Thermal stress measurement and analysis method for composite material and metal connection structure

Abstract

The invention discloses a method for measuring and analyzing thermal stress of a composite material and metal connecting structure, which comprises three level tests, namely a material level thermal expansion coefficient test to obtain thermal expansion coefficients of metal and a composite material; the laminate-level thermal stress test is used as a calibration test; and obtaining the total strain of the structure through a connection structure thermal stress test. According to the invention, the design reliability and efficiency of the hybrid connection structure in an extreme temperature environment are improved.

Description

Technical Field

[0001] This invention belongs to the field of static strength design and analysis technology of aerospace structures, and relates to a method for measuring and analyzing thermal stress in composite and metal connection structures. Background Technology

[0002] Composite materials, with their high strength and light weight, are widely used in the aerospace field. In actual aircraft structures, composite materials are commonly connected to metals. These connections typically rely on bolts / rivets. Due to the significant difference in thermal expansion coefficients between composite materials and metals, temperature changes experienced by the aircraft during takeoff, cruise, and landing can induce internal stresses at the connection points. Long-term exposure to these thermal stresses can lead to thermal fatigue, thereby affecting the structural integrity and safety.

[0003] In aircraft structural strength design, the analytical methods for composite material and metal-connected structures must be experimentally proven effective before being extended to general structures. This requires thermal stress testing to eliminate interference from factors such as material free expansion and measurement errors, and to accurately measure the thermal strain level of the structure, providing accurate and effective data for the analytical methods. Therefore, developing reasonable thermal stress testing methods and studying the thermal stress behavior of composite material and metal-connected structures is of great significance for ensuring the safety and durability of aircraft structures. Summary of the Invention

[0004] The purpose of this invention is to provide a method for measuring and analyzing the thermal stress of composite and metal connection structures. This invention improves the design reliability and efficiency of hybrid connection structures under extreme temperature environments.

[0005] The technical solution adopted in this invention is a method for measuring and analyzing the thermal stress of a composite and metal connection structure, comprising three levels of testing: a material-level thermal expansion coefficient test to obtain the thermal expansion coefficients of the metal and composite; a layer-level thermal stress test as a calibration test; and a connection structure thermal stress test to obtain the total strain of the structure.

[0006] In the aforementioned method for measuring and analyzing the thermal stress of composite and metal connection structures, the test procedure for the material-level coefficient of thermal expansion is as follows:

[0007] S11. Design test specimens based on the material at the connection between the composite material and the metal: n metal specimens, and n specimens of 0° and 90° unidirectional laminates and multidirectional laminates for each material batch.

[0008] S12. Conduct standard thermal expansion coefficient measurement tests using the sample;

[0009] S13. Calculate the coefficient of thermal expansion of each sample using the following general formula:

[0010] α m =ΔLm / (L m ×ΔT m )

[0011] α1 = ΔL1 / (L1 × ΔT1)

[0012] α2 = ΔL2 / (L2 × ΔT2)

[0013] In the formula: α m α1 and α2 are the coefficients of thermal expansion of the metal sample, the 0° unidirectional laminate sample, and the 90° unidirectional laminate sample, respectively.

[0014] ΔL m ΔL1 and ΔL2 represent the length changes in the expansion direction of the corresponding sample, respectively.

[0015] L m L1 and L2 are the initial lengths of the corresponding sample in the expansion direction, respectively;

[0016] ΔT m ΔT1 and ΔT2 are all experimental temperature differences;

[0017] Using α1 and α2 as inputs to the finite element analysis model, the coefficient of thermal expansion α of the multidirectional laminate specimen of the composite material was obtained. c .

[0018] In the aforementioned method for measuring and analyzing thermal stress in composite and metal connection structures, in S11, the sample size is 20mm × 5mm.

[0019] In the aforementioned method for measuring and analyzing thermal stress in composite and metal connection structures, S12 is specifically as follows:

[0020] S121. Use a standard quartz sample and the same test parameters as the sample test to perform baseline calibration;

[0021] S122. Measure the initial length L of the sample expansion test direction at ambient temperature;

[0022] S123. Place the sample under the probe inside the container and place the temperature sensor in contact with the sample;

[0023] S124. Cool down to the lowest temperature of the test, then heat up to the highest temperature of the test at a preset heating rate, and record the sample length-temperature curve.

