Fatigue performance evaluation method for aircraft additive manufacturing structure

Abstract

The invention discloses a fatigue performance evaluation method for an aircraft additive manufacturing structure. The method comprises the following steps: manufacturing a typical part based on additive manufacturing, and sampling in the typical part; sampling comprises two types of sample pieces, namely a low-cycle fatigue sample piece and a high-cycle fatigue sample piece in the forming direction; the low-cycle fatigue samples are divided into three groups, and the high-cycle fatigue samples are divided into four groups; sample pieces are extracted from the three groups to be subjected to a constant-amplitude low-cycle fatigue test, and corresponding strain fatigue analysis material performance parameters are obtained through fitting; sample pieces are extracted from the four groups to be subjected to a constant-amplitude high-cycle fatigue test, and corresponding stress fatigue analysis material performance parameters are obtained through fitting; calculating a surface roughness correction coefficient, a heat treatment size correction coefficient, a residual stress correction coefficient and a defect life influence correction coefficient based on a constant-amplitude high-cycle fatigue test of the high-cycle fatigue sample; performing additive manufacturing on a simulation piece containing structural detail features; and substituting the parameters into a life analysis method, and predicting the fatigue life of the simulation part. According to the invention, the precision of life prediction is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of aircraft additive manufacturing, and in particular relates to a method for evaluating fatigue performance of an aircraft additively manufactured structure. Background Art

[0002] Additive manufacturing technology is called the symbol of the third industrial revolution. It is a revolutionary breakthrough in manufacturing technology. It has many advantages such as high material utilization, small geometric constraints, fast iterative improvement, and high degree of digitization. It is in line with the pursuit of light weight and low cost in aerospace structures and is gradually being used. Represented by laser selective melting and laser melting deposition technology, additive manufacturing technology has been applied to secondary load-bearing structures and a small number of main load-bearing structures of military aircraft. However, due to the obvious anisotropy and greater life dispersion of additive manufacturing structures, the fatigue performance evaluation method of additive manufacturing structures has not yet been fully established for its process and material performance characteristics, which limits its promotion and application in main load-bearing structures. Summary of the invention

[0003] The purpose of the present invention is to provide a fatigue performance evaluation method for aircraft additive manufacturing structures. The present invention improves the accuracy of life prediction.

[0004] The technical solution is as follows: A method for evaluating fatigue performance of aircraft additive manufacturing structures, comprising the following steps:

[0005] Step S1, based on additive manufacturing, typical parts are made, and samples are taken from the typical parts; the samples include two types of samples, namely: low-cycle fatigue samples and high-cycle fatigue samples in the forming direction; the low-cycle fatigue samples are divided into three groups, namely, group 1.1: samples in the forming direction, group 1.2: samples perpendicular to the forming direction, group 1.3: samples at an angle α with the forming direction, 0<α<90°; the high-cycle fatigue samples are divided into four groups, namely, group 2.1: samples with Ra0.32, heat treatment thickness less than 20mm, and no stress relief annealing after polishing, group 2.2: samples with Ra0.32, heat treatment thickness less than 20mm, and stress relief annealing after polishing, group 2.3: samples with Ra as the original surface of additive manufacturing, heat treatment thickness less than 20mm, and no stress relief annealing after sampling, group 2.4: samples with Ra0.32, heat treatment thickness of 200mm, and no stress relief annealing after polishing;

[0006] Step S2, extracting samples from groups 1.1 to 1.3 to conduct a constant amplitude low-cycle fatigue test, obtaining the material strain-life relationship in three directions, and fitting to obtain the corresponding strain fatigue analysis material performance parameters; extracting samples from groups 2.1 to 2.4 to conduct a constant amplitude high-cycle fatigue test, obtaining the stress-life relationship of the four groups of samples, and fitting to obtain the corresponding stress fatigue analysis material performance parameters;

[0007] Step S3, based on the equal amplitude high cycle fatigue tests of groups 2.1 to 2.4, calculate the surface roughness correction factor, the heat treatment dimension correction factor, the residual stress correction factor, and the defect life effect correction factor;

[0008] Step S4, additively manufacturing a simulated part with structural detail features; substituting the surface roughness correction factor, heat treatment size correction factor, residual stress correction factor, and defect influence correction factor obtained in step S3 into the life analysis method to predict the fatigue life of the simulated part;

[0009] Step S5, performing a fatigue test on the simulated part under a random spectrum, and obtaining the crack initiation life by inferring the fracture surface of the fatigue test;

