Ultrahigh-cycle fatigue life prediction method considering orientation characteristics of crack initiation area

Abstract

An ultra-high cycle fatigue life prediction method considering orientation characteristics of a crack initiation area comprises the following steps: acquiring a failure sample, scanning a fatigue fracture surface of the failure sample by using a scanning electron microscope, and when a fatigue crack initiation source of a fine grain area exists in the fatigue fracture surface, repeating the steps; acquiring crack initiation zone parameters of the fatigue crack initiation source of the fine grain zone, wherein the crack initiation zone parameters comprise an initiation zone projection area and an initiation zone inclination angle; determining the real area of the initiation area according to the projection area of the initiation area and the inclination angle of the initiation area; acquiring a stress intensity factor on the inclined plane of the initiation area of the fatigue crack initiation source of the fine grain area; establishing a fatigue life model according to the real area and the stress intensity factor of the initiation area; according to the fatigue life model, the fatigue life of the failure sample is predicted, a more accurate life prediction model is obtained, and the accuracy of fatigue life prediction is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of structural component evaluation, and relates to a method for predicting ultra-high cycle fatigue life taking into account the positional characteristics of crack initiation zones. 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 the 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 the structure based on the crack initiation characteristics. In the past, life prediction models have been obtained based on the crack initiation mode induced by surface scratches or internal inclusions. Since these models are established only based on plane information, the prediction 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 taking into account the positional characteristics of crack initiation zones, thereby solving the problem of inaccurate prediction of fatigue life of structures based on crack initiation characteristics in the prior art.

[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for predicting ultra-high cycle fatigue life considering the positional characteristics of the crack initiation zone, comprising the following steps: 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.

[0005] 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.

[0006] 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.

[0007] In S4, the formula used to determine the true area of ​​the initiation zone is:

[0008] in, represents the actual area of ​​the germination zone, represents the projected area of ​​the germination zone, Indicates the inclination angle of the germination zone.

[0009] 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.

[0010] 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 area; 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] The main beneficial effects of the present invention are: By measuring the three-dimensional parameters of the crack initiation zone and calculating the stress intensity factor, the dominant factors in the formation of the fine grain zone morphology at the crack initiation source location were determined, thereby ensuring the applicability of the fatigue life prediction formula; and the existing life prediction formula was adjusted using three-dimensional morphology parameters instead of plane parameters to obtain a more accurate life prediction model.

[0015] By measuring the three-dimensional parameters of the crack initiation zone, the accurate projection area and inclination angle of the crack initiation zone are obtained, which improves the accuracy of the data.

[0016] The calculation of the stress intensity factor reveals the dominant factor in the formation of the fine grain zone morphology at the crack initiation source, thus ensuring the applicability of the fatigue life prediction formula. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0018] Figure 1 It is a schematic diagram of the process of the present invention.

[0019] Figure 2 Schematic diagram of fatigue crack initiation source in fine grain area of ​​the present invention.

[0020] Figure 3 It is a schematic diagram for comparing the cross-section, profile and height diagram of the present invention.

[0021] Figure 4 It is a schematic diagram of the magnitude of the stress intensity factors of the type I and type II cracks at the edge of the initiation zone of the present invention; Figure 5 This is a schematic diagram of the linear relationship in the experiment designed for the present invention.

[0022] Figure 6 This is a schematic diagram of the fatigue life prediction results of the present invention. DETAILED DESCRIPTION

[0023] like Figure 1 to Figure 6 A method for predicting ultra-high cycle fatigue life considering the location characteristics of crack initiation zone includes the following steps: S1, obtaining a test sample, performing an ultrasonic fatigue test on the test sample, and obtaining a failure sample; Preferably, the main equipment for conducting ultrasonic fatigue testing on the test specimen includes: Ultrasonic signal generator,The ultrasonic signal generator can be used to convert a 50Hz sinusoidal alternating current signal into a 20±0.5kHz ultrasonic signal.

[0024] Piezoelectric transducer: can be used to convert the electrical signal generated by the signal generator into an axial mechanical vibration wave of the same frequency; Displacement amplifier: can be used to increase the vibration displacement by reducing the cross-sectional area at one end; Displacement control system: can be used to control the precise output of vibration displacement amplitude and collect fatigue cycle times and vibration frequency in real time.

[0025] In this embodiment, ultrasonic fatigue is a resonance experimental method based on displacement control, and its essence is to use axial mechanical resonance waves to achieve ultra-high cycle loading on the sample.

