CN114065571A - Low-cycle fatigue life analysis method for engine cylinder cover model - Google Patents
Low-cycle fatigue life analysis method for engine cylinder cover model Download PDFInfo
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Abstract
The invention relates to a low cycle fatigue life analysis method of an engine cylinder cover model, which comprises the steps of establishing a finite element grid model; after the assembly relation is added, a temperature field analysis model is established for temperature field analysis; analyzing the stress of the cylinder cover; acquiring a minimum fatigue safety factor; optimizing a cylinder cover structure, and extracting displacement and temperature boundaries of a cylinder cover model from the full model; processing the boundary according to the cyclic working condition of the low-cycle fatigue test; carrying out stress analysis on the sub-model loading cycle working condition boundary; analyzing the low cycle fatigue life of the cylinder cover; and obtaining a more accurate low cycle fatigue life result. The low cycle fatigue life analysis method of the engine cylinder cover model can accurately analyze the low cycle fatigue life of the cylinder cover; by using a sub-model method, a stress analysis model of the low-cycle fatigue cycle working 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 cover.
Description
Technical Field
The invention relates to the field of engine structure design analysis, in particular to a low cycle fatigue life analysis method for an engine cylinder cover model.
Background
The cylinder cover is complex in structure, bears alternating mechanical load and thermal load, and the cold and hot alternation of the engine easily causes the cylinder cover to generate low-cycle thermal engine fatigue failure, and along with the development of the engine, the working temperature of the cylinder cover is higher and higher, so that the probability of creep failure is increased. The problem of fatigue failure of a heat engine of a cylinder cover is a key point of industrial research and a difficult point in recent years, the existing cylinder cover fatigue mainly adopts high-cycle fatigue analysis, the low-cycle fatigue life prediction is rarely carried out, and the low-cycle fatigue analysis has the defects of large analysis model, long analysis period, low accuracy and the like.
Disclosure of Invention
Aiming at the defects in the prior art, the invention forms the low-cycle fatigue life analysis method of the engine cylinder cover model, which has the advantages of simpler model, consistency with the test working condition, higher analysis efficiency and higher accuracy by calibrating with the low-cycle fatigue test.
The technical scheme adopted by the invention is as follows:
compared with the prior art, the invention has the beneficial effects that:
the invention relates to a method for analyzing the low cycle fatigue life of an engine cylinder cover model,
1) the analysis step S is set to be consistent with the cycle working condition of the cylinder cover low-cycle fatigue test, so that the low-cycle fatigue life of the cylinder cover can be accurately analyzed;
a cycle of the cylinder cover low-cycle fatigue test comprises three processes of heating-heat preservation-cooling of the engine, the fatigue performance of the low-cycle heat engine of the engine cylinder cover under the condition of bearing cold and heat shock is mainly checked, and corresponding time is set for the three processes of heating, heat preservation and cooling in the test. The analysis of the temperature field is consistent with the test cycle course and comprises three processes of temperature rise, heat preservation and temperature reduction, the temperature rise process is divided into four time periods according to a test temperature test curve, the temperature reduction process is divided into three time periods to carry out the temperature field and the strength of the cylinder cover, the temperature and strength analysis result consistent with the test cycle working condition can be obtained, the strength analysis result consistent with the test working condition is obtained by calculation to carry out low-cycle fatigue life analysis, and the low-cycle fatigue life consistent with the low-cycle fatigue test working condition can be obtained.
2) By using a sub-model method, a stress analysis model of the low-cycle fatigue cycle working condition is reduced, and the analysis efficiency is improved;
the cylinder cover is complex in structure, in order to obtain accurate cylinder cover low-cycle fatigue life results, the geometric characteristics of the cylinder cover need to be kept as far as possible in some areas prone to failure, a small grid needs to be manufactured in some places, so that the grid number of the cylinder cover is large, if the whole cylinder cover model is used for calculating the low-cycle fatigue life, the analysis model is very large, the calculation time is long, the calculation result file can reach more than 100Gb, the cycle condition analysis model can be reduced by using the cylinder cover model, the calculation time is shortened, the analysis result file is reduced, and the analysis efficiency is improved. For example, when the full-cylinder-cover model is used for analysis, the calculation time of a workstation is required to be about 60 hours, and the calculation result file is over 100Gb, while the calculation time of the sub-model is only 8-10 hours, and the calculation result file is only about 10 Gb.
The sub-model as the attached figure is selected as an analysis object because the low-cycle fatigue failure of the engine cylinder cover is mostly generated at the position with alternating cold and heat and large temperature change, the thermal load of the thermal surface area of the cylinder cover on the engine is subjected to the alternating cold and heat, the low-cycle thermal fatigue failure is easy to generate, and the low-cycle fatigue failure of the engine in the past is generated in the thermal surface area of the cylinder cover, so the thermal surface area is selected as the sub-model to analyze the low-cycle fatigue life of the cylinder cover.
