Low cycle fatigue life analysis method for engine cylinder head model

Abstract

The present invention relates to a method for analyzing the low-cycle fatigue life of an engine cylinder head model, comprising establishing a finite element mesh model; establishing a temperature field analysis model after adding an assembly relationship, and performing temperature field analysis; performing a cylinder head stress analysis; obtaining a minimum fatigue safety factor; optimizing the cylinder head structure, extracting the displacement and temperature boundary of the cylinder head model from the full model; processing the boundary with reference to the low-cycle fatigue test cycle condition; performing stress analysis on the sub-model loading cycle condition boundary; analyzing the low-cycle fatigue life of the cylinder head; and obtaining a relatively accurate low-cycle fatigue life result. The method for analyzing the low-cycle fatigue life of an engine cylinder head model of the present invention can accurately analyze the low-cycle fatigue life of the cylinder head; using the sub-model method, the stress analysis model of the low-cycle fatigue cycle condition is reduced, and the analysis efficiency is improved; the low-cycle fatigue life analysis can comprehensively consider mechanical, oxidation and creep damage, and accurately analyze the low-cycle fatigue life of the cylinder head.

Description

Technical Field

[0001] The invention relates to the field of engine structure design and analysis, and in particular to a low cycle fatigue life analysis method for an engine cylinder head model. Background Art

[0002] The cylinder head has a complex structure and is subjected to alternating mechanical and thermal loads. The alternating hot and cold conditions of the engine can easily cause the cylinder head to fail due to low-cycle thermal fatigue. With the development of engines, the operating temperature of the cylinder head is getting higher and higher, which increases the probability of creep failure. In recent years, the problem of thermal fatigue failure of cylinder heads has been the focus of industry research and also a difficult point. The existing cylinder head fatigue is mainly based on high-cycle fatigue analysis, and very few low-cycle fatigue life predictions are performed. In addition, low-cycle fatigue analysis has the disadvantages of large analysis model, long analysis cycle, and low accuracy. Summary of the invention

[0003] In view of the above-mentioned deficiencies in the prior art, the present invention forms a low-cycle fatigue life analysis method for an engine cylinder head model with a relatively simplified model that is consistent with the test conditions, improves analysis efficiency, and has higher accuracy by benchmarking with the low-cycle fatigue test.

[0004] The technical solution adopted by the present invention is:

[0005] The beneficial effects of the present invention compared with the prior art are as follows:

[0006] The low cycle fatigue life analysis method of the engine cylinder head model of the present invention is

[0007] 1) Setting the analysis step S to be consistent with the cylinder head low cycle fatigue test cycle condition can more accurately analyze the cylinder head low cycle fatigue life;

[0008] A cycle of the cylinder head low-cycle fatigue test includes three processes: engine heating-keeping-cooling. It mainly tests the low-cycle thermal fatigue performance of the engine cylinder head under hot and cold shocks. In the test, the corresponding time will be set for the three processes of heating, keeping and cooling. The temperature field analysis is consistent with the test cycle process, which is divided into three processes of heating-keeping-cooling. Referring to the test temperature test curve, the heating process is divided into four time periods, and the cooling process is divided into three time periods. The cylinder head temperature field and strength can be analyzed to obtain temperature and strength analysis results consistent with the test cycle conditions. The strength analysis results obtained by calculation that are consistent with the test conditions are used for low-cycle fatigue life analysis, and a low-cycle fatigue life that is relatively consistent with the low-cycle fatigue test conditions can be obtained.

[0009] 2) Use the sub-model method to reduce the stress analysis model of low-cycle fatigue cycle conditions and improve analysis efficiency;

[0010] The cylinder head has a complex structure. In order to obtain accurate results of the low-cycle fatigue life of the cylinder head, the geometric features of the cylinder head need to be retained as much as possible in some areas prone to failure. Some places must be made into relatively fine grids, resulting in a large number of grids in the cylinder head itself. If the entire cylinder head model is used to calculate the low-cycle fatigue life, the analysis model will be very large, the calculation time will be long, and the calculation result file can reach more than 100Gb. Using the cylinder head model can reduce the cycle condition analysis model, shorten the calculation time, reduce the analysis result file, and improve the analysis efficiency. For example, using the full cylinder head model for analysis requires a workstation calculation time of about 60 hours and a calculation result file of more than 100Gb, while using the sub-model only takes 8-10 hours to calculate, and the result file is only about 10Gb.

