Method for predicting fatigue strength of metal material under different stress ratios

Abstract

The invention discloses a method for predicting the fatigue strength of a metal material under different stress ratios, which belongs to the technical field of metal material fatigue strength prediction, and comprises the following steps: carrying out a fatigue experiment on a fatigue sample of the metal material, measuring and calculating the fatigue strength under a certain stress ratio R1, carrying out normalization processing by adopting a prediction model, and calculating the fatigue strength of the metal material under the stress ratio R1; the fatigue strength under R =-1 is obtained; and multiplying by a value corresponding to the target stress ratio R2 to obtain the fatigue strength under the target stress ratio R2. According to the method, based on the energy consumption in the fatigue damage process, the fatigue strength values under different stress ratios are calculated and obtained through equivalence of the effects of different stress ratios; the fatigue strength of the metal material under different stress ratios is predicted through the fatigue strength prediction model, the model explains the influence of the stress ratios on the fatigue strength of the metal material, and an effective strategy is provided for predicting the fatigue strength of the metal material under the variable-amplitude load. The fatigue strength predicted by the method is matched with an experimental value, so that the experimental time and cost are greatly saved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of metal material fatigue strength prediction, and particularly relates to a method for predicting fatigue strength of metal material under different stress ratios. BACKGROUND

[0002] Material fracture is ubiquitous in nature and has a significant impact on many key areas of engineering applications. It is well known that fatigue fracture is the most common failure mode in metal materials and components to date. Fatigue life prediction of metal materials or components has always been a key issue in engineering applications, and material fatigue strength is an important indicator for mechanical structure reliability design. At present, material fatigue testing is commonly tested by constant amplitude fatigue method, although the research of constant amplitude fatigue is helpful for predicting fatigue life, however, the structure components are actually always under variable amplitude load, for example, high-speed trains and airplanes often suffer random load during operation. Extending the knowledge of constant amplitude fatigue to more practical cases, including variable amplitude cyclic stress and strain, is a crucial challenge. However, it is time-consuming, large amount of testing and high cost to predict the fatigue strength of engineering components by using a large number of fatigue experiments, so this method is not feasible. SUMMARY

[0003] In order to solve the above problems, the present application provides a method for predicting fatigue strength of metal material under different stress ratios.

[0004] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0005] A method for predicting fatigue strength of metal material under different stress ratios, comprising the following steps:

[0006] (1) Fatigue experiment is carried out on the fatigue sample of the metal material to measure the fatigue strength under a certain stress ratio R1 , the value range of R1 is [-1, 1);

[0007] (2) The fatigue strength in step (1) is normalized by using the following prediction model, and the fatigue strength is divided by the value of under the stress ratio R1 to obtain the fatigue strength under R=-1 ;

[0008] The fatigue strength ratio of the metal material under different stress ratios R and R=-1 is , and the relationship is as follows:

[0009] ;

[0010] ;

[0011] (3) multiplying the fatigue strength calculated in step (2) by the value corresponding to the target stress ratio R2, i.e. obtaining the fatigue strength under the target stress ratio R2.

[0012] Further, in step (1), the fatigue experiment adopts a room temperature high cycle fatigue experiment.

[0013] Further, the high cycle fatigue sample size and the experiment process of the room temperature high cycle fatigue experiment in step (1) meet the standard GB / T 3075-2021 Metal Material Fatigue Test Axial Force Control Method.

[0014] Further, in step (1), the fatigue strength under a certain stress ratio R1 is measured and calculated by using the ascending and descending method.

[0015] Further, the calculation method of the fatigue strength in step (1) meets the standard GB / T 24176-2009 Metal Material Fatigue Test Data Statistics Scheme and Analysis Method.

[0016] Compared with the prior art, the technical progress achieved by the present application is that:

[0017] Based on the energy consumption of the fatigue damage process, the fatigue damage process under different stress ratios is determined by the consumed energy of the sample loading, which can be equivalent to the effect of different stress ratios, and then the fatigue strength values under different stress ratios are calculated. The present application predicts the fatigue strength of the metal material under different stress ratios through a fatigue strength prediction model, which theoretically explains the influence of the stress ratio on the fatigue strength of the metal material, and provides an effective strategy for the fatigue strength prediction of the metal material under variable amplitude load. The fatigue strength predicted by the present application is consistent with the experimental value, which greatly saves the experimental time and cost. BRIEF DESCRIPTION OF DRAWINGS

[0018] The accompanying drawings are used to provide a further understanding of the present application, and constitute a part of the specification, together with the embodiments of the present application, to explain the present application, and do not constitute a limitation of the present application.