[0010] Step S6. Compare the predicted life obtained in steps S4 and S5 with the crack initiation life, respectively. When 0.5<predicted life / crack initiation life<2, each correction coefficient is reasonable and applicable;

[0011] Step S7, determining the key parts of any additive manufacturing structure;

[0012] Step S8, according to the aircraft design load spectrum, using a multivariate linear regression method or a machine learning method to obtain the stress spectrum of the key parts of the additive manufacturing structure;

[0013] Step S9, according to the placement mode and forming direction in the additive manufacturing structure forming process, and the direction of the maximum principal stress of the key part, calculate the angle β between the forming direction of the key part and the forming direction of the structure; if 0≤β<30°, select the material performance parameters of group 1.1 in step S2; if 60≤β≤90°, select the material performance parameters of group 1.2 in step S2; if 30°≤β<60°, select the material performance parameters of group 1.3 in step S2;

[0014] Step S10, using the stress spectrum of step S8 and the material performance parameters of step S9 as input, using the life analysis method verified by step S4 and the surface roughness correction factor, heat treatment size correction factor, residual stress correction factor, and defect influence correction factor obtained in step S3 to perform life assessment on key parts of the additive manufacturing structure.

[0015] In the aforementioned aircraft additive manufacturing structure fatigue performance evaluation method, in step S1, the typical parts are made as follows: based on the life analysis method and combined with the geometric characteristics of the additive manufacturing structure, typical parts for material performance test sampling are designed.

[0016] In the aforementioned aircraft additive manufacturing structure fatigue performance evaluation method, in step S1, when a typical part is produced, a stress fatigue analysis method or a strain fatigue analysis method is selected as a life analysis method according to the load characteristics, stress level and life requirements of the additive manufacturing structure; when the local linear elastic stress of the structure exceeds the yield strength, the strain fatigue analysis method is used; in addition, the stress fatigue analysis method is used.

[0017] In the aforementioned aircraft additive manufacturing structure fatigue performance evaluation method, in step S1, the number of samples in group 2.1 is no less than 120.

[0018] In the aforementioned aircraft additive manufacturing structure fatigue performance evaluation method, in step S2, the test conditions of the constant amplitude low cycle fatigue test are: room temperature, strain ratio Rε=-1, and no less than 5 strain levels.

[0019] In the aforementioned aircraft additive manufacturing structure fatigue performance evaluation method, in step S2, the test conditions of the constant amplitude high cycle fatigue test are: room temperature, stress ratio R=0.06, and no less than 5 stress levels.

[0020] In the aforementioned aircraft additive manufacturing structure fatigue performance evaluation method, in step S2, in the constant amplitude high cycle fatigue test, each stress level of group 2.1 has no less than 20 pieces of valid data.

[0021] In the aforementioned aircraft additive manufacturing structure fatigue performance evaluation method, step S3 is as follows: based on the stress-life relationship and stress fatigue analysis material performance parameters, the 2.1 to 2.4 groups of samples are calculated and obtained in 10 5 The stress corresponding to the cycle is 1 , σ 2 , σ 3 , σ 4 ; Then: σ 3 / σ 1 is the surface roughness correction factor, σ 4 / σ 1 is the heat treatment size correction factor, σ 2 / σ 1 is the residual stress correction factor; analyze all fatigue test fractures of group 2.1 of samples in step S2, determine their fatigue sources, and observe whether the fatigue sources are manufacturing defects. Divide the stress-life data into two categories: fatigue source without defects and fatigue source with defects, fit their stress-life relationship equations respectively, and calculate the fatigue source without defects and fatigue source with defects respectively. 5 Stress corresponding to the cycle σ 11 , σ 12 , σ 12 / σ 11 It is the correction factor of defect life impact.

[0022] In the aforementioned aircraft additive manufacturing structure fatigue performance assessment method, in step S4, the structural detail features include fillets, holes, and grooves.

[0023] In the aforementioned method for evaluating fatigue performance of aircraft additively manufactured structures, in step S5, when a fatigue test under a random spectrum is performed, an identification load is added at the end of the random spectrum.