3) The program can be compiled to quickly extract and process the analysis boundary of the low-cycle working condition;
extracting fire surface grids from the cylinder cover full model to serve as a sub-model, and setting all grid nodes of the sub-model as a set to be named as set 1;
the grid nodes on the division surfaces of the sub-model and the full model are named as a set 2;
the full model carries out intensity calculation of working conditions of idle speed and rated power points, and temperature information of a set1 set and displacement information of a set2 set are saved in a dat format file in the calculation process;
writing an MATLAB program 1, reading the temperature and displacement information of nodes set1 and set2 of idle speed and rated power point working conditions in a dat file by a keyword searching method, and recording the temperature and displacement information, wherein the temperature information of set1 and the displacement information of set2 can be automatically output as files in abqinp format, so that four files of displacement temperature boundaries of the idle speed and rated power point working conditions are obtained, and the four files are respectively the temperature boundary of set1 and the displacement boundary of set2 under the idle speed working condition; temperature boundary of rated power point operating condition set1 and displacement of set2, and displacement boundary comprises displacement of node X/Y/Z in three directions.
The structure mainly depends on displacement drive to generate stress, and the full model outputs a displacement boundary to the sub-model, so as to ensure that the displacement of the segmentation surface is consistent with the full model.
The temperature information file of set1 under idle condition is named as TBCIThe temperature information file of the rated power point set1 is named TBCRThe displacement information file of set2 under idle condition is named DBCIThe displacement information file of the rated power point set2 is named DBCR；
The working condition from idling to a rated power point represents a temperature rise process from low temperature to high temperature, the temperature rises quickly in the front section of temperature rise, the temperature rises slowly in the rear section of temperature rise, the temperature rise process is divided into a plurality of time sections, and then the temperature and displacement boundaries of a plurality of time points need to be obtained;
writing an MATLAB program 2, processing the four boundary files according to a low-cycle fatigue cycle working condition, wherein the temperatures and displacement boundaries at different time points can be automatically obtained through the program, and the calculation formula of the displacement and temperature boundaries in the program is as follows:
T(i)＝TR-(TR-TI)*Ki
U(i)＝UR-(UR-UI)*Ki
wherein T (i) is the temperature at the time point, TRIs the rated power point temperature, TIFor idle temperature, U (i) displacement of time node, URFor displacement of rated power point, UIFor idle displacement, KiAs a variable parameter, KiObtained through experimental tests and experiences;
the program forms and outputs a temperature boundary file and a displacement boundary file for each time point, saves the temperature of set1 and the displacement information of set2 at different time points, and is named as TBCiAnd DBCi。
4) 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 cover.
Accurate boundary conditions such as a cyclic stress strain curve, a strain life curve, a creep curve, thermal engine fatigue performance parameters and the like of the cylinder head material at different temperatures are obtained through tests.
Drawings
FIG. 1 is a flow chart of a method for low cycle fatigue life analysis of an engine cylinder head model;
FIG. 2 is a low cycle fatigue life analysis cycle behavior diagram of an engine cylinder head model low cycle fatigue life analysis method;
FIG. 3 is a displacement boundary diagram of a method for low cycle fatigue life analysis of an engine cylinder head model;
FIG. 4 is a temperature boundary diagram of a method for low cycle fatigue life analysis of an engine cylinder head model;
FIG. 5 is a diagram of a cylinder head low cycle fatigue test of a method for analyzing the low cycle fatigue life of a model of an engine cylinder head;
FIG. 6 is a graph of a global model and submodels relationship for a low cycle fatigue life analysis method for an engine cylinder head model;
FIG. 7 is a combustion gas temperature and heat transfer coefficient boundary diagram for an engine cylinder head model low cycle fatigue life analysis method;
FIG. 8 is a total damage map of a method for low cycle fatigue life analysis of an engine cylinder head model.
Detailed Description
The invention is described in detail below with reference to the figures and examples:
as can be seen in the attached figures 1-8, the invention discloses a low cycle fatigue life analysis method of an engine cylinder cover model, which comprises the following steps:
step S1, establishing a finite element mesh full model for cylinder cover fatigue analysis, namely acquiring a digital model and related assembly parameters required by the cylinder cover fatigue analysis, and establishing the finite element mesh full model; the method mainly comprises the following steps: acquiring digital models and assembly parameters of a cylinder cover, a cylinder body, a cylinder gasket, a cylinder cover, cylinder cover bolts, a main bearing cover, an air inlet and exhaust valve seat ring, an air inlet and exhaust valve guide pipe and the like, discretizing the digital models to establish a grid model, and establishing an assembly relation among all parts according to an actual assembly boundary; the contact relation is set according to the actual contact area in the area where the parts are installed and contacted, if the cylinder body and the cylinder cover are connected through bolts, the contact relation is set in the area where the nut is contacted with the cylinder cover, the friction coefficient is defined, the contact relation is set between the screw and the cylinder body bolt hole when the screw threads are contacted with the cylinder body, and the bolt axial force is loaded on the screw.
