Method and system for solving fatigue crack growth rate baseline of metal material

Abstract

The invention relates to a method and system for solving a fatigue crack growth rate baseline of a metal material, and relates to the technical field of structural fatigue reliability, and the method comprises the following steps: extracting an elastic-plastic crack opening displacement of a crack body along a crack surface; plastic constraint factors corresponding to different test loads and crack lengths are obtained, and an empirical relational expression between the normalized sample thickness and the plastic constraint factors is established; determining a crack tip cyclic plastic damage initial point load through elastic-plastic response analysis and iterative solution under the action of a cyclic load, so as to establish a mapping relation between stress intensity factor amplitudes and effective driving force under different stress ratios and plastic constraint factors; and replacing the amplitude of the stress intensity factor in each data pair with effective driving force through data conversion processing to obtain baseline data of the fatigue crack growth rate of the metal material, which is not influenced by the stress ratio and the thickness of the sample. The method has the beneficial effect that a fatigue crack growth rate baseline for uniformly describing different stress ratios and sample thickness influences can be established.

Description

Technical Field

[0001] This invention relates to the field of structural fatigue reliability technology, and more specifically, to a method and system for solving the baseline of fatigue crack propagation rate in metallic materials. Background Technology

[0002] In modern engineering, mechanical structures are widely used in rail transportation, aerospace, and automobile manufacturing. These structures frequently endure random vibration loads during service, leading to structural failures due to fatigue fracture. Therefore, establishing a quantitative prediction method for the fatigue crack propagation life of mechanical structures under vibration loading conditions provides an important theoretical tool for damage tolerance design of defective mechanical structures. Existing methods for predicting structural fatigue crack propagation life are based on fracture mechanics theory, relying on solving the relationship between the stress intensity factor and defect size of the defective structure under service loading conditions, and establishing the correlation between the stress intensity factor amplitude and crack propagation rate at the material level—that is, the material's fatigue crack propagation rate curve. Therefore, establishing an efficient and accurate fatigue crack propagation rate curve testing method is crucial for achieving high-precision prediction of structural fatigue crack propagation life.

[0003] Existing methods for testing fatigue crack growth rate involve conducting fatigue crack growth tests under constant amplitude loading on standard fracture specimens (such as compact tensile specimens and central crack tensile specimens) specified by national standards. This establishes the relationship between the stress intensity factor amplitude ΔK and the fatigue crack growth rate da / dN. However, the fatigue crack growth rate data (ΔK ~ da / dN data) obtained by this method show significant changes with variations in the constant amplitude load stress ratio and specimen thickness, exhibiting stress ratio effect and thickness dependence. The fatigue crack growth rate curve, as a material parameter reflecting the fracture performance of metallic materials, does not change with variations in the load stress ratio or specimen thickness. Therefore, existing standard-specified fatigue crack growth rate testing methods cannot obtain a fatigue crack growth rate curve that reflects the fundamental properties of the material, i.e., a baseline fatigue crack growth rate. Summary of the Invention

[0004] The purpose of this invention is to provide a method and system for solving the baseline of fatigue crack propagation rate in metallic materials, by establishing the traditional driving force ΔK and the effective driving force ΔK. CPD The mapping relationship between them is obtained by processing fatigue crack growth rate data (ΔK~da / dN data) obtained from standard test tests to obtain baseline fatigue crack growth rate data (ΔK) that does not have stress ratio effect and thickness dependence. CPD To achieve the above objectives, the technical solution adopted by the present invention is as follows: (~da / dN data). In a first aspect, this application provides a method for solving the baseline of fatigue crack propagation rate in metallic materials, including: Standard fatigue crack propagation specimens were selected, and fatigue crack propagation tests were carried out under constant amplitude load with different stress ratios. Data processing was used to obtain the relationship between the stress intensity factor amplitude and the crack propagation rate under different stress ratios. Based on the sample type, geometric dimensions and load conditions corresponding to the relational data, a three-dimensional finite element model is established. After setting the constitutive model of the elastoplastic material, the elastoplastic crack opening displacement along the crack surface is extracted through elastoplastic finite element analysis under different loads and crack lengths. The above displacements were iteratively calibrated using the strip yield model and a general analytical weight function to obtain the plastic constraint factor corresponding to different test loads and crack lengths. Combined with the specimen thickness and the plane stress crack tip plastic zone size of the crack body under the corresponding test load, an empirical relationship between the normalized specimen thickness and the plastic constraint factor was established through normalization and fitting analysis. An improved strip yield model for the specimen was constructed. This model was used to iteratively solve for the cyclic plastic damage initiation load at the crack tip, which reaches a stable state with crack length under different stress ratios and plastic constraint factors. This established the relationship between the stable cyclic plastic damage initiation load at the crack tip and the ratio of the maximum experimental load under different stress ratios and plastic constraint factors. Based on the relationship between the stress intensity factor amplitude and crack propagation rate under different stress ratios obtained from the experiment, the plastic constraint factor corresponding to each data point under different stress ratios was determined using the empirical relationship between normalized specimen thickness and plastic constraint factor. Based on the stress ratio and plastic constraint factor of each data pair, the mapping relationship between the stress intensity factor amplitude and the effective driving force was determined using the relationship between the cyclic plastic damage initiation load at the crack tip and the ratio of the maximum experimental load. Based on this relationship, the stress intensity factor amplitude in the data pair was replaced with the effective driving force to obtain baseline data on the fatigue crack propagation rate of metallic materials unaffected by stress ratio and specimen thickness. Preferably, the step involves selecting standard fatigue crack propagation specimens according to national standards, conducting fatigue crack propagation tests under constant amplitude load with different stress ratios, and processing the data to obtain the relationship between the stress intensity factor amplitude and crack propagation rate under different stress ratios, including: According to national standards, select a standard fatigue crack propagation specimen (such as a compact tensile specimen or a central crack tensile specimen), conduct fatigue crack propagation tests at multiple stress ratio levels under constant amplitude load, and record the crack length and cycle number data at each stress ratio. The experimental data were processed and analyzed to obtain a basic dataset containing the relationship between stress ratio, stress intensity factor amplitude and crack propagation rate.

[0005] Preferably, after establishing a three-dimensional finite element model based on the sample type, geometric dimensions, and load conditions corresponding to the relational data, and setting the constitutive model of the elastoplastic material, the elastoplastic crack opening displacement along the crack surface is extracted through elastoplastic finite element analysis under different loads and crack lengths, including: Based on the load and crack length covered by the original relational data, a three-dimensional elastoplastic finite element model of the corresponding specimen is established, simulation analysis is performed, and the elastoplastic opening displacement of the crack surface is extracted. For the same load and crack length, the elastic-plastic crack opening displacement is calculated based on the general analytical weight function and strip yield model corresponding to the specimen type. By adjusting the plastic constraint factor, the crack opening displacement calculated by the strip yield model is made to match the opening displacement extracted by the finite element analysis, thereby calibrating the plastic constraint factor for different crack lengths under this test condition and forming a plastic constraint factor dataset.

[0006] Preferably, the displacement is iteratively calibrated using a strip yield model and a general analytical weight function to obtain the plastic constraint factor corresponding to different test loads and crack lengths. Combining the specimen thickness and the plane stress crack tip plastic zone size of the crack body under the corresponding test load, an empirical relationship between the normalized specimen thickness and the plastic constraint factor is established through normalization and fitting analysis, including: For the standard fracture specimen type used, a general analytical weight function expression is determined. Based on the strip yield model and the weight function, the theoretical elastic-plastic crack opening displacement under different test loads and crack lengths is calculated. By adjusting the plastic constraint factor, the theoretical calculated value is made to match the value extracted by the finite element analysis, and the plastic constraint factor is calibrated. Based on the test load, crack length, and plastic constraint factor data obtained from the calibration, the plane stress crack tip plastic zone size corresponding to each data point is calculated. The normalized sample thickness is obtained by normalizing the sample thickness and establishing an empirical relationship between the normalized sample thickness and the plastic constraint factor through fitting analysis, which is used to describe the three-dimensional constraint state of the crack body.

[0007] Preferably, an improved strip yield model for the specimen is constructed. Fatigue crack propagation analysis is performed using this improved strip yield model, iteratively solving for the load at the crack tip cyclic plastic damage initiation point as the crack length enters a stable state. Based on the experimental load conditions and fatigue crack propagation analysis with different plastic constraint factors, fitting relationships are established between the stress ratio, plastic constraint factor, and the ratio of the crack tip cyclic plastic damage initiation point load to the maximum load, including: An improved strip yield model for the cracked body used in the experiment was constructed based on a general analytical weight function. The elastic-plastic response of the cracked body under cyclic external loading was solved by conducting fatigue crack propagation analysis. Under the maximum cyclic external load, the crack opening displacement and stress distribution of the bar element in the crack body are calculated. When the maximum external load is unloaded to the minimum external load, the crack opening displacement, bar element stress and length are updated. Then, the minimum external load is reloaded to the next maximum external load. The load corresponding to the initiation point of cyclic plastic damage at the crack tip is solved iteratively. The above steps are repeated until the fatigue crack extends to the termination crack length corresponding to the test conditions. Thus, the relationship between the external load and the fatigue crack length is obtained. The load at the initiation point of cyclic plastic damage at the crack tip that enters a steady state as the crack length changes is extracted, and the ratio between this load and the maximum load is calculated. Based on the experimental load conditions, multiple fatigue crack propagation analyses were conducted by changing the plastic constraint factor in the improved strip yield model. Polynomial fitting relationships were obtained between the plastic constraint factor and the ratio of the load at the crack tip cyclic plastic damage initiation point to the maximum load under different stress ratios.

[0008] Preferably, the step involves calculating the conversion factor between the stress intensity factor amplitude and the effective driving force based on the relationship data between the stress intensity factor amplitude and the crack propagation rate, the plastic constraint factor, and the stress ratio. This process replaces the stress intensity factor amplitude in each experimental data pair with the effective driving force, ultimately obtaining baseline data on the fatigue crack propagation rate of metallic materials that is unaffected by the stress ratio and sample thickness. This includes: For each data pair in the basic data of fatigue crack propagation test, based on its stress intensity factor amplitude and specimen thickness, the corresponding plastic constraint factor is determined using the empirical relationship between normalized specimen thickness and plastic constraint factor. Combined with its stress ratio level, the conversion parameter is determined using the fitting relationship between the plastic constraint factor and the ratio of the load at the crack tip cyclic plastic damage initiation point to the maximum load. The conversion factor between stress intensity factor amplitude and effective driving force is calculated. The stress intensity factor amplitude in each data pair is converted into an effective driving force by the conversion factor, and finally a dataset consisting of the effective driving force and the crack propagation rate is formed, which is the baseline of fatigue crack propagation rate of metallic materials that is not affected by stress ratio and specimen thickness.