[0024] Beneficial effects: 1) The fatigue performance evaluation method of aircraft additive manufacturing structure established by the present invention fully considers the geometric characteristics and process characteristics of the additive manufacturing structure, designs typical parts for material performance test sampling, obtains samples that can reflect the forming process of the additive manufacturing structure and the anisotropic characteristics of additive manufacturing, and more realistically obtains material performance data that is closer to the performance of the additive manufacturing structure;

[0025] 2) The surface roughness correction coefficient of the existing life analysis method is established for subtractive manufacturing structures, and there are only three types: Ra1.6, Ra3.2, and Ra6.3. However, the surface quality of additive manufacturing structures, especially laser selective melting structures, is controlled by grinding. For complex structures, there are often multiple roughnesses on the surface, and the surface roughness of the internal inaccessible parts even exceeds Ra6.3. Therefore, it is unreasonable to continue to use the existing coefficient. The present invention obtains the surface roughness correction coefficient suitable for fatigue life prediction of additive manufacturing structures by comparing the fatigue test of the original surface samples of additive manufacturing with the standard samples, which can improve the accuracy of life prediction;

[0026] 3) The existing heat treatment dimension correction coefficient is proposed for plates and forgings. The reason for the correction is that after the structure is processed by subtractive processing, the geometric dimensions of the structure are very different from those during heat treatment. The key parts may be in the worst performance area, and correction must be made to ensure that the life prediction is conservative to ensure the safety of the structure. However, the difference between the heat treatment dimensions of the additive manufacturing structure and the final geometric dimensions of the structure is small. It is obviously unreasonable to use the traditional subtractive manufacturing heat treatment dimension correction coefficient for additive manufacturing. Therefore, the present invention determines the heat treatment dimension correction coefficient applicable to the additive manufacturing structure through high-cycle fatigue comparison test for the maximum heat treatment thickness that may exist in the additive manufacturing structure, which is conducive to further ensuring the rationality of the life prediction results;

[0027] 4) Due to the complexity of additive manufacturing process, the current additively manufactured aircraft structure is no longer subjected to stress relief annealing after forming and completing surface treatment or conformal processing of some areas. Residual tensile stress may exist on the surface or near the surface, but the current non-destructive residual stress testing technology cannot accurately measure the residual stress of the structure. The present invention determines the influence of residual stress through high-cycle fatigue comparison test, and also improves the accuracy of the life prediction method;

[0028] 5) Even if the additive manufacturing structure is subjected to hot isostatic pressing, there are still widespread internal micro defects in the additive manufacturing structure. The defects have a significant impact on the fatigue life of the structure. It is necessary to introduce a correction factor to consider this impact to ensure that the structure can meet the life requirements even when there are defects. The present invention classifies the test results according to whether there are defects on the fracture surface through high-cycle fatigue tests on a sufficient number of samples, determines the impact of defects on the life of the structure, and further improves the accuracy of life prediction;

[0029] 6) The present invention innovatively designs simulation parts with structural detail features, verifies the life prediction method of additive manufacturing structures through fatigue tests of simulation parts with different surface conditions, and establishes a method for predicting the fatigue life of additive manufacturing structures through material performance data of additive manufacturing samples, providing support for the durability design of such structures.

[0030] The present invention can be further applied to constructing life prediction methods for new materials and new manufacturing processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Technical process for fatigue performance assessment method of aircraft additively manufactured structures;

[0032] Figure 2 Sampling typical parts for a possible material property test;

[0033] Figure 3 To obtain graphical representations of various correction factors;

[0034] Figure 4 It is a typical simulation part with structural detail features. DETAILED DESCRIPTION

[0035] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] Example 1. A fatigue performance evaluation method for aircraft additive manufacturing structures, see Figure 1-Figure 4 , its possible implementation is as follows:

[0037] Step S1, according to the load characteristics, stress level and life requirements of the additive manufacturing structure, select a suitable life analysis method, including stress fatigue analysis method (high cycle fatigue) and strain fatigue analysis method (low cycle fatigue); when the local linear elastic stress of the structure exceeds the yield strength, the strain fatigue analysis method is used; in addition, the stress fatigue analysis method is used;

