Fatigue crack propagation behavior evaluation method

Abstract

The invention provides a fatigue crack propagation behavior evaluation method, and belongs to the technical field of material mechanics evaluation. According to the method, the microstructure characteristics, the critical energy release rate, the fatigue crack growth rate and the crack tip strain field distribution of a to-be-tested material are obtained through experimental characterization, a progressive fatigue damage model combining a viscoelastic-plastic constitutive region and a cyclic cohesion region is constructed, and the model is verified through a multi-scale verification method. By adopting the fatigue crack propagation behavior evaluation method, the whole-process and multi-scale correlation evaluation from microscopic damage to macroscopic failure is realized, a fatigue damage model with a more complete physical mechanism and higher prediction precision is provided, a key microscopic failure mechanism is disclosed, a clear direction is provided for guiding material improvement and performance optimization, and the method has a good application prospect. The engineering applicability and reliability of the evaluation result are enhanced, and a theoretical basis and a practical tool can be provided for the design of high-performance long-life engineering structures and parts.

Description

Technical Field

[0001] This invention relates to the field of materials mechanics evaluation technology, and in particular to a method for evaluating fatigue crack propagation behavior. Background Technology

[0002] In industrial sectors such as aerospace, transportation, and new energy equipment, the fatigue durability of engineering structural materials and components is a key factor affecting product reliability and service life. Many engineering materials, especially polymer-based composites, coating materials, and thin film structures, can develop fatigue cracks under cyclic loading (such as mechanical vibration, thermal cycling, and pressure fluctuations) due to internal microscopic defects (such as initial microcracks, porosity, and weak interfacial bonding). These cracks continue to propagate during service, eventually leading to functional failure or catastrophic fracture of the components.

[0003] Fatigue crack propagation (FCG) often accounts for the majority of a material's entire service life. Its failure mechanism mainly stems from: (1) the gradual evolution, connection and propagation of micro-defects in the material itself under cyclic stress; (2) the swelling-contraction cycle of the material caused by changes in environmental factors (such as temperature and humidity), which exacerbates local stress concentration; and (3) under complex stress conditions, the accumulation of plastic deformation and damage evolution in the crack tip region dominate the crack propagation behavior.

[0004] To investigate the fatigue fracture behavior of materials, existing studies have mostly employed macroscopic experimental methods, such as measuring crack propagation rates and fitting them using the Paris equation, or observing and analyzing failure morphology through static fracture surfaces. Although these works have improved our understanding of material failure, they mostly remain at the macroscopic level, and systematically reveal the mechanisms by which microscopic damage initiation, the evolution of the microscopic plastic zone, and their correlation with macroscopic mechanical behavior are still lacking.

[0005] In numerical simulation, methods such as the Extended Finite Element Method (XFEM), Phase Field Method, Virtual Closure Technology (VCCT), and Cohesive Z-M (CZM) have been applied to simulate crack propagation. Among them, the CZM is considered particularly suitable for simulating fatigue damage in non-metallic materials and composite interfaces because it can effectively describe the stiffness degradation behavior of materials in the fracture process zone. Existing studies mostly use CZM to calculate energy release rate or stress intensity factor, and then combine it with Paris's law to predict macroscopic crack propagation. However, such methods often neglect microscopic physical processes such as microscopic plastic deformation and local damage accumulation under cyclic loading, resulting in shortcomings in revealing multi-scale damage mechanisms and making it difficult to establish a complete prediction system from microscopic damage to macroscopic failure.

[0006] Therefore, current technologies still lack a universal method that can effectively connect the microscopic, mesoscopic, and macroscopic scales to systematically evaluate the fatigue crack propagation behavior of various materials (especially polymers and composites). There is an urgent need to develop a comprehensive analytical method that combines high-resolution experimental observations with multi-scale physical models to deeply reveal their fatigue failure mechanisms and provide theoretical basis and practical tools for the design of high-performance, long-life structural materials and products. Summary of the Invention

[0007] The purpose of this invention is to provide a method for evaluating fatigue crack propagation behavior, which solves the problem in the prior art of lacking an effective way to systematically evaluate the fatigue crack propagation behavior of materials by connecting the micro, meso and macro scales.