[0024] In the aforementioned method for measuring and analyzing thermal stress in composite and metal connection structures, in step S123, a load of 1mN to 100mN is applied to the temperature sensor to ensure that the sensor contacts the sample.

[0025] In the aforementioned method for measuring and analyzing thermal stress in composite and metal connection structures, the test steps for layer-level thermal stress are as follows:

[0026] S21. Design laminate-level specimens based on the material and ply angle at the connection between the composite material and the metal: n metal specimens, and n composite multi-directional laminate specimens with the same composite ply at the connection, each ply having n specimens.

[0027] S22. Conduct laminate-level thermal stress tests:

[0028] S221. Strain gauges and thermocouples are attached to the specimen to measure the temperature and strain of the specimen;

[0029] S222. The sample temperature is gradually increased from the ambient temperature to the highest test temperature or decreased to the lowest test temperature, with a temperature gradient of ±5℃ or ±10℃. Upon reaching each temperature level, the sample temperature is allowed to stabilize within the allowable error range before measuring the sample temperature and strain: the strain of the metallic sample is... The strain of the multi-directional laminate specimen of the composite material is ;

[0030] S223. Plot the strain-temperature curves for each specimen.

[0031] In the aforementioned method for measuring and analyzing thermal stress in composite and metal connection structures, in S21, the sample size is 100mm × 50mm.

[0032] In the aforementioned method for measuring and analyzing the thermal stress of composite and metal connection structures, in S223, when plotting the strain-temperature curve of the multidirectional laminate specimen, the strain is based on the free expansion strain ε of the composite laminate. c ´drawing, ε c Calculate using the following formula:

[0033] ε c ´ = ε c - ε gague ≈ α c × ΔT

[0034] ε gague = ε m - α m × ΔT

[0035] In the formula, ε gague For the strain output of the strain gauge itself, α c Let α be the coefficient of thermal expansion of the multi-directional laminate specimen. m ΔT is the coefficient of thermal expansion of the metal sample, and ΔT is the test temperature difference.

[0036] In the aforementioned method for measuring and analyzing the thermal stress of composite and metal connection structures, the test steps for the thermal stress of the connection structure are as follows:

[0037] S31. The connection structure test piece is designed as a component-level test piece. In the component-level test piece, the connection between the metal sample and the composite multi-directional laminate sample has 7 or more nails. The thickness of the metal and the composite, the composite layup, the bolts, and the nail spacing are consistent with the actual connection structure.

[0038] S32. Conduct thermal stress tests:

[0039] S321. Strain gauges are attached between the nails on the test specimen, and thermocouples are attached to both the metal and composite surfaces to measure the temperature of the test specimen.

[0040] S322. The test specimen is placed freely in the environmental chamber. Before the formal test, the temperature load is directly applied from the ambient temperature to the highest / lowest temperature in the test temperature. After the temperature of the test specimen stabilizes, it is kept at the temperature for 3 minutes and then unloaded to the ambient temperature.

[0041] S323. Formal Test: The temperature is gradually increased from ambient temperature to the highest temperature or decreased to the lowest temperature, with a temperature gradient of ±5℃ or ±10℃. Upon reaching each temperature level, after the temperatures of both the metal and composite material have stabilized within the allowable error range, the temperature and strain of the test specimen are measured. The strain of the metal specimen is... The strain of the multi-directional composite laminate specimen is ;

[0042] S324. Calculate the thermal strain ε of the metal specimen using the following formula. m L Thermal strain ε of multi-directional composite laminate specimen c L :

[0043] ε m L =ε m t - ε gague - α m × ΔT=ε m t - ε m

[0044] ε c L =ε c t - ε gague - ε c ´ =ε c t - ε c

[0045] In the formula, ε gague For the strain output of the strain gauge itself, α m ε is the coefficient of thermal expansion of the metal sample, ΔT is the test temperature difference, and εm ε is the strain of the metal specimen. c The free expansion strain of the multi-directional laminate specimen, ε c The strain of the multi-directional laminate specimen;

[0046] S325. The structural thermal strain will be obtained. , The results were compared with those in the thermal stress finite element model to verify the effectiveness of the finite element analysis method.

[0047] In the aforementioned method for measuring and analyzing thermal stress in composite and metal connection structures, the design of component-level test specimens in S31 is as follows:

[0048] The thickness of the metal and composite materials, the composite layering, and the spacing of bolts and nails are consistent with typical connection structures on aircraft. The thickness of the metal specimen is also consistent. Thickness of multi-directional composite laminate sample The layup sequence is [45 / -45 / 0 / 90]. 2s The connector diameter is D, the nail spacing is 5D, and the nail edge distance is 0.8D. The nail spacing and edge distance meet the space requirements for strain gauge bonding.

