A method for calculating an environmental fatigue correction factor based on average strain rate

Abstract

The application discloses a method for converting strain rate by using an environmental fatigue correction factor, and the method is based on the peak stress and the valley stress of transient combination in fatigue calculation, and the average strain rate and the strain range are calculated, and the environmental fatigue correction factor under each combination is obtained by weighting. The method can consider the influence of the stress history of the structure on the environmental fatigue correction factor by considering the peak value and the valley value in the stress change history, and the influence of the transient stress fluctuation can be simplified and enveloped by the average effect of the peak value and the valley value stress, so that the method is suitable for a wide range of applications.

Description

TECHNICAL FIELD

[0001] The patent relates to the technical field of reactor structure mechanics, and particularly relates to a calculation method of an environmental fatigue correction factor based on average strain rate. BACKGROUND

[0002] Compared with the fatigue life in air environment, the fatigue life of carbon steel, low alloy steel, austenitic stainless steel and nickel-chromium-iron alloy will be significantly reduced under the reactor coolant environment and low strain rate of metal. In the trial period criterion of nuclear level specification, it is clearly pointed out that if the influence of the pressurized water reactor water environment on the fatigue life of the component cannot be proved to be small enough, the environmental fatigue correction factor needs to be used for the environmental effect analysis of the fatigue life of the structure, the environmental effect in the pressurized water reactor is expressed by the Fen factor, and the factor is defined as the ratio of the life in air at the environmental temperature to the life in the pressurized water reactor water at the working temperature. In view of the influence of the coolant environment on the material life, some nuclear power research institutions, nuclear safety supervision authorities and industry specification institutions abroad have carried out a large number of experimental researches, analyzed various factors and laws influencing the material life, and summarized the influence evaluation method.

[0003] NUREG / CR-6909 determines that material, load and environmental parameters are key factors influencing the fatigue life of carbon steel, low alloy steel and austenitic stainless steel based on the current data analysis of the existing fatigue curve and the consideration of the environmental fatigue influence, and gives the expression of the environmental fatigue correction factor Fen of the nickel-based alloy and the austenitic stainless steel. The calculation of the conversion strain rate needs to be combined with the stress state of the structure. SUMMARY

[0004] The purpose of the present application is to provide a calculation method of an environmental fatigue correction factor, which can calculate the strain rate considering the time history based on the stress state of the structure under the transient action of temperature and pressure, so as to calculate the conversion strain rate and the environmental fatigue correction factor. The method can calculate the structure fatigue correction factor considering the environmental influence, and has important guiding significance for the fatigue analysis of engineering structure design.

[0005] The technical scheme of the present application is as follows: a calculation method of an environmental fatigue correction factor based on average strain rate, comprising the following steps:

[0006] Step 1: only considering the transient load of the structure under the internal pressure and the internal temperature changing with time and other mechanical external load, the stress changing with time of each transient of the structure in the air environment is calculated;

[0007] Step 2: combination of transients, each transient combination includes 2 transients, and the number of transients in all transient events is marked as n, and the number of transient combinations is N;

[0008] Step3: Combine the peaks and valleys of each transient in each transient combination, and each transient combination produces a stress amplitude. The fatigue usage factor Ui of a single transient combination is calculated by combining the fatigue curve characteristics of the material in the air environment, i = 1, 2, 3…N;

[0009] Step4: The fatigue usage factor results of multiple transient combinations are accumulated to obtain the total fatigue usage factor

[0010] Step5: For the N transient combinations formed, the environmental fatigue correction factor of each transient combination is calculated based on the average strain rate method;

[0011] Step6: Multiply each transient combination by F en,m-k , calculate the cumulative fatigue usage coefficient CUF en considering the environmental fatigue correction factor, the calculation formula is as follows

[0012] CUF en = U1×F en,1 +…+U n ×F en,n

[0013] Wherein, CUF en is the fatigue usage coefficient considering the environmental fatigue correction;

[0014] U1, U n is the fatigue usage coefficient of the first and nth transient combination;

[0015] F en,1 and F en,n is the environmental fatigue correction factor of the first and nth transient combination.

