Method and System for Determining Creep Fatigue Damage Level Considering Degradation of Material Properties

Abstract

The present invention relates to the technical field of creep-fatigue damage assessment, and discloses a method and system for determining creep-fatigue damage grades considering material property degradation. By calculating the creep damage D of the target material c and the fatigue damage D f , the first creep-fatigue damage interaction criterion equation is determined. Based on the tensile plastic strain energy U T , the material property degradation parameter D m is determined. Through D m , the first creep-fatigue damage interaction criterion equation, the creep-fatigue damage grade of the material and the second creep-fatigue damage interaction criterion equation related to material property degradation are determined. A three-dimensional creep-fatigue damage assessment diagram is drawn, and the parametric state point (D m , D c , D f ) of the service component is placed in the assessment diagram to determine the creep-fatigue damage grade of the service component and whether the service component has creep-fatigue failure. The present invention can identify the damage grade of the material of the service component used in a high-temperature environment and whether creep-fatigue failure occurs within the designed remaining service time.

Description

Technical Field

[0001] The present invention relates to the technical field of creep-fatigue damage assessment, and particularly to a method and system for determining creep-fatigue damage grades considering material property degradation. Background Art

[0002] For structural components such as steam turbines, heavy gas turbines, heat exchangers, and high-temperature and high-pressure pipelines operating in high-temperature environments, in addition to normal working stresses, they are also subjected to creep-fatigue interaction loads caused by frequent start-stop operations and temperature fluctuations. The creep-fatigue interaction load is one of the main factors affecting the safe operation and premature failure of components. Moreover, during the long-term service under high temperature and variable loads, the strength and load-bearing capacity of component materials will decrease with the extension of service time. However, current component life design methods and damage assessment methods are almost all developed based on the initial material properties. Therefore, on the basis of considering time-dependent material property degradation, evaluating and identifying the creep-fatigue damage of this type of component is of great significance for ensuring safe production.

[0003] Since the 1950s, scholars at home and abroad have successively proposed hundreds of damage assessment and safe life design models under creep-fatigue interaction. Most of these models are derived from the Coffin-Mason plastic strain-life equation, strain range partitioning, or linear damage accumulation rule, and mostly establish the relationship between damage and various mechanical and physical quantities from a macroscopic phenomenological perspective.

[0004] In recent years, although the creep-fatigue damage assessment method based on energy density parameters can effectively improve the accuracy of life prediction, its main limitation is that it has not considered the material property degradation during service. Therefore, the existing creep-fatigue damage assessment based on initial material properties cannot correlate the degree of material property deterioration with damage grading and dynamic assessment, and it is difficult to determine whether creep-fatigue failure will occur in the remaining service time designed for equipment or components used in high-temperature environments, resulting in the possibility that existing life design methods may be non-conservative. Summary of the Invention

[0005] The purpose of the present invention is to provide a method and system for determining creep-fatigue damage grades considering material property degradation, which can identify the material damage grades of service components used in high-temperature environments and whether creep-fatigue failure will occur in the remaining service time designed, and is applicable to the evaluation and maintenance of the high-temperature structural integrity of service component materials.

[0006] To achieve the above purpose, the present invention provides the following solutions:

[0007] A method for determining creep-fatigue damage grades considering material property degradation, the creep-fatigue damage grade determination method comprising:

[0008] Perform multiple sets of interrupted creep-fatigue tests and multiple sets of tensile tests after interruption on the target material to determine the material parameters l, a, and b.

[0009] Based on the material parameters and l, calculate the creep damage D of the target material c .

[0010] Based on the material parameters a and b, calculate the fatigue damage D of the target material f .

[0011] Based on the creep damage D c and fatigue damage D f of the target material, determine the first creep-fatigue damage interaction criterion equation.

[0012] Calculate the tensile plastic strain energy U of each interrupted life T .

[0013] Based on the tensile plastic strain energy U T , determine the material property degradation parameter D T related to the U m .

[0014] Based on the material property degradation parameter D m , divide the creep-fatigue damage grades.

[0015] Based on the material property degradation parameter D m and the first creep-fatigue damage interaction criterion equation, determine the damage critical value D m of the first creep-fatigue damage interaction criterion under different material property degradation parameters D t .

[0016] Based on the damage critical value D t , determine the second creep-fatigue damage interaction criterion equation related to material property degradation.

[0017] Based on the second creep-fatigue damage interaction criterion equation and the creep-fatigue damage grades, with the creep damage D c as the x-axis, the fatigue damage D f as the y-axis, and the material property degradation parameter D m as the z-axis, plot a three-dimensional creep-fatigue damage assessment diagram, which is used to determine the creep-fatigue damage grades of components and determine whether the components have creep-fatigue failure; the three-dimensional creep-fatigue damage assessment diagram includes: a safe area and a non-safe area.

[0018] Calculate the parameters (D m , Dc , D f )。

[0019] Taking the parameters (D m , D c , D f ) of the service component as state points and placing them in the three-dimensional creep-fatigue damage assessment diagram to determine the creep-fatigue damage grade of the service component and whether the service component has creep-fatigue failure.

[0020] Optionally, performing multiple groups of interrupted creep-fatigue tests and multiple groups of tensile tests after interruption on the target material to determine the material parameters l, a, and b specifically include:

[0021] Through high-temperature creep tests at different stress levels, obtaining the creep ductility strain ε f , creep fracture time t R .

[0022] Based on the stress level σ, the creep ductility strain ε f , and the creep fracture time t R , determining the creep strain energy density dissipation rate under creep test conditions

[0023] Based on the creep strain energy density dissipation rate determining the failure strain energy density w f .

[0024] Based on the creep strain energy density dissipation rate and the failure strain energy density w f , determining the material parameters and l.

[0025] Through fatigue tests under strain control with the same temperature and different strain amplitudes, obtaining the plastic strain range Δε p , peak stress σ max , and fatigue test life N0 at each strain amplitude.

[0026] Based on the plastic strain range Δε p , peak stress σ max , and fatigue test life N0 to determine the material parameters a and b.

