Ultra-high cycle fatigue life prediction method considering crack initiation location orientation characteristics

Abstract

A method for predicting ultra-high cycle fatigue life considering the orientation characteristics of crack initiation location. By obtaining a failed specimen, using a scanning electron microscope to scan the fatigue fracture surface of the failed specimen. When there is a fatigue crack initiation source in the fine-grained area of the fatigue fracture surface, repeat the steps; obtain the crack initiation area parameters of the fatigue crack initiation source in the fine-grained area, and the crack initiation area parameters include the projected area of the initiation area and the inclination angle of the initiation area; determine the true area of the initiation area according to the projected area of the initiation area and the inclination angle of the initiation area; obtain the stress intensity factor on the inclined surface of the initiation area of the fatigue crack initiation source in the fine-grained area; establish a fatigue life model according to the true area of the initiation area and the stress intensity factor; predict the fatigue life of the failed specimen according to the fatigue life model to obtain a more accurate life prediction model and improve the accuracy of fatigue life prediction.

Description

Technical Field

[0001] The present invention belongs to the technical field of structural component evaluation, and relates to a method for predicting ultra-high cycle fatigue life considering the orientation characteristics of crack initiation location. Background Art

[0002] Determining the safety and reliability of structural components during long-term use has become an urgent need in modern manufacturing. Fatigue crack nucleation, initiation, and early propagation are decisive factors in determining fatigue performance, especially in ultra-high cycle fatigue, where early failure behavior dominated by local microstructure accounts for more than 95% of the total life. Therefore, it is feasible to predict the fatigue life of a structure based on crack initiation characteristics. In the past, life prediction models have been obtained based on crack initiation methods induced by surface scratches or internal inclusions. Since the establishment of these models is only based on planar information, the predicted results are not ideal. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a method for predicting ultra-high cycle fatigue life considering the orientation characteristics of crack initiation location, and solve the problem that the prediction of the fatigue life of a structure based on crack initiation characteristics in the prior art is inaccurate.

[0004] To solve the above technical problem, the technical solution adopted by the present invention is: a method for predicting ultra-high cycle fatigue life considering the orientation characteristics of crack initiation location, comprising the following steps:

[0005] S1, obtain test specimens, conduct ultrasonic fatigue tests on the test specimens, and obtain failed specimens;

[0006] S2, use a scanning electron microscope to scan the fatigue fracture surface of the failed specimen. When there is a fatigue crack initiation source in the fine grain area on the fatigue fracture surface, execute step S3, otherwise return to step S1;

[0007] S3, obtain the crack initiation zone parameters of the fatigue crack initiation source in the fine grain area. The crack initiation zone parameters include the projected area of the initiation zone and the inclination angle of the initiation zone;

[0008] S4, determine the true area of the initiation zone according to the projected area of the initiation zone and the inclination angle of the initiation zone;

[0009] S5, obtain the stress intensity factor on the inclined surface of the initiation zone of the fatigue crack initiation source in the fine grain area;

[0010] S6, establish a fatigue life model according to the true area of the initiation zone and the stress intensity factor;

[0011] S7, predict the fatigue life of the failed specimen according to the fatigue life model.

[0012] In S3, obtain the projected area of the initiation region of the fatigue crack initiation source in the fine-grained region, specifically including:

[0013] S3A-1. Use a scanning electron microscope to scan the fatigue fracture surface at least twice to obtain at least two scanning electron micrographs;

[0014] S3A-2. Measure at least two scanning electron micrographs to obtain at least two data of the projected area of the initiation region;

[0015] S3A-3. Calculate the average value of at least two data of the projected area of the initiation region as the projected area of the initiation region.

[0016] In S3, obtain the inclination angle of the initiation region of the fatigue crack initiation source in the fine-grained region, specifically including:

[0017] S3B-1. Use a non-contact optical measurement system to identify the crack initiation characteristics of the fatigue crack initiation source in the fine-grained region and import the identified results into 3D topography analysis software;

[0018] S3B-2. In the 3D topography analysis software, obtain a cross-sectional profile height map by taking cross-sections of the highest and lowest points in the longitudinal direction of the initiation region of the fatigue crack initiation source in the fine-grained region;

[0019] S3B-3. Obtain the starting point and ending point of the initiation region in the cross-sectional profile height map according to the longitudinal coordinates of the boundary of the initiation region in the 3D topography map, and the 3D topography map is obtained from the 3D topography analysis software;

[0020] S3B-4. Obtain the inclination angle of the initiation region according to the height and length between the starting point and ending point of the initiation region.

