A method for predicting the high cycle fatigue life of metal materials

Abstract

The present invention is a method for predicting the full life of high-cycle fatigue of metal materials. The method comprises the following steps: firstly, a microstructure model based on a nonlinear dislocation dipole mechanism is used to predict the fatigue initiation life of the metal material; then, the initial crack size is calculated according to a formula; then, small crack and long crack fatigue crack growth tests are carried out on the metal material under different stress ratios, and the crack closure model is used to calculate the effective stress intensity factor of the obtained crack growth data, thereby obtaining a relationship between the growth rate of small cracks and long cracks and the range of effective stress intensity factors. Since the crack growth rates of long cracks and small cracks usually differ greatly near the threshold value, the two sets of data put together present a bilinear feature, and the typical growth curves of long and short cracks intersect at one point, which is defined as the "knee intersection point", and the effective stress intensity factor range corresponding to this point is ΔK * , calculate a under different stresses according to the formula * ; Then according to the fracture toughness K of the material IC and the maximum stress σ max , the critical crack size a under this stress condition is calculated by the formula f Finally, the fatigue life of the material is predicted. The present invention comprehensively considers the influence of the metal material microstructure and small crack behavior on the fatigue life, and provides a more accurate method for predicting the high-cycle fatigue life of metal materials.

Description

Technical Field

[0001] The invention relates to a method for predicting the full life of a high cycle fatigue of a metal material, and belongs to the field of material science and engineering applications. Background Art

[0002] There are two stages before fatigue fracture of materials and components: crack initiation stage and crack propagation stage. Correspondingly, fatigue life is also divided into crack initiation life and crack propagation life. However, the dividing line between crack initiation and propagation is difficult to determine, which is also a major difficulty in establishing fatigue life prediction models. Material scientists who study fatigue micro-mechanisms regard micron-scale crack nucleation of slip bands and grain boundaries, as well as the roughening of the surface of fatigue specimens as the crack initiation stage of fatigue failure. On the other hand, engineers engaged in practical work tend to associate the resolution limit of non-destructive crack testing equipment with fatigue crack nucleation, and regard it as the starting size of cracks in design. There are also different views on the application of fatigue crack propagation. At present, the research on the long crack propagation characteristics of materials and the prediction methods based on long crack propagation performance are very mature. However, with the deepening of the research on small cracks in materials, the existence of small crack effect has been found in more and more metal materials, that is, small cracks can still propagate when they are lower than the long crack threshold value, and at the same nominal stress intensity factor, the propagation rate of small cracks is higher than that of long cracks.

[0003] A large number of studies have shown that the fatigue life of turbine engine materials has great differences, which means that the microstructural properties have a great influence on fatigue life, and the uniqueness of small crack growth behavior also requires that it must be taken into account in fatigue life prediction.

[0004] At present, in the aspect of damage tolerance design, the small crack growth data obtained from the test is usually used to verify the existence of the small crack effect. In the application of life prediction, the crack closure model is usually used to process and obtain the relationship between the effective stress intensity factor and the crack growth rate, which is then compared with the long crack growth data, and then the long crack growth rate or long crack threshold is corrected, or the small crack growth behavior is used to predict the fatigue life. The former is mainly based on long crack growth, and there is no clear dividing point between long and short cracks. The small crack growth data either provides a correction for the long crack threshold value or corrects the expression for the long crack growth rate, and the application of the small crack growth rate is insufficient; the latter believes that the fatigue life is mainly consumed in the small crack growth stage, while the long crack growth stage is ignored. Therefore, both methods have certain limitations and fail to more truly reflect the process of fatigue crack growth. Summary of the invention

[0005] The present invention is designed to target the above-mentioned existing technical conditions and provides a method for predicting the fatigue life of a metal material. The method of the present invention takes into account the influence of the material microstructure on the initiation life, and divides the process of fatigue cracks from microcracks to fracture into two parts: a small crack extension stage and a long crack extension stage. The "knee points" with physical significance in the two stages are determined by analyzing the data of long and short cracks, and a method for calculating the crack length at the "knee point" is given, so as to propose a more accurate high-cycle fatigue life prediction method.