[0049] In the aforementioned method for measuring and analyzing thermal stress in composite and metal connection structures, in S321, the strain gauges and thermocouples are arranged as follows: strain gauges are attached between the metal and composite surfaces of the test piece, at the center of the attachment; and thermocouples are attached to both the metal and composite surfaces to measure the temperature of the test piece.

[0050] Beneficial Effects: Aircraft experience varying temperature changes during takeoff, cruise, and landing. Aircraft structures commonly incorporate composite materials and metal connections. The thermal expansion coefficient of composite materials is significantly lower than that of metals, leading to stress and thermal fatigue at the connection points. This invention provides a method for experimentally measuring the thermal stress of composite-metal connection structures, comprising three levels of testing: 1. Material-level thermal expansion testing to obtain the thermal expansion coefficients of the metal and composite materials; 2. Thermal stress testing of single-layer plate-level metal and composite material specimens as a calibration test; 3. Thermal stress testing of the composite-metal connection structure to obtain the total structural strain. This three-level testing approach provides completeness and systematicity. Through experimental data analysis, the influence of structural free expansion and the strain gauge's own thermal strain output can be eliminated. This method not only obtains the thermal stress level of the composite-metal connection structure but also provides a basis for thermal stress analysis methods for composite-metal connection structures.

[0051] This invention obtains the true thermal strain of the connection structure through systematic modular verification experiments and data processing, thereby improving the design reliability and efficiency of the hybrid connection structure under extreme temperature environments. Attached Figure Description

[0052] Figure 1This is a schematic diagram of the test specimen and measurement points for the laminate-level thermal stress test.

[0053] Figure 2 This is a schematic diagram showing the connection between the structural thermal stress test element / test piece and the measurement point. Detailed Implementation

[0054] The features and illustrative embodiments of various aspects of the present invention will now be described in detail. Numerous specific design details are set forth in the following detailed description to provide a more complete understanding of the invention. However, it will be apparent to those skilled in the art that the invention can be practiced without some of these specific details. The following description of embodiments is merely intended to provide a better understanding of the invention by illustrating examples of the invention. The invention is by no means limited to any specific setup and method set forth below, but covers any improvements, substitutions, and modifications to the structures, methods, and devices without departing from the spirit of the invention. In the drawings and the following description, any parts not exhaustively described are considered to be common knowledge or conventional practices in the art.

[0055] It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other, and the various embodiments can be referenced and cited in each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0056] Example 1. A method for measuring and analyzing thermal stress in composite and metal connection structures, such as... Figures 1-2 As shown, it includes the following steps:

[0057] I. Material-level thermal expansion coefficient test

[0058] Step 1: Specimen design for thermal expansion test:

[0059] Based on the material design of the connection structure between the composite material and the metal, n metal samples are prepared, and n samples of 0° and 90° unidirectional laminates and multidirectional laminates are prepared for each batch of materials. The sample size is approximately 20mm × 5mm.

[0060] Step 2: Conduct a standard thermal expansion coefficient measurement test:

[0061] Step 201: Using a standard quartz sample, perform baseline calibration using the same test parameters as for the metal / composite material sample test.

[0062] Step 202: Measure the initial length L of the sample expansion measurement direction, ambient temperature (20~25℃).

[0063] Step 203: Place the sample under the probe inside the container, and place the temperature sensor so that it contacts the sample or is as close as possible to the test piece. Apply a load of 1mN to 100mN to the sensor to ensure that the sensor contacts the sample.

[0064] Step 204: Cool down to the lowest temperature, then heat up to the highest temperature at a suitable heating rate, and record the sample length-temperature curve.

[0065] Step 3: Calculate the coefficient of thermal expansion:

[0066] According to the formula The coefficient of thermal expansion of the material is calculated using the formula: ΔL represents the change in length of the sample along the measurement direction, and ΔT represents the corresponding temperature difference. This yields the coefficient of thermal expansion of the metal in the connecting structure. The coefficient of thermal expansion of composite materials in the 0° and 90° directions and As input to the finite element analysis model, the coefficient of thermal expansion of the composite laminate is... .

