Fatigue strength prediction method for metal materials with different tensile strengths under different stress ratio conditions

Abstract

The invention discloses a fatigue strength prediction method for metal materials with different tensile strengths under the condition of different stress ratios, and belongs to the technical field of metal material performance testing. According to the method, based on the relation between tensile strength and fatigue strength described by a P-C model, the relation is expanded to the influence relation between stress ratios and the fatigue strength, and the fatigue strength evaluation method of the metal material with multiple strength and multiple stress ratios is established. The method is simple and efficient, the fatigue strength of the same series of materials under each strength and each stress ratio can be predicted through at least two groups of stretching and fatigue experiments, a large amount of time and labor cost are saved, and the accuracy is high. In addition, the influence of an internal factor (material strength) and an external factor (stress ratio) on the fatigue strength of the component during service is comprehensively considered, the method has scientific rationality, has been verified to have a good prediction effect on the steel for wind power, can provide method support for fatigue strength checking of engineering components, and has good application prospects in the industrial field.

Description

Technical Field

[0001] The invention relates to the technical field of metal material performance testing, in particular to a fatigue strength prediction method for metal materials with different tensile strengths under different stress ratio conditions. Background Art

[0002] Predicting component life from material properties is a long-standing topic for scholars and an urgent problem to be solved in the engineering field. The variable and complex stress ratio seriously affects the fatigue life of components and also brings great difficulties to prediction research. Therefore, it is necessary to effectively predict the fatigue strength of materials at different stress ratios and establish an evaluation system for the influence of stress ratio on material fatigue performance in the study of material performance prediction of component life.

[0003] With the improvement of production efficiency and safety standards, people's demand for high strength levels of specific materials has gradually increased. While achieving high strength, scholars found that fatigue strength did not increase accordingly, but a short-term "plateau" appeared and there was a downward trend. Pang proposed the PC model to describe this relationship and established a universal relationship between fatigue strength and tensile strength, which can achieve rapid prediction of fatigue strength. This has led to new problems: What is the relationship between fatigue strength under different strengths and different stress ratios of the same series of materials? Therefore, it is of great significance to establish a description of the relationship between tensile properties and fatigue properties at various stress ratios. Summary of the invention

[0004] In order to improve the efficiency of predicting the fatigue strength of the same series of metal materials under different stress conditions, the present invention provides a method for predicting the fatigue strength of metal materials with different tensile strengths under different stress ratios. The method links the relationship between fatigue strength under different tensile strengths and different stress ratios, and can effectively predict the fatigue strength of the same series of materials under various stress ratios by only two sets of fatigue tests under different states of tension, R=1 and R≠-1.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A fatigue strength prediction method for metal materials with different tensile strengths under different stress ratio conditions, the method specifically comprising the following steps:

[0007] A fatigue strength prediction method for metal materials with different tensile strengths under different stress ratio conditions, the method predicts fatigue strength under different stress ratio conditions for the same series of metal materials with different tensile strengths, and specifically comprises the following steps:

[0008] (1) Perform axial tensile tests on metal material specimens of the same series with different tensile strengths to obtain the tensile strength σ b ;

[0009] (2) Perform fatigue strength tests on target metal material samples with the same tensile strength to obtain the fatigue strength σ under symmetrical axial load (stress ratio R = -1) -1 , and at least one fatigue strength σ under asymmetric axial load (R≠-1) w In this way, the σ of other tensile strength target metal material samples is obtained -1 and σ w ;

[0010] (3) The tensile properties (σ) of the samples with different tensile strengths obtained using steps (1)-(2) b ) and R = -1 fatigue strength data, the tensile strength of each σ -1 / σ b With σ b The values ​​are fitted by formula (1) to obtain the parameters C and P;

[0011]

