Method for establishing fatigue failure prediction model of nose frame structure and prediction method

Abstract

The present application relates to the field of aircraft structure fatigue analysis, in order to realize the fatigue life prediction of nose frame structure under multi-axial load combination, the method and the method for establishing the prediction model of nose frame structure fatigue failure are provided, the entity finite element model of frame vulnerable part is established, the shell element in the fine finite element model is replaced, and the shell-solid coupling model with calculation precision and calculation efficiency is formed;Then all the element internal integration points life exceeds the material life in the next step calculation unit is deleted, to simulate the material fatigue progressive damage, and the inflection point on the combined load analysis step failure unit quantity corresponding relation curve is taken as the nose frame structure fatigue failure criterion;The axial force and bending moment of the nose frame are taken as the independent variable, and the corresponding fatigue life is taken as the dependent variable, and the power function form of the nose frame life expression is fitted, the fatigue life prediction of the nose frame structure under multi-axial load combination is realized, and the calculation efficiency is high.

Description

Technical Field

[0001] This invention relates to the field of aircraft structural fatigue analysis, specifically a method for establishing a fatigue failure prediction model and a prediction method for a nose frame structure. Background Technology

[0002] Fatigue analysis of aircraft structures is a crucial method for evaluating whether structural lifespan meets design requirements. Accurate and rapid calculation of structural fatigue life directly impacts aircraft safety and is a key focus in aircraft structural design. In the field of aircraft structural fatigue strength design, the nose frame structure, as a critical load-bearing component, often experiences complex load patterns. Traditional fatigue analysis of nose frame structures typically uses a stiffness equivalent model to calculate the stress state of the frame structure, assuming that the frame flanges and webs are subjected to different uniaxial loads to calculate the frame structure's lifespan. This differs from the loading patterns of the nose frame structure, resulting in an inaccurate description of the frame structure's fatigue performance under complex load combinations. Summary of the Invention

[0003] To achieve fatigue life prediction of the nose frame structure under multi-axis load combinations, this invention provides a method for establishing a fatigue failure prediction model for the nose frame structure and a prediction method.

[0004] The technical solution adopted by the present invention to solve the above problems is:

[0005] The method for establishing a fatigue failure prediction model for the nose frame structure includes:

[0006] Step 1: Establish a machine head frame-shell coupling model based on the fatigue-vulnerable parts of the frame structure, and determine the load range of the vulnerable parts of the machine head frame;

[0007] Step 2: Generate different load combinations based on the load range of vulnerable parts of the head frame, calculate the shell-solid coupling model of the head frame under different load combinations, obtain the stress state of each solid element, and simulate the fatigue failure process by combining the SN curve of the frame structure material.

[0008] Step 3: Based on the fatigue failure process simulation results, determine the inflection point of the number of failure units corresponding to each load combination, and determine the fatigue life under each load combination based on the inflection point; the inflection point is: before the inflection point, the number of failure units increases slowly, and after the inflection point, the number of failure units increases rapidly.

[0009] Step 4: Using axial force and bending moment as independent variables and fatigue life as the dependent variable, fit the data to obtain a fatigue failure prediction model for the head frame structure.

[0010] Further, step 1 includes:

[0011] Step 11: Establish a refined finite element model of the nose frame structure and perform static strength calculations on the nose frame structure under airtight conditions; the refined finite element model is a full shell element model.

[0012] Step 12: Based on the static strength calculation results and structural characteristics, determine the fatigue-prone parts of the frame structure and their axial force and bending moment ranges;

[0013] Step 13: Model the fatigue-prone parts using solid elements and replace the shell elements in the fine finite element model to form an initial shell-solid coupling model;

[0014] Step 14: Perform static calculations on the initial shell-solid coupling model, keeping the loads and boundary conditions consistent with the refined finite element model;

[0015] Step 15: Adjust the initial shell-solid coupling model based on the static calculation results until it is consistent with the stress distribution pattern and values ​​of the refined finite element model.

[0016] Furthermore, the detailed finite element model includes the head frame, skin panels, and floor beams and columns.

