A method for predicting a fatigue crack propagation threshold value under alternating stress

Abstract

This invention provides a method for predicting the fatigue crack propagation threshold under alternating stress, comprising: S1, obtaining the fatigue failure cycle N and fatigue stress range Δσ of the failed specimen; S2, obtaining the critical crack length a of the failed specimen. f S3. Obtain the fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of the specimen. f S4. Formula for determining the stress intensity factor range ΔK and crack length a of the specimen; S5. Determine the crack propagation Δa for each constant force range; S6. Determine the stress range Δσ for each stress level. i S7. Obtain the stress range Δσ for each level. i Crack propagation rate (da / dN) i S8. Determine the length a of each crack level. i The range of stress intensity factor ΔK i S9. Obtain the fatigue crack propagation threshold value ΔK. th This invention predicts fatigue crack propagation threshold values ​​for different stress ranges based on high-cycle fatigue test results, replacing the fatigue crack propagation rate test which has a long testing cycle and high cost, thus reducing the testing workload and improving the efficiency of data acquisition.

Description

Technical Field

[0001] This invention belongs to the field of fatigue crack propagation technology, specifically relating to a method for predicting the threshold value of fatigue crack propagation under alternating stress. Background Technology

[0002] The fatigue crack propagation threshold is an important indicator reflecting a material's fatigue resistance; it is the threshold value used to determine whether a crack will propagate. Currently, the fatigue crack propagation threshold is obtained through fatigue crack propagation rate testing. The fatigue crack propagation rate test references the standard "GBT 6398-2017 Metallic Materials - Test Method for Fatigue Crack Propagation Rate," employing a graded stress reduction method. During graded stress reduction, each stress level must ensure that the crack propagation increment is greater than the plastic zone size r corresponding to the maximum stress intensity factor of the previous level. y 4 to 6 times, until the average crack propagation rate da / dN approaches 10. -7 The test ends when the crack length reaches mm / cycle. During the test, the termination crack length and corresponding cycle number are recorded for each force level or stress intensity factor range, obtaining at least 5 crack lengths distributed within 10 mm / cycle. -6 mm / cycle~10 -7 The data points (Log(da / dN), LogΔK) between mm / cycle are used to obtain the fatigue crack propagation threshold value by linear regression fitting of the data points.

[0003] However, during fatigue crack propagation rate tests, it was found that the entire test was time-consuming, and different initial crack lengths, initial loads, and load reduction gradients resulted in different fatigue crack propagation threshold values. Furthermore, during the tests, when the crack propagation rate approached 10... -7 When the crack length is measured in mm / cycle, the crack remains almost stationary over a long period. This makes the accuracy of crack length measurement highly dependent on the precision of the testing equipment. However, high-precision testing is expensive, significantly increasing the overall measurement cost. Therefore, fatigue crack propagation rate testing suffers from drawbacks such as long testing cycles and high costs, hindering the widespread adoption of fatigue crack propagation threshold testing and negatively impacting the fatigue design and life prediction of cracked components. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method for predicting the fatigue crack propagation threshold value of engineering components under alternating stress. This method rapidly predicts the fatigue crack propagation threshold value under different stress ranges at a given stress ratio by using high-cycle fatigue test results at different stress ranges under a given stress ratio. This method replaces the fatigue crack propagation rate test, which has a long testing cycle and high cost, effectively reducing the testing workload, improving the efficiency of obtaining the fatigue crack propagation threshold value, and reducing the cost of obtaining the fatigue crack propagation threshold value.

[0005] To achieve the above and other related objectives, the present invention provides a method for predicting the fatigue crack propagation threshold under alternating stress, the method comprising the following steps:

[0006] S1. Conduct high-cycle fatigue tests on multiple specimens made of the target material under the same stress ratio but different fatigue stress ranges, and record the fatigue failure cycle number N and fatigue stress range Δσ of the failed specimens; the failed specimens are those with less than 10 cycles. 7 The specimen in which the crack extends to the critical length after the second failure, wherein the fatigue failure cycle N is the actual number of cycles of the failed specimen; the specimen is a specimen with a circular cross-section;

[0007] S2. After marking the crack lead edge of each failed specimen, pull each failed specimen apart to obtain the critical crack length 'a' of the corresponding failed specimen. f ;

[0008] S3. By analyzing the fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of each failed specimen. f By performing fitting, the fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of the specimen were obtained. f The fitting formula: a f =NA(Δσ) m Where A is the first fitting parameter; m is the first fitting index;

