CN115982958B - Material creep fatigue life prediction method based on engineering damage mechanics - Google Patents
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Abstract
The application discloses a material creep fatigue life prediction method based on engineering damage mechanics, which belongs to the field of material life prediction under high-temperature complex load, and comprises the following steps: and carrying out a plurality of groups of different strain purity control fatigue tests and creep tests at the same temperature, respectively determining fatigue damage and creep damage parameters of the material, and further establishing a creep fatigue damage accumulation model. In strain control at the same temperature and stress-strain hybrid control creep fatigue test, creep equivalent stress is determined for different load forms. And then, predicting the service lives of the pure fatigue test and the creep fatigue test according to the creep fatigue damage accumulation model. The method is simple to operate and convenient to calculate, and can accurately predict the cycle life of different materials under various loads such as strain control low cycle fatigue, stress control low cycle fatigue, strain control creep fatigue, stress and strain mixed control creep fatigue.
Description
Technical Field
The application belongs to the field of material life prediction under high-temperature complex load, and particularly relates to a material creep fatigue life prediction method based on engineering damage mechanics.
Background
In the field of gas turbines and aeroengines, hot-end components such as turbine blades, turbine disks, etc. operate in a severe environment at high temperatures and high rotational speeds. Damage to turbine blades and turbine disks under typical loads such as centrifugal forces caused by high rotational speeds, thermal stresses due to temperature gradients, and additional stresses associated with uncoordinated deformations of the associated parts include low cycle fatigue damage associated with landing and maneuver flights, as well as creep damage associated with operating time. The low cycle fatigue and creep damage have mutual influence under the working conditions of the turbine blade and the turbine disk, the damage accumulation speed is accelerated under the interaction between the creep fatigue, and a great influence can be generated on the service life of the aeroengine. Currently, a laboratory generally adopts a strain-controlled creep fatigue test and a stress-strain hybrid control creep fatigue test. The load-keeping modes in the two loading waveforms adopt constant strain load keeping and constant stress load keeping respectively. While strain-controlled creep fatigue simulates the operating conditions of fasteners at high temperatures, stress-strain hybrid-controlled creep fatigue tests are more suitable for simulating high temperature components of turbine blades, turbine disks, and the like, which are subjected to constant stress in a steady operating state.
In a creep fatigue test of strain control, stress relaxation exists during the load-holding period, the traditional damage mechanical model has complex calculation when predicting creep damage in the strain-holding stage, and the prediction result is also inaccurate, so that it is necessary to develop a damage mechanical life prediction method simultaneously applicable to the two loading waveforms so as to meet the creep fatigue life prediction under complex load.
Disclosure of Invention
The application aims to provide a material creep fatigue life prediction method based on engineering injury mechanics, which aims to solve the problems existing in the prior art.
In order to achieve the above object, the application provides a method for predicting creep fatigue life of a material based on engineering damage mechanics, comprising the following steps of
Step S1, taking a plurality of samples of the same material, performing strain control fatigue tests with the same strain rate and different strain amplitudes on part of the samples at the same temperature to obtain fatigue test data, and turning to step S2; carrying out creep tests of different stresses on part of the samples at the same temperature as that of the strain control fatigue test to obtain creep test data, and turning to step S3;
step S2, determining the cycle life of the fatigue test under any strain amplitude according to the fatigue test data, selecting the cycle stress range of each week as a damage parameter to determine fatigue damage evolution parameters, establishing a fatigue damage accumulation model according to the Lemaitre continuous damage mechanics theory, and then calculating the fatigue life, and turning to step S4;
and step S3, obtaining steady-state creep rate and creep stress according to creep test data, and establishing a Norton equation. Meanwhile, the creep strain rate is used as a damage parameter to determine a creep damage evolution parameter, and the creep life can be calculated after a creep damage accumulation model is established according to the Lemaitre continuous damage mechanics theory, and the step S4 is carried out;
step S4, through the fatigue test and the creep test in the step S2 and the step S3, a creep fatigue damage evolution equation is established according to a damage mechanics theory, and the step S5 is carried out;
step S5, carrying out a plurality of groups of strain control creep fatigue tests and stress strain hybrid control creep fatigue tests at the same temperature and the same strain rate to obtain half life cycles of the creep fatigue tests, wherein the data are used for verifying the accuracy of the application, and the step S6 is carried out;
and S6, respectively determining the equivalent creep stress in the load-maintaining process according to the two loading modes because the load-maintaining modes of the strain control creep fatigue and the strain mixed control creep fatigue are different in the load-maintaining stage.