Step S2, establishing a cylinder cover temperature field analysis model: in pre-processing software, mapping heat exchange coefficients and temperature boundaries of the in-cylinder combustion and cooling water jacket to grids of a cylinder cover structure through grid mapping to perform cylinder cover temperature field analysis;
in this step, in-cylinder combustion is performed in a closed space formed between a cylinder body and a cylinder head, and temperature and heat exchange coefficient mapping is performed according to the following steps:
step S201, generating a layer of thin surface grid on a cylinder cover structure by using partial grid contacted with combustion gas, wherein the layer surface grid and the cylinder cover structure grid share nodes, namely share boundary information, and the surface grid is used as the basis of a temperature field analysis model;
step S202, exporting the face mesh into an inp format file, wherein the inp format file contains mesh numbers and position information;
step S203, mapping the temperature and heat exchange coefficient information obtained by combustion analysis to grid numbers with the same position on the layer grid and storing the grid numbers in an inp file;
step S204, reading the inp file with the mapping temperature and the heat exchange coefficient input in the step S203, and inputting the temperature and the heat exchange coefficient of the number in the inp file into a temperature field analysis model as a thermal boundary of temperature field analysis by searching corresponding grid numbers;
the water jacket temperature and heat exchange coefficient boundary is mapped to the structural grid by adopting the same method to be used as a cold boundary for temperature field analysis; a heat conduction analysis task considering the cold and hot boundaries of in-cylinder combustion and water jacket heat dissipation is submitted in analysis software.
Step S3, calibrating the cylinder cover temperature field analysis temperature and the test temperature under the same working condition:
the cylinder cover temperature field analysis temperature refers to the actual temperature of the structure after the heat conduction analysis is stable after the cold and hot boundaries are comprehensively considered; if the error between the analysis temperature and the test temperature is within 5%, the temperature field calibration condition is considered to be met, and the step S4 is skipped; if the standard aligning condition is not met, jumping to the step S2 after the temperature field analysis boundary is adjusted, and repeating the steps S2-S3 until the standard aligning condition of the temperature field is met; the same working condition refers to idling and rated power point working conditions, namely the temperature distribution of the cylinder cover under the idling and rated power point working conditions is checked after the heat conduction analysis is stable, the temperature distribution is aligned with a temperature field test result under the same working condition, when the temperature alignment condition is not met, the temperature field analysis boundary needs to be adjusted, namely, the combustion and water jacket analysis parameters are adjusted, the combustion analysis and water jacket CFD analysis are carried out on the alignment result again by referring to the temperature field, a new temperature and heat exchange coefficient boundary is obtained after remapping, and then the temperature field is calculated, so that the result output by the combustion and water jacket analysis meets the requirement for adjusting the temperature field alignment condition.
Step S4, extracting the displacement and temperature boundary of the cylinder cover sub-model from the cylinder cover finite element grid full model;
in this step, a fire surface mesh is extracted from the finite element mesh full model established in step S1 as a cylinder head sub-model;
the extraction method comprises the following steps:
s401, making a set of all grid nodes of a cylinder cover sub-model, and naming the set as set 1;
the grid nodes on the cylinder cover submodel and the full model division surface are named as set2 as a set;
s402, carrying out intensity calculation on idle speed and rated power point working conditions by a cylinder cover finite element grid full model, and outputting temperature information of a set1 set and displacement information of a set2 set saved in a dat format file in the calculation process;
s403, writing an MATLAB program 1, reading and recording temperature information of a set1 set and displacement information of a set2 set under idle speed and rated power point working conditions in a dat file by a keyword searching method;
s404, automatically outputting the temperature information of the set1 set and the displacement information of the set2 set into a file in an abqinp format;
therefore, four files of displacement and temperature boundaries of the full-model idling and rated power point working conditions are obtained, wherein the four files are respectively the temperature boundary of set1 and the displacement boundary of set2 under the idling working condition; temperature boundary of rated power point condition set1 and displacement boundary of set 2; the displacement boundary comprises displacement of nodes in X/Y/Z directions; and taking the four files as displacement and boundary temperature files of the cylinder cover submodel.