[0008] To achieve the above objectives, the present invention provides a method for evaluating fatigue crack propagation behavior, comprising the following steps: Step S1: Obtain the microstructure characteristics, critical energy release rate, fatigue crack propagation rate, and strain field distribution at the crack tip of the material through experimental characterization; Step S2: Based on the critical energy release rate, fatigue crack propagation rate and microstructure characteristics obtained from experimental characterization, a progressive fatigue damage model combining viscoelastic-plastic constitutive model and cyclic cohesive zone is constructed. Step S3: Using the strain field distribution at the crack tip and the fatigue crack propagation rate obtained from experimental characterization, the progressive fatigue damage model is verified through a multi-scale verification method. Multi-scale verification includes correlating the simulated macroscopic mechanical response with the experimentally measured microscopic damage evolution. Step S4: Based on the validated progressive fatigue damage model, evaluate the fatigue crack propagation behavior and failure mechanism of the material under cyclic loading.

[0009] Preferably, in step S1, the experimental characterization includes: Bilateral notch tensile tests were conducted, and the critical energy release rate of the material was determined based on the fracture work method. Fatigue crack propagation tests were conducted to measure the crack propagation rate under different stress ratios, and digital image correlation technology was used to monitor the evolution of the strain field at the crack tip in real time. The fracture morphology was observed using scanning electron microscopy to analyze the nucleation, merging, and interfacial debonding behavior of micropores.

[0010] Preferred progressive fatigue damage models include: Quasi-static damage factor, calculated based on a bilinear cohesive region model; Fatigue damage factor, calculated based on the cyclic damage evolution equation; The total damage factor is determined by both the quasi-static damage factor and the fatigue damage factor.

[0011] Preferably, the cyclic damage evolution equation is: ; in, Indicates the rate of change of fatigue damage. , , Both represent parameters that control changes in damage. This represents the rate of change of the opening displacement. Indicates the traction threshold. Indicates fatigue damage. This represents the traction norm.

[0012] Preferably, the rate of change of the total damage factor is expressed as: ; in, Indicates total damage. Indicates the rate of change of total damage. Indicates the rate of change of static damage. Indicates the time.

[0013] Preferably, the parameters of the cohesive model include normal cohesive strength, tangential cohesive strength, normal initial stiffness, tangential initial stiffness, Type I critical energy release rate, Type II critical energy release rate, and material parameters, which are determined based on data from bilateral notched tensile tests and fatigue crack propagation tests.

[0014] Preferred multi-scale validation methods include: Compare the simulated strain distribution at the crack tip with the strain distribution measured using digital image correlation techniques; Compare the simulated crack propagation rate with the experimentally measured crack propagation rate; The simulated damage evolution process was verified by observing the microscopic fracture morphology using a scanning electron microscope.

[0015] Therefore, the present invention employs the above-mentioned fatigue crack propagation behavior evaluation method, and the beneficial technical effects are as follows: (1) This invention systematically combines microscopic fracture morphology (SEM observation), microscopic crack tip strain field (DIC measurement), and macroscopic fatigue crack propagation rate (Paris formula fitting), and utilizes a multi-scale verification method to effectively establish a quantitative correlation between macroscopic mechanical response and microscopic damage evolution. This overcomes the limitations of existing technologies that only focus on macroscopic appearances or isolated scale mechanisms, forming a multi-scale research system covering the entire process from microscopic damage initiation to macroscopic failure and fracture, providing a complete framework for a deep understanding of the fatigue fracture mechanism of materials.

[0016] (2) The progressive fatigue damage model constructed in this invention innovatively combines viscoelastic-plastic constitutive model and cyclic cohesive zone model, and quantifies the cumulative damage under fatigue load through cyclic damage evolution equation. This model can more realistically reflect the stiffness degradation and crack propagation process of materials under actual cyclic load, and improve the physical rationality and accuracy of the prediction. Through experimental testing and simulation verification on complex working conditions such as different stress ratios, the model conclusions are ensured to have stronger engineering applicability and reliability.

