A fatigue damage analysis method of additive manufacturing dot array structure considering roughness influence

Abstract

The application provides a kind of fatigue damage analysis method of additive manufacturing lattice structure considering roughness influence, comprising: carrying out fatigue test on lattice structure test piece, and obtaining fatigue life test data of lattice structure test piece;And based on fatigue life test data, damage evolution equation is constructed;Based on the undulating form of the surface of lattice structure test piece, the surface roughness influence factor of lattice structure test piece is obtained;Surface roughness influence factor is introduced into damage evolution equation, and the damage evolution equation considering surface roughness is obtained;Based on finite element software, the finite element model of lattice structure test piece is constructed, the fatigue life prediction of lattice structure test piece is carried out in combination with the damage evolution equation considering surface roughness, and the fatigue damage analysis of additive manufacturing lattice structure considering roughness influence is completed.The technical scheme of the application can reduce repeated test, reduce calculation cost, and also can provide reference for the design of lattice structure.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of fatigue damage analysis of additive manufacturing lattice structure, and particularly relates to a fatigue damage analysis method for additive manufacturing lattice structure considering the influence of roughness. BACKGROUND

[0002] In recent years, with the rapid development of aerospace technology, the demand for lightweight of aircraft and its components is increasing. Due to the constraints of process accessibility of traditional manufacturing methods, it is increasingly difficult to reduce the weight of the product structure of key components of aircraft. In order to pursue a lighter weight reduction target, the structure becomes more and more complex, and the traditional manufacturing cost is extremely high, or even cannot be manufactured, which has seriously restricted the development of high-performance aircraft components. Lattice structures are attracting attention due to their lightweight, high specific strength and design flexibility. They are usually composed of periodic or non-periodic units, and it is difficult to achieve them by traditional manufacturing methods due to their complex structural state. In recent years, with the development of additive manufacturing technology, lattice structures can be manufactured more easily through reasonable design. This makes lattice structures have wide application prospects in the fields of aerospace, automotive engineering and biomedicine. However, the complex geometric characteristics of lattice structures and the micro defects (such as pores, un-melted areas, etc.) that may be generated in the additive manufacturing process pose a major challenge to the prediction of their fatigue life. These defects and geometric complexities increase the uncertainty of fatigue performance, making it difficult for traditional fatigue life prediction methods to effectively evaluate the fatigue life in actual applications. Existing research shows that the fatigue characteristics of lattice structures under different conditions have not been fully explored, and relevant literature is relatively scarce. Therefore, it is particularly important to conduct systematic research on the fatigue characteristics of lattice structures.

[0003] Under this background, it is urgent to develop a fatigue life prediction method specifically for lattice structures, which takes into account the geometric complexity and defects that may occur in the additive manufacturing process. This method should not only focus on the influence of individual defects, but also consider the interaction between defects and their impact on overall fatigue performance. In addition, the fatigue performance of lattice structures is influenced by a variety of factors, including load mode, material properties and manufacturing parameters, which together further increase the difficulty of fatigue life prediction. Therefore, for the fatigue life prediction of lattice structures, innovative theoretical and technical solutions are urgently needed to improve the accuracy and reliability of the prediction. This is not only crucial for enhancing the application performance of lattice structures, but also will provide strong support for engineering design and optimization in related fields. SUMMARY

[0004] In order to solve the problems existing in the prior art, the application provides a fatigue damage analysis method for additive manufacturing lattice structure considering the influence of roughness, which improves the accuracy of fatigue life prediction through computer simulation, and makes the prediction results closer to engineering practice.

[0005] To achieve the above object, the application provides the following scheme:

[0006] A fatigue damage analysis method of an additive manufacturing lattice structure considering roughness influence, comprising:

[0007] Performing a fatigue test on the lattice structure specimen to obtain fatigue life test data of the lattice structure specimen; and constructing a damage evolution equation based on the fatigue life test data;

[0008] Obtaining a surface roughness influence factor of the lattice structure specimen based on the undulating morphology of the surface of the lattice structure specimen;

[0009] Introducing the surface roughness influence factor into the damage evolution equation to obtain a damage evolution equation considering surface roughness;

[0010] Constructing a finite element model of the lattice structure specimen based on a finite element software, combining the damage evolution equation considering surface roughness to perform fatigue life prediction on the lattice structure specimen, and completing fatigue damage analysis of the additive manufacturing lattice structure considering roughness influence.