[0019] In the drawings:

[0020] Figure 1 is a stress change trend graph of single cycle loading when the stress ratio R of the metal material in the embodiment of the present application is -1;

[0021] Figure 2 is a stress change trend graph of single cycle loading when the stress ratio R of the metal material in the embodiment of the present application is greater than or equal to 0;

[0022] Figure 3 ​​​​This is a graph showing the stress change trend of a single cycle of loading when the stress ratio R of the metallic material is less than 0 in an embodiment of the present invention. Detailed Implementation

[0023] The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments. The embodiments of the present invention will now be described with reference to the accompanying drawings.

[0024] This invention provides a method for predicting the fatigue strength of metallic materials under different stress ratios, comprising the following steps:

[0025] (1) A high-cycle fatigue test at room temperature was conducted on the fatigue specimens of metallic materials, and the fatigue strength under a certain stress ratio R1 was calculated using the rise and fall method. The value range of R1 is [-1, 1].

[0026] The dimensions and experimental procedures for high-cycle fatigue specimens conformed to standard GB / T 3075-2021, "Methods for Controlling Axial Force in Fatigue Testing of Metallic Materials". Fatigue strength... The calculation method conforms to the standard GB / T 24176-2009 "Statistical Scheme and Analysis Method for Fatigue Test Data of Metallic Materials".

[0027] (2) Fatigue strength in step (1) The following prediction model is used for normalization, and then divided by the stress ratio R1. The value is used to obtain the fatigue strength at R=-1. ;

[0028] The ratio of fatigue strength of metallic materials under different stress ratios R and R=-1 is The relationship is as follows:

[0029] ;

[0030] .

[0031] (3) The fatigue strength calculated in step (2) Multiply by the target stress ratio R2 The value is the fatigue strength obtained under the target stress ratio R2.

[0032] During cyclic loading, the energy consumption and fatigue damage process of metallic materials are influenced by many factors, and the effects of each factor vary at different stages. The fatigue damage process of metallic materials under different stress ratios is determined by the energy consumed by the loading of the specimen, which depends on the stress change. Therefore, it can be equivalent to the effect of different stress ratios, and the fatigue strength values ​​under different stress ratios can be calculated.

[0033] The influence of the stress ratio on the fatigue strength is based on the energy loss of the fatigue damage process, and quantifies the actual energy consumption in a single cycle loading process. The energy consumption U is expressed as:

[0034] , (1)

[0035] where U is the energy consumption of the fatigue specimen gage section in a single cycle loading process; L is the length of the fatigue specimen gage section; S is the cross-sectional area of the fatigue specimen gage section; is the strain change in a single loading process; is the movement distance of the fatigue specimen in a single cycle loading process; is the stress amplitude of the loading, which is regarded as a constant stress in the loading process:

[0036] (2)

[0037] where R is the stress ratio of the fatigue specimen in the loading process; σ max is the maximum stress of the fatigue specimen in a single loading process.

[0038] where, The expression is as follows:

[0039] (3)

[0040] where t0 is the initial time of a single cycle loading, t1 is the end time, is the strain in a single loading process, and the expression is as follows:

[0041] (4)

[0042] where E is the elastic modulus of the fatigue specimen; t is the single cycle loading time.

[0043] When the stress ratio R = -1, as shown in FIG. 1, the strain change in a single loading is as follows: Figure 1

[0044] (5)

[0045] When the stress ratio R ≥ 0, as shown in FIG. 2, the strain change in a single loading is as follows: Figure 2

[0046] (6)

[0047] When the stress ratio R < 0, as shown in FIG. 3, the strain change in a single loading is as follows: Figure 3

[0048] ​​​(7)

[0049] Wherein, t c is the position of stress 0 in the cyclic loading process, and its expression is as follows:

[0050] (8)

[0051] Based on the definition of fatigue strength in the fatigue loading process, the energy consumed by the fatigue damage in the same cycle is proportional to the fatigue strength (in stress amplitude), and it is generally considered that the stress ratio R=-1 is the commonly used fatigue strength expression, therefore the fatigue strength under other stress ratios can be expressed as:

[0052] (9)

[0053] Wherein, σ R is the stress amplitude in the single loading process of the fatigue sample under the stress ratio R, that is, the fatigue strength; U R is the energy consumption of the gage section of the fatigue sample in the single cycle loading process under R; σ -1 is the stress amplitude in the single loading process of the fatigue sample when R=-1; U -1 is the energy consumption of the gage section of the fatigue sample in the single cycle loading process when R=-1.

