Life prediction method considering stress relaxation effect based on total tensile strain energy density

Abstract

The present invention discloses a life prediction method based on the total tensile strain energy density taking into account the stress relaxation effect. First, at the same temperature, the test is carried out in a strain control mode to obtain the mechanical response parameters and test life of each material test in the half-life cycle. Then, based on the mechanical response in the load-holding stage, a stress relaxation model is fitted and the area under each region of the half-life cycle hysteresis loop is used as the damage parameter to represent the inelastic strain energy density caused by various loads. At the same time, the material's compensation effect is considered and the influence of the average stress on the life is corrected. Finally, based on the strain energy density-frequency separation method, a life prediction model is established to achieve fatigue-creep life prediction. The model of the method of the present invention can comprehensively consider the influence of multiple important factors such as stress relaxation, loading waveform, loading frequency, and average stress on the life, and uses the area of ​​the hysteresis loop as the damage parameter, so that the model has certain physical significance and has strong versatility in practical engineering applications.

Description

Technical Field

[0001] The present invention belongs to the technical field of material life prediction, and in particular relates to a life prediction method based on total tensile strain energy density and taking stress relaxation effect into consideration. Background Art

[0002] Hot-end components, operating in high-temperature environments, are subject not only to fatigue damage but also to high-temperature creep damage. The specific damage type varies depending on the load waveform and hold time, with three main types occurring: fatigue-dominated, creep-dominated, and creep-fatigue interaction.

[0003] The hot-end components of aviation equipment often operate under high-temperature, cyclic, and complex loads. Unlike fatigue or creep damage caused by a single factor, they often fail due to low-cycle creep fatigue damage. Repeated starts and stops and operating mode changes often cause low-cycle fatigue damage to the material, while cruising under certain operating conditions can also cause creep damage. Due to harsh operating conditions, low-cycle creep fatigue damage has become one of the main causes of failure of hot-end components. When hot-end components fail, they often have serious consequences. Therefore, accurately estimating the low-cycle creep fatigue life of the materials used in hot-end components is of great significance to the safe service of aviation equipment.

[0004] Hundreds of life prediction models have been proposed by scholars for the life prediction of hot-end components. Most of these prediction models are simple combinations of stress, strain, or stress-strain. Models such as the critical plane method, Basquin method, Manson-Coffin method, and strain energy frequency separation (SEFS) method do not consider the impact of loading waveforms on component life. Models such as the ductility exhaustion approach (DE), the continuous damage mechanics Kachanov-Rabotnov method, and the Lemaitre method lack the consideration of important factors such as average stress and loading frequency on component life. At the same time, the nonlinear creep fatigue damage accumulation method (NDS), the strain range partitioning method (SRP), and the strain energy partitioning method (SEP) contain too many material constants, making the models too complex to be used in real scenarios. Therefore, the above-mentioned methods all have some limitations in actual engineering life prediction. Summary of the Invention

[0005] In order to solve the above technical problems, the present invention provides a life prediction method based on the total tensile strain energy density taking into account the stress relaxation effect, constructs a life prediction model with fewer undetermined material constants, lower complexity, and higher prediction accuracy, and realizes low-cycle creep fatigue life prediction.

[0006] The technical solution adopted by the present invention is: a life prediction method based on the total tensile strain energy density and considering the stress relaxation effect, the specific steps are as follows:

[0007] S1. Conduct uniaxial low-cycle fatigue tests and low-cycle fatigue-creep tests on the material in strain-controlled mode at the same temperature to obtain the mechanical response parameters and test life of the material in each test at the half-life cycle;

[0008] The mechanical response parameters include: stress response range, total strain range Δε t , inelastic strain range Δε in , tension and compression holding time t h , loading frequency v, stress relaxation range during load maintenance, inelastic strain energy density range ΔW in .

[0009] Among them, the stress response range includes: the maximum stress σ under half life cycle max , minimum stress σ under half life cycle min ; The stress relaxation range during load holding includes: the stress relaxation range Δσ during tensile load holding r,tension , stress relaxation range Δσ under compression load r,compression .

[0010] S2. Fitting the stress relaxation model based on the mechanical response during the holding stage;

[0011] The load holding stage in the strain control mode is a stage in which the strain load remains constant over a period of time.

