A method for quantitatively characterizing long crack fatigue growth resistance and life prediction based on microstructure and crack branching

Abstract

The application provides a long crack fatigue propagation resistance quantitative characterization and life prediction method based on microstructure and crack bifurcation, comprising the following steps: step one, microstructure characterization and sample preparation; step two, crack propagation experiment and propagation parameter acquisition; step three, Paris deviation propagation behavior identification and organization correlation analysis; step four, crack propagation resistance separation and quantitative characterization; step five, based on the calculation of the hysteresis cycle and life prediction of the propagation resistance. The beneficial effects of the application are as follows: through microstructure characterization, fatigue crack propagation test and fracture analysis, a crack propagation resistance separation model is established, the propagation resistance is decomposed into material intrinsic resistance and obstacle resistance, and the evolution law thereof is inverted, a life prediction model considering the resistance effect is constructed, the quantitative evaluation and life prediction of long crack propagation behavior are realized, the fatigue life prediction precision of complex metal materials is improved, and the application is suitable for engineering structure safety evaluation and life design.

Description

Technical Field

[0001] This invention belongs to the field of fatigue fracture evaluation and life prediction technology of engineering materials, specifically involving a quantitative characterization and life prediction method for fatigue propagation resistance of long cracks based on microstructure and crack bifurcation. Background Technology

[0002] Fatigue crack propagation is one of the most significant failure modes in engineering structures during long-term service. After cracks initiate under cyclic loading, the long crack propagation process continuously depletes the structure's remaining load-bearing capacity, ultimately leading to macroscopic failure. For large, complex structures and high-reliability components, accurately predicting crack propagation life is crucial for ensuring structural mechanical safety and extending service life.

[0003] Currently, the classic Paris model is commonly used to describe the life prediction of long crack propagation behavior. Its basic idea is to establish an empirical relationship between the crack propagation rate and the stress intensity factor range during the steady-state propagation phase, and then obtain the crack propagation life through integral calculation. Due to its simplicity and strong engineering applicability, this method has been widely applied in fatigue design and damage tolerance analysis.

[0004] However, extensive mechanical research and engineering practice have shown that the traditional Paris model, based on the assumption of a homogeneous continuous medium, treats the material's resistance to crack propagation as a fixed parameter, considering only the driving effect of external loads and neglecting the microscopic mechanism influence of the material's internal microstructure on the crack propagation path and propagation energy consumption. Existing patented technologies (such as CN119622892A and CN115458079B) typically address such problems only from a macroscopic phenomenological perspective or treat the material's resistance to crack propagation as a constant, failing to delve into the essential mechanism of rate fluctuations at the microscopic crystallographic level. While some microscopic simulation patents (such as CN119905182B) introduce the Hall-Petch effect, they are limited to short crack scales and cannot quantitatively separate the contributions of the "matrix intrinsic barrier" and the "crack surface spatial deflection barrier" when long cracks traverse heterogeneous structures, leading to a significant decrease in the accuracy of predicting long crack propagation in complex, large-span structures or high-end components.

[0005] For practical engineering materials, especially high-performance metallic materials, there are usually fluctuations in grain size, differences in crystal orientation, texture gradients, second-phase particles, inclusions, and localized regions of inhomogeneity. When long cracks extend into these micro-regions, the crack front undergoes complex evolutionary behaviors such as deflection, bifurcation, local stagnation, and re-merging, resulting in significant fluctuations in the crack propagation rate and exhibiting nonlinear propagation characteristics that deviate significantly from the classical Paris relation. Summary of the Invention

[0006] The purpose of this invention is to provide a quantitative characterization and life prediction method for long crack fatigue propagation resistance based on microstructure and crack bifurcation. This method divides long crack propagation resistance into intrinsic resistance and barrier resistance, performing physical decomposition and quantitative calculation to achieve a unified description of crack propagation rate fluctuations, multi-stage propagation, and deviations from Paris behavior. The influence of microstructure inhomogeneity on long crack propagation resistance is systematically explored. Through quantitative analysis, the intrinsic mechanisms affecting deviations from Paris propagation behavior are elucidated, and a life prediction method based on crack propagation resistance is proposed.

