CN115266433A - Fatigue crack propagation threshold value testing method - Google Patents
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Abstract
The invention discloses a fatigue crack propagation threshold value testing method, and belongs to the technical field of metal material fatigue tests. The method mainly solves the problems of experimental data dispersion and inaccurate test result caused by frequent start/stop of the testing machine in the testing process by combining a flexibility method and a K-lowering method on the basis of meeting the test requirement according to the requirement of a fatigue crack propagation method for the fatigue test of the metal material GB/T6398-2017 in the standard of the fatigue crack propagation method. The method can be used for quickly obtaining the accurate fatigue crack propagation threshold value of the metal material, and meanwhile, the method can also be used for reducing the test operation steps and improving the test efficiency and accuracy when the testing machine is saved. The fatigue test period of the material is shortened, and the test cost of material research and development is reduced.
Description
Technical Field
The invention relates to the technical field of metal material fatigue crack propagation rate tests, in particular to a fatigue crack propagation threshold value test method.
Background
The stress condition of the structural material under the actual service working condition is circularly loaded, so the fatigue failure is one of the main failure modes of the engineering component. Failure failures of engineering components caused by fatigue failure are counted by the relevant research departments to be about 80% of all the failure failures of the components. Fatigue fractures tend to be characterized as asymptomatic, abrupt, and catastrophic, with the resulting losses generally being large. Therefore, the fatigue problem of the material is an inevitable main link before the practical application of the component, and the fatigue performance of the material must be accurately, safely and quickly measured to ensure the service safety of the component. Therefore, accurate acquisition of fatigue data of the material is an essential link before the material is actually applied.
The damage tolerance design method is a main method for designing the structural safety of heavy equipment such as airplanes, high-speed rails, ships and the like, and allows structural parts to have certain defects such as cracks, but the existence of the defects such as the cracks cannot influence the safety of the structural parts within a specified service time. By the method, equipment such as airplanes and the like have stronger structures and lighter weight, so that the comprehensive performance and the carrying capacity are more excellent, and the oil consumption is lower. The fatigue crack propagation threshold value is used as an important parameter in fatigue fracture mechanics and is an important index for damage tolerance design of a structural material. The accuracy of the fatigue crack propagation threshold value data directly determines the service reliability of the heavy equipment. Therefore, before structural design of important structural members such as an aircraft and the like, a fatigue crack propagation threshold value of a material needs to be accurately obtained.
Fatigue crack propagation threshold value delta K th Is defined as follows: fatigue crack propagation rate equal to 10 -7 The stress intensity factor range corresponding to mm/cycle, wherein the fatigue crack propagation rate is represented by da/dN, aFor crack length, N is the number of stress cycles. The fatigue crack growth threshold test is a professional and long test process. The fatigue crack length is taken as a test termination condition in the whole test process, the fatigue crack length can be calculated by a visual method or a flexibility method and the like, the visual method needs repeated measurement by a tester, and the method is complex and has large errors. The flexibility method can automatically record and store by using a computer without manual operation, and the calculation formulas are shown as (1) and (2).
a/w＝C o +C 1 U x +C 2 U x 2 +C 3 U x 3 +C 4 U x 4 +C 5 U x 5 (2)；
In the formula: a represents the fatigue crack length, B represents the sample thickness, W represents the distance from the center of the loading hole to the end of the sample, E represents the elastic modulus, vx represents the deformation change amount, and F represents the load; c0 to C5 represent calculation constants.
In the GB/T6398-2017 metal material fatigue crack propagation rate test method, the test method for the fatigue crack propagation threshold value test is specified in detail as follows:
at a crack propagation rate (da/dN) of between 10 -7 mm/cycle～10 -6 Selecting at least 5 pairs of da/dN-delta K (stress intensity factor) data points in the mm/cycle range, and fitting the data points by using a linear regression method under a log-log coordinate system, wherein the recommended fitting formula is shown as a formula (3):
da/dN＝C(△K) m (3)；
in equation (3): c and m are the intercept and slope, respectively, of the best-fit straight line.