[0021] In S4, the formula for determining the true area of the initiation region is:

[0022]

[0023] Where, represents the true area of the initiation region, represents the projected area of the initiation region, represents the inclination angle of the initiation region.

[0024] In S5, the formula for obtaining the stress intensity factor is:

[0025] ;

[0026] Where, represents the projected component of the stress normal to the inclined plane of the initiation region, represents the true area of the initiation region, represents pi, denotes the stress intensity factor.

[0027] In S6, establishing the fatigue life model specifically includes:

[0028] S6-1, obtaining the propagation formula of the fatigue crack at the fracture surface of the fatigue crack initiation source in the fine grain area;

[0029] S6-2, determining the initial fatigue life model according to the propagation formula, the true area of the initiation zone, and the stress intensity factor;

[0030] S6-3, solving the unknown parameters in the initial fatigue life model according to the designed experiment to obtain the solution results of the unknown parameters;

[0031] S6-4, substituting the solution results into the initial fatigue life model to obtain the fatigue life model.

[0032] In S6-1, the propagation formula is:

[0033] ;

[0034] Wherein, is the propagation rate of the fatigue crack at the fracture surface, is the amplitude of the stress intensity factor, and are constants related to the material.

[0035] In S6-2, the initial fatigue life model is:

[0036] ;

[0037] Wherein, denotes the stress intensity factor, denotes the true area of the initiation zone, denotes the fatigue life, and are constants related to the material.

[0038] In S6-4, the fatigue life model is:

[0039] ;

[0040] Wherein, denotes pi, denotes the projection component of the stress normal to the inclined surface of the initiation zone, denotes the true area of the initiation zone.

[0041] The main beneficial effects of the present invention are as follows:

[0042] By measuring the three-dimensional parameters of the crack initiation zone, calculating the magnitude of the stress intensity factor, and determining the dominant factors for the formation of the fine-grained zone morphology at the crack initiation source location, the applicability of the fatigue life prediction formula is ensured; and the existing life prediction formula is adjusted using three-dimensional morphology parameters instead of planar parameters to obtain a more accurate life prediction model.

[0043] By measuring the three-dimensional parameters of the crack initiation zone, the accurate projected area of the initiation zone and the inclination angle of the initiation zone are obtained, improving the accuracy of the data.

[0044] Calculating the magnitude of the stress intensity factor reveals the dominant factors for the formation of the fine-grained zone morphology at the crack initiation source location, thus ensuring the applicability of the fatigue life prediction formula. Brief Description of the Drawings

[0045] The present invention will be further described below in conjunction with the drawings and embodiments.

[0046] Figure 1 It is a schematic flow chart of the present invention.

[0047] Figure 2 It is a schematic diagram of the fatigue crack initiation source in the fine-grained zone of the present invention.

[0048] Figure 3 It is a schematic diagram for comparing the cross-section, profile and height diagram of the present invention.

[0049] Figure 4 It is a schematic diagram of the magnitude of the stress intensity factor of type I and type II cracks at the edge of the initiation zone of the present invention;

[0050] Figure 5 It is a schematic diagram of the linear relationship in the designed experiment of the present invention.

[0051] Figure 6 It is a schematic diagram of the fatigue life prediction result of the present invention. Detailed Embodiment

[0052] Such as Figures 1 to 6 In [reference], a very high cycle fatigue life prediction method considering the orientation characteristics of the crack initiation location includes the following steps:

[0053] S1. Obtain test specimens, conduct ultrasonic fatigue tests on the test specimens, and obtain failed specimens;

[0054] Preferably, the main equipment for conducting ultrasonic fatigue tests on the test specimens includes:

[0055] An ultrasonic signal generator, which can be used to convert a 50 Hz sinusoidal alternating current signal into an ultrasonic signal of 20 ± 0.5 kHz.