[0007] To achieve the above objectives, the present invention provides the following technical solution: To address the problems that existing technologies cannot effectively explain the deviation from Paris propagation behavior during long crack propagation in non-uniform materials, the unclear source of crack propagation resistance, and the insufficient accuracy of life prediction, this invention proposes a quantitative characterization and life prediction method for long crack fatigue propagation resistance based on microstructure and crack bifurcation.

[0008] This invention constructs a model of the synergistic effect of crack propagation driving force and propagation resistance, decomposes the total crack propagation resistance into intrinsic material resistance and barrier resistance, and establishes a method for resistance inversion and life integral calculation, thereby realizing a physical description of the entire process of long crack propagation in complex materials.

[0009] To achieve the above objectives, the method provided by the present invention includes the following steps: Step 1, Microstructure Characterization and Sample Preparation: Obtain the microstructure information of the metal material to be tested and prepare fatigue crack propagation samples, wherein the microstructure information includes at least parameters that can characterize the material's microstructure characteristics and microstructure inhomogeneity. Step 2, Crack propagation experiment and propagation parameter acquisition: Apply cyclic load to the sample to conduct a crack propagation experiment, obtain the evolution relationship between crack length a and number of cycles N, and calculate the crack propagation rate da / dN and the corresponding crack propagation driving force parameter ΔK to obtain the crack propagation path characteristics. Step 3, Identification of deviation from Paris propagation behavior and analysis of tissue correlation: Based on the changes in crack propagation rate and fracture morphology, the range of deviation from the Paris propagation law is identified, and the crack deflection, crack bifurcation and tissue-induced obstruction areas are determined by combining crack path evolution characteristics and microstructure information. Step 4, Separation and Quantitative Characterization of Crack Propagation Resistance: Establish a crack propagation resistance model, decompose the total crack propagation resistance into intrinsic material resistance and barrier resistance, establish a benchmark propagation relationship using the stable propagation range, and obtain the evolution law of barrier resistance by inverting the deviation between theoretical propagation behavior and actual propagation behavior. Step 5, Calculation of hysteresis cycles and prediction of life based on propagation resistance: The fatigue hysteresis cycles are obtained based on resistance calculation, and the control equation containing the resistance term is introduced into the crack propagation constitutive model. The entire process of crack propagation from the initial length to the critical length is calculated by integral calculation to obtain the fatigue propagation life of long cracks.

[0010] Furthermore, the intrinsic resistance represents the inherent ability of a material to resist crack propagation under the assumption of a homogeneous medium, and it is determined by the C and m values ​​corresponding to the stable propagation stage.

[0011] Furthermore, the barrier resistance refers to the additional resistance formed during crack propagation by crack deflection, crack bifurcation, grain boundary obstruction, microstructure inhomogeneity, and local energy dissipation.

[0012] In real materials, crack propagation paths are not ideally straight lines, but are influenced by grain boundaries, inclusions, second-phase particles, dislocation cells, and local microstructure inhomogeneities, resulting in crack deflection, crack bifurcation, and other phenomena that consume additional crack propagation driving force. Therefore, the additional resistance caused by microstructure is defined as barrier drag. .

[0013] Considering the effect of resistance, due to the presence of barrier resistance, the crack propagation rate will decrease sharply when encountering crack bifurcation and deflection, ultimately resulting in barrier resistance. : ; Furthermore, by establishing an inversion model of resistance through the deviation between the theoretical and actual propagation rates, the dynamic evolution relationship of resistance with crack length and stress intensity factor is obtained.

[0014] Fatigue crack hysteresis cycle calculation: Assuming the crack length at the time of delayed occurrence is... As the crack continues to propagate, the residual stress caused by the barrier resistance is gradually consumed until the crack length reaches a certain value. At that time, the actual rate recovers to the same level as the theoretical reference rate. The entire range affected by the hysteresis effect is... .