The set1 temperature information file under idle condition is named as TBCIThe temperature information file of the set of rated power points set1 is named TBCRThe set2 set displacement information file under idle condition is named DBCIThe displacement information file of the set of rated power points set2 is named DBCR。
Step S5, processing the temperature boundary according to the cyclic working condition of the low-cycle fatigue life test;
FIG. 2 illustrates the cycle behavior of a low cycle fatigue life foot ring as tested in low cycles, dividing the engine idle-rated point into several segments by time; the purpose of segmentation is to keep consistent with a test boundary as much as possible, because the heating rates are inconsistent, a plurality of time periods are added for analyzing a transient temperature field, and the transient temperature field refers to a set of transient temperatures at each point in an engine cylinder;
the working condition from idling to a rated power point represents a heating process from low temperature to high temperature, the temperature rises quickly in the front section of heating, the temperature rises slowly in the rear section, in order to keep the same as a test state as far as possible, a plurality of middle time points are added in the transient temperature field analysis, the heating and cooling process is divided into a plurality of time periods, and thus the temperature and displacement boundaries of a plurality of time points need to be obtained;
writing an MATLAB program 2, processing the four boundary files according to a low-cycle fatigue cycle working condition, wherein the temperatures and displacement boundaries at different time points can be automatically obtained through the program, and the calculation formula of the displacement and temperature boundaries in the program is as follows:
T(i)＝TR-(TR-TI)*Ki
U(i)＝UR-(UR-UI)*Ki
wherein T (i) is the temperature at the time point, TRIs the rated power point temperature, TIIs an idle temperature;
u (i) is the displacement of the time node, URFor displacement of rated power point, UIFor idle displacement, KiIs a known variable parameter, KiObtained through experimental tests and experiences.
The program forms and outputs a temperature boundary file and a displacement boundary file for each time point, saves the temperature of set1 and the displacement information of set2 at different time points, and is named as TBCiAnd DBCi。
Step S6, performing stress analysis on the sub-model loading cycle working condition boundary;
when the cylinder cover model carries out stress analysis, the nodes on the submodels can read the processed displacement and temperature boundary information and load according to the time history to carry out transient stress field analysis;
the cylinder cover model carries out stress analysis, a low-cycle fatigue test cycle is divided into a plurality of time periods, and nodes on the submodel can read the temperature of set1 and the displacement boundary information of set2 at different time points after treatment;
the submodel carries out transient stress field analysis according to the time history loading displacement and the temperature boundary;
in order to make the analysis temperature and the test temperature closer in time history, as shown in fig. 2, referring to the test result of the test, the temperature rising process is divided into four time periods, the temperature lowering process is divided into three time periods, one low-cycle fatigue cycle has 9 time points from P1 to P9, the duration of each time period is consistent with the test time, and the temperature of the time node is also set according to the temperature rising gradient of the test, so that the stress of the cylinder cover in the analysis process is consistent with the test state;
the stress field analysis of the sub-model should analyze a minimum of 3-4 cycles over the time history and use the stress-strain results of the last cycle for low cycle fatigue life analysis.
Step S7, analyzing the low-cycle fatigue life of the cylinder cover; testing to obtain accurate fatigue parameters of the cylinder cover heat engine; the effects of oxidation, creep and mechanical load are comprehensively considered, and a more accurate low-cycle fatigue life result can be obtained;
and judging whether the design requirement is met or not, checking whether the cycle life of the low-cycle fatigue analysis is greater than the cycle number of the test specification or not, wherein the cycle number greater than the test specification meets the low-cycle fatigue design requirement, and the cycle number smaller than the test specification does not meet the design requirement. If the design requirements are not met, optimizing the cylinder cover structure (such as rounding off, thickening and the like), and then jumping to the step S1 until the design requirements are met;
the relationship between the total damage and the mechanical damage, the oxidation damage and the creep damage is shown in a formula 1, and the relationship between the total service life and the mechanical fatigue service life, the oxidation fatigue service life and the creep fatigue service life is shown in a formula 2;
in the analysis process, mechanical damage, oxidation damage and creep damage are respectively calculated according to corresponding material parameters, and then the three damage values are added to obtain a total damage;
the low cycle fatigue life is the reciprocal of the damage, and the low cycle fatigue life considering creep damage, mechanical damage and oxidation damage can be obtained by the formula 2; and checking whether the cycle life result of the low-cycle fatigue analysis is greater than the cycle number of the test specification, wherein the low-cycle fatigue life needs to be greater than the cycle number of the low-cycle test specified in the test specification.
DTMF＝Dfat+Denv+Dcreep.....................I
Wherein DTMFIs total injury, DfatIs a mechanical damage, DenvIs an oxidative damage, DcreepIs creep damage, Nf totalIs the total life, Nf fatIs the mechanical fatigue life, Nf envIs the oxidation fatigue life, Nf creepIs creep fatigue life;
d is damage, expressed as the reciprocal of life;
n is the lifetime, in number of times;
the range of damage is 0-1.
