Method for measuring elastic modulus of unidirectional carbon fiber composite material

Abstract

The invention discloses a method for measuring the elastic modulus of a unidirectional carbon fiber composite material. Whether the carbon fiber and the matrix in the unidirectional carbon fiber composite material to be detected are completely bonded or not is judged according to the error between the longitudinal elastic modulus obtained by test data and the empirical elastic modulus obtained by material parameters under the normal temperature condition, if not, the elastic modulus is corrected and compensated, and if the carbon fiber and the matrix are completely bonded, the carbon fiber and the matrix in the unidirectional carbon fiber composite material to be detected are not completely bonded. Carrying out unidirectional tensile tests at different temperatures; wherein the tensile deformation of the sample does not exceed 0.5%, and the sample is cooled by spraying a preset-temperature mixture composed of liquid nitrogen and air through an air atomizing nozzle. When the elastic modulus is measured, the influence of the bonding state between the carbon fiber and the matrix on the test result is considered, the elastic modulus measurement at different temperatures can be realized, and the material consumption is low.

Description

Technical Field

[0001] The invention belongs to the field of mechanical property detection of composite materials, and particularly relates to a method for measuring the elastic modulus of a unidirectional carbon fiber composite material. Background Art

[0002] With the continuous development of advanced manufacturing, carbon fiber composites have been widely used in aerospace, automobile, wind energy and other fields due to their excellent strength, stiffness, low density and corrosion resistance. Unidirectional carbon fiber composites, in particular, have great potential in mechanical properties. The mechanical properties of unidirectional carbon fiber composites are mainly affected by the direction of the carbon fibers. The elastic modulus, as an important parameter to characterize the stiffness of the material, is crucial for the design, optimization and application of these composites. In unidirectional carbon fiber composites, the bonding quality between the fiber and the matrix will directly affect the test results of the elastic modulus. The traditional elastic modulus test method does not consider the impact of interfacial debonding or fiber slip between the fiber and the matrix on the test results, and the tensile test will completely destroy the sample. When performing multiple elastic modulus tests, the sample needs to be repeatedly replaced, and the amount of consumables is high. In addition, the existing low-temperature tensile test only adjusts the sample temperature by the temperature of a single liquefied gas, which is difficult to accurately and quickly adjust to the required sample temperature. Summary of the Invention

[0003] The purpose of the present invention is to overcome the shortcomings of the prior art and to provide a method for measuring the elastic modulus of unidirectional carbon fiber composite materials.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] The elastic modulus measurement method of the unidirectional carbon fiber composite material of the present invention is as follows:

[0006] Step 1: Under room temperature conditions, process the unidirectional carbon fiber composite material to be tested into a circular specimen and polish the edge of the specimen.

[0007] Step 2: Paste a strain gauge at the center of the sample, and the strain gauge is parallel to the carbon fiber arrangement direction; use the two clamps of the tensile testing machine to clamp the sample, so that the tensile direction of the tensile test is parallel to the carbon fiber arrangement direction, and make the two clamps symmetrical about the center of the sample.

[0008] Step 3: Conduct a uniaxial tensile test. A strain gauge measures the strain of the specimen under the tensile load. After the test, the tensile tester releases the tensile load, returning the specimen to its pre-tensile deformation state. The tensile deformation of the specimen during the uniaxial tensile test does not exceed 0.5%. Next, the elastic modulus (E) of the uniaxial carbon fiber composite material is calculated based on the tensile force applied during the test and the strain measured by the strain gauge.

[0009] Step 4: Repeat step 3 multiple times, take the average of the elastic modulus E of the unidirectional carbon fiber composite material obtained each time, and obtain the elastic modulus E0 of the unidirectional carbon fiber composite material under the tensile test.

[0010] Step 5: Calculate the empirical elastic modulus E1 of the unidirectional carbon fiber composite material based on the material parameters.