[0056] Piezoelectric transducer: It can be used to convert the electrical signal generated by a signal generator into an axial mechanical vibration wave of the same frequency;

[0057] Displacement amplifier: It can be used to increase the vibration displacement by reducing the cross-sectional area at one end;

[0058] Displacement control system: It can be used to control the accurate output of the vibration displacement amplitude and collect the fatigue cycle times and vibration frequencies in real time.

[0059] In this embodiment, ultrasonic fatigue is a resonance-based experimental method based on displacement control. Its essence is to use axial mechanical resonance waves to apply ultra-high cycle loading to the specimen.

[0060] S2. Use a scanning electron microscope to scan the fatigue fracture surface of the failed specimen. When there is a fatigue crack initiation source in the fine grain area on the fatigue fracture surface, perform step S3; otherwise, return to step S1;

[0061] Preferably, the method for judging the existence of a fatigue crack initiation source in the fine grain area on the fatigue fracture surface is: if the morphology of the fatigue crack initiation area is rough and there are many small protrusions, and the size of the small protrusions is smaller than the average size of the small grains, it is the fine grain area. As Figure 2 shown, Figure 2 is a schematic diagram of the fatigue crack initiation source in the fine grain area.

[0062] S3. Obtain the crack initiation area parameters of the fatigue crack initiation source in the fine grain area. The crack initiation area parameters include the projected area of the initiation area and the inclination angle of the initiation area;

[0063] S4. Determine the true area of the initiation area according to the projected area of the initiation area and the inclination angle of the initiation area;

[0064] S5. Obtain the stress intensity factor on the inclined surface of the initiation area of the fatigue crack initiation source in the fine grain area;

[0065] S6. Establish a fatigue life model according to the true area of the initiation area and the stress intensity factor;

[0066] S7. Predict the fatigue life of the failed specimen according to the fatigue life model.

[0067] In a preferred solution, in S3, to obtain the projected area of the initiation area of the fatigue crack initiation source in the fine grain area, it specifically includes:

[0068] S3A-1. Use a scanning electron microscope to scan the fatigue fracture surface at least twice to obtain at least two scanning electron micrographs;

[0069] S3A-2. Measure at least two scanning electron micrographs to obtain at least two projected area data of the initiation area;

[0070] S3A - 3. Calculate the average value of the projected area data of at least two germination areas as the projected area of the germination area.

[0071] In a preferred embodiment, in S3, obtain the inclination angle of the germination area of the fatigue crack initiation source in the fine - grained area, specifically including:

[0072] S3B - 1. Use a non - contact optical measurement system to identify the crack initiation characteristics of the fatigue crack initiation source in the fine - grained area and import the identified results into 3D topography analysis software;

[0073] S3B - 2. In the 3D topography analysis software, obtain a cross - section profile height map by taking the longitudinal highest and lowest points of the germination area of the fatigue crack initiation source in the fine - grained area;

[0074] S3B - 3. Obtain the starting and ending points of the germination area in the cross - section profile height map according to the longitudinal coordinates of the boundary of the germination area in the 3D topography map, where the 3D topography map is obtained from the 3D topography analysis software;

[0075] S3B - 4. Obtain the inclination angle of the germination area according to the height and length between the starting and ending points of the germination area.

[0076] Preferably, after determining that the germination morphology is the fine - grained area, it is necessary to accurately draw the range of the fine - grained area and measure the projected area size of the germination area on the plane perpendicular to the loading stress using a scanning electron micrograph. To reduce errors, the value of this area size can be measured multiple times and averaged. Then use a non - contact optical measurement system to identify the crack initiation characteristics and import the obtained data into 3D topography analysis software. Take a cross - section of the longitudinal highest and lowest points of the germination area to obtain a specified cross - section profile height map, as Figure 3 shown, Figure 3 which is a schematic diagram of the cross - section profile height map. Determine the starting and ending points of the germination area in the cross - section profile height map according to the longitudinal coordinates of the boundary of the germination area in the 3D topography map, measure the height and length between the two points to calculate the inclination angle of the germination area, and calculate the true area of the germination area.

[0077] In a preferred embodiment, in S4, the formula for determining the true area of the germination area is:

[0078]

[0079] where, represents the true area of the germination area, represents the projected area of the germination area, represents the inclination angle of the germination area.

[0080] In a preferred embodiment, in S5, the formula for obtaining the stress intensity factor is:

[0081] ;

[0082] Wherein, represents the normal projection component of the stress on the inclined plane of the germination zone, represents the true area of the germination zone, represents pi, represents the stress intensity factor.

