Experimental and Numerical Investigation on Fatigue Properties of Carbon Fiber Cross-Ply Laminates in Hygrothermal Environments

The fatigue properties of composite materials are degraded seriously in hygrothermal environments, so taking into account their influence is very important when evaluating the fatigue life of composite structures. Tensile fatigue experiments of carbon fiber reinforced resin composite cross-ply laminates were conducted in room temperature/dry (RTD), cool temperature/dry (CTD) and elevated temperature/wet (ETW) conditions. The S-N curves and fatigue failure modes of the cross-ply laminates were obtained in three conditions. On this basis, a finite element model was established to discuss the influence of temperature and moisture content on the fatigue properties, as well as a method for determining environmental factors of fatigue life of cross-ply laminates was established. The results show that the saturation moisture absorption and temperature have a significant influence on the tensile fatigue properties of cross-ply laminates. The high-cycle fatigue property is weakened significantly by the saturation moisture absorption and high temperature, but the low-cycle fatigue properties were strengthened in cool temperature conditions. The delamination failure mode in ETW is the most severe, presenting with an obvious necking phenomenon. The influence of temperature has a greater effect than that of moisture content, but moisture absorption would play its affect obviously when temperature exceeds 40 °C.


Introduction
Carbon fiber reinforced resin matrix composites (CFRP) have been widely used in aircraft structures due to their high specific strength and stiffness [1][2][3]. The composite structure of the aircraft not only needs to stand cyclic fatigue loads during the service period, but also may encounter harsh environmental conditions such as heat and humidity [4]. Due to them being sensitive to heat and humidity [5], an environment with elevated temperature and high moisture could seriously weaken the mechanical properties of the composite material structure, and that will make the fatigue problem more prominent. The durability of the composite structure in severe environment conditions has become a big challenge of their application in aerospace industries [6].
Some studies have been conducted on the fatigue properties of composite laminates in wet and thermal circumstances [7][8][9][10][11][12][13][14][15][16][17][18]. Mismatch in moisture expansion and thermal expansion coefficients between the fiber and matrix will induce wet stress and thermal stress, and then initiate micro-cracks and debonding in the interface of the fiber/matrix, reducing the structural bearing capacity [19,20]. Kawai et al. [21] investigated the effect of moisture absorption on the fatigue strength of plane fabric fiber quasi-isotropic composite laminates under constant stress amplitude and different stress ratios. The results indicate that the fatigue life of wet laminates is lower than that of dry laminates, and the fatigue strength of wet laminates is 11% lower than that of dry laminates at room temperature. Mcbagonluri et al. [22] compared and studied the tensile fatigue properties of vinyl glass fiber composites in a dry state, fresh water and salty water immersion. The S-N curve after moisture absorption is lower than that in the dry state, but the slope of the S-N curve in three environments is almost the same. Malpot et al. [23] investigated the effect of moisture absorption on the fatigue property of composites in three wet conditions (RH0, RH50% and RH100%) and found that when the cycle number was less than 10 4 , the fatigue life decreased with the increase in relative humidity. However, when the number of cycles is greater than 10 4 , the humidity condition hardly affects the fatigue life. Therefore, the effect of hygrothermal environment on high-cycle fatigue far exceeds that of low-cycle fatigue. Kawai et al. [24] investigated the fatigue property of three composites, AS4/PEEK, T800H/Polyimide and T800H/Epoxy, at high temperature (100 • C) with different lay-up angles. The AS4/PEEK composites were found to exhibit stronger fatigue strength except for the 0 • lay-up, while the T800H/Polyimide composites had weaker fatigue strength. The fatigue strength of all the lay-up materials except for the 0 • layup showed a significant decrease when the number of cycles reached 10 5 . In terms of numerical simulation, Shokrieh et al. [25,26] combined stress analysis, failure analysis and material performance attenuation to propose a fatigue model suitable for carbon fiber composite unidirectional bands. Harper et al. [27] established a FEM mode by using cohesive element to simulate fatigue delamination.
All the investigations above focus on the individual effect on the fatigue property of composites about temperature and moisture absorption, so there are few studies on the combined effects of moisture and temperature. That contributes to the insufficiency of experimental verification, a deficient depth of numerical simulation, as well as the lack of the in-depth discussion upon the coupling between moisture and temperature.
We have previously investigated the fatigue property of carbon fiber cross-ply laminates under hygrothermal environmental conditions in the article [28] using the method of experiments and numerical simulations, and then established an analysis model about the environmental influence factor of fatigue life for laminates. The effect of the hygrothermal environment on the properties of laminates largely depends on the influence of the matrix and the matrix/fiber interface. The sensitivity of angle-ply laminates to that under the hygrothermal environment is much stronger than that of cross-ply laminates. Therefore, we continue this investigation to gain a deeper understanding of coupling effect of the temperature and moisture absorption as well as better to improve the analysis model.