[0011] Step 6. Calculate the error between the elastic modulus E0 and the elastic modulus E1; if the error is greater than a preset value, it is determined that the carbon fibers and the matrix in the unidirectional carbon fiber composite material are not completely bonded, and there is interface debonding or fiber slippage, and the elastic modulus in the longitudinal direction of the unidirectional carbon fiber composite material is corrected and compensated; otherwise, it is determined that the carbon fibers and the matrix in the unidirectional carbon fiber composite material are completely bonded, and the elastic modulus E0 is used as the elastic modulus of the unidirectional carbon fiber composite material in the longitudinal direction.

[0012] Step 7. For unidirectional carbon fiber composite materials with complete bonding between carbon fiber and matrix, set a temperature range and select multiple temperatures, and gradually perform unidirectional tensile tests at each temperature from high to low. The specific process of performing unidirectional tensile tests at each temperature is as follows: spray a mixture of liquid nitrogen and air into the insulation box of the tensile testing machine through an air atomizing nozzle to cool the sample so that the sample temperature drops to the selected temperature, and then perform steps 3 and 4; after performing unidirectional tensile tests at each selected temperature, the elastic modulus of the unidirectional carbon fiber composite material with complete bonding between carbon fiber and matrix in the longitudinal direction at each selected temperature is obtained.

[0013] Step 8. Fit the selected temperatures to the elastic modulus in the longitudinal direction of the unidirectional carbon fiber composite material with complete bonding between the corresponding carbon fibers and the matrix, and obtain a functional representation of the elastic modulus in the longitudinal direction of the unidirectional carbon fiber composite material with complete bonding between the carbon fibers and the matrix and temperature.

[0014] Preferably, in step 1, the edge of the sample is polished with wet sandpaper, and after polishing, the surface of the sample is dried in the shade at room temperature.

[0015] Preferably, the elastic modulus E of the unidirectional carbon fiber composite material calculated according to the tensile force F applied during the unidirectional tensile test and the strain ε measured by the strain gauge is

[0016]

[0017] Where σ is stress;

[0018]

[0019] Where A is the average cross-sectional area of the specimen between the two clamps, R is the radius of the specimen, t is the distance from the center of the specimen to the clamps, and b is the thickness of the specimen.

[0020] Preferably, the empirical elastic modulus E1 of the unidirectional carbon fiber composite material calculated according to the material parameters is

[0021] E1=λv f E f +(1-v f )E m

[0022] Where λ is the fiber length correction factor, v f is the volume ratio of carbon fiber in unidirectional carbon fiber composite materials, E f is the elastic modulus of carbon fiber, E m is the elastic modulus of the matrix material;

[0023]

[0024] Where d is the diameter of the sample, d = 2R, and the limit is

[0025] Preferably, the elastic modulus of the unidirectional carbon fiber composite material in the longitudinal direction after correction and compensation is

[0026]

[0027] Where η is the interface effect correction coefficient, and ξ is the slip correction coefficient introduced to consider the effect of slip between carbon fiber and matrix on the elastic modulus of unidirectional carbon fiber composite material, and considering the effect of fiber slip on matrix shear modulus and the effect of sample diameter and thickness on tensile results, it is set Interface slip parameters G m is the shear modulus of the matrix, τ is the shear stress, for uniaxial tensile test, Diameter to thickness ratio of the specimen

[0028] Preferably, the cooling process of the sample is divided into three stages: in stage one, the temperature of the mixture sprayed from the air atomizing nozzle is made 5% to 10% lower than the selected temperature; in stage two, when the difference between the surface temperature of the sample and the selected temperature is less than 10% of the selected temperature, the temperature of the mixture sprayed from the air atomizing nozzle is adjusted to the selected temperature; in stage three, when the surface temperature of the sample is cooled to the selected temperature, the selected temperature is maintained in the insulation box for a preset time.