[0011] Preferably, the method for constructing the damage evolution equation comprises:

[0012] Introducing a damage variable into a constitutive model to obtain a stress-strain relationship of the lattice structure specimen after being stressed; wherein the value of the damage variable is used to judge the damage degree of the material at a representative volume element of the lattice structure specimen;

[0013] Based on the fatigue life test data, combining the damage variable and the stress-strain relationship, the damage evolution equation is constructed.

[0014] Preferably, the constitutive model comprises an elastic constitutive model and a plastic constitutive model; wherein the elastic constitutive model adopts Hooke's law, and the plastic constitutive model adopts Chaboche model.

[0015] Preferably, the method for obtaining the surface roughness influence factor of the lattice structure specimen comprises:

[0016] Based on the length and height of the measurement section of the lattice structure specimen, the average surface roughness of the lattice structure specimen is obtained;

[0017] Polishing the surface of the lattice structure specimen to be smooth to obtain a reference surface roughness;

[0018] Based on the average surface roughness and the reference surface roughness, combining the additive manufacturing direction of the lattice structure specimen, the surface roughness influence factor of the lattice structure specimen is obtained.

[0019] Preferably, the expression of the surface roughness degree influence factor is as follows:

[0020]

[0021] In the formula, γ is an included angle parameter, R a is an average surface roughness, R a0 is a reference surface roughness.

[0022] Preferably, the method for obtaining the damage evolution equation considering the surface roughness comprises the following steps:

[0023] Integrating the damage variable in the damage evolution equation to 0 to 1, a fatigue life calculation formula of the lattice structure specimen is obtained;

[0024] Based on the fatigue life test data of the lattice structure specimen, an S-N curve of the lattice structure specimen with a smooth surface under a preset stress ratio is fitted;

[0025] Based on the S-N curve and the fatigue life calculation formula, the fatigue life of the lattice structure specimen with a smooth surface under the uniaxial loading condition of the preset stress ratio is obtained through the continuous damage mechanics theory;

[0026] Based on the fatigue life under the uniaxial loading condition and the fatigue life test data, the parameters of the damage evolution equation are calibrated by using the MATLAB curve fitting tool, the least square method and the finite element numerical simulation;

[0027] The calibrated parameters of the damage evolution equation and the surface roughness degree influence factor are introduced into the damage evolution equation, and the damage evolution equation considering the surface roughness is obtained.

[0028] Compared with the prior art, the beneficial effects of the present application are that the lattice structure fatigue damage analysis method considering the roughness influence can predict the fatigue life of various lattice configuration specimens under different roughnesses without developing a large number of tests, reduce repeated tests, reduce the calculation cost, and also provide a reference for the design of the lattice structure. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the present application, the following briefly introduces the drawings needed to be used in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor.

[0030] Figure 1 Flow chart of the additive manufacturing lattice structure fatigue damage analysis method considering the roughness influence of the embodiments of the present application;

[0031] Figure 2 Flow chart for calculating fatigue life of lattice structure based on continuum damage mechanics theory for embodiments of the present application;

[0032] Figure 3 Schematic diagram of test piece shape and loading for additive manufacturing material lattice structure for embodiments of the present application;

[0033] Figure 4 Schematic diagram of load peak-life curve for different roughness for embodiments of the present application;

[0034] Figure 5 Schematic diagram of displacement-life curve for lattice structure for embodiments of the present application. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0036] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application will be further described in detail below with reference to the drawings and specific embodiments.

[0037] The terms appearing in the embodiments are explained as follows:

[0038] The S-N curve represents the number of load cycles that the material can withstand at different stress levels. In the S-N curve, S represents the stress level, usually expressed in stress amplitude or maximum stress; N represents the number of load cycles that the material can withstand at this stress level, i.e. fatigue life. This curve is an empirical formula obtained by a large number of fatigue tests on the material, and is the basis for the study of material fatigue performance.

[0039] The Chaboche model is a constitutive model used to describe the inelastic deformation behavior of materials under cyclic loading, especially suitable for material analysis under high temperature and complex stress state. The model was proposed by French engineer Chaboche, and is mainly used to solve the inelastic analysis problem of structures under cyclic loading.