[0009] Secondly, this application also provides a solution system for the baseline fatigue crack propagation rate of metallic materials, including: Test module: Used to select standard fatigue crack propagation specimens, conduct fatigue crack propagation tests under constant amplitude load with different stress ratios, and obtain data on the relationship between stress intensity factor amplitude and crack propagation rate under different stress ratios through data processing; Extraction module: It is used to establish a three-dimensional finite element model based on the sample type, geometric size and load conditions corresponding to the relational data. After setting the constitutive model of the elastoplastic material, it extracts the elastoplastic crack opening displacement along the crack surface of the crack body through elastoplastic finite element analysis under different loads and crack lengths. Calibration module: Used to iteratively calibrate the opening displacement of elastoplastic cracks using a strip yield model and a general analytical weighting function, obtaining the plastic constraint factor λ corresponding to different test loads and crack lengths, combined with the specimen thickness B and the plane stress crack tip plastic zone size r of the crack body under the corresponding test load. p0 The normalized sample thickness B / r was established through normalization and fitting analysis. p0 Empirical relationship between the plastic constraint factor λ and the plastic constraint factor λ; The solver module is used to construct an improved strip yield model for the specimen. This improved strip yield model is used to iteratively solve for the crack tip cyclic plastic damage initiation load P, which is the load at the crack tip as the crack length reaches a steady state under different stress ratios R and plastic constraint factors λ. CPD This establishes the ratio P of the stable crack tip cyclic plastic damage initiation point load to the maximum experimental load under different stress ratios R and plastic constraint factors λ. CPD / P max The relationship between them; Conversion module: Used to convert the stress ratio R and plastic constraint factor λ of each data pair into the ratio P of the load at the crack tip cyclic plastic damage initiation point to the maximum experimental load. CPD / P max The relationship between the stress intensity factor amplitude ΔK and the effective driving force ΔK is determined. CPD The mapping relationship between them; based on this relationship, the stress intensity factor magnitude ΔK in the data pair is replaced with the effective driving force ΔK. CPD We obtained baseline data on fatigue crack propagation rate of metallic materials that are unaffected by stress ratio R and specimen thickness B.

[0010] Thirdly, this application also provides an apparatus for solving the baseline of fatigue crack propagation rate in metallic materials, comprising: Memory, used to store computer programs; A processor, used to execute the computer program, implements the steps of a method for solving the baseline of fatigue crack propagation rate of the metallic material.

[0011] Fourthly, this application also provides a readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the above-described method for solving the baseline of fatigue crack propagation rate based on metallic materials.

[0012] The beneficial effects of this invention are as follows: This invention establishes the relationship between normalized specimen thickness and plastic constraint factor through elastoplastic finite element analysis. It also establishes the relationship between stress ratio, plastic constraint factor, and the ratio of the load at the crack tip's plastic cyclic damage initiation point to the maximum load by improving the strip yield model. This allows for the establishment of a real-time mapping relationship between the stress intensity factor amplitude and the effective driving force for fatigue crack propagation. The baseline fatigue crack propagation rate of the material is indirectly obtained from fatigue crack propagation rate data obtained through national standard testing. Compared to existing testing methods, this method does not require additional measuring equipment and offers advantages such as ease of measurement, low cost, and high data processing efficiency, enabling accurate measurement of the fatigue crack propagation rate baseline.

[0013] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing embodiments of the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description, claims, and drawings. Attached Figure Description

[0014] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 Based on the effective driving force ΔK CPD The flowchart for solving the baseline fatigue crack propagation rate.

[0016] Figure 2 This is a schematic diagram of the displacement extraction during the opening of an elastoplastic crack in a three-dimensional penetrating crack body.

[0017] Figure 3 To obtain the load P at the crack tip cyclic plastic damage initiation point CPD A flowchart showing the relationship between fatigue crack length a and the change in fatigue crack length a.

[0018] Figure 4 Comparison of test data before and after processing for tensile specimens with a central crack in 7475-T7351 aluminum alloy: (a) Fatigue crack propagation rate data (ΔK~da / dN data) with stress ratio and specimen thickness effects; (b) Baseline fatigue crack propagation rate data (ΔK) obtained based on the processing of this invention. CPD ~da / dN data).

[0019] Figure 5 A schematic diagram of a three-dimensional 1 / 8 symmetric elastoplastic finite element model of a tensile specimen with a central crack.

[0020] Figure 6 A schematic diagram of determining the plastic constraint factor of a tensile specimen with a central crack based on the elastic-plastic crack opening displacement: (a) elastic-plastic crack opening displacement determined by finite element and strip yield model; (b) local magnified view of elastic-plastic crack opening displacement.

[0021] Figure 7 Plastic constraint factor λ for a tensile specimen with a central crack and normalized specimen thickness B / r p0 The changing pattern and the fitted curve.

[0022] Figure 8 For a stress ratio R of -1, the load P at the crack tip of a tensile specimen with a central crack is used to determine the cyclic plastic damage initiation point under different plastic constraint factors λ. CPD Graph showing the relationship between crack length a and crack length 'a'.

[0023] Figure 9 P for a tensile specimen with a central crack CPD / P max The graph shows the variation of stress ratio and plastic constraint factor.

[0024] Figure 10 Comparison of test data before and after processing for AISI 4340 steel compact tensile specimens: (a) Fatigue crack propagation rate data (ΔK~da / dN data) with stress ratio and specimen thickness effects; (b) Baseline fatigue crack propagation rate data (ΔK) obtained based on the processing of this invention. CPD ~da / dN data).

[0025] Figure 11 A schematic diagram of a three-dimensional 1 / 8 symmetric elastoplastic finite element model of a compact tensile specimen.

[0026] Figure 12 Schematic diagram for determining the plastic constraint factor of a compact tensile specimen based on the elastic-plastic crack opening displacement: (a) elastic-plastic crack opening displacement determined by finite element and strip yield model; (b) local magnified view of elastic-plastic crack opening displacement.

[0027] Figure 13 The plastic constraint factor λ of the compact tensile specimen and the normalized specimen thickness B / r p0 The changing pattern and the fitted curve.

[0028] Figure 14 For a stress ratio R of 0.1, the load P at the crack tip cyclic plastic damage initiation point of a compact tensile specimen under different plastic constraint factors λ is given. CPD Graph showing the relationship between crack length a and crack length 'a'.

[0029] Figure 15 P for compact tensile specimens CPD / P maxGraph showing the variation of stress ratio R and plastic constraint factor λ; Figure 16 This is a schematic diagram of the system structure for solving the baseline fatigue crack propagation rate of metallic materials as described in this embodiment of the invention. Figure 17 This is a schematic diagram of the equipment structure for solving the baseline fatigue crack propagation rate of metallic materials as described in this embodiment of the invention.

[0030] In the diagram: 701, Test module; 702, Extraction module; 703, Calibration module; 704, Solving module; 705, Conversion module; 800, Solving device for the baseline of fatigue crack propagation rate of metallic materials; 801, Processor; 802, Memory; 803, Multimedia component; 804, I / O interface; 805, Communication component. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0032] It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. Furthermore, in the description of this invention, terms such as "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0033] Example 1: The physical mechanism of fatigue crack propagation is the generation of cyclic plastic damage at the crack tip. Therefore, the effective driving force for crack propagation, ΔK, which characterizes the cyclic plastic damage at the crack tip, is used. CPD Replacing the stress intensity factor amplitude ΔK, the fatigue crack propagation rate curves under different stress ratios and specimen thicknesses can be well unified, thereby establishing a baseline for fatigue crack propagation rate (ΔK) characterizing the basic fracture properties of materials. CPD ~da / dN data). However, in obtaining the effective driving force ΔK for fatigue crack propagation. CPDDuring the experiment, it is necessary to monitor the local deformation and compliance changes at the crack tip in real time during crack propagation to obtain the variation law of the load of cyclic plastic damage at the crack tip with crack length. Existing testing methods use digital image correlation technology or strain gauges to achieve accurate monitoring of local cyclic plastic damage at the crack tip, which has the disadvantages of complex measurement methods and cumbersome data processing. Furthermore, the effective driving force ΔK is limited. CPD It is susceptible to noise in digital signals.

[0034] Therefore, this invention proposes a fatigue crack propagation rate baseline (ΔK). CPD The method for testing (~da / dN data) establishes the relationship between normalized specimen thickness and plastic constraint factor through elastoplastic finite element analysis. It also establishes the relationship between stress ratio, plastic constraint factor, and the ratio of the load at the crack tip cyclic plastic damage initiation point to the maximum load by improving the strip yield model. This, in turn, establishes the relationship between the stress intensity factor amplitude ΔK and the effective driving force ΔK for fatigue crack propagation. CPD The real-time mapping relationship between them. The baseline of the fatigue crack propagation rate of the material is indirectly obtained through ΔK~da / dN obtained by conventional testing.

[0035] Compared to existing testing methods, this method does not require the introduction of additional measurement equipment and has the advantages of simple measurement, low measurement cost and high data processing efficiency, enabling accurate measurement of the baseline fatigue crack propagation rate.

[0036] This embodiment provides a method for solving the baseline of fatigue crack propagation rate in metallic materials.

[0037] See Figure 1 The figure shows that the method includes steps S100, S200, S300, S400 and S500.

[0038] S100. Select standard fatigue crack propagation specimens and conduct fatigue crack propagation tests with different stress ratios under constant amplitude load. After data processing, obtain the relationship between the stress intensity factor amplitude and crack propagation rate under different stress ratios.

[0039] It is understood that step S100 includes S101 and S102, wherein: S101. Select a standard fatigue crack propagation specimen according to the national standard requirements, and carry out fatigue crack propagation tests at multiple stress ratio levels under constant amplitude load, and record the crack length and cycle number data at each stress ratio. S102. Process and analyze the experimental data to obtain a basic dataset containing the relationship between stress ratio, stress intensity factor amplitude and crack propagation rate.

[0040] It should be noted that, based on national standards, a standard fatigue crack propagation specimen (such as a compact tensile specimen or a central crack tensile specimen) is selected to conduct fatigue crack propagation tests under constant amplitude load with different stress ratios. The test data is then processed to obtain the relationship between the stress intensity factor amplitude ΔK and the crack propagation rate da / dN of the metallic material under constant amplitude load with different stress ratios R (R~ΔK~da / dN data).

[0041] S200. Based on the sample type, geometric dimensions and load conditions corresponding to the relational data, a three-dimensional finite element model is established. After setting the constitutive model of the elastoplastic material, the elastoplastic crack opening displacement along the crack surface is extracted through elastoplastic finite element analysis under different loads and crack lengths.

