A method for fastener road spectrum fatigue analysis

Abstract

This invention provides a method for fatigue analysis of fasteners based on road surface parameters, belonging to the field of automotive parts performance evaluation. The method includes: Step 1, establishing a high-precision model to simulate bolt preload and contact nonlinearity, and outputting stress data files after preload locking; Step 2, reading the stress data file, extracting and processing nodal stresses, and outputting the preload stress field; Step 3, fusing the preload stress field with the operating stress and correcting the average stress offset to obtain fatigue life prediction. This method fills the gap in fatigue damage analysis around fasteners; precise positioning via time step frames avoids errors associated with traditional manual truncation; and it is applicable to similar road surface fatigue simulation analysis involving preload.

Description

Technical Field

[0001] This invention relates to the field of automotive parts performance evaluation technology, specifically to a fastener road spectrum fatigue analysis method. Background Technology

[0002] In fatigue life assessment of automotive components, road spectrum fatigue damage analysis is a crucial step. Traditional methods typically involve linearly superimposing the finite element stress results under a road spectrum load after rainflow statistics to predict the cumulative damage of components under this load. However, this method has significant limitations in practical applications. When dealing with fatigue damage around fasteners, the rigid coupling method (Rb2 coupling) simplifies bolt tightening. This simplified fastener connection often reveals abnormal stresses and damage hotspots. When assessing component reliability, these hotspots are often ignored, resulting in gaps in the fatigue analysis contour map. When dealing with stress around fasteners, bolt preload and contact nonlinearity must be considered. However, fatigue analysis only inputs the unit load stress contour map, and conventional methods cannot account for the effects of bolt preload and contact nonlinearity. Furthermore, the impact of bolt preload on component fatigue life is complex and may reduce local lifespan; directly applying biased stress for average stress correction is not advisable. Summary of the Invention

[0003] The purpose of this invention is to solve the problems existing in the prior art and to provide a bolt preload stress mapping method based on the linkage of finite element and fatigue analysis.

[0004] This invention is achieved through the following technical solution: A method for fatigue analysis of fastener road spectrum, the method comprising: Step 1: Build a high-precision model, simulate bolt preload, and output the stress data file after the preload is locked. Step 2: Read the stress data file, extract and process the nodal stress, and output the preload stress field; Step 3: Integrate the preload stress field with the working stress and correct the mean stress offset to obtain fatigue life prediction.

[0005] Furthermore, step 1 includes: Step 101: Perform detailed modeling and preload loading, and output a finite element model that includes preload and contact effects; Step 102: Generate a stress data .odb file based on the finite element model containing preload and contact effect.

[0006] Furthermore, step 101 includes: Step 1011: Construct an assembly model containing bolts, nuts, and connectors in the simulation platform; Step 1012: Based on the global mesh, refine the local mesh for the bolt root fillet, thread engagement area and contact surface to reduce the element size in critical areas; Step 1013: Define the preload section in the bolt cross section and use a step-by-step loading mechanism to simulate the actual tightening process; Step 1014: Configure contact settings to improve convergence.

[0007] Furthermore, the operation of simulating the actual tightening process in step 1013 includes: constraining the fixed point, opening the contact between the connector and the connected part, applying bolt preload; locking the preload length, modifying the boundary conditions of the next analysis step; and applying the working load.

[0008] Furthermore, step 2 includes: Step 201: Analyze the environment and perform initialization; Step 202: Save the key parameters to the state object; Step 203: Edit the Python script to obtain the preload stress field using the stress data .odb file.

[0009] Furthermore, step 203 includes: Step 2031: Read the mesh, material, load step, field variation, and time history data contained in the stress data .odb file; Step 2032: Locate the analysis step after the preload locking is completed, identify the stress data, and locate the state after the preload locking is completed; Step 2033: Lock the last incremental step of the analysis step after completion, and take the last step size; Step 2034: Extract the stress tensor data of all nodes and output the structured data full-node stress field. Step 2035: Associate the stress field of all nodes with the fatigue analysis mesh as the preload stress field.