[0017] Furthermore, in step 13, the replaced solid unit is connected to the original shell unit through shell-solid coupling.

[0018] Furthermore, step 2 uses experimental design methods to generate different combinations of axial force and bending moment.

[0019] Furthermore, for each load combination, the fatigue failure simulation process includes:

[0020] Step 21: In each incremental step, use the isotropic linear elastic material constitutive model to calculate the Von-Mises equivalent stress at all integration points of the head frame structural solid element;

[0021] Step 22: Substitute the Von-Mises stress calculated at each integration point of the machine head frame structural solid unit into the corresponding SN curve of the material to obtain the theoretical fatigue life of each integration point under the current stress level;

[0022] Step 23: Multiply the analysis step time in the finite element simulation by a scaling factor to convert it into the actual number of cyclic loading cycles.

[0023] Step 24: Compare the calculated number of cyclic loading cycles with the theoretical fatigue life of each integration point. If the cumulative number of cycles at an integration point exceeds its theoretical fatigue life, then the integration point is determined to have experienced fatigue failure and is marked as a failure state.

[0024] Step 25: When all integration points inside a certain solid element are marked as failure, delete this element in the shell-solid coupling model.

[0025] Step 26: Repeat steps 21 to 25 until the structure reaches a critical state due to accumulated damage.

[0026] Furthermore, step 3 includes:

[0027] Step 31: For each load combination, iterate through all analysis steps and count the number of failure elements corresponding to each analysis step;

[0028] Step 32: Based on the statistical results, plot the curve showing the relationship between the number of analysis steps and the number of failed units;

[0029] Step 33: Determine the inflection point based on the corresponding relationship curve;

[0030] Step 34: Determine fatigue life based on the inflection point.

[0031] Furthermore, step 3 specifically involves:

[0032] Among all load combinations, select typical load combinations to plot the curves showing the relationship between analysis steps and the number of failed elements and determine the inflection points. Calculate the percentage of failed elements at the inflection point of each typical load combination relative to the total number of elements. Take the average of the percentages corresponding to the typical load combinations as the common rule. Determine the fatigue life under other load combinations based on the common rule.

[0033] Furthermore, step 4 is based on the power function. The data is fitted using a form where Life represents the predicted fatigue life, T represents the axial force, M represents the bending moment, and a, b, and c are fitting parameters.

[0034] Methods for predicting fatigue failure of the nose frame structure include:

[0035] Step 1: Obtain the load combination;

[0036] Step 2: Perform fatigue failure prediction based on the fatigue failure prediction model of the nose frame structure. The fatigue failure prediction model of the nose frame structure is established using the method for establishing a fatigue failure prediction model of the nose frame structure.

[0037] The advantages of this invention compared to existing technologies are as follows: A solid finite element model is established for the vulnerable parts of the frame, replacing the shell elements in the refined finite element model to form a shell-solid coupling model that combines computational accuracy and efficiency. Then, elements whose integral point lifetimes exceed the material lifetime are deleted in the next calculation step to simulate progressive material fatigue damage. The inflection point on the curve corresponding to the number of failed elements in the combined load analysis step is used as the fatigue failure criterion for the nose frame structure. The axial force and bending moment of the nose frame are used as independent variables, and the corresponding fatigue lifetime is used as the dependent variable. A power function form expressing the nose frame lifetime is fitted, realizing fatigue lifetime prediction of the nose frame structure under multi-axis load combinations with high computational efficiency. Attached Figure Description

[0038] Figure 1 A method for establishing a fatigue failure prediction model for the nose frame structure;

[0039] Figure 2 This is a schematic diagram of the SN curve for aluminum alloy materials;

[0040] Figure 3 This is a schematic diagram showing the change in the number of failure units in the headstock frame as the analysis step progresses.