The invention relates to a method for analyzing the low cycle fatigue life of an engine cylinder cover model,
1) the analysis step is set to be consistent with the cycle working condition of the cylinder cover low-cycle fatigue test, so that the low-cycle fatigue life of the cylinder cover can be accurately analyzed;
a cycle of the cylinder cover low-cycle fatigue test comprises three processes of heating-heat preservation-cooling of the engine, the fatigue performance of the low-cycle heat engine of the engine cylinder cover under the condition of bearing cold and heat shock is mainly checked, and corresponding time is set for the three processes of heating, heat preservation and cooling in the test. The analysis of the temperature field is consistent with the test cycle course and comprises three processes of temperature rise, heat preservation and temperature reduction, the temperature rise process is divided into four time periods according to a test temperature test curve, the temperature reduction process is divided into three time periods to carry out the temperature field and the strength of the cylinder cover, the temperature and strength analysis result consistent with the test cycle working condition can be obtained, the strength analysis result consistent with the test working condition is obtained by calculation to carry out low-cycle fatigue life analysis, and the low-cycle fatigue life consistent with the low-cycle fatigue test working condition can be obtained.
2) By using a sub-model method, a stress analysis model of the low-cycle fatigue cycle working condition is reduced, and the analysis efficiency is improved;
the cylinder cover is complex in structure, in order to obtain accurate cylinder cover low-cycle fatigue life results, the geometric characteristics of the cylinder cover need to be kept as far as possible in some areas prone to failure, a small grid needs to be manufactured in some places, so that the grid number of the cylinder cover is large, if the whole cylinder cover model is used for calculating the low-cycle fatigue life, the analysis model is very large, the calculation time is long, the calculation result file can reach more than 100Gb, the cycle condition analysis model can be reduced by using the cylinder cover model, the calculation time is shortened, the analysis result file is reduced, and the analysis efficiency is improved. For example, when the full-cylinder-cover model is used for analysis, the calculation time of a workstation is required to be about 60 hours, and the calculation result file is over 100Gb, while the calculation time of the sub-model is only 8-10 hours, and the calculation result file is only about 10 Gb.
The submodel is selected as an analysis object because the low-cycle fatigue failure of the engine cylinder cover is frequently generated at the positions with alternating cold and heat and large temperature change, the thermal load of the cold and heat alternation is borne on the engine by the thermal surface area of the cylinder cover, the low-cycle thermal fatigue failure is easy to generate, and the low-cycle fatigue failure of the engine in the past is generated in the thermal surface area of the cylinder cover, so the thermal surface area is selected as the submodel to analyze the low-cycle fatigue life of the cylinder cover.
3) Extracting fire surface grids from the cylinder cover full model to be used as sub models for analysis, and compiling a program to quickly extract and process low-cycle working condition analysis boundaries;
4) 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 cover.
Accurate boundary conditions such as a cyclic stress strain curve, a strain life curve, a creep curve, thermal engine fatigue performance parameters and the like of the cylinder head material at different temperatures are obtained through tests, creep and oxidation damage can be calculated by comprehensively considering creep and oxidation property parameters of the material in low-cycle fatigue life analysis, and therefore mechanical damage, oxidation damage and creep damage are comprehensively considered, and the low-cycle fatigue life of the fire surface of the cylinder head is analyzed.
The low-cycle fatigue life analysis of the cylinder cover is based on a strain-cycle (E-N) criterion, and the life condition under the action of high-temperature and low-temperature thermal cycle stress is analyzed and predicted. By means of FEMFAT fatigue analysis software, accurate material parameters obtained by testing are input to carry out low-cycle fatigue life analysis, mechanical damage, oxidation damage and creep damage are considered in analysis, the relation between total damage and the mechanical damage, the relation between the total damage and the oxidation damage and the creep damage are shown in a formula 1, and the relation between the total life and the mechanical fatigue life, the relation between the oxidation fatigue life and the creep fatigue life are shown in a formula 2.
DTMF＝Dfat+Denv+Dcreep.....................1
Wherein DTMFIs total injury, DfatIs a mechanical damage, DenvIs an oxidative damage, DcreepIs creep damage, Nf totalIs the total life, Nf fatIs the mechanical fatigue life, Nf envIs the oxidation fatigue life, Nf creepIs creep fatigue life.
The above description is only for the preferred embodiment of the present invention, and is not intended to limit the structure of the present invention in any way. Any simple modification, equivalent change and modification of the above embodiments according to the technical spirit of the present invention are within the technical scope of the present invention.