[0067] II. Laminate-level thermal stress test

[0068] Step 1: Design of Layer-Level Specimens

[0069] The test specimens are designed based on the material and ply angle at the connection between the composite material and the metal. There are n metal specimens and n multi-directional composite laminate specimens with the same ply as the composite material at the connection. The specimen size is approximately 100mm × 50mm.

[0070] Step 2: Conduct layer-level thermal stress tests

[0071] Step 201: Attach strain gauges and thermocouples to the specimen to measure the temperature and strain of the specimen. The operating temperature range of the strain gauges and thermocouples should cover the test temperature.

[0072] Step 202: Place the laminate-level specimens freely in an environmental chamber. Gradually increase the temperature from room temperature (20~25℃) to the highest temperature or decrease it to the lowest temperature, with a temperature gradient of ±5℃ or ±10℃. Upon reaching each temperature level, wait until the specimen temperature stabilizes within the allowable error range, then measure the specimen's temperature and strain. The strain measured for the metallic specimen is... The strain measured on the composite sample was as follows: .

[0073] Step 203: Plot the strain-temperature curves of the metal and composite laminate specimens. It is worth noting that the measured strain includes the strain output of the strain gauge itself. The strain can be obtained by subtracting the free expansion strain of the metal from the strain measured in the metal plate, i.e. Strain measurements on composite materials include Due to the smaller coefficient of thermal expansion of composite materials, the measured strain is related to the free expansion of composite materials. Will be affected The influence of strain-temperature curves may result in positive values ​​at low temperatures, negative values ​​at high temperatures, and nonlinear strain-temperature curves, misleading the analysis of thermal stress behavior in composite materials. The free expansion strain of composite laminates is... , obtained Should be with Close, and - The temperature curve has good linearity.

[0074] III. Thermal Stress Test of Connection Structure

[0075] Step 1: Design of Connection Structure Test Specimen

[0076] Thermal stress test specimens for connection structures can be designed at the component level or full-size. Component-level specimens facilitate the verification of finite element analysis methods and simplify modeling. For component-level specimens, it is recommended that the connection between the metal plate and the composite laminate be secured with 7 or more nails, and that the thickness of the metal and composite, the composite layup, the bolts, and the nail spacing be consistent with the actual connection structure. Full-size specimens are tested directly on aircraft components.

[0077] Step 2: Conduct thermal stress tests

[0078] Step 201: Attach strain gauges between the nails of the test specimen to measure the strain, and attach thermocouples to both the metal and composite surfaces to measure the temperature of the test specimen.

[0079] Step 202: The test specimen is placed freely in the environmental chamber. Before the formal test, the temperature load is directly applied from room temperature to the highest / lowest temperature. After the temperature of the test specimen stabilizes, it is kept at the temperature for 3 minutes and then unloaded to room temperature to eliminate the influence of gaps between the connecting structures.

[0080] Step 203: Formal Test. The temperature is gradually increased from room temperature (20~25℃) to the highest temperature or decreased to the lowest temperature, with a temperature gradient of ±5℃ or ±10℃. Upon reaching each temperature level, after the temperatures of both the metal and composite material have stabilized within the allowable error range, the temperature and strain of the test specimen are measured. The strain of the metal specimen is measured as follows: The strain measured on the composite sample was as follows: .

[0081] Step 203: The strain measured in the experiment is the sum of thermal strain, free expansion strain, and the strain gauge's own output strain. Calculate the metal's thermal strain as follows: The thermal strain of composite materials is The thermal strain of a structure can be obtained by subtracting the strain measured by the thermal stress test of the corresponding material and the strain measured by the thermal stress test of the laminate level. This not only eliminates the influence of free expansion of the material, but also eliminates the influence of strain gauge measurement.

[0082] Step 204: Obtain the thermal strain of the structure. , The results were compared with those in the thermal stress finite element model to verify the effectiveness of the finite element analysis method.

[0083] Example 2. (As shown) Figures 1 to 2 The specific implementation steps of the method for measuring the thermal stress of the composite material-metal connection structure shown are as follows:

[0084] (1) Statistically analyze the configurations of hybrid connection structures on aircraft, and select typical connection configurations on aircraft as samples or test pieces for this scheme based on the thickness of the metal and composite materials, the composite material layup, and the bolt type. For example, if the metal material is aluminum alloy, the thickness... The composite material is a unidirectional carbon fiber tape laminate with a thickness of [missing information]. The layup sequence is [45 / -45 / 0 / 90]. 2s Bolt diameter D.