[0027] Optionally, the creep damage D c of the target material is specifically calculated by the following formula:

[0028]

[0029] where Dc is creep damage, t h is the holding time at the steady-state cycle of the creep-fatigue test, Δε c is the range of non-recoverable strain generated during the holding time at the steady-state cycle, σ max is the peak stress at the steady-state cycle of the creep-fatigue test, Δσ r is the difference in stress levels between the start and end stages of the holding at the steady-state cycle, and l are material parameters, N e is the remaining creep-fatigue life in cycles.

[0030] Optionally, the fatigue damage D of the target material f is specifically calculated by the following formula:

[0031] D f = 1 / [a·(σ max ·Δε p ) -b ·N e

[0032] where D f is the fatigue damage, Δε p is the plastic strain range at the steady-state cycle of the creep-fatigue test, a and b are material parameters, N e is the remaining creep-fatigue life in cycles.

[0033] Optionally, the tensile plastic strain energy U of each interrupted life T is specifically calculated by the following formula:

[0034]

[0035] where U T is the tensile plastic strain energy, ε p is the true plastic strain, σ(ε p ) is the true stress corresponding to the plastic strain, ε t is the true fracture strain.

[0036] Optionally, the determination formula of the material property degradation parameter D m is as follows:

[0037]

[0038] where U T(0) is the tensile plastic strain energy of the undamaged material, U T(N) is the tensile plastic strain energy of the material at the Nth cycle.

[0039] Optionally, the determination equation of the damage critical value D t is as follows:

[0040] D t = 1 - (D m ) k

[0041] where D t is the damage critical value, and k is the power exponent obtained by fitting with the non - linear least - squares method.

[0042] Optionally, based on the damage critical value D t , determining the second creep - fatigue damage interaction criterion equation related to material property degradation specifically includes:

[0043] Determining the linear damage accumulation criterion equation, and the linear damage accumulation criterion equation is:

[0044] D f + D c = 1 - (D m ) k

[0045] Determining the bilinear interaction criterion equation, and the bilinear interaction criterion equation is:

[0046]

[0047]

[0048] Determining the simplified continuous interaction criterion equation, and the simplified continuous interaction criterion equation is:

[0049]

[0050] where D c is the creep damage of the target material, D f is the fatigue damage of the target material, D m is the material property degradation parameter, k is the power exponent obtained by fitting with the non - linear least - squares method, d f , d c are the fatigue damage and creep damage of a single steady - state cycle, is the turning point of fatigue damage and creep damage in the bilinear interaction criterion, and n is the power exponent of the simplified continuous interaction criterion.

[0051] Optionally, placing the parameters (D m , D c , D f ) of the service component as state points in the three - dimensional creep - fatigue damage assessment diagram, and determining the creep - fatigue damage level of the service component and whether the service component has creep - fatigue failure specifically includes:

[0052] If the state point (D m , D c , Df ) If it is within the safe area of the three-dimensional creep-fatigue damage assessment diagram, the current creep-fatigue damage level of the in-service component can be determined, and the in-service component will not suffer creep-fatigue failure within the remaining designed service time.

[0053] If the state point (D m , D c , D f ) of the in-service component is within the non-safe area of the three-dimensional creep-fatigue damage assessment diagram, it indicates that there is a possibility of creep-fatigue failure of the in-service component within the remaining designed service time, and further maintenance or shortening of the remaining service time is required.

[0054] Based on the above method, the present invention also provides a creep-fatigue damage level determination system considering material property degradation, and the creep-fatigue damage level determination system includes:

[0055] A material parameter determination module, which is used to conduct multiple groups of interrupted creep-fatigue tests and multiple groups of post-interruption tensile tests on the target material to determine the material parameters l, a, and b.

[0056] The creep damage D of the target material c Calculation module, connected to the material parameter determination module, and used to calculate the creep damage D of the target material based on the material parameters and l. c .

[0057] The fatigue damage D of the target material f Calculation module, connected to the material parameter determination module, and used to calculate the fatigue damage D of the target material based on the material parameters a and b. f .

[0058] The first creep-fatigue damage interaction criterion equation determination module, connected to the creep damage D calculation module of the target material c and the fatigue damage D calculation module of the target material f , and is used to determine the first creep-fatigue damage interaction criterion equation based on the creep damage D c and fatigue damage D f of the target material.

[0059] The tensile plastic strain energy U T Calculation module, which is used to calculate the tensile plastic strain energy U of each interrupted life T .

[0060] The material property degradation parameter D m Determination module, connected to the tensile plastic strain energy U T Calculation module, and used to determine based on the tensile plastic strain energy U T, determine the material property degradation parameter D T related to the U m .

[0061] The creep-fatigue damage level classification module, connected to the material property degradation parameter D m determination module, is used to classify the creep-fatigue damage level based on the material property degradation parameter D m .

[0062] The damage critical value D t determination module, connected to the first creep-fatigue damage interaction criterion equation determination module and the material property degradation parameter D m determination module, is used to determine the damage critical value D of the first creep-fatigue damage interaction criterion under different material property degradation parameters D m and the first creep-fatigue damage interaction criterion equation m . t .

[0063] The second creep-fatigue damage interaction criterion equation determination module, connected to the damage critical value D t determination module, is used to determine the second creep-fatigue damage interaction criterion equation related to material property degradation based on the damage critical value D t .

[0064] The three-dimensional creep-fatigue damage assessment diagram drawing module, connected to the second creep-fatigue damage interaction criterion equation determination module and the creep-fatigue damage level classification module, is used to draw a three-dimensional creep-fatigue damage assessment diagram with the creep damage D c as the x-axis, the fatigue damage D f as the y-axis, and the material property degradation parameter D m as the z-axis; the three-dimensional creep-fatigue damage assessment diagram is used to determine the creep-fatigue damage level of the component and determine whether the component has creep-fatigue failure; the three-dimensional creep-fatigue damage assessment diagram includes: a safe area and a non-safe area.

[0065] The parameter calculation module for in-service components, used to calculate the parameters (D m , D c , D f ) of the in-service components.

[0066] The creep-fatigue level and failure determination module for in-service components, connected to the parameter calculation module for in-service components, is used to use the parameters (D m , D c , D f)As a state point is placed in the three-dimensional creep-fatigue damage assessment diagram, the creep-fatigue damage level of the service component is determined, as well as whether the service component has suffered creep-fatigue failure.