[0011] The sub-model shown in the attached figure is selected as the analysis object because the low-cycle fatigue failure of the engine cylinder head often occurs in the position where there is alternating hot and cold and large temperature changes. The cylinder head fire surface area is subjected to the alternating hot and cold thermal load on the engine, and is prone to low-cycle thermal engine fatigue failure. In the past, the low-cycle fatigue failure of the engine all occurred in the cylinder head fire surface area. Therefore, the fire surface area is selected as the sub-model for the cylinder head low-cycle fatigue life analysis object.

[0012] 3) Write a program that can quickly extract and process the low-cycle operating condition analysis boundaries;

[0013] Extract the fire surface mesh from the cylinder head full model as a sub-model, and make a set of all mesh nodes of the sub-model and name it set1;

[0014] The mesh nodes on the dividing surface between the sub-model and the full model are taken as a set and named as set2;

[0015] The full model performs strength calculations at idle and rated power points. During the calculation process, a dat-format file is output to save the temperature information of set1 and the displacement information of set2.

[0016] Write MATLAB program 1. By searching for keywords, you can read the temperature and displacement information of the set1 and set2 nodes in the dat file for the idle and rated power conditions and record them. The temperature information of set1 and the displacement information of set2 can be automatically output as abqinp format files, and four files of displacement temperature boundaries of the idle and rated power conditions are obtained, namely the temperature boundary of set1 and the displacement boundary of set2 under the idle condition; the temperature boundary of set1 and the displacement of set2 under the rated power condition. The displacement boundary includes the displacement of the nodes in the three directions of X / Y / Z.

[0017] The structure mainly generates stress when it is driven by displacement. The full model outputs the displacement boundary to the sub-model to ensure that the displacement of the split surface is consistent with the full model.

[0018] The temperature information file of set1 under idle condition is named TBC I , the temperature information file of rated power point set1 is named TBC R , the displacement information file of set2 under idle condition is named DBC I , the displacement information file of rated power point set2 is named DBC R ;

[0019] The operating condition from idle to rated power point represents the heating process from low temperature to high temperature. The temperature rises quickly in the first half of the heating process and slowly in the second half. If the heating process is divided into several time periods, the temperature and displacement boundaries at several time points need to be obtained.

[0020] Write MATLAB program 2 to process the four boundary files according to the low-cycle fatigue cycle conditions. The temperature and displacement boundaries at different time points can be automatically obtained through the program. The displacement and temperature boundary calculation formulas in the program are as follows:

[0021] T(i)=T R -(T R -T I )*K i

[0022] U(i)=U R -(U R -U I )*K i

[0023] Where T(i) is the temperature at a certain time point, T R is the rated power point temperature, T I is the idle temperature, U(i) is the displacement of the time node, U R is the displacement of the rated power point, U I is the idle displacement, K i is the variable parameter, K i Gained through experimental testing and experience;

[0024] The program generates and outputs a temperature boundary file and a displacement boundary file for each time point, saving the temperature of set1 and the displacement information of set2 at different time points, named TBC i and DBC i .

[0025] 4) Low cycle fatigue life analysis can comprehensively consider mechanical, oxidation and creep damage, and more accurately analyze the low cycle fatigue life of the cylinder head.

[0026] Through experiments, accurate boundary conditions such as cyclic stress-strain curve, strain-life curve, creep curve and thermal fatigue performance parameters of cylinder head materials at different temperatures are obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a flow chart of the low cycle fatigue life analysis method of the engine cylinder head model;

[0028] Figure 2 It is a low cycle fatigue life analysis cycle condition diagram of the low cycle fatigue life analysis method of the engine cylinder head model;

[0029] Figure 3 It is the displacement boundary diagram of the low cycle fatigue life analysis method of the engine cylinder head model;

[0030] Figure 4 It is the temperature boundary diagram of the low cycle fatigue life analysis method of the engine cylinder head model;

[0031] Figure 5 It is the cylinder head low cycle fatigue test diagram of the engine cylinder head model low cycle fatigue life analysis method;

[0032] Figure 6 It is the relationship diagram between the global model and sub-model of the low cycle fatigue life analysis method of the engine cylinder head model;

[0033] Figure 7 It is the combustion gas temperature and heat transfer coefficient boundary diagram of the low cycle fatigue life analysis method of the engine cylinder head model;

[0034] Figure 8 It is the total damage diagram of the low cycle fatigue life analysis method of the engine cylinder head model. DETAILED DESCRIPTION

[0035] The present invention is described in detail below with reference to the accompanying drawings and embodiments:

[0036] Attached Figure 1-8 It can be seen that the present invention discloses a low cycle fatigue life analysis method for an engine cylinder head model, comprising the following steps:

[0037] Step S1, establishing a finite element mesh full model for cylinder head fatigue analysis: obtaining the digital model and related assembly parameters required for cylinder head fatigue analysis, and establishing a finite element mesh full model; mainly including: obtaining the digital model and assembly parameters of the cylinder head, cylinder block, cylinder gasket, cylinder head cover, cylinder head bolts, main bearing cover, intake and exhaust valves, intake and exhaust valve seat rings, intake and exhaust valve guides, etc., discretizing the digital model to establish a mesh model, and establishing the assembly relationship between each part according to the actual assembly boundary; setting the contact relationship of the area where the parts are installed and in contact according to the actual contact area, such as the cylinder block and cylinder head are bolted, the nut is in contact with the cylinder head, then the area where the nut and the cylinder head are in contact is set as a contact relationship, and the friction coefficient is defined, the screw thread is in contact with the cylinder block, then the contact relationship is set between the screw and the cylinder block bolt hole, and the bolt axial force is loaded on the screw.

[0038] Step S2, establishing a cylinder head temperature field analysis model: in the pre-processing software, the cylinder combustion and cooling water jacket heat transfer coefficients and temperature boundaries are mapped to the cylinder head structure grid through grid mapping to perform cylinder head temperature field analysis;

[0039] In this step, the combustion in the cylinder is carried out in the closed space formed between the cylinder block and the cylinder head, and the temperature and heat transfer coefficient mapping are as follows:

[0040] Step S201, generating a very thin surface mesh on the cylinder head structure for the mesh in contact with the combustion gas. The surface mesh and the cylinder head structure mesh share nodes, that is, share boundary information, and the surface mesh is used as the basis of the temperature field analysis model;

[0041] Step S202, exporting the surface mesh into an inp format file, which contains mesh number and position information;

[0042] Step S203, mapping the temperature and heat transfer coefficient information obtained from the combustion analysis to the grid number with the same position on the grid of the layer and saving it in the inp file;

[0043] Step S204, reading the inp file after the mapping temperature and heat transfer coefficient are input in step S203, and by searching for the corresponding grid number, inputting the temperature and heat transfer coefficient of the number in the inp file into the temperature field analysis model as the thermal boundary of the temperature field analysis;

[0044] The water jacket temperature and heat transfer coefficient boundaries are also mapped to the structural grid using the same method as the cold boundary for temperature field analysis; a heat conduction analysis task of the cold and hot boundaries considering the combustion in the cylinder and the heat dissipation of the water jacket is submitted in the analysis software.

[0045] Step S3, the cylinder head temperature field analysis temperature and the test temperature are compared under the same working conditions:

[0046] The cylinder head temperature field analysis temperature refers to the actual temperature of the structure after the heat conduction analysis is stabilized after comprehensive consideration of the hot and cold boundaries; if the error between the analysis temperature and the test temperature is within 5%, it is considered that the temperature field benchmarking condition is met, and jump to step S4; if the benchmarking condition is not met, jump to step S2 after adjusting the temperature field analysis boundary, and repeat steps S2-S3 until the temperature field benchmarking condition is met; the same operating condition refers to the idle speed and rated power point operating conditions, that is, after the heat conduction analysis is stable, check the temperature distribution of the cylinder head at the idle speed and rated power point conditions, and benchmark with the temperature field test results under the same operating conditions. When the temperature benchmarking condition is not met, it is necessary to adjust the temperature field analysis boundary, that is, adjust the combustion and water jacket analysis parameters, refer to the temperature field benchmarking results, re-perform the combustion analysis and water jacket CFD analysis to obtain new temperature and heat transfer coefficient boundaries, and then re-map the temperature field so that the results output by the combustion and water jacket analysis meet the requirements of adjusting the temperature field benchmarking conditions.

[0047] Step S4, extracting the displacement and temperature boundary of the cylinder head cover model from the cylinder head finite element mesh full model;

[0048] In this step, the fire surface mesh is extracted from the finite element mesh full model established in step S1 as the cylinder cover model;

[0049] The specific method of extraction is as follows:

[0050] S401. Make a set of all mesh nodes of the cylinder cover model and name it set1;

[0051] The mesh nodes on the split surface between the cylinder cover model and the whole model are named as a set2;

[0052] S402. Perform strength calculation on the cylinder head finite element mesh full model at idle speed and rated power point, and output a dat format file to save the temperature information of set1 and the displacement information of set2 during the calculation process;

[0053] S403. Write a MATLAB program 1, which can read the temperature information of the set1 set and the displacement information of the set2 set under the idle and rated power point conditions in the dat file by searching for keywords and record them;

[0054] S404. The temperature information of set1 and the displacement information of set2 are automatically output as abqinp format files;

[0055] Thus, four files of displacement and temperature boundaries of the whole model at idle and rated power conditions are obtained, namely, the temperature boundary of set1 and the displacement boundary of set2 under idle condition; the temperature boundary of set1 and the displacement boundary of set2 under rated power condition; the displacement boundary includes the displacement of the node in three directions of X / Y / Z; these four files are used as the displacement and boundary temperature files of the cylinder cover model.