[0042] It is understood that step S200 includes S201, S202, and S203, wherein: S201. Based on the load and crack length covered by the original relational data, establish a three-dimensional elastoplastic finite element model of the corresponding specimen, perform simulation analysis, and extract the elastoplastic opening displacement of the crack surface. S202. For the same load and crack length, based on the general analytical weight function corresponding to the specimen type and the strip yield model, calculate the elastic-plastic crack opening displacement. The calculation formula is as follows: In the formula, d is the virtual crack length of the crack body, that is, the physical crack length a and the crack tip plastic zone size r. p The sum of these values, E΄ being the effective elastic modulus of the material, W being the characteristic width of the crack, x being the distance along the crack line, m(d / W, x / d) being the universal analytical weight function of the crack, and K... ref (τ) is the stress intensity factor at the crack tip under the test load when the crack length is τ, which can be expressed as: In the formula, τ is the crack length, and σ is the crack length. ref (x) represents the stress distribution along the crack line of the cracked body used in the experiment when there is no crack, under the action of a reference external load, m(d / W, x / d) is the general analytical weight function of the cracked body, λ is the plastic constraint factor, x is the distance along the crack line, and σ0 is the flow stress of the cracked body material, which is the average of the yield strength and tensile strength.

[0043] S203. By adjusting the plastic constraint factor, the crack opening displacement calculated by the strip yield model is made to match the opening displacement extracted by the finite element analysis, thereby calibrating the plastic constraint factor for different crack lengths under the test condition and forming a plastic constraint factor dataset.

[0044] It should be noted that the constitutive and geometric dimensions (planar dimensions and specimen thickness) of the elastoplastic material of the model are set, and elastoplastic finite element analysis is carried out on the specimen under different loads and crack lengths within the test range. Based on the elastoplastic finite element calculation results, the elastoplastic crack opening displacement along the crack surface of the crack body under different loads and crack lengths is extracted. The elastoplastic crack opening displacement consists of the real crack opening displacement and the virtual crack opening displacement. The real crack opening displacement is extracted by the nodal displacement (the component perpendicular to the crack surface) of the physical crack surface of the crack body. For the virtual crack opening displacement located on the virtual crack surface (the plastic region at the crack tip), it is obtained by integrating the plastic strain component perpendicular to the crack surface. In the formula, V(x) represents the virtual crack opening displacement at position x along the crack surface, σ0 is the flow stress, and E' is the effective elastic modulus of the material. Let a be the plastic strain component perpendicular to the crack plane, and r be the crack length. p This represents the size of the plastic zone at the crack tip.

[0045] S300. The opening displacement of the elastoplastic crack is iteratively calibrated using a strip yield model and a general analytical weighting function to obtain the plastic constraint factor λ corresponding to different test loads and crack lengths. This is combined with the specimen thickness B and the plane stress crack tip plastic zone size r of the crack body under the corresponding test load. p0 The normalized sample thickness B / r was established through normalization and fitting analysis. p0 The empirical relationship between the plastic constraint factor λ and the plastic constraint factor λ.

[0046] It is understood that step S300 includes S301 and S302, wherein: S301. Determine the general analytical weight function expression for the standard fracture specimen type used, calculate the theoretical elastic-plastic crack opening displacement under different test loads and crack lengths based on the strip yield model and the weight function, and adjust the plastic constraint factor to make the theoretical calculation value match the value extracted by the finite element analysis, thus completing the calibration of the plastic constraint factor. S302. Based on the calibrated test load, crack length, and plastic constraint factor data, calculate the plane stress crack tip plastic zone size corresponding to each data point. Normalize the sample thickness using the normalized sample thickness. Establish an empirical relationship between the normalized sample thickness and the plastic constraint factor through fitting analysis to describe the three-dimensional constraint state of the crack body. The empirical relationship between the normalized sample thickness and the plastic constraint factor is fitted using the following expression: In the formula, λ max For when B / r p0 → The lower limit of λ when it reaches 0 is 1.min For when B / r p0 The upper limit of λ as it approaches infinity is 1 / (1-2v), where v is the material's Poisson's ratio, B is the sample thickness, and r is the upper limit of λ as it approaches infinity. p0 , where is the size of the plastic zone at the crack tip of the plane stress corresponding to the current external load in the experiment, and k and b are coefficients determined by fitting.

[0047] It should be noted that, for the standard fracture specimen used in the test, the Wu-Carlson general analytical weight function m(a / W, x / a) is determined based on the specimen type and geometry. For a centrally cracked specimen (such as a centrally cracked tensile specimen), its general analytical weight function can be expressed as: For specimens with edge cracks (such as compact tensile specimens), the general analytical weighting function can be expressed as: In the formula, a is the crack length, W is the characteristic width of the crack body, x is the distance along the crack line, and β is the crack length. j These are the series coefficients of the weighting function, which are related to the specimen type and the normalized specimen thickness a / W.

[0048] Based on the strip yielding model, the elastoplastic crack opening displacement V(x) of the crack body under different test loads and crack lengths is calculated using the weighting function of the standard fracture specimen: In the formula, d is the virtual crack length of the crack body, that is, the physical crack length a and the crack tip plastic zone size r. p The sum of E' and W. E' is the effective elastic modulus of the material, W is the characteristic width of the crack, x is the distance along the crack line, m(d / W, x / d) is the general analytical weight function of the crack, and K is the sum of E' and W. ref (τ) is the stress intensity factor at the crack tip under the test load when the crack length is τ, which can be expressed as: In the formula, τ is the crack length, and σ is the crack length. ref (x) represents the stress distribution along the crack line of the cracked body used in the experiment under a reference external load when no crack exists. m(d / W, x / d) is the general analytical weight function of the cracked body, λ is the plastic constraint factor, x is the distance along the crack line, and σ0 is the flow stress of the cracked body material, which is the average of the yield strength and tensile strength. When calculating the elastic-plastic crack opening displacement of the cracked body, the virtual crack length d can be solved iteratively based on the strip yield model using the following expression: In the formula, σ ref(x) represents the stress distribution along the crack line of the cracked body used in the experiment when there is no crack, under the action of a reference external load, m(d / W, x / d) is the general analytical weight function of the cracked body, λ is the plastic constraint factor, x is the distance along the crack line, and σ0 is the flow stress of the cracked body material, which is the average of the yield strength and tensile strength.

[0049] Under different test loads and crack lengths, by adjusting the plastic constraint factor λ, the elastic-plastic opening displacement of the crack body predicted by the above formula is made to match the elastic-plastic crack opening displacement extracted based on the elastic-plastic finite element analysis results in the above steps, and the plastic constraint factor λ corresponding to the test load and crack length is obtained.

[0050] S400. Construct an improved strip yield model for the specimen. Iteratively solve the cyclic plastic damage initiation load P at the crack tip as the crack length reaches a steady state under different stress ratios R and plastic constraint factors λ. CPD This establishes the ratio P of the stable crack tip cyclic plastic damage initiation point load to the maximum experimental load under different stress ratios R and plastic constraint factors λ. CPD / P max The relationship between them;

[0051] It is understood that in this step, S400 includes S401, S402, and S403, wherein: S401. Based on the general analytical weight function, an improved strip yield model of the crack body used in the experiment is constructed. By carrying out fatigue crack propagation analysis, the elastic-plastic response of the crack body under cyclic external load is solved. S402. Calculate the crack opening displacement and stress distribution of the bar element in the crack body under the maximum external load of the cyclic load. When unloading from the maximum external load to the minimum external load, update the crack opening displacement, bar element stress and length. Then reload from the minimum external load to the next maximum external load. Iteratively solve the load corresponding to the initiation point of cyclic plastic damage at the crack tip. Repeat the above steps until the fatigue crack extends to the termination crack length corresponding to the test conditions. This will obtain the relationship between the load at the initiation point of cyclic plastic damage at the crack tip and the fatigue crack length. Extract the load at the initiation point of cyclic plastic damage at the crack tip that enters a stable state as the crack length changes, and calculate the ratio between this load and the maximum load. S403. Based on the experimental load conditions, multiple fatigue crack propagation analyses were conducted by changing the plastic constraint factor in the improved strip yield model to obtain the polynomial fitting relationship between the plastic constraint factor and the ratio of the load at the crack tip cyclic plastic damage initiation point to the maximum load under different stress ratios.

[0052] It should be noted that, based on the geometric dimensions of the specimen used in the experiment, the initial crack length corresponding to the experimental conditions was set. The crack surface and crack tip plastic zone of the crack body were discretized into several ideal elastoplastic rod elements. An improved strip yield model corresponding to this crack body was established based on the Wu-Carlsson universal analytical weight function to solve the elastoplastic response of the crack body during the fatigue crack propagation stage under cyclic external loading, representing the experimental load. The maximum external load P of the cyclic load was... max Under the action of the action, the elastic-plastic crack opening displacement of the crack body is solved by the following formula: In the formula, x j V represents the center position of the j-th link element. max (x j ) is the maximum external load P max The crack surface displacement at the center of the j-th bar element during the action, where n is the total number of bar elements, F(d, x) j ) and G(d, x i , x j ) represents the influence function of crack surface displacement, σ max (x i Let be the stress value acting on the i-th link element, which can be expressed as: In the formula, n is the total number of bar elements, k is the number of bar elements in the physical crack surface region, λ is the plastic constraint factor, and σ0 is the flow stress.

[0053] Based on the calculated elastic-plastic crack opening displacement of the crack body under the maximum external load, the length of the rod element in the crack tip plastic zone is determined using the following formula: In the formula, L(x) j ) represents the length of the j-th bar element, E′ represents the effective elastic modulus of the cracked material, λ represents the plastic constraint factor, and σ0 represents the flow stress.

[0054] When the load level of the current cycle in a constant amplitude load changes from the maximum external load P max Become the minimum external load P min At that time, the elastic-plastic crack opening displacement and the stress and length of the rod element are updated using the following formula: In the formula, V min (x j ) is the minimum external load P min The displacement of the crack surface at the center of the j-th rod element during the action is F(d, x). j ) and G(d, x i , xj ) represents the influence function of crack surface displacement, σ min (x j ) is the minimum external load P of the current loop. min The stress in the rod element under action is solved iteratively using the constrained Gauss-Seidel method with the following expression: In the formula, L(x) j The length of the bar element that produces compressive yielding under minimum load is updated as follows: In the formula, σ min (x j ) is the minimum external load P min The stress value V acting on the j-th link element during operation. min (x j ) is the minimum external load P min The displacement of the crack surface at the center of the j-th bar element during action. For bar elements in the plastic wake region (1≤x j ≤a), the following constraints must be satisfied during the iteration process: For bar elements in the crack tip plastic zone (a≤x) j ≤d), the following constraints must be satisfied during the iteration process: When the crack originates from the maximum external load P max Unload to minimum external load P min Then, by the minimum external load P min Reload the maximum external load P to the next loop max At this point, the crack tip will reopen and generate cyclic plastic damage. The load P at the crack tip cyclic plastic damage initiation point... CPD Up to maximum load P max The corresponding stress intensity factor amplitude ΔK CPD As an effective driving force for fatigue crack propagation, the load P at the crack tip cyclic plastic damage initiation point is iteratively solved using the expression shown below. CPD : In the formula, x k Let F(d, x) be the center position of the k-th link element. j ) and G(d, x i , x j L(x) is the influence function of crack surface displacement. k σ is the length of the rod element that produces compressive yielding under minimum load. CPD(x i ) represents the stress value acting on the i-th link element, which still satisfies the above constraints during the iteration process.