According to the result obtained by fitting of the formula (3), calculation is carried out to obtain da/dN =10 -7 The corresponding delta K value at mm/cycle is used as the fatigue crack propagation threshold value delta K th 。
Present fatigue crack propagation thresholdThe compliance method + the constant load method is mainly used in the value test. The dead load method means that a constant load is selected during the fatigue test so that the crack propagation rate is between 10 -7 mm/cycle～10 -6 Stopping the test after the fatigue crack expands for 0.2-0.5 mm crack length between mm/cycle to obtain a pair of da/dN-delta K data points, then continuously reducing the test load, wherein the load change range is not more than 10%, and after the fatigue crack expands for 0.2-0.5 mm crack length, the crack expansion rate approaches 10 -7 After mm/cycle, the load is increased gradually, the load increase range is not more than 10 percent, each crack is expanded by 0.2 to 0.5mm until the crack expansion rate exceeds 10 -6 The test was stopped after mm/cycle.
In the test process, the fatigue testing machine is required to be started/stopped repeatedly, the test is continued after the test load is changed, when the test is stopped, the crack tip of the sample can generate a crack closing effect, so that the error of the crack length calculated by the flexibility method is large, the data fitting error is large due to the fact that the data points da/dN-delta K obtained by the flexibility method are dispersed, and the threshold value delta K obtained by calculation is large th The error is also large, and the maximum error range can reach 40%. Moreover, the test load needs to be changed repeatedly by the tester, the crack length is collected and recorded, and the labor intensity of the tester is high. The test data is unstable, and the error of the test result is large, so that the difficulty in research and development and use of materials is increased, and research and development and equipment of heavy equipment are seriously hindered.
Disclosure of Invention
Aiming at the defects in the existing testing method, the invention provides the fatigue crack propagation threshold value testing method, the method completes the whole fatigue crack propagation threshold value test in one testing program by using a flexibility method and a K-lowering method, avoids the frequent start/stop of a testing machine, avoids the crack closing effect, obtains data points meeting the standard requirements, and has continuous, stable and accurate data.
The K-reducing method is characterized in that a stress intensity factor K is adopted for control in the test process, and the stress intensity factor K is linearly corresponding to the test load, so that the test load can be gradually and slowly reduced at a constant speed under the control of a computer, but the test load is always applied to two ends of a test sample in the whole process until cracks are continuously obtainedThe fringe spreading rate (da/dN) is between 10 -7 mm/cycle～10 - 6 Stopping the assay for at least 5 da/dN- Δ K data points in the mm/cycle range. The fatigue testing machine cannot be stopped to replace the testing load in the whole testing process, and the tip of the testing crack is always pulled, so that the tip of the testing crack cannot be closed. Therefore, the obtained da/dN-delta K data points are continuous and stable, the data linearity is good, the data fitting precision is high, the error does not exceed 10%, and the threshold value delta K obtained by calculation th The method has the advantages of being accurate, greatly improving the efficiency and accuracy of the fatigue crack propagation threshold value test, reducing the test cost of material research and development, providing reliable reference for the design and use of subsequent structural member materials, and ensuring the service reliability of major equipment.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a test method for a fatigue crack propagation threshold value uses a Compact Tension (CT) sample, a method for calculating the crack length by a computer is a flexibility method, and a control mode adopted in a fatigue crack propagation threshold value test process is a constant force control K-drop method.
