 1, characterized in that: In step 5), the coefficient of thermal expansion of the sample to be tested (2) is the average value of multiple results.

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