[0010] Furthermore, step 3 includes: Step 301: Import the stress data .odb file, map the stress of each node based on the preload condition, establish a mapping relationship table between node ID and preload stress, verify the stress distribution, and output the verified preload stress field. Step 302: Perform stress field fusion; Step 303: Compensate for the average stress shift caused by the preload and perform stress correction according to the Goodman rule; Step 304: Obtain fatigue life prediction.

[0011] Furthermore, step 302 includes: Step 3021: Establish a cyclic damage calculation function and calculate fatigue damage based on the stress state; Step 3022: Obtain the nodal stress components and calculate the cyclic maximum and minimum stresses; Step 3023: Perform stress field fusion calculation.

[0012] Furthermore, the stress field fusion calculation in step 3023 includes: (1) Calculate the stress amplitude without considering preload using the following formula: stressAmplitude = abs((maxStress - minStress) / 2) Where MaxStres is the maximum stress value that occurs at a node during a fatigue loading cycle, MinStress is the minimum stress value that occurs at a node during a fatigue loading cycle, and stressAmplitude is the constant stress amplitude. (2) The static preload stress field is superimposed on the constant stress amplitude to achieve the fusion of preload and working stress, using the following formula: EquivStress = stressAmplitude + preloadStress Wherein, PreloadStress is the static average stress generated at the node by the preload, and EquivStress is the equivalent total stress amplitude after fusion.

[0013] Furthermore, step 303 involves stress correction using the following formula: EquivStress = EquivStress scaleFactor + offset Where scaleFactor is the stress scaling factor and offset is the stress offset.

[0014] The beneficial effects of this invention are: it fills the gap in fatigue damage analysis around fasteners; it avoids the errors of traditional manual truncation by accurately positioning the time step frame; and it is applicable to fatigue simulation analysis of road spectrum involving preload of the same type. Attached Figure Description

[0015] Figure 1 This is a flowchart of the overall invention.

[0016] Figure 2 This is a comparison chart of the effects of conventional analysis methods and the analysis method of the present invention, where the left side represents the conventional method and the right side represents the method of the present invention.

[0017] Figure 3 This is a flowchart of the automated stress extraction process in this invention. Detailed Implementation

[0018] The present invention will now be described in further detail with reference to the accompanying drawings: like Figure 1 As shown, this invention provides a bolt preload stress mapping method based on the linkage of finite element and fatigue analysis, specifically including the following steps: Step 1: Finite element modeling and preload calculation A high-precision model is established to simulate bolt preload and contact nonlinearity, and outputs stress data files after preload locking, providing basic data for fatigue analysis. This avoids the loss of stress hotspots caused by the simplification of traditional rigid coupling.

[0019] Step 1-1. Refined Modeling and Preload Loading 1. Geometric Modeling: Construct an assembly model containing bolts, nuts, and connectors in the Abaqus simulation platform, including: removing small chamfer features from the model, performing Boolean operations on the bolts and nuts to treat them as the same entity, and generating a 1mm hexahedral mesh.

[0020] 2. Mesh Generation: Based on the global mesh, local mesh refinement is performed on the bolt root fillet, thread engagement area and contact surface to reduce the element size in critical areas, increase convergence and accuracy. The element size is reduced to 30% of the global mesh to capture high stress gradients.

[0021] Furthermore, the mesh encryption of the present invention adopts a gradual transition, such as refining the mesh size by 1mm, the mesh size of the non-encrypted area by 3mm, and the mesh size of the common surface by 2mm.

[0022] 3. Pretension application: Using the Bolt Load module tool, define the pretension section in the bolt cross-section, and use a step-by-step loading mechanism to simulate the actual tightening process: Step 1: Apply preload, constrain the fixing point, open the contact between the connector and the connected part, and apply bolt preload; Step 2: Lock the preload length and modify the boundary conditions for the next analysis step using the load function in the Abaqus simulation platform; Step 3: Apply the operating load.

