A method and storage medium for predicting the thermomechanical fatigue life of metal materials based on strain rate influence

Abstract

The present application discloses a method and storage medium for predicting the thermomechanical fatigue life of metal materials based on the influence of strain rate, wherein the method comprises: first determining the thermomechanical fatigue working condition and target temperature, conducting low-cycle fatigue and thermomechanical fatigue tests on the metal material, determining the relationship between the low-cycle and thermomechanical fatigue cycle hysteresis energy and the plastic strain and stress range, establishing a low-cycle fatigue energy model and obtaining relevant parameters; then introducing the fatigue-creep damage coupling index, correcting the strain rate, obtaining a thermomechanical fatigue life prediction model, and finally achieving prediction. The present application only requires a small amount of low-cycle fatigue and thermomechanical fatigue tests to achieve accurate prediction of thermomechanical fatigue life, and can also be used for low-cycle fatigue life prediction, while improving accuracy, significantly saving time, manpower and material resources. It can be applied to creep-fatigue coupled life prediction of high-temperature alloys and heat-resistant metal materials such as gas turbine blades and turbine disks, and internal combustion engine pistons.

Description

Technical Field

[0001] The present application relates to a low-cycle fatigue and thermomechanical fatigue performance test of a metal material, belongs to the field of thermomechanical testing, and particularly relates to a thermomechanical fatigue life prediction method for a metal material based on strain rate influence and a computer storage medium. Background Art

[0002] In many fields such as aerospace, energy and power, many hot-end components (internal combustion engine pistons, gas turbine blades and turbine discs, etc.) operate for a long time under high temperatures and cyclic loads. The cyclic loads include three types of loads, namely assembly loads, machine loads and thermal loads. According to failure analysis, the most important failure form is thermomechanical fatigue damage caused by the reciprocating action of thermal loads and mechanical loads. The effect of the interactive coupling of thermal loads and mechanical loads on material fatigue often causes the number of cyclic failures to be less than 105. If a reasonable thermomechanical fatigue life prediction cannot be made, the service cost of the component will be greatly increased, the service safety of the component will be reduced, and irreparable damage will be caused.

[0003] Due to the complexity of thermomechanical fatigue damage, thermomechanical fatigue life has puzzled researchers for nearly half a century. Since the 1950s, researchers at home and abroad have conducted extensive research on the fatigue damage behavior of materials related to thermomechanics. Starting with the Manson-Coffin equation, methods have been developed to consider the effects of frequency and plastic strain on fatigue, including the SRP method and frequency correction method. However, these models require a large number of experimental parameters and lack physical meaning. Another approach is the ductility exhaustion model and time fractional model based on linear cumulative damage. These methods, including the Neu-Sehitoglu model, the Miller model, and the J-integral model, establish physical models by revealing the physical damage behavior during thermomechanical fatigue. However, the limitations of these methods are that, while the theoretical derivations of the formulas are rigorous, they fail to consider the coupling effects between damage, resulting in unconservative or overconservative life predictions. Furthermore, these methods lack applicability to fatigue life predictions for different materials. Therefore, a relatively simple and accurate fatigue life prediction method is urgently needed to address the problem of thermomechanical fatigue failure. Summary of the Invention

[0004] According to one aspect of the present application, a method for predicting the thermomechanical fatigue life of metal materials based on the influence of strain rate is provided. The method introduces a strain rate correction factor and takes into account the fatigue-creep coupled damage behavior. Only a small number of low-cycle fatigue and thermomechanical fatigue tests are required to accurately predict the thermomechanical fatigue life. While improving the prediction accuracy, the test volume requirement is greatly reduced.

[0005] The method for predicting the thermomechanical fatigue life of metal materials based on the influence of strain rate includes:

[0006] (1) Analyze the actual service conditions of service components, obtain the thermomechanical fatigue test load spectrum used to reflect the service behavior of the material, and determine the range of thermomechanical fatigue test control parameters;

[0007] (2) determining a fatigue load range for thermomechanical fatigue of the component to be life predicted, wherein the fatigue load range includes a target strain amplitude and a test temperature range; wherein the fatigue load has the following characteristics: the mechanical load varies periodically with time, and the temperature varies periodically with time; and the target strain amplitude includes thermal strain and mechanical strain amplitude values;

[0008] (3) According to the range of the control parameters of the thermomechanical fatigue test, a constant temperature low cycle fatigue performance test is performed under the conditions of the highest temperature of thermomechanical fatigue and the target strain amplitude to obtain the cyclic plastic strain Δε p , stress range Δσ, single cycle hysteresis energy W i , cycle number i and strain rate Calculate the hysteresis loop shape factor k;

