A method for predicting fatigue life of welded structural steel based on critical distance theory

Abstract

The present application relates to the technical field of welded structural steel fatigue life prediction, and particularly relates to a welded structural steel fatigue life prediction method based on critical distance theory, comprising: obtaining the ultimate tensile strength of a target welded structural steel sample; constructing a reference S-N curve equation of the target welded structural steel sample; determining local stress and maximum stress concentration points; obtaining effective equivalent stress in the effective action area of fatigue damage; and obtaining the predicted fatigue life of the target welded structural steel sample under alternating load. The present application only needs conventional tensile test and basic uniaxial fatigue data to realize life prediction, and gets rid of the dependence on expensive and time-consuming full-size complex component fatigue calibration test.

Description

Technical Field

[0001] This invention relates to the field of fatigue life prediction technology for welded structural steel, specifically to a method and system for predicting fatigue life of welded structural steel based on critical distance theory. Background Technology

[0002] Welded structural components are widely used as critical force-transmitting parts in heavy machinery manufacturing and large load-bearing equipment. However, due to their inherent technological characteristics, complex parts such as butt joints and cross joints inevitably exhibit geometric abrupt changes and local defects. Under long-term alternating loads, these areas will experience severe stress concentration effects, making them highly susceptible to fatigue crack initiation and ultimately leading to early structural failure, seriously threatening the operational safety of equipment. Therefore, accurate fatigue life assessment of complex welded structures is a core pain point that urgently needs to be addressed in the fields of engineering design and safety maintenance.

[0003] Currently, commonly used fatigue life assessment methods in the engineering field face a dilemma of balancing accuracy and cost. Traditional assessment methods struggle to accurately reflect the dramatic local stress gradient changes at the weld toe when dealing with complex welded joints, leading to predictions that often deviate from reality. Furthermore, existing high-precision prediction models typically rely heavily on numerous and complex full-scale fatigue calibration tests on specific structures. This not only incurs high testing costs but also results in lengthy testing cycles, failing to meet the practical needs of modern industry for rapid iteration and low-cost design.

[0004] To address the aforementioned practical problems, this invention proposes a method for predicting the fatigue life of welded structural steel based on critical distance theory. Summary of the Invention

[0005] To address the problems in existing technologies, this invention provides a method for predicting the fatigue life of welded structural steel based on critical distance theory. It uses conventional hardness and tensile tests as a foundation, combined with a small amount of basic fatigue data under specific stress ratios to obtain the constitutive characteristics of the material. Furthermore, it innovatively introduces critical distance theory, transforming the local "point stress" extracted by finite element analysis into "regional effective stress," which better reflects the essence of fatigue damage. This invention significantly reduces reliance on expensive fatigue tests, overcomes the limitations of traditional methods, and achieves low-cost, high-precision prediction of the fatigue life of complex welded structures.

[0006] The first aspect of this invention provides a method for predicting the fatigue life of welded structural steel based on the critical distance theory, employing the following technical solution, including: Uniaxial tensile mechanical tests were conducted on the target welded structural steel specimens to obtain the ultimate tensile strength of the target welded structural steel specimens. The initial maximum cyclic stress is set according to the ultimate tensile strength. Under the preset stress ratio and stress control mode, a multi-gradient fatigue test is carried out on the target welded structural steel specimen. The reference SN curve equation of the target welded structural steel specimen is obtained according to the initial maximum cyclic stress and fatigue failure cycle number. A three-dimensional finite element simulation model of the target welded structural steel specimen was established, and the local stress of the three-dimensional finite element simulation model under fatigue load was extracted and the maximum stress concentration point was determined. Based on local stress, the effective equivalent stress within the effective action area of ​​fatigue damage is calculated using the volume method with the point of maximum stress concentration as the center of the sphere and the critical distance theory. The predicted fatigue life of the target welded structural steel specimen under alternating load is obtained by using the effective equivalent stress and the reference SN curve equation.