Specimen Design and Manufacture
In this work, T300 level CF3052/3238A carbon fiber-reinforced epoxy composite laminates were investigated whose configuration is shown in Figure 1. The specimens were designed based on ASTM D3479M [29] and a typical configuration of a [(45/−45)] 8 lay-up was selected based on the application in rotorcraft industry. Prepregs were supplied by Guangwei Composites Co., Ltd. (Weihai, China). Laminates were manufactured by the AVIC composite corporation then. The maximum temperature for curing of laminates was 120 • C and they were kept for 120 mins under that condition. A pressure of 0.45 MPa was applied for the whole curing cycle.
The test category and number of specimens are presented in Table 1. Among them, there are 3 moisture absorption specimens, which were used to determine the saturated moisture absorption state and time of all specimens. There are three experiment environments, namely in the room temperature and dry condition (RTD), in the cool temperature and dry condition (CTD, −40 • C) as well as in the elevated temperature and wet condition  The test category and number of specimens are presented in Table 1. Amon there are 3 moisture absorption specimens, which were used to determine the sa moisture absorption state and time of all specimens. There are three experime ronments, namely in the room temperature and dry condition (RTD), in the co perature and dry condition (CTD, −40 °C) as well as in the elevated temperature condition (ETW, 55 °C). In order to determine the fatigue test load, the corresp static tensile tests were conducted first.

Experimental Procedures
Three kinds of experiments were conducted, involving moisture absorption and tensile-tensile fatigue tests. All the specimens of ETW were firstly put in water to reach wet condition, and then tensile and tensile-tensile fatigue tests w formed in RTD, RTW and ETW. When testing, the thermal condition was prov the conditioning chamber.

Moisture Absorption
Moisture absorption experiments were designed based on HB7401 [30] and Standard D5229 [31]. The specimens in ETW were immersed in a deionized wa to achieve the moisture adsorption equilibrium as the water temperature was s 70 °C which is lower than the glass transmission of the matrix. Weights of sp were regularly measured by an analytical balance with the accuracy of 0.1 m specimens were cleaned by the absorbent cloth before weighing, and the time of ing was within 5 min. Moisture equilibrium was reached when the moisture cont the following equation [31].

Experimental Procedures
Three kinds of experiments were conducted, involving moisture absorption, tensile and tensile-tensile fatigue tests. All the specimens of ETW were firstly put in the hot water to reach wet condition, and then tensile and tensile-tensile fatigue tests were performed in RTD, RTW and ETW. When testing, the thermal condition was provided by the conditioning chamber.

Moisture Absorption
Moisture absorption experiments were designed based on HB7401 [30] and ASTM Standard D5229 [31]. The specimens in ETW were immersed in a deionized water tank to achieve the moisture adsorption equilibrium as the water temperature was set to be 70 • C which is lower than the glass transmission of the matrix. Weights of specimens were regularly measured by an analytical balance with the accuracy of 0.1 mg. The specimens were cleaned by the absorbent cloth before weighing, and the time of weighing was within 5 min. Moisture equilibrium was reached when the moisture content met the following equation [31].
where M is the moisture absorption content of the test specimens, W 0 and W i are the mass of the test specimen before and after moisture absorption.