The test method specifically comprises the following steps:
(1) Preparing a metal material Compact Tension (CT) sample and measuring the size of the sample;
(2) Installing a sample and a COD gauge on a fatigue testing machine, and inputting parameters such as the size, the material and the like of the sample; a COD gauge is arranged at the position of the sample knife edge;
(3) Performing fatigue preloading, and confirming that the crack length calculated by a flexibility method is consistent with the actual crack length by modifying the elastic modulus, so that the crack length is accurately calculated;
(4) Setting the control mode as a flexibility method and a K reduction method: calculating according to the formula (5) and the formula (6) to obtain the corresponding stress intensity factor range delta K and the sample fatigue crack propagation rate da/dN data, and continuously gradually reducing the stress intensity factor of the tip of the test fatigue crack step by step according to the formula (4) by using a K reduction method; inputting K gradient value C, wherein C is between-0.05 and-0.15;
(5) Setting a greater terminal fatigue crack length to be equalStarting the test at W in the control mode set in the step (4), and applying test loads to the two ends of the test sample until a crack propagation rate (da/dN) of 10 is continuously obtained -7 mm/cycle～10 -6 Stopping the assay for at least 5 da/dN- Δ K data points in the mm/cycle range;
(6) Fitting the data points obtained in the step (5) by using a linear regression method according to a formula (3) in a log-log coordinate system, and calculating and obtaining da/dN =10 according to a fitting result -7 The delta K value corresponding to mm/cycle is used as the fatigue crack propagation threshold value delta K th 。
In the test process in the step (5), the test load is controlled by adopting a stress intensity factor delta K, the stress intensity factor delta K is linearly corresponding to the test load, and the stress intensity factor delta K is controlled by a computer to gradually and slowly reduce the test load at a constant speed (namely a K gradient value C).
The stress intensity factor Δ K is calculated according to formula (5):
in equation (5): Δ K represents the range of stress intensity factors to which the specimen crack tip is subjected, related to specimen size, crack length and load; f represents the test load, B represents the thickness of the sample, and alpha represents the ratio of the fatigue crack length a to W; measuring the variation of the opening displacement of the crack tip of the sample by a COD gauge, and calculating the real-time crack length of the sample by using a flexibility method.
In the test process of the step (5), the fatigue testing machine cannot be stopped to replace the test load, and the tip of the test crack is always pulled, so that the tip of the test crack cannot generate a closing effect.
In the step (4), the fatigue crack growth rate da/dN represents the fatigue crack growth amount of one cycle, and the calculation formula is formula (6):
da/dN＝△a/N f (6)；
in equation (6): Δ a represents the fatigue crack growth (length of increase of fatigue crack in cycle), N f Indicating fatigueCycle life required for crack propagation.
In the step (4), the stress intensity factor is gradually and slowly reduced step by step and is calculated according to the formula (4);
ΔK i(j) ＝ΔK i(j-1) e CΔa(j-1) (4)；
in equation (4): delta K i(j) Representing the j-th level initial stress intensity factor range; delta K i(j-1) Representing the range of the j-1 st level initial stress intensity factor; delta a (j-1) The crack expansion amount corresponding to the previous stage constant load is generally 0.05-0.15 mm; c represents a delta K gradient value, and C = -0.05 to-0.15 (when C is more than 0, the K increasing method is represented, and when C is less than 0, the K decreasing method is represented).
The principle and the beneficial effects of the invention are as follows:
1. the fatigue crack propagation threshold value testing method effectively solves the problem that the fatigue testing machine measures the fatigue crack propagation threshold value delta K at the present stage th The novel test method adopts a mode of a flexibility method and a K reduction method, avoids the problem of repeated start and stop of a test machine in the test process, and gradually and slowly reduces the stress intensity factor K at a constant speed (C = -0.1, C represents a K gradient value) under the control of a computer, so that continuous and stable da/dN-delta K data points are obtained, the data fitting precision is high, and the calculated threshold value delta K is obtained th Is accurate. The whole test process testing machine applies load to the test sample all the time, and the machine cannot be stopped. The problem that the test result is inaccurate due to data dispersion caused by the fact that the crack tip of the test sample is closed due to frequent shutdown of the testing machine is avoided. Practice proves that: by using the method, the error of the test result can be improved to be within 10% from the original 40%, the test accuracy is greatly improved, and the structural design safety and the service reliability of the major equipment design are ensured.