[0015] According to the derivative transformation, in the infinitesimal element Within the specified range, the theoretical number of cycle cycles a component should consume is: In the absence of hysteresis, the crack... Total number of weeks that the interval should have consumed for: Due to the overload hysteresis slowing it down, the cracks struggled to complete the same range. Total weeks actually consumed for: In physics, "hysteresis cycles" are the extra fatigue cycles due to the effect of resistance. Therefore, it equals the actual number of cycles consumed minus the baseline number of cycles that should have been consumed. Substituting into the above equation, we get: ; Lifetime prediction based on crack propagation resistance: For cracks propagating from an initial length a0 to a critical length a f If the resistance term is not considered, the extended life calculation is as follows: ; After introducing the drag term into the Paris equation, according to the Paris formula considering the drag term, the crack length from the initial length is... Extend to critical length Its extended lifespan should be: ; Considering the phased changes in material parameters during crack propagation, the crack propagation process is divided into multiple stages. Therefore, the total lifespan is: .

[0016] in, Represents crack propagation life. The length of the crack. The final crack length within stage i. C is the initial crack length within stage i. i and m i For material-related constants, representing the intrinsic resistance to crack propagation, For the range of stress intensity factors, represents the resistance to crack propagation, and n represents the total number of stages of crack propagation rate.

[0017] Compared with the prior art, the present invention has the following beneficial effects: (1) For the first time, the physical decomposition and quantitative calculation of “material intrinsic resistance (matrix inherent capacity)” and “propagation barrier resistance (microstructure and crack bifurcation induction)” were realized in the mechanical constitutive model of long crack propagation, filling the theoretical gap between macroscopic phenomenological model and microscopic crystal damage, and realizing the physical decomposition and quantitative characterization of the source of crack propagation resistance. (2) Breaking through the limitation of the traditional Paris model that treats material parameters as constants, it achieves a unified description of crack propagation rate fluctuations, multi-stage propagation and deviations from Paris behavior; (3) A method for calculating crack hysteresis cycles is proposed to quantitatively characterize the effect of crack propagation resistance on the propagation rate.

[0018] (4) A life prediction method based on resistance correction is proposed. By introducing the dynamically evolving obstacle resistance into the integral control equation in real time, the accuracy of fatigue extended life prediction and engineering applicability of complex materials are improved. Attached Figure Description

[0019] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein... Figure 1 This is a flowchart illustrating the technical route of the present invention.

[0020] Figure 2 This is a schematic diagram showing the sampling location, orientation, and macroscopic geometric morphology of the fatigue fracture surface.

[0021] Figure 3 The graph shows the relationship between crack propagation rate da / dN and crack length a and stress intensity factor ΔK.

[0022] Figure 4 The image shows the microscopic morphology of the corresponding crack fracture surface.

[0023] Figure 5 The inversion yields the crack propagation barrier resistance. Evolutionary diagram.

[0024] Figure 6 This is a comparison of the original extended curve with the fitting results including and without the resistance term.

[0025] Figure 7 The number of cycles for crack bifurcation lag. Detailed Implementation

[0026] To illustrate the features of the present invention, the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0027] Example: The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] The purpose of this invention is to develop an experimental method combining long crack propagation experiments with microstructure characterization, to quantitatively obtain crack propagation rate and resistance parameters, and to establish experimental and calibration methods for intrinsic resistance and barrier resistance. The influence of microstructure inhomogeneity on long crack propagation resistance is systematically explored. Through quantitative analysis, the intrinsic mechanism affecting deviations from Paris propagation behavior is elucidated, and a lifetime prediction based on crack propagation resistance is proposed. The specific implementation steps are as follows: like Figure 1 As shown, the method for quantitative characterization of fatigue propagation resistance and life prediction of long cracks provided by the present invention includes: Step 1: Characterization of heterogeneous microstructure and sample preparation. Specifically, such as... Figure 2 As shown, taking high-strength aluminum alloy as an example, material was cut from the LS surface of a 12.7 cm thick AA7050-T7651 aluminum alloy plate. The microstructure was scanned using techniques such as OM, SEM, and EBSD to obtain the average space orientation of grains and the distribution of grain boundary orientation differences, and a data matrix was constructed. Fatigue specimens with a central standard groove (such as a four-point bending specimen with a notch created via FIB) were prepared strictly according to the GB / T6398-2017 standard.

