CN117577242A - Preferential method of metal material creep-fatigue life prediction model - Google Patents

Abstract

The invention discloses a method for selecting the best creep-fatigue life prediction model of metal materials. The method includes: 1. Obtaining the time-life curve of the metal material under different load holding times; 2. Establishing a nonlinear cumulative damage model; 3. Determine the creep damage of metal materials; 4. Verify the accuracy of creep-fatigue life prediction. For the creep-fatigue failure of metal materials, the present invention fully considers the interaction between creep-fatigue, establishes a creep-fatigue interaction theory, compares models under different stress, strain and energy damage principles, and considers the average stress and The influence of stress relaxation on modeling accuracy was used to obtain the optimal model for creep-fatigue life prediction of metal materials, and a creep-life fatigue coupling damage assessment method based on failure mechanism was developed to meet the needs of mechanical structural environment-high performance-high safety. Sexual design requirements.

Description

An optimization method for creep-fatigue life prediction model of metal materials

Technical field

The invention belongs to the technical field of material performance prediction, and in particular relates to a method for selecting the best creep-fatigue life prediction model for metal materials.

Background technique

With the advancement of science and technology, mechanical structures are facing the requirements of "high performance", "environmental adaptability" and "long life", because they are directly related to the service performance, service life and safety and reliability of the mechanical mechanism. For mechanical structures, their structural materials will be affected by factors such as complex environments during service, and then creep-fatigue coupling damage will occur, which greatly restricts the development needs of high performance, high durability, and low maintenance costs of mechanical structures. There is an urgent need to make breakthrough progress in material selection, failure diagnosis, and life prediction of aerospace structural parts.

In addition, the traditional life assessment based on pure fatigue or pure creep mechanisms is far from meeting the requirements for life prediction of mechanical structures and materials in complex service environments. Therefore, it is necessary to reveal, analyze and construct the creep-fatigue damage evolution equation of metal materials based on multiple theories and methods on the premise of clarifying the creep-fatigue coupling damage modes, laws and mechanisms of metal structures and materials, and forming a failure mechanism-based Only the creep-life fatigue coupling damage assessment method can meet the mechanical structure environment-high performance-high safety design requirements.

Contents of the invention

The technical problem to be solved by the present invention is to provide an optimal method for the creep-fatigue life prediction model of metal materials in view of the above-mentioned deficiencies in the prior art. For the creep-fatigue failure of metal materials, the creep-fatigue failure is fully considered. Interaction between fatigue, establish creep-fatigue interaction theory, compare models under different stress, strain and energy damage principles, consider the impact of average stress and stress relaxation on modeling accuracy, and obtain the creep-fatigue life of metal materials The predicted optimal model forms a creep-life fatigue coupling damage assessment method based on failure mechanisms to meet the mechanical structural environment-high performance-high safety design requirements.

In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for selecting the best creep-fatigue life prediction model of metal materials, which is characterized in that: the method includes the following steps:

Step 1. Obtain the time-life curve of the metal material under different load holding times: According to the creep-fatigue test standard of metal materials, conduct a creep-fatigue test under uniaxial constant strain conditions, and plot the obtained test data in the coordinate system , obtain the time-life curve of metal materials under uniaxial constant strain conditions;

Step 2: Establish a nonlinear cumulative damage model. The process is as follows:

Step 201. According to the linear damage sum criterion, during the entire service life, when the sum of the fatigue damage and creep damage of the metal material is equal to 1, that is Metal materials are damaged; among them,/> It is the accumulated fatigue damage when the metal material is damaged,/> It is the accumulated creep damage when the metal material is damaged;

Step 202: Based on the fatigue damage d _f and creep damage d _c of each cycle during the entire service cycle, obtain the corresponding fatigue damage during the entire service cycle and creep damage/> Right now

Among them, N ₀ is the number of pure fatigue cycles using the same total strain range in the creep-fatigue interaction test;

Step 203. Substitute the formula in step 201 to obtain the linear damage model:

Step 204. According to the damage function described by Skelton, the linear damage accumulated in step 201 is expanded to a nonlinear damage function. The formula in step 201 is modified as And substitute the formula in step 202 to obtain the nonlinear cumulative damage model/>

Step 3: Determine the creep damage of metal materials. The process is as follows:

Step 301. According to the time fractional model, the integral expression of each creep damage during the load holding stage is obtained: Combined with the stress relaxation model that changes with holding time proposed by Feltham, the creep damage of each cycle is obtained, that is/> Among them, t _h is the tensile holding time of each cycle, t _R (σ, T) is the creep rupture time under different stress levels σ and temperature T; σ ₀ is the initial stress of the stable cycle holding time, B” and b are all fitting parameters, t is the time from the start of the holding time;

Step 302: According to the ductile depletion model, the integral expression of each creep damage during the load holding stage is obtained: Combined with instantaneous inelastic strain rate/> and ductile creep/> The linear relationship between the two, the creep damage of each cycle is obtained, that is/> Among them,/> is the instantaneous inelastic strain rate; is given/> Ductility creep under and T; B" and b are fitting parameters; E is Young's modulus; β and d are both fitting parameters;

Step 303: Modify the strain energy density depletion model according to the strain energy density depletion model, and obtain the integral expression of each creep damage during the load holding stage as Combining the influence of the average stress on the creep-fatigue interaction test, as well as the influence of the plastic strain range Δε _pp and the holding time on the stress relaxation behavior, the creep damage of each cycle is obtained, that is/> in, It is the equation of failure strain energy density determined by the inelastic strain energy density rate and temperature;/> is the inelastic strain energy density rate; ω _crit (T) is the critical failure strain energy density when creep damage does not occur at a certain temperature;/> and n ₁ is the material and temperature correlation coefficient;/>

Step 4. Verify the accuracy of creep-fatigue life prediction: Taking the test fatigue life as the abscissa and the predicted life as the ordinate, establish a coordinate system and combine the three creep damage prediction models with the different maintenance results obtained by the fatigue damage prediction model. The prediction results of creep-fatigue life under time are plotted in the coordinate system. The prediction results of the time fraction model have the largest degree of dispersion and exceed the 5-fold line boundary; the prediction results of the ductile depletion model are all within the 5-fold line boundary. The deviation is smaller than that of the time fraction model; the prediction results of the strain energy density depletion model are all within the 3-fold line boundary; it can be concluded that the strain energy density depletion model has the highest accuracy among the three creep damage assessment models.

The above-mentioned method for selecting the best creep-fatigue life prediction model for metal materials is characterized by: in step 202, the fatigue damage d _f of each cycle during the entire service cycle is obtained by the reciprocal of the fatigue life without holding time, that is,

The above-mentioned method for selecting the best creep-fatigue life prediction model for metal materials is characterized by: in step 301, the relationship between t _R and different stress levels σ is t _R (σ, T) = k·σ ^-α ; where , k and α are coefficients related to the material and temperature, which are obtained by fitting the creep test results t _R and σ; the stress relaxation model proposed by Feltham that changes with the holding time is σ ^F =σ ₀ [1-B”ln( bt+1)].

The above-mentioned method for selecting a metal material creep-fatigue life prediction model is characterized by: in step 302, the instantaneous inelastic strain rate during the stress relaxation process at a given time t And the instantaneous inelastic strain rate and ductile creep/> The linear relationship between Among them,/> is the stress relaxation rate during the stress relaxation process; and/>

The above-mentioned method for selecting the best creep-fatigue life prediction model for metal materials is characterized by: in step 303, and/> The relationship between Among them,/> and n ₁ is the material and temperature correlation coefficient; ω _f is the inelastic strain energy density accumulated to failure, and/>

The above-mentioned method for selecting the best creep-fatigue life prediction model for metal materials is characterized by: in step 303, combined with the influence of the average stress on the creep-fatigue interaction test, when the applied stress is greater than the absolute value of the average stress, the creep -When fatigue cracks begin to gradually initiate and expand, the inelastic strain energy density is And/> Among them,/> is the instantaneous inelastic strain energy density at a given time t; differentiate the inelastic strain energy density to obtain the instantaneous inelastic strain energy density change rate at a given time/> That is/>

The above-mentioned method for selecting the best creep-fatigue life prediction model for metal materials is characterized by: in step 303, combining the influence of the plastic strain range Δε _pp and the holding time on the stress relaxation behavior, a stress relaxation model that changes with the holding time is obtained. is σ ^J =σ ₀ -(A·logΔε _pp +B)·log(1+t); among them, A and B are the parameters of the fitting correction; derivation of the stress relaxation formula with respect to the load holding time t, we get Substituting into the formula of instantaneous inelastic strain energy density change rate, we get

make

but

The above-mentioned method for selecting the best creep-fatigue life prediction model for metal materials is characterized by: in step 303, the influence of the average stress on the creep-fatigue interaction test is combined with the influence of the plastic strain range Δε _pp and the holding time on the stress relaxation. In the improved strain energy density depletion model, the integral expression of creep damage during the load holding stage is modified to Among them, ω _crit (T) is the critical failure strain energy density without creep damage at a certain temperature;