[0041] Figure 4 This is a schematic diagram of the fitted surface. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0043] This invention establishes a solid finite element model for the vulnerable parts of the frame, replacing the shell elements in the refined finite element model to form a shell-solid coupled model that combines computational accuracy and efficiency. Then, elements whose integral point lifetimes exceed the material lifetime are deleted in the next calculation to simulate progressive fatigue damage. The "inflection point" on the curve corresponding to the number of failed elements in the combined load analysis step is used as the fatigue failure criterion for the nose frame structure. The axial force and bending moment of the nose frame are used as independent variables, and the corresponding fatigue lifetime as the dependent variable, fitting a power function form to express the nose frame's lifespan. This method accurately characterizes the structural stress state using a finite element model under multiaxial loads, simulates progressive structural damage by combining material fatigue performance and element deletion methods, characterizes fatigue life using the relationship between load-bearing capacity and the number of failed elements (i.e., load-bearing capacity), and finally fits a function expression using the relationship between multiaxial loads and lifespan. This expression enables rapid prediction of the frame structure's fatigue life, solving the problem that the uniaxial stress SN curve of the nose frame structure cannot accurately represent its multiaxial stress state under actual complex loads. This provides a reliable theoretical tool for improving the safety and design accuracy of aircraft structures.

[0044] like Figure 1 As shown, the method for establishing a fatigue failure prediction model for the nose frame structure includes:

[0045] Step 1: Establish a shell-solid coupling model based on the fatigue-prone parts of the frame structure, and determine the load range of the vulnerable parts of the nose frame, specifically including:

[0046] Step 1-1: Taking the nose frame of a large transport aircraft as an example, the nose frame mainly bears the airtight load. A detailed finite element model of the nose frame structure is established, which mainly includes the nose frame, skin panels, floor beams and columns, etc. Static strength calculation is performed on the nose frame structure under the condition of 2 times airtightness.

[0047] Steps 1-2: Read and analyze the stress state of the machine head frame structure in the fine finite element model, and determine the fatigue-prone parts of the frame structure based on the structural characteristics. In this embodiment, the maximum stress of the machine head frame occurs above the connection with the floor beam.

[0048] Steps 1-3: Use solid elements with a unit size of 3mm (unit type C3D8) to model the fatigue-prone parts of the frame structure and replace the shell elements in the original fine model.

[0049] Steps 1-4: In the fine finite element model of the machine head frame, connect the solid elements of the fatigue-prone parts of the frame with the shell elements of the rest of the frame through shell-solid coupling.

[0050] Steps 1-5: Perform static calculations on the shell-solid coupling model, keeping the loads and boundary conditions consistent with the fine finite element model.

[0051] Steps 1-6: Analyze the static calculation results of the shell-solid coupling model. The results are consistent with the stress distribution pattern (the maximum stress location is above the connection between the frame and the floor beam) and the values ​​(the maximum stress is 324 MPa) of the fine finite element model.

[0052] Step 2: Generate different load combinations based on the load range of vulnerable parts of the head frame. For each load combination, calculate the shell-solid coupling model of the head frame under different load combinations to obtain the stress state of each solid element. Combine the SN curve of the frame structure material to simulate the fatigue failure process.

[0053] In this embodiment, the axial force of the head frame ranges from 5000N to 8000N, and the bending moment ranges from 5N·m to 8N·m; 66 combinations of different axial forces and bending moments are generated using experimental design methods.

[0054] The fatigue failure simulation process specifically includes:

[0055] Step 2-1: In each incremental step, use the isotropic linear elastic material constitutive model to compute the Von-Mises equivalent stress at all integration points of the head frame structural solid element, thereby characterizing the stress state of the material at that point.

[0056] Step 2-2: The head frame material is 7075-T6 aluminum alloy, and its material SN curve is as follows. Figure 2 As shown, the Von-Mises stress calculated at each integration point of the head frame structural solid unit is substituted into the corresponding SN curve of the material to calculate the theoretical fatigue life of each integration point at the current stress level.

[0057] Steps 2-3: Multiply the analysis step time in the finite element simulation by a scaling factor to convert it into the actual number of cyclic loading cycles, i.e., the "cycle skipping" technique, to improve computational efficiency. The specific scaling factor can be selected according to actual needs and is not restricted here.

[0058] Steps 2-4: Compare the actual number of cycles calculated under fatigue conditions with the fatigue life of each integration point in each unit of each analysis step. Once the cumulative number of cycles of an integration point exceeds its fatigue life, it is determined that the integration point has experienced fatigue failure and is marked.