[0029] Step Two: Macroscopic Fatigue Propagation Experiment and Data Acquisition of "Deviating from Paris Propagation Behavior". The above-mentioned specimens were mounted on a fatigue testing machine, and the test parameters were set as follows: stress ratio R = 0.1, loading frequency 20 Hz, and maximum applied cyclic stress 131.5 MPa. The evolution of the main crack length *a* with the number of cycles *N* was monitored in real time. The macroscopic crack propagation rate *da* / *dN* was calculated using a numerical differential algorithm.

[0030] Step 3: Analyze the relationship between the point of decline in the expansion rate and the microstructure. For example... Figure 3 As shown, experimental results indicate that the curve exhibits a two-stage Paris expansion characteristic, with a significant rate drop (displaying a "V"-shaped change) (the local da / dN decrease exceeds 50%). Fracture surface image ( Figure 4 This confirms that such a sharp drop originates from the bifurcation and deflection of the crack tip, which consumes additional propagation driving force and creates resistance. Combined with the fatigue crack propagation rate curve (…), Figure 3 ) and fracture morphology ( Figure 4 The propagation process of long cracks can be clearly divided into two stages: non-penetrating cracks and penetrating cracks.

[0031] 1. Non-penetrating crack stage (a < 26.8 mm): In the early and middle stages, the crack has not yet penetrated the thickness direction and is subject to strong three-dimensional geometric constraints, resulting in slow overall propagation. Multiple local rate decreases occur in this region, the mechanical origin of which lies in the microscopic bifurcation of the crack in the heterogeneous tissue region. This bifurcation behavior constitutes a significant obstacle, dramatically increasing the resistance to local propagation; after overcoming the obstacle, the rate recovers and re-aligns with the theoretical Paris baseline.

[0032] 2. Through-crack stage (a>26.8 mm): After the crack front breaks through the thickness limit, the three-dimensional constraint disappears, the overall resistance drops sharply, and the propagation rate surges. At this time, the curve baseline rises and is accompanied by high-amplitude "V-shaped sawtooth" oscillations, indicating that during high-speed penetration, large-scale crack bifurcation or severe tissue deflection can still cause transient and violent fluctuations in resistance.

[0033] Step 4: Calculate the intrinsic resistance and barrier resistance of long crack propagation. This invention, based on the physical essence of the balance between the driving force at the crack tip and the multi-scale resistance within the material, clarifies that the total resistance to long crack propagation consists of two parts: intrinsic resistance and barrier resistance.

[0034] The phenomenon of deflection in the crack propagation rate curve is usually manifested as multiple stable propagation stages, which are composed of several Paris segments with different slopes in a logarithmic coordinate system, i.e., the multi-stage Paris law. This is due to the difference in intrinsic resistance caused by the microstructure of the material, which is manifested as the difference in the intrinsic constants C and m of the material fatigue crack propagation rate.

[0035] In real materials, crack propagation paths are not ideally straight lines, but are influenced by grain boundaries, inclusions, second-phase particles, dislocation cells, and local microstructure inhomogeneities, resulting in crack deflection, crack bifurcation, and other phenomena. This consumes additional driving force for crack propagation (as shown in the figure, the sharp drop in rate exhibits a "V-shaped" propagation deviating from Paris's law). Therefore, the additional resistance caused by microstructure is defined as barrier drag. .

[0036] 1. Determination of material intrinsic constants C and m and definition of intrinsic drag. First, based on the crack propagation rate curve obtained from the fatigue crack propagation test ( (curve), and during the stable expansion phase, a multi-stage Paris law is used to fit the experimental data: Where a is the crack length and N is the corresponding number of cycles. This represents the range of stress intensity factors. C and m are intrinsic material constants, determined by... and Linear regression fitting was performed to obtain the material intrinsic constants for stage 1: C1 = 1.5883 × 10⁻⁶.-5 The constant power m1 = 1.1497; the material intrinsic constant for stage 2: C2 = 3.3152 × 10⁻⁶. -11 The constant power m2 = 4.6023.