[0023] 4. To improve convergence, the following contact settings are implemented: frictional contact is defined through the bolt-connector interface with a friction coefficient μ=0.17. An augmented Lagrangian algorithm is used to avoid slip distortion, and the contact stiffness factor is set to 0.01 to improve convergence.

[0024] Step 1-2. Solving and Output Configuration Based on the finite element model containing preload and contact effect from step 1-1, generate a stress data file that can be used for subsequent processing.

[0025] Specifically, static analysis is selected to output the steady-state stress field after the preload is locked, while transient analysis is selected for dynamic load superposition conditions.

[0026] Finally, export the stress data file in .odb (Abaqus) format, which must include nodal stress fields, output (stress components such as S11, S12, and S13), and element topology information.

[0027] Step 2: Nodal stress extraction and processing The CustomEngineMethods script file was edited in the advanced edit using the Custcomanalyse module in ncode to perform subsequent preload stress field mapping and fatigue analysis processing steps.

[0028] Step 2-1. Analyze Environment Initialization 1. Open the log file and record the analysis process; 2. Obtain material properties: material name, material type; 3. Obtain method attribute parameters: stress scaling factor, stress offset, calculation options.

[0029] Step 2-2. Save key parameters Save material properties to a state object, including: b1 fatigue strength index, SRI1 fatigue strength coefficient, and uts ultimate tensile strength; Save method parameters to the state object, including: stress scaling factor, stress offset, and calculation options.

[0030] Specifically, the preload stress is read and saved using the following calculation method: preloadStress= CustomEngineMethodState.GetPreloadStressAt() CustomEngineMethodState.AppendVariable(preloadStress) Steps 2-3. Automated stress data extraction A Python script was developed based on the Abaqus API to directly read the nodal stress field of the locked step from a .odb file. The process is as follows: Figure 3 As shown.

[0031] 1. Read .odb files The .odb file is an Abaqus binary results database containing mesh, material, load step, field variables (stress / strain, etc.) and time history data. The .odb file is opened using the Abaqus API's session.openOdb() method.

[0032] 2. Specify the preload analysis step and identify stress data. Specifically, the analysis step is located after the preload locking is completed. This step includes the steady-state stress field caused by the preload, and the stress data is identified to locate the state after the preload locking is completed.

[0033] 3. Take the last step size After the preload is locked, the analysis step may contain multiple incremental steps (such as the loading process), but only the final state after locking is completed (i.e., the last incremental step) is needed, that is, to locate the state after the preload is locked. Specifically, all frames are obtained through lock_step.frames, and the last frame (index -1) is taken.

[0034] 4. Output of stress field at all nodes Extract stress tensor data from all nodes (including the state after preload locking and the final step size), and output as structured data (such as CSV or directly to fatigue analysis software).

[0035] Specifically, first, the nodal stress ('S' field variable) is read from the last frame as a stress field object; then, all nodes are traversed, and the stress components of each node are extracted. The stress components are stored in the form of a matrix or array.

[0036] 5. Stress field mapping The stress field at all nodes is associated with the fatigue analysis mesh to ensure spatial consistency, thus forming a pre-tightening stress field.

[0037] Step 3: Preload stress field mapping and fatigue analysis This step imports the preload stress field into the fatigue analysis software (nCode), merges it with the working stress, and corrects for the mean stress offset, ultimately obtaining a more accurate fatigue life prediction. It also addresses the complex influence of bolt preload on life (which may increase or decrease local life).

[0038] Step 3-1. Introduction and Verification of Preload Stress Field Add the FE Input module to the nCode flow to import stress data (.odb file) results. Map the stress of each node to the offset based on the preload condition, using the formula: preloadStress = CustomEngineMethodState.GetVariable(). Establish a mapping table between node ID and preload stress. The stress distribution is verified using the FE Display visualization tool. The verification criteria are: the bolt root shows symmetrical tensile stress concentration, and the contact surface of the clamped part shows compressive stress distribution; otherwise, contact failure is determined. The final output is the verified preload stress field.

[0039] Step 3-2. Stress field fusion Calculate cyclic damage in the CustomAnalyse module.