[0056] The temperature information file of set1 under idle condition is named TBC I , the temperature information file of the rated power point set1 is named TBC R , the displacement information file of set2 under idle condition is named DBC I , the displacement information file of the rated power point set2 is named DBC R .

[0057] Step S5, processing the temperature boundary with reference to the low cycle fatigue life test cycle conditions;

[0058] Figure 2The low-cycle fatigue life cycle is shown in the cyclic working condition of the low-cycle test, and the engine idle-rated point is divided into several sections according to time; the purpose of segmentation is to keep consistent with the test boundary as much as possible. Due to the inconsistent heating rate, it is necessary to add several time periods for transient temperature field analysis. The transient temperature field refers to the collection of transient temperatures at various points in the engine cylinder;

[0059] The operating condition from idle to rated power point represents the heating process from low temperature to high temperature. The temperature rises quickly in the first part of the heating process and slowly in the second part. In order to keep consistent with the test state as much as possible, several intermediate time points are added in the transient temperature field analysis, and the heating and cooling process is divided into several time periods. In this way, the temperature and displacement boundaries at several time points need to be obtained.

[0060] Write MATLAB program 2 to process the four boundary files according to the low-cycle fatigue cycle conditions. The temperature and displacement boundaries at different time points can be automatically obtained through the program. The displacement and temperature boundary calculation formulas in the program are as follows:

[0061] T(i)=T R -(T R -T I )*K i

[0062] U(i)=U R -(U R -U I )*K i

[0063] Where T(i) is the temperature at a certain time point, T R is the rated power point temperature, T I is the idle temperature;

[0064] U(i) is the displacement of the time node, U R is the displacement of the rated power point, U I is the idle displacement, K i is a known variable parameter, K i Obtained through experimental testing and experience.

[0065] The program generates and outputs a temperature boundary file and a displacement boundary file for each time point, saving the temperature of set1 and the displacement information of set2 at different time points, named TBC i and DBC i .

[0066] Step S6, performing stress analysis on the sub-model loading cycle condition boundary;

[0067] When the cylinder head model is subjected to stress analysis, the nodes on the sub-model can read the processed displacement and temperature boundary information, and perform transient stress field analysis according to time history loading;

[0068] The cylinder head model performs stress analysis and divides a low-cycle fatigue test cycle into several time periods. The nodes on the sub-model can read the temperature of set1 and the displacement boundary information of set2 at different time points after processing;

[0069] The sub-model performs transient stress field analysis based on time history loading displacement and temperature boundary;

[0070] In order to make the analysis temperature and the test temperature closer in time history, as shown in the attached Figure 2 Refer to the test results, divide the heating process into four time periods, and the cooling process into three time periods. A low-cycle fatigue cycle has 9 time points from P1 to P9. The duration of each time period should be consistent with the test time. The temperature of the time node is also set according to the temperature gradient of the test, so that the cylinder head stress during the analysis process is consistent with the test state.

[0071] The stress field analysis of the sub-model should be performed for at least 3-4 cycles in the time history, and the stress-strain results of the last cycle should be used for low-cycle fatigue life analysis.

[0072] Step S7, cylinder head low cycle fatigue life analysis; accurate cylinder head thermal fatigue parameters are obtained by experimental testing; comprehensive consideration of oxidation, creep and mechanical load effects can obtain relatively accurate low cycle fatigue life results;

[0073] Determine whether the design requirements are met by checking whether the low-cycle fatigue analysis cycle life is greater than the number of cycles in the test specification. If it is greater than the number of cycles in the test specification, the low-cycle fatigue design requirements are met, and if it is less than the number of cycles, it does not meet the requirements. If the design requirements are not met, optimize the cylinder head structure (such as rounding, thickening, etc.) and jump to step S1 until they are met;

[0074] The relationship between total damage and mechanical damage, oxidation damage and creep damage is shown in formula 1, and the relationship between total life and mechanical fatigue life, oxidation fatigue life and creep fatigue life is shown in formula 2;

[0075] During the analysis, mechanical damage, oxidation damage, and creep damage are calculated separately according to the corresponding material parameters, and then the three damage values ​​are added together to obtain a total damage;

[0076] The low-cycle fatigue life is the inverse of the damage. Formula 2 can be used to obtain the low-cycle fatigue life that takes creep damage, mechanical damage, and oxidation damage into consideration. Check whether the low-cycle fatigue analysis cycle life result is greater than the number of cycles in the test specification. The low-cycle fatigue life needs to be greater than the number of low-cycle test cycles specified in the test specification.