[0055] Calculate the load P at the crack tip cyclic plastic damage initiation point corresponding to the current crack length. CPD Subsequently, fatigue crack propagation under cyclic external loading is achieved by breaking the bar elements in the crack tip plastic zone. For computational accuracy, the fatigue crack propagation increment Δa is calculated using the following expression: In the formula, ω is the size of the cyclic plastic zone at the crack tip, which is taken as the minimum value between the size of the plastic zone at the crack tip under the maximum and minimum external loads.

[0056] Based on the extended fatigue crack length, the bar elements of the physical crack surface and the crack tip plastic zone are redefined, and the elastic-plastic crack opening displacement, bar element length, and bar element stress of the crack body under the next cyclic load are calculated. The above steps are repeated until the fatigue crack extends to the termination crack length corresponding to the experimental conditions, thereby obtaining the cyclic plastic damage initiation load P at the crack tip. CPD The relationship between fatigue crack length a and the crack length α.

[0057] S500, based on the stress ratio R and plastic constraint factor λ for each data pair, is the ratio P of the load at the crack tip cyclic plastic damage initiation point to the maximum experimental load. CPD / P max The relationship between the stress intensity factor amplitude ΔK and the effective driving force ΔK is determined. CPD The mapping relationship between them; based on this relationship, the stress intensity factor magnitude ΔK in the data pair is replaced with the effective driving force ΔK. CPD We obtained baseline data on fatigue crack propagation rate of metallic materials that are unaffected by stress ratio R and specimen thickness B.

[0058] It is understood that in this step, S500 includes S501 and S502, wherein: S501. For each data pair in the basic data of fatigue crack propagation test, based on its stress intensity factor amplitude and specimen thickness, the corresponding plastic constraint factor is determined using the empirical relationship between the normalized specimen thickness and the plastic constraint factor. Combined with its stress ratio, the conversion parameter is determined using the fitting relationship between the load at the crack tip cyclic plastic damage initiation point and the maximum load. The conversion factor between the stress intensity factor amplitude and the effective driving force is calculated, and its calculation formula is as follows: In the formula, U is the conversion factor, ΔK is the stress intensity factor amplitude, and ΔK CPD R is the corresponding effective driving force, and P is the stress ratio. CPD / P max The load P at the crack tip cyclic plastic damage initiation point CPD With maximum load P max The ratio of .

[0059] S502. The stress intensity factor amplitude in each data pair is converted into effective driving force through the conversion factor, and finally a dataset consisting of effective driving force and crack propagation rate is formed, which is the baseline of fatigue crack propagation rate of metallic materials that is not affected by stress ratio and specimen thickness.

[0060] It should be noted that the improved strip yield model was used to conduct fatigue crack propagation analysis under different plastic constraint factors λ, and the crack length a and the load P at the crack tip cyclic plastic damage initiation point were calculated. CPD The relationship between the load P at the crack tip cyclic plastic damage initiation point is obtained. CPD The values ​​are taken when the system reaches a steady state. Then, the stress ratio R - plastic constraint factor λ - ratio of the load at the crack tip cyclic plastic damage initiation point to the maximum load P is established. CPD / P max The data were used to fit the plastic constraint factor λ and P under different stress ratios R using the polynomial method shown below. CPD / P max The relationship between them: In the formula, p i (i=1,...,4) are the coefficients of the fitted polynomial.

[0061] Based on the fatigue crack propagation rate data (stress ratio R - specimen thickness B - stress intensity factor amplitude ΔK - fatigue crack propagation rate da / dN data) obtained in the above steps, for each data pair, the corresponding plastic constraint factor λ is determined by its corresponding stress intensity factor amplitude ΔK and specimen thickness B; combined with its stress ratio R, the corresponding P is determined. CPD / P max The stress intensity factor amplitude ΔK and the effective driving force ΔK for fatigue crack propagation are calculated using the following expressions. CPD The conversion factor U between them: In the formula, U is the conversion factor, ΔK is the stress intensity factor amplitude, and ΔK CPD R is the corresponding effective driving force, and P is the stress ratio. CPD / P max The load P at the crack tip cyclic plastic damage initiation point CPD With maximum load P max The ratio of .

[0062] The effective driving force ΔK is determined by the stress intensity factor amplitude ΔK in the data pair through the conversion factor U. CPD Replace the stress intensity factor magnitude ΔK in all data pairs with the effective driving force ΔK. CPD Finally, the baseline (ΔK) of the fatigue crack propagation rate of metallic materials, which is unaffected by the stress ratio R and the sample thickness B, is obtained. CPD -da / dN data).

[0063] In this embodiment, the method of the present invention obtains a baseline (ΔK) for the fatigue crack propagation rate of metallic materials that is unaffected by the stress ratio R and the sample thickness B. CPD The process of (-da / dN data) is as follows Figure 1 As shown. First, based on national standards, a standard specimen was selected to conduct fatigue crack propagation tests under constant amplitude loads with different stress ratios. The test data (R~ΔK~da / dN data) were processed. Then, a three-dimensional finite element model of the specimen was established and elastoplastic finite element analysis was performed to extract the elastoplastic crack opening displacement along the crack surface under the corresponding load and crack length. Using the Wu-Carlson universal analytical weight function of the specimen and based on the strip yield model, the plastic constraint factor λ was adjusted to make the predicted crack opening displacement match the elastoplastic crack opening displacement extracted by the finite element analysis. The calibrated test load P-crack length a-plastic constraint factor λ data were obtained and converted into normalized specimen thickness B / r. p0 - Data on the plastic constraint factor λ. Then, based on the size of the test specimen, the load conditions, and the initial crack length, the improved strip yield model is used to solve the elasto-plastic response of the crack body under cyclic external loading, representing the test load, to obtain the load P at the crack tip cyclic plastic damage initiation point. CPD The relationship between stress ratio R and fatigue crack length a is established, thereby establishing the ratio of stress ratio R to plastic constraint factor λ to the maximum load at the crack tip cyclic plastic damage initiation point P. CPD / P max Data. Finally, the relationship between the fatigue crack propagation rate data (RB-ΔK-da / dN data) obtained from experiments and the plastic constraint factor λ, and λ-P... CPD / P max The relationship between the stress intensity factor amplitude ΔK and the effective driving force ΔK for fatigue crack propagation, which characterizes cyclic plastic damage at the crack tip, is established. CPD The relationship between these factors allows for the processing of fatigue crack propagation rate data to obtain the baseline fatigue crack propagation rate (ΔK) for the corresponding metallic material. CPD (~da / dN data). The specific solution process for the baseline fatigue crack propagation rate of metallic materials is as follows: Step S1: Based on national standards, select a standard fatigue crack propagation specimen (such as a compact tensile specimen or a central crack tensile specimen) to conduct fatigue crack propagation tests under constant amplitude load with different stress ratios, and process the test data to obtain the relationship between the stress intensity factor amplitude ΔK and the crack propagation rate da / dN of the metallic material under constant amplitude load with different stress ratios R (R~ΔK~da / dN data).

[0064] Step S2: Establish a three-dimensional finite element model of the specimen used in the fatigue crack propagation test in Step S1, set the elastoplastic material constitutive model and geometric dimensions (planar dimensions and specimen thickness), and conduct elastoplastic finite element analysis of the specimen under different loads and crack lengths within the test range. Figure 2 As shown, based on the results of elastoplastic finite element analysis, the elastoplastic crack opening displacement along the crack surface of the crack body under different loads and crack lengths is extracted. The elastoplastic crack opening displacement consists of the actual crack opening displacement and the virtual crack opening displacement. The actual crack opening displacement is extracted by the nodal displacement (the component perpendicular to the crack surface) of the physical crack surface of the crack body. For the virtual crack opening displacement located on the virtual crack surface (the plastic region at the crack tip), it is obtained by integrating the plastic strain component perpendicular to the crack surface: In the formula, V(x) represents the virtual crack opening displacement at position x along the crack surface, σ0 is the flow stress, and E' is the effective elastic modulus of the material. Let a be the plastic strain component perpendicular to the crack plane, and r be the crack length. p This represents the size of the plastic zone at the crack tip.

[0065] Step S3: For the standard fracture specimen used in the test, determine the Wu-Carlson universal analytical weight function m(a / W, x / a) based on the specimen type and geometry. For a centrally cracked specimen (such as a centrally cracked tensile specimen), its universal analytical weight function can be expressed as: For specimens with edge cracks (such as compact tensile specimens), the general analytical weighting function can be expressed as: In the formula, a is the crack length, W is the characteristic width of the crack body, x is the distance along the crack line, and β is the crack length. j These are the series coefficients of the weighting function, which are related to the specimen type and the normalized specimen thickness a / W.

[0066] For the test load and crack length in step S2, based on the strip yielding model, the elastoplastic crack opening displacement V(x) of the crack body under different test loads and crack lengths is calculated using the weight function of the standard fracture specimen: In the formula, d is the virtual crack length of the crack body, that is, the physical crack length a and the crack tip plastic zone size r. p The sum of these values, E΄ being the effective elastic modulus of the material, W being the characteristic width of the crack, x being the distance along the crack line, m(d / W, x / d) being the universal analytical weight function of the crack, and K... ref (τ) is the stress intensity factor at the crack tip under the test load when the crack length is τ, which can be expressed as: In the formula, τ is the crack length, and σ is the crack length. ref (x) represents the stress distribution along the crack line of the cracked body used in the experiment under a reference external load when no crack exists. m(d / W, x / d) is the general analytical weight function of the cracked body, λ is the plastic constraint factor, x is the distance along the crack line, and σ0 is the flow stress of the cracked body material, which is the average of the yield strength and tensile strength. When calculating the elastic-plastic crack opening displacement of the cracked body, the virtual crack length d can be solved iteratively based on the strip yield model using the following expression: In the formula, σ ref (x) represents the stress distribution along the crack line of the cracked body used in the experiment when there is no crack, under the action of a reference external load, m(d / W, x / d) is the general analytical weight function of the cracked body, λ is the plastic constraint factor, x is the distance along the crack line, and σ0 is the flow stress of the cracked body material, which is the average of the yield strength and tensile strength.

