CN114216803A - High cycle fatigue full-life prediction method for metal material - Google Patents

High cycle fatigue full-life prediction method for metal material Download PDF

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CN114216803A
CN114216803A CN202111168022.5A CN202111168022A CN114216803A CN 114216803 A CN114216803 A CN 114216803A CN 202111168022 A CN202111168022 A CN 202111168022A CN 114216803 A CN114216803 A CN 114216803A
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张丽
刘尧
李旭东
焦泽辉
武亮亮
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AECC Beijing Institute of Aeronautical Materials
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    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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    • G01N3/32Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
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    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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Abstract

The invention relates to a high cycle fatigue full-life prediction method of a metal material, which comprises the following steps: firstly, predicting the fatigue initiation life of a metal material by adopting a microstructure model based on a nonlinear dislocation dipole mechanism; calculating the initial size of the crack according to a formula; then carrying out fatigue crack propagation tests of small cracks and long cracks on the metal material to be tested under different stress ratios, and calculating effective stress intensity factors of the obtained crack propagation data by adopting a crack closed model so as to further obtain a relational expression of the propagation rates of the small cracks and the long cracks and the range of the effective stress intensity factors*Calculating a under different stresses according to a formula*(ii) a Depending on the fracture toughness K of the materialICAnd maximum stress σmaxCalculating the critical crack size a under the stress condition by a formulaf(ii) a And finally, predicting the fatigue total life of the material. The invention comprehensively considers the influence of the microstructure and the small crack behavior of the metal material on the fatigue life, and provides a more accurate prediction method of the high cycle fatigue full life of the metal material.