[0029] More preferably, the temperature of the mixture sprayed from the air atomizing nozzle and the surface temperature of the sample are detected in real time by thermocouples, and the temperature of the mixture sprayed from the air atomizing nozzle T m for

[0030]

[0031] Where m1, c1 and T1 are the mass flow rate, specific heat capacity and temperature of liquid nitrogen before mixing; m2, c2 and T2 are the mass flow rate, specific heat capacity and temperature of air before mixing;

[0032] The temperature of the mixture sprayed from the air atomizing nozzle in each stage is adjusted by controlling the mass flow rate of liquid nitrogen or air in the corresponding stage. When the temperature of the mixture sprayed from the air atomizing nozzle is greater than the set temperature or temperature range of the stage, the mass flow rate of liquid nitrogen is adjusted to increase. When the temperature of the mixture sprayed from the air atomizing nozzle is lower than the set temperature or temperature range of the stage, the mass flow rate of air is adjusted to increase.

[0033] Preferably, the temperature of the mixture sprayed from the air atomizing nozzle is initially set to -100°C, and the mass flow ratio of liquid nitrogen to air is set to 1:3.3.

[0034] The present invention has the following beneficial effects:

[0035] 1. The present invention takes into account the influence of the bonding state between the carbon fiber and the matrix on the test results when measuring the elastic modulus of the unidirectional carbon fiber composite material to be tested, and can also realize the measurement of the elastic modulus of the unidirectional carbon fiber composite material to be tested at different temperatures, and the amount of consumables is low. Specifically, the present invention compares the longitudinal (tensile direction is parallel to the carbon fiber arrangement direction) elastic modulus calculated according to the test data under normal temperature conditions with the empirical elastic modulus calculated according to the material parameters to determine whether there is interface debonding or fiber slippage between the carbon fiber and the matrix in the unidirectional carbon fiber composite material to be tested. If there is interface debonding or fiber slippage between the carbon fiber and the matrix in the unidirectional carbon fiber composite material to be tested, the elastic modulus in the longitudinal direction of the unidirectional carbon fiber composite material is corrected and compensated. Otherwise, the elastic modulus in the longitudinal direction calculated according to the test data is used as the elastic modulus in the longitudinal direction of the unidirectional carbon fiber composite material under normal temperature conditions, and unidirectional tensile tests are carried out at different temperatures to obtain the elastic modulus of the unidirectional carbon fiber composite material. The elastic modulus of the composite material at different temperatures is measured. Furthermore, in the present invention, a mixture of liquid nitrogen and air is sprayed into the insulated box of the tensile testing machine through an air atomizing nozzle to cool the sample. Compared with the existing method of cooling by a single liquefied gas, the temperature of the mixture can be accurately and quickly adjusted by controlling the mass flow rates of liquid nitrogen and air, thereby realizing unidirectional tensile testing at different temperatures. In addition, the elastic deformation range of the carbon fiber material is 0.5%-1%. In the present invention, the tensile deformation of the sample does not exceed 0.5% during the unidirectional tensile test, ensuring that the tensile deformation of the sample is elastic deformation, ensuring that the result of the next tensile test is not affected by the previous unidirectional tensile test, reducing the number of sample replacements, and low consumables.

[0036] 2. The present invention divides the cooling process of the sample into three stages: the first stage is a rapid cooling stage, in which the temperature of the mixture sprayed from the air atomizing nozzle is 5% to 10% lower than the selected temperature, and the sample surface is quickly cooled; the second stage is a slow cooling stage, when the difference between the sample surface temperature and the selected temperature is less than 10% of the selected temperature, the temperature of the mixture sprayed from the air atomizing nozzle is adjusted to the selected temperature, and the sample surface is cooled; the third stage is a heat preservation stage, when the sample surface temperature is cooled to the selected temperature, the selected temperature is maintained in the heat preservation box for a preset time, and the internal temperature of the sample is consistent with the surface temperature through long-term heat preservation, thereby reducing the error of the tensile test results. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of a tensile testing machine clamping a circular specimen.