[0009] S4. Calibrate the coefficients in the relationship between the stress intensity factor range ΔK and the crack length a to determine the final relationship between the stress intensity factor range ΔK and the crack length a of the specimen:

[0010]

[0011] Where ΔF is the range of fatigue loads borne by the specimen;

[0012] D is the diameter of the cross-section at the center of the sample;

[0013] S0 is the original cross-sectional area at the center of the sample, derived from the formula... Sure;

[0014] The geometric factor coefficient of the sample;

[0015] k0~k n The coefficients obtained from calibration;

[0016] n+1 is the number of different cycles in the calibration test, and n is not less than 4;

[0017] S5. Based on the principle of the continuous decreasing K method, calculate the crack propagation Δa corresponding to each constant force range. The calculation formula is as follows:

[0018]

[0019] Where ΔK i The range of the i-th level stress intensity factor;

[0020] ΔK i-1 The range of stress intensity factor for level i-1;

[0021] Δx K The stress intensity factor decreases continuously by a fixed value, satisfying Δx. K ≤2%;

[0022] C is the gradient of decreasing K, which is a fixed value that satisfies C ≥ -0.1 mm. -1 ;

[0023] S6. Determine the initial maximum stress σ0 and the initial crack length a0; calculate the stress range Δσ for each stage based on the principle of the continuous decreasing K method. i The calculation formula is as follows:

[0024]

[0025] In the formula, Δσ i The stress range of level i, and the stress range of level i Δσ1=(1-R)σ0, where R is the stress ratio;

[0026] Δσ i-1 The stress range of level i-1;

[0027] a i Let be the length of the i-th level crack, given by formula a i =a i-1 +Δa is determined, where the length of the first-order crack a1=a0+Δa;

[0028] a i-1 The length of the (i-1)th level crack;

[0029] S7. Based on the fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of the specimen. f The fitting formula is used to obtain the stress range Δσ at each level. i The number of iterations ΔN i Thus, the stress ranges Δσ at each level are obtained. i Crack propagation rate (da / dN) i The calculation formula is as follows:

[0030] S8. Calculate the crack length a at each stage using the relationship between the stress intensity factor range ΔK and the crack length a. i The corresponding stress intensity factor range ΔKi ;

[0031] S9. Draw the stress range Δσ for each level. i Lower crack propagation rate (da / dN) i With the corresponding stress intensity factor range ΔK i The relationship curve is used to obtain the fatigue crack propagation threshold value ΔK. th The fatigue crack propagation threshold value ΔK th For the crack propagation rate da / dN = 10 -7 Range of stress intensity factor at mm / cycle.

[0032] Preferably, the calibration process in step S4 includes the following steps:

[0033] S41. Conduct fatigue tests on n+1 specimens made of the target material under different cycles, the same stress ratio, and the same fatigue load range, and record the number of specimens n+1, the fatigue load range ΔF, and the crack length a of the specimen corresponding to the number of cycles.

[0034] S42. Obtain the stress intensity factor range ΔK for the corresponding number of cycles;

[0035] S43. Using the fatigue load range ΔF, the crack length a of the specimen under different cycles, and the corresponding stress intensity factor range ΔK, calculate the coefficients k0~k in the relationship between the stress intensity factor range ΔK and the crack length a of the specimen. n .

[0036] Preferably, step S41 is replaced by: performing a calibration fatigue test on a specimen made of the target material under the same stress ratio and the same fatigue load range, with the number of cycles increasing sequentially, to obtain the number of different cycle numbers n+1, the fatigue load range ΔF, and the specimen crack length a corresponding to the number of cycles.

[0037] Preferably, the stress intensity factor range ΔK corresponding to the number of cycles is obtained by mathematical analysis, boundary configuration, or finite element method.

[0038] Preferably, the crack leading edge of the failed specimen in step S2 is marked using a heat-coloring method or a secondary fatigue method.

[0039] Preferably, the initial maximum stress σ0 satisfies: σ R ≤σ0≤R p0.2 Among them, R p0.2 σ represents the yield strength of the target material. R The conditional fatigue limit of the target material.

