Method for predicting tensile strength of carbon fiber-reinforced epoxy resin matrix composite at high temperature

Abstract

The present invention relates to the technical field of testing methods. Specifically disclosed is a method for predicting the tensile strength of a carbon fiber-reinforced epoxy resin matrix composite (CFRC) at a high temperature. The prediction method comprises: determining a pyrolysis process of a material during a heating process; determining a peak temperature at each stage; determining a temperature holding time required for a unidirectional CFRC laminate to reach a thermal equilibrium state; determining the tensile strength of the CFRC at different temperature points in a high-temperature environment; and determining a variation law and prediction method of the tensile strength of the material during the pyrolysis process. The prediction method of the present invention takes into account the influence mechanisms of resin matrix thermal softening and thermal decomposition and carbon fiber high-temperature softening on the tensile strength of CFRCs, can truly and effectively characterize the degradation process of the tensile properties of CFRCs at high temperatures, allows for macroscopic prediction of the tensile strength of CFRCs at high temperatures, and adapts to a temperature range of 20-450°C.

Description

A method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperature Technical Field

[0001] This invention belongs to the technical field of testing methods, specifically relating to a method for predicting the tensile strength of carbon fiber epoxy resin-based composite materials at high temperatures. Background Technology

[0002] Carbon fiber reinforced epoxy resin matrix composites (CFRCs) are used to manufacture high-pressure hydrogen storage cylinders for vehicles due to their high strength and flexible handling, effectively increasing hydrogen storage density and reducing weight. However, because CFRC cylinders store hydrogen at pressures up to 70 MPa and are flammable, they undergo irreversible pyrolysis at high temperatures. If a vehicle catches fire in an accident, the mechanical properties of CFRCs will significantly degrade under high temperatures, further affecting the load-bearing capacity of the composite layers and increasing the risk of cylinder failure. Furthermore, studies on the fire thermal response and heat transfer characteristics of high-pressure hydrogen cylinders show that at the thermocouple-indicated temperature (minimum 600°C) required for the fire test, the outermost composite layer decomposes and becomes loose, losing its load-bearing capacity. In the event of cylinder failure, the temperature of the inner composite layer increases to approximately 400°C. Therefore, establishing a method to effectively predict the tensile strength of CFRCs at high temperatures can provide guidance for predicting the fire resistance performance of gas cylinders, lay the foundation for proposing methods to optimize and control the fire resistance performance of gas cylinders, and avoid catastrophic accidents of gas cylinders in fire environments.

[0003] Existing methods for calculating the tensile mechanical properties of CFRCs mainly include:

[0004] Reference 1, “GIBSON AG, WU YS, EVANS JT, et al. Laminate theory analysis of composites under load in fire[J]. Journal of Composite Materials, 2006, 40(7): 639-658,” discloses a method for characterizing the polymer matrix softening process by fitting experimental data with antisymmetric characteristic functions, such as hyperbolic tanh functions. However, this method does not consider the influence of resin thermal decomposition on the strength degradation of composite materials, and therefore is not suitable for strength prediction under high-temperature conditions.

[0005] Reference 2, "LI Y,LI W,TAO Y, et al. Theoretical model for the temperature dependent longitudinal tensile strength of unidirectional fiber reinforced polymer composites[J].Composites Part B:Engineering,2019,161:121-127", discloses a theoretical prediction model for the longitudinal tensile strength of unidirectional fiber composites at different temperatures. However, the experimental data used to verify the model are all at temperatures lower than the melting temperature of the polymer matrix. Its applicability to other temperature ranges still needs to be verified. Moreover, the model involves many parameters of the fiber and the matrix, making it inconvenient for practical application.