[0085] (2) The test temperature load is determined by the aircraft’s operating temperature or storage temperature, which is -55℃ to 70℃.

[0086] (3) Three levels of tests are carried out in sequence. Among them, the thermal stress test of the plate level can be carried out at the same time as the thermal stress test of the hybrid connection structure, as an accompanying component of the thermal stress test of the connection structure.

[0087] I. Material-level thermal expansion coefficient test

[0088] Step 1: Specimen design for thermal expansion test:

[0089] Three aluminum alloy samples, and composite material 0° and 90° unidirectional laminates with [45 / -45 / 0 / 90] angles. 2s Three samples of multidirectional laminate were prepared for each batch of materials, with each sample measuring approximately 20mm × 5mm.

[0090] Step 2: Conduct a standard thermal expansion coefficient measurement test:

[0091] Step 201: Using a standard quartz sample, perform baseline calibration using the same test parameters as for the metal / composite material sample test.

[0092] Step 202: Measure the initial length L of the sample in the direction of expansion test, and the ambient temperature (20~25℃).

[0093] Step 203: Place the sample under the probe inside the container, and place the temperature sensor so that it is in contact with or as close as possible to the sample. Apply a load of 1mN to 100mN to the sensor to ensure that the sensor is in contact with the sample.

[0094] Step 204: Cool down to the lowest temperature, then heat up to the highest temperature at a suitable heating rate, and record the sample length-temperature curve.

[0095] Step 3: Calculate the coefficient of thermal expansion:

[0096] According to the formula The coefficient of thermal expansion of the material is calculated using the formula: ΔL represents the change in length of the sample along the measurement direction, and ΔT represents the corresponding temperature difference. The coefficient of thermal expansion of the aluminum alloy in the connecting structure is then obtained. The coefficient of thermal expansion of the composite material in the 0° and 90° directions is . and As input to the finite element analysis model, the coefficient of thermal expansion of the composite laminate is... .

[0097] II. Laminate-level thermal stress test

[0098] Step 1: Design of Layer-Level Specimens

[0099] Metal sample with [45 / -45 / 0 / 90] 2s Two samples of multi-directional laminated sheets were prepared, each approximately 100mm × 50mm in size.

[0100] Step 2: Conduct layer-level thermal stress tests

[0101] Step 201: Attach strain gauges and thermocouples to the specimen. The operating temperature range of the strain gauges and thermocouples should cover the test temperature. The strain gauges should be attached back-to-back, both laterally and longitudinally. Thermocouples should be attached to both metal and composite specimens. Generally, the temperature change of metals is faster than that of composite materials, but this is also related to the specimen thickness. Refer to the following for the arrangement of strain and temperature measurement points. Figure 1 .

[0102] Step 202: Place the laminate-level specimen freely in the environmental chamber and measure the strain at room temperature, then zero the strain value. For the low-temperature test, gradually decrease the temperature from room temperature (20~25℃) to -55℃ (for the high-temperature test, raise the temperature to 70℃), with a temperature gradient of -5℃ or -10℃. When the environmental chamber reaches each temperature level (10℃, 0℃... -50℃, -55℃), maintain the temperature in the environmental chamber until the specimen temperature stabilizes within ±1℃ of the target temperature. Measure the temperature and strain of the specimen to obtain the strain of the metallic specimen. -Temperature profile, strain of composite sample -Temperature curve, repeat the experiment twice.

[0103] III. Thermal Stress Test of Connection Structure

[0104] Step 1: Design of test specimens for connecting structural components

[0105] The thickness of the metal and composite materials, the composite layering, and the spacing of bolts and nails are consistent with typical connection structures on aircraft, and the aluminum alloy thickness is also consistent. Composite thickness The layup sequence is [45 / -45 / 0 / 90]. 2sThe bolt diameter is D, the bolt spacing is approximately 5D, and the bolt edge distance is approximately 4 / 5D. The bolt spacing and edge distance must meet the space requirements for strain gauge mounting. (Refer to...) Figure 2 .

[0106] Step 2: Conduct thermal stress tests

[0107] Step 201: Strain gauges are attached between the metal and composite surfaces of the test specimen and the nails, positioned at the center of the nails. Thermocouples are attached to both the metal and composite surfaces to measure the temperature of the test specimen. (Refer to...) Figure 2 .