[0040] As described above, the method for predicting the fatigue crack propagation threshold value of engineering components under alternating stress according to the present invention has the following beneficial effects:

[0041] This invention uses high-cycle fatigue test results at different stress ranges under a certain stress ratio to quickly predict the fatigue crack propagation threshold value at different stress ranges under the corresponding stress ratio. This replaces the fatigue crack propagation rate test, which has a long testing cycle and high cost, effectively reducing the testing task, improving the efficiency of obtaining the fatigue crack propagation threshold value, and reducing the cost of obtaining the fatigue crack propagation threshold value.

[0042] The fatigue crack propagation threshold value predicted by the present invention based on high-cycle fatigue test results is basically equal to the fatigue crack propagation threshold value obtained by fatigue crack propagation rate test, and the test verifies the effectiveness of the present invention. Attached Figure Description

[0043] Figure 1 This is a flowchart of the fatigue crack propagation threshold prediction method of the present invention.

[0044] Figure 2 This is a graph showing the relationship between the maximum stress range and the crack length at each initial maximum stress level.

[0045] Figure 3 This is a graph showing the relationship between the crack propagation rate at each initial maximum stress and the range of stress intensity factors at each stress level. Detailed Implementation

[0046] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0047] Please see Figures 1 to 3 It should be understood that the structures, proportions, sizes, etc., illustrated in the accompanying drawings are merely for illustrative purposes to aid those skilled in the art and to facilitate understanding and reading. They are not intended to limit the scope of the invention and therefore have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to size, without affecting the effectiveness and purpose of the invention, should still fall within the scope of the technical content disclosed in this invention. Furthermore, the terms such as "upper," "lower," "left," "right," "middle," and "one" used in this specification are merely for clarity and not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention's implementation.

[0048] like Figure 1 As shown, this invention provides a method for predicting the fatigue crack propagation threshold under alternating stress, the method comprising the following steps:

[0049] S1. Conduct high-cycle fatigue tests on multiple specimens made of the target material under the same stress ratio but different fatigue stress ranges, and record the fatigue failure cycle number N and fatigue stress range Δσ of the failed specimens; the failed specimens are those with less than 10 cycles. 7 The specimen in which the crack extends to the critical length after the second failure, wherein the fatigue failure cycle N is the actual number of cycles of the failed specimen; the specimen is a specimen with a circular cross-section;

[0050] It is understood that the specimen with a circular cross-section can be a cylindrical specimen or an hourglass-shaped specimen, and there is no limitation on this. The present invention preferably uses an hourglass-shaped specimen.

[0051] It is understood that the hourglass-shaped specimen in this invention includes two heads and a connecting shaft for connecting the two heads, and the hourglass-shaped specimen has a centrally symmetrical structure, and its cross-section at any position is circular; wherein, the cross-section at the center of the specimen refers to the cross-section of the connecting shaft.

[0052] It is understood that the high-cycle fatigue test in this invention is controlled by the standard GB / T 3075-2008 Axial Force Control Method for Fatigue Testing of Metallic Materials. The test equipment is the GPS2000 high-frequency vibration fatigue testing machine from Changchun Testing Machine Factory. When the crack in the specimen extends to the critical crack length, the resonant frequency deviates from the set value, causing the fatigue testing machine to automatically stop. At this time, the specimen is considered a failed specimen.

[0053] S2. After marking the crack lead edge of each failed specimen using the heat-dyeing method or the secondary fatigue method, each failed specimen is broken to obtain the critical crack length 'a' of the corresponding failed specimen. f ;

[0054] S3. By analyzing the fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of each failed specimen. f Three-parameter fitting was performed to obtain the fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of the specimen. f The first fitting formula: a f =NA(Δσ) m (1); where A is the first fitting parameter; m is the first fitting index;

[0055] S4. Regarding the coefficients k0~k in the relationship between the stress intensity factor range ΔK and the crack length a, n Calibration was performed to determine the final relationship between the stress intensity factor range ΔK and the crack length a of the specimen:

[0056] (2)

[0057] Where ΔF is the range of fatigue loads borne by the specimen;

[0058] D is the diameter of the cross-section at the center of the sample;

[0059] S0 is the original cross-sectional area at the center of the sample, derived from the formula... Sure;

[0060] The geometric factor coefficient of the sample;

[0061] k0~k n The coefficients obtained from calibration;

[0062] n+1 represents the number of different cycles in the calibration experiment, and n is not less than 4.

[0063] It is understandable that the coefficients k0~k in formula (2) n There are two specific calibration methods, the only difference being the number of test specimens in the fatigue test during calibration.