[0017] (3) By embedding experimentally determined material parameters into a fully validated finite element model, this invention achieves low-cost, high-efficiency numerical simulation of material fatigue crack propagation behavior. This tool can accurately simulate the evolution of the plastic zone at the crack tip and the crack propagation rate, and can replace some complex and expensive in-situ experiments, reducing R&D costs and time. At the same time, the key microscopic failure mechanisms revealed (such as micropore nucleation and merging, and interface debonding) provide a clear theoretical basis and direction for improving fatigue resistance through material modification (such as optimizing interfaces and reducing defects). Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the material structure, microstructure, and sample geometry, in which... Figure 1 (a) in the diagram represents the polymer structure. Figure 1 (b) shows the microstructure of the membrane under TEM observation. Figure 1 (c) in the figure is a double-notched tensile specimen. Figure 1 (d) in the figure represents a fatigue crack propagation specimen; Figure 2 The results are from a bilateral notch tensile test, in which... Figure 2 In the figure, (a) shows the load-displacement curves for different ligament lengths. Figure 2 (b) shows the relationship between specific fracture work and ligament length; Figure 3 The crack propagation rate under different stress ratios; Figure 4 For the evolution of the strain field at the crack tip, the strain changes near the crack tip with stress ratios R = 0.1(a1-f1), 0.2(a2-f2), and 0.3(a3-f3) when the crack length a = 4 mm; Figure 5 The damage mechanism at the crack tip, among which, Figure 5 (a) shows the micropore evolution characteristics at the tip of a proton exchange membrane crack. Figure 5 (b) is the SEM image of the fatigue fracture surface; Figure 6 The microstructure of the fatigue fracture surface, among which, Figure 6 Image (a) is a SEM image of the fatigue fracture surface. Figure 6(b) in the image shows the boundary between the observed crack initiation and propagation zones. Figure 6 (c) in the diagram represents the observed ligament formation area. Figure 6 In the example, (d) represents a ligament with a coiled shape; Figure 7 This is a schematic diagram of the traction-displacement relationship in the cohesive force model, where, Figure 7 (a) in the diagram represents Mode I (normal direction). Figure 7 (b) represents Mode II (Cut). Figure 7 (c) represents the response under the combined effects of quasi-static and fatigue damage. Figure 8 This is a schematic diagram of the damage state of cohesive elements under fatigue loading. Figure 9 For the finite element model of embedded cohesion, where, Figure 9 (a) in the diagram represents a semi-model of crack propagation. Figure 9 (b) in the diagram represents a combined mesh generation. Figure 9 (c) in the diagram represents the cohesive element; Figure 10 The flowchart shows the calculation process for the fatigue damage cohesion model. Figure 11 The results are for strain field verification, where, Figure 11 Simulated cloud diagrams of strain distribution near the tip under maximum strain state when the crack lengths a of (a1), (b1), and (c1) are 3 mm, 4 mm, and 5 mm, respectively. Figure 11 Comparison of near-tip strain distributions between simulation and experimental measurements at crack lengths (a2), (b2), and (c2); Figure 12 For the damage evolution of cohesive units, where, Figure 12 In the diagram, (a) shows the evolution of the normal stress in cohesive elements A, B, and C. Figure 12 (b) in the figure represents the evolution of the total damage rate of cohesive units A, B, and C; Figure 13 This is a comparison chart of experimental and numerical simulation results for crack propagation rate. Detailed Implementation

[0019] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0020] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0021] Example 1 This embodiment uses the perfluorosulfonic acid proton exchange membrane Nafion®115 as the test object to implement a fatigue crack propagation behavior evaluation method.

[0022] 1. Material and sample preparation.