For the creep damage per cycle under tensile load-holding conditions, the instantaneous inelastic strain energy density change rate is substituted into the integral expression of the creep damage in the corrected load-holding stage to obtain

In the process of fitting ω _crit (T), when the value of ω _crit (T) is greater than the value of ω _f , the integral expression of creep damage is simplified to

The beneficial effect of the present invention is that for the creep-fatigue failure of metal materials, the interaction between creep-fatigue is fully considered, the creep-fatigue interaction theory is established, and the models under different stress, strain and energy damage principles are compared. Considering the influence of average stress and stress relaxation on modeling accuracy, the optimal model for creep-fatigue life prediction of metal materials is obtained, and a creep-life fatigue coupling damage assessment method based on the failure mechanism is formed to meet the requirements of mechanical structural environment-high Performance-high security design requirements.

The technical solution of the present invention will be further described in detail below through the accompanying drawings and examples.

Description of the drawings

Figure 1 is a time-life curve diagram of the metal material Nb521 of the present invention under different load holding times.

Figure 2 is a comparison chart of the predicted life and test life of the metal material Nb521 of the present invention based on the time fraction model.

Figure 3 is a comparison chart of the predicted life and test life of the metal material Nb521 of the present invention based on the ductile depletion model.

Figure 4 is a comparison chart between the predicted life and the test life of the metal material Nb521 of the present invention based on the strain energy density depletion model.

Figure 5 is a probability density comparison diagram of three creep damage evaluation models of the metal material Nb521 of the present invention under linear and nonlinear cumulative damage theories.

Figure 6 is a flow chart of the method of the present invention.

Detailed ways

As shown in Figures 1 to 6, a method for selecting the best creep-fatigue life prediction model for metal materials includes the following steps:

Step 1. Obtain the time-life curve of the metal material under different load holding times: According to the creep-fatigue test standard of metal materials, conduct a creep-fatigue test under uniaxial constant strain conditions, and plot the obtained test data in the coordinate system , obtain the time-life curve of metal materials under uniaxial constant strain conditions;

Step 2: Establish a nonlinear cumulative damage model. The process is as follows:

Step 201. According to the linear damage sum criterion, during the entire service life, when the sum of the fatigue damage and creep damage of the metal material is equal to 1, that is Metal materials are damaged; among them,/> It is the accumulated fatigue damage when the metal material is damaged,/> It is the accumulated creep damage when the metal material is damaged;

Step 202: Based on the fatigue damage d _f and creep damage d _c of each cycle during the entire service cycle, obtain the corresponding fatigue damage during the entire service cycle and creep damage/> Right now

Among them, N ₀ is the number of pure fatigue cycles using the same total strain range in the creep-fatigue interaction test;

Step 203. Substitute the formula in step 201 to obtain the linear damage model:

Step 204. According to the damage function described by Skelton, the linear damage accumulated in step 201 is expanded to a nonlinear damage function. The formula in step 201 is modified as And substitute the formula in step 202 to obtain the nonlinear cumulative damage model/>

In step 204, the linear cumulative damage criterion considers creep damage and fatigue damage separately and ignores the interaction between creep damage and fatigue damage. That is, the formation of creep holes will accelerate the initiation and expansion of fatigue cracks, and the fatigue load will also Promote the accumulation of creep damage. According to the damage function described by Skelton, the sum of linear damages accumulated in step 201 can be expanded to a more destructive nonlinear interaction function, which considers the interaction relationship between creep-fatigue and fatigue-creep.