[0012] S3. Determine the inelastic strain energy density generated by fatigue-creep loading, i.e., the area of ​​each region of the hysteresis loop under half-life cycle, and propose a new inelastic strain energy density as a damage parameter;

[0013] The inelastic strain energy density W in Including: Plastic strain energy density W p , tensile creep strain energy density W c , the inelastic strain energy density W generated during the compression and loading stage tc .

[0014] The damage parameter is the sum of the plastic strain energy density, the tensile creep strain energy density, and the inelastic strain energy density generated in the compression holding stage.

[0015] S4. Considering the restoration effect, determine the additional inelastic strain energy density generated by the mean stress and correct the influence of the mean stress on the lifespan;

[0016] Among them, the phenomenon that the average compressive stress makes it difficult for microscopic holes generated in the fatigue-creep process to form and connect is called the restoration effect.

[0017] S5. Based on the strain energy density-frequency separation method SEFS, a fatigue-creep life prediction model is established by combining the sum of the inelastic strain energy density described in step S3 and the additional inelastic strain energy density generated by the average stress in step S4 to achieve fatigue-creep life prediction.

[0018] Furthermore, the step S1 is specifically as follows:

[0019] At the same temperature, low cycle fatigue tests and low cycle fatigue-creep tests of material specimens were carried out in strain control mode.

[0020] Among them, the material specimens are subjected to the same total strain range Δε t and strain rate Loading is performed, and the loading waveforms include: pure fatigue pp type loading without holding time, fatigue-creep cp type loading with holding time in the tensile stage, fatigue-creep pc type loading with holding time in the compression stage, and fatigue-creep cc type loading with holding time in both the tensile and compression stages.

[0021] The holding time t for tension and compression of each test h The mechanical response parameters of the specimens in each group of tests under life and half-life cycles were obtained.

[0022] Furthermore, the step S2 is specifically as follows:

[0023] Combined with the time-dependent stress response during the holding stage, the Feltham stress relaxation model is used to describe the stress relaxation phenomenon, and the expression is as follows:

[0024] σ=σ0[1-B″·ln(bt h +1)] (1)

[0025] Where σ0 represents the initial stress at the beginning of the holding stage; σ represents the stress at the end of the holding stage; t h represents the holding time; B″ and b represent the fitting parameters of the Feltham model.

[0026] Furthermore, the step S3 is specifically as follows:

[0027] S31. Determine the plastic strain energy density resulting from changes in operating conditions;

[0028] The plastic strain energy density generated by the change in working conditions can be described by the area of ​​the hysteresis loop as PSED, which is expressed as follows:

[0029]

[0030] Among them, β represents the fitting parameter, Δσ represents the stress range, and Δε p Indicates the range of plastic strain caused by changes in working conditions.

[0031] Only the tensile inelastic strain energy density will induce fatigue-creep damage, so the plastic strain energy density W generated by the working condition change will be described. p Corrected, the expression is as follows

[0032]

[0033] Among them, σ max Indicates the maximum stress.

[0034] S32, determining the creep strain energy density generated during the tensile load-holding stage, i.e., the tensile creep strain energy density;

[0035] The creep strain energy density W generated during the tensile loading stage is determined by considering the stress relaxation phenomenon. c , under the tensile strain holding state, the material produces creep phenomenon, and the creep strain ε c , and the maximum strain ε max If constant, the creep strain of this part evolves from the elastic strain. Creep strain energy density W c It is expressed by the area of ​​the stress relaxation region in the hysteresis loop under half-life cycle, and the expression is as follows:

[0036]

[0037] Among them, σ max represents the maximum stress, σ rt represents the stress at the end of the tensile holding stage; E represents Young's modulus.

[0038] S33. Determine the inelastic strain energy density generated by the compression and holding stage;

[0039] Stress relaxation during the compression and holding stage produces inelastic strain This results in a decrease in the compressive stress, and the hysteresis loop reaches a stable state at half the life cycle. In the tensile stage, additional tensile inelastic strain energy density is generated, which makes the hysteresis loop reach a stable closed state. The tensile inelastic strain energy density uses the stress relaxation in the compression holding stage to generate inelastic strain. The inelastic strain energy density W generated during the compression and holding stage tcThat is, the area of ​​the inelastic strain energy density generated by the hysteresis loop of the half-life cycle. The expression of this area is as follows:

[0040]

[0041] Among them, σ rc represents the stress at the end of the compression holding stage; σ min Indicates the minimum stress.