[0016] In Step1, the flow and method of fatigue calculation of nuclear grade pressure equipment provided in ASME Volume III NB3200 or RCC-M Volume I B3200 in nuclear energy industry specification are adopted.

[0017] In Step1, the results of stress change with time corresponding to each transient in the air environment are calculated by using finite element software in Step1.

[0018] In Step2, the rain flow method provided by ASME and RCC-M is used for transient combination.

[0019] In Step3, the fatigue usage factors of four transient combinations in the air environment are calculated as 0.72676, 0.16684, 0.0, and 0.0, respectively.

[0020] In Step 4, the total fatigue utilization coefficient U = 0.89361.

[0021] Step 5 includes:

[0022] Step 5.1: Traverse the transient combinations from 1 to N, such as... Figure 3 As shown, the transient numbers of the q-th transient combination are transient m and transient k. The trough of transient m is point A, the peak of transient k is point B, the trough of transient 2 is point C, and the peak of transient 2 is point D. The stress or strain changes over time in this part are saved and then filtered.

[0023] Step 5.2: Based on the time periods during which the increments of stress or strain involved in the combination are positive, save the stress values ​​at points A, B, C, and D of transient states m and k. Point B is the valley point of transient state m, denoted as: σ max，m，i Point A is the peak point of the transient m, denoted as: σ min，m，i Point D is the valley point of the transient k, denoted as: σ max，k，i Point C is the valley point of the transient k, denoted as: σ min，k，i Each point has 6 stress components x, y, z, xy, xz, yz, so i represents x, y, z, xy, xz, yz;

[0024] Step 5.3: Based on the calculated σ values ​​of the six stress components at points A, B, C, and D respectively i The difference, Δσ, is calculated by subtracting the trough from the peak value. m , i =σ max，m，i -σ min，m，i , Δσ k , i =σ max，k，i -σ min，k，i Each point has 6 stress components x, y, z, xy, xz, yz, where i represents x, y, z, xy, xz, yz;

[0025] Step 5.4: Based on the calculated difference Δσ of transient m m，i , Δσ m，y , Δσ m，z , Δσ m，xy , Δσ m，xz , Δσ m，yz The three differential principal stresses Δσ of transient m were calculated. m，1 , Δσ m，2 , Δσ m，3 Differential intensity Δσ k，SI Similarly, it is also based on the calculated difference Δσ of transient k. k，i , Δσk，y , Δσ k，z , Δσ k，xy , Δσ k，xz , Δσ k，yz , the three difference principal stresses Δσ k，1 , Δσ k，2 , Δσ k，3 , the difference intensity Δσ k，SI ;

[0026] Step5.5: Based on the calculated difference principal stress, divided by the time difference Δt of peak and valley m and Δt k , determine the average stress rate of transient m Determine the average stress rate of transient 2 Each point has 6 stress components x, y, z, xy, xz, yz, where i represents x, y, z, xy, xz, yz;

[0027] Step5.6: Based on the calculated six stress rate components Calculate the rate principal stress of transient m and and the rate stress intensity Similarly, based on the calculated six stress rate components Calculate the rate principal stress of transient k and and the rate stress intensity

[0028] Step5.7: Calculate the strain range, the strain range calculation formula of each transient is as follows

[0029]

[0030]

[0031] Where, K e is the simplified elastic-plastic correction factor when the load combination corresponds to ASME NB-3228.5; E is the elastic modulus;

[0032] Step5.8: Calculate the average strain rate, the average strain rate calculation formula of each transient is as follows

[0033]

[0034]

[0035] Step5.9: Based on the average strain rate, calculate the conversion strain rate

[0036] Step 5.10: Based on the calculated conversion strain rate, the environmental fatigue correction factor F of transient m and transient k is calculated en,simp,m and F en,simp,k ;

[0037] Step 5.11: According to the calculated strain range Δε m and Δε k , the environmental fatigue correction factor F of transient m and transient k is weighted to calculate the environmental fatigue correction factor F of transient combination en,simp,m and F en,simp,k . en,n ;

[0038]

[0039] In the Step 5.1, the principle of screening is: removing a section with negative stress increment, and keeping the part with positive stress increment, that is, only keeping the stress results of A to B and C to D.