[0007] A further technical solution of the present invention is that the uniaxial tensile mechanical test adopts a displacement-controlled loading method to obtain the stress and strain of the target welded structural steel specimen in the uniaxial tensile mechanical test, and determines the ultimate tensile strength based on the stress and strain.

[0008] A further technical solution of the present invention is that the preset stress ratio is set to 0.1, the loading waveform is a sine wave, and the loading method adopts a single-axis pulsating cyclic stress loading mode.

[0009] A further technical solution of the present invention is that the initial cyclic maximum stress is:

[0010] In the formula, This indicates the maximum stress during the initial cycle; Indicates the loading coefficient; It represents the ultimate tensile strength.

[0011] A further technical solution of the present invention is that the expression for the reference SN curve equation of the target welded structural steel specimen is:

[0012] In the formula, This indicates the number of fatigue failure cycles for the target welded structural steel specimen under multi-gradient fatigue testing. This indicates the maximum stress during the initial cycle; This represents the intercept constant, which reflects the overall fatigue resistance of the target welded structural steel specimen material. The slope constant of the line represents the degree of sensitivity of a material to load.

[0013] A further technical solution of the present invention is that the base material area corresponding to the three-dimensional finite element simulation model adopts a conventional mesh, and the mesh is refined in the stress concentration and potential dangerous areas of the weld.

[0014] A further technical solution of the present invention is to take the point of maximum stress concentration as the danger point, extract the node coordinates, element volume and first principal stress of the finite element element in the preset area of ​​the danger point, and take the first principal stress as the local stress.

[0015] A further technical solution of the present invention is as follows: the steps of calculating the effective equivalent stress within the effective action area of ​​fatigue damage based on local stress, with the point of maximum stress concentration as the center of the sphere, and using the volume method of critical distance theory are as follows: Using the determined maximum stress concentration point as the center of the sphere and the critical distance of the target welded structural steel sample material as the radius, a hemispherical volume region is constructed and used as the effective area for fatigue damage. The effective equivalent stress within the effective action area is obtained based on the volume and local stress of each finite element in the effective action area.

[0016] A further technical solution of the present invention is that the predicted fatigue life of the target welded structural steel specimen under alternating load is:

[0017] In the formula, This represents the predicted fatigue life value; Indicates effective equivalent force; This represents the intercept constant, which reflects the overall fatigue resistance of the target welded structural steel specimen material. The slope constant of the line represents the degree of sensitivity of a material to load.

[0018] A second aspect of this invention provides a fatigue life prediction system for welded structural steel based on critical distance theory, comprising: The basic test unit is used to conduct uniaxial tensile mechanical tests on the target welded structural steel to obtain the ultimate tensile strength. The fatigue testing unit is used to set the initial maximum cyclic stress based on the ultimate tensile strength, and to conduct multi-gradient fatigue tests on the target welded structural steel specimen under a preset stress ratio and stress control mode. The reference SN curve equation of the target welded structural steel specimen is obtained based on the initial maximum cyclic stress and the fatigue failure cycle number. The local stress acquisition unit is used to establish a three-dimensional finite element simulation model of the target welded structural steel specimen, extract the local stress of the three-dimensional finite element simulation model under fatigue load, and determine the maximum stress concentration point. Effective equivalent stress determination element, used to calculate the effective equivalent stress within the effective action area of ​​fatigue damage based on local stress, with the maximum stress concentration point as the sphere center and using the volume method of critical distance theory; And a prediction unit, used to obtain the predicted fatigue life of the target welded structural steel specimen under alternating load by combining the effective equivalent stress and the reference SN curve equation.