[0009] (4) Based on the energy method, establish the stable hysteresis energy W s and the corresponding fatigue life N f The relationship between and is used to obtain the values ​​of material parameters W0 and β;

[0010] (5) Perform thermomechanical fatigue tests to obtain the median stable hysteresis energy W s , cycle number and strain rate and the hysteresis loop shape factor k;

[0011] (6) Based on the low-cycle fatigue energy model parameters, the strain rate is introduced and Creep-fatigue damage coupling index r, to establish a thermal engine fatigue life prediction model;

[0012] (7) Substituting the data and parameters of steps (4) and (5) into the thermal engine fatigue life prediction model of step (6), obtaining the damage coupling index, and calculating the thermal engine fatigue life in different temperature ranges and mechanical strains.

[0013] Optionally, in step (1), the thermomechanical fatigue test control parameters include: the maximum test temperature, the minimum test temperature, the heating rate, the cooling rate, the high temperature holding time, the low temperature holding time, and the constraint coefficient. The thermomechanical fatigue test waveforms commonly used in laboratories include in-phase (IP, maximum tensile strain corresponding to the maximum temperature) and anti-phase (OP, maximum compressive stress corresponding to the maximum temperature). The method of the present application is applicable to both in-phase and anti-phase thermomechanical fatigue.

[0014] Furthermore, in step (2), the mechanical strain amplitude value range and the test temperature range need to be selected according to actual working conditions, and the mechanical load change cycle is the same as the temperature load change cycle, and the thermal strain amplitude value range is obtained by changing the test temperature.

[0015] Furthermore, in step (3), the single cycle hysteresis energy W i The simplified method obtained is to determine the corresponding plastic strain energy from the strain and stress at a specific temperature. The specific formula is as follows:

[0016] W i =∫σdε=k·Δε p ·Δσ

[0017] Among them, W i is the plastic strain energy of a single hysteresis loop, Δε p and Δσ are the plastic strain and stress ranges, respectively, and k is the hysteresis loop shape factor;

[0018] Or, obtain it directly through the software.

[0019] Furthermore, in step (4), the model based on the energy method is shown in the following formula:

[0020]

[0021] Among them, N f is fatigue life, W s In order to stabilize the hysteresis energy, the hysteresis energy corresponding to the median life was selected in the experiment, and β and W0 were fitting parameters.

[0022] Furthermore, step (6) also includes: selecting different total strain amplitudes and temperature ranges according to actual service conditions.

[0023] Furthermore, in step (6), the thermomechanical fatigue life prediction model is shown in the following formula:

[0024]

[0025] in, represents the thermomechanical fatigue life, the values ​​of the constants W0 and β are obtained through step (4), and the creep-fatigue damage coupling index r is obtained based on the data of low-cycle fatigue performance test and thermomechanical fatigue test.

[0026] Preferably, the data of the low-cycle fatigue performance test and the thermo-mechanical fatigue test include: thermo-mechanical fatigue requiring at least two different total mechanical strains and isothermal low-cycle fatigue corresponding to the highest temperature.

[0027] According to another aspect of the present application, a computer-readable storage medium is provided, wherein the computer-readable storage medium stores computer-executable instructions. When the computer-executable instructions are run on a computer, the computer executes some or all of the steps in the method for predicting the thermomechanical fatigue life of metal materials based on strain rate influence.

[0028] The beneficial effects of this application include:

[0029] 1) The prediction method provided in this application adopts an energy model based on the energy method, which has good applicability for fatigue life prediction (low cycle fatigue and thermomechanical fatigue) under different conditions.

[0030] 2) The thermal engine fatigue life prediction model provided in this application takes into account the influence of strain rate on the basis of the traditional energy method model. According to the damage characteristics of thermomechanical fatigue, taking into account that in addition to fatigue damage, there is also creep damage related to time, the strain rate is corrected by introducing a damage coupling index. This application combines the advantages of the energy model and the frequency correction model, and has the characteristics of clear physical meaning, accurate prediction, and high applicability. Through simple theoretical derivation, using low-cycle fatigue data and correcting with a small amount of thermomechanical data, time, manpower and money costs are greatly saved. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a flow chart of a method for predicting the thermomechanical fatigue life of metal materials based on the influence of strain rate in one embodiment of the present application;

[0032] Figure 2 This is a load spectrum of a same-phase thermomechanical fatigue test of cast aluminum material in one embodiment of the present application;

