Low-cycle fatigue life prediction method considering influence of contact pressure on fatigue strength

Abstract

The invention discloses a low-cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength, and provides a fatigue life prediction method considering the influence of contact pressure on low-cycle fatigue strength by analyzing the influence of contact pressure on fatigue strength. The method comprises the following specific steps: (1) considering the influence of contact pressure on fatigue strength to obtain a correction coefficient; (2) obtaining the influence of fatigue strength change on the fatigue life according to the fatigue life curve; (3) substituting the correction coefficient into a fatigue life formula to obtain a fatigue life prediction formula considering the influence of the contact pressure on the fatigue strength; and (4) carrying out finite element analysis on the material structure, determining a fatigue dangerous point through stress-strain distribution, substituting parameters such as strain, stress, a friction coefficient and contact pressure into a fatigue life prediction formula, and finally calculating to obtain the fatigue life. The fatigue life prediction method is simple and convenient in calculation, accurate in result, suitable for fatigue life prediction under complex working conditions and high in application value.

Description

Technical Field

[0001] The present invention belongs to the technical field of structural fatigue life prediction, and relates to fatigue analysis and life calculation under complex stress states such as multi-axis fatigue and fretting fatigue. Specifically, it is a low-cycle fatigue life prediction method that considers the influence of contact pressure on fatigue strength. The method is suitable for fatigue analysis of components in high-load mechanical systems such as aircraft engines and structural fatigue life assessment. Background Art

[0002] Fatigue refers to the process in which a material is gradually damaged, cracked, and finally breaks under repeated stress or strain. In many engineering fields such as aerospace, mechanical manufacturing, and transportation, fatigue has always been an important topic that has received much attention. Especially in the field of aircraft engines, fatigue has become one of the main causes of component failure. The danger of fatigue failure is manifested in the fact that there is no obvious precursor when the structure reaches its fatigue life, that is, when the structure reaches its fatigue life, it often breaks and disintegrates suddenly without any observable signs such as significant deformation. This sudden fatigue failure greatly threatens the operational safety of the mechanical system. Therefore, it is necessary to focus on the prediction method of fatigue life to prevent major accidents caused by structural fatigue.

[0003] At present, the prediction methods for fatigue life mostly use empirical formula method or experimental method. Both methods are widely used in engineering practice, but each has its limitations. Among them, the empirical formula method is based on the empirical formula summarized from a large amount of experimental data for calculation, and it is necessary to consider the influence of many factors such as stress concentration, average stress, loading frequency, ambient temperature, etc. Although this method is relatively simple, due to the large number of influencing factors and the complex coupling relationship between the factors, the prediction accuracy is often difficult to meet the requirements. The experimental method obtains life data by performing fatigue tests on actual structures or specimens. Although this method can obtain relatively accurate results, it has low universality and complex operation.

[0004] Fatigue strength refers to the maximum stress that a material can withstand without damage under an infinite number of alternating loads. It reflects the material's ability to withstand long-term cyclic loads and is one of the key parameters for measuring the fatigue life of a material. In actual engineering, fatigue strength is affected by many factors, such as material properties, component geometry, surface state, processing technology, environmental conditions, etc., among which load characteristics have a particularly significant impact on fatigue strength. In many engineering applications, such as turbine blades, bearings, and gears in aircraft engines, the fatigue behavior of materials in the contact area will change significantly under the combined effects of high temperature, high pressure, and cyclic loads. Contact pressure not only changes the stress distribution on the material surface, but also affects the initiation and propagation of fatigue cracks. However, most of the existing fatigue life prediction methods fail to fully consider the effect of contact pressure on fatigue strength. For example, traditional fatigue life prediction models usually assume that the material is subjected to cyclic loading under an ideal state of no friction and no contact pressure, while changes in friction coefficient and contact pressure in actual working conditions will significantly affect fatigue strength, resulting in a certain deviation between the prediction results and engineering practice.