[0067] According to the specific embodiments provided by the present invention, the following technical effects are disclosed by the present invention:

[0068] The present invention provides a method and system for determining the creep-fatigue damage level considering material property degradation. By conducting multiple sets of interrupted creep-fatigue tests and multiple sets of tensile tests after interruption on the target material, material parameters , l, a, and b are determined; based on the material parameters and l, the creep damage D c of the target material is calculated; based on the material parameters a and b, the fatigue damage D f of the target material is calculated; based on the creep damage D c and fatigue damage D f of the target material, the first creep-fatigue damage interaction criterion equation is determined; the tensile plastic strain energy U T of each interrupted life is calculated; based on the tensile plastic strain energy U T , the material property degradation parameter D T related to the U m is determined; based on the material property degradation parameter D m , the creep-fatigue damage level is divided; based on the material property degradation parameter D m and the first creep-fatigue damage interaction criterion equation, the damage critical value D m of the first creep-fatigue damage interaction criterion under different material property degradation parameters D t is determined; based on the damage critical value D t , the second creep-fatigue damage interaction criterion equation related to material property degradation is determined; based on the second creep-fatigue damage interaction criterion equation and the creep-fatigue damage level, with the creep damage D c as the x-axis, the fatigue damage D f as the y-axis, and the material property degradation parameter D m as the z-axis, a three-dimensional creep-fatigue damage assessment diagram is drawn. The three-dimensional creep-fatigue damage assessment diagram is used to determine the creep-fatigue damage level of the component and to determine whether the component has suffered creep-fatigue failure; the three-dimensional creep-fatigue damage assessment diagram includes: a safe area and a non-safe area; the parameters (D m , D c , D f ) of the service component are calculated; the parameters (D m , D c , D f)As a state point is placed in the three-dimensional creep-fatigue damage assessment diagram, the creep-fatigue damage grade of the service component and whether the service component has creep-fatigue failure are determined. The present invention can identify the material damage grade of the service component used in a high-temperature environment and whether creep-fatigue failure occurs within the remaining designed service time, and is applicable to the evaluation and maintenance of the high-temperature structural integrity of the service component material. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0070] Figure 1 It is a flowchart of a method for determining the creep-fatigue damage grade considering material property degradation according to an embodiment of the present invention;

[0071] Figure 2 It is a schematic diagram of the tensile plastic strain energy U of each interrupted life according to an embodiment of the present invention T Schematic diagram;

[0072] Figure 3 It is a schematic diagram for determining the damage critical value D according to an embodiment of the present invention t Schematic diagram;

[0073] Figure 4 It is the damage critical value D according to an embodiment of the present invention t Schematic diagram of the equation;

[0074] Figure 5 It is a schematic diagram of the structure of a system for determining the creep-fatigue damage grade considering material property degradation according to an embodiment of the present invention;

[0075] Figure 6 It is a schematic diagram of the damage critical values of the nickel-based GH4169 alloy provided in Embodiment 1 of the present invention at 10%, 50%, and 80% interrupted life fractions;

[0076] Figure 7 It is the damage critical value D of the nickel-based GH4169 alloy provided in Embodiment 1 of the present invention t Schematic diagram of the equation;

[0077] Figure 8 It is a three-dimensional creep-fatigue damage assessment diagram of the nickel-based GH4169 alloy provided in Embodiment 1 of the present invention;

[0078] Figure 9 It is a schematic diagram of the damage critical values of the martensitic heat-resistant steel P92 provided in Embodiment 2 of the present invention at 20%, 50%, and 70% interrupted life fractions;

[0079] Figure 10 The critical damage value D of the martensitic heat-resistant steel P92 provided in the second embodiment of the present invention t Schematic diagram of the equation;

[0080] Figure 11 The three-dimensional creep-fatigue damage assessment diagram of the martensitic heat-resistant steel P92 provided in the second embodiment of the present invention. Specific implementation manners

[0081] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0082] The purpose of the present invention is to provide a method and system for determining creep-fatigue damage grades considering material property degradation, which can identify the material damage grades of service components used in high-temperature environments and whether creep-fatigue failure occurs within the designed remaining service time.

[0083] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation manners.

[0084] Referring to the accompanying drawings Figure 1 , the present invention provides a method for determining creep-fatigue damage grades considering material property degradation, which is characterized in that the creep-fatigue damage grade determination method includes:

[0085] S1. Conduct multiple groups of interrupted creep-fatigue tests and multiple groups of post-interruption tensile tests on the target material to determine material parameters , l, a, and b. The S1 specifically includes:

[0086] S11. Through high-temperature creep tests at different stress levels, obtain the creep ductility strain ε f , creep fracture time t R of the target material at each stress level σ.

[0087] S12. Based on the stress level σ, the creep ductility strain ε f and the creep fracture time t R , determine the creep strain energy density dissipation rate under creep test conditions

[0088] Specifically, the creep strain energy density dissipation rate is determined by the following formula:

[0089]

[0090] S13. Determine the failure strain energy density w based on the creep strain energy density dissipation rate f .

[0091] Specifically, the failure strain energy density w f is determined by the following formula:

[0092]

[0093] S14. Based on the creep strain energy density dissipation rate and the failure strain energy density w f , determine the material parameters and l.

[0094] Specifically, by fitting using the least squares method, the numerical values of the material parameters and l are determined.

[0095] S15. Through fatigue tests under strain control with the same temperature but different strain amplitudes, obtain the plastic strain range Δε p , peak stress σ max and fatigue test life N0 at each strain amplitude.

[0096] S16. Based on the plastic strain range Δε p , peak stress σ max and fatigue test life N0, determine the material parameters a and b.

[0097] Specifically, the relationship between σ max ·Δε p and N0 is: N0 = a·(Δε p ·σ max ) b . By fitting N0 - σ max ·Δε p using the least squares method, the material parameters a and b are determined.

[0098] Specifically, the multi - group interrupted creep - fatigue tests and multi - group post - interruption tensile tests on the target material are interrupted creep - fatigue tests under strain control at the same temperature, with at least 3 interruption life fractions and the span of the 3 fractions covering at least 70% of the full life. At each interruption life fraction, there are at least ten sets of working condition data to calculate creep damage and fatigue damage.