[0083] Preferably, the germination zone presents an inclined angle, and this inclined surface IA is subjected to cyclic loading of type I and type II stresses. For the magnitudes of the stress intensity factors of type I and type II cracks at the edge of the germination zone ( , ), they are calculated by using the following formula:

[0084]

[0085]

[0086] Wherein, and are respectively the normal projection component and the tangential projection component of the stress on IA, is the true area of the germination zone.

[0087] Preferably, as Figure 4 shown, Figure 4 is a schematic diagram of the magnitudes of the stress intensity factors of type I and type II cracks at the edge of the germination zone. Whether it is high-cycle or very-high-cycle fatigue loading, the stress intensity factor of the type I crack in IA is dominant. Therefore, in the present invention, the stress intensity factor of the type I crack is used as the stress intensity factor on the inclined surface of the germination zone where the fatigue crack initiation source in the crystal region is located.

[0088] In a preferred embodiment, in S6, establishing the fatigue life model specifically includes:

[0089] S6-1, obtaining the propagation formula of the fatigue crack at the fracture surface of the fatigue crack initiation source in the fine crystal region;

[0090] S6-2, determining the initial fatigue life model according to the propagation formula, the true area of the germination zone, and the stress intensity factor;

[0091] S6-3, solving the unknown parameters in the initial fatigue life model according to the designed experiment to obtain the solution results of the unknown parameters;

[0092] S6-4, substituting the solution results into the initial fatigue life model to obtain the fatigue life model.

[0093] In a preferred embodiment, in S6-1, under extremely high cyclic fatigue conditions, more than 95% of the fatigue life is used to form the initiation region, which is defined as the fatigue life N in the present invention. The extended formula is:

[0094] ;

[0095] where is the propagation rate of the fatigue crack at the fracture surface, is the amplitude of the stress intensity factor, and are material-related constants.

[0096] In a preferred embodiment, in S6-2, since the formation of the fine-grained region morphology is dominated by the mode I stress intensity factor, when the fatigue crack initiates from the interior to fatigue failure, the initial fatigue life model is:

[0097] ;

[0098] where represents the stress intensity factor, represents the true area of the initiation region, represents the fatigue life, and are material-related constants.

[0099] Preferably, and are unknown parameters in step S6-3. The unknown parameters and are evaluated according to the designed experiment. As shown in Figure 5 , Figure 5 is a schematic diagram of the linear relationship in the designed experiment. According to Figure 5 and by using the fitting algorithm, the estimated values of the parameters and are 6.96 and 5.13E-13 respectively. Therefore, substituting and into the initial fatigue life model, the fatigue life model can be obtained, and the fatigue life model in step S6-4 can be obtained.

[0100] In a preferred embodiment, in S6-4, the fatigue life model is:

[0101] ;

[0102] where represents pi, represents the projection component of the stress normal to the inclined plane of the initiation region, represents the true area of the initiation region.

[0103] S7. Predict the fatigue life of the failed specimen according to the fatigue life model.

[0104] In the above method, the prediction of the fatigue life of the failed specimen is as Figure 6 shown Figure 6 in the schematic diagram of the fatigue life prediction result. The predicted life data of the failed specimen mainly fall within the distribution band 2, and the error is within a reasonable range. Therefore, this ultra-high cycle fatigue life prediction method considering the crack initiation location orientation characteristics can provide meaningful predictions for preventive maintenance decisions.

[0105] The above embodiments are only the preferred technical solutions of the present invention and should not be regarded as limitations on the present invention. The embodiments in this application and the features in the embodiments can be arbitrarily combined with each other without conflict. The protection scope of the present invention should be the technical solutions recorded in the claims, including the equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, the equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A method for predicting ultra-high cycle fatigue life considering the location characteristics of crack initiation zone, characterized in that: The steps include: S1, obtaining a test sample, performing an ultrasonic fatigue test on the test sample, and obtaining a failure sample; S2, scanning the fatigue fracture surface of the failed sample using a scanning electron microscope, and when there is a fatigue crack initiation source in a fine grain area on the fatigue fracture surface, executing step S3, otherwise returning to step S1; S3, obtaining crack initiation zone parameters of fatigue crack initiation sources in the fine grain region, the crack initiation zone parameters including the initiation zone projection area and the initiation zone inclination angle; S4, determining the real area of ​​the initiation zone according to the projection area of ​​the initiation zone and the inclination angle of the initiation zone; S5, obtain the stress intensity factor on the inclined surface of the initiation zone of fatigue crack initiation source in the fine grain zone; S6, establish a fatigue life model based on the actual area of ​​the initiation zone and the stress intensity factor; S7, predict the fatigue life of the failed specimen based on the fatigue life model.