[0033] Figure 3 The schematic diagram of the hysteresis loop and plastic strain energy and the stable hysteresis energy W in one embodiment of the present application is shown in FIG. s and Δε p .Δσ relationship;

[0034] Figure 4 It is the evolution of hysteresis energy of low cycle fatigue cycle of cast aluminum material at 350℃ and 425℃;

[0035] Figure 5 It is the prediction result of thermomechanical fatigue life of cast aluminum material at 120-350℃ and 120-425℃ in the same direction. DETAILED DESCRIPTION

[0036] The present application is described in detail below with reference to embodiments, but the present application is not limited to these embodiments.

[0037] like Figure 1As shown in the figure, the prediction method of thermomechanical fatigue life of metal materials based on the influence of strain rate is as follows:

[0038] Step (1): Analyze the temperature, stress-strain characteristics of the service component to obtain a thermomechanical fatigue test load spectrum that can reasonably reflect the service behavior of the material. The important control parameters of the thermomechanical fatigue test, such as the maximum test temperature, minimum test temperature, heating rate, cooling rate, high temperature holding time, low temperature holding time and constraint coefficient, are determined through the test load spectrum. The thermomechanical fatigue test waveforms commonly used in the laboratory include in-phase (IP, maximum tensile strain corresponding to maximum temperature) and anti-phase (OP, maximum compressive stress corresponding to maximum temperature). This method is applicable to both in-phase and anti-phase thermomechanical fatigue.

[0039] Step (2): Determine the target strain amplitude and service temperature, that is, determine the thermal strain and mechanical strain amplitude values ​​for life prediction, as well as the test temperature range.

[0040] Step (3): Select the constant temperature low cycle fatigue performance test at the highest temperature of thermal mechanical fatigue to obtain the cyclic plastic strain Δε p , stress range Δσ, single cycle hysteresis energy W i (area of ​​hysteresis loop per cycle), number of cycles and strain rate etc. A simplified method for obtaining plastic strain energy is to determine the corresponding plastic strain energy by strain and stress at a specific temperature, as shown in the following formula:

[0041] W i =∫σdε=k·Δε p ·Δσ

[0042] Where: W i Plastic strain energy of a single hysteresis loop, Δε p and Δσ are the plastic strain and stress ranges, respectively, and k is the shape factor.

[0043] Step (4): First, establish an energy-based damage model and fatigue damage characterization parameters. If the fatigue damage of the sample is regarded as the result of the input of plastic work, the combined effect of stress and strain can be considered at the same time. Therefore, the fatigue damage amount of the sample in each cyclic deformation process can be defined as: D i =(W i / W0) β .

[0044] When the material reaches the maximum damage D = 1 that it can bear, it is considered to be fatigue fracture, that is:

[0045]

[0046] Where: N f is fatigue life, Ws is the stable cyclic plastic strain energy under this condition, W0 and β are material constants independent of the material loading conditions.

[0047] Step (5): Based on the energy method in step (4), establish the median life hysteresis energy W s and the corresponding fatigue life N f , and obtain the values ​​of material parameters W0 and β.

[0048] Step (6): Perform thermomechanical fatigue test to obtain the median hysteresis energy W s , cycle number s and strain rate and hysteresis loop shape factor k, etc.

[0049] Step (7): Under high temperature conditions, due to the dependence of time and deformation on the entire fatigue cycle loading process, under thermomechanical fatigue conditions, in addition to fatigue damage, the material will also have time-related damage behavior (creep and oxidation). More sensitive, in order to take these factors into account, based on the energy model, the strain rate corrected damage parameter is introduced, as shown in the following formula:

[0050]

[0051] When the damage accumulates to the point of fracture, it is expressed as:

[0052]

[0053] Step (8): Substitute the data and parameters in steps (5) and (6) into step (7) to obtain the damage coupling index r, and obtain the thermomechanical fatigue life prediction equation and parameters. Then, the thermomechanical fatigue life in different temperature ranges and mechanical strains can be calculated.