[0019] The beneficial effects of this invention are: This invention first introduces the critical distance theory to transform the traditional "point stress" into "regional effective stress," effectively addressing the severe stress gradient problem in complex areas such as butt joints and cross joints in welded structures, and avoiding underestimation of lifespan caused by a single high-stress point. Secondly, only uniaxial fatigue data from conventional tensile tests (such as data at a stress ratio of 0.1) are needed for evaluation, combined with finite element simulation, completely eliminating the reliance on expensive and time-consuming fatigue calibration tests for full-size complex components. Furthermore, this invention is not limited to a single material and can be widely applied to welded structural steel systems of different strength grades, providing a universal solution for fatigue-resistant design in fields such as heavy machinery and building structures. Finally, it directly connects theoretical damage mechanisms with the geometrical abrupt changes in engineering practice, providing a reliable quantitative basis for structural optimization and early risk prevention. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a flowchart illustrating a method for predicting the fatigue life of welded structural steel based on the critical distance theory according to the present invention. Figure 2 This is a sample size diagram of the target welded structural steel in an embodiment of the present invention; Figure 3 This is a schematic diagram of the fitted basic SN curve in an embodiment of the present invention; Figure 4 This is a stress field distribution diagram of the finite element model in an embodiment of the present invention; Figure 5 This is a comparison chart of the fatigue life predicted using the method of the invention and the actual fatigue life. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] In a first aspect, the present invention provides an embodiment of a method for predicting the fatigue life of welded structural steel based on critical distance theory, such as... Figure 1 As shown, it includes: S1. Obtain the ultimate tensile strength of the target welded structural steel specimen; Specifically, a uniaxial tensile mechanical test is performed on the target welded structural steel specimen to obtain the ultimate tensile strength of the target welded structural steel specimen.

[0024] S11, Uniaxial tensile mechanical test: For example, in one specific embodiment, a uniaxial tensile mechanical test is performed on the target welded structural steel specimen to generate stress-strain data. Specifically, in this embodiment, firstly, the target welded structural steel specimen is prepared as a dog-bone shaped specimen and its original dimensions are accurately measured. The specimen dimension diagram is shown below. Figure 2 As shown. The specimen is mounted on a universal testing machine. At the start of the test, a tensile load is applied at a constant rate (usually strain-controlled). The axial tensile force F and the elongation ΔL of the gauge length are recorded synchronously by the internal sensors of the testing machine until the specimen fractures, thus obtaining the original load-displacement curve. Subsequently, during the entire fracture process of applying the axial tensile load to the specimen, based on the original cross-sectional area A0 and the original gauge length L0 of the specimen, the tensile force-displacement data are converted into stress and strain. The specific calculation formula is as follows:

[0025]

[0026] In the formula, Indicates stress; Indicates strain; Indicates axial tensile force; Indicates the original cross-sectional area of ​​the sample; Indicates the elongation of the gauge length of the specimen; This indicates the original gauge length of the sample.

[0027] S12. Obtain the ultimate tensile strength of the target welded structural steel specimen: For example, based on step S11, the ultimate tensile strength of the target welded structural steel specimen can be calculated by dividing the maximum recorded load by the original cross-sectional area of ​​the specimen, according to the load and displacement data recorded in the tensile test.

[0028] S2. Construct the reference SN curve equation for the target welded structural steel specimen; Specifically, the initial maximum cyclic stress is set according to the ultimate tensile strength. Under the preset stress ratio and stress control mode, a multi-gradient fatigue test is carried out on the target welded structural steel specimen. The reference SN curve equation of the target welded structural steel specimen is obtained according to the initial maximum cyclic stress and fatigue failure cycle number. S21. Set the initial maximum cyclic stress based on the ultimate tensile strength: For example, in one specific embodiment, the expression for the initial cyclic maximum stress is:

[0029] In the formula, This indicates the maximum stress during the initial cycle; Indicates the loading coefficient; Indicates ultimate tensile strength; in this embodiment, the initial loading coefficient... The value is taken within the range of 0.6 to 0.7. During the test, the initial loading coefficient for subsequent tests is dynamically adjusted based on the fatigue life results of the preceding samples. The values ​​are adjusted appropriately to ensure that the fatigue failure cycles are evenly distributed across different lifespan ranges, thereby significantly improving test efficiency and data validity.