For a creep fatigue test of strain control, taking a half life cycle of the creep fatigue test as a characteristic cycle, calculating a creep rate during stress relaxation through a stress-time relation during the half life cycle load maintaining, finding a steady-state creep rate during the stress relaxation through a creep rate change rule, and calculating a creep stress at the steady-state strain rate by using a Norton equation established in the step S3, namely an equivalent stress during the strain load maintaining, and turning to the step S7;
for a stress-strain hybrid control creep fatigue test, taking the non-elastic strain amplitude of half life cycle under the same strain amplitude obtained by the fatigue test as a key parameter for determining weekly fatigue damage, taking the load-holding stress and the load-holding time in the creep fatigue test as key parameters for determining weekly creep damage, and turning to step S7;
and S7, calculating to obtain the cycle when the damage is 1 by utilizing the creep fatigue damage evolution equation established in the step S4, wherein the cycle is the predicted creep fatigue life.
The application has the technical effects that:
according to the method for predicting the creep fatigue life of the material based on engineering damage mechanics, based on the Lemaitre continuous damage mechanics theory, after damage related parameters are determined through a fatigue test and a creep test respectively, a creep fatigue damage evolution model is established to describe the damage evolution process of the material under the creep fatigue interaction, so that the creep fatigue life is predicted. On the basis of the method, the characteristics of creep deformation under different loading waveforms are considered, and a method for determining the creep equivalent stress suitable for strain control creep fatigue load is provided. Based on the method provided by the application, the creep damage in the strain load-keeping process is accurately calculated, so that the creep fatigue damage mechanical model provided by the application can be simultaneously applied to the life prediction of different materials under the conditions of strain control creep fatigue and stress-strain mixed control creep fatigue load.
The method is simple to operate, convenient to calculate and wide in applicability, widens the application range of the life prediction method based on damage mechanics, and can accurately predict the cycle life of different materials under the strain control and stress-strain mixed creep control fatigue load.
Drawings
The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and together with the description serve to explain the application. In the drawings:
FIG. 1 is a flow chart of a method in an embodiment of the application;
FIG. 2 is a graph of strain versus time and stress versus time for an embodiment of the present application;
FIG. 3 is a graph of creep strain rate versus time in an embodiment of the present application;
FIG. 4 is a graph of fatigue life prediction results in an embodiment of the present application;
FIG. 5 is a graph of strain control creep fatigue life prediction results in an embodiment of the present application;
FIG. 6 is a graph of predicted stress-strain hybrid control creep fatigue life in an embodiment of the application.
Detailed Description
It should be noted that, without conflict, the embodiments of the present application and features of the embodiments may be combined with each other. The application will be described in detail below with reference to the drawings in connection with embodiments. It should be noted that the steps illustrated in the flowcharts of the figures may be performed in a computer system such as a set of computer executable instructions, and that although a logical order is illustrated in the flowcharts, in some cases the steps illustrated or described may be performed in an order other than that illustrated herein.
Example 1
As shown in fig. 1, the embodiment provides a method for predicting creep fatigue life of a material based on engineering damage mechanics, which includes:
step S1, taking a plurality of samples of the same material, and performing strain control fatigue tests with the same strain rate and different strain amplitudes on part of the samples at the same temperature to obtain fatigue test data [ national standard GB/T26077-2021, "axial strain control method for fatigue test of metallic materials ], and turning to step S2; creep tests of different stresses are carried out on part of the samples at the same temperature as the strain control fatigue test [ national standard GB/T2039-2012, method for uniaxial tensile creep test of metallic materials ], creep test data are obtained, and the step S3 is carried out.