[0054] Among them, N f is fatigue life, W s To stabilize the plastic strain energy (taken from the hysteresis loop area corresponding to the median life), β and W0 are fitting parameters. This equation can be used to predict the thermomechanical fatigue life based on the parameters obtained from low-cycle fatigue test data, and to obtain and correct the test parameters using a small amount of thermomechanical fatigue data:

[0055]

[0056] Among them, the constants W0 and β are obtained through low-cycle fatigue tests at the highest temperature of thermomechanical fatigue; and the parameter r is obtained through a small amount of thermomechanical fatigue test data. Macroscopically speaking, the intrinsic fatigue toughness W0 represents the resistance to fatigue crack growth; the fatigue cracking index β characterizes the fatigue cracking resistance, which is the ability to convert the degree of external action into energy that damages the material; the coupled damage index r represents the effect of time-related damage (oxidation, creep, etc.) on fatigue life. Microscopically speaking, W0 is related to the fracture toughness of the material. An increase in W0 indicates an increase in the fatigue damage limit that the material can accommodate. The β value is related to the evolution of the material's microstructure (such as dislocations, twins, holes and cracks). When β becomes smaller, under the same external plastic work, it is easy to generate defects inside the material and expand the original defects (such as holes, cracks, etc.), thereby increasing its degree of damage. W i W is the plastic work done by a single deformation that can be considered as material failure. s The hysteresis loop area corresponding to the median life is represented by the equation. This model has clear physical meaning and good fitting accuracy. After fitting, the specific values ​​of β and W0 are obtained, which reveals the exponential relationship between the thermomechanical fatigue life and the hysteresis energy. However, since thermomechanical fatigue testing at the target strain amplitude was not performed, it is not possible to directly predict the thermomechanical fatigue life based on the specific values ​​of β and W0.

[0057] The present application also provides a computer-readable storage medium, which stores computer-executable instructions. When the computer-executable instructions are run on a computer, the computer executes some or all of the steps in the method for predicting the thermomechanical fatigue life of metal materials based on strain rate influence.

[0058] Example 1

[0059] This embodiment predicts the life of cast aluminum materials under isotropic thermomechanical fatigue conditions.

[0060] (1) The cast aluminum material is taken from the diesel engine piston. According to the working conditions, the required predicted thermomechanical fatigue temperature load is determined to be 120~350℃ and 120~425℃.

[0061] (2) Low cycle fatigue test, test conditions: temperature is the highest temperature of thermomechanical cycle 350℃ and 425℃, mechanical strain is ±0.2%, ±0.3%, ±0.4% respectively.

[0062] (3) Establish the plastic strain energy W i and plastic strain Δε p and the relationship between the stress range Δσ (such as Figure 3 As shown), and the plastic strain energy W is obtained i (Hysteresis loop area).

[0063] Wi =∫σdε=k·Δε p ·Δσ

[0064] For 350°C low cycle fatigue relationship: k = 0.79.

[0065] For 425°C low cycle fatigue relationship: k = 0.84.

[0066] (4) Figure 4 As shown, an energy damage model is established to calculate the median life hysteresis loop area W of each test data. s And the corresponding fatigue life (cycle life) N f For the above tests, the following equations are used for fitting:

[0067]

[0068] For low cycle fatigue at 350°C: β = 2.23, W0 = 12.2 MJ / m 3 ;

[0069] For 425℃ low cycle fatigue relationship: β=1.66,W0=1.66MJ / m 3 .

[0070] (5) Based on the low-cycle fatigue prediction formula, the strain rate factor is introduced to establish the following formula, and the formula is corrected by the thermomechanical fatigue life.

[0071]

[0072] (6) According to the actual working conditions, establish the loading conditions of the thermomechanical fatigue test such as Figure 2 As shown in the figure, for operating conditions between 120°C and 350°C, the target strain amplitude is ±0.2% to ±0.3%; for operating conditions between 120°C and 425°C, the target strain amplitude is ±0.3% to ±0.8%. Multiple strain amplitudes are selected here as target strain amplitudes for comparison with experimental results.

[0073] (7) Substitute the thermomechanical fatigue life data into the formula in step (5) to obtain the damage coupling index r at different temperatures.

[0074] For 120-350°C thermomechanical fatigue relationship: r = -0.63;

[0075] For the thermomechanical fatigue relationship between 120 and 425°C: r = -0.31.

[0076] (8) Based on the exponential relationship in step (3) and the frequency correction in step (5), the ultimate life under the target mechanical strain amplitude under the thermomechanical fatigue conditions of 120-350℃ and 120-425℃ is calculated respectively. Compared with the actual measured test data, Figure 5 The results of the predicted and actual values ​​of the thermomechanical fatigue life and the error band diagram are shown, that is, the actual life distribution is within a 2-fold error range.