Step 3: Determine the creep damage of metal materials. The process is as follows:

Step 301. According to the time fractional model, the integral expression of each creep damage during the load holding stage is obtained: Combined with the stress relaxation model that changes with holding time proposed by Feltham, the creep damage of each cycle is obtained, that is/> Among them, t _h is the tensile holding time of each cycle, t _R (σ, T) is the creep rupture time under different stress levels σ and temperature T; σ ₀ is the initial stress of the stable cycle holding time, B” and b are all fitting parameters, t is the time from the start of the holding time;

Step 302: According to the ductile depletion model, the integral expression of each creep damage during the load holding stage is obtained: Combined with instantaneous inelastic strain rate/> and ductile creep/> The linear relationship between the two, the creep damage of each cycle is obtained, that is/> Among them,/> is the instantaneous inelastic strain rate; is given/> Ductility creep under and T; B" and b are fitting parameters; E is Young's modulus; β and d are both fitting parameters;

Step 303: Modify the strain energy density depletion model according to the strain energy density depletion model, and obtain the integral expression of each creep damage during the load holding stage as Combining the influence of the average stress on the creep-fatigue interaction test, as well as the influence of the plastic strain range Δε _pp and the holding time on the stress relaxation behavior, the creep damage of each cycle is obtained, that is/> in, It is the equation of failure strain energy density determined by the inelastic strain energy density rate and temperature;/> is the inelastic strain energy density rate; ω _crit (T) is the critical failure strain energy density when creep damage does not occur at a certain temperature;/> and n ₁ is the material and temperature correlation coefficient;/>

In step three, the current calculation of creep damage is mainly based on the integral equation of stress, strain and energy, namely the time fraction model, the ductility depletion model and the strain energy density depletion model, which are widely used in the assessment of creep damage. In step 202, the calculation formula of fatigue damage d _f for each cycle during the entire service life illustrates the value of fatigue damage for each cycle. Therefore, how to evaluate the creep damage generated by metal materials during the load-holding stage is to predict creep-fatigue. The hard part about longevity.

In step 301, the stress change and temperature during the test are used as variables to calculate the creep damage of the metal material. In step 302, the inelastic strain rate and creep ductility are used as the main parameters to calculate the creep damage of the metal material. In step 303, the energy is combined Density dissipation criterion calculates creep damage of metallic materials.

In step 303, in the strain energy density depletion model, the inelastic strain energy density is used as a parameter to evaluate the creep damage of each cycle during the retention period. The Payten energy method uses the inelastic strain energy density dissipation rate and the inelastic strain energy density as the main control parameters to define one cycle of creep damage. According to the strain energy density depletion model, the value of each creep damage during the load holding stage is obtained. The integral expression is Based on a large amount of experimental data, Takahashi et al. revised the strain energy density depletion model and proposed an energy-based ductility depletion model, namely/> Wang studied the influence of mean stress on creep-fatigue interaction tests based on Takahashi, and synthesized the theory that when the applied stress is greater than the absolute value of mean stress, creep-fatigue cracks begin to gradually initiate and expand. In this model, The inelastic strain energy density can be expressed as

Step 4. Verify the accuracy of creep-fatigue life prediction: Taking the test fatigue life as the abscissa and the predicted life as the ordinate, establish a coordinate system and combine the three creep damage prediction models with the different maintenance results obtained by the fatigue damage prediction model. The prediction results of creep-fatigue life under time are plotted in the coordinate system. The prediction results of the time fraction model have the largest degree of dispersion and exceed the 5-fold line boundary; the prediction results of the ductile depletion model are all within the 5-fold line boundary. The deviation is smaller than that of the time fraction model; the prediction results of the strain energy density depletion model are all within the 3-fold line boundary; it can be concluded that the strain energy density depletion model has the highest accuracy among the three creep damage assessment models.

For the creep-fatigue failure of metal materials, the present invention fully considers the interaction between creep-fatigue, establishes a creep-fatigue interaction theory, compares models under different stress, strain and energy damage principles, and considers the average stress and The influence of stress relaxation on modeling accuracy is used to obtain the optimal model for creep-fatigue life prediction of metal materials, and a creep-life fatigue coupling damage assessment method based on failure mechanism is formed to meet the requirements of mechanical structural environment-high performance-high safety. Sexual design requirements.

It should be noted that, as shown in Figures 2 and 3, the prediction results of the ductility depletion model are all within the 5-fold line boundary, some prediction results are lower than the experimental data, and the prediction deviation is smaller than the time fraction model; the strain energy density depletion model prediction The results are all distributed within the 3-fold line boundary and mainly concentrated within the 1.5-fold line boundary.