[0059] Steps 2-5: When all integration points inside a certain solid element are marked as failure, it is considered that the element has completely lost its load-bearing capacity. This element is then deleted from the model to simulate the progressive damage caused by material fatigue.

[0060] Step 2-6: Repeat steps 2-1 to 2-5. As failure elements are continuously generated and deleted, the stiffness and load-bearing capacity of the high-stress level area continue to decrease. This cycle continues until the structure reaches a critical state due to accumulated damage and suffers overall fatigue instability failure, at which point the calculation terminates.

[0061] Step 3: Based on the simulation results of the fatigue failure process, determine the inflection point of the number of failure units corresponding to each load combination, and determine the fatigue life under each load combination based on the inflection point; the inflection point is: before the inflection point, the number of failure units increases slowly, and after the inflection point, the number of failure units increases rapidly.

[0062] In this embodiment, four working conditions were set for the vulnerable parts of the machine head frame for calculation. Under each working condition, the axial force and bending moment of the vulnerable part frame were (5000N, 5N·m), (6000N, 6N·m), (7000N, 7N·m), and (8000N, 8N·m), respectively. The number of failure elements corresponding to each analysis step was counted, and the relationship curve between the analysis step and the number of failure elements was plotted. The change of the number of failure elements of the machine head frame with the analysis step under the above four working conditions is as follows: Figure 3 As shown, the relationship between the analysis step and the number of failed units is analyzed to determine the "inflection point" on the curve. Before this inflection point, the number of failed units increases slowly, but once this point is crossed, the damage accelerates rapidly, the number of failed units rises sharply, and eventually the overall structure becomes unstable. The inflection point can be determined by the slope of the curve. In this embodiment, the position where the slope of the first analysis step exceeds 1 is determined as the inflection point. This can be adjusted according to actual needs and is not limited here. Under the above four load conditions, the number of failed units at the inflection point accounts for approximately 2% of the total number of units. This is taken as a common rule, and based on this common rule, "2% unit failure" is established as the critical criterion for the fatigue life of the head frame. The shell-solid coupling model of the vulnerable parts of the head frame under different combinations of axial force and bending moment is calculated, and the fatigue life under each load combination is calculated using the critical criterion. Typical load conditions can be set based on experience. By analyzing typical load conditions, the critical criterion is determined, and the fatigue life of other load combinations can be directly calculated based on the critical criterion, effectively improving the calculation efficiency.

[0063] Step 4: Using axial force and bending moment as independent variables and fatigue life as the dependent variable, fit the data to obtain a fatigue failure prediction model for the head frame structure.

[0064] In this embodiment, based on the power function The data is fitted using the following format: Life represents the predicted fatigue life, T represents the axial force, M represents the bending moment, and a, b, and c are the fitting parameters. The fitted surface is shown in Figure 1. Figure 4 As shown, the expression for the power function is: It was used as a fatigue failure prediction model for the head frame structure of the aircraft.

[0065] Correspondingly, this embodiment also provides a method for predicting fatigue failure of the head frame structure, including:

[0066] Step 1: Obtain the load combination;

[0067] Step 2: Perform fatigue failure prediction based on the fatigue failure prediction model of the nose frame structure. The fatigue failure prediction model of the nose frame structure is established using the method for establishing a fatigue failure prediction model of the nose frame structure.