[0077] D TMF =D fat +D env +D creep .....................I

[0078]

[0079] Where D TMF is the total damage, D fat It is mechanical damage, D env Oxidative damage, D creep is the creep damage, N f total is the total lifespan, N f fat is the mechanical fatigue life, N f env is the oxidation fatigue life, N f creep is the creep fatigue life;

[0080] D is the damage, expressed as the inverse of the life span;

[0081] N is the life span, which is the number of times;

[0082] The damage range is 0-1.

[0083] The low cycle fatigue life analysis method of the engine cylinder head model of the present invention is

[0084] 1) The analysis steps are set to be consistent with the cylinder head low cycle fatigue test cycle conditions, which can accurately analyze the cylinder head low cycle fatigue life;

[0085] A cycle of the cylinder head low-cycle fatigue test includes three processes: engine heating-keeping-cooling. It mainly tests the low-cycle thermal fatigue performance of the engine cylinder head under hot and cold shocks. In the test, the corresponding time will be set for the three processes of heating, keeping and cooling. The temperature field analysis is consistent with the test cycle process, which is divided into three processes of heating-keeping-cooling. Referring to the test temperature test curve, the heating process is divided into four time periods, and the cooling process is divided into three time periods. The cylinder head temperature field and strength can be analyzed to obtain temperature and strength analysis results consistent with the test cycle conditions. The strength analysis results obtained by calculation that are consistent with the test conditions are used for low-cycle fatigue life analysis, and a low-cycle fatigue life that is relatively consistent with the low-cycle fatigue test conditions can be obtained.

[0086] 2) Use the sub-model method to reduce the stress analysis model of low-cycle fatigue cycle conditions and improve analysis efficiency;

[0087] The cylinder head has a complex structure. In order to obtain accurate results of the low-cycle fatigue life of the cylinder head, the geometric features of the cylinder head need to be retained as much as possible in some areas prone to failure. Some places must be made into relatively fine grids, resulting in a large number of grids in the cylinder head itself. If the entire cylinder head model is used to calculate the low-cycle fatigue life, the analysis model will be very large, the calculation time will be long, and the calculation result file can reach more than 100Gb. Using the cylinder head model can reduce the cycle condition analysis model, shorten the calculation time, reduce the analysis result file, and improve the analysis efficiency. For example, using the full cylinder head model for analysis requires a workstation calculation time of about 60 hours and a calculation result file of more than 100Gb, while using the sub-model only takes 8-10 hours to calculate, and the result file is only about 10Gb.

[0088] The sub-model is selected as the analysis object because the low-cycle fatigue failure of the engine cylinder head often occurs in the position where there is alternating hot and cold and large temperature changes. The cylinder head fire surface area is subjected to the thermal load of alternating hot and cold on the engine, which is prone to low-cycle thermal engine fatigue failure. In the past, the low-cycle fatigue failure of the engine all occurred in the cylinder head fire surface area. Therefore, the fire surface area is selected as the sub-model for the cylinder head low-cycle fatigue life analysis object.

[0089] 3) Extract the fire surface mesh from the full cylinder head model as a sub-model for analysis, and write a program that can quickly extract and process the low-cycle operating condition analysis boundary;

[0090] 4) Low cycle fatigue life analysis can comprehensively consider mechanical, oxidation and creep damage, and more accurately analyze the low cycle fatigue life of the cylinder head.

[0091] Through experiments, accurate boundary conditions such as cyclic stress-strain curve, strain-life curve, creep curve and thermal-mechanical fatigue performance parameters of cylinder head materials at different temperatures are obtained. The creep and oxidation property parameters of the material are comprehensively considered in the low-cycle fatigue life analysis, and the creep and oxidation damage can be calculated. Thus, the low-cycle fatigue life of the cylinder head fire surface can be analyzed by comprehensively considering mechanical damage, oxidation damage and creep damage.

[0092] The low-cycle fatigue life analysis of the cylinder head is based on the strain-cycle (EN) criterion to analyze and predict the life under high and low temperature thermal cycle stress. With the help of FEMFAT fatigue analysis software, accurate material parameters obtained by input test are used for low-cycle fatigue life analysis. The analysis takes into account mechanical damage, oxidation damage and creep damage. The relationship between total damage and mechanical damage, oxidation damage and creep damage is shown in formula 1, and the relationship between total life and mechanical fatigue life, oxidation fatigue life and creep fatigue life is shown in formula 2.