The prediction results of Nb521 creep-fatigue interaction life based on three creep damage assessment models are calculated. As shown in Figure 2, Figure 3 and Figure 4, the prediction results of the time fraction model (TF) have the largest degree of dispersion. Beyond the 5-fold line boundary; the prediction results of the ductility depletion model (DE) are all within the 5-fold line boundary, some prediction results are lower than the experimental data, and the prediction deviation is smaller than the time fraction model; the prediction results of the strain energy density depletion model (SEDE) All are distributed within the 3-fold line boundary and mainly concentrated within the 1.5-fold line boundary; among the three creep damage assessment models, the strain energy density depletion model has the highest accuracy.

Compared with the linear cumulative damage and nonlinear cumulative damage creep-fatigue life prediction results, the nonlinear cumulative damage criterion takes into account the interaction between creep and fatigue, making the nonlinear cumulative damage life prediction more accurate. From the prediction results of the three models, it can be found that when the holding time is 90s and 120s, the prediction capabilities of the three models are significantly overestimated. For this phenomenon, it is mainly due to the weak oxidation resistance of the Nb521 alloy, so for long-term Creep-fatigue life predictions for load-holding conditions are overestimated. In response to this situation, the distribution of prediction error P _error is quantified by using the probability density function and P _error =log ₁₀ (N _p )-log ₁₀ (N _e ); where N _p and N _e represent the predicted life and experimental life respectively. .

Figure 5 shows a comparison of the probability densities of three creep damage assessment models under linear and nonlinear cumulative damage theories. Compared with the time fractional model, ductility depletion model and strain energy density depletion model, their prediction error dispersion is getting smaller and smaller, and the prediction accuracy of the model is getting better and better; compared with linear and nonlinear cumulative damage, nonlinear cumulative damage conditions The P _error values of the following three creep damage assessment models are closer to 0, indicating that the creep-fatigue life prediction accuracy of the alloy based on the nonlinear cumulative damage criterion is higher. For the strain energy density depletion model, the probability density distribution function has a higher mean value when using the linear cumulative damage model, indicating that its creep-fatigue life prediction results are strongly non-conservative, rather than the linear cumulative damage theoretical conditions. The forecast results below are relatively more conservative. In summary, for the prediction of creep-fatigue life of metals, the strain energy density depletion model under the nonlinear damage summation criterion should be preferred, as this model has the highest prediction accuracy.

In this embodiment, in step 202, the fatigue damage d _f of each cycle during the entire service cycle is obtained by the reciprocal of the fatigue life without holding time, that is,

In this embodiment, in step 301, the relationship between t _R and different stress levels σ is t _R (σ, T) = k·σ ^-α ; where k and α are coefficients related to the material and temperature. Through fitting The creep test results t _R and σ are obtained; the stress relaxation model proposed by Feltham that changes with the holding time is σ ^F =σ ₀ [1-B”ln(bt+1)].

In step 301, the determination of t _R (σ, T) requires conducting uniaxial tensile creep tests at different stress levels at a given temperature.

In this embodiment, in step 302, the instantaneous inelastic strain rate during the stress relaxation process at a given time t And the instantaneous inelastic strain rate/> and ductile creep/> The linear relationship between Among them,/> is the stress relaxation rate during the stress relaxation process; and/>

In step 302, Priest and Ellison proposed a ductility loss model based on the research on Cr-Mo steel and combined with Lagneberg's modified model of the linear cumulative damage model. In this model, inelastic strain rate and creep ductility are considered to have the greatest influence on creep damage per cycle. The determination also requires uniaxial tensile creep tests at different stress levels at a given temperature.

In this embodiment, in step 303, and/> The relationship between in, and n ₁ is the material and temperature correlation coefficient; ω _f is the inelastic strain energy density accumulated to failure, and/>

In this embodiment, in step 303, combined with the influence of average stress on the creep-fatigue interaction test, when the applied stress is greater than the absolute value of the average stress, when creep-fatigue cracks begin to gradually initiate and expand, the inelastic strain energy density is and/> Among them,/> is the instantaneous inelastic strain energy density at a given time t; differentiate the inelastic strain energy density to obtain the instantaneous inelastic strain energy density change rate at a given time/> That is/>

