Method for establishing influence of fatigue stress concentration coefficient and temperature on fatigue performance of alloy material and application

Abstract

The invention provides a method for establishing the influence of a fatigue stress concentration coefficient and temperature on the fatigue performance of an alloy material, and the method comprises the following steps: S1) defining the given fatigue life of an alloy, and obtaining a fatigue stress concentration coefficient Kf, N; under the condition that the stress concentration coefficient Kt, the stress ratio and the temperature of the alloy are determined, a two-parameter stress fatigue life model of the alloy is obtained; s2) if the stress concentration coefficient Kt is equal to 1, obtaining the fatigue limit of the given fatigue life of the smooth sample on the basis of the given fatigue life; s3) on the basis of the given fatigue life, obtaining a fatigue limit and Kf, N when Kt is not equal to 1 of the given fatigue life; and S4) establishing a relationship with the given fatigue life according to the Kf, N of the given fatigue life and the temperature in the step S1). According to the establishment method provided by the invention, the influence of the combined action of two factors of Kf, N and temperature on the fatigue performance can be analyzed.

Description

Technical Field

[0001] The invention relates to the field of fatigue testing, and in particular to a method for establishing the influence of stress concentration factor and temperature on the fatigue performance of alloy materials and its application. Background Art

[0002] Temperature and stress concentration factor are important factors affecting the high-cycle fatigue performance of materials. The SN curve is a curve that uses the fatigue strength of the standard specimen of the material as the ordinate and the logarithm of the fatigue life lg N as the abscissa to represent the relationship between the fatigue strength and fatigue life of the standard specimen under certain cycle characteristics. It is also called the stress-life curve. When a single factor of temperature or stress concentration changes, its impact on fatigue performance can be qualitatively compared by plotting multiple SN curves in the same coordinate system, such as plotting SN curves of different test temperatures in one graph for comparison, or plotting multiple SN curves of different stress concentration factors in one graph for comparison.

[0003] Although the method of plotting the data to be compared in the same graph is simple, it can only be used for qualitative comparison. When considering quantitative comparison, in order to illustrate the influence of stress concentration on fatigue performance, fatigue strength reduction factor is usually defined for analysis. Usually, the reduction in fatigue strength of notched parts is expressed by fatigue strength reduction factor K. f Indicates that K f Also called fatigue notch factor or fatigue stress concentration factor; K f is the fatigue limit of the smooth specimen σ -1 Fatigue limit σ of notched specimen -1N The calculation formula is shown in formula (1):

[0004] K f =σ -1 / σ -1N (1);

[0005] Fatigue stress concentration factor K f It is related to the material properties and the geometry of the notched specimen, that is, the stress concentration factor K t The fatigue notch sensitivity index q can represent the fatigue notch sensitivity of the material, and its definition is as follows:

[0006] q=(K f -1) / (K t -1) (2);

[0007] When q=1, K f =K t , indicating that the fatigue strength of the material is reduced to the greatest extent due to the presence of the notch, that is, the sensitivity to the notch is the greatest; when q = 0, K f=1, indicating that the fatigue strength of the material is not reduced by the presence of the notch, that is, it is insensitive to the notch.

[0008] As mentioned above, regardless of the fatigue stress concentration factor K f , or fatigue notch sensitivity index q, both represent a single factor K t Influence on fatigue performance; in addition, the definitions of these parameters are all based on fatigue limits and cannot characterize the impact on the middle life region or even the short life region.

[0009] In order to better understand the fatigue performance of the material, the material is analyzed under temperature (T) and fatigue stress concentration factor (K f )The combined effect of the two factors on the regularity of the high-cycle fatigue performance of the material, and the method of simultaneously analyzing the short-life zone, medium-life zone and long-life zone of the material is of great significance. Summary of the invention

[0010] The technical problem solved by the present invention is to provide a method for establishing the influence of fatigue stress concentration coefficient and temperature on the fatigue performance of alloy materials. The establishment method provided by the present application can analyze the influence of the two factors of temperature and fatigue stress concentration coefficient on the regularity of high-cycle fatigue performance of alloy materials under the combined action of the two factors, and can also analyze the short-life zone, medium-life zone and long-life zone of the alloy materials.