2. The method for predicting ultra-high cycle fatigue life considering the positional characteristics of the crack initiation zone according to claim 1 is characterized by: In S3, the projection area of ​​the initiation zone of fatigue crack initiation sources in the fine grain region is obtained, specifically including: S3A-1, use a scanning electron microscope to scan the fatigue fracture surface at least twice to obtain at least two scanning electron micrographs; S3A-2, measuring at least two scanning electron micrographs to obtain at least two initiation zone projection area data; S3A-3, calculating an average value of at least two initiation zone projection area data as the initiation zone projection area.

3. The method for predicting ultra-high cycle fatigue life considering the positional characteristics of the crack initiation zone according to claim 1 is characterized by: In S3, the inclination angle of the initiation zone of fatigue crack initiation source in the fine grain zone is obtained, specifically including: S3B-1. Use a non-contact optical measurement system to identify the crack initiation characteristics of fatigue crack initiation sources in the fine grain region, and import the identification results into the three-dimensional morphology analysis software; S3B-2. In the three-dimensional morphology analysis software, the highest and lowest points in the longitudinal direction of the initiation area of ​​fatigue crack initiation sources in the fine grain region are sectioned to obtain a cross-sectional height diagram; S3B-3, obtaining the starting point and the end point of the initiation region in the cross-sectional height map according to the longitudinal coordinates of the initiation region boundary in the three-dimensional topography map, the three-dimensional topography map being obtained from the three-dimensional topography analysis software; S3B-4. According to the height and length between the starting point and the end point of the germination zone, the inclination angle of the germination zone is obtained.

4. The method for predicting ultra-high cycle fatigue life considering the positional characteristics of the crack initiation zone according to claim 1 is characterized by: In S4, the formula used to determine the true area of ​​the initiation zone is: in, represents the actual area of ​​the germination zone, represents the projected area of ​​the germination zone, Represents the inclination angle of the germination zone.

5. The method for predicting ultra-high cycle fatigue life considering the positional characteristics of crack initiation zone according to claim 1 is characterized by: In S5, the formula for obtaining the stress intensity factor is: ； in, represents the normal projection component of the stress on the inclined surface of the germination zone, represents the actual area of ​​the germination zone, represents pi, represents the stress intensity factor.

6. The method for predicting ultra-high cycle fatigue life considering the positional characteristics of crack initiation zone according to claim 1 is characterized by: In S6, the fatigue life model is established by: S6-1, obtain the propagation formula of the fatigue crack at the fracture initiation source of fatigue crack in the fine grain region; S6-2, determine the initial fatigue life model based on the expansion formula, the true area of ​​the initiation zone and the stress intensity factor; S6-3, solving unknown parameters in the initial fatigue life model according to the designed experiment to obtain solution results of the unknown parameters; S6-4, substitute the solution result into the initial fatigue life model to obtain the fatigue life model.

7. The method for predicting ultra-high cycle fatigue life considering the positional characteristics of the crack initiation zone according to claim 6 is characterized in that: In S6-1, the expanded formula is: ； in, is the growth rate of the fracture fatigue crack, is the magnitude of the stress intensity factor, and is a constant related to the material.

8. The method for predicting ultra-high cycle fatigue life considering the positional characteristics of the crack initiation zone according to claim 6 is characterized by: In S6-2, the initial fatigue life model is: ； in, represents the stress intensity factor, represents the actual area of ​​the germination zone, represents fatigue life, and is a constant related to the material.

9. The method for predicting ultra-high cycle fatigue life considering the positional characteristics of the crack initiation zone according to claim 6 is characterized in that: In S6-4, the fatigue life model is: ； in, represents pi, represents the normal projection component of the stress on the inclined surface of the germination zone, Represents the actual area of ​​the germination zone.

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