[0042] S34. Based on steps S31-S33, a new inelastic strain energy density is proposed as a damage parameter;

[0043] The damage parameter W in =W p +W c +W tc , the expression is as follows:

[0044]

[0045] Furthermore, the step S4 is specifically as follows:

[0046] First, the mean stress σ m The area enclosed by the inelastic strain on the half-life hysteresis loop takes into account the influence of the mean stress, and the expression is as follows:

[0047]

[0048] Among them, W m represents the additional inelastic strain energy density generated by the mean stress, Δε in Represents the inelastic strain range under half life cycle.

[0049] Combined with the damage parameter W proposed in step S3 in , the inelastic strain energy density damage parameter considering the average stress is proposed as W in,improved =W in +W m , the expression is as follows:

[0050]

[0051] Furthermore, the step S5 is specifically as follows:

[0052] The SEFS model is constructed based on the strain energy density-frequency separation method, which is used as the basis for building a life prediction model. The expression is as follows:

[0053]

[0054] Among them, N f represents the predicted life of the SEFS model, W represents the strain energy density, vt represents the frequency of the stretching process, v c The frequency of the compression process can be expressed by the strain rate of tension and compression, respectively and holding time t h Calculated to consider the effect of loading frequency on life prediction, C, φ, m, k are all fitting parameters of SEFS model;

[0055] Then the strain energy density-frequency separation method is combined with W in,improved , predicting the fatigue-creep life of materials The expression is as follows:

[0056]

[0057] Among them, N c-f Indicates the fatigue-creep life of the material.

[0058] Beneficial effects of the present invention: The method of the present invention first conducts uniaxial low-cycle fatigue tests and low-cycle fatigue-creep tests of the material in a strain-controlled mode at the same temperature, obtains the mechanical response parameters and test life of the material in each test in the half-life cycle, and then, based on the mechanical response in the holding stage, fits the stress relaxation model and uses the area under each region of the half-life cycle hysteresis loop as the damage parameter to represent the inelastic strain energy density caused by various loads, while considering the material's healing effect and correcting the influence of the average stress on the life. Finally, based on the strain energy density-frequency separation method, a fatigue-creep life prediction model considering the loading waveform, loading frequency, holding time, stress relaxation, and average stress is established to achieve fatigue-creep life prediction. The life prediction model proposed by the method of the present invention can comprehensively consider the influence of multiple important factors such as stress relaxation, loading waveform, loading frequency, average stress, etc. on the life, and uses the area of ​​the hysteresis loop as the damage parameter, so that the model has certain physical meaning and has strong versatility in actual engineering applications. In addition to the mechanical response parameters under the half-life cycle, some parameters included in the life prediction model, such as the holding time, loading frequency, and strain range, are determined when the load is applied, making the model easy to use. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 The present invention is a flowchart of a life prediction method based on the total tensile strain energy density and considering the stress relaxation effect.

[0060] Figure 2 Schematic diagram of loading waveform during strain control in an embodiment of the present invention.

[0061] Figure 3 Schematic diagram of the pp-type loading waveform and the corresponding inelastic strain energy density in an embodiment of the present invention.

[0062] Figure 4 Schematic diagram of the CP-type loading waveform and the corresponding inelastic strain energy density in an embodiment of the present invention.

[0063] Figure 5 Schematic diagram of the PC-type loading waveform and the corresponding inelastic strain energy density in an embodiment of the present invention.

[0064] Figure 6 Schematic diagram of the CC-type loading waveform and the corresponding inelastic strain energy density in an embodiment of the present invention.

[0065] Figure 7 Schematic diagram of the inelastic strain energy density considering the mean stress of the CC-type loading waveform in an embodiment of the present invention.

[0066] Figure 8 Schematic diagram of the prediction effect of the life prediction model constructed in an embodiment of the present invention on GH4169 material. DETAILED DESCRIPTION

[0067] The method of the present invention is further described below with reference to the accompanying drawings and embodiments.

[0068] like Figure 1 As shown in FIG, a flow chart of a life prediction method based on the total tensile strain energy density and considering the stress relaxation effect of the present invention is shown, and the specific steps are as follows:

[0069] S1. Conduct uniaxial low-cycle fatigue tests and low-cycle fatigue-creep tests on the material in strain-controlled mode at the same temperature to obtain the mechanical response parameters and test life of the material in each test at the half-life cycle;

[0070] The mechanical response parameters include: stress response range, total strain range Δε t , inelastic strain range Δε in , tension and compression holding time t h , loading frequency v, stress relaxation range during load maintenance, inelastic strain energy density range ΔW in .