[0023] The material selected in this embodiment is the Nafion® 115 perfluorosulfonic acid proton exchange membrane with a thickness of 127 μm manufactured by DuPont.

[0024] The polymer structure of a proton exchange membrane consists of a hydrophobic polytetrafluoroethylene backbone and hydrophilic sulfonic acid side chains, such as... Figure 1 As shown in (a) of the diagram. Figure 1 (b) shows the microstructure of the Nafion® 115 film observed under a transmission electron microscope (TEM). The dark area is the hydrophilic phase, namely the sulfonic acid-rich structural domain; the bright area is the hydrophobic phase, namely the polytetrafluoroethylene-rich framework structural domain.

[0025] Reference Figure 1 (c) and Figure 1 In (d), two types of rectangular specimens were prepared: a double-notched tensile (DENT) specimen (30 mm long, 15 mm wide, with a ligament length L between 1.7 mm and 6.7 mm) for determining fracture properties, and a centrally pre-cracked specimen (30 mm long, 15 mm wide, with a pre-crack length of approximately 2 mm) for fatigue crack propagation (FCG) testing. All cracks were prepared using a sharp tool.

[0026] 2. Experimental characterization.

[0027] 2.1. Bilateral notch tensile test.

[0028] Quasi-static tensile tests were conducted on a series of DENT specimens with different ligament lengths (from 1.7 mm to 6.7 mm). The tests were strain-controlled, using a strain rate of 0.003 / s to obtain stable load-displacement curves. Figure 2 As shown in (a), the total fracture energy is calculated based on the obtained load-displacement curve. Subsequently, based on the fundamental fracture work (EWF) method, the fracture work was linearly fitted to the ratio of fracture work. With ligament length L Relationships (such as) Figure 2 As shown in (b) above, the intercept of the fitted line on the ordinate axis is obtained, and this intercept is the Type I critical energy release rate G. IC In this embodiment, G is determined by this method. IC It is 6.608 kJ·m -2 .

[0029] Total fracture energy Sum of fracture work The calculation formula is as follows: ; ; Among them, total fracture energy It can be divided into fracture zones. and plastic zone , This indicates the necessary foundation for breaking the chain. This indicates the specific plastic work that is not necessary for fracture. Indicates the thickness of the sample. This represents the shape factor associated with the formation of the plastic zone.

[0030] 2.2 Fatigue crack propagation test.

[0031] Fatigue crack propagation tests were conducted on specimens with a pre-existing central crack. Test parameters were set as follows: a triangular wave load waveform at 1 Hz, a stress amplitude of 3.4 MPa, and stress ratios R of 0.1, 0.2, and 0.3. Before the test, a low load was applied to the specimen to induce stable crack propagation of 100-200 μm at the crack tip. In the formal test, the crack length was estimated using the seven-point incremental polynomial method recommended by ASTM E647-11, and the number of cycles was recorded to calculate the crack propagation rate. ; in, Indicates the length of the central crack. Indicates the number of loops. , Indicates material parameters, Indicates the range of stress intensity factor.

[0032] Based on linear elastic fracture mechanics, the range of stress intensity factors is calculated. : ; in, Indicates the stress range, Indicates the width of the specimen.

[0033] Ultimately, to obtain Figure 3 The relationship curves shown are fitted using the Paris formula to obtain the material parameters. and Coefficients under three stress ratio conditions The values ​​are 1.12 × 10 -14 2.78×10 -14 and 14.3×10 -14 That is, it increases significantly with the increase of stress ratio and shows a non-linear increasing trend. The values ​​are similar, at 9.72, 9.29 and 8.87 respectively.

[0034] 2.3 Digital Image Correlation (DIC) Measurement.

[0035] During fatigue crack propagation tests, an integrated DIC system was used to monitor the strain field at the crack tip. The specimen observation surface was pre-sprayed with fine speckle patterns approximately 80 μm in diameter. Crack tip morphology was photographed at 0%, 20%, 40%, 60%, 80%, and 100% of the maximum strain state within a single loading cycle when the crack had propagated to different lengths (e.g., 3 mm, 4 mm, 5 mm). Analysis using VIC 2.0 software yielded the following results: Figure 4 The strain field cloud map near the crack tip and the strain distribution data along the crack propagation direction are shown.