Step S2, determining the cycle life of the fatigue test under any strain amplitude according to the fatigue test data, selecting the cycle stress range of each week as a damage parameter to determine the fatigue damage evolution parameter, and establishing a fatigue damage accumulation model according to the Lemaitre continuous damage mechanics theory to calculate the fatigue life, wherein the method comprises the following steps:
for low cycle fatigue of strain control, the fatigue damage is determined by:
the formula (1) represents a test value of fatigue damage determined by using the weekly cyclic stress range as a damage parameter, wherein D f For fatigue damage, Δσ is the cyclic stress range of the material, Δσ * Is the steady state cyclic stress range of the material. The theoretical value of fatigue damage is shown in the formula (2), wherein N is the cycle of fatigue and N f For fatigue life, alpha 1 Fatigue damage evolution parameters, the fatigue damage evolution parameters alpha can be determined by the test value formula (1) of the fatigue damage and the theoretical value formula (2) of the fatigue damage 1 . The fatigue damage accumulation in each cycle is represented by the formula (3), wherein N is the cycle number, Δε in R is the inelastic strain amplitude of half life cycles in the test ν Is a triaxial factor, is 1 in a uniaxial test, and omega and gamma are fatigue damage related parameters, and are determined by a fatigue test.
Based on the fatigue damage accumulation relationship in each cycle, the fatigue life of the material under the test strain amplitude can be obtained, as shown in the formula (4):
the process proceeds to step S4.
And step S3, obtaining steady-state creep rate and creep stress according to creep test data, and establishing a Norton equation. Meanwhile, the creep strain rate is used as a damage parameter to determine a creep damage evolution parameter, and the creep life can be calculated after a creep damage accumulation model is established according to the Lemaitre continuous damage mechanics theory, and the method specifically comprises the following steps:
from the creep test data, a relationship (Norton equation) between the steady-state creep rate and the creep stress is established as shown in formula (5):
wherein ,for steady state creep rate, σ is the creep stress in the test, B is the material constant for the second stage of creep, n is the creep index, determined from the creep test data.
For the creep test, the creep damage parameters were determined by:
the formula (6) shows that the creep strain rate is used as a damage parameter to determine a creep damage test value, wherein D c In order to provide a creep damage,creep of the third stageVariable rate. The theoretical value of creep damage is shown in formula (7), wherein t is creep time, t r For creep life, the fatigue creep damage evolution parameter alpha can be determined by the test value formula (6) of creep damage and the theoretical value formula (7) of creep damage 2 . Formula (8) shows the relationship between creep damage and creep time in creep test, sigma is creep stress in test, R ν And the three-axis factor is 1 in a uniaxial test, and lambda and r are creep damage related parameters, and are determined by creep life and creep stress in a creep test.
Based on the relation (8) between creep damage and creep time in the above-mentioned transformation test, the creep life under the creep stress of the material test can be obtained as shown in the formula (9):
the process proceeds to step S4.
Step S4, through the fatigue test and the creep test in the step S2 and the step S3, and according to the Lemaitre continuous damage mechanics theory, a creep fatigue damage evolution equation is established, and the method specifically comprises the following steps:
in the creep fatigue test, the total creep time is shown as formula (10):
t＝t d ·N (10)
wherein t is creep time in creep fatigue test, t d For the dwell time, N is the cycle of creep fatigue.
Then, the creep damage accumulation per week can be expressed as:
thus, the accumulation of damage per week in creep fatigue test can be expressed as:
wherein D is the total damage in creep fatigue test, sigma eq For equivalent creep stress in the load-maintaining process, the calculation process is shown in step S6 aiming at different creep fatigue control waveforms.
The process proceeds to step S5.
Step S5, carrying out a plurality of groups of strain control creep fatigue tests and stress strain hybrid control creep fatigue tests at the same temperature and the same strain rate to obtain half life cycles of the creep fatigue tests, wherein the data are used for verifying the accuracy of the application, and specifically comprise the following steps:
wherein, strain control creep fatigue test adopts strain control trapezoidal wave to load, and the retention time t d To input parameters, the stress sigma is maintained d Along with the evolution of the load retention time, the strain control is adopted in the fatigue loading part of the stress-strain hybrid control creep fatigue test, and the stress control is adopted in the creep loading part of the stress-strain hybrid control creep fatigue test. Fig. 2 shows a graph of strain versus time and stress versus time for a stress-strain hybrid control creep fatigue load.