[0026] 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; Preferably, the method for judging the presence of a fine-grained fatigue crack initiation source in the fatigue fracture surface is as follows: if the fatigue crack initiation zone has a rough morphology and has many small protrusions, and the size of the small protrusions is smaller than the average size of the small grains, then it is a fine-grained zone, such as Figure 2 As shown, Figure 2 Schematic diagram of fatigue crack initiation source in fine grain area.

[0027] 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.

[0028] In a preferred solution, in S3, obtaining the projection area of ​​the initiation zone of fatigue crack initiation sources in the fine grain region specifically includes: 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.

[0029] In a preferred solution, in S3, obtaining the inclination angle of the initiation zone of fatigue crack initiation sources in the fine grain region specifically includes: 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.

[0030] Preferably, after judging that the initiation morphology is a fine-grained area, it is necessary to accurately draw the range of the fine-grained area, and use a scanning electron micrograph to measure the projected area of ​​the initiation area on a plane perpendicular to the applied stress. In order to reduce the error, the area value of this area can be measured multiple times to take the average value. Then use a non-contact optical measurement system to identify the crack initiation characteristics, and import the obtained data into a three-dimensional morphology analysis software. The highest and lowest points in the longitudinal direction of the initiation area are sectioned to obtain a specified cross-sectional profile height map, such as Figure 3 As shown, Figure 3 The schematic diagram of the cross-sectional height map is shown in Figure 1. The starting point and end point of the germination area in the cross-sectional height map are determined according to the longitudinal coordinates of the boundary of the germination area in the three-dimensional topography map, and the height and length between the two points are measured to calculate the inclination angle of the germination area, and the actual area of ​​the germination area is calculated.

[0031] In the preferred solution, in S4, the formula for determining the actual area of ​​the germination zone is:

[0032] in, represents the actual area of ​​the germination zone, represents the projected area of ​​the germination zone, Indicates the inclination angle of the germination zone.

[0033] In a preferred solution, 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.

[0034] Preferably, the initiation zone presents an inclined angle, and the inclined surface IA is subjected to cyclic loading of type I and type II stress. The magnitude of the stress intensity factor of type I and type II cracks at the edge of the initiation zone ( , ), calculated by using the following formula:

[0035]

[0036] in, and are the normal projection component and tangential projection component of stress on IA, respectively. is the actual area of ​​the germination zone.

[0037] Preferably, if Figure 4 As shown, Figure 4 The schematic diagram of the magnitude of the stress intensity factors of type I and type II cracks at the edge of the initiation zone. Regardless of high-cycle or ultra-high-cycle fatigue loading, the stress intensity factor of type I crack of IA is dominant, so in the present invention, the stress intensity factor of type I crack is used as the stress intensity factor on the inclined surface of the initiation zone of the fatigue crack initiation source in the crystal region.

[0038] In a preferred solution, in S6, establishing the fatigue life model specifically includes: S6-1, obtain the propagation formula of the fatigue crack at the fracture initiation source of fatigue crack in the fine grain area; 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.

[0039] In the preferred solution, in S6-1, under extremely high cycle fatigue conditions, more than 95% of the fatigue life is used to form the initiation region, which is equal to the fatigue life N in the present invention. The extended 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.

[0040] In the preferred solution, in S6-2, since the formation of the fine grain region morphology is dominated by the I-type stress intensity factor, when the fatigue crack initiates from the inside to fatigue failure, 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.

[0041] Preferably, and is the unknown parameter in step S6-3, and the unknown parameter is subjected to the designed experiment. and Conduct an assessment. Figure 5 As shown, Figure 5 This is a schematic diagram of the linear relationship in the designed experiment. Figure 5 The parameters are estimated by fitting algorithm and are 6.96 and 5.13E-13 respectively. and Substituting the initial fatigue life model into the fatigue life model, the fatigue life model in step S6-4 can be obtained.

[0042] In the preferred solution, 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.

[0043] S7. Predict the fatigue life of the failed specimen based on the fatigue life model.

[0044] In the above method, the fatigue life of the failed specimen is predicted as follows: Figure 6 As shown, Figure 6 The diagram is a schematic diagram of fatigue life prediction results. The predicted life data of the failed specimen mainly fall within distribution band 2, and the error is within a reasonable range. Therefore, this ultra-high cycle fatigue life prediction method that considers the location characteristics of the crack initiation zone can provide meaningful predictions for preventive maintenance decisions.

[0045] The above-mentioned embodiments are only preferred technical solutions of the present invention and should not be regarded as limitations of the present invention. The embodiments and features in the embodiments of the present application can be arbitrarily combined with each other without conflict. The protection scope of the present invention shall 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, 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 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 boundary of the initiation region 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 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, Indicates 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 the 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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