[0012] (4) Using the tensile properties of the same tensile strength specimen (σ b ) and fatigue strength data of R≠-1 (σ w ), calculate the average stress σ of fatigue strength under the condition of R≠-1 m , with σ m / σ b is the horizontal axis, σ w / σ -1 Plot σ on the ordinate w / σ -1 With σ m / σ b Relationship diagram, draw (σ m / σ b ,σ w / σ -1 ) coordinates, and fit the data points through the (0,1) coordinates using formula (2) to obtain the parameter G;

[0013]

[0014] (5) The parameter G under various tensile strengths and the corresponding σ -1 / σ b The fitting coefficients G0 and k are obtained by fitting formula (3);

[0015]

[0016] (6) Substitute the obtained parameter values ​​into formula (4),

[0017]

[0018] In formula (4), σ-1 =(CP·σ b )·σ b , The strength of the same series can be obtained by calculation as σ b The fatigue strength prediction value σ of the same series of metal materials under stress ratio R w .

[0019] In the above step (1), the same series of metal materials with different tensile strengths refer to metal materials with the same chemical composition but different tensile strength values; the same series of metal materials with different tensile strengths are samples prepared by adopting different processing methods.

[0020] In the above step (1), at least two samples with different tensile strengths are selected from the same series of metal materials for testing; to ensure data repeatability, at least three samples for each tensile strength are required to perform the tensile test.

[0021] In the above step (2), the fatigue test must maintain the same environmental conditions (temperature and atmosphere, etc.) as the tensile test. To ensure the accuracy of the prediction results, 1 to 2 asymmetric axial loads can be selected for fatigue strength testing. Each set of fatigue strength must be obtained by at least 3 pairs of lifting and lowering methods, and the fatigue strength is represented by the maximum stress.

[0022] The advantages and beneficial effects of the present invention are as follows:

[0023] 1. The prediction method of the present invention relates the relationship between the tensile strength and the fatigue strength of each stress ratio of the same series of materials.

[0024] 2. The prediction method of the present invention is applicable to different tensile strengths of the same series of metal materials. As the strength changes, the values ​​of parameters C and P change, which can well describe the relationship between tensile strength and fatigue strength.

[0025] 3. The prediction method of the present invention is applicable to a variety of loading conditions. As the environmental conditions such as the number of cycles, temperature and stress concentration state of the material change, the parameter G changes, which can better describe the relationship between stress ratio and fatigue strength.

[0026] 4. The prediction method of the present invention has high accuracy and can perform effective predictions through a small amount of tensile and fatigue tests, and has the advantages of high efficiency and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The flowchart of fatigue strength prediction method for different stress ratios of metal materials with different tensile strengths;

[0028] Figure 2 is the σ of 18Ni maraging steel -1 / σ b –σb Relationship diagram;

[0029] Figure 3 is the σ of 18Ni maraging steel in two states m / σ b –σ w / σ -1 Relationship diagram;

[0030] Figure 4 is the G–σ of 18Ni maraging steel -1 / σ b Relationship diagram;

[0031] Figure 5 Verify the accuracy of the prediction results for 18Ni martensite;

[0032] Figure 6 is -1 / σ b Relationship diagram of tensile strength with its corresponding state;

[0033] Figure 7 is the σ of Q460 and Q500 m / σ b and σ w / σ -1 Value relationship diagram;

[0034] Figure 8 is the G value and σ -1 / σ b Relationship diagram;

[0035] Fig. 9 The comparison between the calculated and experimental values ​​is shown in the figure. DETAILED DESCRIPTION

[0036] The present invention is further described below in conjunction with embodiments and drawings.

[0037] The present invention is a method for predicting fatigue strength of metal materials with different tensile strengths under different stress ratio conditions. The operation process is as follows: Figure 1 .

[0038] Embodiment 1:

[0039] This example predicts the fatigue strength of 18Ni maraging steel with different tensile strengths at various stress ratios. The tensile strength and fatigue strength at stress ratios R=-1 and 0.1 of 18Ni maraging steel with two different processing technologies: 500℃ / 5h and 630℃ / 3h are measured (experimental data), and used to predict the fatigue strength at various stress ratios at 500℃ / 5h, R=0.5, 550℃ / 5h and 600℃ / 3h (verification data).