[0071] Among them, the stress response range includes: the maximum stress σ under half life cycle max , minimum stress σ under half life cycle min ; The stress relaxation range during load holding includes: the stress relaxation range Δσ during tensile load holding r,tension , stress relaxation range Δσ under compression load r,compression .

[0072] S2. Fitting the stress relaxation model based on the mechanical response during the holding stage;

[0073] The load holding stage in the strain control mode is a stage in which the strain load remains constant over a period of time.

[0074] S3. Determine the inelastic strain energy density generated by fatigue-creep loading, i.e., the area of ​​each region of the hysteresis loop under half-life cycle, and propose a new inelastic strain energy density as a damage parameter;

[0075] The inelastic strain energy density W in Including: Plastic strain energy density W p , tensile creep strain energy density W c , the inelastic strain energy density W generated during the compression and loading stage tc .

[0076] The damage parameter is the sum of the plastic strain energy density, the tensile creep strain energy density, and the inelastic strain energy density generated in the compression holding stage.

[0077] S4. Considering the restoration effect, determine the additional inelastic strain energy density generated by the mean stress and correct the influence of the mean stress on the lifespan;

[0078] Among them, the phenomenon that the average compressive stress makes it difficult for microscopic holes generated in the fatigue-creep process to form and connect is called the restoration effect.

[0079] S5. Based on the strain energy density-frequency separation method (SEFS), a fatigue-creep life prediction model is established by combining the sum of the inelastic strain energy density described in step S3 and the additional inelastic strain energy density generated by the average stress in step S4 to achieve fatigue-creep life prediction.

[0080] In this embodiment, step S1 is specifically as follows:

[0081] In this embodiment, low cycle fatigue test and low cycle fatigue-creep test of nickel-based alloy GH4169 specimen were carried out in strain control mode at 650°C. The loading waveform is as follows: Figure 2 As shown, it includes: pure fatigue pp type loading without holding time, fatigue-creep cp type loading with holding load in the tension stage, fatigue-creep pc type loading with holding load in the compression stage, and fatigue-creep cc type loading with holding load in both tension and compression stages.

[0082] Among them, the material specimens are subjected to the same total strain range Δε t = 1% and strain rate Loading, tension and compression holding time t for each test h Different, the holding time t in this embodiment h The mechanical response parameters of the specimens in each test group under the life and half-life cycles were obtained in a time range from 60s to 1800s.

[0083] In this embodiment, step S2 is specifically as follows:

[0084] Combined with the time-dependent stress response during the holding stage, the Feltham stress relaxation model is used to describe the stress relaxation phenomenon, and the expression is as follows:

[0085] σ=σ0[1-B″·ln(bt h +1)] (1)

[0086] Where σ0 represents the initial stress at the beginning of the load holding stage; σ represents the stress at the end of the load holding stage; t h represents the holding time; B″ and b represent the fitting parameters of the Feltham model.

[0087] In this embodiment, step S3 is specifically as follows:

[0088] In step S3 of this embodiment, a new inelastic strain energy density is proposed as a damage parameter. The area of ​​the hysteresis loop under half-life cycle is used to describe the inelastic strain energy density generated by fatigue and fatigue creep loads. For different loading waveforms (pp, cp, pc, cc) and holding times, the hysteresis loop under half-life cycle will have different shapes and areas. The new inelastic strain energy density calculation method can calculate the area of ​​the fatigue and creep corresponding hysteresis loop regions based on fatigue and creep characteristics, respectively, thereby considering the influence of loading waveform and holding time. This damage parameter modifies the plastic strain energy density (PSED) theory based on the tensile inelastic strain energy density that triggers crack growth, and considers stress relaxation during the tensile holding stage. The inelastic strain energy density generated during the compression holding stage is innovatively introduced to consider fatigue damage caused by the compression holding stage.

[0089] S31. Determine the plastic strain energy density resulting from changes in operating conditions;

[0090] The plastic strain energy density generated by the change in working conditions can be described by the area of ​​the hysteresis loop as PSED, which is expressed as follows:

[0091]

[0092] Among them, β represents the fitting parameter, Δσ represents the stress range, and Δε p Indicates the range of plastic strain caused by changes in working conditions.