[0067] Step S4: Based on the test load P-crack length a-plastic constraint factor λ data obtained through calibration in step S3, calculate the corresponding plane stress crack tip plastic zone size r. p0 : In the formula, K is the stress intensity factor of the cracked body under the test external load when the crack length is a, and σ0 is the flow stress of the cracked body material, which is the average of the yield strength and tensile strength.

[0068] Based on the specimen thickness B and the calculated plane stress crack tip plastic zone size r p0 The test load P-crack length a-plastic constraint factor λ data were converted into normalized specimen thickness B / r. p0 - Plastic constraint factor λ data. The normalized specimen thickness B / r is fitted using the expression shown below. p0 Relationship with plastic constraint factor λ: In the formula, λ maxFor when B / r p0 → The lower limit of λ when it reaches 0 is 1; min For when B / r p0 The upper limit of λ as it approaches infinity is 1 / (1-2v), where v is the material's Poisson's ratio, B is the sample thickness, and r is the upper limit of λ as it approaches infinity. p0 Let k and b be the dimensions of the plastic zone at the crack tip under the plane stress corresponding to the current experimental external load, and let b be the coefficients determined by fitting. By fitting, the values ​​of parameters k and b can be determined, thereby establishing an empirical expression describing the three-dimensional constraint state of the cracked body used in the experiment.

[0069] Step S5, refer to... Figure 3 The flowchart shown illustrates how, based on the geometry of the test specimen, the initial crack length corresponding to the test conditions is set. The crack surface and crack tip plastic zone of the crack body are discretized into several ideal elasto-plastic rod elements. An improved strip yield model corresponding to this crack body is established based on the Wu-Carlsson universal analytical weight function to solve the elasto-plastic response of the crack body during the fatigue crack propagation stage under cyclic external loading, representing the test load. The maximum external load P of the cyclic load... max Under the action of the action, the elastic-plastic crack opening displacement of the crack body is solved by the following formula: In the formula, x j V represents the center position of the j-th link element. max (x j ) is the maximum external load P max The crack surface displacement at the center of the j-th bar element during the action, where n is the total number of bar elements, F(d, x) j ) and G(d, x i , x j ) represents the influence function of crack surface displacement, σ max (x i Let be the stress value acting on the i-th link element, which can be expressed as: In the formula, n is the total number of bar elements, k is the number of bar elements in the physical crack surface region, λ is the plastic constraint factor, and σ0 is the flow stress.

[0070] Based on the calculated elastic-plastic crack opening displacement of the crack body under the maximum external load, the length of the rod element in the crack tip plastic zone is determined using the following formula: In the formula, L(x) j ) represents the length of the j-th bar element, E′ represents the effective elastic modulus of the cracked material, and σ0 represents the flow stress.

[0071] When the load level of the current cycle in a constant amplitude load changes from the maximum external load Pmax Become the minimum external load P min At that time, the elastic-plastic crack opening displacement and the stress and length of the rod element are updated using the following formula: In the formula, V min (x j ) is the minimum external load P min The displacement of the crack surface at the center of the j-th rod element during the action is F(d, x). j ) and G(d, x i , x j ) represents the influence function of crack surface displacement, σ min (x j ) is the minimum external load P of the current loop. min The stress in the rod element under action is solved iteratively using the constrained Gauss-Seidel method with the following expression: Where, L(x) j The length of the bar element that produces compressive yielding under minimum load is updated as follows: In the formula, σ min (x j ) is the minimum external load P min The stress value V acting on the j-th link element during operation. min (x j ) is the minimum external load P min The displacement of the crack surface at the center of the j-th bar element during action. For bar elements in the plastic wake region (1≤x j ≤a), the following constraints must be satisfied during the iteration process: For bar elements in the crack tip plastic zone (a≤x) j ≤d), the following constraints must be satisfied during the iteration process: When the crack originates from the maximum external load P max Unload to minimum external load P min Then, by the minimum external load P min Reload the maximum external load P to the next loop max When the crack tip reopens, cyclic plastic damage occurs. The load P at the initiation point of cyclic plastic damage at the crack tip when it undergoes tensile plastic deformation again is... CPD Up to maximum load P max The corresponding stress intensity factor amplitude ΔK CPDAs an effective driving force for fatigue crack propagation, the load P at the crack tip cyclic plastic damage initiation point is iteratively solved using the expression shown below. CPD : In the formula, x k Let F(d, x) be the center position of the k-th link element. j ) and G(d, x i , x j L(x) is the influence function of crack surface displacement. k σ is the length of the rod element that produces compressive yielding under minimum load. CPD (x i ) represents the stress value acting on the i-th link element, which still satisfies the above constraints during the iteration process.

[0072] Calculate the load P at the crack tip cyclic plastic damage initiation point corresponding to the current crack length. CPD Subsequently, fatigue crack propagation under cyclic external loading is achieved by breaking the bar elements in the crack tip plastic zone. For computational accuracy, the fatigue crack propagation increment Δa is calculated using the following expression: In the formula, ω is the size of the cyclic plastic zone at the crack tip, which is taken as the minimum value between the size of the plastic zone at the crack tip under the maximum and minimum external loads.

[0073] Based on the extended fatigue crack length, the bar elements of the physical crack surface and the crack tip plastic zone are redefined, and the elastic-plastic crack opening displacement, bar element length, and bar element stress of the crack body under the next cyclic load are calculated. The above steps are repeated until the fatigue crack extends to the termination crack length corresponding to the experimental conditions, thereby obtaining the cyclic plastic damage initiation load P at the crack tip. CPD The relationship between fatigue crack length a and the crack length α.

[0074] Step S6: Based on the test load conditions in Step S1, fatigue crack propagation analysis is conducted under different plastic constraint factors λ using the improved strip yield model established in Step S5. The crack length a and the load P at the crack tip cyclic plastic damage initiation point are calculated. CPD The relationship between the load P at the crack tip cyclic plastic damage initiation point is obtained. CPD The values ​​at which the system reaches a steady state are determined. Then, the stress ratio R – the plastic constraint factor λ-P – is established. CPD / P max The data were used to fit the plastic constraint factor λ and P under different stress ratios R using the polynomial method shown below. CPD / P max The relationship between them: In the formula, p i (i=1,...,4) are the coefficients of the fitted polynomial.

[0075] Step S7: Based on the fatigue crack propagation rate data obtained in Step S1 (stress ratio R - specimen thickness B - stress intensity factor amplitude ΔK - fatigue crack propagation rate da / dN data), for each data pair, using its corresponding stress intensity factor amplitude ΔK and test thickness B, the corresponding plastic constraint factor λ is determined in Step S4. Combined with its stress ratio R, the corresponding P is determined in Step S6. CPD / P max The relationship between the stress intensity factor amplitude ΔK and the effective driving force ΔK for fatigue crack propagation is calculated using the expression shown below. CPD The conversion factor U between them: In the formula, U is the conversion factor, ΔK is the stress intensity factor amplitude, and ΔK CPD R is the corresponding effective driving force, and P is the stress ratio. CPD / P max The load P at the crack tip cyclic plastic damage initiation point CPD With maximum load P max The ratio of .

[0076] The effective driving force ΔK is determined by the stress intensity factor amplitude ΔK in the data pair through the conversion factor U. CPD Replace the stress intensity factor magnitude ΔK in all data pairs with the effective driving force ΔK. CPD Finally, the baseline (ΔK) of the fatigue crack propagation rate of metallic materials, which is unaffected by the stress ratio R and the sample thickness B, is obtained. CPD -da / dN data).

[0077] Example 2: The following experimental examples will further illustrate the method for obtaining the baseline of fatigue crack propagation rate of metallic materials that is unaffected by stress ratio R and specimen thickness B, as described in this invention.

[0078] This experiment uses a 7475-T7351 aluminum alloy tensile specimen with a central crack as the target specimen. The specimen has an elastic modulus E = 200 GPa, a Poisson's ratio ν = 0.32, and a yield stress σ. Y =440 MPa, tensile strength σ b =530 MPa. Based on the fatigue crack propagation rate test data of this specimen, the baseline fatigue crack propagation rate of this specimen is determined using the method of this invention, specifically including the following steps: Step S1, as follows Figure 4As shown in -a, based on national standards, fatigue crack propagation tests were conducted on tensile specimens with center cracks and thicknesses B of 6.36 mm and 12.7 mm, respectively, under constant amplitude loads with stress ratios R of -1, 0.06, and 0.5. The test data were processed to obtain the relationship between the stress intensity factor amplitude ΔK and the fatigue crack propagation rate da / dN under different specimen thicknesses B and different stress ratios R (R~ΔK~da / dN data).

[0079] Step S2, as follows Figure 5 As shown, a three-dimensional finite element model of a tensile specimen with a central crack was established. The constitutive model and geometric dimensions (planar dimensions and specimen thickness) of the elastoplastic material were set, and elastoplastic finite element analysis of the specimen was carried out under different loads and crack lengths in the test. Figure 2 As shown, based on the results of elastoplastic finite element analysis, the elastoplastic crack opening displacement along the crack surface of the crack body under different loads and crack lengths is extracted.

[0080] Step S3: Determine the Wu-Carlson universal analytical weight function for the tensile specimen with a central crack based on the specimen size, as shown in the following equation: In the formula, β j The coefficients of the weight function can be expressed as: In the formula, b in The values ​​of the fitting polynomial coefficients are shown in Table 1.

[0081] Table 1. Weighting function series coefficients β of tensile specimens with central crack i (a / W) The fitting polynomial coefficients b in Values ​​(0 ≤ a / W ≤ 0.9, β1 = 2.0) For the test load and crack length in step S2, based on the strip yielding model, the elastoplastic crack opening displacement V(x) of the crack body under different test loads and crack lengths is calculated using the analytical weight function of the tensile specimen with a central crack. Under different test loads and crack lengths, by adjusting the plastic constraint factor λ, the predicted elastoplastic crack opening displacement is made to match the elastoplastic crack opening displacement extracted from the elastoplastic finite element analysis results in step S2, thus obtaining the plastic constraint factor λ corresponding to the tensile specimen with a central crack under that test load and crack length (test load P - crack length a - plastic constraint factor λ data). Figure 6 As shown in Figure -a, a schematic diagram is presented for the calibration of the plastic constraint factor of a tensile specimen with a central crack, a thickness B of 6.36 mm, and a crack length of 25 mm, under test load. Figure 6 -b is a magnified view of the displacement of the elastoplastic crack opening.