[0011] In view of this, the present application provides a method for establishing the influence of fatigue stress concentration factor and temperature on the fatigue performance of alloy materials, comprising the following steps:

[0012] S1) defines a given fatigue life of the alloy, then the fatigue stress concentration factor K under a given fatigue life is obtained as shown in formula (3): f，N ;

[0013] K f，N = σ D / σ DN (3);

[0014] Among them, σ D is the fatigue limit of the alloy smooth specimen, σ DN is the fatigue limit of the alloy notched specimen;

[0015] The stress concentration factor K in the alloy t Under the conditions of , stress ratio and temperature being determined, a two-parameter stress-fatigue life model of the alloy is obtained as shown in formula (4);

[0016] N f =C f (S a -S ac ) -2 (4);

[0017] Among them, N f is stress fatigue life, C f is the stress fatigue resistance coefficient, S a is the fatigue limit at any level, S ac is the theoretical fatigue limit;

[0018] S2) Stress concentration factor K t =1 is a smooth specimen. On the basis of a given fatigue life, the fatigue limit of the given fatigue life of the smooth specimen is obtained according to formula (4);

[0019] S3) Based on the given fatigue life, according to the fatigue limit of the given fatigue life of the smooth specimen and formula (4), K is obtained. t ≠1 Fatigue limit and K for a given fatigue life f，N ;

[0020] S4) According to the given fatigue life K f，N The relationship between the temperature in step S1) and the given fatigue life is established.

[0021] Preferably, the given fatigue life is 10 4 ~10 7 Any lifetime within the range of weeks.

[0022] Preferably, the temperature is room temperature to 1100°C.

[0023] Preferably, the alloy is a high-temperature alloy.

[0024] The present application also provides the application of the establishment method in analyzing the influence of temperature and fatigue stress concentration factor on fatigue performance.

[0025] The present application provides a method for establishing the influence of fatigue stress concentration factor and temperature on the fatigue performance of alloy materials. The method first defines a given fatigue life of the alloy, and then obtains the fatigue stress concentration factor K under a given fatigue life as shown in formula (3): f，N ; The stress concentration factor K of the alloy t Under the conditions of determining the stress ratio and temperature, the two-parameter stress fatigue life model of the alloy is obtained as shown in formula (4); due to the stress concentration factor K t = 1 for a smooth specimen. On the basis of a given fatigue life, the fatigue limit of the given fatigue life of the smooth specimen is obtained according to formula (4); then, on the basis of a given fatigue life, the fatigue limit of the given fatigue life of the smooth specimen and formula (4) are used to obtain the notched specimen (K t ≠1) Fatigue limit and K for a given fatigue life f，N ; Finally, according to the given fatigue life K f，Nand temperature to establish a relationship with a given fatigue life. The establishment method provided in this application establishes the "K f，N -Temperature" curve, K can be obtained under a given life f，N The temperature change curve can intuitively display the fatigue notch stress concentration factor (K f，N The influence of the combined effect of temperature and the fatigue life of the alloy is analyzed to analyze the influence of the combined effect of the two factors on the fatigue performance; at the same time, by defining the fatigue stress concentration factor K under a given life f The value can characterize the influence of the notch on the fatigue strength under different given lifespans. The given lifespan can be given arbitrarily according to the needs, thereby obtaining the fatigue stress concentration factor in the short, medium and long life zones. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The alloy of the present invention is given under the life span (N=10 5 Weekly, 10 6 Week and 10 7 Cycle) Fatigue strength and temperature relationship curve;

[0027] Figure 2 The alloy of the present invention is given under the life span (N=10 5 Weekly, 10 6 Week and 10 7 The relationship between fatigue stress concentration factor (cycles) and temperature. DETAILED DESCRIPTION

[0028] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0029] In view of the fact that in the prior art, fatigue strength reduction factor or fatigue notch sensitivity index is the influence of a single factor on fatigue performance, and there is a lack of research on the regularity of the combined effect of multiple factors such as temperature and fatigue stress concentration factor on the high cycle fatigue performance of materials, specifically, the present application provides a method for establishing the influence of the combined effect of fatigue stress concentration factor and temperature on the fatigue performance of alloy materials, which can analyze the influence of temperature (T) and fatigue stress concentration factor (K) on the fatigue performance of alloy materials. f ) The influence of the combined action of the two factors on the regularity of the high-cycle fatigue performance of the material can also be realized, and at the same time, the analysis of the short-life zone, medium-life zone and long-life zone of the material can be realized. Specifically, the embodiment of the present invention discloses a method for establishing the influence of the combined action of fatigue stress concentration factor and temperature on the fatigue performance of alloy materials, comprising the following steps:

[0030] S1) defines a given fatigue life of the alloy, then the fatigue stress concentration factor K under a given fatigue life is obtained as shown in formula (3): f，N ;

[0031] K f，N = σ D / σ DN (3);

[0032] Among them, σ D is the fatigue limit of the alloy smooth specimen, σ DN is the fatigue limit of the alloy notched specimen;

[0033] The stress concentration factor K in the alloy t Under the conditions of , stress ratio and temperature being determined, a two-parameter stress fatigue life model of the alloy is obtained as shown in formula (4);

[0034] N f =C f (S a -S ac ) -2 (4);

[0035] Among them, N f is stress fatigue life, C f is the stress fatigue resistance coefficient, S a is the fatigue limit at any level, S ac is the theoretical fatigue limit;

[0036] S2) Stress concentration factor K t =1 is a smooth specimen. On the basis of a given fatigue life, the fatigue limit of the given fatigue life of the smooth specimen is obtained according to formula (4);

[0037] S3) Based on the given fatigue life, according to the fatigue limit of the given fatigue life of the smooth specimen and formula (4), K is obtained. t ≠1 Fatigue limit and K for a given fatigue life f，N ;

[0038] S4) According to the given fatigue life K f，N The relationship between the temperature in step S1) and the given fatigue life is established.

[0039] The establishment process of the influence of the combined effect of fatigue stress concentration factor and temperature on the fatigue performance of alloy materials in this application is specifically described as follows:

[0040] If the temperature (T) changes and the stress concentration factor (K t) changes simultaneously, which has an important influence on the regularity of the high cycle fatigue performance of the alloy, and is a good way to quantitatively evaluate the temperature (T) and fatigue stress concentration factor (K f ) combined effect on the high cycle fatigue performance of alloys, a parameter is first defined: the stress concentration fatigue strength influence coefficient or fatigue stress concentration coefficient K under a given life. f，N , and secondly, given K f，N Then use the calculation method of “K f，N -Temperature" curve realizes the analysis of the combined effect of temperature and fatigue stress concentration factor on fatigue performance; the above process is specifically as follows:

[0041] (1) K under a given lifespan f，N Definition

[0042] There are always notches in structural parts, which cause stress concentration. Under a given stress ratio or average stress, different stress concentration factors have a significant impact on the fatigue life curve. As the stress concentration factor increases, the fatigue life shortens and the fatigue limit decreases.

[0043] The reduction in fatigue strength of notched specimens is expressed by the stress concentration fatigue strength influence coefficient K. f Indicates; K f，N is defined as the ratio of the fatigue strength of a smooth specimen to the fatigue strength of a notched specimen at a given life:

[0044] K f，N = σ D / σ DN (3);

[0045] K f，N It may also be called the fatigue notch factor or fatigue stress concentration factor.

[0046] Fatigue stress concentration factor K f，N Characterizes the reduction in fatigue strength under a given life due to the presence of a notch, K f，N The value of is divided into the following three cases:

[0047] 1) K f，N =1: indicates that the fatigue strength of the material is not reduced by the presence of the notch, that is, the material is not sensitive to the notch;

[0048] 2) K f，N >1: Indicates that the fatigue strength of the material is reduced due to the presence of the notch, K f，N The larger the value, the greater the fatigue strength loss caused by the notch. f，N Theoretically, it is greater than 1. The magnitude of stress concentration at the notch is usually expressed by the stress concentration factor K. t In theory, K fis less than the stress concentration factor K t of;

[0049] 3) K f，N <1: Indicates that the fatigue strength of the material increases due to the presence of the notch. K f，N The smaller the value, the greater the increase in fatigue strength caused by the notch. f，N The situation where the value is less than 1 is only true mathematically. Considering the actual fatigue situation, this situation does not exist.

[0050] In the prior art, σ in formula (3) D and σ DN It is usually defined as the fatigue limit of smooth and notched specimens of the material; at this time, K f，N It is called the fatigue limit reduction factor for a given life, also known as the fatigue notch factor or fatigue stress concentration factor.

[0051] Furthermore, if we change σ in equation (3) D and σ DN Defined as the fatigue strength under a given life, the fatigue stress concentration factor K under any given life can be obtained f，N For example, for high cycle fatigue, the fatigue life N = 10 5 Weekly, 10 6 Week and 10 7 cycles, so that the fatigue stress concentration factor under a given life can be obtained:

[0052] i) Given a lifetime of 10 5 Fatigue stress concentration factor at cycle: K f,10 5 ;

[0053] ii) Given a lifetime of 10 6 Fatigue stress concentration factor at cycle: K f,10 6 ;

[0054] iii) Given a lifetime of 10 7 Fatigue stress concentration factor at cycle: K f,10 7 .