[0038] Figure 2 for Figure 1 side view. DETAILED DESCRIPTION

[0039] The elastic modulus measurement method of the unidirectional carbon fiber composite material of the present invention is as follows:

[0040] Step 1: At room temperature (25°C), the unidirectional carbon fiber composite material to be tested is processed into a circular specimen. To avoid stress concentration during the test, the edge of the specimen is polished with water sandpaper. After polishing, the surface of the specimen is dried in the shade at room temperature to avoid freezing and expansion of the water remaining on the surface of the specimen due to the temperature drop during the subsequent unidirectional tensile test in a low-temperature environment, which affects the tensile test results.

[0041] Step 2: Paste the strain gauge at the center of the sample, and the strain gauge is parallel to the carbon fiber arrangement direction. Use the two clamps of the tensile testing machine to clamp the sample, so that the tensile direction of the tensile test is parallel to the carbon fiber arrangement direction, and make the two clamps symmetrical about the center of the sample to avoid the influence of the tensile test results due to misalignment. Figure 1 and Figure 2 shown.

[0042] Step three, carry out a unidirectional tensile test, and use a strain gauge to measure the strain value of the sample under the tensile load. After completing the unidirectional tensile test, the tensile testing machine releases the tensile load applied to the sample to restore the sample to its state before tensile deformation. When performing unidirectional tensile testing, the tensile loading rate is set to 2mm / min in accordance with the GB / T3354-2014 standard. The elastic deformation range of carbon fiber materials is generally between 0.5% and 1%. In order to ensure that the results of the next tensile test are not affected by the previous unidirectional tensile test, the tensile deformation of the sample during the unidirectional tensile test is set to no more than 0.5%, ensuring that the tensile deformation of the sample is elastic deformation. Next, based on the tensile force F applied during the unidirectional tensile test (which can be directly obtained by the tensile testing machine) and the strain ε measured by the strain gauge, the elastic modulus E of the unidirectional carbon fiber composite material is calculated:

[0043]

[0044] Where σ is stress;

[0045]

[0046] Where A is the average cross-sectional area of the specimen between the two clamps, R is the radius of the specimen, t is the distance from the center of the specimen to the clamps, and b is the thickness of the specimen. b and t can be measured before the uniaxial tensile test.

[0047] Step 4: Repeat step 3 multiple times, take the average of the elastic modulus E of the unidirectional carbon fiber composite material obtained each time, and obtain the elastic modulus E0 of the unidirectional carbon fiber composite material under the tensile test.

[0048] Step 5. Calculate the empirical elastic modulus E1 of the unidirectional carbon fiber composite material based on the material parameters:

[0049] E1=λv f E f +(1-v f )E m

[0050] Where λ is the fiber length correction factor, v f is the volume ratio of carbon fiber in unidirectional carbon fiber composite materials, E f is the elastic modulus of carbon fiber, E m It is the elastic modulus of the matrix material (the material other than carbon fiber in the unidirectional carbon fiber composite material).

[0051] Among them, since the elastic modulus of the composite material (referring to the longitudinal elastic modulus) is also affected by the fiber aspect ratio, when the fiber aspect ratio is less than 10, the elastic modulus of the composite material increases rapidly with the increase of the fiber aspect ratio. When the fiber aspect ratio is greater than 10, the elastic modulus of the composite material increases gently to a certain value with the increase of the fiber aspect ratio. Therefore, the fiber length correction factor λ is substituted into the sample. Since the carbon fiber lengths at different positions in the sample are inconsistent, the present invention homogenizes the unidirectional carbon fiber composite material and replaces the fiber aspect ratio in the existing fiber length correction factor formula with the diameter-to-thickness ratio of the sample to obtain

[0052]

[0053] Where d is the diameter of the sample, d = 2R, and the limit is

[0054] Step 6. Calculate the error between the elastic modulus E0 and the elastic modulus E1. If the error is within 10%, it is judged that the carbon fiber and the matrix in the unidirectional carbon fiber composite material are completely bonded, and the elastic modulus E0 is used as the elastic modulus of the unidirectional carbon fiber composite material in the longitudinal direction (the tensile direction is parallel to the carbon fiber direction); if the error is greater than 10%, it is judged that the carbon fiber and the matrix in the unidirectional carbon fiber composite material are not completely bonded, and there is interface debonding or fiber slippage. The elastic modulus of the unidirectional carbon fiber composite material in the longitudinal direction is corrected using the effective medium theory. After correction and compensation, the elastic modulus of the unidirectional carbon fiber composite material in the longitudinal direction is