The process proceeds to step S6.
In step S6, since two loading modes of stress-strain hybrid control creep fatigue and strain control creep fatigue are different in load-maintaining mode in the load-maintaining stage, the equivalent creep stress in the load-maintaining process is determined for the two loading modes respectively, which is specifically as follows:
step S61, for stress-strain hybrid control test, the non-elastic strain amplitude delta epsilon of half life cycle at the same strain amplitude obtained by fatigue test in As a key parameter for determining weekly fatigue damage, the retention stress and retention time in the creep fatigue test are used as key parameters for determining weekly creep damage, wherein the retention stress in the creep fatigue test is equivalent stress sigma of weekly creep damage in step S4 eq Step S7 is carried out;
step S62, regarding a creep fatigue test of strain control, taking a half life cycle of the creep fatigue test as a characteristic cycle, calculating a creep rate during stress relaxation through a stress-time relation during a half life cycle load maintaining period, finding a steady-state creep rate during the stress relaxation process through a creep rate change rule, and calculating a creep stress under the steady-state strain rate by using a Norton equation established in the step S2, namely an equivalent stress during the strain load maintaining process. :
in the creep fatigue test of strain control, half life cycle is taken as characteristic cycle, and some characteristic parameters are very representative. For example, creep damage during half life cycle load-holding may be taken as an average of creep damage throughout the creep fatigue cycle, representing creep damage every week of the full life cycle. The application calculates the equivalent creep stress by adopting the stress-time relation in stress relaxation during the half-life period load retention period, thereby determining the creep fatigue damage under the strain control.
The stress-time relationship in the strain control creep fatigue test half life cycle stress relaxation is described by an empirical formula proposed by Feltham, as shown in formula (13):
σ＝σ 0 ·[1-B·ln(bt+1)] (13)
wherein ,σ0 For relaxation stress at the beginning of the load retention, t is the relaxation time from the beginning of the load retention phase, and B are parameters of the Feltham stress relaxation model, which can be obtained by fitting a stress relaxation curve.
Stress relaxation is a phenomenon in which a material is strained at high temperatures and changes in stress over time. In the stress relaxation process, the material is gradually transformed into permanent creep deformation under the condition that the strain is kept unchanged, and creep damage is generated in the process. The creep strain rate, which is an important parameter in the creep process, may generally represent the rate at which creep damage accumulates. The Norton equation describes the relation between the steady creep strain and the creep stress, finds out the evolution rule of the creep strain in the stress relaxation process, obtains the steady creep rate in the stress relaxation process, and can obtain the creep driving force in the process through the Norton equation to be used as the equivalent creep stress to describe the creep damage accumulation in the stress relaxation process.
In the load-maintaining stage of the creep fatigue test of strain control, the load-maintaining strain epsilon 0 From elastic strain epsilon e Plastic strain epsilon p And creep strain ε c Three parts are composed as shown in a formula (14):
ε e +ε p +ε c ＝ε 0 (14)
wherein the elastic strain ε e And plastic strain ε p Represented by the following formulas (15) and (16):
hooke's law epsilon e ＝σ/E (15)
Holomon relationship
Wherein E is the elastic modulus of the material at the test temperature, K is the strength coefficient, n is the strain hardening index, and sigma is the actual stress during stress relaxation, and is described by formula (13).