[0055] Fatigue strength σ at a given life D and σ DN It can be calculated using the material fatigue life expression.

[0056] (2) K under a given lifespan f，N Calculation

[0057] If the fatigue life is given as N = 10 5 Weekly, 10 6Week and 10 7 Weeks, then for a given life span, K f，N The calculation of is as follows:

[0058] K f,10 5 =σ D,10 5 / σ DN,10 5 (5);

[0059] K f,10 6 =σ D,10 6 / σ DN,10 6 (6);

[0060] K f,10 7 =σ D,10 7 / σ DN,10 7 (7);

[0061] Of course, the given life span is any number of cycles and can be given by yourself.

[0062] For the high cycle fatigue test data of alloy materials, the two-parameter stress fatigue life model can be used to characterize the high cycle fatigue data of the alloy as follows:

[0063] N f =C f (S a -S ac ) -2 (4);

[0064] N f is stress fatigue life, C f is the stress fatigue resistance coefficient, S a is any horizontal fatigue limit, S ac is the theoretical fatigue limit; where the stress fatigue resistance coefficient and the theoretical fatigue limit are obtained by linear fitting of the test data, that is, C f and S ac At temperature, the stress concentration factor K of the alloy t , stress ratio R is determined on the basis of a fixed value. At the same time, under different temperatures, different stress concentration factors and different stress ratios, C f and S ac Also different, then the above formula (4) is N f and S a Taking Table 1 as an example, N f The specific expression is:

[0065] Table 1 Expression table of high cycle fatigue two-parameter life model of a nickel-based single crystal high temperature alloy

[0066]

[0067] Here, a two-parameter stress fatigue life model is used as an example, and other life formulas that can well characterize fatigue test data can be used.

[0068] When the fatigue life expression (4) of the smooth specimen and the notched specimen is known, the given life N f (For example, 10 5 Weekly, 10 6 Week and 10 7 The fatigue strength is calculated by back-calculating the corresponding fatigue strength, and then the fatigue stress concentration factor under a given life is calculated using formulas (5) to (7); specifically, K t =1 corresponds to the stress concentration factor of the smooth specimen. Using formula (4), we can calculate S at any temperature. a Fatigue limit, at other K t Under certain conditions, based on a certain temperature and stress ratio, S can be calculated using formula (4) a Fatigue limit, and then use formula (3) to get K at the above temperature f，N The above-mentioned certain temperature may be any temperature value within the range of room temperature to 1100° C., and in a specific embodiment, it may be 500° C., 700° C., 760° C., 850° C., or other temperature points within the range.

[0069] (3) Method for analyzing the combined effect of temperature and fatigue stress concentration factor on the high-cycle fatigue performance of materials

[0070] When analyzing the influence of fatigue stress concentration factor and temperature on the high cycle fatigue performance of alloys, first, a smooth specimen (K t =1) and notched specimen (K t =3) The variation of fatigue strength with temperature at each given life. This step gives the absolute variation of fatigue strength with temperature for smooth and notched specimens. Secondly, due to the K f，N The effect of stress concentration factor changes on fatigue strength has been characterized. f，N -temperature” curve can intuitively show how fatigue strength changes with temperature due to the presence of notches; K f，N >1 means that the fatigue strength of the material is reduced due to the presence of the notch, K f，N The larger the value, the greater the fatigue strength loss caused by the notch. From this, we can intuitively see the influence of the fatigue stress concentration factor and temperature on high cycle fatigue performance.

[0071] The establishment method provided in the present application can be applied to the fatigue stress concentration factor and the influence of temperature on fatigue performance of high-temperature alloys, titanium alloys, and steels in high-temperature environments. In a specific embodiment, the establishment method is applied to nickel-based single crystal high-temperature alloys.

[0072] The establishment method provided in this application has application significance in analyzing the combined effect of temperature and fatigue stress concentration factor on fatigue performance.

[0073] In this application, the "cycle" in high cycle fatigue and the "cycle number" in a given life span refer to the number of cycles for those skilled in the art.