[0055]

[0056] Among them, η is the interface effect correction coefficient, which is used to correct the influence of the interface between the fiber and the matrix on the elastic modulus of the unidirectional carbon fiber composite material, and ξ is the slip correction coefficient introduced to consider the effect of slip between carbon fiber and matrix on the elastic modulus of unidirectional carbon fiber composites. It is used to quantify the effect of slip between carbon fiber and matrix on the elastic modulus of unidirectional carbon fiber composites. In addition, considering that fiber slip has a greater effect on the matrix shear modulus, and considering the influence of the diameter and thickness of the specimen on the tensile results, it is set Interface slip parameters G m is the shear modulus of the matrix, which can be obtained by querying the data, and τ is the shear stress. For the uniaxial tensile test, the shear stress is half of the stress measured in the tensile test, that is, Diameter to thickness ratio of the specimen

[0057] Step 7. Under low temperature conditions, the difference in thermal expansion coefficient between the carbon fiber and the matrix is large, which will amplify the original defects between the carbon fiber and the matrix. The distribution of stress between the carbon fiber and the matrix will become unstable and difficult to predict, seriously affecting the accuracy of the test results. Therefore, when the carbon fiber and the matrix are not completely bonded in the unidirectional carbon fiber composite material, the unidirectional tensile test at different temperatures is not performed. For the unidirectional carbon fiber composite material with complete bonding between the carbon fiber and the matrix, a temperature range is set, and multiple temperatures are selected. The unidirectional tensile test is gradually performed at each temperature from high to low. The specific process of performing the unidirectional tensile test at each temperature is as follows: a mixture of liquid nitrogen and air is sprayed into the insulation box of the tensile testing machine through an air atomizing nozzle to cool the sample until the sample temperature drops to the selected temperature, and then steps 3 and 4 are performed; after performing the unidirectional tensile test at each selected temperature, the elastic modulus of the unidirectional carbon fiber composite material in the longitudinal direction with complete bonding between the carbon fiber and the matrix at each selected temperature is obtained.

[0058] Among them, in order to improve the cooling efficiency of the sample, the cooling process is divided into three stages: the first stage is the rapid cooling stage, in which the temperature of the mixture sprayed by the air atomizing nozzle is 5% to 10% lower than the selected temperature; the second stage is the slow cooling stage. When the difference between the surface temperature of the sample and the selected temperature is less than 10% of the selected temperature, the temperature of the mixture sprayed by the air atomizing nozzle is adjusted to the selected temperature, and the surface of the sample is cooled using the mixture of the selected temperature; the third stage is the insulation stage. Since the carbon fiber material is anisotropic, there is a difference in the heat conduction rate along the fiber direction and the perpendicular fiber direction. When the surface temperature of the sample is cooled to the selected temperature, the selected temperature is maintained in the insulation box for 5-10 minutes. The long-term insulation ensures that the internal temperature of the sample is consistent with the surface temperature, thereby reducing the error of the tensile test. The temperature of the mixture sprayed by the air atomizing nozzle and the surface temperature of the sample are detected in real time by thermocouples, and the temperature of the mixture sprayed by the air atomizing nozzle T m for

[0059]

[0060] Where m1, c1 and T1 are the mass flow rate, specific heat capacity and temperature of liquid nitrogen before mixing; m2, c2 and T2 are the mass flow rate, specific heat capacity and temperature of air before mixing.

[0061] The temperature of the mixture ejected from the air atomizing nozzle at each stage is adjusted by controlling the mass flow rate of liquid nitrogen or air in the corresponding stage. When the temperature of the mixture ejected from the air atomizing nozzle is greater than the set temperature or temperature range for that stage, the mass flow rate of liquid nitrogen is increased. When the temperature of the mixture ejected from the air atomizing nozzle is less than the set temperature or temperature range for that stage, the mass flow rate of air is increased. In this embodiment, the initial temperature of the mixture ejected from the air atomizing nozzle is set to -100°C, and the mass flow rate ratio of liquid nitrogen to air is 1:3.3.