[0038] Step S2: Based on the method selected in step S1 and in combination with the geometric features of the additive manufacturing structure, a typical sample of material performance test is designed and additively manufactured. A possible typical sample embodiment is shown in Figure 1 ; Samples were taken from typical parts of additive manufacturing; The samples included two types of samples. The first type was low-cycle fatigue samples. The heat-treated thickness of these samples was less than 20 mm. The surface roughness reached Ra0.32 by polishing. No stress relief annealing was performed after polishing. There were three groups of samples: the first group (denoted as group 1.1) was samples in the forming direction, the second group (denoted as group 1.2) was samples perpendicular to the forming direction, and the third group (denoted as group 1.3) was samples at an angle α to the forming direction, 0<α<90°; The second type was high-cycle fatigue samples, all of which were samples in the forming direction, and included four groups of samples: the first group (denoted as group 2.1) was samples with a surface roughness of Ra0.3 2. The samples with heat treatment thickness less than 20mm and no stress relief annealing after polishing. The second group (referred to as group 2.2) are samples with surface roughness reaching Ra0.32, heat treatment thickness less than 20mm, and stress relief annealing after polishing. The third group (referred to as group 2.3) are samples with surface roughness of the original surface of additive manufacturing (not polished), heat treatment thickness less than 20mm, and no stress relief annealing after sampling. The fourth group (referred to as group 2.4) is samples with surface roughness reaching Ra0.32, heat treatment thickness of 200mm, and no stress relief annealing after polishing; among them, the samples of group 2.1 are not less than 120 pieces, and the sampling positions of group 2.4 are shown in Figure 2 ; A recommended number of samples is shown in Table 1;

[0039] Table 1 A possible number of samples to be sampled

[0040]

[0041] Step S3, using the low-cycle fatigue specimens taken out in step S2, carry out a low-cycle fatigue test of equal amplitude at room temperature, with a strain ratio of Rε=-1 and a strain level of not less than 5 levels, obtain the material strain-life relationship in three directions, and fit the material performance parameters of the corresponding strain fatigue analysis model; using the four groups of high-cycle fatigue specimens taken out, carry out a high-cycle fatigue test of equal amplitude at room temperature, with a stress ratio of R=0.06 and a stress level of not less than 5 levels, obtain the stress-life relationship of the four groups of specimens, and fit the material performance parameters of the corresponding stress fatigue analysis model; wherein 2.1 groups have no less than 20 valid data at each stress level;

[0042] Step S4, using the stress-life relationship and stress fatigue analysis parameters of the 2.1 to 2.4 groups of samples obtained in step S3, respectively calculate the stress-life relationship and stress fatigue analysis parameters of the 2.1 to 2.4 groups of samples in 10 5 The stress corresponding to the cycle is 1 , σ2 , σ 3 , σ 4 , σ 3 / σ 1 is the surface roughness correction factor, σ 4 / σ 1 is the heat treatment size correction factor, σ 2 / σ 1 is the residual stress correction coefficient; analyze the fracture surfaces of all samples in group 2.1 in step S3 after fatigue test, determine the fatigue source, and observe whether the fatigue source is a manufacturing defect. Divide the stress-life data into two categories: fatigue source without defect and fatigue source with defect, fit their stress-life relationship equations respectively, and calculate the fatigue source without defect and fatigue source with defect 10 5 Stress corresponding to the cycle σ 11 , σ 12 , σ 12 / σ 11 It is the correction coefficient of the effect of defects on life. Taking the correction coefficient of defect effect as an example, the principle of obtaining each correction coefficient is shown in Figure 3 ;

[0043] Step S5: Design and use additive manufacturing technology to manufacture a simulation part with structural detail features (see Figure 4 ), there are less than 10 non-simulated parts, 5 parts are not polished after manufacturing, and the other 5 parts are polished using the same polishing process as the actual additive manufacturing structure; the surface roughness correction factor, heat treatment size correction factor, residual stress correction factor, and defect influence correction factor obtained in step S4 are substituted into the life analysis method selected in step S1 to predict the fatigue life of the simulated parts with two different surface conditions;

[0044] Step S6, performing a fatigue test on the simulated part under a random spectrum, and obtaining the crack initiation life (median test life) of the simulated parts with two surface states by inverse calculation of the fracture surface of the fatigue test and statistical calculation; when performing the fatigue test under a random spectrum, adding an identification load at the end of the random spectrum;

[0045] Step S7. Compare the predicted life obtained in step S5 with the median test life obtained in step S6. When 0.5<predicted life / median test life<2, the correction coefficients in step S4 are reasonable and applicable.