Tensile Fatigue Tests
As shown in Figure 2a, the fatigue tests were conducted on the Instron 8801 servohydraulic testing system (Instron Inc., Shanghai, China). Parameters of the sinusoidal waveform and stress ratio R = 0.0526 were set. RTD specimens were directly exposed to the environment and CTD/ETW specimens were placed in the conditioning chamber during testing. Hang extensometers with a maximum measurement range of 2.5 m in the longitudinal and transverse directions, respectively, were used to measure strains. The standard test method [29] points out that the loading frequency should ensure the change of the surface temperature of the test specimens does not exceed 10 • C. Therefore, an infrared temperature tester was used to monitor the temperature, and the loading frequency is set to 1 Hz according to the monitoring results. Due to the phenomenon of heating and water loss in the fatigue test of composite materials, the test specimens were moisturized by the method shown in Figure 2b under the ETW environment. Four stress levels which represent the maximum fatigue stress were selected based on some certain percentages of static strength. The test was terminated once the specimen was broken or the dynamic stiffness of the specimen was reduced by 10%.
waveform and stress ratio R = 0.0526 were set. RTD specimens were directly exposed to the environment and CTD/ETW specimens were placed in the conditioning chamber during testing. Hang extensometers with a maximum measurement range of 2.5 m in the longitudinal and transverse directions, respectively, were used to measure strains. The standard test method [29] points out that the loading frequency should ensure the change of the surface temperature of the test specimens does not exceed 10 °C. Therefore, an infrared temperature tester was used to monitor the temperature, and the loading frequency is set to 1 Hz according to the monitoring results. Due to the phenomenon of heating and water loss in the fatigue test of composite materials, the test specimens were moisturized by the method shown in Figure 2b under the ETW environment. Four stress levels which represent the maximum fatigue stress were selected based on some certain percentages of static strength. The test was terminated once the specimen was broken or the dynamic stiffness of the specimen was reduced by 10%.

Basic Property Degradation
The wet and thermal environment seriously weaken the properties of the matrix and matrix/fiber interface, so its basic mechanical properties are deteriorated. The mechanical properties in RTD are shown in Table 2. The model established by Shan et al. [32] is used to describe the change law of basic mechanical properties in wet and thermal environments, in which the dimensionless T* considers the influence of temperature and moisture absorption on basic mechanical properties. T* can be described as In which T is the current temperature, T0 is the starting temperature, Tgw is the glass transition temperature of resin when the current moisture absorption is C, and Tg0 is the glass transition temperature of resin in dry state. Tgw and Tg0 satisfy the following relationship, In which, C is the current moisture absorption of resin and g is the constant. The resin moisture absorption C is calculated from the moisture absorption content M of laminates, resin density m ρ , laminate density ρ and resin volume content m V according to Equation (4),

Basic Property Degradation
The wet and thermal environment seriously weaken the properties of the matrix and matrix/fiber interface, so its basic mechanical properties are deteriorated. The mechanical properties in RTD are shown in Table 2. The model established by Shan et al. [32] is used to describe the change law of basic mechanical properties in wet and thermal environments, in which the dimensionless T* considers the influence of temperature and moisture absorption on basic mechanical properties. T* can be described as Table 2. Material parameters [28]. In which T is the current temperature, T 0 is the starting temperature, T gw is the glass transition temperature of resin when the current moisture absorption is C, and T g0 is the glass transition temperature of resin in dry state. T gw and T g0 satisfy the following relationship, In which, C is the current moisture absorption of resin and g is the constant. The resin moisture absorption C is calculated from the moisture absorption content M of laminates, resin density ρ m , laminate density ρ and resin volume content V m according to Equation (4), Polymers 2022, 14, 1857