[0062] In addition, the air atomizing nozzle sprays the mixture at an upward angle of 45 degrees to avoid the air atomizing nozzle being directly aimed at the sample surface. An insulation layer is set on the outside of the control valve of the liquid nitrogen delivery pipeline to prevent water vapor in the air from freezing on the control valve and affecting the control results. A drying system is set in the pipe of the air delivery pipeline to prevent a large amount of water vapor in the mixture from affecting the sample and further affecting the tensile test results. Silica gel is added to the insulation box to absorb water vapor in the insulation box to further avoid the influence of water vapor on the tensile test results.

[0063] Step 8. Fit the selected temperatures to the elastic modulus in the longitudinal direction of the unidirectional carbon fiber composite material with complete bonding between the corresponding carbon fibers and the matrix, and obtain a functional representation of the elastic modulus in the longitudinal direction of the unidirectional carbon fiber composite material with complete bonding between the carbon fibers and the matrix and temperature.

Claims

1. A method for measuring the elastic modulus of unidirectional carbon fiber composite materials, characterized by: The details are as follows: Step 1: Under normal temperature conditions, process the unidirectional carbon fiber composite material to be tested into a circular specimen and polish the edge of the specimen; Step 2: Paste a strain gauge at the center of the sample, and the strain gauge is parallel to the carbon fiber arrangement direction; use the two clamps of the tensile testing machine to clamp the sample, so that the tensile direction of the tensile test is parallel to the carbon fiber arrangement direction, and the two clamps are symmetrical about the center of the sample; Step 3: Perform a uniaxial tensile test, using a strain gauge to measure the strain of the sample under the tensile load. After the uniaxial tensile test is completed, the tensile testing machine releases the tensile load applied to the sample to restore the sample to its pre-tensile deformation state. The tensile deformation of the sample during the uniaxial tensile test does not exceed 0.5%. Next, calculate the elastic modulus E of the uniaxial carbon fiber composite material based on the tensile force applied during the uniaxial tensile test and the strain measured by the strain gauge. Step 4: Repeat step 3 multiple times, take the average of the elastic modulus E of the unidirectional carbon fiber composite material obtained each time, and obtain the elastic modulus E0 of the unidirectional carbon fiber composite material under the tensile test; Step 5, calculating the empirical elastic modulus E1 of the unidirectional carbon fiber composite material based on the material parameters; Step 6. Calculate the error between the elastic modulus E0 and the elastic modulus E1; if the error is greater than a preset value, it is determined that the carbon fibers and the matrix in the unidirectional carbon fiber composite material are not fully bonded, and there is interface debonding or fiber slippage, and the elastic modulus in the longitudinal direction of the unidirectional carbon fiber composite material is corrected and compensated; otherwise, it is determined that the carbon fibers and the matrix in the unidirectional carbon fiber composite material are fully bonded, and the elastic modulus E0 is used as the elastic modulus of the unidirectional carbon fiber composite material in the longitudinal direction; Step 7. For a unidirectional carbon fiber composite material with complete bonding between the carbon fibers and the matrix, set a temperature range and select multiple temperatures, and gradually perform unidirectional tensile tests at each temperature from high to low. The specific process of performing the unidirectional tensile test at each temperature is as follows: spray a mixture of liquid nitrogen and air into an insulation box of a tensile testing machine through an air atomizing nozzle to cool the sample until the sample temperature drops to a selected temperature, and then perform steps 3 and 4; after performing unidirectional tensile tests at each selected temperature, obtain the elastic modulus in the longitudinal direction of the unidirectional carbon fiber composite material with complete bonding between the carbon fibers and the matrix at each selected temperature; Step 8. Fit the selected temperatures to the elastic modulus in the longitudinal direction of the unidirectional carbon fiber composite material with complete bonding between the corresponding carbon fibers and the matrix, and obtain a functional representation of the elastic modulus in the longitudinal direction of the unidirectional carbon fiber composite material with complete bonding between the carbon fibers and the matrix and temperature.