[0006] Therefore, it is urgent to establish a method to predict the tensile strength of unidirectional carbon fiber epoxy resin matrix composites under high temperature (20℃~450℃) conditions, taking into account the thermal softening and decomposition of the CFRCs matrix and the softening of carbon fibers. Summary of the Invention

[0007] This invention addresses the limitations of existing prediction methods in considering the impact of resin matrix softening, thermal decomposition, and carbon fiber thermal softening on the tensile strength of CFRCs during heating. It proposes a method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperatures. This method is based on thermogravimetric analysis results, aligns with the actual conditions of high-temperature pyrolysis of materials, and solves the problem of predicting the tensile strength of unidirectional CFRCs under the combined effects of high temperature and stress.

[0008] To address the aforementioned technical problems, this invention proposes a method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperatures, comprising the following steps:

[0009] (1) Based on thermogravimetric analysis of materials, the pyrolysis process of materials is determined by the thermogravimetric curves of materials at different heating rates;

[0010] (2) Based on the derivative thermogravimetric curves of the material at different heating rates, the peak temperature is obtained and the temperature range of each decomposition stage is determined.

[0011] (3) Determine the pyrolysis kinetic parameters of the carbon fiber epoxy resin matrix composite material based on the non-isothermal differential thermogravimetric curve and mechanism function, wherein the kinetic parameters include activation energy and pre-exponential factor;

[0012] (4) Preparation of unidirectional carbon fiber epoxy resin matrix composite laminate;

[0013] (5) The heating time required for the unidirectional carbon fiber epoxy resin matrix composite laminate to reach a stable thermal equilibrium state was calculated using the thermal analysis module of ANSYS software.

[0014] (6) Determine the maximum tensile load of carbon fiber epoxy resin matrix composite material at different temperature points in a high-temperature environment;

[0015] (7) Calculate the tensile strength of the carbon fiber epoxy resin matrix composite material according to standard GB / T 3354-2014, and obtain the test value;

[0016] (8) Based on the pyrolysis degradation law of carbon fiber epoxy resin matrix composite obtained in steps (1) to (5), the tensile strength degradation model of the material at high temperature is determined by hyperbolic function; based on the fitting of the experimental values ​​calculated in step (7), the curve of the change law of the tensile strength of the carbon fiber epoxy resin matrix composite with temperature is obtained.

[0017] In some embodiments of the present invention, step (1), determining the pyrolysis process of the material, includes the following steps:

[0018] Carbon fiber epoxy resin-based composite material samples were cut and dried before thermogravimetric analysis was performed. During the thermogravimetric analysis, the samples were heated from 30°C to 950°C in air at heating rates of 5°C / min, 10°C / min, 20°C / min, and 30°C / min, respectively.

[0019] In some embodiments of the present invention, the drying temperature is 50-60°C, preferably 55°C.

[0020] In some embodiments of the present invention, the drying time is 20-28 hours, preferably 24 hours.

[0021] In some embodiments of the present invention, in step (3), the determination of the pyrolysis kinetic parameters of the material is carried out using the Arrhenius formula to describe the change of the material reaction rate constant with temperature, the reaction rate, and the conversion function:

[0022] Where: α is the conversion rate, %; A is the pre-exponential factor; E is the activation energy, kJ / mol; R is the ideal gas constant, 8.314 J / (mol·K); T is the absolute temperature, K; n is the reaction order; f(α) is the differential form of the mechanism function; m0, m t and m f Let g be the initial mass, the mass at any given time, and the final mass of the sample, respectively.

[0023] The activation energy is calculated using the Kissinger-Akahira-Sunose method, and its basic form is as follows:

[0024] Where: the heating rate β is a constant, β=dT / dt; g(α) is the integral function of the conversion rate. Where T0 is the initial temperature;

[0025] The temperature points T corresponding to each peak of the DTG curve from the thermogravimetric analysis experiment were used. p To calculate the activation energy:

[0026] Where: β i The heating rate for the i-th experiment is expressed in °C / min; T pi The peak temperature for each stage is expressed in absolute temperature, in °C.

