Method for predicting thermal fatigue life of engine piston based on plastic strain
By using finite element analysis based on plastic strain and a thermal fatigue life model, this paper solves the problems of high cost and long time consumption in the prediction of engine piston thermal fatigue life in the prior art, and provides an efficient and accurate prediction method that is suitable for asymmetric cyclic thermal loads.
Patent Information
- Application Number
- CN202511064065.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-07-31
- Publication Date
- 2025-11-07
AI Technical Summary
Existing technologies are costly and time-consuming in predicting the thermal fatigue life of engine pistons, and the constraint rate method has a complicated evaluation process, which is not conducive to its practical application in production.
A plastic strain-based method is used to analyze the temperature field and stress-strain field of the piston through finite element modeling, calculate the plastic strain difference, and predict the thermal fatigue life of the piston by combining it with a thermal fatigue life model.
It achieves efficient and accurate prediction of piston thermal fatigue life, with a prediction accuracy higher than that of the restraint rate method. It is low-cost and time-saving, and is suitable for asymmetric cyclic thermal loads.
Smart Images
Figure CN120911028A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the technical field of piston fatigue performance, and particularly relates to a method for predicting the thermal fatigue life of an engine piston based on plastic strain. BACKGROUND
[0002] In the fields of ships, vehicles, nuclear power, aerospace, etc., the piston will be subject to thermal fatigue failure under the action of cyclic thermal load, and then lead to the overall failure of the engine. Therefore, accurately predicting the thermal fatigue life of the engine piston is of great significance to prolong the service life of the engine.
[0003] At present, there are two methods for evaluating the thermal fatigue failure of the engine piston, one is the test method, which has the advantage of reliable results, but in the case of complex test pieces or multiple working conditions, the test method needs to consume a lot of manpower, material resources and financial resources, and the test cycle is also relatively long; the other is the prediction method, which uses the constraint ratio method to evaluate the thermal fatigue failure of the structure. The process of predicting the thermal fatigue life by using the constraint ratio method is as follows: (1) using the thermal elastoplastic finite element analysis method to determine the fatigue dangerous position of the structure; (2) calculating the constraint ratio of the fatigue dangerous position according to the thermal elastoplastic finite element analysis result; (3) combining the constraint ratio-fatigue life curve of the material under the corresponding cyclic temperature and the fatigue cumulative damage theory to obtain the fatigue life of the structure.
[0004] In order to predict the thermal fatigue life of the piston, the thermal fatigue test of the piston material is needed to obtain the thermal fatigue life model based on the constraint ratio under the corresponding cyclic temperature, which is high in cost and time-consuming, and the damage parameter in the existing constraint ratio method is complex, which makes the evaluation process very cumbersome and not conducive to popularization and use in production practice. SUMMARY
[0005] In order to solve the problem of high cost and long time consumption of the constraint ratio method for measuring the thermal fatigue life of the engine piston, the present application provides a high-efficiency and accurate method for predicting the thermal fatigue life of the engine piston based on plastic strain.
[0006] The present application is realized by the following technical solutions: A method for predicting the thermal fatigue life of an engine piston based on plastic strain is provided, comprising the following steps: S1, finite element modeling and analysis: establishing a finite element model of the piston, obtaining the temperature field distribution of the piston through transient thermal analysis; performing steady-state mechanical analysis with the temperature field as the thermal load to obtain the stress and strain field of the piston and locate the maximum tensile stress region as the fatigue dangerous position of the piston.
[0007] S2, plastic strain difference calculation: extract the plastic strain at the highest temperature, the plastic strain at the intermediate temperature and the plastic strain at the lowest temperature of the fatigue dangerous position respectively, eliminate the initial temperature influence by using the following formula and calculate the plastic strain difference of the fatigue dangerous position, ; In the formula, is the plastic strain difference, is the plastic strain at the highest temperature, is the plastic strain at the lowest temperature, is the plastic strain at the intermediate temperature.
[0008] S3, thermal fatigue life model establishment: design not less than 5 groups of material reverse thermal mechanical fatigue test, each group contains 3 tests of same temperature cycle and different mechanical strain cycle; fit the plastic strain difference and fatigue life curve according to the test data, and establish the thermal fatigue life model in logarithmic coordinate system.
[0009] S4, thermal fatigue life prediction: substitute the plastic strain difference obtained in step S2 into the model established in step S3, and calculate the piston thermal fatigue life.