2. The method for measuring the elastic modulus of a unidirectional carbon fiber composite material according to claim 1, wherein: In the step 1, the edge of the sample is polished with water sandpaper, and after polishing, the surface of the sample is dried in the shade at room temperature.

3. The method for measuring the elastic modulus of a unidirectional carbon fiber composite material according to claim 1, wherein: The elastic modulus E of the unidirectional carbon fiber composite material is calculated based on the tensile force F applied during the unidirectional tensile test and the strain ε measured by the strain gauge: Where σ is stress; Where A is the average cross-sectional area of the specimen between the two clamps, R is the radius of the specimen, t is the distance from the center of the specimen to the clamps, and b is the thickness of the specimen.

4. The method for measuring the elastic modulus of a unidirectional carbon fiber composite material according to claim 1, wherein: The empirical elastic modulus E1 of the unidirectional carbon fiber composite material calculated according to the material parameters is: E1=λv f E f +(1-v f )E m Where λ is the fiber length correction factor, v f is the volume ratio of carbon fiber in unidirectional carbon fiber composite materials, E f is the elastic modulus of carbon fiber, E m is the elastic modulus of the matrix material; Where d is the diameter of the sample, d = 2R, and the limit is 5. The method for measuring the elastic modulus of a unidirectional carbon fiber composite material according to claim 1, wherein: The elastic modulus of the unidirectional carbon fiber composite material in the longitudinal direction after correction and compensation is: Where η is the interface effect correction coefficient, and ξ is the slip correction coefficient introduced to consider the effect of slip between carbon fiber and matrix on the elastic modulus of unidirectional carbon fiber composite material, and considering the effect of fiber slip on matrix shear modulus and the effect of sample diameter and thickness on tensile results, it is set Interface slip parameters G m is the shear modulus of the matrix, τ is the shear stress, for uniaxial tensile test, Diameter to thickness ratio of the specimen 6. The method for measuring the elastic modulus of a unidirectional carbon fiber composite material according to claim 1, wherein: The cooling process of the sample is divided into three stages: in stage one, the temperature of the mixture sprayed from the air atomizing nozzle is made 5% to 10% lower than the selected temperature; in stage two, when the difference between the surface temperature of the sample and the selected temperature is less than 10% of the selected temperature, the temperature of the mixture sprayed from the air atomizing nozzle is adjusted to the selected temperature; in stage three, when the surface temperature of the sample is cooled to the selected temperature, the selected temperature is maintained in the insulation box for a preset time.

7. The method for measuring the elastic modulus of a unidirectional carbon fiber composite material according to claim 6, wherein: The temperature of the mixture sprayed from the air atomizing nozzle and the surface temperature of the sample are detected in real time by thermocouples, and the temperature of the mixture sprayed from the air atomizing nozzle T m for Where m1, c1 and T1 are the mass flow rate, specific heat capacity and temperature of liquid nitrogen before mixing; m2, c2 and T2 are the mass flow rate, specific heat capacity and temperature of air before mixing; The temperature of the mixture sprayed from the air atomizing nozzle in each stage is adjusted by controlling the mass flow rate of liquid nitrogen or air in the corresponding stage. When the temperature of the mixture sprayed from the air atomizing nozzle is greater than the set temperature or temperature range of the stage, the mass flow rate of liquid nitrogen is adjusted to increase. When the temperature of the mixture sprayed from the air atomizing nozzle is lower than the set temperature or temperature range of the stage, the mass flow rate of air is adjusted to increase.

8. The method for measuring the elastic modulus of a unidirectional carbon fiber composite material according to claim 7, wherein: The initial temperature of the mixture sprayed from the air atomizing nozzle was set to -100 °C, and the mass flow ratio of liquid nitrogen to air was set to 1:3.3.

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