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After the dimensionless T* is obtained, the strength and stiffness of laminates are attenuated according to Equation (5), In which, E 11 and E 22 are the elastic modulus of laminates along longitudinal and transverse directions under the current environment, respectively, E 0 11 and E 0 22 are elastic modulus along two directions in RTD, respectively, X T and Y T are the tensile strength along two directions under the current environment, respectively, X 0 T and Y 0 T are the tensile strength along two directions in RTD, respectively, G 12 and S 12 are the in-plane shear modulus and strength of laminates under the current environment, respectively, G 0 12 and S 0 12 are the in-plane shear modulus and strength in RTD, respectively. Finally, a, b, c, and d are the degradation constants of material properties in wet and thermal environments.
The parameters used in Equations (2)-(5) are shown in Table 2. Based on the test data in RTD, the calculated value of material properties in CTD and ETW are shown in Table 3. Compared with the simulation results, it was found that the theoretical value is in good agreement with the test value, and the correlation coefficients R 2 is greater than 0.99, indicating that the prediction of mechanical property parameters for laminates is reliable. Table 3. Empirical constants [28].

Fatigue Failure Criterion and Property Degradation Model
In the process of fatigue loading, the residual strength and stiffness will show a downward trend. The degradation law is described as follows [25,26], where X T (n,σ,R), X C (n,σ,R), Y T (n,σ,R), Y C (n,σ,R) denote the residual strength along the transverse and longitudinal direction, respectively, as well as the subscript T and C is for tensile and compression. E 11 (n,σ,R), S 12 (n,σ,R), G 12 (n,σ,R) are the residual elastic modulus, in-plane shear strength and in-plane shear modulus respectively after n cycles, X T , X C , Y T , Y C , E 11 , S 12 , G 1 are the corresponding strength and modulus, respectively, before fatigue testing, σ 1 and τ 12 are the normal stress and in-plane shear stress, respectively, and N is the fatigue life. Damage criteria of composites were selected according to Shokrieh [25,26] (Equations (7)-(11)).
Fiber-matrix shear failure: where σ i and τ ij are the components of normal and shear stress. S 13 (n, σ, R) and S 23 (n, σ, R) are the ris1ual shear strength in the 1-3 and 2-3 planes after n cycles. The stress-strain relationship of a single-layer lamina can be given as, where ε i and γ ij represent linear and shear strain, respectively, and [C] denotes the threedimensional stiffness matrix. The damage quantity D is introduced to characterize the damage degree of the material. D = 0 means that the material is not damaged, and d = 1 means that the material is completely damaged. When damage occurs, the propagation was described by stiffness degradation as, where Ĉ is the reduced stiffness matrix,Ĉ ij is given by, C 44 ,Ĉ 55 = C 55 , andĈ 66 = C 66 . d it and d ic (i = 1, 2), respectively, express the damage degree of tensile and compression along 2 directions, and d 12 is for the change of in-plane shear damage. The damage variable d a is a continuous function of f a (a = 1t, 1c, 2t, 2c, 12), where G a (a = 1t, 1c, 2t, 2c, 12) is the fracture energy density of materials under different damage forms.

Fatigue Finite Element Model
As shown in Figure 3, Abaqus 6.14 is used to establish the finite element analysis model (FEA). The model is divided into three parts: a fixed support section, working section and loading section and laminates contains eight layers along the thickness direction. In order to improve the calculation accuracy, the grid element type is set to C3D8R, which contributes to 39,820 units in total. The load and constraint conditions are as follows: the load is applied through the in-plane shear force on the upper and lower surfaces of the loading section; translational constraints are applied to all nodes on the upper and lower surfaces of the fixed support section. The material property parameters in the FEA model are as shown in Table 4.   A UMAT subroutine written in the Fortran language according to fatigue criterion and properties degradation model was used to carry out the progressiv age evolution of laminates, the flow chart of which is shown in Figure 4.  A UMAT subroutine written in the Fortran language according to fatigue failure criterion and properties degradation model was used to carry out the progressive damage evolution of laminates, the flow chart of which is shown in Figure 4.  Table 4.  A UMAT subroutine written in the Fortran language according to fatigue failur criterion and properties degradation model was used to carry out the progressive dam age evolution of laminates, the flow chart of which is shown in Figure 4.