[0036] 2.4 Microscopic morphology analysis.

[0037] After the experiment, the fatigue fracture morphology was observed using a scanning electron microscope (SEM). For example... Figure 5 and Figure 6 As shown, micropores preferentially nucleate and merge in the high-stress zone at the tip of the pre-existing crack, and a stepped morphology is formed by gradual debonding at the fracture interface, revealing a progressive crack propagation mode of interlayer peeling.

[0038] 3. Construction of a progressive fatigue damage model.

[0039] Based on the data obtained from the above experimental characterization, a progressive fatigue damage model combining viscoelastic-plastic constitutive model and cyclic cohesive region was constructed. The core of this model lies in describing the stiffness degradation process of the material under cyclic loading through cohesive elements.

[0040] 3.1. Fundamentals of the Quasi-Static Cohesive Model

[0041] A bilinear cohesive zone model is used to describe the traction-separation behavior of the interface. This model is based on the following physical foundation: when the interfacial stress reaches the cohesive strength, the material begins to damage; as the displacement increases, the load-bearing capacity decreases linearly until complete failure.

[0042] The damage evolution of the model is controlled by the following key parameters: Damage initiation displacement: ; in, Indicates the initial displacement of the normal damage. Represents normal cohesive strength. Indicates the initial normal stiffness. Indicates the initial displacement of tangential damage. Indicates tangential cohesive strength. This represents the initial tangential stiffness.

[0043] The area of ​​the triangle enclosed by the traction-separation curves, i.e. Figure 7The area of ​​the triangle in (a) and Figure 7 In (b), the area of ​​the triangle represents the type I fracture energy G. IC and Type II fracture energy G IIC Therefore, the displacement is ultimately destroyed: ; in, Indicates the normal critical fracture displacement. This indicates the critical tangential fracture displacement.

[0044] For the mixed-mode situation in actual crack propagation, the following relationship is established: Equivalent displacement of the hybrid model : ; in, , This indicates tangential displacement.

[0045] In the hybrid model, the proportion of different crack types is compared with the model ratio. Represented as: ; By combining the above equations, the critical damage initiation displacement of the hybrid model can be calculated. : ; Calculation of mixed-mode fracture energy using the BK criterion : ; in, These are empirical parameters determined through experiments. The critical damage initiation displacement of the hybrid model's traction displacement can be determined from the above formula. and critical fracture displacement .

[0046] ; Among them, characters K Indicates the stiffness of the material. Indicates the initial normal stiffness. This represents the initial tangential stiffness.

[0047] Therefore, the quasi-static damage factor of the hybrid model can be derived as follows: ; in, This indicates the opening displacement within the historical loading period ( , The maximum value of the interval; ; in, express t The maximum value of the equivalent displacement of the hybrid mode within the historical loading cycle at the given time. This represents the time difference between two adjacent increment steps in finite element analysis. express t - The maximum value of the equivalent displacement of the hybrid mode within the historical loading cycle at the given time. express t The equivalent displacement of the hybrid mode at time t.

[0048] 3.2 Mechanism of cyclic damage accumulation.

[0049] To describe the damage evolution under fatigue loading, a cyclic damage component is introduced into the quasi-static model: The total damage evolution rate is expressed as: ; in, Indicates total damage. Indicates the rate of change of total damage. Indicates the rate of change of static damage. This represents the rate of change in fatigue damage.

[0050] Cyclic damage evolution takes the following forms: ; in, , Parameters that control for changes in damage. This represents the rate of change of the opening displacement. Indicates the traction threshold. This indicates fatigue damage.

[0051] Traction Norm Defined as: ; in, Indicates the normal residual strength. This indicates the tangential residual strength.

[0052] The stiffness of the cohesive element after damage is expressed as follows: .