[0040] 1. First, establish a cyclic stress analysis framework: construct a cyclic damage calculation function and calculate fatigue damage based on the stress state.

[0041] 2. Traverse all nodes and load cases to obtain the nodal stress components, including: obtaining the stress components of all nodes: stress in direction 1, stress in direction 2, and shear stress in direction 12; calculate the cyclic maximum and minimum stresses using the formula: maxStress, minStress = GetCycleExtremes().

[0042] 3. Perform stress field fusion calculations, including: 1) Calculate the stress variation amplitude caused only by external working loads (such as vibration and pressure fluctuations) without considering preload, i.e., equivalent stress (stress amplitude), using the formula: stressAmplitude = abs((maxStress - minStress) / 2) Among them, MaxStress and MinStress are the maximum and minimum stress values ​​that occur at a node during a fatigue load cycle, and stressAmplitude represents the intensity of the dynamic load.

[0043] 2) Based on the options, the equivalent stress is processed by using the static preload stress field as a "reference" or "offset" and directly superimposed on the dynamic working stress amplitude to achieve the fusion of preload and working stress. The formula is: EquivStress = stressAmplitude + preloadStress Among them, PreloadStress comes from finite element analysis and is the static average stress generated by the preload at the node; EquivStress is the equivalent total stress amplitude after fusion.

[0044] Step 3-3. Mean stress correction: Enable the Goodman model to compensate for the mean stress offset caused by the preload. The mean stress is corrected according to the Goodman rules.

[0045] This invention uses a stress scaling factor and an offset to correct the Goodman mean stress, using the following formula: EquivStress = EquivStress scaleFactor + offset Where EquivStress is the equivalent total stress amplitude after correction by the Goodman model, scaleFactor is the stress scaling factor, and offset is the stress offset.

[0046] Steps 3-4. Obtain fatigue life prediction.

[0047] Damage is calculated and fatigue life is predicted using the modified equivalent stress (including preload stress) and the material SN curve.

[0048] Conventional road fatigue simulation methods can only use unit load. The fatigue stress obtained using the road spectrum coefficient method cannot reflect the influence of preload, such as bolt preload, on fatigue damage. This invention's new method uses Python to customize and modify the fatigue stress calculation method, mapping preload to fatigue stress, thereby optimizing the fatigue results in the average stress correction method. Figure 2 As shown in the figure, the numerical labels directly reveal the difference in the effectiveness of the two methods in quantitative analysis. The data in the figure are summarized in the table below.

[0049] Table 1

[0050] This comparison chart clearly illustrates: 1. Conventional analytical methods have fatal flaws: Figure 2 The visualization of the model on the left side, using a conventional method, shows that all colors are concentrated in the green range and below, completely excluding the red area. This means that, according to this model, there are no "extremely high damage" points on the part.

[0051] Conventional analysis methods offer a false sense of security, showing a low overall level of damage to the part, which may lead engineers to believe the design is safe. Due to its methodological limitations, it completely fails to identify the weakest link in the part, namely the high-stress concentration area at the edge of the central hole.

[0052] 2. The new method has revolutionary advantages: Figure 2 The visualization of the new method of this invention on the right side of the model jumps directly into the red zone, clearly indicating the existence of a fatal weakness where the damage value is close to the upper limit.

[0053] The novel method of this invention provides precise localization, enabling it to extremely accurately capture the highest damage areas that are overlooked by conventional methods. It not only pinpoints the risk but also provides damage values ​​that are two orders of magnitude higher and closer to the actual damage (48.54 vs 0.89). Based on the results of this new method, engineers can accurately reinforce the structure of this high-damage area (e.g., by chamfering optimization, increasing wall thickness), thereby preventing potential product failures.

[0054] The above technical solution is only one embodiment of the present invention. For those skilled in the art, based on the principles disclosed in the present invention, it is easy to make various types of improvements or modifications, and not limited to the technical solutions described in the specific embodiments of the present invention. Therefore, the foregoing description is only a preferred option and is not restrictive.