[0099] Specifically, obtain the true stress - strain curve through post - interruption tensile tests at the same temperature. ​

[0100] S2. Calculate the creep damage D of the target material based on the material parameters and l c . The creep damage D of the target material c is specifically calculated by the following formula:

[0101]

[0102] where D c is the creep damage, t h is the holding time of the steady-state cycle of the creep-fatigue test, Δε c is the range of non-recoverable strain generated during the holding time of the steady-state cycle, σ max is the peak stress of the steady-state cycle of the creep-fatigue test, Δσ r is the difference in stress levels between the start and end stages of the holding of the steady-state cycle, and l are material parameters, and N e is the remaining number of cycles of creep-fatigue life

[0103] S3. Calculate the fatigue damage D of the target material based on the material parameters a and b f . The fatigue damage D of the target material f is specifically calculated by the following formula:

[0104] D f = 1 / [a·(σ max ·Δε p ) -b ·N e

[0105] where D f is the fatigue damage, Δε p is the plastic strain range of the steady-state cycle of the creep-fatigue test, a and b are material parameters, and N e is the remaining number of cycles of creep-fatigue life

[0106] S4. Determine the first creep-fatigue damage interaction criterion equation based on the creep damage D c and fatigue damage D f of the target material

[0107] Specifically, the determination of the first creep-fatigue damage interaction criterion equation includes:

[0108] Obtain the first linear damage accumulation criterion equation, and the first linear damage accumulation criterion equation is:

[0109] D f + D c = 1

[0110] Obtain the bilinear interaction criterion equation, and the bilinear interaction criterion equation is:

[0111]

[0112] Obtain the simplified continuous interaction criterion equation, and the simplified continuous interaction criterion equation is

[0113]

[0114] where D c is the creep damage of the target material, D f is the fatigue damage of the target material, d f , d c is the fatigue damage and creep damage of a single steady-state cycle, is the turning point of fatigue damage and creep damage in the bilinear interaction criterion, and n is the power exponent of the simplified continuous interaction criterion.

[0115] S5. Calculate the tensile plastic strain energy U of each interrupted life T , and the tensile plastic strain energy U of each interrupted life T is specifically calculated by the following formula:

[0116]

[0117] where U T is the tensile plastic strain energy, ε p is the true plastic strain, σ(ε p ) is the true stress corresponding to the plastic strain, and ε t is the true fracture strain. Specifically, see the attached drawing Figure 2 Schematic diagram of the tensile plastic strain energy U of each interrupted life T .

[0118] S6. Based on the tensile plastic strain energy U T , determine the material property degradation parameter D T related to the U m . The determination formula of the material property degradation parameter D m is as follows:

[0119]

[0120] where U T(0) is the tensile plastic strain energy of the undamaged material, and U T(N ) is the tensile plastic strain energy of the material at the Nth cycle.

[0121] S7. Based on the material property degradation parameter D m , divide the creep-fatigue damage grade.

[0122] Specifically, the creep-fatigue damage grades are classified as: Grade I, Grade II, Grade III, Grade IV, and Grade V.

[0123] Specifically, the service life corresponding to Grade I is within 20% of the total life; the service life corresponding to Grade II is within 20% - 40% of the total life; the service life corresponding to Grade III is within 40% - 60% of the total life; the service life corresponding to Grade IV is within 60% - 80% of the total life; and the service life corresponding to Grade V is within 80% - 100% of the total life.

[0124] Specifically, referring to the attached drawings Figure 3 , taking the linear damage accumulation criterion as an example, according to D at each interrupted life cycle c and D f to determine the safety domain (lower envelope domain), and obtain the critical value D of different D m under the damage interaction criterion t .

[0125] S8. Based on the material property degradation parameter D m and the first creep-fatigue damage interaction criterion equation, determine the damage critical value D of the first creep-fatigue damage interaction criterion under different material property degradation parameters D m . The determination equation of the damage critical value D t is as follows: t D

[0126] D t = 1 - (D m ) k

[0127] where D t is the damage critical value, and k is the power exponent obtained by fitting with the non-linear least squares method. Specifically, refer to the attached drawings Figure 4 .

[0128] S9. Based on the damage critical value D t , determine the second creep-fatigue damage interaction criterion equation related to material property degradation. The specific content of S9 includes:

[0129] Determine the linear damage accumulation criterion equation, and the linear damage accumulation criterion equation is:

[0130] D f + D c = 1 - (D m ) k

[0131] Determine the bilinear interaction criterion equation, and the bilinear interaction criterion equation is:

[0132]

[0133]

[0134] Determine the simplified continuous interaction criterion equation, and the simplified continuous interaction criterion equation is:

[0135]

[0136] where D c is the creep damage of the target material, D f is the fatigue damage of the target material, D m is the material property degradation parameter, k is the power exponent obtained by fitting with the non-linear least squares method, d f , d c is the fatigue damage and creep damage of a single steady-state cycle, is the turning point of fatigue damage and creep damage in the bilinear interaction criterion, and n is the power exponent of the simplified continuous interaction criterion.

[0137] S10. Based on the second creep-fatigue damage interaction criterion equation and the creep-fatigue damage grade, with the creep damage D c as the x-axis, the fatigue damage D f as the y-axis, and the material property degradation parameter D m as the z-axis, plot a three-dimensional creep-fatigue damage assessment diagram, which is used to determine the creep-fatigue damage grade of the component and determine whether the component has creep-fatigue failure; the three-dimensional creep-fatigue damage assessment diagram includes: a safe area and a non-safe area. Specifically, the area below the envelope of the three-dimensional creep-fatigue damage assessment diagram is the safe area, and the area above the envelope is the non-safe area.

[0138] S11. Calculate the parameters (D m , D c , D f ) of the in-service component.