[0027] In some embodiments of the present invention, step (4) of preparing the unidirectional carbon fiber epoxy resin matrix composite laminate includes the following steps:

[0028] First, a single-layer composite prepreg was made using carbon fiber and epoxy resin matrix; then, a unidirectional composite preform was prepared using a vacuum bag-autoclave method; aluminum alloy reinforcing sheets were then attached to the ends of the prepared unidirectional composite preform; finally, the unidirectional composite preform with attached aluminum alloy reinforcing sheets was divided into several samples, with the length, width and thickness of the samples being 330 mm, 25 mm and 2 mm, respectively.

[0029] In some embodiments of the present invention, the carbon fiber is carbon fiber used to manufacture vehicle-mounted composite hydrogen storage cylinders. The carbon fiber used to manufacture vehicle-mounted composite hydrogen storage cylinders has the characteristics of high strength and high modulus, which can meet the special requirements of hydrogen storage cylinders. For example, Toray Industries' T700 carbon fiber.

[0030] In some embodiments of the present invention, the epoxy resin matrix is ​​a thermosetting epoxy resin, such as 0164 resin from Wuxi Resin Factory.

[0031] In some embodiments of the present invention, step (6), determining the maximum tensile load of the carbon fiber epoxy resin matrix composite material at different temperature points under high temperature conditions, includes the following steps:

[0032] Tensile testing equipment equipped with a high-temperature heating furnace was used to heat the specimen to the required temperature, hold it at that temperature, and then stretch it. The crossbeam displacement control mode was used to stretch the specimen under the loading speed until it broke. The load-displacement curves of carbon fiber epoxy resin matrix composites at different temperature points were obtained through tensile tests.

[0033] In some embodiments of the present invention, the heat preservation time is 15-25 minutes; preferably 20 minutes.

[0034] In some embodiments of the present invention, the loading speed is 0.5-1.5 mm / min; preferably 1 mm / min.

[0035] In some embodiments of the present invention, in step (7), the formula for calculating the tensile strength is:

[0036] Where: σ t P represents tensile strength, in MPa; max The maximum load the specimen could withstand before failure is N; w is the specimen width in mm; h is the specimen thickness in mm.

[0037] In some embodiments of the present invention, in step (8), the tensile strength degradation model of the material at high temperature is as follows:

[0038] Where P1, P2 and P3 represent the tensile strength at room temperature, the tensile strength between the two transformation processes and the final tensile strength, respectively, in MPa; k1 and k2 represent the relaxation widths, obtained through fitting; T1 and T2 are the transformation temperatures, in °C.

[0039] In some embodiments of the present invention, in step (8), the applicable temperature for the curve of the tensile strength of the carbon fiber epoxy resin matrix composite material as a function of temperature is 20℃~450℃.

[0040] Compared with the prior art, the above-described technical solution of the present invention has at least the following technical effects or advantages:

[0041] (1) This invention provides a method for calculating the tensile strength of carbon fiber epoxy resin matrix composites under high-temperature conditions. The method determines the pyrolysis process of the material during heating, the peak temperature at each stage, the holding time required for the unidirectional laminate of the carbon fiber epoxy resin matrix composite to reach thermal equilibrium, the tensile strength of the carbon fiber epoxy resin matrix composite at different temperature points under high-temperature conditions, and the variation law and prediction method of the tensile strength of the material during pyrolysis. The prediction method of this invention can accurately and effectively characterize the degradation process of the tensile properties of carbon fiber epoxy resin matrix composites at high temperatures and macroscopically predict the high-temperature tensile strength of the carbon fiber epoxy resin matrix composite.