[0030] S22. Obtain the reference SN curve equation for the target welded structural steel specimen: For example, in one specific embodiment, fatigue testing is the data foundation for obtaining the true fatigue resistance of materials and constructing theoretical prediction models. In this embodiment, the fatigue test of welded structural steel specimens is performed in accordance with the national standard GB / T3075-2021 (equivalent to the international standard ISO 1099:2017). The fatigue test adopts a uniaxial pulsating cyclic stress loading mode, and the input load waveform is set to a sine wave. To accurately ensure that the stress amplitude borne by the specimen is constant, the testing machine adopts a force-controlled loading method throughout the process. To avoid excessive heating of the specimen caused by high-frequency friction or internal friction, which would affect the true fatigue mechanical properties of the material, the test loading frequency is strictly controlled between 5-10Hz. The stress ratio of the cyclic load is set to R=0.1, and the loading waveform is a sine wave. This preset stress ratio ensures that the specimen is always in a tensile state throughout the entire alternating cyclic loading process, effectively eliminating the interference of compressive stress on crack closure and accurately simulating the typical service conditions guided by tension in actual engineering.

[0031] Specifically, in this embodiment, five different stress conditions were set for each type of target welded structural steel to be tested, forming five continuous stress gradients. To ensure the statistical significance of the data, fatigue tests were independently performed on at least three parallel specimens under each stress condition. During the test, the number of cycles N for each specimen from initial loading to macroscopic fracture failure was monitored and recorded in real time. Simultaneously, the corresponding fatigue crack initiation and final failure location (such as the base metal region or weld region) were recorded in detail, providing experimental evidence for the subsequent location of the critical plane and danger point. Based on the collected initial cycle maximum stress... The corresponding fatigue failure cycle The experimental data points were fitted using linear regression in a double logarithmic coordinate system using the least squares method to obtain the following results: Figure 3 The table shows a baseline SN curve fitted to a set of fatigue test data. The commonly used baseline fitting model uses a power function form (i.e., the standard Basquin fatigue equation), meaning the expression for the baseline SN curve equation of the target welded structural steel specimen is:

[0032] In the formula, This indicates the number of fatigue failure cycles for the target welded structural steel specimen under multi-gradient fatigue testing. This indicates the maximum stress during the initial cycle; This represents the intercept constant, which reflects the overall fatigue resistance of the target welded structural steel specimen material. The slope constant of the line representing the material's sensitivity to load; the intercept constant. and the slope constant of the line All material constants were obtained through fatigue-based testing. A baseline SN curve for the target welded structural steel under a stress ratio R=0.1 was constructed based on the baseline SN curve equation. Furthermore, based on statistical bias, the upper and lower boundaries of the probability distribution containing a 95% survival rate were determined, thus providing a high-precision benchmark for subsequent correction and life prediction using the critical distance theory (TCD).

[0033] S3. Determine the local stress and the point of maximum stress concentration; Specifically, a three-dimensional finite element simulation model of the target welded structural steel specimen is established, and the local stress of the three-dimensional finite element simulation model under fatigue load is extracted and the maximum stress concentration point is determined.

[0034] For example, in one specific embodiment, to accurately capture the local stress response of complex welding morphologies under alternating loads, finite element numerical simulation technology is used to analyze the stress field of the welded joints of the target welded structural steel specimen. Based on the actual dimensions of the welded structure in the test or engineering application, a full-size geometric model of the target welded structural steel specimen, including butt joints or cross joints, is established using 3D solid modeling software. Figure 4 As shown, after importing the full-size geometric model of the target welded structural steel specimen into the finite element analysis software, the three-dimensional finite element simulation model is meshed. To ensure the accuracy of stress gradient capture and control the computational scale, in this embodiment, a sub-model technique or a mesh gradient partitioning strategy is used to mesh the three-dimensional finite element simulation model: that is, a coarser mesh is used in the base material area far from the weld, while a high-density mesh is used in the dangerous areas of weld toe and weld root where there are significant geometrical abrupt changes and stress concentrations; the mesh size of the refined area needs to be verified by mesh independence to ensure that its size is much smaller than the theoretical critical distance characteristic value of the material.