[0053] Figure 8This is a schematic diagram of fatigue loading element damage. The element at the crack tip has completely failed and is located in the failure zone in the image. The area between the element at the crack tip and the element that has just been damaged is called the damage zone. In this area, the damage to the element gradually accumulates until it completely fails. The area after the element that has just been damaged is the elastic zone. The loading path and unloading path of the elements in this area are exactly the same, and no damage has occurred yet.

[0054] 3.3 Determination of model parameters and numerical implementation.

[0055] Constitutive model: The proton exchange membrane is modeled as an anisotropic viscoelastic-plastic material, with its elastic modulus and yield stress set to 198.8 MPa and 9.8 MPa, respectively.

[0056] Cohesive Model (CZM) Parameter Determination: Cohesive model parameters were set based on experimental data.

[0057] The cohesive strength in the normal and tangential directions is taken as 1 / 10 of the Young's modulus of the material.

[0058] The initial normal and tangential stiffness are set to sufficiently large values ​​(2 × 10⁻⁶). 12 N·mm -3 To avoid artificial softening, the initial stiffness values ​​in the normal and tangential directions are the same.

[0059] Type I critical energy release rate G IC The results of the double-notch tensile test (6.608 kJ·m) were adopted. -2 ).

[0060] Type II critical energy release rate G IIC Set to 2kJ·m -2 .

[0061] For the proton exchange membrane Nafion® 115, the Poisson's ratio was set to 0.4. On the other hand, parameters controlling damage variation were: Take 10 9 , It is 15. The value is 2.35, material parameters. It is then set to 2. The constitutive parameters of all materials were selected under conditions of 25°C and 30% relative humidity.

[0062] Model Implementation: In the finite element software ABAQUS, a model is established as follows... Figure 9 The model is a semi-symmetric plane stress model with embedded zero-thickness cohesive elements. The bottom of the model is fixed, and a triangular wave cyclic load with a stress ratio R=0.1 is applied to the top. This is achieved by writing a User Material Subroutine (UMAT). Figure 10 The fatigue damage CZM calculation flowchart shown is used to comprehensively calculate the quasi-static damage factor. and fatigue damage factors Thus, the total damage factor is obtained. Based on this, the element stiffness is updated to simulate the complete fracture process from damage initiation to element removal. Figure 10 middle, express t + Quasi-static damage factor over time, express t + The rate of change of quasi-static damage factors over time express t + Fatigue damage factors over time express t + The rate of change of fatigue damage factors over time.

[0063] 4. Multi-scale model validation.

[0064] The accuracy of the constructed numerical model was verified using the following multi-scale method: Mesoscopic strain field verification: Extracting the strain distribution near the crack tip under the maximum strain state at different crack lengths (a=3mm, 4mm, 5mm) from numerical simulations. Figure 11 (a1)-(c1)), and the corresponding results of the DIC experimental measurements ( Figure 11 The results show that the strain distribution curves of the simulation and experiment are in good agreement in terms of their variation trend, both exhibiting a logarithmic form that decreases with increasing distance from the crack tip, thus verifying the model's ability to predict the evolution of the crack tip plastic zone (CPZ).

[0065] Verification of macroscopic crack propagation rate: The relationship between crack propagation rate and stress intensity factor range obtained from numerical simulation was compared with experimental data. Figure 13 The results show that the simulation results agree well with the experimental fitting lines, indicating that the model can effectively evaluate the macroscopic fatigue crack propagation behavior.

[0066] Microscopic mechanism correlation: The correlation analysis between the microscopic failure features observed by SEM (such as micropore nucleation and merging, and stepped fracture surface) and the progressive stiffness degradation law and traction-separation behavior described in the cohesive force model was carried out, which verified the rationality of the physical basis of the model from the mechanism level.

[0067] 5. Results Analysis and Evaluation.