[0010] The present scheme utilizes the thermal elastoplastic finite element analysis method to calculate the temperature field and stress-strain field of the structure under the action of cyclic thermal load, find the position of the maximum tensile stress, and determine the fatigue dangerous position of the structure; according to the thermal elastoplastic finite element analysis result, extract the characteristic plastic strain (the plastic strain at the highest temperature, the plastic strain at the intermediate temperature and the plastic strain at the lowest temperature) of the fatigue dangerous position, calculate the plastic strain difference of the fatigue dangerous position according to the plastic strain difference calculation formula; combine the plastic strain difference of the dangerous position of the structure and the thermal fatigue life model to predict the thermal fatigue life of the structure. The piston life predicted by the method is relatively small compared with the measured life, and the prediction accuracy of the plastic strain method is higher than that of the constraint ratio method.
[0011] Further, in step S2, the intermediate temperature is the arithmetic mean of the highest temperature and the lowest temperature.
[0012] By introducing the plastic strain at the intermediate temperature, not only the influence of the initial temperature can be eliminated, but also the actual thermal load history can be better reflected, so that the prediction result is more accurate.
[0013] Further, in step S4, if the piston involves multiple working conditions, the life under each working condition needs to be calculated first, and the total thermal fatigue life of the piston is evaluated combining the linear cumulative damage theory.
[0014] A piston thermal fatigue life prediction system, comprising: a finite element analysis module for performing the temperature field and stress-strain field calculation of step S1; a data processing module for performing the plastic strain difference calculation of step S2; a test fitting module for performing the thermal fatigue life model establishment of step S3; a life prediction module for outputting the life evaluation result of step S4.
[0015] Advantages of the present application: The present application is more accurate in the prediction method: the plastic strain difference directly represents the damage accumulation of each cycle, and is more in line with the physical mechanism than the constraint ratio method (indirect parameter), and the present application is applicable to asymmetric cycles: by introducing the plastic strain of the intermediate temperature, the actual thermal load history can be better reflected.
[0016] The present application provides a new means different from the constraint ratio piston life prediction method based on the constraint ratio, and the piston life predicted by the method is less different from the measured life, and the prediction accuracy of the plastic strain method is higher than that of the constraint ratio method. BRIEF DESCRIPTION OF DRAWINGS
[0017] Figure 1 The present application is based on the plastic strain of the thermal fatigue life prediction flowchart.
[0018] Figure 2 The circumferential tensile stress distribution of the piston throat.
[0019] Figure 3 The plastic strain of the fatigue dangerous position.
[0020] Figure 4 The linear relationship diagram of the plastic strain difference-fatigue life in the present application.
[0021] Figure 5 The error comparison table of the life prediction method of the present application and the constraint ratio method life prediction of the prior art. DETAILED DESCRIPTION
[0022] In order to clearly illustrate the technical features of the present application, the present application will be described below through specific embodiments.
[0023] Example 1: A piston thermal fatigue life prediction method based on plastic strain of an engine, comprising the following steps: S1, finite element modeling and analysis: a finite element model of the piston is established, and the temperature field distribution of the piston is obtained through transient thermal analysis; the temperature field is used as a thermal load for steady-state mechanical analysis to obtain the stress and strain field of the piston and locate the maximum tensile stress region as the fatigue dangerous position of the piston.
[0024] Taking an aluminum alloy piston as an example, the circumferential tensile stress distribution at the piston throat is calculated by using the thermo-elastic-plastic finite element method, as shown in Figure 2 From Figure 2 it can be seen that the circumferential tensile stress in different directions of the piston throat is obviously different, the circumferential tensile stress in the T direction of the throat is the smallest, and the circumferential tensile stress at 210° in the F direction of the throat is the largest, indicating that this is a fatigue dangerous position.
[0025] S2, plastic strain difference calculation: the plastic strain at the highest temperature, the plastic strain at the intermediate temperature and the plastic strain at the lowest temperature of the fatigue dangerous position are extracted respectively, and the intermediate temperature is the arithmetic average of the highest temperature and the lowest temperature. The initial temperature influence is eliminated and the plastic strain difference of the fatigue dangerous position is calculated by using the following formula, In the formula, is the plastic strain difference, is the plastic strain at the highest temperature, is the plastic strain at the lowest temperature, is the plastic strain at the intermediate temperature.
[0026] According to the results of thermo-elastic-plastic finite element calculation, as shown in Figure 3 , the plastic strain of the fatigue dangerous position is extracted, and according to the definition of plastic strain difference, the plastic strain difference of the fatigue dangerous position is calculated as 0.00268.
[0027] S3, establishment of thermal fatigue life model: design not less than 5 groups of material reverse thermal mechanical fatigue test, each group contains 3 tests of same temperature cycle and different mechanical strain cycle; fit the plastic strain difference and fatigue life curve according to the test data, and establish the thermal fatigue life model in logarithmic coordinate system.