Description

High cycle fatigue full-life prediction method for metal material
Technical Field
The invention relates to a high cycle fatigue full-life prediction method for a metal material, and belongs to the field of material science and engineering application.
Background
The material and the component before fatigue fracture comprise two stages: the crack initiation stage and the crack propagation stage, and accordingly the fatigue life, are also divided into a crack initiation life and a crack propagation life. However, the boundary between crack initiation and propagation is difficult to determine, which is also a major difficulty in establishing a fatigue life prediction model. The material scientists studying the microscopic mechanism of fatigue consider the micron-scale crack nucleation of slip bands and grain boundaries, and the roughening of the fatigue test piece surface as the crack initiation phase of fatigue failure. On the other hand, engineers working in practice tend to associate the resolution limit of a non-destructive testing crack apparatus with fatigue crack nucleation, which is considered in the design as the starting size of the crack. There are different views on the application of fatigue crack propagation, and the research on the propagation characteristics of long cracks of materials and the method for predicting the propagation performance of long cracks are mature, but as the research on small cracks of materials is deepened, the existence of small crack effect is found in more and more metal materials, namely, the small cracks can still propagate under the condition of being lower than the threshold value of the long cracks, and the propagation rate of the small cracks is higher than that of the long cracks under the same nominal stress intensity factor.
Numerous studies have shown that turbine engine materials have a large variability in fatigue life, which indicates that microstructure properties have a large impact on fatigue life, and the uniqueness of the small crack propagation behavior also requires that it must be taken into account in the fatigue life prediction.
At present, in the aspect of damage tolerance design, aiming at small crack propagation data obtained by a test, the small crack propagation data is generally used for verifying the existence of a small crack effect, a crack closing model is generally adopted for processing in the aspect of service life prediction to obtain the relation between an effective stress intensity factor and a crack propagation rate, the relation is compared with long crack propagation data, and then the long crack propagation rate or a long crack threshold value is corrected, or the small crack propagation behavior is adopted to predict the fatigue full life. The former mainly takes long crack propagation, no clear demarcation point exists between long cracks and short cracks, and small crack propagation data provides correction of a long crack threshold value or corrects a long crack propagation rate expression, so that the application of the small crack propagation rate is insufficient; the latter considers that the fatigue life is mainly consumed in the small crack growth stage, and the long crack growth stage is neglected. Therefore, both methods have certain limitations, and both methods fail to reflect the progress of fatigue crack propagation more truly.
Disclosure of Invention
The invention provides a method for predicting the fatigue total life of a metal material, which is designed and provided aiming at the prior art situation, the method considers the influence of the microstructure of the material on the initiation life, divides the process from micro-crack to fracture of a fatigue crack into a small crack expansion stage and a long crack expansion stage, determines a knee intersection point with physical significance in the two stages by analyzing the data of the long crack and the short crack, and provides a method for calculating the length of the crack at the knee intersection point so as to provide a more accurate method for predicting the fatigue total life of the high cycle.
The purpose of the invention is realized by the following technical scheme:
the method for predicting the high cycle fatigue full life of the metal material comprises the following steps:
step one, calculating the crack initiation life N according to a formula (1)i
Figure BDA0003288759650000021
In the formula: Δ σ is the stress range, Δ σeTo the fatigue limit, aiFor the initial crack size, calculated according to the formula (2), μ is the shear modulus, M is the Taylor constant, λ is the constant (0.005), h is the width of the slip band, D is the grain size, ν is the Poisson's ratio, α is the dependence on the slip angle and the layerThe value of the energy error constant is more than 0 and less than or equal to 1;
step two, calculating the initial crack size a according to a formula (2)i
Figure BDA0003288759650000022
In the formula:
Figure BDA0003288759650000023
for long crack threshold values, F (a) as boundary correction factors, a for defined conditions, such as temperature and stress ratio determinationiIs a determined constant;
thirdly, performing fatigue crack propagation tests of long cracks and small cracks on the metal material to be tested under different stress ratios, calculating crack propagation rates of the small cracks and the long cracks by adopting a secant method and a seven-point increasing method respectively, calculating corresponding delta K values according to stress intensity factor calculation formulas given in standards or stress intensity factor manuals aiming at different sample shapes, and further determining material parameters C and n of the material at the stages of the long cracks and the short cracks respectively;
3.1 analyzing the propagation data of the long and short cracks under different stress ratios by adopting a crack closing model to obtain an effective stress intensity factor delta KeffAnd further obtaining the relation between the small crack propagation rate and the effective stress intensity factor, and an expression thereof:
Figure BDA0003288759650000031
and the relationship between the propagation rate of long crack and the effective stress intensity factor, and the expression thereof
Figure BDA0003288759650000032
3.2 putting the data of the propagation rates of the long and short cracks under the same coordinate system, finding that the typical propagation curves of the long and short cracks usually intersect at the knee intersection point, and the point pairThe effective stress intensity factor should be in the range of Δ K*According to the formula:
Figure BDA0003288759650000033
can calculate the crack length value a corresponding to the knee intersection point under different stress conditions*Parts of the material may not have "knee intersections";
step four, calculating the critical crack length a according to a formula (6)f
Figure BDA0003288759650000034
KICThe fracture toughness of the material under the test condition;
step five, respectively obtaining the germination life N from the step one to the step fouriInitial crack size aiSmall crack and length crack propagation curve "knee intersection" crack length value a*Critical crack size afAnd the propagation rates of the small cracks and the long cracks and the effective stress intensity factor, and calculating the fracture life of the test sample under different stress conditions according to a formula (7):
Figure BDA0003288759650000035
the technical scheme of the invention is characterized in that firstly, a microstructure model based on a nonlinear dislocation dipole mechanism is adopted to predict the fatigue initiation life of a metal material; according to the formula
Figure BDA0003288759650000041
Calculating the initial size of the crack; then, carrying out small crack and long crack fatigue crack propagation tests under different stress ratios on the metal material to be tested, and calculating effective stress intensity factors of the obtained crack propagation data by adopting a crack closed model so as to obtain a relational expression of the small crack and long crack propagation rates and the range of the effective stress intensity factorsBecause the crack propagation rates of the long crack and the small crack are generally greatly different near a threshold value, two groups of data are put together to present a bilinear characteristic, the typical propagation curves of the long crack and the small crack intersect at one point, which is defined as a knee intersection point, and the effective stress intensity factor range corresponding to the point is delta K*According to the formula