[0042] (2) The method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperatures provided by this invention considers the influence mechanism of thermal softening, thermal decomposition of the resin matrix and high-temperature softening of carbon fibers on the tensile strength of CFRCs, providing a theoretical basis for the structural design and strength prediction of carbon fiber epoxy resin matrix composites. Simultaneously, based on experimental data, this invention establishes a high-temperature tensile mechanical property degradation model for carbon fiber epoxy resin matrix composites, which can predict the tensile strength of carbon fiber epoxy resin matrix composites within the temperature range of 20℃ to 450℃, with high reliability. Attached Figure Description

[0043] Figure 1 shows the thermal decomposition process of carbon fiber epoxy resin matrix composite material in air;

[0044] Figure 2 shows the DTG curves (air) of carbon fiber epoxy resin matrix composites at different heating rates;

[0045] Figure 3 shows the pyrolysis process of carbon fiber epoxy resin matrix composites. With (1 / T) pi The fitted curve of (air);

[0046] Figure 4 shows the load-displacement curves of CFRCs unidirectional laminates at 20℃~450℃.

[0047] Figure 5 is a fitted curve showing the variation of tensile strength of CFRCs unidirectional laminate with temperature.

[0048] Figure 6 is a flowchart of the carbon fiber epoxy resin matrix composite material process. Detailed Implementation

[0049] The present invention will now be described in detail with reference to embodiments to facilitate understanding of the invention by those skilled in the art. It is particularly important to note that the embodiments are merely illustrative of the invention and should not be construed as limiting the scope of protection of the invention. Non-essential improvements and adjustments made to the invention by those skilled in the art based on the above description should still fall within the scope of protection of the present invention.

[0050] In the following embodiments, the environment is an air environment under standard atmospheric pressure; the carbon fiber is Toray Industries' T700 carbon fiber, and the epoxy resin is Wuxi Resin Factory's 0164 resin.

[0051] The carbon fiber epoxy resin matrix composite material in this embodiment of the invention is shown in Figure 6. Specifically, S100 represents the pyrolysis process of the material determined by thermogravimetric analysis (TGA) experiments using thermogravimetric curves at different heating rates; S200 represents the temperature range of each decomposition stage determined by obtaining the peak temperature from the derivative TGA curves at different heating rates; S300 represents the pyrolysis kinetic parameters of the material determined by its non-isothermal derivative TGA curves and mechanistic function; S400 represents the preparation of a unidirectional carbon fiber epoxy resin matrix composite laminate; S500 represents the heating time required for the unidirectional carbon fiber epoxy resin matrix composite laminate to reach a stable thermal equilibrium state calculated using the ANSYS software thermal analysis module; S600 represents the determination of the maximum tensile load of the carbon fiber epoxy resin matrix composite material at different temperature points under high-temperature conditions; and S700 represents the process according to standard GB / T... 3354-2014 Calculate the tensile strength of the carbon fiber epoxy resin matrix composite material to obtain the experimental value; S800 represents the pyrolysis degradation law of the carbon fiber epoxy resin matrix composite material obtained in steps S100 to S500, and the tensile strength degradation model of the material at high temperature is determined by using a hyperbolic function; Based on the fitting of the experimental value calculated in step 700, the curve of the change law of the tensile strength of the carbon fiber epoxy resin matrix composite material with temperature is obtained.

[0052] Example 1

[0053] A method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperature includes the following steps:

[0054] (1) Based on the thermogravimetric analysis of the material, the pyrolysis process of the material is determined by the thermogravimetric curves of the material at different heating rates.

[0055] Carbon fiber epoxy resin matrix composite samples were taken from the gas cylinder. The coarse samples were first crushed and then sieved to small particles of approximately 0.2 mm. The sample mass was 10 mg–20 mg. The samples were dried at 55 °C for 24 h. During thermogravimetric analysis (TGA), the sample was heated from 30 °C to 950 °C at heating rates of 5 °C / min, 10 °C / min, 20 °C / min, and 30 °C / min, respectively, to obtain the thermal decomposition process of the carbon fiber epoxy resin matrix composite in air, as shown in Figure 1 (heating rate 10 °C / min, sample mass 19.657 mg). The DTG curves show that the thermal decomposition of the carbon fiber epoxy resin matrix composite roughly went through three stages. The first stage (300 °C to 420 °C) was mainly the oxidative decomposition of the resin, gradually forming a charred body; the second stage (430 °C to 600 °C) was the oxidative decomposition process of the resin decomposing to form the charred body; the third stage (700 °C to 950 °C) was the oxidative decomposition of the carbon fiber.