[0108] Step 202: The test specimen is placed freely in the environmental chamber. Before the formal test, the low temperature test load is directly applied from room temperature to -55℃ (high temperature test load is applied to 70℃). After the test specimen temperature stabilizes, it is kept at that temperature for 3 minutes and then unloaded to room temperature to eliminate the influence of gaps between the connecting structures.

[0109] Step 203: Formal Test. Measure strain at room temperature and zero the strain value. For the low-temperature test, gradually decrease the temperature from room temperature (20~25℃) to -55℃ (for the high-temperature test, raise the temperature to 70℃), with a temperature gradient of -5℃ or -10℃. When the environmental chamber reaches each temperature level (10℃, 0℃... -50℃, -55℃), maintain the temperature in the environmental chamber until the temperature of both the metal and composite material stabilizes within ±1℃ of the target temperature. Measure the temperature and strain of the sample. The strain measured for the metal sample is... The strain measured on the composite sample was as follows: Repeat the experiment 3 times.

[0110] (4) Experimental data processing

[0111] Step 1: Discard data with poor repeatability from the test strain data, and take the average value of the data obtained from multiple test samples or repeated tests as the result.

[0112] Step 2: Subtract the free expansion strain of the metal from the measured strain of the laminate-level thermal stress test to obtain the strain output of the strain gauge itself. To determine the influence of strain gauge measurements on the test results, the free expansion strain of the composite laminate is... The results should be consistent with the coefficient of thermal expansion of the composite material. near.

[0113] Step 3: Subtract the strain measured from the strain of the hybrid connection structure's metal and composite materials from the strain measured by the lamination-level thermal stress test. The thermal strain of the metal is then obtained. The thermal strain of composite materials is .

[0114] Step four: Compare the structural thermal strain calculated in step three with the elastic strain results in the finite element model to further verify the effectiveness of the analysis method.

[0115] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements without departing from the principle of the present invention, and these improvements should also be considered within the scope of protection of the present invention.

Claims

1. A method for measuring and analyzing thermal stress in composite and metal connection structures, characterized in that, The test includes three levels: material-level thermal expansion coefficient test to obtain the thermal expansion coefficient of metals and composites; plate-level thermal stress test as a calibration test; and connection structure thermal stress test to obtain the total strain of the structure.

2. The method for measuring and analyzing thermal stress in composite and metal connection structures according to claim 1, characterized in that, The test procedure for the coefficient of thermal expansion of materials is as follows: S11. Design test specimens based on the material at the connection between the composite material and the metal: n metal specimens, and n specimens of 0° and 90° unidirectional laminates and multidirectional laminates for each material batch. S12. Conduct standard thermal expansion coefficient measurement tests using the sample; S13. Calculate the coefficient of thermal expansion of each sample using the following general formula: a m =ΔL m / (L m ×ΔT m ) α1 = ΔL1 / (L1 × ΔT1) α2 = ΔL2 / (L2 × ΔT2) In the formula: α m α1 and α2 are the coefficients of thermal expansion of the metal sample, the 0° unidirectional laminate sample, and the 90° unidirectional laminate sample, respectively. ΔL m ΔL1 and ΔL2 represent the length changes in the expansion direction of the corresponding sample, respectively. L m L1 and L2 are the initial lengths of the corresponding sample in the expansion direction, respectively; ΔT m ΔT1 and ΔT2 are the test temperature differences; Using α1 and α2 as inputs to the finite element analysis model, the coefficient of thermal expansion α of the multidirectional laminate specimen of the composite material was obtained. c .

3. The method for measuring and analyzing thermal stress in composite and metal connection structures according to claim 2, characterized in that, In S11, the sample size is 20mm × 5mm.

4. The method for measuring and analyzing thermal stress in composite and metal connection structures according to claim 2, characterized in that, S12 is as follows: S121. Use a standard quartz sample and the same test parameters as the sample test to perform baseline calibration; S122. Measure the initial length L of the sample expansion test direction at ambient temperature; S123. Place the sample under the probe inside the container and place the temperature sensor in contact with the sample; S124. Cool down to the lowest temperature of the test, then heat up to the highest temperature of the test at a preset heating rate, and record the sample length-temperature curve.

5. The method for measuring and analyzing thermal stress in composite and metal connection structures according to claim 4, characterized in that, In S123, a load of 1mN to 100mN is applied to the temperature sensor to ensure that the sensor is in contact with the sample.