[0037] 2. Microstructural barriers and resistance Separation and quantization The traditional Paris formula does not consider the drag term, resulting in a perfectly linear crack propagation. However, due to the presence of drag, the propagation rate decreases sharply when encountering crack bifurcation and deflection. To distinguish it from the traditional Paris formula, crack length is expressed as... To represent the number of loops, use The presence of resistance means a decrease in driving force; therefore, the crack propagation equation considering resistance should be expressed as: in, The length of the crack. For the corresponding number of loops, This represents the range of stress intensity factors. C and m are the material's intrinsic constants.

[0038] Obtaining obstacles and resistance The calculation is as follows: in, The stress intensity factor range is given, where C and m are material intrinsic constants representing the intrinsic resistance to crack propagation. Obtained from the traditional Paris law After considering the resistance term The difference.

[0039] exist Figure 6 The paper further demonstrates the comparison between the original data and the fitting results with and without considering resistance. It can be seen that the fitting results after considering the resistance term are almost in agreement with the experimentally measured original data, further confirming the accuracy of the formula.

[0040] Step 5: Hysteresis Cycle Calculation and Lifetime Prediction Based on Extended Resistance Assuming the crack length at the time of delayed occurrence is As the crack continues to propagate, the residual stress caused by the barrier resistance is gradually consumed until the crack length reaches a certain value. At that time, the actual rate recovers to the same level as the theoretical reference rate. The entire range affected by the hysteresis effect is... .

[0041] exist Within the range, the hysteresis cycles are the additional fatigue cycles caused by resistance, which equals the actual number of cycles consumed minus the baseline number of cycles that should have been consumed. in, As the starting point of the interval, The endpoint of the interval. To assume the expansion rate without considering resistance, To account for the rate of crack propagation, 'a' represents the crack length.

[0042] From the image, the delay week It should be the area enclosed by the actual number of cycles and the theoretical number of cycles. For example... Figure 7 The calculated number of delay cycles is shown. It can be clearly seen that the number of delayed cycles caused by crack bifurcation are 2694, 1069, 461, 1047, 1771, 1623, 2630, and 1664, respectively.

[0043] After introducing the drag term into the Paris equation, according to the Paris formula considering the drag term, the crack length from the initial length is... Extend to critical length Its extended lifespan should be: Considering the phased changes in material parameters during crack propagation, the crack propagation process is divided into two stages. Therefore, the total lifespan is: Where C and m are material-related constants, representing the intrinsic resistance to crack propagation. For the range of stress intensity factors, This represents the resistance to crack propagation, where 'a' is the crack length. The initial crack length is... The critical crack length. This represents the final crack length of stage 1. Let C1, m1 and C2, m2 be the initial crack length of stage 2, and C1, m1 and C2, m2 be the material intrinsic constants of stage 1 and stage 2 respectively: C1 = 1.5883 × 10-5, constant power m1 = 1.1497; material intrinsic constants of stage 2: C2 = 3.3152 × 10-11, constant power m2 = 4.6023.

[0044] The above embodiments and accompanying drawings are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. The present invention has been described in detail with reference to preferred embodiments. Those skilled in the art should understand that any changes, modifications, additions, or substitutions made by those skilled in the art within the scope of the present invention do not depart from the spirit of the present invention and should also fall within the protection scope of the claims of the present invention. Other related technical structures not disclosed in detail in the present invention are existing technologies in the art.