Claims

1. A method for establishing a fatigue failure prediction model for a machine head frame structure, characterized in that, include: Step 1: Establish a machine head frame-shell coupling model based on the fatigue-vulnerable parts of the frame structure, and determine the load range of the vulnerable parts of the machine head frame; Step 2: Generate different load combinations based on the load range of vulnerable parts of the nose frame, calculate the shell-solid coupling model of the nose frame under different load combinations, obtain the stress state of each solid element, and simulate the fatigue failure process by combining the SN curve of the frame structure material; for each load combination, the fatigue failure simulation process includes: Step 21: In each incremental step, use the isotropic linear elastic material constitutive model to calculate the Von-Mises equivalent stress at all integration points of the head frame structural solid element; Step 22: Substitute the Von-Mises stress calculated at each integration point of the machine head frame structural solid unit into the corresponding SN curve of the material to obtain the theoretical fatigue life of each integration point under the current stress level; Step 23: Multiply the analysis step time in the finite element simulation by a scaling factor to convert it into the actual number of cyclic loading cycles. Step 24: Compare the calculated number of cyclic loading cycles with the theoretical fatigue life of each integration point. If the cumulative number of cycles at an integration point exceeds its theoretical fatigue life, then the integration point is determined to have experienced fatigue failure and is marked as a failure state. Step 25: When all integration points inside a certain solid element are marked as failure, delete this element in the shell-solid coupling model. Step 26: Repeat steps 21 to 25 until the structure reaches a critical state due to accumulated damage; Step 3: Based on the fatigue failure process simulation results, determine the inflection point of the number of failure units corresponding to each load combination, and determine the fatigue life under each load combination based on the inflection point; the inflection point is: before the inflection point, the number of failure units increases slowly, and after the inflection point, the number of failure units increases rapidly. Step 4: Using axial force and bending moment as independent variables and fatigue life as the dependent variable, fit the data to obtain a fatigue failure prediction model for the head frame structure.

2. The method for establishing a fatigue failure prediction model for the head frame structure according to claim 1, characterized in that, Step 1 includes: Step 11: Establish a refined finite element model of the nose frame structure and perform static strength calculations on the nose frame structure under airtight conditions; the refined finite element model is a full shell element model. Step 12: Based on the static strength calculation results and structural characteristics, determine the fatigue-prone parts of the frame structure and their axial force and bending moment ranges; Step 13: Model the fatigue-prone parts using solid elements and replace the shell elements in the fine finite element model to form an initial shell-solid coupling model; Step 14: Perform static calculations on the initial shell-solid coupling model, keeping the loads and boundary conditions consistent with the refined finite element model; Step 15: Adjust the initial shell-solid coupling model based on the static calculation results until it is consistent with the stress distribution pattern and values ​​of the refined finite element model.

3. The method for establishing a fatigue failure prediction model for the head frame structure according to claim 2, characterized in that, The detailed finite element model includes the head frame, skin panels, and floor beams and columns.

4. The method for establishing a fatigue failure prediction model for the head frame structure according to claim 2, characterized in that, In step 13, the replaced solid unit is connected to the original shell unit through shell-solid coupling.

5. The method for establishing a fatigue failure prediction model for the head frame structure according to claim 2, characterized in that, Step 2 uses experimental design methods to generate different combinations of axial force and bending moment.

6. The method for establishing a fatigue failure prediction model for the head frame structure according to claim 1, characterized in that, Step 3 includes: Step 31: For each load combination, iterate through all analysis steps and count the number of failure elements corresponding to each analysis step; Step 32: Based on the statistical results, plot the curve showing the relationship between the number of analysis steps and the number of failed units; Step 33: Determine the inflection point based on the corresponding relationship curve; Step 34: Determine fatigue life based on the inflection point.

7. The method for establishing a fatigue failure prediction model for the head frame structure according to claim 6, characterized in that, Step 3 specifically involves: Among all load combinations, select typical load combinations to plot the curves showing the relationship between analysis steps and the number of failed elements and determine the inflection points. Calculate the percentage of failed elements at the inflection point of each typical load combination relative to the total number of elements. Take the average of the percentages corresponding to the typical load combinations as the common rule. Determine the fatigue life under other load combinations based on the common rule.

8. The method for establishing a fatigue failure prediction model for the head frame structure according to claim 7, characterized in that, Step 4 is based on the power function. The data is fitted using a form where Life represents the predicted fatigue life, T represents the axial force, M represents the bending moment, and a, b, and c are fitting parameters.

9. A method for predicting fatigue failure of a machine head frame structure, characterized in that, include: Step 1: Obtain the load combination; Step 2: Perform fatigue failure prediction based on the fatigue failure prediction model of the nose frame structure. The fatigue failure prediction model of the nose frame structure is established using the fatigue failure prediction model establishment method of the nose frame structure as described in any one of claims 1-8.

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