[0064] First calibration method:

[0065] A fatigue test (i.e., calibration test) is conducted on n+1 specimens made of the target material under different cycles, the same stress ratio, and the same fatigue load range. The number of specimens n+1 (i.e. the number of different cycles n+1), the fatigue load range ΔF, and the crack length a of the specimen for the corresponding number of cycles are recorded.

[0066] The stress intensity factor range ΔK for the corresponding number of cycles can be obtained by mathematical analysis, boundary configuration, or finite element method.

[0067] Substituting the obtained n value, fatigue load range ΔF, crack length a of the specimen under different cycles, and corresponding stress intensity factor range ΔK back into formula (2), we obtain the coefficients k0~k n :

[0068] Where n is determined by the experimenters, it is generally not less than 4.

[0069] The second calibration method:

[0070] A specimen made of the target material is subjected to a fatigue test (i.e., calibration test) with an increasing number of cycles under the same stress ratio and the same fatigue load range to obtain the number of different cycles n+1, the fatigue load range ΔF, and the specimen crack length a corresponding to the number of cycles.

[0071] The stress intensity factor range ΔK for the corresponding number of cycles can be obtained by mathematical analysis, boundary configuration, or finite element method.

[0072] Substituting the obtained n value, fatigue load range ΔF, crack length a of the specimen under different cycles, and corresponding stress intensity factor range ΔK back into formula (2), we obtain the coefficients k0~k n :

[0073] Where n is determined by the experimenters, it is generally not less than 4.

[0074] S5. Based on the principle of the continuous decreasing K method, calculate the crack propagation Δa corresponding to each constant force range. The calculation formula is as follows:

[0075] (3)

[0076] Where ΔK i The range of the i-th level stress intensity factor;

[0077] ΔK i-1 The range of stress intensity factor for level i-1;

[0078] Δx K The stress intensity factor continuously decreases by a fixed value, determined by the user, satisfying Δx. K ≤2%;

[0079] C represents the gradient of K-descent, which is a fixed value determined by the user and satisfies C ≥ -0.1 mm. -1 ;

[0080] S6. Determine the initial maximum stress σ0 and the initial crack length a0; calculate the stress range Δσ for each stage based on the principle of the continuous decreasing K method. i The calculation formula is as follows:

[0081] (4)

[0082] In the formula, Δσ i The stress range of level i, and the stress range of level i Δσ1=(1-R)σ0, where R is the stress ratio;

[0083] Δσ i-1 The stress range of level i-1;

[0084] a i Let be the length of the i-th level crack, given by formula a i =a i-1 +Δa is determined, where the length of the first-order crack a1=a0+Δa;

[0085] a i-1 The length of the (i-1)th level crack;

[0086] It is understandable that the initial maximum stress σ0 satisfies: σ R ≤σ0≤R p0.2 Among them, Rp0.2 σ represents the yield strength of the target material. R The conditional fatigue limit of the target material.

[0087] S7. Based on the fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of the specimen. f The fitting formula is used to obtain the stress range Δσ at each level. i The number of iterations ΔN i Thus, the stress ranges Δσ at each level are obtained. i Crack propagation rate (da / dN) i The calculation formula is as follows:

[0088] Now, let's take the stress range of level i, Δσ i Next loop count ΔN i The calculation method is illustrated with an example:

[0089] Since the crack length at the end of the i-th level load is a i The crack length at the start of the i-th level load is a. i-1 (i.e., the crack length at the end of the (i-1)th level load), substituting these values ​​into formula (1) yields the number of cycles at the end of the i-th level load. Obtain the number of cycles at the start of the i-th level load. Thus, Δσ is obtained under the i-th stress range. i The number of loops below

[0090] S8. Calculate the crack length a at each stage using the relationship between the stress intensity factor range ΔK and the crack length a. i The corresponding stress intensity factor range ΔK i ;

[0091] S9. Draw the stress range Δσ for each level. i Lower crack propagation rate (da / dN) i With the corresponding stress intensity factor range ΔK i The relationship curve is used to obtain the fatigue crack propagation threshold value ΔK. th The fatigue crack propagation threshold value ΔK th For the crack propagation rate da / dN = 10 -7 Range of stress intensity factor at mm / cycle.

[0092] The following are specific embodiments of the present invention:

[0093] The target material is 15Cr alloy, and its room temperature yield strength R p0.2 =1180MPa.