[0040] Embodiment one

[0041] As shown in Figure 1 A fatigue damage analysis method for additive manufacturing lattice structure considering the influence of roughness, comprising:

[0042] S1: fatigue test is performed on the lattice structure test piece to obtain fatigue life test data of the lattice structure test piece; and a damage evolution equation is constructed based on the fatigue life test data; a further implementation is that the method for constructing the damage evolution equation comprises:

[0043] S11: introducing the damage variable into the constitutive model to obtain the stress-strain relationship of the lattice structure test piece after being stressed; wherein the value of the damage variable is used to judge the damage degree of the material at the representative volume element of the lattice structure test piece; the constitutive model comprises an elastic constitutive model and a plastic constitutive model. In this embodiment, a user-defined material subroutine is written based on the continuous damage mechanics theory, and the constitutive equation and the damage evolution equation are defined.

[0044] Specifically, damage mechanics realizes the description of damage in the material by introducing a damage variable, and simultaneously considers the rationality of the damage description and the convenience of engineering application. A one-dimensional damage variable D is introduced in the representative volume element, and when D = 1, it means that the material at the representative volume element is completely damaged. The damage variable is introduced into the constitutive model, the elastic constitutive adopts Hooke's law, and the plastic constitutive adopts the Chaboche model, and the stress-strain relationship is as follows:

[0045]

[0046] C k = p k t k (2)

[0047] γ k = t k (3)

[0048] Q = p4 (4)

[0049] b = t4 (5)

[0050] In the formula, σ y represents the yield stress of the material, p k represents the hardening parameter, t k represents the time constant of hardening, C k represents the elastic modulus of the material, Q represents the saturation value of material hardening, b represents the rate of controlling hardening evolution, ε p represents the plastic strain, and M represents the number of nonlinear regression hardening terms, which is usually 3.

[0051] S12: based on the fatigue life test data, combining the damage variable and the stress-strain relationship, a damage evolution equation is constructed.

[0052] In the embodiment, the continuous damage mechanics theory contains various fatigue analysis models, the high-cycle fatigue damage model is suitable for the case that the loading stress level is low, and the damage evolution is mainly dominated by stress. In the multi-axial loading, the damage evolution equation is shown in equation (6):

[0053]

[0054] a, m, n, β represent material parameters, which are obtained by fatigue test.

[0055] wherein, σ * is the damage equivalent stress:

[0056]

[0057]

[0058]

[0059] σ a * is the equivalent stress amplitude, σ m * is the equivalent stress mean, σ H represents hydrostatic pressure, σ eq represents Mises equivalent stress, σ ij represents stress tensor, and v represents Poisson's ratio.

[0060]

[0061] S2: observing the surface of the rough test piece by an electron microscope, obtaining the surface roughness influencing factor of the test piece based on the fluctuation form of the surface of the lattice structure test piece.

[0062] Further implementation manner is that the method for obtaining the surface roughness influencing factor of the test piece comprises the following steps:

[0063] S21: obtaining the average surface roughness of the test piece based on the length and height of the measurement section of the test piece; specifically, for the material of the additive manufacturing process, there is no method for polishing and grinding the surface due to the particularity of some structures, and the roughness of the outer surface has a certain influence on the fatigue life, the average surface roughness R a of the test piece can be expressed as:

[0064]

[0065] wherein, L is the length of the measurement section of the test piece, and Z(x) is the height of the point x along the measurement line.

[0066] S22: polish the surface of the lattice structure sample to be smooth to obtain a reference surface roughness; specifically, define the roughness of the polished sample as R a0 , as a reference for the roughness of the sample surface during analysis, for subsequent fatigue damage analysis and life prediction calculation.

[0067] S23: based on the average surface roughness and the reference surface roughness, and combined with the additive manufacturing direction of the lattice structure sample, obtain the surface roughness influencing factor of the lattice structure sample.

[0068] S3: introduce the surface roughness influencing factor into the damage evolution equation to obtain a damage evolution equation considering surface roughness. Specifically, based on the definition of the average surface roughness, the surface roughness influencing factor can be further defined to describe the influence of different additive manufacturing technologies on the fatigue performance of the manufacturing material, and further reflect the influence of different processes on the fatigue damage evolution. Considering the convenience of application, define the roughness influencing factor R * :

[0069]

[0070] where γ is the included angle parameter, which is related to the additive manufacturing direction of the sample.

[0071] Further embodiments are directed to a method for obtaining a damage evolution equation considering surface roughness, comprising:

[0072] S31: integrate the damage variable in the damage evolution equation from 0 to 1 to obtain a fatigue life calculation formula for the lattice structure sample; in this embodiment, by integrating equation (6) from D=0 to D=1, the following fatigue life calculation formula is obtained:

[0073]

[0074] S32: based on the fatigue life test data of the lattice structure sample, fit the S-N curve of the lattice structure sample with smooth surface under the preset stress ratio;

[0075] S33: based on the S-N curve and the fatigue life calculation formula, obtain the fatigue life of the lattice structure sample with smooth surface under the preset stress ratio under uniaxial loading by the theory of continuum damage mechanics.