[0139] S12. Place the parameters (D m , D c , D f ) of the in-service component as state points in the three-dimensional creep-fatigue damage assessment diagram to determine the creep-fatigue damage grade of the in-service component and whether the in-service component has creep-fatigue failure. The S12 specifically includes:

[0140] If the state point (D m , D c , D f)If it is within the safe area of the three-dimensional creep-fatigue damage assessment diagram, the current creep-fatigue damage level of the in-service component can be determined, and the in-service component will not suffer creep-fatigue failure within the remaining designed service time.

[0141] If the state point (D m , D c , D f ) of the in-service component is within the non-safe area of the three-dimensional creep-fatigue damage assessment diagram, it indicates that there is a possibility of creep-fatigue failure of the in-service component within the remaining designed service time, and further maintenance is required or the remaining service time needs to be shortened.

[0142] Example 1:

[0143] The No. 2 gas turbine of the Eastern Power Plant was put into operation in 2007, and the gas turbine load is 150 - 240 MW. The compressor tie rod of this gas has been in service for about 3 years. Due to the requirements of the production process, this equipment needs to continue running for 5 years before it can be overhauled. Through finite element software, it is estimated that the number of creep-fatigue cycles caused by start-stop and temperature fluctuations during the service period of this tie rod is about 87 cycles, and it still needs to continue running for 145 cycles.

[0144] To ensure the reliability of the compressor during the next 5 years of operation, the damage determination method of the present invention is now adopted, and a three-dimensional creep-fatigue damage assessment diagram with the degradation of material mechanical properties over service time is used to evaluate the current damage level of the component material and its safety within the remaining service time of 5 years.

[0145] The compressor tie rod material is made of nickel-based GH4169 alloy, and the working temperature is 650 °C. The creep-fatigue damage interaction criterion and material parameters without considering the degradation of material mechanical properties can refer to the published patent with the application number CN106202913A.

[0146] First step, obtain GH4169 material, and conduct axial equal-amplitude creep-fatigue interaction tests with a strain range of 1.0% - 2.0% and a hold time of 60 s - 3600 s at the maximum strain range according to the service conditions of the component. Conduct interrupted creep-fatigue tests at 10%, 50%, and 80% life fractions, and conduct tensile tests on the interrupted specimens until fracture.

[0147] Second step, according to the tensile true stress-strain curves at different interrupted life fractions, calculate the tensile plastic strain energy U T and the material mechanical property degradation parameter D m .

[0148] Third step, determine the safe domain (lower envelope domain) according to D c and D f at 10%, 50%, and 80% interrupted life cycles, and obtain different Dm Critical value D of the lower damage interaction criterion t , see the attached drawings Figure 6 .

[0149] Fourth step, see the attached drawings Figure 7 , and obtain D by fitting with the non-linear least squares method m and the damage critical value D t The functional relationship is as follows:

[0150] D t = 1 - (D m ) 1.8

[0151] Fifth step, according to the evolution trend of D m , define the damage level as five levels. The service life corresponding to level I is within 20% of the total life; the service life corresponding to level II is within 20% - 40% of the total life; the service life corresponding to level III is within 40% - 60% of the total life; the service life corresponding to level IV is within 60% - 80% of the total life; the service life corresponding to level V is within 80% - 100% of the total life.

[0152] Sixth step, according to the functional relationship between the said D m and the damage critical value D t , obtain the simplified continuous interaction criterion equation of creep-fatigue damage for the degradation of the mechanical properties of GH4169 material with service time as follows:

[0153]

[0154] According to the above results, generate a creep-fatigue damage interaction surface and obtain a three-dimensional creep-fatigue damage assessment diagram. For details, see the attached drawings Figure 8 .

[0155] Seventh step, conduct multiple sets of tensile tests until fracture on the GH4169 material of the serviced component. The test parameters are the same as those of the interrupted tensile test above. Calculate the material mechanical property damage parameter D m .

[0156] Eighth step, conduct multiple sets of creep-fatigue tests on the GH4169 material of the serviced component under the same working conditions as above until steady state can be stopped. Obtain the plastic strain range Δε p , peak stress σ max , the non-recoverable strain range Δε c during the hold time, the difference Δσ r in the stress levels at the start and end of the hold time. Calculate the creep damage D c and fatigue damage D f in the remaining service time of the design according to the said S2 and the said S3.

[0157] Step 9: Place the parameters (D m , D c , D f ) obtained above as state points in the three-dimensional damage assessment diagram. For details, see the attached drawings Figure 8 . It can be seen that the compressor tie rod material that has served for 3 years is located in the Grade II damage plane, and the assessment point below the damage interaction curve indicates that the component is safe during the remaining 5-year service life.

[0158] In fact, the compressor tie rod did not undergo creep-fatigue failure during the major overhaul inspection after 5 years, which is consistent with the assessment result obtained by using the damage assessment method of the present invention.

[0159] Example 2:

[0160] A power plant boiler was put into operation in 2010. During the shutdown and maintenance process in 2018, the main steam pipeline was tested to obtain the overall condition of these high-temperature and high-pressure pipelines and the reliability of the next overhaul after another 2 years of operation. Through finite element software, the creep-fatigue cycle times of the furnace tubes during service due to start-stop and temperature fluctuations were estimated to be about 720 cycles, and 180 more cycles were required to continue running.

[0161] To ensure the reliability of the main steam pipeline of the boiler during the next 2 years of operation, the damage determination method of the present invention is now adopted, and a three-dimensional creep-fatigue damage assessment diagram with the degradation of material mechanical properties over service time is used to assess the current damage level of the component material and its safety during the remaining 2-year service life.

[0162] The material of the main steam pipeline of the boiler is made of martensitic heat-resistant steel P92, and the working temperature is 630 °C.

[0163] Step 1: Obtain P92 material and conduct axial equal-amplitude creep-fatigue interaction tests with a strain range of 0.4% - 1.2% and a hold time of 30 s - 600 s at the maximum strain range according to the service conditions of the component. Conduct interrupted creep-fatigue tests at 20%, 50%, and 70% life fractions, and conduct tensile tests on the interrupted specimens until fracture.

[0164] Step 2: According to the tensile true stress-strain curves at different interrupted life fractions, calculate the tensile plastic strain energy U using the formulas in S5 and S6 described above T and the material mechanical property degradation parameter D m .