[0093] D TMF =D fat +D env +D creep.....................I

[0094]

[0095] Where D TMF is the total damage, D fat It is mechanical damage, D env Oxidative damage, D creep is the creep damage, N f total is the total lifespan, N f fat is the mechanical fatigue life, N f env is the oxidation fatigue life, N f creep is the creep fatigue life.

[0096] The above is only a preferred embodiment of the present invention, and does not limit the structure of the present invention in any form. Any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention are within the scope of the technical solution of the present invention.

Claims

1. A method for analyzing low cycle fatigue life of an engine cylinder head model, characterized in that: Step S100, start; Step S101, obtaining the digital model and related assembly parameters required for cylinder head fatigue analysis, and establishing a finite element mesh model; Step S102, after adding the assembly relationship, a temperature field analysis model is established, and the combustion in the cylinder and the heat exchange boundary of the water jacket are mapped to perform temperature field analysis; Obtain the digital model and related assembly parameters required for cylinder head fatigue analysis, perform mesh discretization on the 3D digital model in the finite element pre-processing software, and establish a finite element mesh model; After the grid model is established, the assembly relationship between each part is established according to the actual assembly boundary in the pre-processing software; Set up temperature field analysis, establish a cylinder head temperature field analysis model, and map the cylinder combustion and cooling water jacket heat transfer coefficients and temperature boundaries onto the cylinder head structure grid through grid mapping to perform cylinder head temperature field analysis; Step S103, checking the cylinder head temperature field analysis results and comparing them with the cylinder head temperature field test results under the same working conditions; the cylinder head temperature field analysis results refer to the actual temperature of the structure after the heat conduction analysis is stabilized after comprehensive consideration of the hot and cold boundaries; Step S104, determining whether the temperature field benchmarking condition is met; If the temperature field benchmarking condition is met, jump to step S106; if the benchmarking condition is not met, jump to step S105; the same operating conditions refer to: idle speed and rated power point operating conditions; Step S105, adjust the combustion and water jacket heat exchange boundaries, jump to step S102; recalculate the temperature field until the temperature field benchmarking condition is met; Step S106, performing cylinder head stress analysis; Step S107, obtaining the cylinder head stress analysis result to perform cylinder head high cycle fatigue analysis to obtain the minimum fatigue safety factor; In the FEMFAT fatigue analysis software, the stress results calculated by the four cylinder burst pressure analysis steps are used as a cycle to perform high cycle fatigue strength analysis; Step S108, judging whether the high cycle fatigue design requirements are met, and seeing whether the high cycle fatigue analysis safety factor is greater than 1.

1. If it is greater than 1.1, the fatigue design requirements are met, and if it is less than 1.1, it does not meet the requirements; if it meets the high cycle fatigue design requirements, jump to step S110; if it does not meet the high cycle fatigue design requirements, jump to step S109; then it is necessary to optimize the weak position of the cylinder head until the high cycle fatigue design requirements are met; Step S109, optimizing the cylinder head structure, thickening and strengthening the weak position structure with a small safety factor, and jumping to step S101; Step S110, extracting the displacement and temperature boundary of the cylinder head model from the full model; Step S111, processing the boundary with reference to the low cycle fatigue test cycle condition; The temperature and displacement information is processed, and the engine idle-rated point is divided into several time segments according to the low-cycle test cycle conditions; the purpose of segmentation is to keep consistent with the test boundary. Due to the inconsistent heating rate, it is necessary to add several time periods for transient temperature field analysis; According to different coefficients and the temperature and displacement data of idle and rated points obtained above, the temperature and displacement boundary of each time point is obtained; it is used to analyze the displacement and temperature boundary of different time points in transient temperature field; The temperature at each time point is inconsistent and needs to be calculated separately using different coefficients; The coefficient of each time point is obtained through the previous temperature field test data and experience processing. The relationship between different temperature values ​​is analyzed through the idle condition temperature, rated power point temperature and temperature values ​​at different time points to determine the variable parameter K; The operating condition from idle to rated power point represents the heating process from low temperature to high temperature. The temperature rises quickly in the first part of the heating process and slowly in the second part. When analyzing the transient temperature field, several intermediate time points are added to divide the heating and cooling process into several time periods. In this way, the temperature and displacement boundaries at several time points need to be obtained. Write MATLAB program 2 to process the four boundary files according to the low-cycle fatigue cycle conditions. The temperature and displacement boundaries at different time points can be automatically obtained through the program. The calculation formulas for displacement and temperature boundaries in the program are as follows: ； ； in T(i) is the temperature at the time node, T R is the rated power point temperature, T I is the idle temperature, U(i) is the displacement of the time node, U R is the displacement of the rated power point, U I is the idle displacement, K i is the variable parameter, K i Gained through experimental testing and experience; The program generates and outputs a temperature boundary file and a displacement boundary file for each time point, saving the temperature of set1 and the displacement information of set2 at different time points, named TBC i and DBC i ; Step S112, performing stress analysis on the sub-model loading cycle condition boundary; Step S113, low cycle fatigue life analysis of the cylinder head; accurate thermal fatigue parameters of the cylinder head are obtained by experimental testing; by comprehensively considering the effects of oxidation, creep and mechanical load, a relatively accurate low cycle fatigue life result can be obtained; Step S114, judging whether the design requirements are met, to see whether the low-cycle fatigue analysis cycle life is greater than the number of cycles in the test specification, if it is greater than the number of cycles in the test specification, the low-cycle fatigue design requirements are met, if it is less than the number of cycles in the test specification, it does not meet the requirements; if it does not meet the design requirements, jump to step S109, if it meets the design requirements, jump to step S115; Step S115, end.