[0056] (2) Based on the differential thermogravimetric curves of the material at different heating rates, the peak temperature is obtained and the temperature range of each decomposition stage is determined.

[0057] The thermal decomposition process of carbon fiber epoxy resin matrix composites at different heating rates is shown in Figure 2. The thermal decomposition parameters are shown in Table 1.

[0058] Table 1: Thermal decomposition parameters of carbon fiber epoxy resin matrix composites in air

[0059] Note: MLR max This represents the maximum rate of weightlessness.

[0060] (3) Determine the pyrolysis kinetic parameters of the material, including activation energy and pre-exponential factor, based on the non-isothermal differential thermogravimetric curve and mechanism function of carbon fiber epoxy resin matrix composite.

[0061] The Arrhenius equation is used to describe the change of the material reaction rate constant with temperature, and the reaction rate and conversion function:

[0062] Where: α is the conversion rate, %; A is the pre-exponential factor; E is the activation energy, kJ / mol; R is the ideal gas constant, 8.314 J / (mol·K); T is the absolute temperature, K; n is the reaction order; f(α) is the differential form of the mechanism function; m0, m t and m f Let g represent the initial mass, mass at any given time, and final mass of the sample, respectively.

[0063] The activation energy is calculated using the Kissinger-Akahira-Sunose method, and its basic form is as follows:

[0064] Where: the heating rate β is a constant, β=dT / dt; g(α) is the integral function of the conversion rate. Where T0 is the initial temperature;

[0065] This method allows the temperature points T corresponding to the peak values ​​of the DTG curve from the thermogravimetric analysis (TG) test to be determined. p To calculate the activation energy, we can transform it into the following formula:

[0066] Where: β i The heating rate for the i-th experiment is expressed in °C / min; T pi The peak temperature for each stage is expressed in absolute temperature, in °C.

[0067] The peak temperature of the thermogravimetric curve was selected according to the KAS method (see Table 1). For (1 / T) piPlot the graph to obtain the carbon fiber epoxy resin matrix composite material during the pyrolysis process in air. With (1 / T) pi The fitted curve of the composite material at different stages under air is shown in Figure 3. The calculated pyrolysis kinetic parameters of the composite material at different stages under air are shown in Table 2.

[0068] Table 2: Pyrolysis kinetic parameters of composite materials (air) calculated using the KAS method

[0069] Note: Ea is the apparent activation energy, which is derived from the parameter E in the Arrhenius formula shown in formula (1) and obtained through experimental data. It is also called the experimental activation energy.

[0070] The pyrolysis kinetics analysis of CFRCs further corroborated its three-step decomposition process and quantitatively provided the activation energy. In the first stage, epoxy resin is the main raw material for the decomposition reaction. After decomposition, it forms a relatively stable char product. In the second stage, under the action of oxygen, the char product of the initial decomposition of epoxy resin can be further decomposed. As the temperature increases, the carbon fiber also oxidizes and decomposes in the third stage.

[0071] (4) Prepare unidirectional carbon fiber epoxy resin matrix composite laminate samples.

[0072] First, a single-layer composite prepreg was prepared using carbon fiber T700 (used for manufacturing vehicle-mounted composite hydrogen storage cylinders) and thermosetting epoxy resin matrix 0164. Then, a unidirectional composite preform was prepared using a vacuum bag-autoclave method. Next, aluminum alloy reinforcing plates were attached to the ends of the specimens to increase friction and prevent slippage during tensile testing. Finally, the preform with the reinforcing plates attached was divided into several specimens with a length, width, and thickness of 330 mm, 25 mm, and 2 mm, respectively.

[0073] (5) The heating time required for the composite laminate to reach a stable thermal equilibrium state was calculated using the thermal analysis module of ANSYS software.