[0005] Based on the above analysis, the existing fatigue life prediction methods fail to fully consider the impact of contact pressure on fatigue strength under complex working conditions, such as occasions with large contact pressure, resulting in a large deviation between the results of fatigue life prediction and the fatigue life in actual engineering applications. In equipment such as aircraft engines and turbomachinery that are subjected to high loads and high temperatures, accurate fatigue life prediction is of great significance for improving structural safety and reducing the occurrence of fatigue failure accidents. Therefore, how to fully consider the impact of contact pressure on fatigue strength in the process of low-cycle fatigue life prediction, and then improve the accuracy of fatigue life prediction, is a technical problem that needs to be solved urgently. Summary of the invention

[0006] (I) Purpose of the invention

[0007] In view of the defects and shortcomings of the existing fatigue life prediction methods, such as the failure to fully consider the influence of contact pressure on fatigue strength, resulting in a large deviation between the fatigue life prediction results and the actual engineering practice, in order to solve at least one of the above-mentioned and other technical problems in the prior art, the present invention aims to provide a low-cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength. By introducing a contact pressure correction coefficient, the influence of contact pressure on fatigue strength and the influence of changes in fatigue strength on the fatigue life curve are considered, and a calculation method for the low-cycle fatigue life considering the influence of contact pressure on fatigue strength is given. The low-cycle fatigue life under different contact pressures can be obtained, thereby improving the accuracy of the prediction results and the engineering application value, and solving the problem that the existing low-cycle fatigue life prediction methods do not consider the influence of contact pressure on fatigue strength, resulting in a certain deviation between the low-cycle fatigue life prediction results and the actual engineering practice.

[0008] (II) Technical solution

[0009] In order to achieve the purpose of the invention and solve the technical problems, the present invention adopts the following technical solutions:

[0010] A low cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength is used to predict the fatigue life of a structure under a low cycle fatigue condition containing contact pressure, and at least comprises the following steps:

[0011] SS1. The correction coefficient α considering the influence of contact pressure on fatigue strength is introduced, the relationship between contact pressure and fatigue strength is established, and the correction coefficient α(p) considering the influence of contact pressure on fatigue strength is obtained:

[0012]

[0013] Among them, σ f is the fatigue strength coefficient of the material without considering the contact pressure, σ' f is the fatigue strength coefficient of the material under consideration of contact pressure and p is the contact pressure, w is the width of the contact area of ​​the cylindrical micro-motion pad, μ is the friction coefficient, and q is the notch sensitivity coefficient;

[0014] SS2. Based on the fatigue life curve of the material, analyze the influence of fatigue strength change on fatigue life, and establish the fatigue strength σ without considering the influence of contact pressure f The relationship function f(σ f ,N):

[0015]

[0016] Where Δε is the total strain amplitude, Δε e is the elastic strain amplitude, Δε p is the plastic strain amplitude, N is the fatigue life, E is the elastic modulus, σ f is the fatigue strength coefficient without considering the contact pressure, b is the fatigue strength index, ε f is the fatigue plasticity coefficient, c is the fatigue plasticity index;

[0017] SS3. Considering the influence of contact pressure on fatigue strength, the correction coefficient is substituted into the relationship function between fatigue strength and fatigue life to obtain the low-cycle fatigue life prediction formula considering the influence of contact pressure on fatigue strength:

[0018]

[0019] SS4. Establish a three-dimensional finite element model based on the material structure parameters, and calculate the stress and strain distribution of the structure based on the finite element model. Determine the fatigue danger point of the structure through stress and strain distribution analysis and extract relevant parameters of the area, including at least the fatigue strength coefficient σ f , total strain amplitude Δε, elastic strain amplitude Δε e , plastic strain amplitude Δε p , friction coefficient μ and contact pressure p;

[0020] SS5. Substitute the fatigue danger point related parameters obtained in step SS4 into the low-cycle fatigue life prediction formula considering the influence of contact pressure on fatigue strength constructed in step SS3 to calculate the low-cycle fatigue life considering the influence of contact pressure on fatigue strength;

[0021] SS6. Repeat steps SS4 and SS5 to analyze multiple potential fatigue risk points in the structure, obtain the fatigue life of each point, and compare the fatigue life of all analysis points to determine the minimum fatigue life of the structure as the low-cycle fatigue life prediction result of the structure considering the influence of contact pressure.