2. The low cycle fatigue life analysis method of the engine cylinder head model according to claim 1 is characterized by: In step S102, the combustion in the cylinder is carried out in the closed space formed between the cylinder block and the cylinder head, and the temperature and heat transfer coefficient mapping is carried out according to the following steps: Step S201, generating a layer of meshes from the meshes in contact with the combustion gas on the cylinder block and cylinder head structure, wherein the layer of meshes shares nodes with the structural mesh, that is, shares boundary information; Step S202, exporting the surface mesh into an inp format file, which contains mesh number and position information; Step S203, the inp file is input to the combustion analysis to map the temperature and heat transfer coefficient of the gas measured, and the temperature and heat transfer coefficient information obtained by the combustion analysis is mapped to the grid number with the same position on the grid of the layer by matching the position relationship and saved in the inp file; Step S204: during temperature field analysis, read the inp file after mapping the temperature and heat transfer coefficient, and input the temperature and heat transfer coefficient of the number in the inp file into the temperature field analysis model as the thermal boundary of the temperature field analysis by searching for the same grid number; The water jacket temperature and heat transfer coefficient boundaries are also mapped to the structural grid using the same method as the cold boundary for temperature field analysis; a heat conduction analysis task of the cold and hot boundaries considering the combustion in the cylinder and the heat dissipation of the water jacket is submitted in the analysis software.

3. The low cycle fatigue life analysis method of the engine cylinder head model according to claim 1 is characterized by: In step S104, after the heat conduction analysis is stable, the temperature distribution of the cylinder head at the idle speed and rated power point is checked, and the temperature field test results under the same working conditions are compared. If the error between the analysis result and the test result is within 5%, it is considered that the temperature field comparison condition is met. If the temperature error exceeds 5%, it is considered that the temperature comparison condition is not met, and the temperature field analysis boundary conditions need to be checked. Referring to the temperature field benchmarking results, combustion analysis and water jacket CFD analysis are re-performed to obtain new temperatures and heat transfer coefficient boundaries, and the temperature field is calculated after remapping until the temperature field benchmarking conditions are met.

4. The low cycle fatigue life analysis method of the engine cylinder head model according to claim 1 is characterized by: In step S106 and step S107, after the temperature field analysis meets the benchmarking conditions, the next step of cylinder head stress field analysis is performed; Each part of the analysis model needs to establish accurate contact properties according to the actual assembly relationship; The stress field analysis sets up the following analysis steps: Step S301, applying a pre-tightening force to the cylinder head bolts; Step S302, the stress analysis model reads the idle condition temperature field analysis result. Just like reading the mapped combustion boundary, by reading the grid node information in the temperature field analysis result, the node numbers in the stress analysis model that are the same as those in the temperature field analysis model are matched one by one, and the temperature results after the temperature field analysis are transferred to the stress model to analyze the thermal stress of the cylinder head; Step S303, the stress analysis model reads the rated power point working condition temperature field analysis result to analyze the cylinder head thermal stress; Step S304, applying an explosion pressure load in the first cylinder, and applying a pressure load equal to the maximum explosion pressure on the fire surface and the cylinder hole; Step S305, applying an explosion pressure load in the second cylinder, and applying a pressure load equal to the maximum explosion pressure on the fire surface and the cylinder hole; Step S306, applying an explosion pressure load in the third cylinder, and applying a pressure load equal to the maximum explosion pressure on the fire surface and the cylinder hole; Step S307, applying an explosion pressure load in the fourth cylinder, and applying a pressure load equal to the maximum explosion pressure on the fire surface and the cylinder hole; The cylinder head stress analysis comprehensively considers the impact of assembly, thermal stress and gas explosion pressure on the cylinder head; After the cylinder head stress analysis calculation is completed, the high-cycle fatigue strength analysis is performed using the fourth to seventh analysis steps in the stress analysis, i.e., the 1-4 cylinder ignition condition, as a high-cycle cycle to obtain the minimum fatigue safety factor of the cylinder head structure. If the high-cycle fatigue design requirements are not met, it is necessary to optimize the weak positions of the cylinder head until the high-cycle fatigue design requirements are met.