[0028] From the aluminum alloy thermal mechanical fatigue test, the plastic strain difference-fatigue life curve is obtained, as shown in Figure 4 From Figure 4 it can be seen that in the single logarithmic coordinate system, the plastic strain difference or constraint ratio and the fatigue life show a linear relationship, and the fatigue life decreases with the increase of the plastic strain difference or constraint ratio.
[0029] S4, thermal fatigue life prediction: the plastic strain difference obtained in step S2 is substituted into the thermal fatigue life model to predict the thermal fatigue life of the piston. Substitute the data into the model established in step S3 to calculate the piston's thermal fatigue life. If the piston is involved in multiple operating conditions, the life under each condition must be calculated separately, and the total thermal fatigue life of the piston can be evaluated by combining the linear cumulative damage theory.
[0030] The calculated plastic strain difference was substituted into the thermal fatigue life model, and the calculated fatigue life was 7476 cycles.
[0031] A thermal fatigue test was conducted on the piston using a constructed thermal fatigue testing rig with cyclic temperatures ranging from 100 to 350 ℃. The results showed that after 8000 to 8900 cycles of thermal fatigue loading, cracks first appeared around the piston throat in the F direction. An average of 8450 cycles was taken as the piston's test life. Figure 5 As shown, the test life of the piston predicted by the restraint rate method is 9582 cycles. Therefore, the prediction error of the plastic strain method is -12%, while the prediction error of the restraint rate method is 13%. This method meets the practical requirement that the prediction error of thermal fatigue life does not exceed 20%.
[0032] Example 2: A piston thermal fatigue life prediction system includes: The finite element analysis module is used to perform the temperature field and stress-strain field calculations in step S1. The data processing module is used to perform the plastic strain difference calculation in step S2; The experimental fitting module is used to perform the thermal fatigue life model establishment in step S3. The lifespan prediction module is used to output the lifespan assessment results of step S4.
[0033] Of course, the above description is not limited to the examples above. Technical features not described in this invention can be implemented by or using existing technology, and will not be repeated here. The above embodiments and drawings are only used to illustrate the technical solutions of this invention and are not intended to limit this invention. This invention has been described in detail with reference to preferred embodiments. Those skilled in the art should understand that any changes, modifications, additions or substitutions made by those skilled in the art within the scope of this invention do not depart from the spirit of this invention and should also fall within the scope of protection of the claims of this invention.
Claims
1. A method of predicting the thermal fatigue life of an engine piston based on plastic strain, characterized by: The method comprises the following steps: S1, finite element modeling and analysis: a finite element model of the piston is established, and a temperature field distribution of the piston is obtained through transient thermal analysis; A steady-state mechanical analysis is performed with the temperature field as a thermal load, a stress and strain field of the piston is obtained, and a maximum tensile stress region is located as a fatigue dangerous position of the piston; S2, plastic strain difference calculation: plastic strains of the fatigue dangerous position at a highest temperature, at an intermediate temperature and at a lowest temperature are extracted respectively, an initial temperature influence is eliminated and a plastic strain difference of the fatigue dangerous position is calculated by using the following formula, ； wherein is the plastic strain difference, is the plastic strain at the highest temperature, is the plastic strain at the lowest temperature, is the plastic strain at the intermediate temperature; S3, thermal fatigue life model establishment: no less than 5 groups of reverse thermal mechanical fatigue tests of the material are designed, each group contains 3 tests of the same temperature cycle and different mechanical strain cycles; a plastic strain difference and a fatigue life curve are fitted according to test data, and a thermal fatigue life model in a logarithmic coordinate system is established; S4. Thermal fatigue life prediction: The result obtained in step S2... Substitute the model established in step S3 into the calculation of the piston thermal fatigue life.
2. The plastic strain based engine piston thermal fatigue life prediction method of claim 1, wherein: In step S2, the intermediate temperature is an arithmetic average of the highest temperature and the lowest temperature.
3. The plastic strain based engine piston thermal fatigue life prediction method of claim 1, wherein: In step S4, if the piston involves multiple working conditions, the life under each working condition needs to be calculated respectively, and the total thermal fatigue life of the piston is evaluated in combination with a linear cumulative damage theory.
4. A piston thermal fatigue life prediction system characterized by, The method comprises the following steps: A finite element analysis module is used to perform the temperature field and stress and strain field calculation of step S1 in claim 1; A data processing module is used to perform the plastic strain difference calculation of step S2 in claim 1; A test fitting module is used to perform the thermal fatigue life model establishment of step S3 in claim 1; A life prediction module is used to output the life evaluation result of step S4 in claim 1.