[0022] (III) Technical Effect

[0023] Compared with the prior art, the low-cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength of the present invention has the following beneficial and significant technical effects:

[0024] (1) The present invention takes into account the influence of contact pressure on fatigue strength and the influence of changes in fatigue strength on the low-cycle fatigue life curve by introducing a correction coefficient, thereby overcoming the problem of calculation deviation caused by failure to consider contact pressure in existing low-cycle fatigue life prediction methods. The present invention provides in detail a calculation method for low-cycle fatigue life that takes into account the influence of contact pressure on fatigue strength, and can obtain low-cycle fatigue life under different contact pressures. This method not only improves the accuracy of fatigue life prediction, but also expands its scope of application, making it applicable to a variety of high contact pressure conditions.

[0025] (2) The method of the present invention conducts an in-depth analysis of the relationship between fatigue strength change and fatigue life curve, and accurately describes the relationship between fatigue strength and fatigue life through the modified Manson-Coffin formula. This method is particularly suitable for fatigue life prediction under high stress amplitude and complex working conditions, and can better reflect the law of fatigue strength change with strain amplitude, thereby improving the accuracy of life prediction.

[0026] (3) The present invention provides a simple calculation method, which greatly reduces the difficulty of calculation by reasonably simplifying the complex multi-axial stress state and contact pressure calculation, making the method easy to apply in practical engineering. In addition, the stress and strain distribution of fatigue danger points in the structure are accurately calculated using finite element analysis (FEA), making the present invention not only suitable for fatigue life prediction under ideal conditions, but also suitable for life assessment under actual complex working conditions.

[0027] (4) The present invention has high engineering application value. Its method is relatively accurate and simple to calculate. It is suitable for application in the fields of aviation, mechanical manufacturing, etc., especially for the prediction of structural fatigue life under high contact pressure and complex load conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the implementation process of the low cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength of the present invention;

[0029] Figure 2 It is the fatigue life curve. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical scheme and advantages of the implementation of the present invention clearer, the technical scheme in the embodiment of the present invention will be described in more detail below in conjunction with the drawings in the embodiment of the present invention. The described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. The embodiments described below with reference to the drawings are exemplary and are intended to be used to explain the present invention, and should not be understood as limiting the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0031] The present invention aims to provide a low-cycle fatigue life prediction method taking into account the influence of contact pressure on fatigue strength. By introducing a contact pressure correction coefficient, the influence of contact pressure on fatigue strength and the influence of changes in fatigue strength on fatigue life curves are taken into account. A calculation method for low-cycle fatigue life taking into account the influence of contact pressure on fatigue strength is provided. The low-cycle fatigue life under different contact pressures can be obtained, thereby improving the accuracy of the prediction results and the engineering application value, and solving the problem that the existing low-cycle fatigue life prediction methods do not take into account the influence of contact pressure on fatigue strength, resulting in a deviation between the low-cycle fatigue life prediction results and the actual engineering practice.

[0032] As a specific example, Figure 1 As shown, the low cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength in the embodiment of the present invention mainly includes the following steps when implemented:

[0033] SS1. Introducing a correction factor to consider the effect of contact pressure on fatigue strength

[0034] In order to obtain the fatigue life prediction formula considering the influence of contact pressure on fatigue strength, the correction coefficient α is introduced to establish the relationship between contact pressure and fatigue strength, which can be expressed as:

[0035]

[0036] according to p is the contact pressure, b is the width of the contact area of ​​the cylindrical micro-motion pad, w is the friction coefficient, q is the notch sensitivity coefficient, and we can get:

[0037]

[0038] Preferably, in this step, the calculation of the correction coefficient α can also comprehensively consider the influence of contact pressure under multi-axis fatigue conditions, and use contact pressure correction coefficients in different directions to perform multi-axis correction on fatigue strength based on the anisotropic fatigue behavior of the material. Different friction coefficients and contact pressures are introduced for the stress and strain distribution in each axis direction, so as to ensure that under complex load conditions, the fatigue life prediction results can more accurately reflect the actual working conditions.