[0082] Step S4: For the tensile specimen with a central crack, based on the test load P-crack length a-plastic constraint factor λ data obtained through calibration in step S3, calculate the corresponding plane stress crack tip plastic zone size r. p0 Based on the specimen thickness B, the test load P-crack length a-plastic constraint factor λ data are converted into normalized specimen thickness B / r. p0 - Plastic constraint factor λ data, the transformed data and fitting results are as follows: Figure 7 As shown. The empirical formula obtained from the fitting is as follows: Step S5, refer to the process Figure 3 Based on the geometry of the tensile specimen with a central crack, the initial crack length corresponding to the test conditions is set. The crack surface and crack tip plastic region of the crack body are discretized into several ideal elastic-plastic rod elements. An improved strip yield model corresponding to this crack body is established based on the Wu-Carlsson universal analytical weight function to solve the elastic-plastic response of the crack body during the fatigue crack propagation stage under cyclic external loading, representing the test load. Figure 8 As shown, the load P characterizing the initiation point of cyclic plastic damage at the crack tip under different plastic constraint factors λ with a stress ratio R of -1 was obtained. CPD The relationship between fatigue crack length a and the curve.

[0083] Step S6: Based on the test load conditions of the tensile specimen with a central crack in Step S1, fatigue crack propagation analysis is carried out under different plastic constraint factors λ using the improved strip yield model established in Step S5. The crack length a and the load P at the crack tip cyclic plastic damage initiation point are calculated. CPD The relationship between the load P at the crack tip cyclic plastic damage initiation point is obtained. CPD The value taken when entering a steady state. Establish the stress ratio R - plastic constraint factor λ - ratio of the load at the crack tip cyclic plastic damage initiation point to the maximum load P. CPD / P max The data was fitted using the polynomial shown below, and the fitting results are shown in [the original text]. Figure 9 The curve in the figure is shown.

[0084] In the formula, the polynomial coefficients p i As shown in Table 2.

[0085] Table 2 P CPD / P max Fitting polynomial coefficients showing the variation of stress ratio R and plastic constraint factor λ Step S7: Based on the fatigue crack propagation rate data obtained in Step S1 (stress ratio R - specimen thickness B - stress intensity factor amplitude ΔK - fatigue crack propagation rate da / dN data), for each data pair, using its corresponding stress intensity factor amplitude ΔK and test thickness B, the corresponding plastic constraint factor λ is determined in Step S4. Combined with its stress ratio R, the corresponding P is determined in Step S6. CPD / P max The stress intensity factor amplitude ΔK and the effective driving force ΔK for fatigue crack propagation are calculated using the corresponding expressions. CPD The conversion factor U between the data pairs is used to determine the corresponding effective driving force ΔK based on the stress intensity factor amplitude ΔK in the data pair. CPD .

[0086] Will Figure 4 -a All data from constant amplitude load fatigue crack propagation tests of central crack tensile specimens with thicknesses B of 6.36 mm and 12.7 mm and stress ratios R of -1, 0.06, and 0.5, respectively, are replaced with the effective driving force ΔK. CPD Ultimately, the result is as follows Figure 4 -b shows the baseline fatigue crack propagation rate (ΔK) of the tensile specimen with a central crack. CPD -da / dN data). Comparison Figure 4 -a and Figure 4 As can be seen from -b, for fatigue crack propagation rate data based on the stress intensity factor amplitude ΔK, the fatigue crack propagation rate data under different sample thicknesses and stress ratios exhibits significant dispersion. When converted to fatigue crack propagation rate data based on effective driving force using the method described in this invention, the fatigue crack propagation rate data under different stress ratios and sample thicknesses converge onto the same dispersion band, and there is no effect from stress ratio and sample thickness. This verifies the effectiveness and reliability of the method of this invention in solving the baseline of fatigue crack propagation rate.

[0087] Example 3: The following experimental examples will further illustrate the method for obtaining the baseline of fatigue crack propagation rate of metallic materials that is unaffected by stress ratio R and specimen thickness B, as described in this invention.

[0088] This experiment uses a compact tensile specimen of AISI 4340 steel as the target specimen. The specimen has an elastic modulus E = 200 GPa, a Poisson's ratio ν = 0.3, and a yield stress σ. Y =1391 MPa, tensile strength σ b=2131 MPa. Based on the fatigue crack growth rate test data of this specimen, the baseline fatigue crack growth rate of this specimen is indirectly obtained using the method of this invention, specifically including the following steps: Step S1, as follows Figure 10 As shown in -a, based on national standards, fatigue crack propagation tests were conducted on tensile specimens with a center crack and a thickness of 10 mm under constant amplitude loading with stress ratios R of 0.1, 0.3, and 0.5. The test data were processed to obtain the relationship between the stress intensity factor amplitude ΔK and the crack propagation rate da / dN of the metallic material under constant amplitude loading with different stress ratios R (R~ΔK~da / dN data).

[0089] Step S2, as follows Figure 11 As shown, a three-dimensional finite element model of a compact tensile specimen was established. The constitutive model and geometric dimensions (planar dimensions and specimen thickness) of the elastoplastic material were set, and elastoplastic finite element analysis of the specimen under different loads and crack lengths within the test range was carried out. Figure 2 As shown, based on the results of elastoplastic finite element analysis, the elastoplastic crack opening displacement along the crack surface of the crack body under different loads and crack lengths is extracted.

[0090] Step S3: Determine the Wu-Carlson universal analytical weight function for the compact tensile specimen based on its dimensions. The function is shown in the following equation: In the formula, β j The coefficients of the weight function can be expressed as: In the formula, b in The values ​​of the fitting polynomial coefficients are shown in Table 3.

[0091] Table 3 Weight function series coefficients β for compact tensile specimens i (a / W) fitting polynomial coefficients b in Value (0≤a / W≤0.9, β1=2.0) For the test load and crack length in step S2, based on the strip yielding model, the elastoplastic crack opening displacement V(x) of the crack body under different test loads and crack lengths is calculated using the analytical weight function of the compact tensile specimen. Under different test loads and crack lengths, by adjusting the plastic constraint factor λ, the predicted elastoplastic crack opening displacement is made to match the elastoplastic crack opening displacement extracted from the elastoplastic finite element analysis results in step S2, thus obtaining the plastic constraint factor λ corresponding to the compact tensile specimen under the test load and crack length (test load P - crack length a - plastic constraint factor λ data). Figure 12-a provides an exemplary schematic diagram of plastic constraint factor calibration under test load when the thickness of the compact tensile specimen is 10 mm and the crack length is 25 mm. Figure 12 -b is a magnified view of the displacement of the elastoplastic crack opening.

[0092] Step S4: For the compact tensile specimen, based on the test load P-crack length a-plastic constraint factor λ data obtained through calibration in step S3, calculate the corresponding plane stress crack tip plastic zone size r. p0 Based on the specimen thickness B, the test load P-crack length a-plastic constraint factor λ data are converted into normalized specimen thickness B / r. p0 - Plastic constraint factor λ data, the transformed data and fitting results are as follows: Figure 13 As shown. The empirical formula obtained from the fitting is as follows: Step S5, refer to the process Figure 3 Based on the geometry of the compact tensile specimen, the initial crack length corresponding to the test conditions is set. By discretizing the crack surface and crack tip plastic zone of the crack body into several ideal elastic-plastic rod elements, an improved strip yield model corresponding to the crack body is established based on the Wu-Carlsson general analytical weight function to solve the elastic-plastic response of the crack body in the fatigue crack propagation stage under cyclic external loading that represents the test load. Figure 14 An example is given showing the load P characterizing the initiation point of cyclic plastic damage at the crack tip under different plastic constraint factors λ when the stress ratio R is 0.1. CPD The relationship between fatigue crack length a and the curve.

[0093] Step S6: Based on the test load conditions of the compact tensile specimen in Step S1, fatigue crack propagation analysis is carried out under different plastic constraint factors λ using the improved strip yield model established in Step S5. The crack length a and the load P at the crack tip cyclic plastic damage initiation point are calculated. CPD The relationship between the load P at the crack tip cyclic plastic damage initiation point is obtained. CPD The value at which the system reaches a steady state. Establish the stress ratio R - plastic constraint factor λ-P. CPD / P max The data was fitted using the polynomial shown below, and the fitting results are shown in [the original text]. Figure 15 The curve in the figure is shown.

[0094] In the formula, the polynomial coefficients p i As shown in Table 4.

[0095] Table 4 P CPD / P maxFitting polynomial coefficients showing the variation of stress ratio R and plastic constraint factor λ Step S7: Based on the fatigue crack propagation rate data obtained in Step S1 (stress ratio R - specimen thickness B - stress intensity factor amplitude ΔK - fatigue crack propagation rate da / dN data), for each data pair, using its corresponding stress intensity factor amplitude ΔK and test thickness B, the corresponding plastic constraint factor λ is determined in Step S4. Combined with its stress ratio R, the corresponding P is determined in Step S6. CPD / P max The relationship between the stress intensity factor amplitude ΔK and the effective driving force ΔK for fatigue crack propagation is calculated using the corresponding expression. CPD The conversion factor U between the data pairs is used to determine the corresponding effective driving force ΔK based on the stress intensity factor amplitude ΔK in the data pair. CPD .

[0096] Will Figure 10 In the data set of all data from constant amplitude load fatigue crack propagation tests of compact tensile specimens in type -a under constant amplitude load conditions with a thickness B of 10 mm and stress ratios R of 0.1, 0.3, and 0.5, the stress intensity factor amplitude ΔK is replaced with the effective driving force ΔK. CPD Ultimately, the result is as follows Figure 10 -b shows the baseline fatigue crack propagation rate (ΔK) of a compact tensile specimen unaffected by stress ratio R and specimen thickness B. CPD -da / dN data), thereby verifying the effectiveness and reliability of the method of the present invention in obtaining the baseline of fatigue crack propagation rate of compact tensile specimens.