[0074] A three-dimensional geometric model of CFRCs was established based on the unidirectional laminate specimen. Boundary conditions for the three-dimensional heat conduction model were set according to the high-temperature furnace heating method of the MTS tensile testing machine used in the experiment. Heat conduction mainly occurs through convection and conduction. Solid90 elements were selected to mesh the physical model. Loads were applied, and analysis and solutions were performed. Calculations showed that it takes approximately 18 minutes for the specimen to heat to the maximum experimental temperature of 450℃ and reach equilibrium; therefore, the heating time was set to 20 minutes.

[0075] (6) Determine the maximum tensile load of carbon fiber epoxy resin matrix composite material at different temperature points under high temperature environment.

[0076] Tensile testing equipment equipped with a high-temperature heating furnace was used. The ambient chamber was first heated to the required temperature; then, after holding at that temperature for 20 minutes, tensile testing was started directly. A beam displacement control mode was adopted, and the specimen was continuously loaded at a loading rate of 1 mm / min until the specimen broke. Load-displacement curves of carbon fiber epoxy resin composites at different temperature points (with gradually increasing temperature) were obtained through tensile testing. Figure 4 shows the load-displacement curves of CFRCs unidirectional laminates at 20℃~450℃. As shown in Figure 4, the fracture process of the sample can be roughly divided into three stages. The first stage is the elastic linear segment, and the curve is relatively smooth. In the second stage, the curve begins to show "ripples (sawtooths)," but it still basically presents a linear relationship. Since the fiber modulus is higher than the matrix modulus, the stress-strain relationship in this segment depends on the mechanical properties of the fiber. Small strands of fiber begin to break, and the load is affected. The surrounding resin matrix redistributes the fracture stress to other fibers, increasing the load on the fibers and causing them to break successively. The third stage is the fracture stage, where the curve suddenly drops, indicating that the sample breaks instantaneously. Under the cumulative effect of damage such as matrix cracking, fiber pull-out, and interface debonding, the sample undergoes macroscopic damage.

[0077] (7) Calculate the tensile strength according to the formula provided in standard GB / T 3354-2014.

[0078] Where: σ t P represents tensile strength, in MPa; max The maximum load the specimen could withstand before failure is N; w is the specimen width in mm; h is the specimen thickness in mm.

[0079] The average tensile strength and coefficient of variation of CFRCs unidirectional laminates at different temperatures are shown in Table 3.

[0080] Table 3: Tensile strength of unidirectional laminate specimens at different temperatures

[0081] (8) Based on the pyrolysis degradation law of carbon fiber epoxy resin matrix composite obtained in steps (1) to (5), the tensile strength degradation model of the material at high temperature is determined by using an improved hyperbolic function.

[0082] Hyperbolic functions can be used to simulate a significant relaxation process that occurs in thermosetting resin matrix composites within a certain temperature range:

[0083] Where: P U and P RThese represent the performance data at the "non-relaxation" (low temperature) and "relaxation" (high temperature) temperatures, respectively. k represents the relaxation width, obtained through fitting, and T1 represents the "transition" temperature, the temperature at which the tensile properties decrease to 50% of their initial value. The performance-temperature curve described by this equation is generally antisymmetric on both sides of the transition temperature.

[0084] As shown in Figure 5 and the foregoing analysis, although the tensile strength of the composite material measured in this invention does not show a significant sharp decrease near the glass transition temperature, it exhibits a continuous decreasing trend in the temperature ranges of 20℃ to 200℃ and 250℃ to 450℃, respectively. However, the change is minimal in the 200℃ to 250℃ range, showing a small "plateau," dividing the entire mechanical property degradation process into two stages. Therefore, this invention proposes to establish a degradation model using hyperbolic functions to characterize and sum the properties, based on the premise that the effect of temperature on performance is cumulative. The degradation process is divided into two stages according to the corresponding temperature: the first stage is 20℃ to 250℃, and the second stage is 250℃ to 450℃. The improved hyperbolic function expression characterizing the tensile property degradation law of the composite material across the entire temperature range is as follows:

[0085] Where: P1, P2 and P3 represent the tensile strength at room temperature, the tensile strength between the two transformation processes and the final tensile strength, respectively, in MPa; k1 and k2 represent the relaxation widths, obtained through fitting; T1 and T2 are the transformation temperatures, in °C.