[0006] The objective of the present invention is achieved through the following technical solutions:

[0007] The steps of the high cycle fatigue life prediction method for this metal material are as follows:

[0008] Step 1: Calculate the crack initiation life N according to formula (1) i ;

[0009]

[0010] Where: Δσ is the stress range, Δσ e is the fatigue limit, a i is the initial crack size, calculated according to formula (2), μ is the shear modulus, M is the Taylor constant, λ is a constant (0.005), h is the slip band width, D is the grain size, ν is the Poisson's ratio, and α is a constant that depends on the slip angle and stacking fault energy, and its value is in the range of 0<α≤1;

[0011] Step 2: Calculate the initial crack size a according to formula (2): i :

[0012]

[0013] Where: is the long crack threshold, F(a) is the boundary correction factor, under certain conditions, such as temperature and stress ratio, a i is a certain constant;

[0014] Step 3: Perform fatigue crack growth tests of long cracks and small cracks under different stress ratios on the metal material to be tested. The crack growth rates of small cracks and long cracks are calculated by using the secant method and the seven-point increment method, respectively. The corresponding ΔK value is calculated according to the stress intensity factor calculation formula given in the standard or stress intensity factor manual for different specimen shapes, and then the material parameters C and n of the material at the long and short crack stages are determined;

[0015] 3.1 The crack closure model is used to analyze the long and short crack growth data under different stress ratios to obtain the effective stress intensity factor ΔK eff, and then obtain the relationship between small crack growth rate and effective stress intensity factor, and its expression:

[0016]

[0017] And the relationship between the long crack growth rate and the effective stress intensity factor, and its expression

[0018]

[0019] 3.2 Placing the long and short crack growth rate data in the same coordinate system, it is found that the typical growth curves of long and short cracks usually intersect at the "knee intersection point", and the effective stress intensity factor range corresponding to this point is ΔK * , according to the formula:

[0020]

[0021] The crack length value a corresponding to the "knee intersection" under different stress conditions can be calculated * , some materials may not have a “knee intersection”;

[0022] Step 4: Calculate the critical crack length a according to formula (6): f

[0023]

[0024] K IC is the fracture toughness of the material under the test conditions;

[0025] Step 5: Obtain the initiation life N from steps 1 to 4 respectively i , initial crack size a i , small cracks and long crack extension curve "knee intersection" crack length value a * , critical crack size a f , the relationship between the growth rate of small cracks and long cracks and the effective stress intensity factor, and then calculate the fracture life of the specimen under different stress conditions according to formula (7):

[0026]

[0027] The technical solution of the present invention is characterized in that the fatigue initiation life of metal materials is predicted by using a microstructure model based on the nonlinear dislocation dipole mechanism; according to the formula Calculate the initial crack size; then carry out fatigue crack growth tests of small cracks and long cracks under different stress ratios on the metal material to be tested, and use the crack closure model to calculate the effective stress intensity factor of the obtained crack growth data, and then obtain the relationship between the growth rate of small cracks and long cracks and the range of effective stress intensity factors. Since the crack growth rates of long cracks and small cracks usually differ greatly near the threshold value, the two sets of data put together show bilinear characteristics. The typical growth curves of long and short cracks intersect at one point, which is defined as the "knee intersection point". The effective stress intensity factor range corresponding to this point is ΔK * , according to the formula Calculate a under different stresses * ; Then according to the fracture toughness K of the material IC and the maximum stress σ max , through the formula Calculate the critical crack size a under this stress condition f ; Finally, the fatigue life of the material is predicted.

[0028] In the technical solution of the present invention, the fatigue initiation life, small crack extension life and long crack extension life of the material are comprehensively considered in the fatigue full life prediction method, and the fatigue full life prediction method is characterized and staged.