[0040] In the Step 5, the environmental fatigue correction factor of each transient combination is calculated according to material mechanics.

[0041] In the Step 5.9, the formula provided by NUREG / CR-6909 is used to calculate the conversion strain rate

[0042] In the Step 5.9, the formula provided by NUREG / CR-6909 is used to calculate the environmental fatigue correction factor F of transient m and transient k en,simp,m and F en,simp,k .

[0043] In the Step 6, the environmental fatigue correction factor F of transient combination en,n is 6.547, 4.847 and 3.542, and the fatigue use factor CUF of the corresponding transient combination considering the environmental fatigue correction factor en,i is 4.758, 0.809 and 0.0 respectively, and the total fatigue use life factor CUF considering the environmental fatigue correction factor is calculated en as 5.567.

[0044] The significant effect of the present application is that: the method for calculating the environmental fatigue correction factor conversion strain rate provided by the present application can consider the influence of the stress history of the structure on the environmental fatigue correction factor by considering the peak value and the valley value in the stress change history, and the influence of transient stress fluctuation can be simplified and enveloped by the average effect of the peak value and the valley value stress, so that the application range is wide. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1The finite element model, load, and boundary condition diagram provided for the embodiment of the present application are shown in the following figure:

[0046] Figure 2 The time history diagram of transient 1 stress component provided for the embodiment of the present application is shown in the following figure:

[0047] Figure 3 The transient combination diagram provided for the embodiment of the present application is shown in the following figure:

[0048] Table 1 is a transient data diagram provided for the embodiment of the present application:

[0049] Table 2 is a fatigue calculation result diagram in air environment provided for the embodiment of the present application:

[0050] Table 3 is a conversion strain rate calculation formula diagram provided for the embodiment of the present application.

[0051] Table 4 is a cross-section 1 environmental fatigue calculation result diagram provided for the embodiment of the present application. DETAILED DESCRIPTION

[0052] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. However, it will be apparent to one skilled in the art that the present application can be practiced without the specific details presented herein. In other instances, well-known methods have not been described in detail in order to avoid obscuring aspects of the present application.

[0053] The terminology used in this disclosure of one or more embodiments is for the purpose of describing particular embodiments only and is not intended to be limiting of one or more embodiments. As used in this disclosure and the appended claims, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term "and / or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items.

[0054] It will be understood that, although the terms first, second, etc. can be used herein to describe various information, these terms are not intended to denote a temporal or chronological order. Rather, these terms are used solely to distinguish one from another only. For example, without departing from the scope of one or more embodiments, first can be termed second, and similarly, second can be termed first.

[0055] The transient or transient in the present application is understood as an event, or the process of an event.

[0056] A calculation method of an environmental fatigue correction factor based on average strain rate, the steps are as follows:

[0057] Step 1: Only consider the structure under the instantaneous load of internal pressure and internal temperature change with time and other mechanical external load, calculate the stress of each transient of the structure in the air environment with time.

[0058] 1. As preferred, the fatigue calculation process and method of nuclear class pressure equipment in Step 1 is provided in ASME Volume III NB3200 or RCC-M Volume I B3200 of the nuclear energy industry standard;

[0059] 2. As preferred, the stress change with time corresponding to each transient of the structure in the air environment is calculated by using finite element software in Step 1, and the maximum stress value is called the crest, and the minimum stress value is called the trough. After calculating a large number of transients, a large number of stress crests and troughs can be obtained. Here, the transient refers to the load of pressure and temperature change with time.