5. The low cycle fatigue life analysis method of the engine cylinder head model according to claim 1 is characterized by: In the step S110, the fire surface mesh is extracted from the cylinder head full model as a sub-model; Make a set of all mesh nodes of the sub-model and name it set1; The mesh nodes on the dividing surface between the sub-model and the full model are taken as a set and named as set2; The full model performs strength calculations at idle and rated power points. During the calculation process, a dat-format file is output to save the temperature information of set1 and the displacement information of set2. Write MATLAB program 1, which can read the temperature and displacement information of the set1 and set2 nodes of the idle and rated power conditions in the dat file by searching for keywords and record them; The temperature information of set1 and the displacement information of set2 are automatically output as files in abqinp format; Four files are obtained for the displacement temperature boundaries of the idle and rated power point conditions, which are the temperature boundary of set1 and the displacement boundary of set2 under the idle condition; The temperature boundary of the rated power point condition set1 and the displacement of set2; the displacement boundary includes the displacement of the node in the three directions of X / Y / Z; The temperature information file of set1 under idle condition is named TBC I , the temperature information file of rated power point set1 is named TBC R , the displacement information file of set2 under idle condition is named DBC I , the displacement information file of rated power point set2 is named DBC R .

6. The low cycle fatigue life analysis method of the engine cylinder head model according to claim 1, characterized in that: In step S112, when the cylinder head model is subjected to stress analysis, the nodes on the sub-model can read the processed displacement and temperature boundary information, and perform transient stress field analysis according to time history loading; The cylinder head model performs stress analysis and divides a low-cycle fatigue test cycle into several time periods. The nodes on the sub-model can read the temperature of set1 and the displacement boundary information of set2 at different time points after processing; The sub-model performs transient stress field analysis based on time history loading displacement and temperature boundary; In order to make the analysis temperature and the test temperature closer in time history, the heating process is divided into four time periods, and the cooling process is divided into three time periods. A low-cycle fatigue cycle has 9 time points from P1 to P9. The duration of each time period should be consistent with the test time. The temperature of the time node is also set according to the temperature rise gradient of the test, so that the cylinder head stress during the analysis process is consistent with the test state. The stress field analysis of the sub-model should be performed for at least 3-4 cycles in the time history, and the stress-strain results of the last cycle should be used for low-cycle fatigue life analysis.

7. The low cycle fatigue life analysis method of the engine cylinder head model according to claim 1, characterized in that: To determine whether the design requirements are met, check whether the cycle life results of the low-cycle fatigue analysis are greater than the number of cycles in the test specification. If the number of cycles is greater than the test specification, the low-cycle fatigue design requirements are met; if it is less than the number of cycles, it does not meet the requirements; The step S114 verifies and guides the low cycle fatigue design of the cylinder head. If the design requirements are not met, it is necessary to optimize the structural area of ​​the cylinder head structure with a smaller low cycle fatigue life, and recalculate the low cycle fatigue life according to the process until the design requirements are met; Optimize the area with small low-cycle fatigue life in the cylinder head structure. Recalculate the low-cycle fatigue life according to the process. The main purpose is to optimize the structural area where the low-cycle fatigue cycle life does not meet the design requirements, and thicken and strengthen the weak position structure with a small safety factor; The relationship between total damage and mechanical damage, oxidation damage and creep damage is shown in formula 1, and the relationship between total life and mechanical fatigue life, oxidation fatigue life and creep fatigue life is shown in formula 2; During the analysis, mechanical damage, oxidation damage, and creep damage are calculated separately according to the corresponding material parameters, and then the three damage values ​​are added together to obtain a total damage; The low-cycle fatigue life is the inverse of the damage. The low-cycle fatigue life considering creep damage, mechanical damage and oxidation damage can be obtained by formula 2. Check whether the cycle life result of low-cycle fatigue analysis is greater than the number of cycles in the test specification. The low-cycle fatigue life needs to be greater than the number of low-cycle test cycles specified in the test specification; ； ； Where D TMF is the total damage, D fat It is mechanical damage, D env Oxidative damage, D creep is the creep damage, N f total is the total lifespan, N f fat is the mechanical fatigue life, N f env is the oxidation fatigue life, N f creep is the creep fatigue life; D is the damage, expressed as the inverse of the life span; N is the life span, which is the number of times; The damage range is 0-1.

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