[0040] Step 1: tensile tests were performed on 500°C / 5h and 630°C / 3h tensile specimens of 18Ni maraging steel, and the tensile strengths were 1939MPa and 1358MPa, respectively.

[0041] Step 2: Fatigue test was carried out on 18Ni maraging steel. The fatigue strength at 500℃ / 5h under the conditions of R=-1 and 0.1 was 595MPa and 1056MPa respectively; the fatigue strength at 630℃ / 3h under the conditions of R=-1 and 0.1 was 588MPa and 858MPa respectively, as shown in Table 1.

[0042] Step 3: Calculate the σ of the two states of 18Ni maraging steel -1 / σ b , and the tensile strength of its corresponding state is expressed by the formula Fitting (such as Figure 2 As shown), C and P were obtained to be 0.728 and 2.171×10 -4 (See Table 1).

[0043] Step 4: Calculate the σ of 18Ni maraging steel at 500℃ / 5h and 630℃ / 3h under R=-1 and 0.1 respectively. m / σ b and σ w / σ -1 Values, plotted on σ m / σ b -σ w / σ -1 In the relationship diagram, the coordinates (0,1) are passed through the formula Perform a linear fit (such as Figure 3 As shown), the G values ​​obtained were 2.585 and 1.321 respectively (see Table 1).

[0044] Step 5: Compare the G value obtained in step 4 with σ -1 / σ b By formula Fitting (such as Figure 4 As shown), G0 and k were obtained to be 5.662 and -10.026 respectively.

[0045] Step 6: Substitute the above parameter values ​​into the formula In which σ -1 =(CP·σ b )·σ b , The fatigue strength of 18Ni maraging steel with different strengths at various stress ratios can be predicted.

[0046] Step 7: To evaluate the accuracy of the prediction results, the fatigue strength of 18Ni maraging steel at 500℃ / 5h at R=0.5, 550℃ / 5h and 600℃ / 3h at various stress ratios was measured and calculated using the above prediction method. The prediction accuracy is shown in Table 1, and the relationship between the calculated value and the experimental value is compared in Table 1. Figure 5 (This step is to verify the accuracy of the method and can be ignored in actual operation).

[0047] Table 1 Summary of prediction data for 18Ni maraging steel

[0048]

[0049] Embodiment 2:

[0050] This example predicts the fatigue strength of wind power steel with different tensile strengths at various stress ratios. The tensile strength and fatigue strength of two different processing technologies: Q460 and Q500 steel at stress ratios R = -1 and 0.5 are measured (experimental data), and used to predict the fatigue strength of Q500 at R = 0.5, Q390 and Q420 at various stress ratios (verification data).

[0051] Step 1: Conduct tensile tests on Q460 and Q500 tensile specimens of wind power steel, and the tensile strengths are 649MPa and 734MPa respectively.

[0052] Step 2: Fatigue tests are carried out on Q460 and Q500 steels for wind power. The fatigue strength of Q460 under the conditions of R=-1 and 0.5 is 326MPa and 648MPa respectively; the fatigue strength of Q500 under the conditions of R=-1 and 0.5 is 340MPa and 668MPa respectively, as shown in Table 2.

[0053] Step 3: Calculate the σ of two grades of wind power steel -1 / σ b , and the tensile strength of its corresponding state is expressed by the formula Fitting (such as Figure 6 As shown), C and P were obtained to be 0.801 and 4.600×10 -4 (See Table 2).

[0054] Step 4: Calculate the σ of Q460 and Q500 for wind power steel under the conditions of R = -1 and 0.5 respectively. m / σ b and σ w / σ -1 Values, plotted on σ m / σ b -σ w / σ -1 In the relationship diagram, the coordinates (0,1) are passed through the formula Perform a linear fit (such as Figure 7 As shown), the G values ​​obtained were 1.319 and 1.413 respectively (see Table 2).