The time was obtained on both the left and right sides of equation (14):
creep rate during stress relaxationCan be expressed as:
the stress relaxation curve of the material is generally divided into two stages, the internal stress is quickly relaxed in an initial time period, and the relaxation rate is higher; the second stage relaxation rate gradually slows down, and the stress relaxation curve is flattened. Correspondingly, the short creep rate of the initial stage is larger, and the creep rate tends to be stable along with the stress relaxation rate in the first stage and the second stage of the creep test, and corresponds to the steady creep stage of the creep test. For general materials, after 10 to 30 hours of relaxation, the stress relaxation rate tends to be stable, the stress relaxation enters the second stage, and the creep rate at this time is the "steady state creep" in the stress relaxation processVariable rate). Substituting the strain into the Norton equation established in the step S3 to calculate the creep stress at the steady-state strain rate, namely equivalent stress sigma in the strain retention process eq . The equivalent stress is calculated by the equation (19).
wherein ,for "steady state creep rate" during stress relaxation, B and n are the material constant and creep index, respectively, of the second stage of creep determined in step S3.
The process proceeds to step S7.
Step S7, calculating to obtain the cycle of the damage of 1 as the predicted creep fatigue life by utilizing the creep fatigue damage evolution equation established in the step S4, wherein the cycle is specifically as follows:
from formula (12):
integrating equation (20) yields creep fatigue life as shown in equation (21):
wherein ,Δεin Inelastic strain amplitude for half life cycles at the same strain amplitude obtained for strain control fatigue test; t is t d Is the load-keeping time; loading waveforms sigma aiming at different creep fatigue tests eq Determined by step S6.
Compared with the prior art, the embodiment has the remarkable advantages that:
(1) In the embodiment, the equivalent creep stress in the strain load-holding process is determined by calculating the steady creep rate in the stress relaxation process, and the real driving force for generating creep deformation in the strain load-holding process is disclosed;
(2) The method is simple to operate, convenient to calculate and wide in applicability, widens the application range of the life prediction method based on damage mechanics, and can accurately predict the cycle life of different materials under strain control and stress-strain mixed creep control fatigue load.
Example two
The method for predicting the creep fatigue life of the material based on engineering damage mechanics in the embodiment 1 is adopted to predict the cycle life of different temperatures and different materials under the fatigue and strain control of strain control and stress-strain mixed creep fatigue load control.
Selecting stress-strain mixed control creep fatigue test of GH4169 high-temperature alloy with data of 8 groups at 650 ℃, wherein the holding time is 180s,600s and the holding stress of 180s is 600MPa-750MPa; the stress-strain hybrid control creep fatigue test of the 16 groups of P92 steel at 625 ℃ is carried out, the holding time is 300s,600s and 180s, and the holding stress is 140MPa-180MPa; the stress-strain hybrid control creep fatigue test of the 16 groups of P92 steel at 650 ℃ has the holding time of 180s,600s and 180s and the holding stress of 115MPa and 140MPa, and the test method is described in the specification.
Strain control creep fatigue test of 9 groups of P92 steel at 650 ℃ and holding time of 180s,300s and 600s; the strain control creep fatigue test of the 3 groups of 316L stainless steel at 550 ℃ is carried out for 60s,180s and 600s, and the test method is described in the specification. The GH4169 superalloy was subjected to strain control creep fatigue testing at 650 ℃ using paper data.
Group 3 low cycle fatigue tests of GH4169 superalloy strain control at 650 ℃ with strain amplitudes of 0.6%,0.7%,0.8% and test methods as described in the specification. The 4 groups of low-cycle fatigue tests of GH4169 high-temperature alloy strain control at 650 ℃ adopt paper data, the 27 groups of low-cycle fatigue tests of GH4169 high-temperature alloy strain control at 650 ℃ adopt paper data, and the fatigue tests of P92 steel at different temperatures and strain amplitudes adopt published paper data. 6 groups of 316L stainless steel are subjected to strain control fatigue test at 550 ℃, and the strain amplitude is 0.3% -1.2%; the stress amplitude of the 6 groups of 316L stainless steel in a fatigue test under the stress control of 550 ℃ is 230MPa-300 MPa; the test method is as described in the specification. The fatigue test of P92 steel at different temperatures and strain amplitudes uses published paper data.
Creep test of P92 steel at 625 ℃ in 4 groups, wherein creep stress is 150-180 MPa;3 groups of P92 steel creep tests at 650 ℃ have creep stress of 115MPa,130MPa and 140MPa. Creep test of group 5 stainless steel of 316L at 550 ℃ and creep stress of 280MPa-320MPa GH4169 superalloy adopts paper data in the creep test at 650 ℃.