Claims (8)
1. A low cycle fatigue life analysis method of an engine cylinder cover model is characterized in that,
step S100, start;
step S101, acquiring a digital model and related assembly parameters required by cylinder head fatigue analysis, and establishing a finite element grid model;
step S102, building a temperature field analysis model after adding an assembly relation, and mapping in-cylinder combustion and water jacket heat exchange boundaries to perform temperature field analysis;
acquiring a digital model and related assembly parameters required by cylinder head fatigue analysis, carrying out grid discrete processing on the 3D digital model in finite element preprocessing software, and establishing a finite element grid model;
after a grid model is established, establishing an assembly relation among all parts according to an actual assembly boundary in front processing software;
setting temperature field analysis, establishing a cylinder body and cylinder cover temperature field analysis model, and mapping the in-cylinder combustion and cooling water jacket heat exchange coefficients and temperature boundaries to the structural grids of the cylinder body and the cylinder cover through grid mapping to perform cylinder cover temperature field analysis;
step S103, checking the analysis result of the cylinder cover temperature field and performing benchmarking with the test result of the cylinder cover temperature field under the same working condition; the analysis result of the cylinder cover temperature field refers to the actual temperature of the structure after the heat conduction analysis is stable after the cold and hot boundaries are comprehensively considered;
step S104, judging whether the temperature field benchmarking condition is met;
the temperature field is met and the calibration conditions are met, and the step S106 is skipped; skipping step S105 under the condition that the benchmarking condition is not met; the same working condition refers to: idle and rated power point conditions;
step S105, adjusting the combustion and water jacket heat exchange boundary, and skipping to step S102; recalculating the temperature field until the temperature field calibration condition is met;
step S106, analyzing the stress of the cylinder cover;
s107, obtaining a cylinder cover stress analysis result to perform cylinder cover high cycle fatigue analysis, and obtaining a minimum fatigue safety coefficient;
in FEMFAT fatigue analysis software, the stress result calculated by four cylinder explosion pressure analysis steps is used as a cycle to carry out high cycle fatigue strength analysis;
step S108, judging whether the high cycle fatigue design requirement is met, and judging whether the high cycle fatigue analysis safety coefficient is more than 1.1, if so, meeting the fatigue design requirement, and if not, meeting the fatigue design requirement; if the high cycle fatigue design requirement is met, skipping to the step S110; skipping to step S109 when the high cycle fatigue design requirement is not met; optimizing the weak position of the cylinder cover until the design requirement of high cycle fatigue is met;
s109, optimizing a cylinder cover structure, thickening and reinforcing the weak position structure with a small safety coefficient, and jumping to S101;
step S110, extracting displacement and temperature boundaries of a cylinder cover model from the full model;
step S111, processing the boundary according to the cyclic working condition of the low-cycle fatigue test;
step S112, carrying out stress analysis on the sub-model loading cycle working condition boundary;
step S113, analyzing the low-cycle fatigue life of the cylinder cover; testing to obtain accurate fatigue parameters of the cylinder cover heat engine; the effects of oxidation, creep and mechanical load are comprehensively considered, and a more accurate low-cycle fatigue life result can be obtained;
and step S114, judging whether the design requirement is met, and judging whether the cycle life of the low-cycle fatigue analysis is greater than the cycle number of the test specification, wherein the cycle number greater than the test specification meets the design requirement of the low-cycle fatigue and the cycle number less than the test specification does not meet the design requirement of the low-cycle fatigue. Skipping to step S109 when the design requirement is not met, and skipping to step S115 when the design requirement is met;
and step S115, ending.
2. The engine cylinder head model low cycle fatigue life analysis method of claim 1, wherein:
in the step S102, in-cylinder combustion is performed in a closed space formed between the cylinder body and the cylinder cover, and the temperature and heat exchange coefficient mapping comprises the following steps:
step S201, generating a layer of thin surface grid on a cylinder body and cylinder cover structure for partial grid contacting with combustion gas, wherein the surface grid and the structure grid share nodes, namely sharing boundary information;
step S202, exporting the face mesh into an inp format file, wherein the inp format file contains mesh numbers and position information;
step S203, inputting an inp file to the temperature and heat exchange coefficient measured by the combustion analysis mapping gas, mapping the temperature and heat exchange coefficient information obtained by the combustion analysis to grid numbers with consistent positions on the layer grid according to the matching position relation, and storing the grid numbers in the inp file;
step S204, reading the inp file after mapping the temperature and the heat exchange coefficient during temperature field analysis, and inputting the temperature and the heat exchange coefficient of the number in the inp file into a temperature field analysis model as a thermal boundary of the temperature field analysis by searching the same grid number;
the water jacket temperature and heat exchange coefficient boundary is mapped to the structural grid by adopting the same method to be used as a cold boundary for temperature field analysis; a heat conduction analysis task considering the cold and hot boundaries of in-cylinder combustion and water jacket heat dissipation is submitted in analysis software.