In this embodiment, in step 303, by combining the influence of the plastic strain range Δε _pp and the holding time on the stress relaxation behavior, the stress relaxation model that changes with the holding time is obtained as σ ^J =σ ₀ -(A·logΔε _pp +B)· log(1+t); among them, A and B are the parameters of fitting correction; derivation of the stress relaxation formula with respect to the load holding time t, we get Substituting into the formula of instantaneous inelastic strain energy density change rate, we get

make

but

It should be noted that in the time fractional model and the ductile depletion model, the Feltham stress relaxation formula is usually used to describe the stress drop during the load holding period. Fitting the stress relaxation curve under holding time, it was found that although the fitting curve under this formula has a high degree of fitting, the model parameters (especially parameter b) are highly sensitive under different holding times, so Reducing its applicability in complex load conditions. Wang's model modifies the stress relaxation formula proposed by Jeong et al., which also considers the influence of plastic strain range and holding time on stress relaxation behavior. Compared with the Feltham stress relaxation formula, Jeong's fitting effect is better and the dispersion of the fitting parameters is lower.

In this embodiment, in step 303, combined with the influence of the average stress on the creep-fatigue interaction test and the influence of the plastic strain range Δε _pp and the holding time on the stress relaxation behavior, the improved strain energy density depletion model will maintain The integral expression of creep damage during the loading stage is modified to Among them, ω _crit (T) is the critical failure strain energy density without creep damage at a certain temperature;

For the creep damage per cycle under tensile load-holding conditions, the instantaneous inelastic strain energy density change rate is substituted into the integral expression of the creep damage in the corrected load-holding stage to obtain

In the process of fitting ω _crit (T), when the value of ω _crit (T) is greater than the value of ω _f , the integral expression of creep damage is simplified to

It should be noted that if in the process of fitting ω _crit (T) it is found that its value is much larger than the value of ω _f , that is, there is no "no creep damage" caused by the higher inelastic strain energy density in the early stage of the load holding stage. Stage", the formula of creep damage can be simplified as:

The above are only preferred embodiments of the present invention and do not limit the present invention in any way. Any simple modifications, changes and equivalent structural changes made to the above embodiments based on the technical essence of the present invention still belong to the technology of the present invention. within the protection scope of the scheme.

Claims (8)

1. A preferential method of a metal material creep-fatigue life prediction model, which is characterized by comprising the following steps:

step one, obtaining time-life curves of metal materials under different holding time: according to a creep-fatigue test standard of the metal material, performing a creep-fatigue test under a uniaxial constant strain condition, and drawing the obtained test data in a coordinate system to obtain a time-life curve of the metal material under the uniaxial constant strain condition;

step two, a nonlinear accumulated damage model is established, and the process is as follows:

step 201, according to the linear damage sum criterion, when the sum of the fatigue damage and the creep damage of the metal material is equal to 1 in the whole service period, namelyThe metal material is destroyed; wherein (1)>For accumulated fatigue damage when the metal material is damaged, < ->Is accumulated creep damage when the metal material is damaged;

step 202, according to fatigue damage d of each cycle in the whole service period _f And creep damage d _c Obtaining the corresponding fatigue damage in the whole service cycleAnd creep damage->I.e.

Wherein N is ₀ For pure fatigue cycles using the same total strain range in the creep-fatigue interaction test;

step 203, substituting the formula in step 201 to obtain a linear damage model:

step 204, expanding the accumulated linear damage in step 201 to a nonlinear damage function according to the damage function described by Skelton, and modifying the formula in step 201 toAnd substituting the formula in the step 202 to obtain a nonlinear cumulative damage model ++>

Step three, determining creep damage of the metal material, wherein the process is as follows:

step 301, obtaining an integral expression of each creep damage in the load-keeping stage according to the time score model asCombining with the stress relaxation model proposed by Feltham and changing along with the retention time to obtain creep damage of each cycle, namely +.>Wherein t is _h For the draw hold time per cycle, t _R (sigma, T) is the creep rupture time at different stress levels sigma and temperatures T; sigma (sigma) ₀ For stabilizing the initial stress of the period holding time, B' and B are fitting parameters, and t is the time from the holding time;

step 302, obtaining an integral expression of each creep damage in the load-keeping stage according to the ductile exhaustion model asCombined transient inelastic strain rate->And ductile creep->The linear relation between the two is obtained, and creep damage of each cycle is obtained>Wherein (1)>Is the instantaneous inelastic strain rate;is given +.>And ductile creep at T; b' and B are fitting parameters; e is Young's modulus; beta and d are fitting parameters;

step 303, according to the strain energy density depletion model, correcting the strain energy density depletion model to obtain an integral expression of each creep damage in the load-maintaining stage asIn combination with the influence of the average stress on the creep-fatigue interaction test, the plastic strain range delta epsilon _pp And the effect of the holding time on the stress relaxation behavior, resulting in creep damage per cycle +.>Wherein (1)>Is an equation for the failure strain energy density determined by the inelastic strain energy density rate and temperature; />Is the inelastic strain energy density rate; omega _crit (T) is critical failure strain energy density when creep damage does not occur at a certain temperature; />And n ₁ Is a material and temperature related coefficient; />