2. The invention mainly utilizes a flexibility method and a K reduction method to meet the requirement of test standard, and crack propagation rate (da/dN) is between 10 -7 mm/cycle～10 -6 Selecting at least 5 pairs of dadN-delta K (stress intensity factor range) in mm/cycle rangeFitting the data points by linear regression under a log-log coordinate system to obtain da/dN =10 -7 Fatigue crack propagation threshold value delta K corresponding to mm/cycle th . On the premise of ensuring the accuracy of the test result, the time required by the test is greatly reduced, the test efficiency is improved, and the research and development and service cycle of materials are reduced, so that the production and use of heavy equipment are accelerated.
Drawings
Figure 1 is a compact tensile specimen size diagram.
FIG. 2 is a schematic diagram of a constant force controlled K-drop method.
FIG. 3 is a schematic diagram of crack tip stress of a sample by a K-lowering method.
Detailed Description
For further understanding of the present invention, the present invention will be described with reference to the following examples, which are provided for the purpose of further illustrating the features and advantages of the present invention, and are not intended to limit the scope of the present invention as set forth in the appended claims.
Example 1:
in this embodiment, the fatigue crack propagation threshold is measured, and the test method is used for measuring the fatigue crack propagation threshold of the metal material. The test method uses Compact Tensile (CT) samples, compact tensile tests with different sizes are common to the test method, and the shapes and the sizes of the samples are shown in figure 1.
The test method specifically comprises the following steps:
(1) Preparing a Compact Tensile (CT) sample of a metal material and measuring the size of the sample as shown in FIG. 1;
(2) Mounting a sample and a COD gauge on a fatigue testing machine, and inputting the size and material parameters of the sample; a COD gauge is arranged at the position of a sample knife edge; the flexibility method calculation parameters corresponding to different COD gauge mounting positions are different;
(3) Performing fatigue preloading, and modifying the elastic modulus to enable the crack length obtained by calculation by adopting a flexibility method and a constant load method to be consistent with the actual crack length, so as to confirm that the crack length calculation is accurate;
(4) Setting the control mode as a flexibility method and a K reduction method: calculating according to the formula (5) and the formula (6) to obtain the corresponding stress intensity factor range delta K and the sample fatigue crack propagation rate da/dN data, and continuously gradually reducing the stress intensity factor of the tip of the test fatigue crack step by step according to the formula (4) by using a K reduction method; inputting a K gradient value C, wherein the C is-0.1; as shown in fig. 2.
(5) Setting a larger terminal fatigue crack length equal to W, and starting the test in the control manner set in the step (4), applying test loads to both ends of the test piece until a crack propagation rate (da/dN) of 10 is continuously obtained -7 mm/cycle～10 -6 Stopping the assay for at least 5 da/dN- Δ K data points in the mm/cycle range;
in the test process of the step (5), the test load is controlled by a stress intensity factor delta K, the stress intensity factor delta K is linearly corresponding to the test load, and the stress intensity factor delta K is controlled by a computer to gradually and slowly reduce the test load at a constant speed (namely a K gradient value C). The stress intensity factor Δ K is calculated according to formula (5):
in equation (5): Δ K represents the range of stress intensity factors to which the sample crack tip is subjected, in relation to the sample size, crack length and load; f represents the test load, B represents the thickness of the sample, and alpha represents the ratio of the fatigue crack length a to W; measuring the variation of the opening displacement of the crack tip of the sample by a COD gauge, and calculating the real-time crack length of the sample by using a flexibility method.
In the test process of the step (5), the fatigue testing machine cannot be stopped to replace the test load, and the tip of the test crack is always pulled, so that the tip of the test crack cannot generate a closing effect.