[0055] Step 5: Compare the G value obtained in step 4 with σ -1 / σ b By formula Fitting (such as Figure 8 As shown), G0 and k are obtained as 2.527 and -2.404 respectively.

[0056] Step 6: Substitute the above parameter values ​​into the formula In which σ -1 =(CP·σ b )·σ b , The fatigue strength of wind power steel with different strengths can be predicted at various stress ratios.

[0057] Step 7: To evaluate the accuracy of the prediction results, the fatigue strength of Q500, Q390 and Q420 steel for wind power generation at R = 0.5 and various stress ratios was measured and calculated using the above prediction method. The prediction accuracy is shown in Table 2, and the relationship between the calculated value and the experimental value is compared in Table 2. Fig. 9 (This step is to verify the accuracy of the method and can be ignored in actual operation).

[0058] Table 2 Summary of forecast data for steel used in wind power

[0059]

[0060]

[0061] The above embodiments are merely illustrative of the principles and performances of the present invention, and are not exhaustive. People can also obtain other embodiments based on the present embodiments without creative work, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A fatigue strength prediction method for metal materials with different tensile strengths under different stress ratio conditions, characterized by: This method predicts fatigue strength under different stress ratio conditions for the same series of metal materials with different tensile strengths, and specifically includes the following steps: (1) Perform axial tensile tests on metal material specimens of the same series with different tensile strengths to obtain the tensile strength σ b ; (2) Perform fatigue strength tests on target metal material samples with the same tensile strength to obtain the fatigue strength σ under symmetrical axial load (stress ratio R = -1) -1 , and at least one fatigue strength σ under asymmetric axial load (R≠-1) w In this way, the σ of other tensile strength target metal material samples is obtained -1 and σ w ; (3) The tensile properties (σ) of the samples with different tensile strengths obtained using steps (1)-(2) b ) and R = -1 fatigue strength data, the tensile strength of each σ -1 / σ b With σ b The values ​​are fitted by formula (1) to obtain the parameters C and P; (4) Using the tensile properties of the same tensile strength specimen (σ b ) and fatigue strength data of R≠-1 (σ w ), calculate the average stress σ of fatigue strength under the condition of R≠-1 m , with σ m / σ b is the horizontal axis, σ w / σ -1 Plot σ on the ordinate w / σ -1 With σ m / σ b Relationship diagram, draw (σ m / σ b ,σ w / σ -1 ) coordinates, and fit the data points through the (0,1) coordinates using formula (2) to obtain the parameter G; (5) The parameter G under various tensile strengths and the corresponding σ -1 / σ b The fitting coefficients G0 and k are obtained by fitting formula (3); (6) Substitute the obtained parameter values ​​into formula (4), In formula (4), σ -1 =(CP·σ b )·σ b , The strength of the same series can be obtained by calculation as σ b The fatigue strength prediction value σ of the same series of metal materials under stress ratio R w .

2. The fatigue strength prediction method for metal materials with different tensile strengths under different stress ratio conditions according to claim 1, characterized in that: In step (1), the same series of metal materials with different tensile strengths refer to metal materials with the same chemical composition but different tensile strength values; the same series of metal materials with different tensile strengths are samples prepared by adopting different processing methods.

3. The fatigue strength prediction method for metal materials with different tensile strengths under different stress ratio conditions according to claim 1, characterized in that: In step (1), at least two samples with different tensile strengths are selected from the same series of metal materials for testing; to ensure data repeatability, at least three samples for each tensile strength are required to perform the tensile test.

4. The fatigue strength prediction method for metal materials with different tensile strengths under different stress ratio conditions according to claim 1, characterized in that: In step (2), the fatigue test needs to maintain the same environmental conditions (temperature and atmosphere, etc.) as the tensile test; to ensure the accuracy of the prediction results, 1 to 2 asymmetric axial loads can be selected for fatigue strength testing; each set of fatigue strength needs to be obtained by at least 3 pairs of lifting and lowering methods, and the fatigue strength is represented by the maximum stress.

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