[0165] Step 3: Refer to the attached drawings Figure 9 , and determine the safety domain (lower envelope domain) according to D c and D f at 20%, 50%, and 70% interrupted life cycle times, and obtain different Dm Critical value D of the lower damage interaction criterion t .

[0166] Fourth step, refer to the attached drawings Figure 10 , and obtain D by fitting through the non - linear least - squares method m and the damage critical value D t The functional relationship is as follows:

[0167] D t = 1 - (D m ) 2.5

[0168] Fifth step, according to the evolution trend of D m , define the damage level as five levels. The service life corresponding to level I is within 20% of the total life; the service life corresponding to level II is within 20% - 40% of the total life; the service life corresponding to level III is within 40% - 60% of the total life; the service life corresponding to level IV is within 60% - 80% of the total life; the service life corresponding to level V is within 80% - 100% of the total life.

[0169] Sixth step, according to the above - mentioned functional relationship, obtain the simplified continuous interaction criterion equation of creep - fatigue damage for the degradation of the mechanical properties of P92 material with service time as follows:

[0170] D c + D f = 1 - (D m ) 2.5

[0171] According to the above results, generate a creep - fatigue damage interaction surface and obtain a three - dimensional creep - fatigue damage assessment diagram. Refer to the attached drawings Figure 11 .

[0172] Seventh step, conduct multiple sets of tensile - to - fracture tests on the used component material P92. The test parameters are the same as those of the interrupted tensile test above. Calculate the mechanical property damage parameter D of the material according to the above - mentioned S6 m .

[0173] Eighth step, conduct multiple sets of creep - fatigue tests on the used component material P92 under the same working conditions as above until steady state is reached and then stop. Obtain the plastic strain range Δε p , peak stress σ max , non - recoverable strain range Δε c during the hold time, difference in stress levels Δσ r between the start and end of the hold time. Calculate the creep damage D c and fatigue damage D f during the designed remaining service time according to the above - mentioned S2 and S3

[0174] Step 9: Use the parameters (D m , D c , D f ) obtained above as state points in the three-dimensional damage assessment diagram. For details, see the attached drawings Figure 11 . It can be seen that the material of the main steam pipeline of the boiler that has been in service for 8 years is located in the IV-level damage plane, and the assessment point below the damage interaction curve indicates that the component is safe during the remaining 2 years of service.

[0175] Moreover, no creep-fatigue failure occurred during the major overhaul inspection of the main steam pipeline of the boiler after 2 years, which is consistent with the assessment results obtained by using the damage assessment method of the present invention.

[0176] In both Example 1 and Example 2, the simplified continuous interaction criterion equation is used to draw the three-dimensional creep-fatigue damage assessment diagram. However, for different materials under different damage calculation methods, the creep-fatigue damage interaction criterion equations that can be used include: the linear damage accumulation criterion equation, the bilinear interaction criterion equation, and the simplified continuous interaction criterion equation.

[0177] Based on the above method, see the attached drawings Figure 5 , the present invention also provides a creep-fatigue damage level determination system considering material property degradation. The creep-fatigue damage level determination system includes: a material parameter determination module 1, a target material creep damage D c calculation module 2, a target material fatigue damage D f calculation module 3, a first creep-fatigue damage interaction criterion equation determination module 4, a tensile plastic strain energy U T calculation module 5, a material property degradation parameter D m determination module 6, a creep-fatigue damage level division module 7, a damage critical value D t determination module 8, a second creep-fatigue damage interaction criterion equation determination module 9, a three-dimensional creep-fatigue damage assessment diagram drawing module 10, a parameter calculation module 11 for service components, and a creep-fatigue level and failure determination module 12 for service components.

[0178] The material parameter determination module 1 is used to conduct multiple groups of interrupted creep-fatigue tests and multiple groups of tensile tests after interruption on the target material to determine the material parameters , l, a, and b.

[0179] The target material creep damage D c calculation module 2 is connected to the material parameter determination module 1 and is used to calculate the creep damage D of the target material based on the material parameters c .

[0180] The target material fatigue damage D fThe calculation module 3 is connected to the material parameter determination module 1 and is configured to calculate the fatigue damage D of the target material based on the material parameters a and b f .

[0181] The first creep-fatigue damage interaction criterion equation determination module 4 is connected to the creep damage D of the target material c the calculation module 2, and the fatigue damage D of the target material f The calculation module 3 is connected and is configured to determine the first creep-fatigue damage interaction criterion equation based on the creep damage D c and the fatigue damage D f of the target material.

[0182] The tensile plastic strain energy U T The calculation module 5 is configured to calculate the tensile plastic strain energy U of each interrupted life T .

[0183] The material property degradation parameter D m The determination module 6 is connected to the tensile plastic strain energy U T The calculation module 5 and is configured to determine the material property degradation parameter D related to the U T based on the tensile plastic strain energy U T . m .

[0184] The creep-fatigue damage level division module 7 is connected to the material property degradation parameter D m The determination module 6 and is configured to divide the creep-fatigue damage level based on the material property degradation parameter D m .

[0185] The damage critical value D t The determination module 8 is connected to the first creep-fatigue damage interaction criterion equation determination module 4 and the material property degradation parameter D m The determination module 6 and is configured to determine the damage critical value D of the first creep-fatigue damage interaction criterion under different material property degradation parameters D m based on the material property degradation parameter D m and the first creep-fatigue damage interaction criterion equation t .

[0186] The second creep-fatigue damage interaction criterion equation determination module 9 is connected to the damage critical value D t The determination module 8 and is configured to determine the second creep-fatigue damage interaction criterion equation related to the material property degradation based on the damage critical value D t .

[0187] The three-dimensional creep-fatigue damage assessment diagram drawing module 10 is connected to the second creep-fatigue damage interaction criterion equation determination module 9 and the creep-fatigue damage level division module 7, and is used to, based on the second creep-fatigue damage interaction criterion equation and the creep-fatigue damage level, with the creep damage D c as the x-axis, with the fatigue damage D f as the y-axis, and with the material property degradation parameter D m as the z-axis, draw a three-dimensional creep-fatigue damage assessment diagram. The three-dimensional creep-fatigue damage assessment diagram is used to determine the creep-fatigue damage level of a component and to determine whether the component has suffered creep-fatigue failure. The three-dimensional creep-fatigue damage assessment diagram includes: a safe area and a non-safe area.