[0060] Step 2: According to the rain flow method provided by ASME and RCC-M, each transient combination is composed of two transients. Here, the number of transients in all transient events is marked as n, and the number of transient combinations is N.

[0061] Step 3: The crest and trough of each transient in each transient combination are combined, and each transient combination produces a stress amplitude. This stress amplitude, combined with the inherent fatigue curve characteristics of the material in the air environment, calculates the fatigue usage coefficient Ui of a single transient combination, i = 1.. N;

[0062] Step 4: The fatigue usage coefficient results of multiple transient combinations are added up to obtain the total fatigue usage coefficient

[0063] Step 5: For the N transient combinations formed, the environmental fatigue correction factor of each transient combination is calculated based on the average strain rate method.

[0064] Step 5.1: Traverse the transient combinations from 1 to N. Assume that the transient number of the qth transient combination is: transient m and transient k, the trough (minimum value) of transient m is point A, the crest (maximum value) of transient k is point B, the trough (minimum value) of transient 2 is point C, and the crest (maximum value) of transient 2 is point D. Save the stress or strain change with time results of this part, and select the principle: remove the part with negative stress increment, and keep the part with positive stress increment, that is, only keep the stress results from A to B and from C to D;

[0065] Step5.2: According to the time period in which the stress or strain change in the combination process is positive, the stress values of the A, B, C, and D points of the transient m and transient k are saved, the B point is the valley point of the transient m (i.e. the low point), denoted as: σ max，m，i , the A point is the peak point of the transient m (i.e. the high point), denoted as: σ min，m，i , the D point is the valley point of the transient k (i.e. the low point), denoted as: σ max，k，i , and the C point is the valley point of the transient k (i.e. the low point), denoted as: and σ min，k，i , According to material mechanics, each point has 6 stress components, so i represents x, y, z, xy, xz, and yz.

[0066] Step5.3: Based on the calculated six stress components of the A, B, C, and D points, respectively, each adopts the peak value minus the valley value to calculate the difference, i.e. Δσ i , m i = σ max，m，i - σ min，m，i , Δσ k , i = σ max，k，i - σ min，k，i , According to material mechanics, each point has 6 stress components, where i represents x, y, z, xy, xz, and yz.

[0067] Step5.4: Based on the calculated difference Δσ m，i , Δσ m，y , Δσ m，z , Δσ m，xy , Δσ m，xz , Δσ m，yz of the transient m, according to the method of calculating the principal stress provided in material mechanics, the three difference principal stresses Δσ m，1 , Δσ m，2 , Δσ m，3 of the transient m are calculated, and the difference intensity Δσ k，SI ; Similarly, based on the calculated difference Δσ k，i , Δσ k，y , Δσ k，z , Δσ k，xy , Δσ k，xz , Δσ k，yz of the transient k, according to the method of calculating the principal stress provided in material mechanics, the three difference principal stresses Δσ k，1 , Δσ k，2 , Δσ k，3 of the transient k are calculated, and the difference intensity Δσ k，SI ;

[0068] ​Step5.5: Based on the calculated difference principal stress, divide by the time difference of peak and valley Δt m and Δt k , determine the average stress rate of transient m Determine the average stress rate of transient 2 According to the mechanics of materials, each point has 6 stress components, where i represents x, y, z, xy, xz, yz;

[0069] Step5.6: Based on the calculated six stress rate components According to the calculation method of the mechanics of materials, the rate principal stress of transient m is calculated and and the rate stress intensity Similarly, based on the calculated six stress rate components According to the calculation method of the mechanics of materials, the rate principal stress of transient k is calculated and and the rate stress intensity

[0070] Step5.7: Calculate the strain range, the strain range calculation formula of each transient is as follows

[0071]

[0072]

[0073] Where, K e is the simplified elastic-plastic correction factor when the load is combined, corresponding to ASME NB-3228.5; E is the elastic modulus.