[0076] Specifically, process the test data to fit the S-N curve of the smooth material under stress ratio R=-1 and the S-N curve under another stress ratio (fitted from the longest life point in the material test fatigue life). According to the theory of continuum damage mechanics, the fatigue life calculation formula under uniaxial loading with stress ratio R=-1 is:

[0077]

[0078] S34: Based on the fatigue life under uniaxial loading and the fatigue life test data, the parameters of the damage evolution equation are calibrated by using the MATLAB curve fitting tool, the least square method and the finite element numerical simulation. Specifically, the values of the parameters m and a(1+β) are obtained by combining the MATLAB curve fitting toolbox. Then, the parameter n is fitted by using the test data under other stress ratios and the least square method. According to the fatigue test data of a notched piece and by using the finite element numerical calculation method, the specific value of β is determined. Finally, the R a , γ is obtained by comparing the finite element numerical calculation with the rough piece fatigue test data.

[0079] S35: The calibrated parameters of the damage evolution equation and the surface roughness degree influence factor are introduced into the damage evolution equation to obtain the damage evolution equation considering the surface roughness. Specifically, based on the roughness degree influence factor, the existing damage evolution law can be modified to reflect the influence of different roughness on damage evolution, so that the damage evolution equation considering roughness is established:

[0080]

[0081] S4: Based on the finite element software, the finite element model of the lattice structure specimen is constructed, and the fatigue life of the lattice structure specimen is predicted by combining the damage evolution equation considering the surface roughness, and the fatigue damage analysis of the additive manufacturing lattice structure considering the roughness influence is completed. Specifically, based on the above-mentioned material damage analysis and life prediction method, the finite element calculation platform and its subroutine interface of ABAQUS can be used to realize the program customization of damage evolution and the numerical simulation of fatigue process, and the fatigue life prediction results of the specific additive manufacturing material lattice structure are obtained by combining the failure form of the lattice structure.

[0082] S41: According to the specific configuration of the lattice structure specimen, the finite element model for numerical simulation is established, and the beam element mesh is used for the lattice structure, and the grid density for convergence is determined, so that the damage process of the lattice structure can be more accurately simulated, and the correct boundary conditions and the load conforming to the test conditions are applied.

[0083] S42: The fatigue life calculation method process of the lattice structure based on the continuous damage mechanics theory is shown in Figure 2 After each cycle, the stress field and the strain field of the whole model are calculated, and then the damage increment ΔD is calculated. According to the above method, the damage increment at the integral point after each cycle is calculated, and the damage degree at the integral point under the current cycle number ΔN is obtained by accumulating the damage increment:

[0084] Di+1 = D i + ΔD i · ΔN (17)

[0085] When calculated using the finite element method, the entire model is discretized into a finite number of elements, and the stress and strain values at the integration points within all elements are calculated after each cycle. Then, the damage increment is calculated based on the modified damage evolution equation (considering the damage evolution variance of surface roughness) in the previous section. The damage increment at the integration point after each cycle is calculated according to the above method, and the damage increment is accumulated to obtain the damage size at the integration point at the current cycle number. After N i cycles, the damage reaches 1, the branch rod element is deleted, and it is determined whether the overall displacement U of the structure reaches the failure value U c . If not, repeat the above process until failure, and the fatigue life of the structure is:

[0086]

[0087] N i is the fatigue life when the i-th element is deleted.

[0088] Numerical simulation is carried out using finite element software to complete the fatigue life calculation of multiple point lattice structures, and fatigue life data of the components under various stress level distributions are obtained.

[0089] In this embodiment, it also includes step S5: data processing and life comparison, marking the fatigue life data points obtained by numerical simulation on the S-N curve, and observing the agreement between the fatigue life simulation results and the test results under the same stress level.

[0090] Example Two

[0091] This embodiment uses the method described in Example One to provide a specific test analysis process:

[0092] Assume that the existing point lattice structure test piece as shown in Figure 3 is subjected to fatigue test under fatigue load with stress ratio R = -1, and the roughness influence factor composed of surface roughness is quantitatively calculated. Based on the method described in Example One, the fatigue damage and life prediction of the additive manufacturing point lattice structure are analyzed and calculated.