The fatigue crack growth rate da/dN represents the fatigue crack growth amount of one cycle, and the calculation formula is formula (6):
da/dN＝△a/N f (6)；
in equation (6): Δ a represents the fatigue crack growth (length of increase of fatigue crack in cycle), N f Means for indicating fatigue crack propagationThe required cycle life.
(6) Fitting the data points obtained in the step (5) by a linear regression method according to a formula (3) under a log-log coordinate system, and calculating and obtaining da/dN =10 according to a fitting result -7 The delta K value corresponding to mm/cycle is used as the fatigue crack propagation threshold value delta K th . Calculating the stress intensity factor gradually and slowly decreased step by step according to a formula (4);
ΔK i(j) ＝ΔK i(j-1) e CΔa(j-1) (4)；
in equation (4): delta K i(j) Representing the j-th level initial stress intensity factor range; delta K i(j-1) Representing the range of the initial stress intensity factor of the j-1 st level; delta a (j-1) The crack expansion amount corresponding to the previous stage constant load is generally 0.05-0.15 mm; c represents a Delta K gradient value, and C is-0.1.
By adopting the embodiment for testing, the crack closure effect does not occur, an accurate fatigue crack propagation rate threshold value result is obtained through calculation, processing and fitting, and the test data is continuous, stable and high in accuracy. As shown in fig. 3.
When the testing method is used, the COD gauge qualified by the third party is required to be used, and the load precision of the fatigue testing machine meets the testing requirement. The shape of the sample is a Compact Tensile (CT) sample, the size of the sample meets the standard requirement, and the test process cannot be stopped midway. This test method cannot be used in corrosive environments.
The testing method meets the requirement of the fatigue crack propagation threshold value test in the fatigue crack propagation method of the GB/T6398-2017 metal material fatigue test.
The testing method is based on the crack propagation rate testing software, so that the testing time can be saved, the testing efficiency can be improved, and the testing cost can be reduced.
Claims (8)
1. A fatigue crack propagation threshold value test method is characterized by comprising the following steps: the test method uses a Compact Tension (CT) sample as a sample, a method for calculating the crack length by a computer is a flexibility method, and a control mode adopted in the fatigue crack propagation threshold value test process is a constant force control K-drop method.
2. The fatigue crack propagation threshold test method according to claim 1, characterized in that: the test method specifically comprises the following steps:
(1) Preparing a metal material Compact Tension (CT) sample and measuring the size of the sample;
(2) Mounting a sample and a COD gauge on a fatigue testing machine, and inputting parameters such as the size, the material and the like of the sample; a COD gauge is arranged at the position of the sample knife edge;
(3) Performing fatigue preloading, and confirming that the crack length calculated by a flexibility method is consistent with the actual crack length by modifying the elastic modulus, so that the crack length is accurately calculated;
(4) Setting the control mode as a flexibility method and a K-lowering method: calculating according to the formula (5) and the formula (6) to obtain the corresponding stress intensity factor range delta K and the sample fatigue crack propagation rate da/dN data, and continuously gradually reducing the stress intensity factor of the tip of the test fatigue crack step by step according to the formula (4) by using a K reduction method; wherein, the gradient value C of the input K is between-0.05 and-0.15;
(5) Setting a larger terminal fatigue crack length equal to W, starting the test in the control mode set in the step (4), and applying test loads to two ends of the test sample until a crack propagation rate (da/dN) of 10 is continuously obtained -7 mm/cycle～10 -6 Stopping the assay for at least 5 da/dN- Δ K data points in the mm/cycle range;
(6) Fitting the data points obtained in the step (5) by a linear regression method according to a formula (3) under a log-log coordinate system, and calculating and obtaining da/dN =10 according to a fitting result -7 The delta K value corresponding to mm/cycle is used as the fatigue crack propagation threshold value delta K th 。
3. The fatigue crack growth rate threshold test method according to claim 1, wherein: the crack length of the sample is calculated by a flexibility method as shown in formulas (1) and (2):
a/w＝C o +C 1 U x +C 2 U x 2 +C 3 U x 3 +C 4 U x 4 +C 5 U x 5 (2)；
in equations (1) to (2): b represents the specimen thickness, W represents the distance from the center of the loading hole to the end of the specimen, E represents the elastic modulus, vx represents the amount of change in strain, F represents the load, C0 to C5 represent calculation constants, and a represents the fatigue crack length.