[0054] The formula (9) is brought into the formula (6-7), and the fatigue strength ratio relationship of different stress ratios and R=-1 can be obtained:

[0055] (10)

[0056] In the formula (10), the is a parameter related to the stress ratio, in the fatigue life prediction of metal materials, the fatigue strength is calculated by using the model for normalization, and the fatigue strength under different stress ratios can be predicted through the formula (10).

[0057] In view of the problem that the fatigue strength test process of the metal material in the prior art is complex and time-consuming, the purpose of the present application is to provide a method for predicting the fatigue strength of metal materials under different stress ratios, which can simply use the fatigue strength under a certain stress ratio, based on the normalization formula of the stress ratio, so as to predict the fatigue strength under other stress ratios, avoid complicated experimental operation, and reduce the time cost and money cost.

[0058] The following is one specific embodiment of the present application:

[0059] In this embodiment, the fatigue strength under other stress ratios is predicted by using a set of fatigue strength under different stress ratios of steel, and the specific steps are as follows:

[0060] Step 1: Perform a room temperature high cycle fatigue experiment for a certain steel stress ratio R1, and calculate the fatigue strength value at the stress ratio based on the lifting method;

[0061] Step 2: Based on formula (10), normalize and divide by the value at R1 to obtain the fatigue strength at R=-1;

[0062] Step 3: Multiply the fatigue strength calculated in step (2) by the value corresponding to the target stress ratio R2 to obtain the fatigue strength value at the stress ratio R2. The predicted value and the experimental value are compared as shown in Table 1, and they are consistent, with an error of less than 10%.

[0063] Table 1 Comparison of fatigue strength prediction value and experimental value of a certain steel based on the model

[0064]

[0065] In summary, the beneficial effects of the present application are as follows:

[0066] 1. The present application proposes a simple and convenient method to predict the fatigue strength of metal materials at different stress ratios. The main principle is that the fatigue damage process at different stress ratios is determined by the energy consumption of the sample loading, which depends on the stress change, so that it can be equivalent to the action of different stress ratios, and the fatigue strength value at different stress ratios can be calculated.

[0067] 2. The present application is based on the energy consumption of the fatigue damage process, and the predicted fatigue strength at different stress ratios is consistent with the experimental value. Through the present application, the fatigue strength at different stress ratios can be simply and effectively predicted, which greatly saves the experimental time and cost.

[0068] 3. In the present application, a fatigue strength prediction model is proposed based on the energy principle. The model can theoretically explain the influence of stress ratio on the fatigue strength of metal materials, and provides an effective strategy for the fatigue strength prediction of metal materials under variable amplitude loading.

[0069] Finally, it should be noted that the above description is only for the preferred embodiments of the present application and is not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent replacements to some technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the claims of the present application.​​

Claims

1. A method of predicting fatigue strength of a metal material under different stress ratios, characterized by, Comprising the following steps: (1) Conduct fatigue tests on fatigue specimens of metallic materials and calculate the fatigue strength under a certain stress ratio R1. The range of R1 is [-1, 1). (2) The fatigue strength in step (1) is normalized by the following prediction model. The fatigue strength is normalized by dividing the value of the stress ratio R1 by the value of the fatigue strength at R = -1. ;​​ The ratio of the fatigue strength of the metal material at different stress ratios R to the fatigue strength at R = -1 is , and the relationship is as follows: ； ； (3) multiply the fatigue strength calculated in step (2) by the value corresponding to the target stress ratio R2, i.e. obtain the fatigue strength at the target stress ratio R2. ​​ 2. The method of claim 1, wherein the method is characterized by: In step (1), the fatigue experiment adopts room temperature high cycle fatigue experiment.

3. The method of claim 2, wherein the method is characterized by: The high cycle fatigue sample size and experiment process of the room temperature high cycle fatigue experiment in step (1) conform to the standard GB / T 3075-2021 "Metal Material Fatigue Test Axial Force Control Method".

4. The method of claim 3, wherein the method is characterized by: In step (1), the fatigue strength under a certain stress ratio R1 is measured by the step-up method .

5. The method of claim 4, wherein the method is characterized by: Fatigue strength in step (1) The calculation method conforms to the standard GB / T 24176-2009 "Statistical Scheme and Analysis Method for Fatigue Test Data of Metallic Materials".

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