[0035] Boundary conditions identical to those in the actual fatigue test (step S2) are applied to the three-dimensional finite element simulation model. All degrees of freedom at one end of the three-dimensional finite element simulation model are fixed, and a corresponding uniaxial tensile load is applied to the other end of the model. The load magnitude corresponds to multiple preset fatigue stress gradients. A linear or elastoplastic finite element solver is run to obtain the overall stress and strain distribution of the three-dimensional finite element simulation model. The maximum stress concentration point in the three-dimensional finite element simulation model is located, and this maximum stress concentration point is taken as the critical point for fatigue crack initiation. The nodal coordinates, finite element volume, and corresponding first principal stress of each finite element within the preset critical point region are extracted to form local stress, providing input parameters for subsequent critical distance calculation. The preset critical point region is a sphere with a radius of 0.05 mm to 1.5 mm. In this embodiment, a sphere with a radius of 1 mm is taken as the preset critical point region.

[0036] S4. Obtain the effective equivalent stress within the effective area of ​​fatigue damage; Specifically, based on local stress, the effective equivalent stress within the effective action area of ​​fatigue damage is calculated using the volume method with the point of maximum stress concentration as the center of the sphere and the critical distance theory.

[0037] For example, in one specific embodiment, the fatigue failure of the welded joint of the target welded structural steel specimen is mainly controlled by the local stress-strain field within a certain range around the critical point due to the notch effect. This embodiment adopts the more advanced "volume method" in critical distance theory to transform the three-dimensional local stress into a regional effective equivalent stress. The critical distance... The critical distance is an inherent fatigue characteristic length parameter of the material; for the target structural steel specimen, it is derived and calculated through the material's basic fatigue limit and fracture mechanics parameters (such as crack propagation threshold), or obtained by reverse calibration through comparing fatigue test data of smooth and notched specimens. The critical distance of the target structural steel specimen material is established. Then, the critical distance It is considered as a material constant within a certain lifespan range.

[0038] For example, in one specific embodiment, the steps of calculating the effective equivalent stress within the effective area of ​​fatigue damage based on local stress, with the point of maximum stress concentration as the center of the sphere and using the volume method of critical distance theory, are as follows: Using the maximum stress concentration point (critical point) determined in step S3 as the center of the sphere, and the critical distance of the target structural steel specimen material as the sphere's center... A hemispherical region is constructed with radius [radius] along the plate thickness or notch depth. The volume of this hemispherical region is the effective area affected by fatigue damage. The calculation formula is:

[0039] Extract the first principal stress and volume of all finite element elements within the effective region; calculate the weighted average of the absolute values ​​of the first principal stresses within the effective region using voxel integration or weighted arithmetic mean method, which is taken as the effective equivalent stress driving fatigue failure of the welded structure. The effective equivalent stress is:

[0040] In the formula, The total number of finite element elements that fall completely within the effective action area; Let be the volume of the i-th finite element. The first principal stress of the i-th finite element is... This represents the sum of the volumes of all finite element elements within the effective operating region. It should be noted that the highly concentrated weld toe peak stress is smoothed using the volume method formula in step S4. This method avoids the extreme sensitivity of the traditional point stress method to mesh size, comprehensively considers the stress gradient effect caused by the complex three-dimensional morphology of the weld, and thus obtains structural fatigue characterization parameters with more rigorous physical meaning and more stable numerical values.

[0041] S5. Obtain the predicted fatigue life of the target welded structural steel specimen under alternating load; Specifically, the predicted fatigue life of the target welded structural steel specimen under alternating load is obtained by using the effective equivalent stress and the reference SN curve equation.