[0046] Step S8, using the finite element method to carry out detailed stress analysis of the additively manufactured structure under typical load conditions, obtain its three-dimensional stress distribution, and determine the key parts; the typical load conditions at least include two conditions where the key parts are subjected to the largest compressive stress and tensile stress;

[0047] Step S9: According to the aircraft design load spectrum, based on the key part stress obtained in S8 and the corresponding component load, a mapping relationship between the key part stress and the component load is established by using a multivariate linear regression method or a machine learning method; the mapping relationship and the component loads of other working conditions in the load spectrum are used to obtain the stress spectrum of the key part of the additive manufacturing structure;

[0048] Step S10, according to the placement mode and forming direction of the additive manufacturing structure during the forming process, as well as the direction of the maximum principal stress of the key part, calculate the angle β between the direction of the maximum principal stress of the key part and the forming direction of the structure; if 0≤β<30°, select the material performance parameters of group 1.1 in step S3; if 60≤β≤90°, select the material performance parameters of group 1.2 in step S3; if 30°≤β<60°, select the material performance parameters of group 1.3 in step S3;

[0049] Step S11, using the stress spectrum of step S9 and the material performance parameters of step S10 as input, using the life analysis method selected in step S1 after verification in step S5 and the surface roughness correction factor, heat treatment size correction factor, residual stress correction factor, and defect influence correction factor obtained in step S4, to perform life assessment on key parts of the additive manufacturing structure.

[0050] Example 2. A fatigue performance evaluation method for aircraft additive manufacturing structures, see Figure 1-Figure 4 , including the following steps:

[0051] Step S1, according to the load characteristics, stress level and life requirements of the additive manufacturing structure, select a suitable life analysis method, including stress fatigue analysis method (high cycle fatigue) and strain fatigue analysis method (low cycle fatigue); when the local linear elastic stress of the structure exceeds the yield strength, the strain fatigue analysis method is used; in addition, the stress fatigue analysis method is used;

[0052] Step S2, according to the method selected in step S1, combined with the geometric characteristics of the additive manufacturing structure, design a typical sample for material performance testing, and perform additive manufacturing; sample from the typical parts of additive manufacturing; the sampling includes two types of samples, the first type is low-cycle fatigue samples, the heat treatment thickness of such samples is less than 20mm, and the surface roughness reaches Ra0.32 by polishing. No stress relief annealing is performed after polishing, including three groups of samples: the first group (denoted as group 1.1) is samples in the forming direction, the second group (denoted as group 1.2) is samples perpendicular to the forming direction, and the third group (denoted as group 1.3) is samples at an angle α to the forming direction, 0<α<90°; the second type is high-cycle fatigue samples, all of which are samples in the forming direction, including Contains four groups of samples: the first group (referred to as Group 2.1) is the samples with a surface roughness of Ra0.32, a heat treatment thickness of less than 20mm, and no stress relief annealing after polishing; the second group (referred to as Group 2.2) is the samples with a surface roughness of Ra0.32, a heat treatment thickness of less than 20mm, and stress relief annealing after polishing; the third group (referred to as Group 2.3) is the samples with a surface roughness of the original surface of additive manufacturing (not polished), a heat treatment thickness of less than 20mm, and no stress relief annealing after sampling; the fourth group (referred to as Group 2.4) is the samples with a surface roughness of Ra0.32, a heat treatment thickness of 200mm, and no stress relief annealing after polishing; among them, the samples in Group 2.1 are no less than 120 pieces;

[0053] Step S3, using the low-cycle fatigue specimens taken out in step S2, carry out a low-cycle fatigue test of equal amplitude at room temperature, with a strain ratio of Rε=-1 and a strain level of not less than 5 levels, obtain the material strain-life relationship in three directions, and obtain the corresponding strain fatigue analysis material performance parameters by fitting; using the four groups of high-cycle fatigue specimens taken out, carry out a high-cycle fatigue test of equal amplitude at room temperature, with a stress ratio of R=0.06 and a stress level of not less than 5 levels, obtain the stress-life relationship of the four groups of specimens, and obtain the corresponding stress fatigue analysis material performance parameters by fitting; wherein 2.1 groups have no less than 20 valid data at each stress level;

[0054] Step S4, using the stress-life relationship and stress fatigue analysis parameters of the 2.1 to 2.4 groups of samples obtained in step S3, respectively calculate the stress-life relationship and stress fatigue analysis parameters of the 2.1 to 2.4 groups of samples in 10 5 The stress corresponding to the cycle is 1 , σ 2 , σ 3 , σ 4 , σ 3 / σ 1 is the surface roughness correction factor, σ 4 / σ 1 is the heat treatment size correction factor, σ 2 / σ 1is the residual stress correction coefficient; analyze the fracture surfaces of all samples in group 2.1 in step S3 after fatigue test, determine the fatigue source, and observe whether the fatigue source is a manufacturing defect. Divide the stress-life data into two categories: fatigue source without defect and fatigue source with defect, fit their stress-life relationship equations respectively, and calculate the fatigue source without defect and fatigue source with defect 10 5 Stress corresponding to the cycle σ 11 , σ 12 , σ 12 / σ 11 It is the correction factor of the effect of defects on life;