Claims

1. A method for fatigue analysis of fastener profiles, characterized in that: The method includes: Step 1: Build a high-precision model, simulate bolt preload, and output the stress data file after the preload is locked. Step 2: Read the stress data file, extract and process the nodal stress, and output the preload stress field; Step 3: Integrate the preload stress field with the working stress and correct the mean stress offset to obtain fatigue life prediction.

2. The fastener path spectrum fatigue analysis method according to claim 1, characterized in that: Step 1 includes: Step 101: Perform detailed modeling and preload loading, and output a finite element model that includes preload and contact effects; Step 102: Generate a stress data .odb file based on the finite element model containing preload and contact effect.

3. The fastener path spectrum fatigue analysis method according to claim 2, characterized in that: Step 101 includes: Step 1011: Construct an assembly model containing bolts, nuts, and connectors in the simulation platform; Step 1012: Based on the global mesh, refine the local mesh for the bolt root fillet, thread engagement area and contact surface to reduce the element size in critical areas; Step 1013: Define the preload section in the bolt cross section and use a step-by-step loading mechanism to simulate the actual tightening process; Step 1014: Configure contact settings to improve convergence.

4. The fastener path spectrum fatigue analysis method according to claim 3, characterized in that: The simulated tightening process in step 1013 includes: constraining the fixed point, opening the contact between the connector and the connected part, applying bolt preload; locking the preload length, modifying the boundary conditions for the next analysis step; and applying the working load.

5. The fastener path spectrum fatigue analysis method according to claim 3 or 4, characterized in that: Step 2 includes: Step 201: Analyze the environment and perform initialization; Step 202: Save the key parameters to the state object; Step 203: Edit the Python script to obtain the preload stress field using the stress data .odb file.

6. The fastener path spectrum fatigue analysis method according to claim 5, characterized in that: Step 203 includes: Step 2031: Read the mesh, material, load step, field variation, and time history data contained in the stress data .odb file; Step 2032: Locate the analysis step after the preload locking is completed, identify the stress data, and locate the state after the preload locking is completed; Step 2033: Lock the last incremental step of the analysis step after completion, and take the last step size; Step 2034: Extract the stress tensor data of all nodes and output the structured data full-node stress field. Step 2035: Associate the stress field of all nodes with the fatigue analysis mesh as the preload stress field.

7. The fastener path spectrum fatigue analysis method according to claim 6, characterized in that: Step 3 includes: Step 301: Import the stress data .odb file, map the stress of each node based on the preload condition, establish a mapping relationship table between node ID and preload stress, verify the stress distribution, and output the verified preload stress field. Step 302: Perform stress field fusion; Step 303: Compensate for the average stress shift caused by the preload and perform stress correction according to the Goodman rule; Step 304: Obtain fatigue life prediction.

8. The fastener path spectrum fatigue analysis method according to claim 7, characterized in that: Step 302 includes: Step 3021: Establish a cyclic damage calculation function and calculate fatigue damage based on the stress state; Step 3022: Obtain the nodal stress components and calculate the cyclic maximum and minimum stresses; Step 3023: Perform stress field fusion calculation.

9. The fastener path spectrum fatigue analysis method according to claim 8, characterized in that: The stress field fusion calculation in step 3023 includes: (1) Calculate the stress amplitude without considering preload using the following formula: stressAmplitude = abs((maxStress - minStress) / 2) Where MaxStres is the maximum stress value that occurs at a node during a fatigue loading cycle, MinStress is the minimum stress value that occurs at a node during a fatigue loading cycle, and stressAmplitude is the constant stress amplitude. (2) The static preload stress field is superimposed on the constant stress amplitude to achieve the fusion of preload and working stress, using the following formula: EquivStress = stressAmplitude + preloadStress Wherein, PreloadStress is the static average stress generated at the node by the preload, and EquivStress is the equivalent total stress amplitude after fusion.

10. The fastener path spectrum fatigue analysis method according to claim 9, characterized in that: Step 303 involves stress correction using the following formula: EquivStress = EquivStress scaleFactor + offset Where scaleFactor is the stress scaling factor and offset is the stress offset.

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