6. The method for measuring and analyzing thermal stress in composite and metal connection structures according to claim 1, characterized in that, The procedure for layer-level thermal stress testing is as follows: S21. Design laminate-level specimens based on the material and ply angle at the connection between the composite material and the metal: n metal specimens, and n composite multi-directional laminate specimens with the same composite ply at the connection, each ply having n specimens. S22. Conduct laminate-level thermal stress tests: S221. Strain gauges and thermocouples are attached to the specimen to measure the temperature and strain of the specimen; S222. The sample temperature is gradually increased from the ambient temperature to the highest test temperature or decreased to the lowest test temperature, with a temperature gradient of ±5℃ or ±10℃. Upon reaching each temperature level, the sample temperature is allowed to stabilize within the allowable error range before measuring the sample temperature and strain: the strain of the metallic sample is... The strain of the multi-directional laminate specimen of the composite material is ; S223. Plot the strain-temperature curves for each specimen.

7. The method for measuring and analyzing thermal stress in composite and metal connection structures according to claim 6, characterized in that, In S21, the sample size is 100mm × 50mm.

8. The method for measuring and analyzing thermal stress in composite and metal connection structures according to claim 6, characterized in that, In S223, when plotting the strain-temperature curve of the multidirectional laminate specimen, the strain ε of the composite laminate under free expansion is used as the basis. c ´drawing, ε c Calculate using the following formula: e c ´ = e c - eh gague ≈ a c × ΔT e gague = e m - a m × ΔT In the formula, ε gague For the strain output of the strain gauge itself, α c Let α be the coefficient of thermal expansion of the multi-directional laminate specimen. m ΔT is the coefficient of thermal expansion of the metal sample, and ΔT is the test temperature difference.

9. The method for measuring and analyzing thermal stress in composite and metal connection structures according to claim 1, characterized in that, The thermal stress test procedure for the connection structure is as follows: S31. The connection structure test piece is designed as a component-level test piece. In the component-level test piece, the connection between the metal sample and the composite multi-directional laminate sample has 7 or more nails. The thickness of the metal and the composite, the composite layup, the bolts, and the nail spacing are consistent with the actual connection structure. S32. Conduct thermal stress tests: S321. Strain gauges are attached between the nails on the test specimen, and thermocouples are attached to both the metal and composite surfaces to measure the temperature of the test specimen. S322. The test specimen is placed freely in the environmental chamber. Before the formal test, the temperature load is directly applied from the ambient temperature to the highest / lowest temperature in the test temperature. After the temperature of the test specimen stabilizes, it is kept at the temperature for 3 minutes and then unloaded to the ambient temperature. S323. Formal Test: The temperature is gradually increased from ambient temperature to the highest temperature or decreased to the lowest temperature, with a temperature gradient of ±5℃ or ±10℃. Upon reaching each temperature level, after the temperatures of both the metal and composite material have stabilized within the allowable error range, the temperature and strain of the test specimen are measured. The strain of the metal specimen is... The strain of the multi-directional composite laminate specimen is ; S324. Calculate the thermal strain ε of the metal specimen using the following formula. m L Thermal strain ε of multi-directional composite laminate specimen c L : e m L =e m t - eh gague - a m × ΔT=ε m t - eh m e c L =e c t - eh gague - eh c ´ =e c t - eh c In the formula, ε gague For the strain output of the strain gauge itself, α m ε is the coefficient of thermal expansion of the metal sample, ΔT is the test temperature difference, and ε m ε is the strain of the metal specimen. c The free expansion strain of the multi-directional laminate specimen, ε c The strain of the multi-directional laminate specimen; S325. The structural thermal strain will be obtained. , The results were compared with those in the thermal stress finite element model to verify the effectiveness of the finite element analysis method.

10. The method for measuring and analyzing thermal stress in composite and metal connection structures according to claim 9, characterized in that, In S31, the design of the component-level test piece is as follows: The thickness of the metal and composite materials, the composite layering, and the spacing of bolts and nails are consistent with typical connection structures on aircraft. The thickness of the metal specimen is also consistent. Thickness of multi-directional composite laminate sample The layup sequence is [45 / -45 / 0 / 90]. 2s The connector diameter is D, the nail spacing is 5D, and the nail edge distance is 0.8D. The nail spacing and edge distance meet the space requirements for strain gauge bonding.

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