[0094] A set (i.e., multiple) of hourglass-shaped specimens with a center diameter D = 6 mm are processed from the target material. Each specimen includes two heads and a connecting shaft for connecting the two heads. The hourglass-shaped specimen has a centrally symmetrical structure, and its cross-section is circular at any position. The center diameter of the hourglass-shaped specimen refers to the diameter of the cross-section of the connecting shaft.

[0095] like Figure 1 As shown, this invention provides a method for predicting the fatigue crack propagation threshold under alternating stress, the method comprising the following steps:

[0096] S1. Multiple hourglass-shaped specimens were subjected to stress ratios R=0 and different maximum stresses σ. max High-cycle fatigue tests were conducted, and the fatigue failure cycle N and fatigue stress range Δσ (i.e., Δσ = σ) of the failed specimens were recorded. max -σ min =σ max -Rσ max =σ max The failed sample was one with fewer than 10 cycles. 7 The fatigue failure cycle N is the actual number of cycles of the failed specimen when the crack extends to the critical length.

[0097] The high-cycle fatigue test was conducted using a GPS2000 high-frequency vibration fatigue testing machine manufactured by Changchun Testing Machine Factory. When the crack in the specimen extends to the critical crack length, the resonant frequency deviates beyond the set value, causing the fatigue testing machine to automatically stop. At this point, if the number of cycles for the specimen is less than 10... 7 If the test specimen fails, it is considered a failed specimen.

[0098] If the number of cycles for the sample is greater than 10 7 If the fatigue testing machine does not automatically stop at this time, the specimen is considered to be a non-failed specimen; the conditional fatigue limit σ is then obtained. R =820 MPa; Conditional fatigue limit σ R For corresponding to 10 7 The critical stress amplitude applied to the specimen during each cycle without causing specimen failure.

[0099] S2. After marking the crack lead edge of each failed specimen using the heat-dyeing method or the secondary fatigue method, each failed specimen is broken to obtain the critical crack length 'a' of the corresponding failed specimen. f ;

[0100] S3. The fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of each failed specimen. f The first fitting parameter A = 1.26 × 10⁻⁶ was obtained by fitting the data. -54With the first fitting exponent m = 16.211, the fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of the specimen were obtained. f The first fitting formula: a f =NA(Δσ) m (1);

[0101] S4. Perform five fatigue tests (high-cycle fatigue test or low-cycle fatigue test) on the hourglass-shaped specimen with a fixed stress ratio and fixed load range at five different cycle numbers. Record the number of different cycle numbers (n+1=5), the fatigue load range ΔF, and the specimen crack length a for the corresponding cycle number. Then, use the finite element method to obtain the stress intensity factor range ΔK of the crack for the corresponding cycle number. Finally, substitute the n value obtained from the test, the fatigue load range ΔF, the specimen crack length a for different cycle numbers, and the stress intensity factor range ΔK obtained from the finite element method into the relationship between the stress intensity factor range ΔK and the crack length a, and calculate the coefficients k0=0.0461, k1=-0.2416, k2=1.5233, k3=-3.386, k4=3.1884, thereby obtaining the relationship between the specimen stress intensity factor range ΔK and the specimen crack length a.

[0102]

[0103] S7. Based on the principle of the continuous decreasing K method, calculate the crack propagation Δa corresponding to each constant force range. The calculation formula is as follows:

[0104] (3)

[0105] Where ΔK i The range of the i-th level stress intensity factor;

[0106] ΔK i-1 The range of stress intensity factor for level i-1;

[0107] Δx K The stress intensity factor decreases continuously by a fixed value, satisfying Δx. K ≤2%;

[0108] C is the gradient of decreasing K, which is a fixed value that satisfies C ≥ -0.1 mm. -1 ;

[0109] In this embodiment, Δx is preferred. K =0.1%, C = -0.1mm -1 At this point, the crack propagation amount Δa = 0.01 mm corresponding to each constant force range can be calculated using formula (3).