[0074] Step5.8: Calculate the average strain rate, the average strain rate calculation formula of each transient is as follows

[0075]

[0076]

[0077] Step5.9: Based on the average strain rate, the conversion strain rate is calculated by using the formula provided by NUREG / CR-6909

[0078] Step5.10: Based on the calculated conversion strain rate, the environmental fatigue correction factor F of transient m and transient k is calculated by using the formula provided by NUREG / CR-6909 en,simp,m and F en,simp,k ;

[0079] Step5.11: According to the calculated strain range Δε m and Δεk , the environmental fatigue correction factor F of transient m and transient k en,simp,m and F en,simp,k , the environmental fatigue correction factor F of transient combination is calculated by weighting en,m-k .

[0080]

[0081] Step6: multiply each transient combination by F en,m-k , the cumulative fatigue usage factor CUF considering environmental fatigue correction factor is calculated en , the calculation formula is as follows

[0082] CUF en = U1 x F en,1 +…+ U n x F en,n

[0083] Wherein, CUF en is the fatigue usage factor considering environmental fatigue correction; U1, U n is the fatigue usage factor of the first and the nth transient combination; F en,1 and F en,n is the environmental fatigue correction factor of the first and the nth transient combination.

[0084] Taking the reactor pressure vessel nozzle as an example, the method of the present application is calculated and used as follows:

[0085] Step1: only considering the transient load of the reactor pressure vessel nozzle structure under the internal pressure and the internal temperature changing with time and other mechanical external load, the stress changing with time of each transient of the structure in the air environment is calculated, and the transient load adopted is shown in Table 1.

[0086] Wherein, the number of transient 1 is 20 times, the number of transient 2 is 50 times, and the number of transient 3 is 500 times.

[0087] 1. As preferred, the process and method of nuclear class pressure equipment fatigue calculation provided in ASME III NB3200 or RCC-M I B3200 of nuclear energy industry specification are adopted in Step1 of the present application;

[0088] 2. As preferred, the finite element software is adopted to calculate the stress changing with time corresponding to each transient of the structure in the air environment in Step1 of the present application, and the maximum stress value in the result is called the crest, and the minimum stress value is called the trough, after calculating a plurality of transients, a plurality of stress crests and troughs can be obtained, and here the transient refers to the load of pressure and temperature changing with time. The finite element model of the reactor pressure vessel nozzle is as follows Figure 1The axial stress component versus time curve under the action of transient 1 is shown in Fig. 1. Figure 2

[0089] Step 2: According to the method provided by ASME and RCC-M, each transient combination is composed of 2 transients, and the number of transients is 3, and the number of transient combinations is 3, as shown in Table 2. It should be noted that the transient combination can be a combination of the same transient itself, or a combination of two different transients.

[0090] Step 3: The peak and valley of each transient in each transient combination are combined, and each transient combination produces a stress amplitude. This stress amplitude, combined with the inherent fatigue curve characteristics of the material in the air environment, is used to calculate the fatigue usage factor U of a single transient combination. i U i is defined as the fatigue usage factor of the ith transient combination.

[0091] As shown in Table 2, the fatigue usage factors of the four transient combinations in the air environment are 0.72676, 0.16684, 0.0, and 0.0, respectively.

[0092] Step 4: The fatigue usage factor results of multiple transient combinations are accumulated to obtain the total fatigue usage factor U in the air environment, which is 0.89361, as shown in Table 2.

[0093] Step 5: For the N transient combinations formed, the environmental fatigue correction factor of each transient combination is calculated based on the average strain rate method.