[0188] The parameter calculation module 11 of the in-service component is used to calculate the parameters (D m , D c , D f ) of the in-service component.

[0189] The creep-fatigue level and failure determination module 12 of the in-service component is connected to the parameter calculation module 11 of the in-service component, and is used to use the parameters (D m , D c , D f ) of the in-service component as a state point in the three-dimensional creep-fatigue damage assessment diagram to determine the creep-fatigue damage level of the in-service component and whether the in-service component has suffered creep-fatigue failure.

[0190] The creep-fatigue damage level determination method and system of the present invention that consider material property degradation improve the damage interaction criterion. As the service time increases, the safe area defined by the damage interaction criterion decreases, which can reasonably reflect the influence of the material mechanical property degradation mechanism.

[0191] Compared with the existing prediction technologies, the present invention can use the three-dimensional creep-fatigue damage assessment diagram to evaluate the damage level of materials in real time. By comparing the position of the state point in the damage assessment diagram, it is possible to intuitively judge whether the current component is safe and the remaining creep-fatigue endurance performance.

[0192] The present invention has strong applicability. Different components can use different damage interaction criteria and material mechanical property degradation parameters to conduct creep-fatigue damage assessment.

[0193] In this specification, each embodiment is described in a progressive manner. The key point of each embodiment is to illustrate the differences from other embodiments. For the same and similar parts between each embodiment, reference can be made to each other. For the system disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and for the relevant parts, reference can be made to the description in the method part.

[0194] In this article, specific examples are used to illustrate the principles and implementation manners of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manners and application scopes. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A method for determining the creep-fatigue damage level considering material property degradation, characterized in that, The method for determining the creep-fatigue damage level includes: Perform multiple groups of interrupted creep-fatigue tests and multiple groups of tensile tests after interruption on the target material to determine the material parameters l, a, and b; Based on the material parameters and l, calculate the creep damage D of the target material c ; Calculate the fatigue damage D of the target material based on the material parameters a and b f ; Based on the creep damage D of the target material c and fatigue damage D f , determine the first creep-fatigue damage interaction criterion equation, specifically including: Obtaining a first linear damage accumulation criterion equation, where the first linear damage accumulation criterion equation is: D f +D c = 1 Obtaining a bilinear interaction criterion equation, where the bilinear interaction criterion equation is: Obtaining a simplified continuous interaction criterion equation, where the simplified continuous interaction criterion equation is Among them, d f , d c are the fatigue damage and creep damage of a single steady-state cycle, is the turning point of fatigue damage and creep damage in the bilinear interaction criterion, and n is the power exponent of the simplified continuous interaction criterion; Calculate the tensile plastic strain energy U for each interrupted life T ; Based on the tensile plastic strain energy U T , determine the material property degradation parameter D T associated with the U m ; Based on the material property degradation parameter D m , classify the creep-fatigue damage level; Based on the material property degradation parameter D m and the first creep-fatigue damage interaction criterion equation, determine the damage critical value D m of the first creep-fatigue damage interaction criterion under different material property degradation parameters D t ; Based on the damage threshold D t , a second creep-fatigue damage interaction criterion equation related to the degradation of material properties is determined, specifically including: Determining a linear damage accumulation criterion equation, where the linear damage accumulation criterion equation is: D f +D c =1-(D m ) k ; Determining a bilinear interaction criterion equation, where the bilinear interaction criterion equation is: Determining a simplified continuous interaction criterion equation, where the simplified continuous interaction criterion equation is: where k is the power exponent obtained by fitting with the nonlinear least squares method; Based on the second creep-fatigue damage interaction criterion equation and the creep-fatigue damage level, with the creep damage D c as the x-axis, with the fatigue damage D f as the y-axis, and with the material property degradation parameter D m as the z-axis, a three-dimensional creep-fatigue damage assessment diagram is plotted. The three-dimensional creep-fatigue damage assessment diagram is used to determine the creep-fatigue damage level of a component and to determine whether the component has suffered creep-fatigue failure; the three-dimensional creep-fatigue damage assessment diagram includes: a safe area and a non-safe area; Calculating parameters of the in-service component (D m , D c , D f ); Taking the parameters (D m , D c , D f ) of the in-service component as state points in the three-dimensional creep-fatigue damage assessment diagram, to determine the creep-fatigue damage level of the in-service component and whether the in-service component has suffered creep-fatigue failure.

2. The method for determining the creep-fatigue damage level considering material property degradation according to claim 1, characterized in that, Perform multiple sets of interrupted creep-fatigue tests and multiple sets of tensile tests after interruption on the target material to determine the material parameters l, a, and b specifically include: Through high-temperature creep tests at different stress levels, the creep ductility strain ε and creep rupture time t of the target material at each stress level σ are obtained. f , creep rupture time t R ; Based on the stress level σ, the creep ductility strain ε f and the creep rupture time t R , determine the creep strain energy density dissipation rate under creep test conditions Based on the dissipation rate of the creep strain energy density Determine the failure strain energy density w f ; Based on the dissipation rate of the creep strain energy density and the failure strain energy density w f , determine the material parameters and l; Through fatigue tests under strain control with the same temperature but different strain amplitudes, the plastic strain range Δη of the steady-state cycle number at each strain amplitude, p the peak stress σ, max and the fatigue test life N0 are obtained. Based on the plastic strain range Δε p , the peak stress σ max and the fatigue test life N0, determine the material parameters a and b.

3. The method for determining the creep-fatigue damage level considering material property degradation according to claim 1, characterized in that,The creep damage D of the target material c is specifically calculated by the following formula: Among them, D c is creep damage, t h is the holding time at the steady-state cycle of the creep-fatigue test, Δε c is the range of non-recoverable strain generated during the holding time at the steady-state cycle, σ max is the peak stress at the steady-state cycle of the creep-fatigue test, Δσ r is the difference in stress levels between the start and end stages of the holding at the steady-state cycle, and l are material parameters, N e is the remaining number of cycles of creep-fatigue life.