3. The engine cylinder head model low cycle fatigue life analysis method of claim 1, wherein:
step S104, checking the temperature distribution of the cylinder cover under the working conditions of idle speed and rated power point after the heat conduction analysis is stable, and performing benchmarking with the temperature field test result under the same working condition, wherein if the error between the analysis result and the test result is within 5%, the benchmarking condition of the temperature field is considered to be met, and if the error between the temperature and the test result exceeds 5%, the benchmarking condition of the temperature field is considered not to be met, and the analysis boundary condition of the temperature field needs to be checked;
and (4) carrying out combustion analysis and water jacket CFD analysis again on the reference temperature field calibration result to obtain new temperature and heat exchange coefficient boundaries, remapping and calculating the temperature field until the calibration condition of the temperature field is met.
4. The engine cylinder head model low cycle fatigue life analysis method of claim 1, wherein:
in the step S106 and the step S107, after the temperature field analysis meets the standard condition, the next step of cylinder cover stress field analysis is carried out;
analyzing each part of the model to establish accurate contact attribute according to the actual assembly relation;
the stress field analysis sets the following analysis steps:
s301, applying pretightening force of a cylinder cover bolt;
step S302, reading an idling condition temperature field analysis result by a stress analysis model, wherein the idling condition temperature field analysis result is the same as the combustion boundary read and mapped, and the node numbers in the stress analysis model, which are the same as those in the temperature field analysis model, are in one-to-one correspondence by reading grid node information in the temperature field analysis result, and the temperature result after the temperature field analysis is transmitted to the stress model to analyze the thermal stress of the cylinder cover;
s303, reading a working condition temperature field analysis result of a rated power point by a stress analysis model to analyze thermal stress of the cylinder cover;
step S304, applying an explosion pressure load in the first cylinder, and loading a pressure load which is the same as the maximum explosion pressure on a fire surface and a cylinder hole;
step S305, applying an explosion pressure load in the second cylinder, and loading a pressure load which is the same as the maximum explosion pressure on a fire surface and a cylinder hole;
step S306, applying an explosion pressure load in the third cylinder, and loading a pressure load which is the same as the maximum explosion pressure on a fire surface and a cylinder hole;
step S307, applying an explosion pressure load in the fourth cylinder, and loading a pressure load which is the same as the maximum explosion pressure on a fire surface and a cylinder hole;
the influence of assembly, thermal stress and gas detonation pressure on the cylinder cover is comprehensively considered in the stress analysis of the cylinder cover;
after the stress analysis and calculation of the cylinder cover are completed, four to seven analysis steps in the stress analysis, namely 1-4 cylinder ignition working conditions, are used as a high-cycle to carry out high-cycle fatigue strength analysis, the minimum fatigue safety coefficient of the cylinder cover structure is obtained, and if the high-cycle fatigue design requirement is not met, the weak position of the cylinder cover needs to be optimized until the high-cycle fatigue design requirement is met.
5. The engine cylinder head model low cycle fatigue life analysis method of claim 1, wherein:
in the step S110, a fire surface grid is extracted from the cylinder cover full model to be used as a sub model;
all grid nodes of the sub-model are named as set 1;
the grid nodes on the division surfaces of the sub-model and the full model are named as a set 2;
the full model carries out intensity calculation of working conditions of idle speed and rated power points, and temperature information of a set1 set and displacement information of a set2 set are saved in a dat format file in the calculation process;
writing an MATLAB program 1, reading and recording the temperature and displacement information of idle speed and rated power point working conditions set1 and set2 nodes in a dat file by a keyword searching method;
the temperature information of set1 and the displacement information of set2 are automatically output as files in abqinp format;
obtaining four files of displacement temperature boundaries of idle speed and rated power point working conditions, wherein the four files are respectively a set1 temperature boundary and a set2 displacement boundary under the idle speed working condition; temperature boundary of rated power point condition set1 and displacement of set 2; the displacement boundary comprises displacement of nodes in X/Y/Z directions;
the temperature information file of set1 under idle condition is named as TBCIThe temperature information file of the rated power point set1 is named TBCRThe displacement information file of set2 under idle condition is named DBCIThe displacement information file of the rated power point set2 is named DBCR。
6. The engine cylinder head model low cycle fatigue life analysis method of claim 1, wherein:
step S111, processing temperature and displacement information, and dividing the idling-rated point of the engine into a plurality of sections according to time according to the low-cycle test cycle working condition; the segmentation aims to keep consistent with a test boundary as much as possible, and because the temperature rise speed is inconsistent, a plurality of time periods are increased for transient temperature field analysis;
obtaining the temperature and displacement boundary of each time point according to different coefficients and the obtained temperature and displacement data of the idle speed and the rated point; the transient temperature field is used for analyzing the displacement and temperature boundary of different time points;