[0074] This example takes a nickel-based single crystal high-temperature alloy high-cycle fatigue as an example. When the stress ratio R = -1, the smooth sample (K t =1) and notched specimen (K t =3) High cycle fatigue test temperature (T) and fatigue stress concentration factor (K) at 500℃~850℃ f，N ) The influence of two factors changing simultaneously on the regularity of high cycle fatigue performance. Table 1 provides the results of fitting the test data of the alloy using a two-parameter fatigue life model.

[0075] Figure 1 The given life of the alloy at R = -1 is 10 calculated according to the expression in Table 1. 5 Weekly, 10 6 Week and 10 7 The curve of fatigue strength changing with temperature under cycles; Figure 1 (a) It can be seen that the smooth sample (K t =1) When R = -1, the given lifespan is 10 5 Week and 10 6 The fatigue strength is highest at 500℃ and lowest at 700℃. 7 The fatigue strength is highest at 700℃ and lowest at 850℃. Figure 1 (b) It can be seen that for the notched specimen (K t =3), with the increase of temperature, the fatigue strength at each given life first increases and then decreases, and the fatigue strength at 700℃ is the highest.

[0076] Given a life span (N = 10 5 Weekly, 10 6 Week and 10 7 The fatigue stress concentration factor K under a given life can be obtained by ratioing the fatigue strength of the smooth specimen and the fatigue strength of the notched specimen under a given life. f，N , the results are shown in Table 2.

[0077] Table 2 Given life of a nickel-based single crystal high temperature alloy when R = -1 (N = 10 5 Weekly, 10 6 Week and 10 7 Fatigue stress concentration factor data table under (cycles)

[0078]

[0079] It can be seen from Table 2 that at 500°C, as the given life increases, the fatigue stress concentration factor gradually decreases. t =3 The fatigue strength loss caused by the notch gradually decreases; at 700℃ and 850℃, the fatigue stress concentration factor changes little with the increase of the given life, indicating that in the short, medium and long life ranges, due to K t =3The fatigue strength loss ratio caused by the notch is the same.

[0080] Figure 2 The alloy has a lifespan of 10 5 Weekly, 10 6 Week and 10 7 The relationship between the fatigue stress concentration factor and temperature at the cycle. It can be clearly seen from the figure that due to the stress concentration factor K t =3, the relationship between fatigue stress concentration factor and temperature. As mentioned above, K f，N When K > 1, it means that the fatigue strength of the material is reduced due to the presence of the notch. f，N The larger the value, the greater the fatigue strength loss caused by the notch. Figure 2 It can be seen that at 700℃, K f，N The value is the lowest, about 1.35. Compared with 500℃ and 850℃, the fatigue notch sensitivity of the alloy is the lowest at this time, and the fatigue strength loss caused by the notch is the smallest.

[0081] The above embodiments are only used to help understand the method and core idea of ​​the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

[0082] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for establishing the influence of fatigue stress concentration factor and temperature on the fatigue performance of alloy materials, comprising the following steps: S1) defines a given fatigue life of the alloy, then the fatigue stress concentration factor K under a given fatigue life is obtained as shown in formula (3): f，N ; K f，N = s D / p DN (3); Among them, σ D is the fatigue limit of the alloy smooth specimen, σ DN is the fatigue limit of the alloy notched specimen; The stress concentration factor K in the alloy t Under the conditions of , stress ratio and temperature being determined, a two-parameter stress-fatigue life model of the alloy is obtained as shown in formula (4); N f =C f (S a -S ac ) -2 (4); Among them, N f is stress fatigue life, C f is the stress fatigue resistance coefficient, S a is the fatigue limit at any level, S ac is the theoretical fatigue limit; S2) Stress concentration factor K t =1 is a smooth specimen. On the basis of a given fatigue life, the fatigue limit of the given fatigue life of the smooth specimen is obtained according to formula (4); S3) Based on the given fatigue life, according to the fatigue limit of the given fatigue life of the smooth specimen and formula (4), K is obtained. t ≠1 Fatigue limit and K for a given fatigue life f，N ; S4) According to the given fatigue life K f，N The relationship between the temperature in step S1) and the given fatigue life is established.

2. The establishment method according to claim 1, characterized in that: The given fatigue life is 10 4 ~10 7 Any lifetime within the range of weeks.

3. The establishment method according to claim 1, characterized in that: The temperature is room temperature to 1100°C.

4. The establishment method according to claim 1, characterized in that: The alloy is a high temperature alloy.

5. Application of the establishment method according to any one of claims 1 to 4 in analyzing the influence of temperature and fatigue stress concentration factor on fatigue performance.

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