4. The method for determining the creep-fatigue damage level considering material property degradation according to claim 1, wherein, The fatigue damage D of the target material f is specifically calculated by the following formula: D f = 1 / [a·(σ max ·Δε p ) -b ·N e Among them, D f is the fatigue damage, Δε p is the plastic strain range at the steady state cycle of the creep-fatigue test, a and b are material parameters, σ max is the peak stress at the steady state cycle of the creep-fatigue test, N e is the remaining creep-fatigue life cycle.

5. The method for determining the creep-fatigue damage level considering material property degradation according to claim 1, wherein, The tensile plastic strain energy U of each interrupted life T is specifically calculated by the following formula: Among them, U T is the tensile plastic strain energy, ε p is the true plastic strain, σ(ε p ) is the true stress corresponding to the plastic strain, and ε t is the true fracture strain.

6. The method for determining the creep-fatigue damage level considering material property degradation according to claim 1, wherein, The material property degradation parameter D m is determined by the following formula: Among them, U T(0) is the tensile plastic strain energy of the undamaged material, and U T(N) is the tensile plastic strain energy of the material at the Nth cycle.

7. The method for determining the creep-fatigue damage level considering material property degradation according to claim 1, wherein, The damage critical value D t is determined by the following equation: D t = 1 - (D m ) k Among them, D t is the damage critical value, and k is the power exponent obtained by fitting with the nonlinear least squares method.

8. The method for determining the creep-fatigue damage level considering material property degradation according to claim 1, wherein, The parameters (D m , D c , D f ) of the in-service component are used as state points and placed in the three-dimensional creep-fatigue damage assessment diagram. Determining the creep-fatigue damage level of the in-service component and whether the in-service component has suffered creep-fatigue failure specifically includes: If the state points (D m , D c , D f ) of the in-service component are within the safe area of the three-dimensional creep-fatigue damage assessment diagram, the current creep-fatigue damage level of the in-service component can be determined, and the in-service component will not suffer creep-fatigue failure within the remaining designed in-service time; If the state point (D m , D c , D f ) of the in-service component is in the non-safe area of the three-dimensional creep-fatigue damage assessment diagram, it indicates that there is a possibility of creep-fatigue failure of the in-service component within the designed remaining service time, and further maintenance is required or the remaining service time needs to be shortened.

9. The system for determining the creep-fatigue damage level considering material property degradation, wherein, The system for determining the creep-fatigue damage level includes: The material parameter determination module is used to conduct multiple groups of interrupted creep fatigue tests and multiple groups of tensile tests after interruption on the target material to determine the material parameters l, a, and b; Creep damage D of the target material c A calculation module, connected to the material parameter determination module, for calculating the creep damage D of the target material based on the material parameters and l c ; Target material fatigue damage D f A calculation module, connected to the material parameter determination module, for calculating the fatigue damage D of the target material based on the material parameters a and b f ; The first creep-fatigue damage interaction criterion equation determination module, which is connected to the creep damage D of the target material c calculation module, the fatigue damage D of the target material f calculation module, and is used to determine the first creep-fatigue damage interaction criterion equation based on the creep damage D c and fatigue damage D f , specifically including: Obtaining a first linear damage accumulation criterion equation, where the first linear damage accumulation criterion equation is: D f +D c =1 Obtaining a bilinear interaction criterion equation, where the bilinear interaction criterion equation is: Obtaining a simplified continuous interaction criterion equation, where the simplified continuous interaction criterion equation is Among them, d f , d c are the fatigue damage and creep damage of a single steady-state cycle, is the turning point of fatigue damage and creep damage in the bilinear interaction criterion, and n is the power exponent of creep damage D c and fatigue damage D f in the simplified continuous interaction criterion; Tensile plastic strain energy U T Calculation module for calculating the tensile plastic strain energy U of each interruption life T ; Material property degradation parameter D m Determination module, associated with the tensile plastic strain energy U T Calculation module, connected to be used for based on the tensile plastic strain energy U T , determine the one associated with the U T Related material property degradation parameter D m ; Creep-fatigue damage level classification module, connected to the material property degradation parameter D m Determination module, for classifying the creep-fatigue damage level based on the material property degradation parameter D m , and classifying the creep-fatigue damage level; Damage critical value D t A determination module, which is connected to the first creep-fatigue damage interaction criterion equation determination module and the material property degradation parameter D m and is used to determine, based on the material property degradation parameter D m and the first creep-fatigue damage interaction criterion equation, the damage critical value D of the first creep-fatigue damage interaction criterion under different material property degradation parameters D m ; t ; The second creep-fatigue damage interaction criterion equation determination module, connected to the damage critical value D t determination module, is used to determine the second creep-fatigue damage interaction criterion equation related to the degradation of material properties based on the damage critical value D t , specifically including: Determining a linear damage accumulation criterion equation, where the linear damage accumulation criterion equation is: D f +D c =1-(D m ) k ; Determining a bilinear interaction criterion equation, where the bilinear interaction criterion equation is: Determining a simplified continuous interaction criterion equation, where the simplified continuous interaction criterion equation is: where k is the power exponent obtained by fitting with the nonlinear least squares method; The three-dimensional creep-fatigue damage assessment diagram drawing module, which is connected to the second creep-fatigue damage interaction criterion equation determination module and the creep-fatigue damage level division module, is used to draw a three-dimensional creep-fatigue damage assessment diagram based on the second creep-fatigue damage interaction criterion equation and the creep-fatigue damage level, with the creep damage D c as the x-axis, the fatigue damage D f as the y-axis, and the material property degradation parameter D m as the z-axis; the three-dimensional creep-fatigue damage assessment diagram is used to determine the creep-fatigue damage level of the component and determine whether the component has suffered creep-fatigue failure; the three-dimensional creep-fatigue damage assessment diagram includes: a safe area and a non-safe area; Parameter calculation module for in-service components, used to calculate the parameters (D m , D c , D f ); Service component creep-fatigue level and failure determination module, connected to the parameter calculation module of the service component, for using the parameters (D m , D c , D f ) of the service component as state points in the three-dimensional creep-fatigue damage assessment diagram to determine the creep-fatigue damage level of the service component and whether the service component has suffered creep-fatigue failure.

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