[0094] Step 5.1: Traverse the transient combinations from 1 to N, as shown in Figure 3 , the transient numbers of the qth transient combination are transient m and transient k, the valley (minimum value) of transient m is point A, the peak (maximum value) of transient k is point B, the valley (minimum value) of transient 2 is point C, and the peak (maximum value) of transient 2 is point D. Save the stress or strain change results of this part, and select the principle: remove the part with negative stress increment, and keep the part with positive stress increment, i.e. only keep the stress results from A to B and from C to D;

[0095] Step 5.2: According to the time period with positive increment found in the stress or strain change process involved in the combination, save the stress values of points A, B, C, and D of transients m and k. The value of point B is the valley point (i.e. the low point) of transient m, denoted as σ max，m，i , the value of point A is the peak point (i.e. the high point) of transient m, denoted as σ min，m，i , and the value of point D is the valley point (i.e. the low point) of transient k, denoted as σ max，k，i ​, C point is the valley point of transient k (i.e. low point), denoted as: σ min，k，i , According to the mechanics of materials, each point has 6 stress components, so i represents x, y, z, xy, xz, yz;

[0096] Step5.3: Based on the calculated six stress components of A, B, C, D four points respectively i , respectively, the difference is calculated by subtracting the peak value from the valley value, that is, Δσ m , i =σ max，m，i -σ min，m，i , Δσ k , i =σ max，k，i -σ min，k，i , According to the mechanics of materials, each point has 6 stress components, where i represents x, y, z, xy, xz, yz;

[0097] Step5.4: Based on the calculated difference Δσ m，i , Δσ m，y , Δσ m，z , Δσ m，xy , Δσ m，xz , Δσ m，yz , According to the method of calculating the principal stress provided in the mechanics of materials, the three difference principal stresses Δσ m，1 , Δσ m，2 , Δσ m，3 , difference intensity Δσ k，SI ; Similarly, based on the calculated difference Δσ k，i , Δσ k，y , Δσ k，z , Δσ k，xy , Δσ k，xz , Δσ k，yz , According to the method of calculating the principal stress provided in the mechanics of materials, the three difference principal stresses Δσ k，1 , Δσ k，2 , Δσ k，3 , difference intensity Δσ k，SI ;

[0098] Step5.5: Based on the calculated difference principal stress, divided by the time difference Δt m and Δt k , determine the average stress rate of transient m determine the average stress rate of transient 2 According to the mechanics of materials, each point has 6 stress components, where i represents x, y, z, xy, xz, yz;

[0099] Step 5.6: Based on the calculated six stress rate components According to the calculation method of material mechanics, the rate principal stress of transient m is calculated and and the rate stress intensity Similarly, based on the calculated six stress rate components According to the calculation method of material mechanics, the rate principal stress of transient k is calculated and and the rate stress intensity

[0100] Step 5.7: Calculate the strain range, the strain range calculation formula of each transient is as follows

[0101]

[0102]

[0103] Where, K e is the simplified elastic-plastic correction factor when the load is combined, corresponding to ASME NB-3228.5; E is the elastic modulus.

[0104] Step 5.8: Calculate the average strain rate, the average strain rate calculation formula of each transient is as follows

[0105]

[0106]

[0107] Step 5.9: Based on the average strain rate, the conversion strain rate is calculated by using the formula provided by NUREG / CR-6909 The calculation formula is shown in Table 3.

[0108] Step 5.10: Based on the calculated conversion strain rate, the environmental fatigue correction factor F of transient m and transient k is calculated by using the formula provided by NUREG / CR-6909 en,simp,m and F en,simp,k ;

[0109] Step 5.11: According to the calculated strain range Δε m and Δε k , the environmental fatigue correction factor F en,simp,m and F en,simp,k of transient m and transient k, the environmental fatigue correction factor F en,n of transient combination is weighted calculated.

[0110]

[0111] Step 6: multiply each transient combination by F en,n , calculate the cumulative fatigue usage factor CUF considering the environmental fatigue correction factor en , the calculation formula is as follows

[0112] CUF en = U1 x F en,1 +…+ U n x F en,n

[0113] Wherein, CUF en is the fatigue usage factor considering the environmental fatigue correction factor; U1, U n is the fatigue usage factor of the first and nth transient combination; F en,1 and F en,n is the environmental fatigue correction factor of the first and nth transient combination.