4. The fatigue crack growth rate threshold test method according to claim 2, characterized in that: in the test process in the step (5), the test load is controlled by a stress intensity factor delta K, the stress intensity factor delta K is linearly corresponding to the test load, and the stress intensity factor delta K is controlled by a computer to gradually and slowly reduce the test load at a constant speed (namely a K gradient value C).
5. The fatigue crack growth rate threshold test method of claim 4, wherein: the stress intensity factor Δ K is calculated according to formula (5):
in equation (5): Δ K represents the range of stress intensity factors to which the specimen crack tip is subjected, related to specimen size, crack length and load; f represents the test load, B represents the thickness of the sample, and alpha represents the ratio of the fatigue crack length a to W; and measuring the variation of the opening displacement of the crack tip of the sample by a COD gauge, and calculating the real-time crack length of the sample by using a compliance method.
6. The fatigue crack growth rate threshold test method according to claim 2, characterized in that: in the test process of the step (5), the fatigue testing machine cannot be stopped to replace the test load, and the tip of the test crack is always pulled, so that the tip of the test crack cannot generate a closing effect.
7. The fatigue crack growth rate threshold test method according to claim 2, wherein: in the step (4), the fatigue crack growth rate da/dN represents the fatigue crack growth amount of one cycle, and the calculation formula is formula (6):
da/dN＝△a/N f (6)；
in equation (6): Δ a represents the fatigue crack growth (length of increase of fatigue crack in cycle), N f Indicating the cycle life required for the amount of fatigue crack propagation.
8. The fatigue crack growth rate threshold test method according to claim 2, characterized in that: in the step (4), the stress intensity factor is gradually and slowly reduced step by step and is calculated according to the formula (4);
ΔK i(j) ＝ΔK i(j-1) e CΔa(j-1) (4)；
in equation (4): delta K i(j) Representing the j-th level initial stress intensity factor range; delta K i(j-1) Representing the range of the j-1 st level initial stress intensity factor; delta a (j-1) The crack expansion amount corresponding to the previous stage constant load is generally 0.05-0.15 mm; c represents a Delta K gradient value, and C is-0.05 to-0.15.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN116773667A (en) * | 2023-06-15 | 2023-09-19 | 上海发电设备成套设计研究院有限责任公司 | Method and device for monitoring crack safety of rotor blade root groove of nuclear turbine |
| CN117954022A (en) * | 2024-03-25 | 2024-04-30 | 洛阳船舶材料研究所(中国船舶集团有限公司第七二五研究所) | Estimation method of fatigue crack growth rate of metal material under elastoplastic condition |
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN116773667A (en) * | 2023-06-15 | 2023-09-19 | 上海发电设备成套设计研究院有限责任公司 | Method and device for monitoring crack safety of rotor blade root groove of nuclear turbine |
| CN116773667B (en) * | 2023-06-15 | 2024-05-24 | 上海发电设备成套设计研究院有限责任公司 | Method and device for monitoring crack safety of rotor blade root groove of nuclear turbine |
| CN117954022A (en) * | 2024-03-25 | 2024-04-30 | 洛阳船舶材料研究所(中国船舶集团有限公司第七二五研究所) | Estimation method of fatigue crack growth rate of metal material under elastoplastic condition |
| CN117954022B (en) * | 2024-03-25 | 2024-06-07 | 洛阳船舶材料研究所(中国船舶集团有限公司第七二五研究所) | Estimation method of fatigue crack growth rate of metal material under elastoplastic condition |
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