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

Abstract

The invention discloses a tensile total strain energy density-based life prediction method considering a stress relaxation effect, which comprises the following steps of: firstly, performing tests in a strain control mode at the same temperature to obtain mechanical response parameters and test life of a material in each test in a half-life cycle, and then calculating the tensile total strain energy density based on the mechanical response in a load holding stage. Fitting a stress relaxation model, representing the inelastic strain energy density caused by various loads by taking the area under each region of a half-life cycle hysteresis loop as a damage parameter, meanwhile, considering the complex effect of a material, correcting the influence of average stress on the life, and finally, establishing a life prediction model based on a strain energy density-frequency separation method. And the fatigue-creep life prediction is realized. The model of the method can comprehensively consider the influence of various important factors such as stress relaxation, loading waveform, loading frequency, average stress and the like on the service life, and the area of the hysteresis loop serves as a damage parameter, so that the model has certain physical significance and has high universality in practical engineering application.

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 tensile total strain energy density and considering stress relaxation effect. Background Art

[0002] Hot end components are not only damaged by fatigue loads but also by high temperature creep loads when they are in service in high temperature environments. The specific types of damage will be different depending on the waveform and load holding time when the load is applied, and there will be three types of damage: fatigue-dominated, creep-dominated, and creep-fatigue interaction.

[0003] The hot end components of aviation equipment often operate under high temperature and complex cyclic loads. Different from fatigue or creep damage caused by a single factor, they are often damaged by low-cycle creep fatigue damage. Repeated start-stop and working condition conversion of components often cause low-cycle fatigue damage of materials, while cruising under certain working conditions will cause creep damage of materials. Due to the harsh working conditions, low-cycle creep fatigue damage has become one of the main causes of failure of hot end components, and when hot end components are damaged, it often causes serious consequences. Therefore, accurate low-cycle creep fatigue life of materials used in hot end components is of great significance to the safe service of aviation equipment.

[0004] For the life prediction of hot end components, scholars have proposed hundreds of life prediction models, most of which are simple combinations of stress, strain or stress-strain, including: critical plane method, Basquin method, Manson-Coffin method, strain energy frequency separation method (SEFS) and other models do not consider the impact of loading waveform on component life, while ductility exhaustion approach (DE), continuous damage mechanics Kachanov-Rabotnov method and Lemaitre method lack the consideration of important factors such as average stress and loading frequency on component life. At the same time, nonlinear creep fatigue damage accumulation method (NDS), strain range partitioning method (SRP), strain energy range partitioning method (SEP) contain too many material constants, making the model too complex to be used in real scenarios. Therefore, the above methods 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 result 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 considering the stress relaxation effect, and the specific steps are as follows:

[0007] S1. Under the same temperature, carry out uniaxial low-cycle fatigue test and low-cycle fatigue-creep test of the material in strain control mode 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 holding, 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 of tensile load holding Δσ r,tension , stress relaxation range of compression load holding Δσ r,compression .

[0010] S2, fitting the stress relaxation model based on the mechanical response in the load 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 load, that is, the area of ​​each region of the hysteresis loop under the 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 and load-holding stage.

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

[0016] Among them, the phenomenon that the compressive average stress makes it difficult for the 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 test and low cycle fatigue-creep test of material specimens are 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 of tension and compression for each test h The mechanical response parameters of the specimens in each group of tests under life and half-life cycle were obtained.

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

[0023] Combined with the stress response over time in 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 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.

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

[0027] S31. Determine the plastic strain energy density resulting from the change in working 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, and the expression is 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, and the plastic strain energy density W generated by the change of working conditions will be described p After correction, the expression is as follows

[0032]

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

[0034] S32, determining the creep strain energy density generated in the tensile load-holding stage, that is, 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, then this part of the creep strain 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 the 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, determining the inelastic strain energy density generated by the compression and load-holding stage;

[0039] Stress relaxation during the compression and holding stage produces inelastic strain This leads to a decrease in 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 to make 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 in the compression and loading 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 average stress σ m The area enclosed by the inelastic strain on the half-life cycle hysteresis loop considers the influence of the average 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, and 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 tensile and compressive strain rates, respectively. and holding time t h Calculated, in order to consider the effect of loading frequency on life prediction, C, φ, m, k are the 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 firstly performs uniaxial low-cycle fatigue test and low-cycle fatigue-creep test of the material in strain control mode at the same temperature, obtains the mechanical response parameters and test life of each test of the material in the half-life cycle, and then fits the stress relaxation model based on the mechanical response in the holding stage, 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 compensation effect, correcting the influence of the average stress on the life, and finally based on the strain energy density-frequency separation method, establishes a fatigue-creep life prediction model that considers loading waveform, loading frequency, holding time, stress relaxation, and average stress, and realizes 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 significance and has strong versatility in practical 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 flow chart 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 pp-type loading waveform and 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 average stress of the cc-type loading waveform in an embodiment of the present invention.

[0066] Figure 8 It is a schematic diagram of the prediction effect of the life prediction model constructed in the embodiment of the present invention on the GH4169 material. DETAILED DESCRIPTION

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

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

[0069] S1. Under the same temperature, carry out uniaxial low-cycle fatigue test and low-cycle fatigue-creep test of the material in strain control mode 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 holding, 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 of tensile load holding Δσ r,tension , stress relaxation range of compression load holding Δσ r,compression .