[0039] Preferably, the calculation of the correction coefficient α may also take into account the nonlinear effect of the contact pressure, and introduce an exponential function to describe the relationship between the contact pressure and the fatigue strength:

[0040]

[0041] Where n is a nonlinear exponent obtained by fitting experimental data.

[0042] SS2. Analyze the effect of fatigue strength on life

[0043] Based on Figure 2 The fatigue life curve of the material shown in the figure is used to analyze the influence of fatigue strength change on fatigue life, and the fatigue strength σ expressed by the Manson-Coffin formula without considering the influence of contact pressure is established. f The relationship function f(σ f ,N):

[0044]

[0045] Where Δε is the total strain amplitude, Δε is e is the elastic strain amplitude, Δε p is the plastic strain amplitude, N is the fatigue life, σ f is the fatigue strength coefficient without considering the contact pressure, b is the fatigue strength index, ε fis the fatigue plasticity coefficient and c is the fatigue plasticity index.

[0046] According to this formula, fatigue strength σ f It has a nonlinear relationship with fatigue life N. The first term is the elastic part, corresponding to Figure 2 The elastic line a in the figure reflects the strain amplitude of the material within the elastic range, which is mainly composed of the elastic modulus E and fatigue strength σ of the material. f Determine; the latter is the plastic strain part, corresponding to Figure 2 The plastic line b in the figure represents the strain amplitude of the material within the plastic range, which is determined by the fatigue plasticity coefficient ε f and fatigue plasticity index c. The total strain line is Figure 2 The line c in the figure corresponds to the fatigue strength Figure 1 The ordinate of the midpoint A. The fatigue life decreases with the increase of the total strain amplitude Δε.

[0047] Preferably, in step SS2, for the multiaxial stress state, the equivalent strain amplitude Δε is used. eq Instead of the uniaxial strain amplitude Δε, it can better reflect the fatigue behavior of the material under complex load conditions and transform the fatigue life prediction under complex stress state into the fatigue life prediction corresponding to the uniaxial stress state:

[0048] Δεeq={[(2 / 3)*((Δε 1 -Δε 2 ) 2 +(Δε 2 -Δε 3 ) 2 +(Δε 3 -Δε 1 ) 2 )]} 12

[0049] Among them, Δε 1 , Δε 2 , Δε 3 are the strain amplitudes of the structure in the three principal strain directions.

[0050] SS3. Consider the effect of contact pressure on fatigue life

[0051] When considering the effect of contact pressure on fatigue strength, the fatigue strength decreases and the corresponding fatigue strength is Figure 2 The vertical coordinate of the midpoint B reflects the change of fatigue strength under the contact pressure. Correspondingly, when the contact pressure is not considered, the fatigue strength σ f Corresponds to Figure 2 The ordinate of point A in the graph reflects the fatigue behavior of the material under no contact pressure.

[0052] The elastic line is represented by:

[0053] The strain-life relationship corresponding to point A is:

[0054] The strain-life relationship corresponding to point B is:

[0055] From the strain-life relationship corresponding to points A and B, we can get:

[0056] In summary, the low-cycle fatigue life prediction formula considering the influence of contact pressure on fatigue strength is:

[0057]

[0058] Preferably, in this step, the influence of the average stress on the fatigue life can be considered, and the Smith-Watson-Topper (SWT) parameter can be further introduced to correct the fatigue life prediction formula, and the corrected formula is:

[0059]

[0060] Among them, σ max is the maximum stress of the structure under working load conditions, Δε is the total strain amplitude, E is the elastic modulus of the material, σ' f is the corrected fatigue strength coefficient, ε f is the fatigue plasticity coefficient, b is the fatigue strength index, c is the fatigue plasticity index, and N is the fatigue life. The SWT parameter combines the maximum stress and strain amplitude, and is applicable to working conditions with both cyclic loads and static loads, which significantly improves the accuracy of fatigue life prediction, especially for fatigue life calculation under high stress amplitude and complex stress state.