Firstly, according to the step S2 of the application, according to the fatigue test data, the fatigue damage model parameters are determined, and the fatigue life calculation model established by the damage mechanics theory is applied to calculate the cycle life of the fatigue test under any strain amplitude. Then determining a creep damage accumulation model according to the creep test data in the step S3, and then determining a fatigue creep damage accumulation model in a creep fatigue test according to the step S4; according to step S6, the parameters in equation (13) are fitted to the stress relaxation data of the characteristic cycles for strain-controlled creep fatigue to obtain the creep rate during stress relaxationAs shown in FIG. 3, the creep rate is extrapolated from the time relationship to the point in time (at the dashed line in the figure) at which the stress relaxation reaches steady state, where the creep rate is "steady state creep rate +.>", the equivalent creep stress is obtained by substituting formula (19). And for a stress-strain hybrid control test, the retention stress is creep equivalent stress. Finally, according to the step S7, the creep fatigue cycle life of different materials under different test loads is predicted by using a creep fatigue damage accumulation model.
From the results of fig. 4-6, it can be seen that the life predicted by the present application is within a 2-fold error band for the fatigue test, the strain control creep fatigue, and the stress-strain hybrid control creep fatigue test. Therefore, the life prediction model of the application has universality for different test materials, different test temperatures, different fatigue loads and creep fatigue loads.
The present application is not limited to the above-mentioned embodiments, and any changes or substitutions that can be easily understood by those skilled in the art within the technical scope of the present application are intended to be included in the scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.
Claims (6)
1. The method for predicting the creep fatigue life of the material based on engineering injury mechanics is characterized by comprising the following steps of:
performing a fatigue test of strain control on the material, obtaining the cycle life of the fatigue test under any strain amplitude based on fatigue test data, and selecting a weekly cycle stress range as a damage parameter to determine a fatigue damage evolution parameter;
carrying out creep tests of different stresses on the material, obtaining steady-state creep rate and creep stress based on creep test data, establishing a Norton equation, and obtaining creep damage evolution parameters based on the steady-state creep rate;
based on the fatigue damage evolution parameters and the creep damage evolution parameters, constructing a creep fatigue damage evolution equation according to a Lemaitre continuous damage mechanics theory;
developing a plurality of groups of strain control creep fatigue tests and stress strain hybrid control creep fatigue tests, obtaining key parameters in half life cycles of the creep fatigue tests, and respectively determining equivalent creep stress in the load maintaining process according to different load forms based on the key parameters;
the process for respectively determining the equivalent creep stress in the load-holding process comprises the following steps:
carrying out a creep fatigue test of strain control on a material, calculating a creep rate during stress relaxation through a stress-time relation during the retention period of half life cycle based on the half life cycle of the creep fatigue test, obtaining the creep rate through a change rule of the creep rate, establishing a Norton equation based on the relation between the creep rate and the creep stress, and calculating the creep stress through the Norton equation to obtain the equivalent creep stress;
for the stress-strain mixed control creep fatigue test, the retention stress in the creep fatigue test is equivalent creep stress sigma eq ；
Based on the equivalent creep stress, predicting the creep fatigue life under different load forms through the creep fatigue damage evolution equation to obtain the creep fatigue life.
2. The method for predicting creep fatigue life of a material based on engineering injury mechanics according to claim 1, wherein after obtaining the fatigue injury evolution parameter, further comprises:
and obtaining a cycle life based on the fatigue test data, selecting a cycle stress range in the cycle life as a damage parameter, determining a fatigue damage evolution parameter based on the damage parameter, establishing a fatigue damage accumulation model according to a Lemaitre continuous damage mechanics theory, and calculating fatigue damage and predicting the fatigue life based on the fatigue damage accumulation model.
3. The method for predicting creep fatigue life of a material based on engineering injury mechanics according to claim 1, wherein after obtaining creep injury evolution parameters, further comprises:
and obtaining a steady-state creep rate based on the creep test data, obtaining creep damage evolution parameters based on the steady-state creep rate, establishing a creep damage accumulation model according to a Lemaitre continuous damage mechanics theory, and calculating creep damage based on the creep damage accumulation model.