[0072] S2, fitting the stress relaxation model based on the mechanical response in the load 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 load, that is, the area of ​​each region of the hysteresis loop under the 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 and load-holding stage.

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

[0078] Among them, the phenomenon that the compressive average stress makes it difficult for the 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, the 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 specimens 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 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.

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

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

[0084] Combined with the stress response over time in 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, and the inelastic strain energy density generated by fatigue and fatigue creep loads is described by the area of ​​the hysteresis loop under the half-life cycle. For different loading waveforms (pp, cp, pc, cc) and holding time, the hysteresis loop under the half-life cycle will have different shapes and areas. The new inelastic strain energy density calculation method can calculate the area of ​​the hysteresis loop area corresponding to fatigue and creep according to the fatigue and creep characteristics, respectively, so as to consider the influence of the loading waveform and the holding time. The damage parameter uses the tensile inelastic strain energy density to trigger crack propagation to modify the plastic strain energy density theory (Plastic Strain Energy Density, PSED), and considers stress relaxation in the tensile holding stage. The inelastic strain energy density generated by the compression holding stage is innovatively introduced to consider the fatigue damage caused by the compression holding stage.

[0089] S31. Determine the plastic strain energy density resulting from the change in working 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, and the expression is 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 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 generated by the inelastic strain range Δε under the half-life cycle under pp type loading. in Determine, and at the same time can be loaded by pp half life cycle of 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, and the plastic strain energy density W generated by the change of working conditions will be described p After correction, the expression is as follows

[0095]

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

[0097] S32, determining the creep strain energy density generated in the tensile load-holding stage, that is, 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, then this part of the creep strain 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 the 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, determining the inelastic strain energy density generated by the compression and load-holding stage;

[0102] Stress relaxation during the compression and holding stage produces inelastic strain This leads to a decrease in 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 to make 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 in the compression and loading 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 the 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 average stress, the compressive average stress will make it difficult for the microscopic holes generated in the fatigue-creep process to form and connect. This phenomenon is called the complex effect. The complex effect will reduce creep fatigue damage, while the tensile average stress will further expand the microscopic holes and promote the formation and connection of holes, thereby increasing 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 cycle hysteresis loop considers the influence of the average stress, and the expression is as follows:

[0112]

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

[0114] Combined with the damage parameter W proposed in step S3 inThe 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 average stress inelastic strain energy density 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, and 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 tensile and compressive strain rates, respectively. and holding time t h Calculated, in order to consider the effect of loading frequency on life prediction, C, φ, m, k are the 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 also includes step S6, evaluating the life prediction model parameters and verifying the model accuracy, 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, 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 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. The prediction results have high accuracy, and the model has fewer undetermined material constants, which meets the requirements of complex load environment-high accuracy-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 should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

Claims

1. A life prediction method based on tensile total strain energy density considering stress relaxation effect, the specific steps are as follows: S1. Under the same temperature, carry out uniaxial low-cycle fatigue test and low-cycle fatigue-creep test of the material in strain control mode 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 holding, 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 of tensile load holding Δσ r,tension , stress relaxation range of compression load holding Δσ r,compression ; S2, fitting the stress relaxation model based on the mechanical response in the load 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 load, that is, the area of ​​each region of the hysteresis loop under the 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 in the compression holding stage; S4. Consider the restoration effect, determine the additional inelastic strain energy density generated by the average stress, and correct the influence of the average stress on the life; Among them, the phenomenon that the compressive average stress makes it difficult for the microscopic holes generated in the fatigue-creep process to form and connect 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 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.

2. The life prediction method based on the total tensile strain energy density 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 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; The holding time t of tension and compression for each test h The mechanical response parameters of the specimens in each group of tests under life and half-life cycle were obtained.

3. The life prediction method based on tensile total strain energy density considering stress relaxation effect according to claim 1, characterized in that: The step S2 is specifically as follows: Combined with the stress response over time in 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)] (1) 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.

4. The life prediction method based on tensile total strain energy density 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 the change in working 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, and the expression is 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, and the plastic strain energy density W generated by the change of working conditions will be described p After correction, the expression is as follows Among them, σ max represents the maximum stress; S32, determining the creep strain energy density generated in the tensile load-holding stage, that is, 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 is 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 the 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, determining the inelastic strain energy density generated by the compression and load-holding stage; Stress relaxation during the compression and holding stage produces inelastic strain This leads to a decrease in 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 to make 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 in the compression and loading 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 considering stress relaxation effect according to claim 1, characterized in that: The step S4 is specifically as follows: First, the average stress σ m The area enclosed by the inelastic strain on the half-life cycle hysteresis loop considers the influence of the average 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 Represents 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:

6. The life prediction method based on tensile total strain energy density considering stress relaxation effect according to claim 1, characterized in that: The step S5 is specifically as follows: The SEFS model is constructed based on the strain energy density-frequency separation method, and 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 tensile and compressive strain rates, respectively. and holding time t h Calculated, in order to consider the effect of loading frequency on life prediction, C, φ, m, k are the 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.

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