Figure BDA0003288759650000042
Calculating a under different stresses*(ii) a Depending on the fracture toughness K of the materialICAnd maximum stress σmaxBy the formula
Figure BDA0003288759650000043
Calculating the critical crack size a under the stress conditionf(ii) a And finally, predicting the fatigue total life of the material.
In the technical scheme of the invention, the fatigue initiation life, the small crack propagation life and the long crack propagation life of the material are comprehensively considered in the fatigue total life prediction method, and the fatigue total life prediction method is subjected to characteristic stage decomposition.
In the technical scheme of the invention, the fatigue life prediction formula adopts a dislocation dipole mechanism method to characterize the slip band on the surface or the subsurface of the material, which accords with most of material crack initiation modes, and the model comprehensively considers factors such as stress conditions, grain sizes, crack shapes and the like.
In the technical scheme of the invention, the influence of the material fatigue small crack effect on the fatigue life is considered, the behavior of the small crack in the expansion stage is represented by a Paris formula, the method has wide application significance, and the method is suitable for the material with the fatigue small crack still expanding in the area lower than the threshold value and the crack expansion rate higher than that of the material with the long crack.
In the technical scheme of the invention, the knee intersection point a of the long and short cracks is determined*Value and critical crack length afThe calculation of (A) is more pertinent, and different F are selected according to samples with different shapes and sizesIThe obtained crack length characteristic value is more accurate, so that the crack length characteristic value is predictedThe fatigue life of the device is more accurate.
In the implementation of the technical scheme, the method is more suitable for aeroengine materials, the initiation life of the aeroengine materials is usually not negligible, and most of the aeroengine materials have a fatigue small crack effect, namely, the fatigue small crack still extends in a region lower than a long crack threshold value, or the propagation rate of the small crack is higher than that of the long crack.
The invention comprehensively considers the influence of the microstructure and the small crack behavior of the metal material on the fatigue life, and provides a more accurate prediction method of the high cycle fatigue full life of the metal material.
Drawings
FIG. 1 is a flow chart of the high cycle fatigue life prediction method for metal materials of the present invention
FIG. 2 is a typical propagation curve of long and short cracks and the definition of knee intersection point according to the present invention
FIG. 3 is a typical microstructure morphology of Ti-6Al-4V alloy in the examples
FIG. 4 is a graph showing the crack growth rate and the effective stress intensity factor according to the example and the knee intersection point determination
FIG. 5 is a graph of fatigue test data and fatigue life predicted by the method of the present invention in examples
The specific implementation mode is as follows:
the invention is further invented by combining the embodiment and the attached drawings.
In the embodiment of the invention, a method for predicting the fatigue full life of a material is shown in a flow chart of the method in figure 1, and the method takes Ti-6Al-4V alloy as an example and comprises the following steps:
first, the information of each parameter of the material is determined. Microstructure observation can be carried out on the tested material Ti-6Al-4V alloy to obtain material grain size information, as shown in figure 3, a typical microstructure of the Ti-6Al-4V alloy is given, as can be seen from the figure, the microstructure contains 60% of initial alpha phase and 40% of alpha + beta phase, and the equivalent size of the alpha phase and the alpha + beta phase is comprehensively considered to be 13.7 mu m for calculating the germination life. The Taylor constant is taken to be 2 and α is taken to be 0.6. The shear modulus mu of the material is 4.4e +04MPa under the condition of room temperature stress ratio of 0.1, the Poisson ratio v is 0.333, the width h of a slip band is 5.00e-02, and the fatigue limit delta sigma iseIs 340MPaThreshold value of long crack
Figure BDA0003288759650000051
Is composed of
Figure BDA0003288759650000052
Fracture toughness KICIs composed of
Figure BDA0003288759650000053
Step one, calculating the crack initiation life N according to a formula (1)i
Figure BDA0003288759650000054
Different attainable germination lives, a, for different stress ranges Δ σiThe initial crack size was calculated according to equation (2), and λ was constant (0.005).
Step two, calculating the initial crack size a according to a formula (2)i
Figure BDA0003288759650000061
In the formula:
Figure BDA0003288759650000062
is the threshold value of the long crack, F (a) is a boundary correction factor, and the initial crack size a is determined under the determined test conditionsiFor the determination of the values, a is calculated by the above formula for the conditions of room temperature and stress ratio R equal to 0.1iAbout 9 μm;
thirdly, performing fatigue crack propagation tests of long cracks and small cracks on the metal material to be tested under different stress ratios, calculating crack propagation rates of the small cracks and the long cracks by adopting a secant method and a seven-point increasing method respectively, calculating corresponding delta K values according to stress intensity factor calculation formulas given in standards or stress intensity factor manuals aiming at different sample shapes, and further determining material parameters C and n of the material at the stages of the long cracks and the short cracks respectively;
3.1 analyzing the propagation data of the long and short cracks under different stress ratios by adopting a crack closing model to obtain an effective stress intensity factor delta KeffAnd further obtaining the relation between the small crack propagation rate and the effective stress intensity factor, and an expression thereof:
Figure BDA0003288759650000063
and the relationship between the propagation rate of long crack and the effective stress intensity factor, and the expression thereof
Figure BDA0003288759650000064
3.2 putting the data of the propagation rate of the long and short cracks in the same coordinate system, finding that the typical propagation curves of the long and short cracks usually intersect at the knee intersection point, and the corresponding effective stress intensity factor range of the point is delta K*According to the formula:
Figure BDA0003288759650000065
can calculate the crack length value a corresponding to the knee intersection point under different stress conditions*FIG. 4 shows the data of the effective stress intensity factor for the propagation rates of small and long cracks, and the effective stress intensity factor Δ K corresponding to the knee intersection of the long and short cracks*Is about
Figure BDA0003288759650000066
The crack length a corresponding to different stress amplitudes can be calculated by the formula (5).
Step four, calculating the critical crack length a according to a formula (6)f
Figure BDA0003288759650000071
KICThe fracture toughness of the material under the test condition;
step five, respectively obtaining the germination life N from the step one to the step fouriInitial crack size aiSmall crack and length crack propagation curve "knee intersection" crack length value a*Critical crack size afAnd the propagation rates of the small cracks and the long cracks and the effective stress intensity factor, and calculating the fracture life of the test sample under different stress conditions according to a formula (7):
Figure BDA0003288759650000072
FIG. 5 shows the experimental points of the Ti-6Al-4V alloy at room temperature and with a stress ratio R of 0.1, and the life prediction curve obtained by the method of the present invention.