[0086] Figure 5 shows the curve of the degradation of the tensile strength of the carbon fiber epoxy resin matrix composite material with temperature, obtained by fitting the experimental values ​​using equation (6). Table 4 shows the parameter values ​​in equation (6).

[0087] Table 4: Parameter values ​​in equation (6)

[0088] As can be seen from Figure 5, the curve fit is relatively high, and R0 2 The value is 0.9854, which is consistent with the variation of tensile strength test values ​​for unidirectional laminate specimens between 20℃ and 450℃. For the first stage (20℃ to 250℃), since epoxy resin, an amorphous polymer, only undergoes a glass transition before reaching its decomposition temperature, the hyperbolic function is used to characterize the degradation of mechanical properties with temperature. For the second stage, since there is currently a lack of tensile mechanical property test data covering the thermal decomposition temperature range, some studies have used a simple multiplication of the hyperbolic function by a power factor R. nWhile methods can be used for correction, the power function characterization is not suitable for studying the degradation law of tensile strength of carbon fiber epoxy resin matrix composites in the second stage (250℃~450℃) according to this invention. Furthermore, as can be seen from the above analysis of the mechanical property degradation mechanism, at around 300℃, epoxy resin begins to decompose, the carbon fiber-resin interface is destroyed, stress cannot be effectively transferred, and tensile strength decreases significantly with increasing temperature. Since the effect of temperature on tensile strength degradation is a cumulative process, it is more appropriate to use a hyperbolic tangent function relationship to fit the degradation law of this stage.

[0089] The final prediction model for the tensile strength of unidirectional carbon fiber epoxy resin matrix composites in high-temperature (20℃~450℃) environments is as follows:

[0090] P(T)=2019-180{1+tanh[0.01(T-112)]}-350{1+tanh[0.0167(T-339)]} (7);

[0091] For those skilled in the art, several simple deductions or substitutions can be made without departing from the inventive concept, without requiring creative effort. Therefore, any simple improvements made to this invention by those skilled in the art based on the disclosure of this invention should be within the scope of protection of this invention. The above embodiments are preferred embodiments of this invention, and all prediction methods similar to this invention and their equivalent variations should fall within the scope of protection of this invention.

Claims

1. A method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperatures, characterized in that, Includes the following steps: (1) Based on thermogravimetric analysis of materials, the pyrolysis process of materials is determined by the thermogravimetric curves of materials at different heating rates; (2) Based on the derivative thermogravimetric curves of the material at different heating rates, the peak temperature is obtained and the temperature range of each decomposition stage is determined. (3) Determine the pyrolysis kinetic parameters of the carbon fiber epoxy resin matrix composite material based on the non-isothermal differential thermogravimetric curve and mechanism function, wherein the kinetic parameters include activation energy and pre-exponential factor; (4) Preparation of unidirectional carbon fiber epoxy resin matrix composite laminate; (5) The heating time required for the unidirectional carbon fiber epoxy resin matrix composite laminate to reach a stable thermal equilibrium state was calculated using the thermal analysis module of ANSYS software. (6) Determine the maximum tensile load of carbon fiber epoxy resin matrix composite material at different temperature points in a high-temperature environment; (7) Calculate the tensile strength of the carbon fiber epoxy resin matrix composite material according to standard GB / T 3354-2014, and obtain the test value; (8) Based on the pyrolysis degradation law of carbon fiber epoxy resin matrix composite obtained in steps (1) to (5), the tensile strength degradation model of the material at high temperature is determined by hyperbolic function; based on the experimental value calculated in step (7), the curve of the tensile strength of the carbon fiber epoxy resin matrix composite with temperature is obtained.