[0042] For example, in one specific embodiment, the effective equivalent stress calculated in step S4 is... The equivalent nominal load, considered as driving fatigue fracture of the welded structure, is directly substituted into the baseline SN curve equation (Basquin equation) of the target welded structural steel established in step S2. The corresponding life prediction calculation formula for the target welded structural steel is then transformed into:

[0043] In the formula, This represents the predicted fatigue life value; Indicates effective equivalent force; This represents the intercept constant, which reflects the overall fatigue resistance of the target welded structural steel specimen material. The slope constant of the straight line reflects the material's sensitivity to load; the fatigue life prediction value can be obtained by solving the fatigue life prediction formula. fatigue life prediction value This refers to the theoretically predicted lifespan of the complex welded structure under actual service or a preset load spectrum. For example... Figure 5 As shown, the fatigue life prediction value Comparing the data with the actual fatigue life obtained from the fatigue test in step S2, the vast majority of the data are within 1.5 times the discrete band, and all data are within 2 times the discrete band, indicating that the fatigue life predicted by the method of this invention has high reliability. The method provided in this embodiment effectively connects the basic mechanical properties of materials, macroscopic load conditions, and microscopic geometric abrupt change characteristics, realizing a low-cost, high-precision closed-loop fatigue life prediction and evaluation process.

[0044] Secondly, some embodiments of the present invention provide a fatigue life prediction device for welded structural steel based on critical distance theory. The device includes: a basic test unit, a fatigue test unit, a local stress acquisition unit, an effective equivalent stress determination unit, and a prediction unit. The basic testing unit is used to conduct uniaxial tensile mechanical tests on the target welded structural steel to obtain the ultimate tensile strength; the fatigue testing unit is used to set the initial cyclic maximum stress based on the ultimate tensile strength, and conduct multi-gradient fatigue tests on the target welded structural steel specimen under a preset stress ratio and stress control mode, and obtain the reference SN curve equation of the target welded structural steel specimen based on the initial cyclic maximum stress and fatigue failure cycle number; the local stress acquisition unit is used to establish a three-dimensional finite element simulation model of the target welded structural steel specimen, extract the local stress of the three-dimensional finite element simulation model under fatigue load, and determine the maximum stress concentration point; the effective equivalent stress determination unit is used to calculate the effective equivalent stress in the effective action area of ​​fatigue damage based on the local stress, with the maximum stress concentration point as the center of the sphere, and using the volume method of critical distance theory; the prediction unit is used to obtain the predicted fatigue life of the target welded structural steel specimen under alternating load by combining the effective equivalent stress and the reference SN curve equation.

[0045] Thirdly, the present invention provides a computer device, the computer device including a processor, a memory and a computer program stored in the memory and executable by the processor, wherein when the computer program is executed by the processor, it implements the steps of the method for predicting the fatigue life of welded structural steel based on the critical distance theory provided in the first aspect.

[0046] Fourthly, the present invention provides a computer-readable storage medium storing a computer program, wherein when the computer program is executed by a processor, it implements the steps of the method for predicting the fatigue life of welded structural steel based on the critical distance theory provided in the first aspect.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for predicting the fatigue life of welded structural steel based on critical distance theory, characterized in that, include: Uniaxial tensile mechanical tests were conducted on the target welded structural steel specimens to obtain the ultimate tensile strength of the target welded structural steel specimens. The initial maximum cyclic stress is set according to the ultimate tensile strength. Under the preset stress ratio and stress control mode, a multi-gradient fatigue test is carried out on the target welded structural steel specimen. The reference SN curve equation of the target welded structural steel specimen is obtained according to the initial maximum cyclic stress and fatigue failure cycle number. A three-dimensional finite element simulation model of the target welded structural steel specimen was established, and the local stress of the three-dimensional finite element simulation model under fatigue load was extracted and the maximum stress concentration point was determined. Based on local stress, the effective equivalent stress within the effective action area of ​​fatigue damage is calculated using the volume method with the point of maximum stress concentration as the center of the sphere and the critical distance theory. The predicted fatigue life of the target welded structural steel specimen under alternating load is obtained by using the effective equivalent stress and the reference SN curve equation.