Step four, verifying creep-fatigue life prediction accuracy: establishing a coordinate system by taking the test fatigue life as an abscissa and the predicted life as an ordinate, respectively combining the creep-fatigue life prediction results under different retention times obtained by the fatigue damage prediction models of the three creep damage prediction models in the coordinate system, and marking the maximum discrete degree of the prediction results of the time fraction model beyond the 5-time line boundary; the predicted results of the ductile exhaustion model are all within 5 times line boundaries, and the predicted deviation is smaller than that of the time fraction model; the prediction results of the strain energy density depletion model are all within a 3-fold line boundary; and obtaining the highest precision of the strain energy density depletion model in the three creep damage evaluation models.

2. The method for optimizing a metal material creep-fatigue life prediction model according to claim 1, wherein: in step 202, fatigue damage d is detected for each cycle throughout the life cycle _f Obtained by reciprocal fatigue life without retention time, i.e

3. The method for optimizing a metal material creep-fatigue life prediction model according to claim 1, wherein: in step 301, t _R The relation with the different stress level sigma is t _R (σ,T)＝k·σ ^-α The method comprises the steps of carrying out a first treatment on the surface of the Where k and α are coefficients related to material and temperature by fitting the creep test result t _R And sigma; the stress relaxation model of Feltham with change of holding time is sigma ^F ＝σ ₀ [1-B”ln(bt+1)]。

4. The method for optimizing a metal material creep-fatigue life prediction model according to claim 1, wherein: in step 302, an instantaneous inelastic strain rate during stress relaxation at a given time tAnd instantaneous inelastic strain rate->And ductile creep->The linear relation between them is->Wherein (1)>Is the stress relaxation rate during stress relaxation; and->

5. The method for optimizing a metal material creep-fatigue life prediction model according to claim 1, wherein: in step 303 of the process of the present invention,and->The relation between->Wherein (1)>And n ₁ Is a material and temperature related coefficient; omega _f To accumulate the inelastic strain energy density of the failure, and +.>

6. The method for optimizing a metal material creep-fatigue life prediction model according to claim 1, wherein: in step 303, in combination with the effect of the average stress on the creep-fatigue interaction test, when the applied stress is greater than the absolute value of the average stress, the inelastic strain energy density becomes equal toAnd->Wherein (1)>Is the instantaneous inelastic strain energy density at a given instant t; differentiating the inelastic strain energy density to obtain the instantaneous inelastic strain energy density change rate +.>I.e. < ->

7. The method for optimizing a metal material creep-fatigue life prediction model according to claim 1, wherein: in step 303, the plastic strain range Δε is combined _pp And the effect of retention time on stress relaxation behavior, resulting in a retention time dependent stress relaxation model σ ^J ＝σ ₀ -(A·logΔε _pp +B)·log(1+t) The method comprises the steps of carrying out a first treatment on the surface of the Wherein A and B are parameters for fitting correction; the stress relaxation formula is used for deriving the retention time t to obtainSubstituting into the formula of the instantaneous inelastic strain energy density change rate to obtain

Order the

Then

8. The method for optimizing a metal material creep-fatigue life prediction model according to claim 1, wherein: in step 303, the effect of the average stress on the creep-fatigue interaction test is combined with the plastic strain range Δε _pp And the effect of retention time on stress relaxation behavior, and in the improved strain energy density depletion model, correcting the integral expression of creep damage in the retention stage to beWherein omega _crit (T) is the critical failure strain energy density without creep damage at a certain temperature;

for creep damage per cycle under tensile load-maintaining conditions, substituting the instantaneous inelastic strain energy density change rate into an integral expression of creep damage in the corrected load-maintaining stage to obtain

At fitting omega _crit In the process of (T), ω _crit The value of (T) is greater than omega _f The integral expression of creep damage is reduced to