[0093] Implementation process of the scheme:

[0094] (1) Find literature or carry out multiple groups of fatigue tests under different stress ratios, stress peak values, and various surface roughnesses, and arrange the data according to the fatigue test conditions and test results;

[0095] (2) Take a group of test pieces with polished smooth surfaces, mark them as the reference group, and count them as Ra0 , calculate the R * ;

[0096] (3) Based on the uniaxial tensile test results curve, according to the least square method or particle swarm calibration algorithm, the constitutive model parameters and damage evolution equation parameters of the material are fitted;

[0097] (4) The above parameters calibrated by the standard test piece are used in the fatigue life prediction process of the lattice structure;

[0098] (5) In ABAQUS software, the finite element calculation model of the test piece is built to carry out numerical simulation of the fatigue loading process, and the damage variable at the integration point of the unit is updated continuously by means of VUMAT subroutine;

[0099] (6) When the damage variable of a certain unit integration point reaches 1, the unit is deleted and the above cycle is repeated until the overall structure displacement is greater than the tensile failure displacement, at which time the calculation stops, and the fatigue life prediction results of the lattice structure with different roughness caused by different process conditions are obtained.

[0100] Result analysis:

[0101] According to the test results, the fatigue test data is analyzed, and the material constitutive and fatigue parameters are calibrated, and the expressions of load and fatigue life are obtained. The fatigue load stress peak value and the calculated life value of the same manufacturing process under different treatment methods in the results are taken, and the Figure 4 .

[0102] According to the Figure 2 flow, the numerical calculation of the lattice structure is carried out, and the displacement-life curve as shown in Figure 5 is drawn.

[0103] Example three

[0104] The application also provides a fatigue damage analysis system of an additive manufacturing lattice structure considering the influence of roughness, which is used to realize the method and comprises:

[0105] A damage evolution equation construction module is configured to perform a fatigue test on the lattice structure test piece, obtain fatigue life test data of the lattice structure test piece, and construct a damage evolution equation based on the fatigue life test data;

[0106] An influence factor acquisition module is configured to obtain a surface roughness degree influence factor of the lattice structure test piece based on the undulating morphology of the surface of the lattice structure test piece;

[0107] A modified damage evolution equation construction module is configured to introduce the surface roughness degree influence factor into the damage evolution equation to obtain a damage evolution equation considering the surface roughness;

[0108] The life prediction module is configured to construct a finite element model of the lattice structure specimen based on finite element software, and to perform fatigue life prediction on the lattice structure specimen by combining a damage evolution equation considering surface roughness, thereby completing fatigue damage analysis of the additive manufacturing lattice structure considering the influence of roughness.

[0109] In a further implementation, the damage evolution equation construction module includes:

[0110] The stress-strain relationship acquisition unit is configured to introduce the damage variable into a constitutive model to obtain a stress-strain relationship of the lattice structure specimen after being subjected to stress, wherein the value of the damage variable is used to determine the damage degree of the material at the representative volume element of the lattice structure specimen, and the constitutive model includes an elastic constitutive model and a plastic constitutive model.

[0111] The equation construction unit is configured to construct the damage evolution equation based on fatigue life test data, in combination with the damage variable and the stress-strain relationship.

[0112] In a further implementation, the influence factor acquisition module includes:

[0113] The average roughness degree acquisition unit is configured to obtain the average surface roughness degree of the lattice structure specimen based on the length and height of the measurement section of the lattice structure specimen.

[0114] The reference roughness degree acquisition unit is configured to polish the surface of the lattice structure specimen to be smooth to obtain a reference surface roughness degree.

[0115] The influence factor acquisition unit is configured to obtain the surface roughness degree influence factor of the lattice structure specimen based on the average surface roughness degree and the reference surface roughness degree, in combination with the additive manufacturing direction of the lattice structure specimen.

[0116] In a further implementation, the modified damage evolution equation construction module includes:

[0117] The integral calculation unit is configured to perform 0-1 integration on the damage variable in the damage evolution equation to obtain a fatigue life calculation formula of the lattice structure specimen.

[0118] The curve acquisition unit is configured to fit an S-N curve of the lattice structure specimen with a smooth surface under a preset stress ratio based on fatigue life test data of the lattice structure specimen.

[0119] The fatigue life calculation unit is configured to obtain the fatigue life of the lattice structure specimen with a smooth surface under uniaxial loading at a preset stress ratio by the continuous damage mechanics theory based on the S-N curve and the fatigue life calculation formula.