[0110] S6. Determine the initial maximum stress σ0 and the initial crack length a0; calculate the stress range Δσ for each stage based on the principle of the continuous decreasing K method. i The calculation formula is as follows:

[0111] (4)

[0112] In the formula, Δσ i The stress range of level i, and the stress range of level i Δσ1=(1-R)σ0, where R is the stress ratio;

[0113] Δσ i-1 The stress range of level i-1;

[0114] a i Let be the length of the i-th level crack, given by formula a i =a i-1 +Δa is determined, where the length of the first-order crack a1=a0+Δa;

[0115] a i-1 The length of the (i-1)th level crack;

[0116] Due to the length a of each level of crack i The crack length a from the previous level i-1 Determine (i.e., satisfy formula a) i =a i-1 +Δa), therefore, when the initial crack length a0=0mm, the first-order crack length a1=0.01mm; the second-order crack length a2=0.02mm; the third-order crack length a3=0.03mm; the fourth-order crack length a4=0.04mm;

[0117] Since the initial maximum stress σ0 (i.e., the first-level maximum stress σ1) satisfies: σ R ≤σ0≤R p0.2 Among them, R p0.2 =1180 MPa, where σ is the yield strength of the target material. R =820MPa, which is the conditional fatigue limit of the target material. Therefore, in this embodiment, the initial maximum stress σ0 can be 1100MPa, 950MPa, or 820MPa; according to the maximum stress σ at each level i =Δσ i / (1-R)=Δσ i (R=0) The first-level maximum stress range Δσ1 under different initial maximum stresses σ0 can be calculated, and then the maximum stress ranges Δσ of the other levels can be obtained according to formula (4). i ; Stress range Δσ at different initial maximum stresses σ0 i As shown in Table 1:

[0118] Table 1: Stress range Δσ at different initial maximum stresses σ0 i

[0119]

[0120] Based on the obtained crack lengths a at each level i and the maximum stress range Δσ at each level i It can plot the crack length a at different levels under different initial maximum stresses σ0. i With the maximum stress Δσ at each level i Relationship curves, such as Figure 2 As shown.

[0121] S7. Based on the fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of the specimen. f The fitting formula is used to obtain the stress range Δσ at each level. i The number of loops below This allows us to obtain the stress range Δσ at each level. i Crack propagation rate (da / dN) i The calculation formula is as follows:

[0122] Substituting the data from Table 1 into formula (5), we can obtain the stress range Δσ for each stage under different initial maximum stresses σ0. i The corresponding crack propagation rate (da / dN) i The calculation results are shown in Table 2.

[0123] Table 2: Crack propagation rates at different initial maximum stresses σ0 (da / dN) i

[0124]

[0125] S8. Using the relationship between the stress intensity factor range ΔK and the crack length a of the specimen, calculate the crack length a at each stage under different initial maximum stresses σ0. i The range of stress intensity factor ΔK i (i.e., the range of stress intensity factors ΔK at each level) i As shown in Table 3.

[0126] Table 3: Range of stress intensity factor ΔK at different initial maximum stresses σ0 i

[0127]

[0128] S9. Draw the stress range Δσ for each level. i Lower crack propagation rate (da / dN) i With the corresponding stress intensity factor range ΔKi Relationship curves (such as) Figure 3 As shown, the fatigue crack propagation threshold value ΔK is obtained. th The fatigue crack propagation threshold value ΔK th For the crack propagation rate da / dN = 10 -7 Range of stress intensity factor at mm / cycle.

[0129] Fatigue crack propagation rate tests were conducted on CT specimens made of 15Cr alloy at a stress ratio R=0, and at a fatigue crack propagation rate of 10... -6 mm / cycle~10 -7 Data points within the mm / cycle range were fitted using linear regression, and slope extrapolation was performed to obtain da / dN = 1 × 10⁻⁶. -7 The fatigue crack propagation threshold value corresponding to mm / cycle is 6.1 MPa.m 0.5 The value deviates from the fatigue crack propagation threshold value predicted by the present invention under the condition of maximum initial stress σ0 = 820 MPa and stress ratio R = 0 by no more than 8.9%, which meets the deviation requirement and verifies the effectiveness of the method of the present invention.