2. The method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperature according to claim 1, characterized in that, In step (1), determining the pyrolysis process of the material includes the following steps: Carbon fiber epoxy resin-based composite material samples were cut and dried before thermogravimetric analysis was performed. During the thermogravimetric analysis, the samples were heated from 30°C to 950°C in air at heating rates of 5°C / min, 10°C / min, 20°C / min, and 30°C / min, respectively.

3. The method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperature according to claim 2, characterized in that, The drying temperature is 50-60℃; and / or the drying time is 20-28h.

4. The method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperature according to claim 1, characterized in that, In step (3), the pyrolysis kinetic parameters of the material are determined using the Arrhenius equation to describe the change of the material reaction rate constant with temperature, the reaction rate, and the conversion function: Where: α is the conversion rate, %; A is the pre-exponential factor; E is the activation energy, kJ / mol; R is the ideal gas constant, 8.314 J / (mol·K); T is the absolute temperature, K; n is the reaction order; f(α) is the differential form of the mechanism function; m0, m t and m f Let g be the initial mass, the mass at any given time, and the final mass of the sample, respectively. The activation energy is calculated using the Kissinger-Akahira-Sunose method, and its basic form is as follows: Where: the heating rate β is a constant, β=dT / dt; g(α) is the integral function of the conversion rate. Where T0 is the initial temperature; The temperature points T corresponding to each peak of the DTG curve from the thermogravimetric analysis experiment were used. p To calculate the activation energy: Where: β i The heating rate for the i-th experiment is expressed in °C / min; T pi The peak temperature for each stage is expressed in absolute temperature, in °C.

5. The method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperature according to claim 1, characterized in that, In step (4), the preparation of the unidirectional carbon fiber epoxy resin matrix composite laminate includes the following steps: First, a single-layer composite prepreg was made using carbon fiber and epoxy resin matrix; then, a unidirectional composite preform was prepared using a vacuum bag-autoclave method; aluminum alloy reinforcing sheets were then attached to the ends of the prepared unidirectional composite preform; finally, the unidirectional composite preform with attached aluminum alloy reinforcing sheets was divided into several samples, with the length, width and thickness of the samples being 330 mm, 25 mm and 2 mm, respectively.

6. The method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperature according to claim 5, characterized in that, The carbon fiber is used to manufacture composite hydrogen storage cylinders for vehicles; and / or, the epoxy resin matrix is ​​a thermosetting epoxy resin.

7. The method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperature according to claim 1, characterized in that, In step (6), determining the maximum tensile load of the carbon fiber epoxy resin matrix composite material at different temperature points under high temperature conditions includes the following steps: Tensile testing equipment equipped with a high-temperature heating furnace was used to heat the specimen to the required temperature, hold it at that temperature, and then stretch it. The crossbeam displacement control mode was used to stretch the specimen under the loading speed until it broke. The load-displacement curves of carbon fiber epoxy resin matrix composites at different temperature points were obtained through tensile tests.

8. The method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperature according to claim 1, characterized in that, In step (7), the formula for calculating the tensile strength is: Where: σ t P represents tensile strength, in MPa; max The maximum load the specimen could withstand before failure is N; w is the specimen width in mm; h is the specimen thickness in mm.

9. The method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperature according to claim 1, characterized in that, In step (8), the tensile strength degradation model of the material at high temperature is as follows: Where P1, P2 and P3 represent the tensile strength at room temperature, the tensile strength between the two transformation processes and the final tensile strength, respectively, in MPa; k1 and k2 represent the relaxation widths, obtained through fitting; T1 and T2 are the transformation temperatures, in °C.

10. The method for predicting the tensile strength of carbon fiber epoxy resin matrix composites at high temperature according to claim 1, characterized in that, In step (8), the applicable temperature for the curve showing the change of tensile strength of the carbon fiber epoxy resin matrix composite material with temperature is 20℃~450℃.

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