[0097] In summary, this invention provides a method for testing the baseline fatigue crack propagation rate of metallic materials. Unlike other methods for testing fatigue crack propagation rate curves, this invention uses the effective driving force ΔK for crack propagation, which characterizes cyclic plastic damage at the crack tip. CPD Instead of the stress intensity amplitude ΔK, it effectively unified the fatigue crack propagation rate curves of metallic materials under different stress ratios and sample thicknesses, thus establishing a baseline for fatigue crack propagation rate (ΔK) characterizing the basic fracture properties of materials. CPD The method of this invention (using ~da / dN data) overcomes the shortcomings of traditional methods in measuring fatigue crack propagation rate data (ΔK~da / dN data), which suffers from significant stress ratio effects and specimen thickness dependence. Furthermore, in terms of measurement methods, this invention establishes the relationship between the stress intensity factor amplitude ΔK and the effective driving force ΔK for fatigue crack propagation. CPD The real-time mapping relationship between them, through ΔK~da / dN obtained by national standard testing, indirectly yields the baseline of the fatigue crack propagation rate of the material, overcoming the limitations of existing ΔK... CPDTraditional testing methods, such as digital image correlation (DIC) technology or strain gauges, suffer from drawbacks including complex measurement techniques, cumbersome data processing, and susceptibility to digital signal noise. The method of this invention eliminates the need for additional measurement equipment, offering advantages such as ease of measurement, low cost, and high data processing efficiency. When applied to structural fatigue reliability, this method enables accurate measurement of the baseline fatigue crack propagation rate.

[0098] Example 4: like Figure 16 As shown, this embodiment provides a solution system for the baseline fatigue crack propagation rate of metallic materials. See [link to documentation]. Figure 16 The system includes: Test module 701: Used to select standard fatigue crack propagation specimens according to national standards, conduct fatigue crack propagation tests under constant amplitude load with different stress ratios, and obtain data on the relationship between stress intensity factor amplitude and crack propagation rate under different stress ratios; Extraction module 702: is used to establish a three-dimensional finite element model based on the sample type, geometric size and load conditions corresponding to the relational data, and after setting the constitutive model of the elastoplastic material, extract the elastoplastic crack opening displacement along the crack surface of the crack body through elastoplastic finite element analysis under different loads and crack lengths. Calibration module 703: It is used to solve the crack elastic-plastic opening displacement by combining the strip yield model with the general analytical weight function of the crack body, and to iteratively calibrate the elastic-plastic crack opening displacement obtained by finite element analysis to obtain the plastic constraint factor corresponding to different test loads and crack lengths. Then, combined with the specimen thickness and the plane stress crack tip plastic zone size of the crack body under the corresponding test load, the empirical relationship between the normalized specimen thickness and the plastic constraint factor is established through normalization processing and fitting analysis. Solver Module 704: Used to construct an improved strip yield model of the cracked body used in the experiment. By conducting fatigue crack propagation analysis, it solves the relationship between the stress ratio used in the experiment and different plastic constraint factors and the ratio of the load at the crack tip cyclic plastic damage initiation point to the maximum load. Conversion module 705: Used to calculate the conversion factor between stress intensity factor amplitude and effective driving force based on the relationship data between stress intensity factor amplitude and crack propagation rate under different stress ratios and specimen thicknesses, combined with the relationship between normalized specimen thickness and plastic constraint factor, and the relationship between stress ratio and plastic constraint factor and the ratio of crack tip cyclic plastic damage initiation point load to maximum load.

[0099] It should be noted that the specific methods by which each module performs operations in the system described in the above embodiments have been described in detail in the embodiments related to the method, and will not be elaborated here.

[0100] In summary, traditional methods using the national standard (GB / T6398 Metallic Materials—Fatigue Testing—Fatigue Crack Propagation Methods) to conduct fatigue crack propagation rate tests on standard fracture specimens show significant stress ratio effects and specimen thickness dependence in the measured fatigue crack propagation rate data (ΔK~da / dN data). The effective driving force ΔK for crack propagation, which characterizes cyclic plastic damage at the crack tip, needs further analysis. CPD Established baseline for fatigue crack propagation rate (ΔK) CPD ~da / dN data can effectively unify fatigue crack propagation rate curves under different stress ratios and specimen thicknesses. However, existing testing methods rely on digital image correlation (DIC) technology or strain gauges to accurately monitor the plastic deformation and local compliance at the crack tip, thereby achieving effective driving force ΔK. CPD The measurement of this type of test method has the disadvantages of being complex in measurement and cumbersome in data processing, and the effective driving force of the measurement ΔK is limited. CPD It is susceptible to noise in digital signals.

[0101] This invention proposes a baseline test method for fatigue crack propagation rate. Based on elastoplastic finite element analysis combined with an improved strip yield model, the relationship between the normalized thickness of the specimen, stress ratio, and plastic constraint factor is established, thereby establishing the relationship between the stress intensity factor amplitude ΔK and the effective driving force ΔK for fatigue crack propagation. CPD The real-time mapping relationship between the two is obtained. The baseline of the fatigue crack propagation rate of the material is indirectly obtained through ΔK~da / dN obtained by national standard testing. Compared with existing testing methods, this method does not require the introduction of additional measurement equipment and has the advantages of simple measurement, low measurement cost and high data processing efficiency, and can achieve accurate measurement of the fatigue crack propagation rate baseline.

[0102] Example 5: Corresponding to the above method embodiments, this embodiment also provides a device for solving the fatigue crack propagation rate baseline of metallic materials. The device for solving the fatigue crack propagation rate baseline of metallic materials described below can be referred to in correspondence with the method for solving the fatigue crack propagation rate baseline of metallic materials described above.

[0103] Figure 17 This is a block diagram of a device 800 for solving the baseline fatigue crack propagation rate of a metallic material, according to an exemplary embodiment. Figure 17 As shown, the device 800 for solving the baseline fatigue crack propagation rate of the metallic material includes a processor 801 and a memory 802. The device 800 also includes one or more of a multimedia component 803, an I / O interface 804, and a communication component 805.

[0104] The processor 801 controls the overall operation of the fatigue crack growth rate baseline solving device 800 for the metallic material to complete all or part of the steps in the aforementioned method for solving the fatigue crack growth rate baseline of the metallic material. The memory 802 stores various types of data to support the operation of the fatigue crack growth rate baseline solving device 800 for the metallic material. This data may include, for example, instructions for any application or method operating on the fatigue crack growth rate baseline solving device 800 for the metallic material, as well as application-related data, such as contact data, sent and received messages, images, audio, video, etc. The memory 802 can be implemented using any type of volatile or non-volatile storage device or a combination thereof, such as Static Random Access Memory (SRAM), Electrically Erasable Programmable Read-Only Memory (EEPROM), Erasable Programmable Read-Only Memory (EPROM), Programmable Read-Only Memory (PROM), Read-Only Memory (ROM), magnetic storage, flash memory, magnetic disk, or optical disk. The multimedia component 803 may include a screen and an audio component. The screen may be, for example, a touchscreen, and the audio component is used to output and / or input audio signals. For example, the audio component may include a microphone for receiving external audio signals. The received audio signals may be further stored in the memory 802 or transmitted via the communication component 805. The audio component also includes at least one speaker for outputting audio signals. I / O interface 804 provides an interface between processor 801 and other interface modules, such as a keyboard, mouse, or buttons. These buttons can be virtual or physical. Communication component 805 enables wired or wireless communication between the fatigue crack propagation rate baseline calculation device 800 and other devices. Wireless communication includes, for example, Wi-Fi, Bluetooth, Near Field Communication (NFC), 2G, 3G, or 4G, or a combination thereof. Therefore, the corresponding communication component 805 may include a Wi-Fi module, a Bluetooth module, or an NFC module.

[0105] In an exemplary embodiment, the device 800 for solving the baseline fatigue crack propagation rate of a metallic material may be implemented by one or more application-specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field-programmable gate arrays (FPGAs), controllers, microcontrollers, microprocessors, or other electronic components to perform the aforementioned method for solving the baseline fatigue crack propagation rate of a metallic material.

[0106] In another exemplary embodiment, a computer-readable storage medium including program instructions is also provided. When executed by a processor, these program instructions implement the steps of the method for solving the baseline of fatigue crack propagation rate of metallic materials described above. For example, the computer-readable storage medium may be the memory 802 including the program instructions described above. These program instructions may be executed by the processor 801 of the apparatus 800 for solving the baseline of fatigue crack propagation rate of metallic materials to complete the method for solving the baseline of fatigue crack propagation rate of metallic materials described above.

[0107] Example 6: Corresponding to the above method embodiments, this embodiment also provides a readable storage medium. The readable storage medium described below and the method for solving the baseline of fatigue crack propagation rate of a metallic material described above can be referred to in correspondence.

[0108] A computer program is stored on a readable storage medium, and when the computer program is executed by a processor, it implements the steps of the method for solving the baseline of fatigue crack propagation rate of metallic materials in the above method embodiments.

[0109] Specifically, the readable storage medium can be a USB flash drive, a portable hard drive, a read-only memory (ROM), a random access memory (RAM), a magnetic disk, or an optical disk, or any other readable storage medium capable of storing program code.

[0110] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for determining the baseline fatigue crack propagation rate of a metallic material, characterized in that, include: Standard fatigue crack propagation specimens were selected, and fatigue crack propagation tests were carried out under constant amplitude load with different stress ratios. Data processing was used to obtain the relationship between the stress intensity factor amplitude and the crack propagation rate under different stress ratios. Based on the sample type, geometric dimensions and load conditions corresponding to the relational data, a three-dimensional finite element model is established. After setting the constitutive model of the elastoplastic material, the elastoplastic crack opening displacement along the crack surface is extracted through elastoplastic finite element analysis under different loads and crack lengths. The opening displacement of the elastoplastic crack was iteratively calibrated using a strip yield model and a general analytical weighting function to obtain the plastic constraint factor λ corresponding to different test loads and crack lengths. This was combined with the specimen thickness B and the plane stress crack tip plastic zone size r of the crack body under the corresponding test load. p0 The normalized sample thickness B / r was established through normalization and fitting analysis. p0 Empirical relationship between the plastic constraint factor λ and the plastic constraint factor λ; An improved strip yield model for the specimen was constructed. This improved strip yield model was used to iteratively solve for the crack tip cyclic plastic damage initiation load P, which occurs as the crack length progresses to a steady state under different stress ratios R and plastic constraint factors λ. CPD This establishes the ratio P of the stable crack tip cyclic plastic damage initiation point load to the maximum experimental load under different stress ratios R and plastic constraint factors λ. CPD / P max The relationship between the stress intensity factor amplitude ΔK and the crack propagation rate da / dN under different stress ratios R obtained from experiments, and the relationship between the normalized specimen thickness B / r. p0 The empirical relationship between the plastic constraint factor λ and the stress ratio R determines the plastic constraint factor λ for each data point under different stress ratios R. Based on the stress ratio R and plastic constraint factor λ for each data pair, the ratio P of the load at the crack tip cyclic plastic damage initiation point to the maximum experimental load is used. CPD / P max The relationship between the stress intensity factor amplitude ΔK and the effective driving force ΔK is determined. CPD The mapping relationship between them; Based on this relationship, the stress intensity factor magnitude ΔK in the data pair is replaced with the effective driving force ΔK. CPD We obtained baseline data on fatigue crack propagation rate of metallic materials that are unaffected by stress ratio R and specimen thickness B.