[0061] In addition, considering the influence of the average stress, the modified Morrow formula can also be used in this step to correct the fatigue life prediction. The modified formula is:

[0062]

[0063] Among them, σ' f is the corrected fatigue strength coefficient, σ m is the average stress, E is the elastic modulus of the material, ε fis the fatigue plasticity coefficient, b is the fatigue strength index, c is the fatigue plasticity index, N is the fatigue life, and Δε is the total strain amplitude. The presence of mean stress usually reduces the fatigue life of the material, especially under high mean stress conditions. The modified Morrow formula can effectively correct the life prediction results of low-cycle fatigue and high-cycle fatigue by considering the influence of mean stress, and is particularly suitable for conditions with significant mean stress.

[0064] SS4. Establish finite element model to calculate and determine the fatigue danger point of the structure

[0065] According to the material structure parameters, a three-dimensional finite element model is established and the model is meshed. The mesh model is imported into ANSYS APDL for finite element calculation, and the fatigue danger point is determined by the stress-strain distribution to obtain the strain Δε, Δε e , Δε p , fatigue strength coefficient σ f , friction coefficient μ, contact pressure p and other parameters.

[0066] Preferably, this step can use sub-model technology to improve the computational efficiency and accuracy of finite element analysis. First, a coarse grid model of the overall structure is established, and then a fine grid sub-model is established in the fatigue risk area. By applying the displacement boundary conditions of the overall model to the sub-model, efficient and accurate stress-strain analysis can be achieved, so as to significantly reduce the computing time and resource consumption while ensuring the calculation accuracy.

[0067] In addition, the nonlinear yield behavior of the material can be further considered when establishing a three-dimensional finite element model. By introducing the plastic flow rules and hardening model of the material, the nonlinear stress-strain relationship in actual working conditions can be simulated. Especially in the fatigue life prediction under high strain amplitude, the strain changes of fatigue danger points can be more accurately predicted through finite element calculation.

[0068] SS5. Substitute into fatigue life prediction formula

[0069] The strain, stress, friction coefficient, contact pressure and other parameters of the fatigue danger point obtained in step SS4 are brought into the low-cycle fatigue life prediction formula considering the influence of contact pressure on fatigue strength constructed in step SS3, so as to obtain the fatigue life considering the influence of contact pressure on fatigue strength.

[0070] Preferably, in step SS5, the discreteness and uncertainty of material parameters can be considered by introducing a probability statistics method, and the material parameters (such as fatigue strength coefficient, fatigue plasticity coefficient, etc.) can be randomly sampled by using a Monte Carlo simulation method to generate multiple sets of parameter sets, and the fatigue life can be calculated for each set of parameter sets to finally obtain the probability distribution of fatigue life. This optimization method can provide more comprehensive fatigue life prediction results, including information such as average life and reliability.

[0071] SS6. Analyze multiple fatigue risk points

[0072] Repeat steps SS4 and SS5 to analyze multiple potential fatigue risk points in the structure, obtain the fatigue life of each point, and compare the fatigue life of all analysis points to determine the minimum fatigue life of the structure as the low-cycle fatigue life prediction result of the structure considering the influence of contact pressure.

[0073] Preferably, in this step, the damage accumulation theory can be introduced to consider the influence of variable amplitude loads. The Miner linear cumulative damage theory is used to decompose the variable amplitude load spectrum into several equal amplitude load blocks, and the fatigue life corresponding to each load block is calculated respectively. Then, the total fatigue life is calculated according to the linear cumulative damage theory:

[0074]

[0075] Where D is the cumulative damage, n i is the stress level σ i The actual number of cycles under i is the stress level σ i The fatigue life under , n is the total number of load blocks, and when the cumulative damage D reaches 1, the structure is considered to have fatigue failure. This method can effectively deal with the problem of fatigue life prediction under variable amplitude load conditions, especially under complex conditions. By discretizing the load spectrum into multiple equal amplitude load blocks and calculating the fatigue life separately, the damage accumulation effect of the structure under different stress levels can be accurately predicted, thereby improving the accuracy of fatigue life prediction.