[0029] In the technical solution of the present invention, the fatigue life prediction formula adopts the dislocation dipole mechanism method to characterize the slip bands on the surface or sub-surface of the material, which is consistent with the crack initiation mode of most materials, and the model comprehensively considers factors such as stress conditions, grain size and crack shape.

[0030] In the technical solution of the present invention, the influence of the small fatigue crack effect of the material on the fatigue life is taken into account, and the behavior of the small crack extension stage is characterized by the Paris formula, which has a wider applicability. This method is suitable for materials in which small fatigue cracks still extend in an area below the threshold value and the crack extension rate is higher than that of long cracks.

[0031] In the technical solution of the present invention, the "knee intersection" of the long and short cracks is * Value and critical crack length a f The calculation is more targeted. For samples of different shapes and sizes, different F I The calculation formula of the crack length characteristic value obtained is more accurate, and the fatigue life predicted is more accurate.

[0032] The technical solution of the present invention is more suitable for aviation engine materials during implementation, which usually have a non-negligible initiation life and most of which have a small fatigue crack effect, that is, small fatigue cracks still expand in a region below the long crack threshold, or the small crack expansion rate is higher than that of the long crack.

[0033] The present invention comprehensively considers the influence of the microstructure and small crack behavior of metal materials on fatigue life, and provides a more accurate method for predicting the full life of high-cycle fatigue of metal materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Flow chart of the high cycle fatigue life prediction method for metal materials of the present invention

[0035] Figure 2 The definition of the typical extension curve of long and short cracks and the knee intersection point of the present invention is

[0036] Figure 3 The typical microstructure of Ti-6Al-4V alloy in the embodiment

[0037] Figure 4 Determination of the corresponding curves of crack growth rate and effective stress intensity factor and the knee intersection point in the embodiment

[0038] Figure 5 The fatigue test data in the embodiment and the fatigue life curve predicted by the method of the present invention are shown in FIG. DETAILED DESCRIPTION

[0039] The present invention is further described below in conjunction with the embodiments and drawings.

[0040] In an embodiment of the present invention, a method for predicting the fatigue life of a material is shown in the flowchart of the method. Figure 1 As shown, taking Ti-6Al-4V alloy as an example, the following steps are included:

[0041] First, the various material parameter information must be determined. The microstructure of the material Ti-6Al-4V alloy can be observed to obtain the material grain size information, such as Figure 3 As shown in the figure, the typical microstructure of Ti-6Al-4V alloy is given. It can be seen from the figure that the organization contains 60% of the initial α phase and 40% of the α+β phase. The equivalent size of the α phase size and the α+β phase size of 13.7μm is used to calculate the initiation life. The Taylor constant is taken as 2 and α is taken as 0.6. The shear modulus μ of the material at room temperature and a stress ratio of 0.1 is 4.4e+04MPa, the Poisson's ratio ν is 0.333, the slip band width h is 5.00e-02, and the fatigue limit Δσ e 340MPa, long crack threshold for Fracture toughness K IC for

[0042] Step 1: Calculate the crack initiation life N according to formula (1) i ;

[0043]

[0044] For different stress ranges Δσ, different initiation lives can be obtained, a i is the initial crack size, calculated according to formula (2), and λ is a constant (0.005).

[0045] Step 2: Calculate the initial crack size a according to formula (2): i :

[0046]

[0047] Where: is the long crack threshold, F(a) is the boundary correction factor, and when the test conditions are determined, the initial crack size a i To determine the value, for room temperature and stress ratio R = 0.1, calculate by the above formula, a i About 9μm;

[0048] Step 3: Perform fatigue crack growth tests of long cracks and small cracks under different stress ratios on the metal material to be tested. The crack growth rates of small cracks and long cracks are calculated by using the secant method and the seven-point increment method, respectively. The corresponding ΔK value is calculated according to the stress intensity factor calculation formula given in the standard or stress intensity factor manual for different specimen shapes, and then the material parameters C and n of the material at the long and short crack stages are determined;