[0055] Step S5, designing and using additive manufacturing technology to manufacture a simulated part with structural detail features, the structural detail features including fillets, holes, grooves, etc.; substituting the surface roughness correction factor, heat treatment size correction factor, residual stress correction factor, and defect influence correction factor obtained in step S4 into the life analysis method selected in step S1 to predict the fatigue life of the simulated part;

[0056] Step S6, performing a fatigue test on the simulated part under a random spectrum, and obtaining the crack initiation life (test life) by inverse calculation of the fracture surface of the fatigue test; when performing the fatigue test under a random spectrum, adding an identification load at the end of the random spectrum;

[0057] Step S7. Compare the predicted life obtained in step S5 with the crack initiation life obtained in step S6. When 0.5<predicted life / crack initiation life<2, each correction coefficient is reasonable and applicable;

[0058] Step S8, determining the key parts of any additive manufacturing structure by analysis;

[0059] Step S9, according to the aircraft design load spectrum, using a multivariate linear regression method or a machine learning method to obtain the stress spectrum of the key parts of the additive manufacturing structure;

[0060] Step S10, according to the placement mode and forming direction in the additive manufacturing structure forming process, and the direction of the maximum principal stress of the key part, calculate the angle β between the forming direction of the key part and the forming direction of the structure; if 0≤β<30°, select the material performance parameters of group 1.1 in step S3; if 60≤β≤90°, select the material performance parameters of group 1.2 in step S3; if 30°≤β<60°, select the material performance parameters of group 1.3 in step S3;

[0061] Step S11, using the stress spectrum of step S9 and the material performance parameters of step S10 as input, using the life analysis method selected in step S1 after verification in step S5 and the surface roughness correction factor, heat treatment size correction factor, residual stress correction factor, and defect influence correction factor obtained in step S4, to perform life assessment on key parts of the additive manufacturing structure.

[0062] The above is only a specific embodiment of the present invention, and the present invention is described in detail. The unexplained part is a conventional technology. However, the protection scope of the present invention is not limited to this. Any changes or substitutions that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. The protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A method for evaluating fatigue performance of aircraft additive manufacturing structures, characterized in that: The following steps are involved: Step S1, based on additive manufacturing, typical parts are made, and samples are taken from the typical parts; the samples include two types of samples, namely: low-cycle fatigue samples and high-cycle fatigue samples in the forming direction; the low-cycle fatigue samples are divided into three groups, namely, group 1.1: samples in the forming direction, group 1.2: samples perpendicular to the forming direction, group 1.3: samples at an angle α with the forming direction, 0<α<90°; the high-cycle fatigue samples are divided into four groups, namely, group 2.1: samples with Ra0.32, heat treatment thickness less than 20mm, and no stress relief annealing after polishing, group 2.2: samples with Ra0.32, heat treatment thickness less than 20mm, and stress relief annealing after polishing, group 2.3: samples with Ra as the original surface of additive manufacturing, heat treatment thickness less than 20mm, and no stress relief annealing after sampling, group 2.4: samples with Ra0.32, heat treatment thickness of 200mm, and no stress relief annealing after polishing; Step S2, extracting samples from groups 1.1 to 1.3 to conduct a constant amplitude low-cycle fatigue test, obtaining the material strain-life relationship in three directions, and fitting to obtain the corresponding strain fatigue analysis material performance parameters; extracting samples from groups 2.1 to 2.4 to conduct a constant amplitude high-cycle fatigue test, obtaining the stress-life relationship of the four groups of samples, and fitting to obtain the corresponding stress fatigue analysis material performance parameters; Step S3, based on the equal amplitude high cycle fatigue tests of groups 2.1 to 2.4, calculate the surface roughness correction factor, the heat treatment dimension correction factor, the residual stress correction factor, and the defect life effect correction factor; Step S4, additive manufacturing of a simulation part containing structural detail features; Substituting the surface roughness correction factor, heat treatment dimension correction factor, residual stress correction factor, and defect influence correction factor obtained in step S3 into the life analysis method to predict the fatigue life of the simulated component; Step S5, performing a fatigue test on the simulated part under a random spectrum, and obtaining the crack initiation life by inferring the fracture surface of the fatigue test; Step S6. Compare the predicted life obtained in steps S4 and S5 with the crack initiation life, respectively. When 0.5<predicted life / crack initiation life<2, each correction coefficient is reasonable and applicable; Step S7, determining the key parts of any additive manufacturing structure; Step S8, according to the aircraft design load spectrum, using a multivariate linear regression method or a machine learning method to obtain the stress spectrum of the key parts of the additive manufacturing structure; Step S9, according to the placement mode and forming direction in the additive manufacturing structure forming process, and the direction of the maximum principal stress of the key part, calculate the angle β between the forming direction of the key part and the forming direction of the structure; if 0≤β<30°, select the material performance parameters of group 1.1 in step S2; if 60≤β≤90°, select the material performance parameters of group 1.2 in step S2; if 30°≤β<60°, select the material performance parameters of group 1.3 in step S2; Step S10, using the stress spectrum of step S8 and the material performance parameters of step S9 as input, using the life analysis method verified by step S4 and the surface roughness correction factor, heat treatment size correction factor, residual stress correction factor, and defect influence correction factor obtained in step S3 to perform life assessment on key parts of the additive manufacturing structure.