Tensile Tests
The average tensile properties in three environments are shown in Table 5. It can be found that, compared with the elastic modulus and tensile strength in RTD, they increase by 12.67% and 12.84%, respectively, in CTD, and decrease by −8.54% and −37.11%, respectively, in ETW. Therefore, it can draw a conclusion that the increase in matrix brittleness at cool temperature increases the elastic modulus and tensile strength correspondingly, while the coupling effect of high temperature and high moisture seriously weakens them. The fatigue test results in three environments are shown in Table 6. A total of 16 specimens were conducted in each environment, which are divided into four groups according to the stress level, with an average of four in each group. The life is listed in the column of fatigue life and the stress level refers to the relative value of the average tensile strength. The S-N curve, also known as the Wöhler curve, is obtained after a number of fatigue tests at different stress levels. In this paper, the two-parameter linear mode is applied [33], where A and B are material constants, S is max fatigue stress, N is the cycle number when composites are fractured. As shown in Figure 5, the fatigue S-N curves in RTD, CTD and ETW are fitted as Equations  Figure 5 shows that, for the same cycle times, when the number of cycl than 14,715, the fatigue strength sequence from large to small is RTD, CT When the number is less than 14,715, that sequence is CTD, RTD, and ETW cool temperature makes the high-cycle fatigue strength decrease, but it is be low-cycle fatigue. In addition, the fatigue dispersion in RTD and ETW is com it is obviously reduced in CTD, which should be related to the embrittlem trix at a cool temperature.
The decreasing rate of fatigue strength from large to small is RTD > which means that the fatigue strength decreases most rapidly at cool Taking the fitting curve results when the fatigue life is 10 6 times for comp tigue strength in RTD, CTD and ETW are 135.9 MPa, 102.1 MPa and 66.2 tively. Compared with in RTD, the fatigue strength in CTD and ETW i 24.88% and 51.28%, respectively. The coupling effect of high temperature on fatigue strength is significantly stronger than that of cool temperature al

Failure Mode Analysis
The fatigue failure morphology in three environments is shown in Figure 7. From Figures 6a and 7a, it can be found that the fatigue failure is cracking and fiber fracture in RTD, accompanied by a certain 45° delamina tion failure. At the same time, it can be seen from Figure 6b, Figure 7b, F Figure 7c that failure modes such as fiber fracture, matrix cracking and del be found in CTD and ETW as well. Delamination runs through the whole rection and basically expands along the 0° direction in CTD and ETW. In obvious necking phenomenon can be seen in ETW which means the prope were weakened seriously. At last, it can be seen from Figure 7 that the failure of ETW is the most severe in three environments.  Figure 5 shows that, for the same cycle times, when the number of cycles n is greater than 14,715, the fatigue strength sequence from large to small is RTD, CTD, and ETW. When the number is less than 14,715, that sequence is CTD, RTD, and ETW. It means the cool temperature makes the high-cycle fatigue strength decrease, but it is beneficial to the low-cycle fatigue. In addition, the fatigue dispersion in RTD and ETW is comparable, and it is obviously reduced in CTD, which should be related to the embrittlement of the matrix at a cool temperature.
The decreasing rate of fatigue strength from large to small is RTD > ETW > CTD which means that the fatigue strength decreases most rapidly at cool temperatures. Taking the fitting curve results when the fatigue life is 10 6 times for comparison, the fatigue strength in RTD, CTD and ETW are 135.9 MPa, 102.1 MPa and 66.2 MPa, respectively. Compared with in RTD, the fatigue strength in CTD and ETW is reduced by 24.88% and 51.28%, respectively. The coupling effect of high temperature and moisture on fatigue strength is significantly stronger than that of cool temperature alone.