[0120] A parameter calibration unit is configured to calibrate parameters of the damage evolution equation based on fatigue life under single-axis loading and fatigue life test data, by using a MATLAB curve fitting tool, a least square method and finite element numerical simulation.

[0121] A correction equation acquisition unit is configured to introduce the calibrated parameters of the damage evolution equation and the surface roughness influence factor into the damage evolution equation to obtain a damage evolution equation considering surface roughness.

[0122] The above-described embodiments are merely intended to describe the preferred modes of the present application, and are not intended to limit the scope of the present application. Various modifications and improvements to the technical solutions of the present application made by those skilled in the art without departing from the design spirit of the present application shall fall within the protection scope of the present application as defined by the claims.

Claims

1. A method for fatigue damage analysis of additive manufacturing lattice structures considering roughness effect, characterized in that, The method comprises the following steps: performing a fatigue test on the lattice structure test piece to obtain fatigue life test data of the lattice structure test piece; constructing a damage evolution equation based on the fatigue life test data; obtaining a surface roughness degree influence factor of the lattice structure test piece based on the undulating morphology of the surface of the lattice structure test piece; introducing the surface roughness degree influence factor into the damage evolution equation to obtain a damage evolution equation considering surface roughness; constructing a finite element model of the lattice structure test piece based on a finite element software, combining the damage evolution equation considering surface roughness, performing fatigue life prediction on the lattice structure test piece, and completing fatigue damage analysis of the additive manufacturing lattice structure considering the influence of roughness; the expression of the surface roughness degree influence factor is as follows: ， wherein is an included angle parameter, is an average surface roughness, is a reference surface roughness; the method for obtaining the damage evolution equation considering surface roughness comprises the following steps: integrating the damage variable in the damage evolution equation from 0 to 1 to obtain a fatigue life calculation formula of the lattice structure test piece; Damage evolution equation: , m, n, represent material parameters, obtained from fatigue tests; is the equivalent stress amplitude, is the equivalent stress mean; D represents the damage variable; Fatigue life calculation formula: ​ fitting an S-N curve of a surface-smoothed lattice structure test piece under a preset stress ratio based on the fatigue life test data of the lattice structure test piece; Based on the S-N curve and the fatigue life calculation formula, the fatigue life of the surface smooth dot matrix structure specimen under the uniaxial loading of the preset stress ratio is obtained through the continuous damage mechanics theory; the fatigue life calculation formula under the uniaxial loading of the stress ratio R=-1 is as follows: ; Based on the fatigue life under uniaxial loading and the fatigue test data, the parameters of the damage evolution equation are calibrated by using MATLAB curve fitting tool, least square method and finite element numerical simulation. The parameters are obtained by using MATLAB curve fitting toolbox and the value of . The parameter n is fitted by using test data under other stress ratios and least square method. The specific value of is determined according to the fatigue test data of a notched specimen and by using finite element numerical calculation method. The average surface roughness of the specimen is calculated according to the measured data , and the calculation formula is as follows: ； wherein L is the length of the measurement section of the test piece, and Z(x) is the height of point x along the measurement line; By finite element numerical calculation and rough fatigue test data comparison, it is concluded that; introducing the parameters of the calibrated damage evolution equation and the surface roughness degree influence factor into the damage evolution equation to obtain the damage evolution equation considering surface roughness: 。 2. The method of claim 1, wherein, the method for constructing the damage evolution equation comprises the following steps: introducing the damage variable into a constitutive model to obtain the stress-strain relationship of the lattice structure test piece after being stressed; wherein the value of the damage variable is used to judge the damage degree of the material at the representative volume element of the lattice structure test piece; constructing the damage evolution equation based on the fatigue life test data, combining the damage variable and the stress-strain relationship.

3. The method of claim 2, wherein, The constitutive model comprises an elastic constitutive model and a plastic constitutive model; wherein the elastic constitutive model adopts Hooke's law, and the plastic constitutive model adopts the Chaboche model.

4. The method of claim 3, wherein, The method for obtaining the surface roughness degree influence factor of the lattice structure test piece comprises the following steps: obtaining the average surface roughness degree of the lattice structure test piece based on the length and height of the measurement section of the lattice structure test piece; polishing the surface of the lattice structure test piece to be smooth to obtain a reference surface roughness degree; obtaining the surface roughness degree influence factor of the lattice structure test piece based on the average surface roughness degree and the reference surface roughness degree, and combining the additive manufacturing direction of the lattice structure test piece.

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