[0076] In summary, the present invention proposes a method for predicting low-cycle fatigue life that considers the influence of contact pressure on fatigue strength. This method combines the fatigue life curve, analyzes the influence of contact pressure on fatigue strength, and provides a fatigue life prediction method that considers the influence of contact pressure on low-cycle fatigue strength. This method is compatible with other methods, has clear physical meaning, and a simple and clear calculation process. The subsequent optimization results obtained based on the proposed method are more effective, laying a foundation for predicting fatigue life considering multiple other factors.

[0077] Through the above embodiments, the purpose of the present invention is fully and effectively achieved. Those skilled in the art can understand that the present invention includes but is not limited to the contents described in the drawings and the above specific embodiments. Although the present invention has been described with respect to the most practical and preferred embodiments currently considered, it should be understood that the present invention is not limited to the disclosed embodiments, and any modification that does not deviate from the functional and structural principles of the present invention will be included in the scope of the claims.

Claims

1. A low-cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength, characterized in that: The fatigue prediction method comprises at least the following steps: SS1. The correction coefficient α considering the influence of contact pressure on fatigue strength is introduced, the relationship between contact pressure and fatigue strength is established, and the correction coefficient α(p) considering the influence of contact pressure on fatigue strength is obtained: Among them, σ f is the fatigue strength coefficient of the material without considering the contact pressure, σ' f is the fatigue strength coefficient of the material under consideration of contact pressure and p is the contact pressure, w is the width of the contact area of ​​the cylindrical micro-motion pad, μ is the friction coefficient, and q is the notch sensitivity coefficient; SS2. Based on the fatigue life curve of the material, analyze the influence of fatigue strength change on fatigue life, and establish the fatigue strength σ without considering the influence of contact pressure f The relationship function f(σ f ,N): Where Δε is the total strain amplitude, Δε e is the elastic strain amplitude, Δε p is the plastic strain amplitude, N is the fatigue life, E is the elastic modulus, σ f is the fatigue strength coefficient without considering the contact pressure, b is the fatigue strength index, ε f is the fatigue plasticity coefficient, c is the fatigue plasticity index; SS3. Considering the influence of contact pressure on fatigue strength, the correction coefficient is substituted into the relationship function between fatigue strength and fatigue life to obtain the low-cycle fatigue life prediction formula considering the influence of contact pressure on fatigue strength: SS4. Establish a three-dimensional finite element model based on the material structure parameters, and calculate the stress and strain distribution of the structure based on the finite element model. Determine the fatigue danger point of the structure through stress and strain distribution analysis and extract relevant parameters of the area, including at least the fatigue strength coefficient σ f , total strain amplitude Δε, elastic strain amplitude Δε e , plastic strain amplitude Δε p , friction coefficient μ and contact pressure p; SS5. Substitute the fatigue danger point related parameters obtained in step SS4 into the low-cycle fatigue life prediction formula considering the influence of contact pressure on fatigue strength constructed in step SS3 to calculate the low-cycle fatigue life considering the influence of contact pressure on fatigue strength; SS6. Repeat steps SS4 and SS5 to analyze multiple potential fatigue risk points in the structure, obtain the fatigue life of each point, and compare the fatigue life of all analysis points to determine the minimum fatigue life of the structure as the low-cycle fatigue life prediction result of the structure considering the influence of contact pressure.