[0114] As shown in Table 4, the environmental fatigue correction factors F en,n of the three transient combinations are 6.547, 4.847 and 3.542 respectively, and the fatigue usage factors of the three transient combinations considering the environmental fatigue correction factors are 4.758, 0.809 and 0.0 respectively, so the total fatigue usage life coefficient considering the environmental fatigue correction factor is calculated to be 5.567.

[0115] The above only describes the preferred embodiments of the patent and is not intended to limit the patent. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the patent should be included in the protection scope of the patent.

[0116] It should be noted that, for the foregoing method embodiments, in order to facilitate description, they are all expressed as a series of action combinations, but those skilled in the art should know that the present application is not limited by the action sequence described, because according to the present application, certain steps can be performed in other sequences or simultaneously. Secondly, those skilled in the art should know that the embodiments described in the specification all belong to preferred embodiments, and the actions and modules involved are not necessarily essential to the present application.

[0117] In the above embodiments, the description of each embodiment has its own emphasis, and the parts not described in detail in a certain embodiment can be referred to the relevant description of other embodiments.

[0118] The preferred embodiments of the application disclosed above are only used to illustrate the present application. Alternative embodiments do not describe all the details and limit the application to the specific embodiments described. Obviously, many modifications and changes can be made according to the content of the present application. The present application selects and describes these embodiments in order to better explain the principles and practical applications of the present application, so that those skilled in the art can well understand and utilize the present application.

[0119] Table 1

[0120]

[0121] Table 2

[0122]

[0123] Table 3

[0124]

[0125] Table 4

[0126]

Claims

1. A method for calculating an environmental fatigue correction factor based on mean strain rate, characterized by: The method comprises the following steps: Step 1: only considering the instantaneous load of the structure under the internal pressure and the internal temperature change with time and other mechanical external loads, the stress change with time of each transient state of the structure in the air environment is calculated; Step 2: the transient state combination is performed, each transient state combination comprises two transient states, the number of transient states in all transient state events is marked as n, and the number of transient state combinations is N; Step 3: the peak and the trough of each transient state in each transient state combination are combined, a stress amplitude is generated for each transient state combination, and the fatigue usage coefficient Ui of the single transient state combination is calculated by combining the fatigue curve characteristics of the material in the air environment, wherein i=1, 2, 3…N; Step 4: Accumulate the fatigue usage factor results of the multiple transient groups to obtain the total fatigue usage factor Step 5: for the N transient state combinations formed, the environmental fatigue correction factor of each transient state combination is calculated based on the average strain rate method; comprising: Step 5.1: the transient state combinations from 1 to N are traversed, as shown in FIG. 3, the transient state numbers of the qth transient state combination are transient state m and transient state k, the trough of the transient state m is point A, the peak of the transient state k is point B, the trough of the transient state 2 is point C, and the peak of the transient state 2 is point D, the stress or strain change with time of this part is saved and selected, Step5.2: According to finding out the time period with positive increment in the process of stress or strain change participating in combination, the stress values of A, B, C, D four points of transient m and transient k are saved, B point is the valley point of transient m, recorded as: σ max，m，i , A point is the peak point of transient m, recorded as: σ min，m，i , D point is the valley point of transient k, recorded as: σ max，k，i , C point is the valley point of transient k, recorded as: σ min，k，i , Each point has 6 stress components x, y, z, xy, xz, yz, so i represents x, y, z, xy, xz, yz; Step5.3: Based on the calculated six stress components of A, B, C, D four points respectively i , respectively using the peak minus the valley to calculate the difference, namely Δσ m，i = σ max，m，i - σ min，m，i , Δσ k，i = σ max，k，i - σ min，k，i , each point has 6 stress components x, y, z, xy, xz, yz, where i represents x, y, z, xy, xz, yz; Step 5.4: Based on the calculated difference values Δσ m，i , Δσ m，y , Δσ m，z , Δσ m，xy , Δσ m，xz , Δσ m，yz , the three difference principal stresses Δσ m，1 , Δσ m，2 , Δσ m，3 , the difference intensity Δσ m，SI ; likewise, based on the calculated difference values Δσ k，i , Δσ k，y , Δσ k，z , Δσ k，xy , Δσ k，xz , Δσ k，yz , the three difference principal stresses Δσ k，1 , Δσ k，2 , Δσ k，3 , the difference intensity Δσ k，SI ; Step 5.5: Based on the calculated difference principal stress, divide by the time difference Δt of the peak and valley m and Δt k , determine the average stress rate of transient m Determine the average stress rate of transient 2 Each point has 6 stress components x, y, z, xy, xz, yz, where i represents x, y, z, xy, xz, yz; Step 5.6: Based on the calculated six stress rate components The rate principal stresses of the transient m are calculated and and the rate stress intensity Similarly, based on the calculated six stress rate components The rate principal stresses of the transient k are calculated and and the rate stress intensity Step 5.7: the strain range is calculated, and the strain range calculation formula of each transient state is as follows where K e is the simplified elastic-plastic correction factor for load combination, corresponding to ASME NB-3228.5; E is the modulus of elasticity; Step 5.8: the average strain rate is calculated, and the average strain rate calculation formula of each transient state is as follows Step 5.9: Based on the average strain rate, calculate the conversion strain rate Step 5.10: Based on the calculated conversion strain rate, calculate the environmental fatigue correction factor F for transient m and transient k en,simp,m and F en,simp,k ; Step 5.11: Calculate the strain range Δε according to the calculated strain range Δε m and Δε k , the environmental fatigue correction factor F transient m and F transient k en,simp,m and F en,simp,k , the environmental fatigue correction factor F transient m and F transient k en，n ; Step 6: multiply each transient combination by F en,m-k , the cumulative fatigue usage factor CUF taking into account the environmental fatigue correction factor en , the formula is as follows CUF en = U1 x F en,1 +... + U n x F en,n CUF en is the fatigue usage factor considering environmental fatigue correction; U1, U n Fatigue usage factor for 1st and nth transient combination; F en,1 and F en,n is the environmental fatigue correction factor for the 1st and nth transient combination.