Failure Mode Analysis
The fatigue failure morphology in three environments is shown in Figures 6 and 7. From Figures 6a and 7a, it can be found that the fatigue failure is mainly matrix cracking and fiber fracture in RTD, accompanied by a certain 45 • delamination propagation failure. At the same time, it can be seen from Figures 6b,c and 7b,c that failure modes such as fiber fracture, matrix cracking and delamination can be found in CTD and ETW as well. Delamination runs through the whole thickness direction and basically expands along the 0 • direction in CTD and ETW. In addition, an obvious necking phenomenon can be seen in ETW which means the properties of matrix were weakened seriously. At last, it can be seen from Figure 7 that the delamination failure of ETW is the most severe in three environments. Table 7 presents the comparison of the fatigue life between the predicted values and the experimental values in three environments to verify the validity of the FEM model. It can be found that there is a larger error in the low-cycle section. However, in the high-cycle section, the simulation results are in good agreement with the experimental results, and the error is 13.2% at most. In general, the logarithmic life error is relatively small, which is less than 7%.  Table 7 presents the comparison of the fatigue life between the predicted the experimental values in three environments to verify the validity of the FE can be found that there is a larger error in the low-cycle section. Howe high-cycle section, the simulation results are in good agreement with the ex results, and the error is 13.2% at most. In general, the logarithmic life error small, which is less than 7%.

Model Verification
The failure morphology obtained by finite element calculation is shown From Figure 8a,b, it can be found that the fiber fracture damage occurs in a sm the left and right of the specimen in RTD and CTD, and the damage distributio is at a certain angle with the edge of the specimen. From Figure 8c, it can be the specimen in ETW will have fiber failure in a wider range, which is sim failure mode shown in the test.
In conclusion, the model can effectively calculate the fatigue life of sp different environments and reflect their failure modes.  Table 7 presents the comparison of the fatigue life between the predicted val the experimental values in three environments to verify the validity of the FEM m can be found that there is a larger error in the low-cycle section. However high-cycle section, the simulation results are in good agreement with the exper results, and the error is 13.2% at most. In general, the logarithmic life error is re small, which is less than 7%.

Model Verification
The failure morphology obtained by finite element calculation is shown in F From Figure 8a,b, it can be found that the fiber fracture damage occurs in a small the left and right of the specimen in RTD and CTD, and the damage distribution d is at a certain angle with the edge of the specimen. From Figure 8c, it can be fou the specimen in ETW will have fiber failure in a wider range, which is simila failure mode shown in the test.
In conclusion, the model can effectively calculate the fatigue life of specim different environments and reflect their failure modes.
(a)  The failure morphology obtained by finite element calculation is shown in Figure 8. From Figure 8a,b, it can be found that the fiber fracture damage occurs in a small area on the left and right of the specimen in RTD and CTD, and the damage distribution direction is at a certain angle with the edge of the specimen. From Figure 8c, it can be found that the specimen in ETW will have fiber failure in a wider range, which is similar to the failure mode shown in the test.
In conclusion, the model can effectively calculate the fatigue life of specimens in different environments and reflect their failure modes. the left and right of the specimen in RTD and CTD, and the damage distribution direction is at a certain angle with the edge of the specimen. From Figure 8c, it can be found that the specimen in ETW will have fiber failure in a wider range, which is similar to the failure mode shown in the test.
In conclusion, the model can effectively calculate the fatigue life of specimens in different environments and reflect their failure modes.

Failure Process Analysis
According to the test results, the final fatigue failure of the specimen is mainly fiber fracture. Therefore, the development process of fiber fracture damage is observed with the help of the finite element model, so as to analyze the fatigue damage mechanism of the specimen.
The damage evolution process of fiber fracture in three environments is shown in Figure 9 with the stress being set to 60% of their respective static strength. From Figure 9a in RTD, after 190,000 cycles of loading there is a small area of damage in the middle of the specimen, and then the damage extends roughly along the 45 • direction of the edge of the specimen. After 474,000 cycles, the damage extends from the inside to the edge, and finally to the whole thickness and width of the specimen. From Figure 9b in CTD, the results of the fatigue failure process of the specimen are similar to those in RTD. from Figure 9c in ETW, after the initial damage occurs the damage propagation range is relatively larger, and finally a large area of fiber fracture occurs in the middle of the specimen then the overall life decreases significantly.