Claims (1)

1. A high cycle fatigue full-life prediction method of a metal material is characterized by comprising the following steps: the method comprises the following steps:
step one, calculating the crack initiation life N according to a formula (1)i
Figure FDA0003288759640000011
In the formula: Δ σ is the stress range, Δ σeTo the fatigue limit, aiThe initial crack size is obtained by calculation according to a formula (2), mu is a shear modulus, M is a Taylor constant, lambda is a constant (0.005), h is the width of a slip band, D is the size of a crystal grain, v is a Poisson ratio, alpha is a constant depending on a slip angle and stacking fault energy, and the value is within the range of more than 0 and less than or equal to 1;
step two, calculating the initial crack size a according to a formula (2)0
Figure FDA0003288759640000012
In the formula:
Figure FDA0003288759640000013
for long crack threshold values, F (a) as boundary correction factors, a for defined conditions, such as temperature and stress ratio determinationiIs a determined constant;
thirdly, performing fatigue crack propagation tests of long cracks and small cracks on the metal material to be tested under different stress ratios, calculating crack propagation rates of the small cracks and the long cracks by adopting a secant method and a seven-point increasing method respectively, calculating corresponding delta K values according to stress intensity factor calculation formulas given in standards or stress intensity factor manuals aiming at different sample shapes, and further determining material parameters C and n of the material at the stages of the long cracks and the short cracks respectively;
3.1 analyzing the propagation data of the long and short cracks under different stress ratios by adopting a crack closing model to obtain an effective stress intensity factor delta KeffAnd further obtaining the relation between the small crack propagation rate and the effective stress intensity factor, and an expression thereof:
Figure FDA0003288759640000014
and the relationship between the propagation rate of long crack and the effective stress intensity factor, and the expression thereof
Figure FDA0003288759640000015
3.2 putting the data of the propagation rate of the long and short cracks in the same coordinate system, finding that the typical propagation curves of the long and short cracks usually intersect at the knee intersection point, and the corresponding effective stress intensity factor range of the point is delta K*According to the formula:
Figure FDA0003288759640000021
different stress conditions can be calculatedLower "knee intersection" corresponding crack length value a*Parts of the material may not have "knee intersections";
step four, calculating the critical crack length a according to a formula (6)f
Figure FDA0003288759640000022
KICThe fracture toughness of the material under the test condition;
step five, respectively obtaining the germination life N from the step one to the step fouriInitial crack size aiSmall crack and length crack propagation curve "knee intersection" crack length value a*Critical crack size afAnd the propagation rates of the small cracks and the long cracks and the effective stress intensity factor, and calculating the fracture life of the test sample under different stress conditions according to a formula (7):
Figure FDA0003288759640000023
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Cited By (4)

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Publication number Priority date Publication date Assignee Title
CN114840943A (en) * 2022-05-16 2022-08-02 北京航空航天大学 Fatigue crack propagation simulation piece design method based on consistency of crack propagation path and stress intensity factor
CN114878374A (en) * 2022-07-11 2022-08-09 天津大学 Characterization method for short crack and long crack propagation of metal material
CN115019913A (en) * 2022-05-12 2022-09-06 中国航发四川燃气涡轮研究院 Method for calculating fatigue crack propagation life of dual-performance powder disc
CN116049922A (en) * 2023-01-12 2023-05-02 中国工程物理研究院研究生院 Metal structure fatigue reliability prediction method for probability modeling of stress intensity factor threshold value

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CN110987676A (en) * 2019-12-23 2020-04-10 北京工业大学 Full-life prediction method considering crack closure effect under random multi-axis load

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CN106886663A (en) * 2017-03-29 2017-06-23 北京理工大学 Tooth bending Prediction method for fatigue life and device
CN108920792A (en) * 2018-06-21 2018-11-30 北京工业大学 A kind of agitating friction weldering Fatigue Life Prediction method based on crackle extension
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CN115019913A (en) * 2022-05-12 2022-09-06 中国航发四川燃气涡轮研究院 Method for calculating fatigue crack propagation life of dual-performance powder disc
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CN114878374A (en) * 2022-07-11 2022-08-09 天津大学 Characterization method for short crack and long crack propagation of metal material
CN114878374B (en) * 2022-07-11 2022-09-20 天津大学 Characterization method for short crack and long crack propagation of metal material
CN116049922A (en) * 2023-01-12 2023-05-02 中国工程物理研究院研究生院 Metal structure fatigue reliability prediction method for probability modeling of stress intensity factor threshold value
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