Claims

1. A method for quantitative characterization and lifetime prediction of fatigue propagation resistance of long cracks based on microstructure and crack bifurcation, characterized in that, Includes the following steps: Step 1, Microstructure Characterization and Sample Preparation: Obtain the microstructure information of the metal material to be tested and prepare fatigue crack propagation samples, wherein the microstructure information includes at least parameters that can characterize the material's microstructure characteristics and microstructure inhomogeneity. Step 2, Crack propagation experiment and propagation parameter acquisition: Apply cyclic load to the sample to conduct a crack propagation experiment, obtain the evolution relationship between crack length a and number of cycles N, and calculate the crack propagation rate da / dN and the corresponding crack propagation driving force parameter ΔK to obtain the crack propagation path characteristics. Step 3, Identification of deviation from Paris propagation behavior and analysis of tissue correlation: Based on the changes in crack propagation rate and fracture morphology, the range of deviation from the Paris propagation law is identified. Combined with crack path evolution characteristics and microstructure information, crack deflection, crack bifurcation and tissue-induced resistance areas are determined. Combined with fatigue crack propagation rate curve and fracture morphology, the propagation process of long cracks is significantly divided into two stages: non-penetrating crack and penetrating crack. Step 4, Crack Propagation Resistance Separation and Quantitative Characterization: A crack propagation resistance model is established, decomposing the total crack propagation resistance into intrinsic material propagation resistance and barrier resistance. A baseline propagation relationship is established using the stable propagation range. The evolution law of barrier resistance is obtained through the deviation inversion between theoretical and actual propagation behavior. Based on the crack propagation rate curve obtained from fatigue crack propagation tests, the experimental data are fitted using the multi-stage Paris law during the stable propagation stage to obtain the material's intrinsic constant C for the non-penetrating crack stage and the penetrating crack stage, and the barrier resistance R is calculated. b ; ； in, The stress intensity factor range is given, where C and m are material intrinsic constants representing the intrinsic resistance to crack propagation. Obtained from the traditional Paris law After considering the resistance term The difference; Step 5, Hysteresis Cycle Calculation and Life Prediction Based on Crack Propagation Resistance: Based on the resistance-corrected crack propagation model, the crack length is calculated from the initial length... Extend to critical length Perform integral calculations to obtain the fatigue lag cycles. ; in, To assume the expansion rate without considering resistance, To account for the rate of crack propagation, a is the crack length; Furthermore, the control equations containing resistance terms are introduced into the crack propagation constitutive model, and integral calculations are performed on the entire process of crack propagation from the initial length to the critical length to obtain the fatigue life of long cracks. ; ； Considering the phased changes in material parameters during crack propagation, the crack propagation process is divided into multiple stages. Therefore, the total lifespan is: ； in, Represents crack propagation life. The length of the crack. The final crack length within stage i. C is the initial crack length within stage i. i and m i For material-related constants, representing the intrinsic resistance to crack propagation, The range of stress intensity factors. represents the resistance to crack propagation, and n represents the total number of stages of crack propagation rate.

2. The prediction method according to claim 1, characterized in that, The microstructure information mentioned in step one includes at least one of the following: grain size, grain spatial orientation, grain boundary orientation difference, texture characteristics, microstructure gradient distribution, second phase particle distribution, inclusion distribution, and microstructure heterogeneity parameters.

3. The prediction method according to claim 1, characterized in that, The crack propagation driving force parameter mentioned in step two is represented by the stress intensity factor range ΔK obtained from linear elastic fracture mechanics calculations, and its calculation formula is as follows: Where Y is the geometric correction factor, Δσ is the cyclic stress range, and a is the crack length.

4. The prediction method according to claim 1, characterized in that, The deviation from Paris propagation behavior mentioned in step three includes at least one of the following: local decrease in crack propagation rate, multi-stage propagation relationship, rate oscillation propagation relationship, sawtooth propagation relationship, and hysteretic propagation behavior.

5. The prediction method according to claim 1, characterized in that, In step three, crack deflection, crack bifurcation, and tissue-induced barrier regions are determined by the correspondence between crack propagation paths and fracture morphology features.

6. The prediction method according to claim 1, characterized in that, The intrinsic propagation resistance of the material mentioned in step four is defined as the inherent ability of the material to resist the continued propagation of the crack under stable crack propagation conditions, and is obtained by calibration using the stable propagation range.

7. The prediction method according to claim 1, characterized in that, The obstacle resistance mentioned in step four is defined as the additional resistance generated during crack propagation by crack deflection, crack bifurcation, grain boundary obstruction, microstructure inhomogeneity, and local energy dissipation.

8. The prediction method according to claim 1, characterized in that, In step four, the barrier resistance is obtained by inversion of the deviation between the reference propagation rate and the actual propagation rate, and a mapping relationship is established between the barrier resistance and the crack length, stress intensity factor range, and microstructure characteristics.

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