2. The method for solving the baseline of fatigue crack propagation rate of metallic materials according to claim 1, characterized in that, The selected standard fatigue crack propagation specimens were subjected to fatigue crack propagation tests under constant amplitude loading and different stress ratios. Data processing yielded the relationship between the stress intensity factor amplitude and crack propagation rate under different stress ratios, including: According to the national standard requirements, a standard fatigue crack propagation specimen was selected, and fatigue crack propagation tests were carried out under constant amplitude load with multiple stress ratios. The crack length and number of cycles under each stress ratio were recorded. The experimental data were processed and analyzed to obtain a basic dataset containing the relationship between stress ratio, stress intensity factor amplitude and crack propagation rate.

3. The method for solving the baseline of fatigue crack propagation rate of metallic materials according to claim 1, characterized in that, The three-dimensional finite element model is established based on the sample type, geometric dimensions, and load conditions corresponding to the relational data. After setting the constitutive model of the elastoplastic material, the elastoplastic crack opening displacement along the crack surface is extracted through elastoplastic finite element analysis under different loads and crack lengths, including: Based on the load and crack length covered by the original relational data, a three-dimensional elastoplastic finite element model of the corresponding specimen is established, simulation analysis is performed, and the elastoplastic opening displacement of the crack surface is extracted. For the same load and crack length, the elastic-plastic crack opening displacement is calculated based on the general analytical weight function corresponding to the specimen type and the strip yield model. The calculation formula is as follows: In the formula, d is the virtual crack length of the crack body, that is, the physical crack length a and the crack tip plastic zone size r. p The sum of; E΄ is the effective elastic modulus of the material, W is the characteristic width of the crack, x is the distance along the crack line, m(d / W, x / d) is the general analytical weight function of the crack, K ref (τ) is the stress intensity factor at the crack tip under the test load when the crack length is τ, which can be expressed as: In the formula, τ is the crack length, and σ is the crack length. ref (x) represents the stress distribution along the crack line of the cracked body used in the experiment when there is no crack, under the action of a reference external load, m(d / W, x / d) is the general analytical weight function of the cracked body, λ is the plastic constraint factor, x is the distance along the crack line, and σ0 is the flow stress of the cracked body material, which takes the average value of yield strength and tensile strength. By adjusting the plastic constraint factor λ, the crack opening displacement calculated by the strip yield model is made to match the opening displacement extracted by the finite element analysis, thereby calibrating the plastic constraint factor for different crack lengths under this test condition and forming a plastic constraint factor dataset.

4. The method for solving the baseline of fatigue crack propagation rate of metallic materials according to claim 1, characterized in that, The formula for calculating the size of the plastic zone at the plane stress crack tip is as follows: In the formula, K is the stress intensity factor of the cracked body under the test external load when the crack length is a, and σ0 is the flow stress of the cracked body material, which is the average of the yield strength and tensile strength.

5. The method for solving the baseline of fatigue crack propagation rate of metallic materials according to claim 1, characterized in that, The elastic-plastic crack opening displacement is iteratively calibrated using a strip yield model and a general analytical weighting function to obtain the plastic constraint factor λ corresponding to different test loads and crack lengths. This is combined with the specimen thickness B and the plane stress crack tip plastic zone size r of the crack body under the corresponding test load. p0 The normalized sample thickness B / r was established through normalization and fitting analysis. p0 The empirical relationship between the plastic constraint factor λ and the plastic constraint factor includes: For the standard fracture specimen type used, a general analytical weight function expression is determined. Based on the strip yield model and the weight function, the theoretical elastic-plastic crack opening displacement under different test loads and crack lengths is calculated. By adjusting the plastic constraint factor, the theoretical calculated value is made to match the value extracted by the finite element analysis, and the plastic constraint factor is calibrated. Based on the calibrated test load, crack length, and plastic constraint factor data, the plane stress crack tip plastic zone size corresponding to each data point was calculated. The normalized specimen thickness was obtained by normalizing the specimen thickness and then using fitting analysis to establish an empirical relationship between the normalized specimen thickness and the plastic constraint factor. This relationship describes the three-dimensional constraint state of the crack body. The empirical relationship between the normalized specimen thickness and the plastic constraint factor is fitted using the following expression: In the formula, λ max For when B / r p0 → The lower limit of λ when it reaches 0 is 1; min For when B / r p0 The upper limit of λ as it approaches infinity is 1 / (1-2v), where v is the material's Poisson's ratio, B is the sample thickness, and r is the upper limit of λ as it approaches infinity. p0 , where is the size of the plastic zone at the crack tip of the plane stress corresponding to the current external load in the experiment, and k and b are coefficients determined by fitting.

6. The method for solving the baseline of fatigue crack propagation rate of metallic materials according to claim 1, characterized in that, The improved strip yield model for the constructed specimen is used to iteratively solve the crack tip cyclic plastic damage initiation load P under different stress ratios R and plastic constraint factors λ, which determines the point of steady-state development as the crack length increases. CPD This establishes the ratio P of the stable crack tip cyclic plastic damage initiation point load to the maximum experimental load under different stress ratios R and plastic constraint factors λ. CPD / P max The relationships between them include: An improved strip yield model for the cracked body used in the experiment was constructed based on a general analytical weight function. The elastic-plastic response of the cracked body under cyclic external loading was solved by conducting fatigue crack propagation analysis. Under the maximum cyclic external load, the crack opening displacement and stress distribution of the bar elements in the crack body are calculated. When the load is unloaded from the maximum to the minimum external load, the crack opening displacement, bar element stress, and length are updated. Then, the load is reloaded from the minimum external load to the next maximum external load. The load corresponding to the initiation point of cyclic plastic damage at the crack tip is solved iteratively. The above steps are repeated until the fatigue crack extends to the termination crack length corresponding to the test conditions, thereby obtaining the relationship between the external load and the fatigue crack length. The load at the initiation point of cyclic plastic damage at the crack tip that enters a steady state as the crack length changes is extracted, and the ratio between this load and the maximum load is calculated. Based on the experimental load conditions, multiple fatigue crack propagation analyses were conducted by changing the plastic constraint factor in the improved strip yield model. Polynomial fitting relationships were obtained between the plastic constraint factor and the ratio of the load at the crack tip cyclic plastic damage initiation point to the maximum load under different stress ratios.

7. The method for solving the baseline of fatigue crack propagation rate of metallic materials according to claim 1, characterized in that, The stress ratio R and plastic constraint factor λ, based on each data pair, are derived from the ratio P of the load at the crack tip cyclic plastic damage initiation point to the maximum experimental load. CPD / P max The relationship between the stress intensity factor amplitude ΔK and the effective driving force ΔK is determined. CPD The mapping relationship between them; Based on this relationship, the stress intensity factor magnitude ΔK in the data pair is replaced with the effective driving force ΔK. CPD Baseline data on fatigue crack propagation rates of metallic materials, unaffected by stress ratio R and specimen thickness B, were obtained, including: For each data pair in the fatigue crack propagation test baseline data, based on its stress intensity factor and specimen thickness, the corresponding plastic constraint factor is determined using the empirical relationship between the normalized specimen thickness and the plastic constraint factor. Combined with its stress ratio level, the conversion parameter is determined using the fitting relationship between the plastic constraint factor and the ratio of the load at the crack tip cyclic plastic damage initiation point to the maximum load. The conversion factor between the stress intensity factor amplitude and the effective driving force is calculated using the following formula: In the formula, U is the conversion factor, ΔK is the stress intensity factor amplitude, and ΔK CPD R is the corresponding effective driving force, and P is the stress ratio. CPD / P max The load P at the crack tip cyclic plastic damage initiation point CPD With maximum load P max The ratio; The stress intensity factor amplitude in each data pair is converted into an effective driving force by the conversion factor, and finally a dataset consisting of the effective driving force and the crack propagation rate is formed, which is the baseline of fatigue crack propagation rate of metallic materials that is not affected by stress ratio and specimen thickness.

8. A system for solving the baseline of fatigue crack propagation rate in metallic materials, based on the method for solving the baseline of fatigue crack propagation rate in metallic materials as described in claim 1, characterized in that, include: Test module: Used to select standard fatigue crack propagation specimens, conduct fatigue crack propagation tests under constant amplitude load with different stress ratios, and obtain data on the relationship between stress intensity factor amplitude and crack propagation rate under different stress ratios through data processing; Extraction module: It is used to establish a three-dimensional finite element model based on the sample type, geometric size and load conditions corresponding to the relational data. After setting the constitutive model of the elastoplastic material, it extracts the elastoplastic crack opening displacement along the crack surface of the crack body through elastoplastic finite element analysis under different loads and crack lengths. Calibration module: Used to iteratively calibrate the opening displacement of elastoplastic cracks using a strip yield model and a general analytical weighting function, obtaining the plastic constraint factor λ corresponding to different test loads and crack lengths, combined with the specimen thickness B and the plane stress crack tip plastic zone size r of the crack body under the corresponding test load. p0 The normalized sample thickness B / r was established through normalization and fitting analysis. p0 Empirical relationship between the plastic constraint factor λ and the plastic constraint factor λ; The solver module is used to construct an improved strip yield model for the specimen. This improved strip yield model is used to iteratively solve for the crack tip cyclic plastic damage initiation load P, which is the load at the crack tip as the crack length reaches a steady state under different stress ratios R and plastic constraint factors λ. CPD This establishes the ratio P of the stable crack tip cyclic plastic damage initiation point load to the maximum experimental load under different stress ratios R and plastic constraint factors λ. CPD / P max The relationship between them; Conversion module: Used to convert the stress ratio R and plastic constraint factor λ of each data pair into the ratio P of the load at the crack tip cyclic plastic damage initiation point to the maximum experimental load. CPD / P max The relationship between the stress intensity factor amplitude ΔK and the effective driving force ΔK is determined. CPD The mapping relationship between them; Based on this relationship, the stress intensity factor magnitude ΔK in the data pair is replaced with the effective driving force ΔK. CPD We obtained baseline data on fatigue crack propagation rate of metallic materials that are unaffected by stress ratio R and specimen thickness B.

9. An apparatus for determining the baseline fatigue crack propagation rate of metallic materials, characterized in that, include: Memory, used to store computer programs; A processor, configured to implement, when executing the computer program, a method for solving the baseline of fatigue crack propagation rate of the metallic material as described in any one of claims 1 to 7.

10. A readable storage medium, characterized in that: The readable storage medium stores a computer program that, when executed by a processor, implements a method for solving the baseline fatigue crack propagation rate of the metallic material as described in any one of claims 1 to 7.

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