[0077] The above description is merely an embodiment of the present application and does not constitute any form of limitation to the present application. Although the present application discloses the preferred embodiments as above, it is not intended to limit the present application. Any technical personnel familiar with the present profession, without departing from the scope of the technical solution of the present application, using the technical content disclosed above to make slight changes or modifications are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for predicting the thermomechanical fatigue life of metal materials based on the influence of strain rate, characterized in that: The method includes: (1) Analyze the actual service conditions of service components, obtain the thermomechanical fatigue test load spectrum used to reflect the service behavior of the material, and determine the range of thermomechanical fatigue test control parameters; (2) determining a fatigue load range for thermomechanical fatigue of the component to be life predicted, wherein the fatigue load range includes a target strain amplitude and a test temperature range; wherein the fatigue load has the following characteristics: the mechanical load varies periodically with time, and the temperature varies periodically with time; and the target strain amplitude includes thermal strain and mechanical strain amplitude values; (3) According to the range of the control parameters of the thermomechanical fatigue test, a constant temperature low cycle fatigue performance test is performed under the conditions of the highest temperature of thermomechanical fatigue and the target strain amplitude to obtain the cyclic plastic strain Δε p , stress range Δσ, single cycle hysteresis energy W i , cycle number i and strain rate (4) Based on the energy method, establish the stable hysteresis energy W s and the corresponding fatigue life N f The relationship between and is used to fit the values ​​of material parameters W0 and β; (5) Perform thermomechanical fatigue tests to obtain the median stable hysteresis energy W s , cycle number and strain rate and the hysteresis loop shape factor k; (6) Based on the low-cycle fatigue energy model parameters, the strain rate is introduced and Creep-fatigue damage coupling index r, to establish a thermal engine fatigue life prediction model; (7) Substituting the data and parameters of steps (4) and (5) into the thermal engine fatigue life prediction model of step (6), obtaining the damage coupling index, and calculating the thermal engine fatigue life in different temperature ranges and mechanical strains.

2. The method for predicting the thermomechanical fatigue life of metal materials based on strain rate influence according to claim 1, characterized in that: In step (1), the control parameters of the thermomechanical fatigue test include: the maximum test temperature, the minimum test temperature, the heating rate, the cooling rate, the high temperature holding time, the low temperature holding time and the constraint coefficient.

3. The method for predicting the thermomechanical fatigue life of metal materials based on strain rate influence according to claim 1, characterized in that: In step (2), the mechanical strain amplitude value range and the test temperature range need to be selected according to the actual working conditions, and the mechanical load change cycle is the same as the temperature load change cycle, and the thermal strain amplitude value range is obtained by the change of the test temperature.

4. The method for predicting the thermomechanical fatigue life of metal materials based on strain rate influence according to claim 1, characterized in that: In step (3), the single cycle hysteresis energy W i The method for obtaining is: calculated by the following formula: W i =∫σdε=k·De p ·Board Among them, W i is the plastic strain energy of a single hysteresis loop, Δε p and Δσ are the plastic strain and stress ranges, respectively, and k is the hysteresis loop shape factor; Or, obtain it directly through the software.

5. The method for predicting the thermomechanical fatigue life of metal materials based on strain rate influence according to claim 1, characterized in that: In step (4), the model based on the energy method is shown as follows: Among them, N f is fatigue life, W s In order to stabilize the hysteresis energy, the hysteresis energy corresponding to the median life was selected in the experiment, and β and W0 were fitting parameters.

6. The method for predicting the thermomechanical fatigue life of metal materials based on strain rate effect according to claim 1, characterized in that: Step (6) also includes: selecting different total strain amplitudes and temperature ranges according to actual service conditions.

7. The method for predicting the thermomechanical fatigue life of metal materials based on strain rate effect according to claim 1, characterized in that: In step (6), the thermal engine fatigue life prediction model is shown in the following formula: in, represents the thermomechanical fatigue life, the values ​​of the constants W0 and β are obtained through step (4), and the creep-fatigue damage coupling index r is obtained based on the data of low-cycle fatigue performance test and thermomechanical fatigue test.

8. The method for predicting the thermomechanical fatigue life of metal materials based on strain rate effect according to claim 7, characterized in that: The data of the low-cycle fatigue performance test and the thermo-mechanical fatigue test include: thermo-mechanical fatigue requiring at least two different total mechanical strains and isothermal low-cycle fatigue corresponding to the highest temperature.

9. A computer-readable storage medium, characterized in that The computer-readable storage medium stores computer-executable instructions, which, when executed on a computer, enable the computer to execute the method for predicting the thermomechanical fatigue life of a metal material based on strain rate influence according to any one of claims 1 to 8.

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