4. The method for predicting creep fatigue life of a material based on engineering injury mechanics according to claim 1, wherein the process of constructing the creep fatigue injury evolution equation comprises:
the fatigue damage accumulation model and the creep damage accumulation model are established according to the Lemaitre continuous damage mechanics theory and are respectively as follows:
the above formula represents a model of fatigue damage accumulation in each cycle, where D f For fatigue damage, alpha 1 For fatigue damage evolution parameters, N is cycle number, delta epsilon in R is the inelastic strain amplitude of half life cycles in the test ν Is a triaxial factor, 1 in a uniaxial test, and omega and gamma are fatigue damage related parameters;
the above formula shows the relationship between creep damage and creep time in creep test, D c For creep damage, alpha 2 For creep damage evolution parameters, σ is the creep stress in the test, R ν Is a triaxial factor, 1 is adopted in a uniaxial test, and lambda and r are creep damage related parameters;
in the creep fatigue test, the total creep time per week is shown in the following formula:
t＝t d ·N
wherein t is creep time in creep fatigue test, t d N is the cycle number for the load retention time;
creep damage accumulation per week is expressed as:
the damage accumulation per week in the creep fatigue test according to the Lemaitre continuous damage mechanics theory is expressed as:
wherein D is the total damage in creep fatigue test.
5. The method for predicting creep fatigue life of a material based on engineering injury mechanics according to claim 1, wherein the process of calculating the equivalent creep stress comprises:
the stress-time relationship in the strain control creep fatigue test half life cycle stress relaxation is described by an empirical formula proposed by Feltham, as shown in the following formula:
σ＝σ 0 ·[1-A·ln(bt+1)]
wherein ,σ0 In order to relax stress at the beginning of load retention, t is relaxation time from the beginning of load retention, A and b are parameters of a Feltham stress relaxation model, and the stress relaxation model is obtained by fitting a stress relaxation curve;
in the load-maintaining stage of the creep fatigue test of strain control, the load-maintaining strain epsilon 0 From elastic strain epsilon e Plastic strain epsilon p And creep strain ε c Three parts are composed, and the following formula is shown:
ε e +ε p +ε c ＝ε 0
wherein the elastic strain ε e And plastic strain ε p Expressed by the following stress-strain relationship, respectively:
hooke's law epsilon e ＝σ/E
Holomon relationship
Wherein E is the elastic modulus of the material at the test temperature, K is the strength coefficient, n is the strain strengthening index, and sigma is the stress in the stress relaxation of the strain control creep fatigue test half life cycle;
the left and right sides of the strain expression during the load retention are respectively derived from time:
creep rate during stress relaxationCan be expressed as:
the stress relaxation curve of the material is divided into two stages, the internal stress is quickly relaxed in the initial time period, and the relaxation rate is high; the relaxation rate of the second stage is gradually slowed down, and the stress relaxation curve is gradually flattened; correspondingly, in the first stage and the second stage of the corresponding creep tests, the creep rate tends to be stable along with the stress relaxation rate, and corresponds to the steady creep stage of the creep test; for materials such as heat-resistant steel, austenitic stainless steel, nickel-based alloy and the like, after relaxation for 10 to 30 hours, the stress relaxation rate tends to be stable, the stress relaxation enters a second stage, the creep rate at the moment is the steady-state creep rate in the stress relaxation process, and the creep stress at the steady-state strain rate is obtained through Norton equation calculation, namely the equivalent creep stress sigma eq The method comprises the steps of carrying out a first treatment on the surface of the The calculation formula of the equivalent creep stress is as follows:
wherein ,for "steady state creep rate" during stress relaxation, B and n 1 The material constant and creep index of the second stage of creep are respectively.
6. The method for predicting creep fatigue life of a material based on engineering injury mechanics according to claim 1, wherein the process of obtaining creep fatigue life comprises:
and calculating to obtain the cycle when the damage is 1 according to the creep fatigue damage evolution equation, namely the predicted creep fatigue life.
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