[0093] The plastic strain energy density generated by the change in working conditions is the inelastic strain range Δε generated under PP-type loading at half the life cycle. inWhen the total strain range and strain rate remain unchanged, the plastic strain energy density generated by the other loading waveforms (cp type, pc type, cc type) due to the change of working conditions is all derived from the inelastic strain range Δε under the half-life cycle generated by the pp type loading. in Determine, and at the same time can be loaded by pp half life cycle inelastic strain energy density range ΔW in , find the value of β.

[0094] Only the tensile inelastic strain energy density will induce fatigue-creep damage, so the plastic strain energy density W generated by the working condition change will be described. p Corrected, the expression is as follows

[0095]

[0096] Among them, σ max Indicates the maximum stress.

[0097] S32, determining the creep strain energy density generated during the tensile load-holding stage, i.e., the tensile creep strain energy density;

[0098] The creep strain energy density W generated during the tensile loading stage is determined by considering the stress relaxation phenomenon. c , under the tensile strain holding state, the material produces creep phenomenon, and the creep strain ε c , and the maximum strain ε max If constant, the creep strain of this part evolves from the elastic strain. Creep strain energy density W c It is expressed by the area of ​​the stress relaxation region in the hysteresis loop under half-life cycle, and the expression is as follows:

[0099]

[0100] Among them, σ max represents the maximum stress, σ rt represents the stress at the end of the tensile holding stage; E represents Young's modulus.

[0101] S33. Determine the inelastic strain energy density generated by the compression and holding stage;

[0102] Stress relaxation during the compression and holding stage produces inelastic strain This results in a decrease in the compressive stress, and the hysteresis loop reaches a stable state at half the life cycle. In the tensile stage, additional tensile inelastic strain energy density is generated, which makes the hysteresis loop reach a stable closed state. The tensile inelastic strain energy density uses the stress relaxation in the compression holding stage to generate inelastic strain. The inelastic strain energy density W generated during the compression and holding stage tcThat is, the area of ​​the inelastic strain energy density generated by the hysteresis loop of the half-life cycle. The expression of this area is as follows:

[0103]

[0104] Among them, σ rc represents the stress at the end of the compression holding stage; σ min Indicates the minimum stress.

[0105] S34. Based on steps S31-S33, a new inelastic strain energy density is proposed as a damage parameter;

[0106] The damage parameter W in =W p +W c +W tc , the expression is as follows:

[0107]

[0108] The schematic diagrams of damage parameters of pp, cp, pc and cc loading modes on the half-life cycle hysteresis loop are shown as follows: Figure 3 、 Figure 4 、 Figure 5 、 Figure 6 As shown, where Δε t Indicates the strain range, Δσ r,tension Indicates the tensile stress relaxation range, Δσ r,compression Indicates the compressive stress relaxation range.

[0109] In this embodiment, step S4 is specifically as follows:

[0110] Considering the additional inelastic strain energy density generated by the mean stress, the compressive mean stress makes it difficult for the microscopic holes generated in the fatigue-creep process to form and connect. This phenomenon is called the restoration effect, which reduces creep fatigue damage. The tensile mean stress will further expand the microscopic holes, promote the formation and connection of holes, and thus increase creep fatigue damage.

[0111] First, according to the material's complex effect, the average stress σ m The area enclosed by the inelastic strain on the half-life hysteresis loop takes into account the influence of the mean stress, and the expression is as follows:

[0112]

[0113] Among them, W m The additional inelastic strain energy density generated by the mean stress

[0114] Combined with the damage parameter W proposed in step S3 in, the inelastic strain energy density damage parameter considering the average stress is proposed as W in,improved =W in +W m , the expression is as follows:

[0115]

[0116] like Figure 7 As shown, the CC loading waveform represents the inelastic strain energy density considering the average stress on the hysteresis loop of the half-life cycle.

[0117] In this embodiment, step S5 is specifically as follows:

[0118] The SEFS model is constructed based on the strain energy density-frequency separation method, which is used as the basis for building a life prediction model. The expression is as follows:

[0119]

[0120] Among them, N f represents the predicted life of the SEFS model, W represents the strain energy density, v t represents the frequency of the stretching process, v c The frequency of the compression process can be expressed by the strain rate of tension and compression, respectively and holding time t h Calculated to consider the effect of loading frequency on life prediction, C, φ, m, k are all fitting parameters of SEFS model;

[0121] Then the strain energy density-frequency separation method is combined with W in,improved , predicting the fatigue-creep life of materials The expression is as follows:

[0122]

[0123] Among them, N c-f Indicates the fatigue-creep life of the material.