the temperature of each time point is inconsistent and needs to be respectively calculated by different coefficients;
the coefficient of each time point is obtained by processing the previous temperature field test data and experience, and the relation among different temperature values is analyzed through the idle working condition temperature, the rated power point temperature and the temperature values of different time points to determine a variable parameter K;
the working condition from idling to a rated power point represents a heating process from low temperature to high temperature, the temperature rises quickly in the front section of heating, the temperature rises slowly in the rear section, in order to keep the same as a test state as far as possible, a plurality of middle time points are added in the transient temperature field analysis, the heating and cooling process is divided into a plurality of time periods, and thus the temperature and displacement boundaries of a plurality of time points need to be obtained;
writing an MATLAB program 2, processing the four boundary files according to a low-cycle fatigue cycle working condition, wherein the temperatures and displacement boundaries at different time points can be automatically obtained through the program, and the calculation formula of the displacement and temperature boundaries in the program is as follows:
T(i)＝TR-(TR-TI)*Ki
U(i)＝UR-(UR-UI)*Ki
where T (i) is the temperature of the time node, TRIs the rated power point temperature, TIFor idle temperature, U (i) displacement of time node, URFor displacement of rated power point, UIFor idle displacement, KiAs a variable parameter, KiObtained through experimental tests and experiences;
the program forms and outputs a temperature boundary file and a displacement boundary file for each time point, saves the temperature of set1 and the displacement information of set2 at different time points, and is named as TBCiAnd DBCi。
7. The engine cylinder head model low cycle fatigue life analysis method of claim 1, wherein:
in step S112, when the cylinder head model performs stress analysis, the node on the sub-model may read the processed displacement and temperature boundary information, and perform transient stress field analysis by loading according to the time history;
the cylinder cover model carries out stress analysis, a low-cycle fatigue test cycle is divided into a plurality of time periods, and nodes on the submodel can read the temperature of set1 and the displacement boundary information of set2 at different time points after treatment;
the submodel carries out transient stress field analysis according to the time history loading displacement and the temperature boundary;
in order to enable the analysis temperature to be closer to the test temperature in the time course, the temperature rise process is divided into four time periods, the temperature reduction process is divided into three time periods, 9 time points from P1 to P9 exist in a low-cycle fatigue cycle, the duration of each time period is consistent with the test time, the temperature of a time node is also set according to the temperature rise gradient of the test, and the stress of a cylinder cover in the analysis process is consistent with the test state;
the stress field analysis of the sub-model should analyze a minimum of 3-4 cycles over the time history and use the stress-strain results of the last cycle for low cycle fatigue life analysis.
8. The engine cylinder head model low cycle fatigue life analysis method of claim 1, wherein:
judging whether the design requirement is met, checking whether the cycle life result of the low-cycle fatigue analysis is greater than the cycle number of the test specification, wherein the cycle number greater than the test specification meets the low-cycle fatigue design requirement, and the cycle number smaller than the test specification does not meet the design requirement;
step S114, verifying and guiding the low-cycle fatigue design of the cylinder cover, if the design requirement is not met, optimizing a structural region with a smaller low-cycle fatigue life of the cylinder cover structure, and recalculating the low-cycle fatigue life according to the process until the design requirement is met;
optimizing a region with a smaller low cycle fatigue life of the cylinder cover structure, recalculating the low cycle fatigue life according to a flow, mainly optimizing a structural region with the low cycle fatigue life not meeting design requirements, and optimally designing by means of thickening, chamfering and the like the structural region not meeting analysis requirements;
the relationship between the total damage and the mechanical damage, the oxidation damage and the creep damage is shown in a formula 1, and the relationship between the total service life and the mechanical fatigue service life, the oxidation fatigue service life and the creep fatigue service life is shown in a formula 2;
in the analysis process, mechanical damage, oxidation damage and creep damage are respectively calculated according to corresponding material parameters, and then the three damage values are added to obtain a total damage;
the low cycle fatigue life is the reciprocal of the damage, and the low cycle fatigue life considering creep damage, mechanical damage and oxidation damage can be obtained by the formula 2;
checking whether the cycle life result of the low-cycle fatigue analysis is greater than the cycle number of the test specification, wherein the low-cycle fatigue life needs to be greater than the cycle number of the low-cycle test specified in the test specification;
DTMF＝Dfat+Denv+Dcreep.....................1
wherein DTMFIs total injury, DfatIs a mechanical damage, DenvIs an oxidative damage, DcreepIs creep damage, Nf totalIs the total life, Nf fatIs the mechanical fatigue life, Nf envIs the oxidation fatigue life, Nf creepIs creep fatigue life;
d is damage, expressed as the reciprocal of life;
n is the lifetime, in number of times;
the range of damage is 0-1.
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