2. The low cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength according to claim 1 is characterized in that: In the above step SS1, the calculation of the correction coefficient α also comprehensively considers the influence of contact pressure under multi-axis fatigue conditions. Based on the anisotropic fatigue behavior of the material, the contact pressure correction coefficients in different directions are used to perform multi-axis correction on the fatigue strength. Different friction coefficients and contact pressures are introduced for the stress and strain distribution in each axis direction to ensure that under complex load conditions, the fatigue life prediction results can more accurately reflect the actual working conditions.

3. The low cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength according to claim 1 is characterized in that: In the above step SS1, the calculation of the correction coefficient α also takes into account the nonlinear effect of the contact pressure, and introduces an exponential function to describe the relationship between the contact pressure and the fatigue strength: Where n is a nonlinear exponent obtained by fitting experimental data.

4. The low cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength according to claim 1 is characterized in that: In the above step SS2, for the multiaxial stress state, the equivalent strain amplitude Δε is used eq Instead of the uniaxial strain amplitude Δε, it can better reflect the fatigue behavior of the material under complex load conditions and transform the fatigue life prediction under complex stress state into the fatigue life prediction corresponding to the uniaxial stress state: Δεeq={[(2 / 3)*((Δε1-Δε2) 2 +(D2-D3) 2 +(D3-D1) 2 )]} 12 Among them, Δε1, Δε2, and Δε3 are the strain amplitudes of the structure in the three principal strain directions, respectively.

5. The low cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength according to claim 1, characterized in that: In the above step SS3, considering the influence of average stress on fatigue life, SWT parameters are further introduced to correct the fatigue life prediction formula. The corrected formula is: Among them, σ max is the maximum stress of the structure under working load conditions, Δε is the total strain amplitude, E is the elastic modulus of the material, σ' f is the corrected fatigue strength coefficient, ε f is the fatigue plasticity coefficient, b is the fatigue strength index, c is the fatigue plasticity index, and N is the fatigue life.

6. The low cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength according to claim 1 is characterized in that: In the above step SS3, the influence of the mean stress is considered and the modified Morrow formula is used to correct the fatigue life prediction. The corrected formula is: Among them, σ' f is the corrected fatigue strength coefficient, σ m is the average stress, E is the elastic modulus of the material, ε f is the fatigue plasticity coefficient, b is the fatigue strength index, c is the fatigue plasticity index, N is the fatigue life, and Δε is the total strain amplitude.

7. The low cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength according to claim 1 is characterized in that: In the above step SS4, sub-model technology is used to improve the calculation efficiency and accuracy of finite element analysis. First, a coarse grid model of the overall structure is established, and then a fine grid sub-model is established in the fatigue risk area. By applying the displacement boundary conditions of the overall model to the sub-model, efficient and accurate stress-strain analysis is achieved, so as to significantly reduce the calculation time and resource consumption while ensuring the calculation accuracy.

8. The low cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength according to claim 1 is characterized in that: In the above step SS4, the nonlinear yield behavior of the material is further considered when establishing the three-dimensional finite element model. By introducing the plastic flow rules and hardening model of the material, the nonlinear stress-strain relationship in the actual working condition is simulated to more accurately predict the strain change of the fatigue danger point.

9. The low cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength according to claim 1, characterized in that: In the above step SS5, a probability statistical method is introduced to consider the discreteness and uncertainty of material parameters. The Monte Carlo simulation method is used to randomly sample material parameters and generate multiple parameter sets. The fatigue life is calculated for each parameter set to finally obtain the fatigue life probability distribution.

10. The low cycle fatigue life prediction method considering the influence of contact pressure on fatigue strength according to claim 1, characterized in that: In the above step SS6, the damage accumulation theory is introduced to consider the influence of variable amplitude load. The Miner linear cumulative damage theory is used to decompose the variable amplitude load spectrum into several equal amplitude load blocks, and the fatigue life corresponding to each load block is calculated respectively. Then, the total fatigue life is calculated according to the linear cumulative damage theory: Where D is the cumulative damage, n i is the stress level σ i The actual number of cycles under i is the stress level σ i Fatigue life under load, n is the total number of load blocks, when the cumulative damage D reaches 1, the structure is considered to have fatigue failure.

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