2. The fatigue life prediction method for welded structural steel based on critical distance theory according to claim 1, characterized in that, The uniaxial tensile mechanical test uses displacement-controlled loading to obtain the stress and strain of the target welded structural steel specimen during the uniaxial tensile mechanical test, and determines the ultimate tensile strength based on the stress and strain.

3. The fatigue life prediction method for welded structural steel based on critical distance theory according to claim 1, characterized in that, The preset stress ratio is set to 0.1, the loading waveform is a sine wave, and the loading method adopts a single-axis pulsating cyclic stress loading mode.

4. The fatigue life prediction method for welded structural steel based on critical distance theory according to claim 1, characterized in that, The initial cyclic maximum stress is: In the formula, This indicates the maximum stress during the initial cycle; Indicates the loading coefficient; It represents the ultimate tensile strength.

5. The fatigue life prediction method for welded structural steel based on critical distance theory according to claim 1, characterized in that, The expression for the reference SN curve equation of the target welded structural steel specimen is as follows: In the formula, This indicates the number of fatigue failure cycles for the target welded structural steel specimen under multi-gradient fatigue testing. This indicates the maximum stress during the initial cycle; This represents the intercept constant, which reflects the overall fatigue resistance of the target welded structural steel specimen material. The slope constant of the line represents the degree of sensitivity of a material to load.

6. The fatigue life prediction method for welded structural steel based on critical distance theory according to claim 1, characterized in that, The base material region corresponding to the three-dimensional finite element simulation model uses a conventional mesh, while the mesh is refined in areas of stress concentration and potential danger of welds.

7. The fatigue life prediction method for welded structural steel based on critical distance theory according to claim 1, characterized in that, The point of maximum stress concentration is taken as the danger point. The node coordinates, element volume and first principal stress of the finite element elements in the preset area of ​​the danger point are extracted, and the first principal stress is taken as the local stress.

8. The fatigue life prediction method for welded structural steel based on critical distance theory according to claim 1, characterized in that, The steps for calculating the effective equivalent stress within the effective action area of ​​fatigue damage based on local stress, using the point of maximum stress concentration as the center of the sphere and the volume method of critical distance theory, are as follows: Using the determined maximum stress concentration point as the center of the sphere and the critical distance of the target welded structural steel sample material as the radius, a hemispherical volume region is constructed and used as the effective area for fatigue damage. The effective equivalent stress within the effective action area is obtained based on the volume and local stress of each finite element in the effective action area.

9. The fatigue life prediction method for welded structural steel based on critical distance theory according to claim 1, characterized in that, The predicted fatigue life of the target welded structural steel specimen under alternating load is: In the formula, This represents the predicted fatigue life value; Indicates effective equivalent force; This represents the intercept constant, which reflects the overall fatigue resistance of the target welded structural steel specimen material. The slope constant of the line represents the degree of sensitivity of a material to load.

10. The fatigue life prediction system for welded structural steel based on critical distance theory according to claim 1, characterized in that, include: The basic test unit is used to conduct uniaxial tensile mechanical tests on the target welded structural steel to obtain the ultimate tensile strength. The fatigue testing unit is used to set the initial maximum cyclic stress based on the ultimate tensile strength, and to conduct multi-gradient fatigue tests on the target welded structural steel specimen under a preset stress ratio and stress control mode. The reference SN curve equation of the target welded structural steel specimen is obtained based on the initial maximum cyclic stress and the fatigue failure cycle number. The local stress acquisition unit is used to establish a three-dimensional finite element simulation model of the target welded structural steel specimen, extract the local stress of the three-dimensional finite element simulation model under fatigue load, and determine the maximum stress concentration point. Effective equivalent stress determination element, used to calculate the effective equivalent stress within the effective action area of ​​fatigue damage based on local stress, with the maximum stress concentration point as the sphere center and using the volume method of critical distance theory; And a prediction unit, used to obtain the predicted fatigue life of the target welded structural steel specimen under alternating load by combining the effective equivalent stress and the reference SN curve equation.

Images