[0068] Using a validated model, the fatigue crack propagation behavior of the proton exchange membrane was further analyzed: The evolution law of normal stress in cohesive elements during cyclic loading was revealed. Figure 12 The characteristics of (a) and the total damage change rate (in the text) Figure 12 (b) in the text clarifies that interface failure is the result of the competition between the gradual instability of the bearing element and the stress rebalancing.

[0069] The study revealed the multi-scale characteristics of fatigue failure in proton exchange membrane materials: microscopic unit failure leads to the reconstruction of the microscopic stress field, and the redistribution of the stress field increases the overall damage rate, thereby accelerating macroscopic crack propagation.

[0070] It is worth noting that all contents not described in detail in this invention are existing technologies and are well known to those skilled in the art.

[0071] Therefore, the present invention adopts the above-mentioned fatigue crack propagation behavior evaluation method, which solves the problem in the prior art of lacking a cross-scale systematic evaluation of material fatigue crack propagation behavior that can effectively connect the micro, meso and macro scales.

[0072] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for evaluating fatigue crack propagation behavior, characterized in that, Includes the following steps: Step S1: Obtain the microstructure characteristics, critical energy release rate, fatigue crack propagation rate, and strain field distribution at the crack tip of the material through experimental characterization; Step S2: Based on the critical energy release rate, fatigue crack propagation rate and microstructure characteristics obtained from experimental characterization, a progressive fatigue damage model combining viscoelastic-plastic constitutive model and cyclic cohesive zone is constructed. Step S3: Using the strain field distribution at the crack tip and the fatigue crack propagation rate obtained from experimental characterization, the progressive fatigue damage model is verified through a multi-scale verification method. Multi-scale verification includes correlating the simulated macroscopic mechanical response with the experimentally measured microscopic damage evolution. Step S4: Based on the validated progressive fatigue damage model, evaluate the fatigue crack propagation behavior and failure mechanism of the material under cyclic loading.

2. The fatigue crack propagation behavior evaluation method according to claim 1, characterized in that, In step S1, the experimental characterization includes: Bilateral notch tensile tests were conducted, and the critical energy release rate of the material was determined based on the fracture work method. Fatigue crack propagation tests were conducted to measure the crack propagation rate under different stress ratios, and digital image correlation technology was used to monitor the evolution of the strain field at the crack tip in real time. The fracture morphology was observed using scanning electron microscopy to analyze the nucleation, merging, and interfacial debonding behavior of micropores.

3. The fatigue crack propagation behavior evaluation method according to claim 1, characterized in that, Progressive fatigue damage models include: Quasi-static damage factor, calculated based on a bilinear cohesive region model; Fatigue damage factor, calculated based on the cyclic damage evolution equation; The total damage factor is determined by both the quasi-static damage factor and the fatigue damage factor.

4. The fatigue crack propagation behavior evaluation method according to claim 3, characterized in that, The cyclic damage evolution equation is: ； in, Indicates the rate of change of fatigue damage. , , Both represent parameters that control changes in damage. This represents the rate of change of the opening displacement. Indicates the traction threshold. Indicates fatigue damage. This represents the traction norm.

5. The fatigue crack propagation behavior evaluation method according to claim 4, characterized in that, The rate of change of the total damage factor is expressed as: ； in, Indicates total damage. Indicates the rate of change of total damage. Indicates the rate of change of static damage. Indicates the time.

6. The fatigue crack propagation behavior evaluation method according to claim 1, characterized in that, The parameters of the cohesive model include normal cohesive strength, tangential cohesive strength, normal initial stiffness, tangential initial stiffness, Type I critical energy release rate, Type II critical energy release rate, and material parameters. The parameters are determined based on data from bilateral notched tensile tests and fatigue crack propagation tests.

7. The fatigue crack propagation behavior evaluation method according to claim 1, characterized in that, Multi-scale validation methods include: Compare the simulated strain distribution at the crack tip with the strain distribution measured using digital image correlation techniques; Compare the simulated crack propagation rate with the experimentally measured crack propagation rate; The simulated damage evolution process was verified by observing the microscopic fracture morphology using a scanning electron microscope.

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