[0049] 3.1 The crack closure model is used to analyze the long and short crack growth data under different stress ratios to obtain the effective stress intensity factor ΔK eff , and then obtain the relationship between small crack growth rate and effective stress intensity factor, and its expression:

[0050]

[0051] And the relationship between the long crack growth rate and the effective stress intensity factor, and its expression

[0052]

[0053] 3.2 Placing the long and short crack growth rate data in the same coordinate system, it is found that the typical growth curves of long and short cracks usually intersect at the "knee intersection point", and the effective stress intensity factor range corresponding to this point is ΔK * , according to the formula:

[0054]

[0055] The crack length value a corresponding to the "knee intersection" under different stress conditions can be calculated * , Figure 4The data of small crack and long crack growth rate - effective stress intensity factor are given, and the effective stress intensity factor ΔK corresponding to the "knee intersection" of long and short cracks is given. * Approx. The crack length a* corresponding to different stress amplitudes can be calculated by formula (5).

[0056] Step 4: Calculate the critical crack length a according to formula (6): f

[0057]

[0058] K IC is the fracture toughness of the material under the test conditions;

[0059] Step 5: Obtain the initiation life N from steps 1 to 4 respectively i , initial crack size a i , small cracks and long crack extension curve "knee intersection" crack length value a * , critical crack size a f , the relationship between the growth rate of small cracks and long cracks and the effective stress intensity factor, and then calculate the fracture life of the specimen under different stress conditions according to formula (7):

[0060]

[0061] Figure 5 Several test points of Ti-6Al-4V alloy at room temperature and a stress ratio R of 0.1 are given, as well as a life prediction curve obtained by the method of the present invention.

Claims

1. A method for predicting the high cycle fatigue life of a metal material, characterized by: The steps of this method are as follows: Step 1: Calculate the crack initiation life N according to formula (1) i ; Where: Δσ is the stress range, Δσ e is the fatigue limit, a i is the initial crack size, calculated according to formula (2), μ is the shear modulus, M is the Taylor constant, λ is a constant of 0.005, h is the slip band width, D is the grain size, ν is the Poisson's ratio, and α is a constant that depends on the slip angle and stacking fault energy, and its value is in the range of 0<α≤1; Step 2: Calculate the initial crack size a according to formula (2): i : Where: is the long crack threshold, F(a) is the boundary correction factor; Step 3: Perform fatigue crack growth tests of long cracks and small cracks under different stress ratios on the metal material to be tested. The crack growth rates of small cracks and long cracks are calculated by using the secant method and the seven-point increment method, respectively. The corresponding ΔK value is calculated according to the stress intensity factor calculation formula given in the standard or stress intensity factor manual for different specimen shapes, and then the material parameters C and n of the material at the long and short crack stages are determined; 3.1 The crack closure model is used to analyze the long and short crack growth data under different stress ratios to obtain the effective stress intensity factor ΔK eff , and then obtain the relationship between small crack growth rate and effective stress intensity factor, and its expression: And the relationship between the long crack growth rate and the effective stress intensity factor, and its expression 3.2 Placing the long and short crack growth rate data in the same coordinate system, it is found that the typical growth curves of long and short cracks usually intersect at the "knee point", and the effective stress intensity factor range corresponding to this point is ΔK * , according to the formula: The crack length a corresponding to the "knee intersection" under different stress conditions can be calculated * , some materials may not have a "knee intersection"; Step 4: Calculate the critical crack length a according to formula (6): f K IC is the fracture toughness of the material under the test conditions; Step 5: Obtain the initiation life N from steps 1 to 4 respectively i , initial crack size a i , small crack and long crack extension curve "knee intersection" crack length value a * , critical crack size a f , the relationship between the growth rate of small cracks and long cracks and the effective stress intensity factor, and then calculate the fracture life of the specimen under different stress conditions according to formula (7):

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