2. The method for calculating the environmental fatigue correction factor based on average strain rate according to claim 1, characterized in that: In the Step 1, the process and method of nuclear grade pressure equipment fatigue calculation provided in ASME Volume III NB3200 or RCC-M Volume I B3200 in the nuclear energy industry are adopted.

3. The method of claim 1, wherein: In the Step 1, the stress change with time corresponding to each transient state of the structure in the air environment is calculated by using the finite element software in the Step 1.

4. The method of claim 1, wherein: In the Step 2, the rain flow method provided in ASME and RCC-M is used for transient state combination.

5. The method of claim 1, wherein: In the Step 3, the fatigue usage coefficients of the four transient state combinations in the air environment are 0.72676, 0.16684, 0.0 and 0.0 respectively.

6. The method of claim 1, wherein: In the Step 4, the total fatigue usage coefficient U is 0.89361.

7. The method of claim 1, wherein: In the Step 5.1, the principle of selection is that a section with a negative stress increment is removed, and only the stress results of the A to B section and the C to D section are reserved.

8. The method of claim 1, wherein: In the Step 5, the environmental fatigue correction factor of each transient state combination is calculated according to material mechanics.

9. The method of claim 1, wherein: In Step 5.9, the conversion strain rate is calculated using the formula provided in NUREG / CR-6909 10. The method of claim 1, wherein: In Step 5.9, the environmental fatigue correction factor F for transient m and transient k is calculated using the formula provided in NUREG / CR-6909 en,simp,m and F en,simp,k .

11. The method of claim 1, wherein: The environmental fatigue correction factor F of the transient combination in Step 6 en，n are 6.547, 4.847 and 3.542, and the fatigue usage factors CUF of the corresponding transient combinations considering the environmental fatigue correction factor are en,i are 4.758, 0.809 and 0.0, respectively, and the total fatigue usage life factor CUF considering the environmental fatigue correction factor is calculated to be en 5.567.

Images