2. The method for evaluating fatigue performance of aircraft additive manufacturing structures according to claim 1, characterized in that: In step S1, the typical parts are made as follows: based on the life analysis method and combined with the geometric characteristics of the additive manufacturing structure, typical parts for material performance test sampling are designed.

3. The method for evaluating fatigue performance of aircraft additive manufacturing structures according to claim 2, characterized in that: In step S1, when a typical part is manufactured, a stress fatigue analysis method or a strain fatigue analysis method is selected as a life analysis method according to the load characteristics, stress level and life requirements of the additive manufacturing structure; when the local linear elastic stress of the structure exceeds the yield strength, the strain fatigue analysis method is used; In addition, stress fatigue analysis method is used.

4. The method for evaluating fatigue performance of aircraft additive manufacturing structures according to claim 1, characterized in that: In step S1, the number of samples in group 2.1 is no less than 120.

5. The method for evaluating fatigue performance of aircraft additive manufacturing structures according to claim 1, characterized in that: In step S2, the test conditions of the constant amplitude low cycle fatigue test are: room temperature, strain ratio Rε=-1, and no less than 5 strain levels.

6. The method for evaluating fatigue performance of aircraft additive manufacturing structures according to claim 1, characterized in that: In step S2, the test conditions of the constant amplitude high cycle fatigue test are: room temperature, stress ratio R=0.06, and stress level of not less than 5 levels.

7. The method for evaluating fatigue performance of aircraft additive manufacturing structures according to claim 1, characterized in that: In step S2, in the constant amplitude high cycle fatigue test, there are no less than 20 valid data for each stress level in group 2.

1.

8. The method for evaluating fatigue performance of aircraft additive manufacturing structures according to claim 1, characterized in that: Step S3 is as follows: Based on the stress-life relationship and stress fatigue analysis material performance parameters, the 2.1 to 2.4 groups of samples are calculated and obtained in 10 5 The corresponding stresses σ1, σ2, σ3, and σ4 during the cycle; then: σ3 / σ1 is the surface roughness correction factor, σ4 / σ1 is the heat treatment size correction factor, and σ2 / σ1 is the residual stress correction factor; analyze all fatigue test fractures of the 2.1 group of samples in step S2 to determine their fatigue sources, and observe whether the fatigue sources are manufacturing defects. The stress-life data are divided into two categories: fatigue source without defects and fatigue source with defects, and their stress-life relationship equations are fitted respectively. The fatigue source without defects and fatigue source with defects are calculated respectively. 5 Stress corresponding to the cycle σ 11 , σ 12 , σ 12 / σ 11 It is the correction factor of defect life impact.

9. The method for evaluating fatigue performance of aircraft additive manufacturing structures according to claim 1, characterized in that: In step S4, the structural detail features include fillets, holes, and grooves.

10. The method for evaluating fatigue performance of aircraft additive manufacturing structures according to claim 1, characterized in that: In step S5, when the fatigue test under the random spectrum is carried out, an identification load is added at the end of the random spectrum.

Images