Failure Process Analysis
According to the test results, the final fatigue failure of the specimen is mainly fiber fracture. Therefore, the development process of fiber fracture damage is observed with the help of the finite element model, so as to analyze the fatigue damage mechanism of the specimen.
The damage evolution process of fiber fracture in three environments is shown in Figure 9 with the stress being set to 60% of their respective static strength. From Figure 9a in RTD, after 190,000 cycles of loading there is a small area of damage in the middle of the specimen, and then the damage extends roughly along the 45° direction of the edge of the specimen. After 474,000 cycles, the damage extends from the inside to the edge, and finally to the whole thickness and width of the specimen. From Figure 9b in CTD, the results of the fatigue failure process of the specimen are similar to those in RTD. from Figure 9c in ETW, after the initial damage occurs the damage propagation range is relatively larger, and finally a large area of fiber fracture occurs in the middle of the specimen then the overall life decreases significantly.

Effect of Different Environments on Fatigue Life
In order to explore the coupling effect of temperature and moisture on the fatigue life of angel-ply laminates, the RTD environmental fatigue life of 10 6 was taken as a fixed point, and the load at this point was kept unchanged at 135.5 MPa. The fatigue life of laminates at different temperatures (−40 • C, −20 • C, 0 • C, 20 • C, 40 • C and 55 • C) and different moisture absorption (0%, 0.5%, 1%, 1.5% and 1.8%) was calculated by the finite element model. Figure 10 shows the fatigue life obtained by finite element calculation for different temperature and moisture combinations. Figure 10a is for fatigue life, and Figure 10b is the normalization type of logarithmic life. Taking the moisture absorption as the horizontal axis and the temperature as the vertical axis, the calculated values under different temperature and moisture absorption are drawn in a palace diagram. Taking the number of life cycles down to one tenth of the value at room temperature in dry state as the critical value, the effect of the hygrothermal environment on fatigue life is divided into a strong influence area and weak influence area, marked with red and blue, respectively. From the calculation results, when the moisture absorption content reaches 1.0% or the temperature exceeds 40 • C, all results are in the strong influence area.

Effect of Different Environments on Fatigue Life
In order to explore the coupling effect of temperature and moisture on the fatigue life of angel-ply laminates, the RTD environmental fatigue life of 10 6 was taken as a fixed point, and the load at this point was kept unchanged at 135.5 MPa. The fatigue life of laminates at different temperatures (−40 °C, −20 °C, 0 °C, 20 °C, 40 °C and 55 °C) and different moisture absorption (0%, 0.5%, 1%, 1.5% and 1.8%) was calculated by the finite element model. Figure 10 shows the fatigue life obtained by finite element calculation for different temperature and moisture combinations. Figure 10a is for fatigue life, and Figure 10b is the normalization type of logarithmic life. Taking the moisture absorption as the horizontal axis and the temperature as the vertical axis, the calculated values under different temperature and moisture absorption are drawn in a palace diagram. Taking the number of life cycles down to one tenth of the value at room temperature in dry state as the critical value, the effect of the hygrothermal environment on fatigue life is divided into a strong influence area and weak influence area, marked with red and blue, respectively. From the calculation results, when the moisture absorption content reaches 1.0% or the temperature exceeds 40 °C, all results are in the strong influence area. Assuming that the effects of temperature and moisture are independent of each other, by fitting the above data, the environmental factors of fatigue life can be expressed as following, where F is the environmental factors of fatigue life, N and N0 are the fatigue life in hygrothermal environment and in RTD, respectively, and M and M∞ are the current and maximum moisture absorption content, respectively. T is the current temperature, T0 is Assuming that the effects of temperature and moisture are independent of each other, by fitting the above data, the environmental factors of fatigue life can be expressed as following, where F is the environmental factors of fatigue life, N and N 0 are the fatigue life in hygrothermal environment and in RTD, respectively, and M and M ∞ are the current and maximum moisture absorption content, respectively. T is the current temperature, T 0 is the reference temperature, and this formula takes 20 • C. T 1 is the maximum temperature and 55 • C is taken in this formula.

Conclusions
In this paper, the tests of moisture absorption, static tensile and tensile-tensile fatigue were conducted on carbon fiber CF3250/3238A angle-ply laminates. Fatigue S-N curves and failure damage modes in three environments were obtained. Based on the experimental study, the finite element analysis model of progressive damage was established, and the influence of temperature and humidity on fatigue performance was discussed. Finally, the