[0124] This embodiment further includes step S6, evaluating the life prediction model parameters to verify the accuracy of the model, as follows:

[0125] By calculating the inelastic strain energy density W of each test in,improved , the frequency of the stretching process v t , the frequency of the compression process v c , combined with the specimen life of each test, the parameters C, φ, m, k of the SEFS model are fitted to obtain the life prediction model for life prediction. The prediction results are as follows Figure 8 As shown in Figure 2, all prediction results are within the 1.5-fold error band.

[0126] Among them, the stretching process frequency v in the SEFS model t According to the loading conditions (maximum strain ε max , minimum strain ε min , strain rate Holding time t h ) calculation, the expression is as follows:

[0127]

[0128] Similarly, the compression process frequency v c The calculation expression is as follows:

[0129]

[0130] In summary, this embodiment compares the prediction results of the life prediction model with the test data, and the prediction results are all within the error band of ±1.5 times. In view of the changes in damage types caused by different waveforms and holding times, the life prediction model proposed by the method of the present invention comprehensively considers the influence of multiple important factors such as stress relaxation, loading waveform, loading frequency, average stress, etc. on life, and uses the area of ​​the hysteresis loop as the damage parameter, so that the model has certain physical significance and has strong versatility in actual engineering applications. The prediction results have high accuracy, and the model has fewer undetermined material constants, which meets the requirements of complex load environment, high accuracy and universality.

[0131] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and it should be understood that the scope of protection of the present invention is not limited to such specific descriptions and embodiments. Those skilled in the art can make various other specific variations and combinations based on the technical teachings disclosed in the present invention without departing from the essence of the present invention, and such variations and combinations are still within the scope of protection of the present invention.

Claims

1. A life prediction method based on the total tensile strain energy density and considering the stress relaxation effect, the specific steps are as follows: S1. Conduct uniaxial low-cycle fatigue tests and low-cycle fatigue-creep tests on the material in strain-controlled mode at the same temperature to obtain the mechanical response parameters and test life of the material in each test at the half-life cycle; The mechanical response parameters include: Stress response range, total strain range Δε t , inelastic strain range Δε in , tension and compression holding time t h , loading frequency v, stress relaxation range during load maintenance, inelastic strain energy density range ΔW in ; Among them, the stress response range includes: the maximum stress σ under half life cycle max , minimum stress σ under half life cycle min ; The stress relaxation range during load holding includes: the stress relaxation range Δσ during tensile load holding r,tension , stress relaxation range Δσ under compression load r,compression ; S2. Fitting the stress relaxation model based on the mechanical response during the holding stage; Among them, the load holding stage in the strain control mode is a stage in which the strain load remains constant over a period of time; S3. Determine the inelastic strain energy density generated by fatigue-creep loading, i.e., the area of ​​each region of the hysteresis loop under half-life cycle, and propose a new inelastic strain energy density as a damage parameter; The inelastic strain energy density W in Including: Plastic strain energy density W p , tensile creep strain energy density W c , the inelastic strain energy density W generated during the compression and loading stage tc ; The damage parameter is the sum of the plastic strain energy density, the tensile creep strain energy density, and the inelastic strain energy density generated during the compression holding stage; S4. Considering the restoration effect, determine the additional inelastic strain energy density generated by the mean stress and correct the influence of the mean stress on the lifespan; Among them, the phenomenon that the compressive average stress makes it difficult for microscopic holes to form and connect during the fatigue-creep process is called the restoration effect; S5. Based on the strain energy density-frequency separation method SEFS, a fatigue-creep life prediction model is established by combining the sum of the inelastic strain energy density in step S3 and the additional inelastic strain energy density generated by the average stress in step S4 to achieve fatigue-creep life prediction; The step S5 is specifically as follows: The SEFS model is constructed based on the strain energy density-frequency separation method, which is used as the basis for building a life prediction model. The expression is as follows: Among them, N f represents the predicted life of the SEFS model, W represents the strain energy density, v t represents the frequency of the stretching process, v c The frequency of the compression process can be expressed by the strain rate of tension and compression, respectively and holding time t h Calculated to consider the effect of loading frequency on life prediction, C, φ, m, k are all fitting parameters of SEFS model; Then the strain energy density-frequency separation method is combined with W in,improved , predicting the fatigue-creep life of materials The expression is as follows: Among them, N c-f Indicates the fatigue-creep life of the material, W in,improved represents the inelastic strain energy density damage parameter considering the average stress, β represents the fitting parameter, Δε p represents the plastic strain range caused by the change of working conditions, E represents Young's modulus, σ rt represents the stress at the end of the tensile holding stage, σ rc represents the stress at the end of the compression holding stage, σ m represents the mean stress.