[0130] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for predicting the fatigue crack propagation threshold under alternating stress, characterized in that, The method includes the following steps: S1. Conduct high-cycle fatigue tests on multiple specimens made of the target material under the same stress ratio but different fatigue stress ranges, and record the fatigue failure cycle number N and fatigue stress range Δσ of the failed specimens; the failed specimens are those with less than 10 cycles. 7 The specimen in which the crack extends to the critical length after the second failure, wherein the fatigue failure cycle N is the actual number of cycles of the failed specimen; the specimen is a specimen with a circular cross-section; S2. After marking the crack lead edge of each failed specimen, pull each failed specimen apart to obtain the critical crack length 'a' of the corresponding failed specimen. f ; S3. By analyzing the fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of each failed specimen. f By performing fitting, the fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of the specimen were obtained. f The fitting formula: a f =NA(Δσ) m Where A is the first fitting parameter; m is the first fitting index; S4. Calibrate the coefficients in the relationship between the stress intensity factor range ΔK and the crack length a to determine the final relationship between the stress intensity factor range ΔK and the crack length a of the specimen: Where ΔF is the range of fatigue loads borne by the specimen; D is the diameter of the cross-section at the center of the sample; S0 is the original cross-sectional area at the center of the sample, derived from the formula... Sure; The geometric factor coefficient of the sample; k0~k n The coefficients obtained from calibration; n+1 is the number of different cycles in the calibration test, and n is not less than 4; S5. Based on the principle of the continuous decreasing K method, calculate the crack propagation Δa corresponding to each constant force range. The calculation formula is as follows: Where ΔK i The range of the i-th level stress intensity factor; ΔK i-1 The range of stress intensity factor for level i-1; Δx K The stress intensity factor decreases continuously by a fixed value, satisfying Δx. K ≤2%; C is the gradient of decreasing K, which is a fixed value that satisfies C ≥ -0.1 mm. -1 ; S6. Determine the initial maximum stress σ0 and the initial crack length a0; calculate the stress range Δσ for each stage based on the principle of the continuous decreasing K method. i The calculation formula is as follows: In the formula, Δσ i The stress range of level i, and the stress range of level i Δσ1=(1-R)σ0, where R is the stress ratio; Δσ i-1 The stress range of level i-1; a i Let be the length of the i-th level crack, given by formula a i =a i-1 +Δa is determined, where the length of the first-order crack a1=a0+Δa; a i-1 The length of the (i-1)th level crack; S7. Based on the fatigue stress range Δσ, fatigue failure cycle N, and critical crack length a of the specimen. f The fitting formula is used to obtain the stress range Δσ at each level. i The number of iterations ΔN i Thus, the stress ranges Δσ at each level are obtained. i Crack propagation rate (da / dN) i The calculation formula is as follows: S8. Calculate the crack length a at each stage using the relationship between the stress intensity factor range ΔK and the crack length a. i The corresponding stress intensity factor range ΔK i ; S9. Draw the stress range Δσ for each level. i Lower crack propagation rate (da / dN) i With the corresponding stress intensity factor range ΔK i The relationship curve is used to obtain the fatigue crack propagation threshold value ΔK. th The fatigue crack propagation threshold value ΔK th For the crack propagation rate da / dN = 10 -7 Range of stress intensity factor at mm / cycle.

2. The method for predicting the fatigue crack propagation threshold under alternating stress according to claim 1, characterized in that, The calibration process in step S4 includes the following steps: S41. Conduct fatigue tests on n+1 specimens made of the target material under different cycles, the same stress ratio, and the same fatigue load range, and record the number of specimens n+1, the fatigue load range ΔF, and the crack length a of the specimen corresponding to the number of cycles. S42. Obtain the stress intensity factor range ΔK for the corresponding number of cycles; S43. Using the fatigue load range ΔF, the crack length a of the specimen under different cycles, and the corresponding stress intensity factor range ΔK, calculate the coefficients k0~k in the relationship between the stress intensity factor range ΔK and the crack length a of the specimen. n .

3. The method for predicting the fatigue crack propagation threshold under alternating stress according to claim 2, characterized in that, Replace step S41 with: Perform a calibration fatigue test on a specimen made of the target material under the same stress ratio and the same fatigue load range, with the number of cycles increasing sequentially, to obtain the number of different cycle numbers n+1, the fatigue load range ΔF, and the specimen crack length a corresponding to the number of cycles.

4. The method for predicting the fatigue crack propagation threshold under alternating stress according to claim 2 or 3, characterized in that, The stress intensity factor range ΔK corresponding to the number of cycles can be obtained by mathematical analysis, boundary configuration, or finite element method.

5. The method for predicting the fatigue crack propagation threshold under alternating stress according to claim 1, characterized in that, The crack leading edge of the failed specimen in step S2 is marked using either the heat coloring method or the secondary fatigue method.

6. The method for predicting the fatigue crack propagation threshold under alternating stress according to claim 1, characterized in that, The initial maximum stress σ0 satisfies: σ R ≤σ0≤R p0.2 ; Among them, R p0.2 σ represents the yield strength of the target material. R The conditional fatigue limit of the target material.

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