2. The life prediction method based on the total tensile strain energy density and considering the stress relaxation effect according to claim 1, characterized in that: The step S1 is specifically as follows: At the same temperature, low cycle fatigue test and low cycle fatigue-creep test of material specimens are carried out in strain control mode; Among them, the material specimens are subjected to the same total strain range Δε t and strain rate Loading is performed, and the loading waveforms include: pure fatigue pp type loading without holding time, fatigue-creep cp type loading with holding time in the tension stage, fatigue-creep pc type loading with holding time in the compression stage, and fatigue-creep cc type loading with holding time in both tension and compression stages; The holding time t for tension and compression of each test h The mechanical response parameters of the specimens in each group of tests under life and half-life cycles were obtained.

3. The life prediction method based on the total tensile strain energy density and considering the stress relaxation effect according to claim 1, characterized in that: The step S2 is specifically as follows: Combined with the time-dependent stress response during the holding stage, the Feltham stress relaxation model is used to describe the stress relaxation phenomenon, and the expression is as follows: σ=σ0[1-B″·ln(bt h +1)] (3) Where σ0 represents the initial stress at the beginning of the holding stage; σ represents the stress at the end of the holding stage; t h represents the holding time; B″ and b represent the fitting parameters of the Feltham model.

4. The life prediction method based on tensile total strain energy density and considering stress relaxation effect according to claim 1, characterized in that: The step S3 is specifically as follows: S31. Determine the plastic strain energy density resulting from changes in operating conditions; The plastic strain energy density generated by the change in working conditions can be described by the area of ​​the hysteresis loop as PSED, which is expressed as follows: Among them, β represents the fitting parameter, Δσ represents the stress range, and Δε p Indicates the range of plastic strain caused by changes in working conditions; Only the tensile inelastic strain energy density will induce fatigue-creep damage, so the plastic strain energy density W generated by the working condition change will be described. p Corrected, the expression is as follows Among them, σ max represents the maximum stress; S32, determining the creep strain energy density generated during the tensile load-holding stage, i.e., the tensile creep strain energy density; The creep strain energy density W generated during the tensile loading stage is determined by considering the stress relaxation phenomenon. c , under the tensile strain holding state, the material produces creep phenomenon, and the creep strain ε c , and the maximum strain ε max constant, then this part of the creep strain evolves from the elastic strain; the creep strain energy density W c It is expressed by the area of ​​the stress relaxation region in the hysteresis loop under half-life cycle, and the expression is as follows: Among them, σ max represents the maximum stress, σ rt represents the stress at the end of the tensile holding stage; E represents Young's modulus; S33. Determine the inelastic strain energy density generated by the compression and holding stage; Stress relaxation during the compression and holding stage produces inelastic strain This results in a decrease in the compressive stress, and the hysteresis loop reaches a stable state at half the life cycle. In the tensile stage, additional tensile inelastic strain energy density is generated, which makes the hysteresis loop reach a stable closed state. The tensile inelastic strain energy density uses the stress relaxation in the compression holding stage to generate inelastic strain. The inelastic strain energy density W generated during the compression and holding stage tc That is, the area of ​​the inelastic strain energy density generated by the hysteresis loop of the half-life cycle. The expression of this area is as follows: Among them, σ rc represents the stress at the end of the compression holding stage; σ min represents the minimum stress; S34. Based on steps S31-S33, a new inelastic strain energy density is proposed as a damage parameter; The damage parameter W in =W p +W c +W tc , the expression is as follows:

5. The life prediction method based on tensile total strain energy density and considering stress relaxation effect according to claim 1, characterized in that: The step S4 is specifically as follows: First, the mean stress σ m The area enclosed by the inelastic strain on the half-life hysteresis loop takes into account the influence of the mean stress, and the expression is as follows: Among them, W m represents the additional inelastic strain energy density generated by the mean stress, Δε in Indicates the inelastic strain range under half life cycle; Combined with the damage parameter W proposed in step S3 in , the inelastic strain energy density damage parameter considering the average stress is proposed as W in,improved =W in +W m , the expression is as follows:

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