A method for calculating the life of fastener holes under complex loading conditions

Abstract

This invention belongs to the field of aircraft structural fatigue life technology, specifically relating to a method for calculating the life of fastening holes under complex loading conditions. It includes the following steps: Step 1: Establishing a finite element model of the test structure and surrounding structures; Step 2: Simplifying the fasteners within the test structure and surrounding structures, adding nail connection information, and performing finite element analysis; Step 3: Extracting the upper and lower surface stresses—nail shear force P—of the fastening holes in the test structure from the finite element analysis results. pin剪力 and the axial tensile force P of the nail pin轴向拉力 Step 4: Obtain the stress concentration factor K of the bypass load in the fastening hole. TG Step 5: Obtain the equivalent stud stress concentration factor K for the MPC connection. tPinMPC Step 6: Calculate the reference stress of the fastening hole; Step 7: Calculate the fastening hole life based on the nail shear force and the reference stress of the fastening hole.

Description

Technical Field

[0001] This invention belongs to the field of aircraft structural fatigue life technology, specifically relating to a method for calculating the life of fastening holes under complex load conditions. Background Technology

[0002] The engineering calculation method for fastening hole life is based on fatigue tests of typical fastening hole details. When using it, only the relevant geometric dimensions of the fastening hole connection, bypass loads, stud shear force, and out-of-plane bending moment need to be considered. Due to its ease of use and sufficient analytical accuracy for typical structures, it is widely used in engineering practice. However, actual aircraft structures are often subjected to complex loads, especially structural parts with large curvature. In these parts, the shear and tensile forces on the fasteners are relatively large, and the fastening holes also bear additional bending moments due to their shape. Furthermore, various load components are large and cannot be ignored. Currently, there is a lack of effective methods to comprehensively consider all load components. Due to the complexity of the structural loads and the obstruction of the holes by the fasteners, it is difficult to obtain accurate stress-strain states of the fastening holes through methods such as strain measurement. Using simplified engineering methods results in significant errors. There is an urgent need for a fastening hole life calculation method that, based on traditional methods, is applicable to complex geometric shapes and complex stress states, to solve practical engineering application problems. Summary of the Invention

[0003] The purpose of this invention is to provide a method for calculating the life of fastening holes under complex loading conditions, based on simulation analysis of the stress-strain history of the fastening holes, in order to calculate the life of fastening holes under complex loading conditions more accurately.

[0004] This invention considers fastener connections through simplified connections that are feasible in engineering. It studies stress concentration calculations under various loading conditions such as compression and bending by using simulation analysis that takes into account the contact between parts, materials, and geometric nonlinearity. It establishes corresponding engineering correction algorithms and proposes a method for calculating the life of fastener holes under complex loading conditions.

[0005] The technical solution of the present invention:

[0006] A method for calculating the life of fastening holes under complex loading conditions includes the following steps:

[0007] Step 1: Establish a finite element model of the assessment structure and its surrounding structures;

[0008] Step 2: Simplify the fasteners in the test structure and surrounding structures, add nail connection information, and perform finite element analysis;

[0009] Step 3: Extract the upper and lower surface stresses of the fastening holes in the test structure from the finite element analysis results. nail load shear force P pin剪力 and the axial tensile force P of the nail pin轴向拉力 ;

[0010] Step 4: Obtain the stress concentration factor K of the bypass load in the fastening hole. TG ;

[0011] Step 5: Obtain the equivalent stud stress concentration factor K for the MPC connection. tPinMPC ;

[0012] Step Six: Calculate the reference stress of the fastening hole;

[0013] Step 7: Calculate the life of the fastening hole based on the shear force of the nail and the reference stress of the fastening hole.

[0014] Furthermore, in step one, the material properties of the assessment structure and its surrounding structure are defined, as well as the contact relationships between the various parts in the assessment structure and its surrounding structure.

[0015] Boundary loads are applied to the model boundaries to constrain rigid body displacements;

[0016] The output of the finite element model of the assessment structure and its surrounding structures includes: mesh, load, contact relationship, and constraint information.

[0017] Furthermore, in step two, when the fastener is simplified, it is simplified into a beam element, and multi-point constraint MPC is used to connect the beam element and the connection structure in layers.

[0018] The nail connection information includes: the material, element, node, and node information of the beam element and the MPC connection.

[0019] Furthermore, in step three, the upper surface of the fastening hole refers to the upper surface of the test structure, and the lower surface refers to the lower surface of the test structure.

[0020] Furthermore, in step four, based on the structural form and load conditions of the location of the fastening hole in the assessment structure, the equivalent geometric dimensions of the structure corresponding to the engineering calculation method for the life of the fastening hole are determined, and the stress concentration factor handbook is consulted to obtain the stress concentration factor of the bypass load of the fastening hole.

[0021] Furthermore, in step five, based on the equivalent geometric dimensions of the structure, the equivalent stud stress concentration curve is found to obtain the equivalent stud stress concentration factor KtPinMPC for the MPC connection;

[0022] The equivalent nail load stress concentration curve is simplified using MPC. The maximum principal stress of the fastening hole under different plate widths, edge distances, and hole diameters is calculated using the finite element method and plotted after dimensionless processing.

[0023] Furthermore, in step six, the reference stress of the fastening hole includes: the reference stress S on the upper and lower surfaces of the fastening hole. 上 S 下 The calculation process is as follows:

[0024] 1) Establish a finite element model of the test structure and the local details of the solid fastener. Define the contact relationship between the solid fastener and the test structure. Apply an axial tensile force P to the solid fastener. pin轴向拉力 Constraints were defined on the test structure, and finite element analysis was performed to obtain the stress on the upper and lower surfaces under the actual connection condition of the fastening holes. The locations of the stress points on the upper and lower surfaces of the hole and the directions of the principal stresses are the same as in step three.

[0025] 2) Based on 1), remove the solid fasteners and replace them with simplified connections. Apply a nail-loaded axial tensile force P to the MPC. pin轴向拉力 Constraints were defined on the test structure, and finite element analysis was performed to obtain the stresses on the upper and lower surfaces of the fastening holes under the simplified MPC connection. The locations of the stress points on the upper and lower surfaces of the hole and the directions of the principal stresses are the same as in step three.

[0026] 3) Calculate the equivalent stud stress of the MPC connection:

[0027]

[0028] In the formula, D is the diameter of the fastener and t is the thickness of the plate of the test structure;

[0029] 4) Calculate the reference stress generated by the nailed shear force.

[0030]

[0031] In the formula, W is the width of the test structure and t is the thickness of the test structure.

[0032] 5) Correct the stress on the upper and lower surfaces of the fastening holes in the test structure using the following correction formula:

[0033]

[0034]

[0035] 6) Calculate the reference stress generated by the bypass load.

[0036]

[0037] 7) Calculate the reference stress generated by out-of-plane bending.

[0038]

[0039] In the formula K Bending It can be obtained from relevant empirical formulas;

[0040] 8) Calculate the reference stresses on the upper and lower surfaces of the fastening hole:

[0041] S上 =S Bypass +S Bending +S pin

[0042] S 下 =S Bypass -S Bending +S pin

[0043] Furthermore, in step seven, the reference stress S on the upper and lower surfaces of the fastening hole is determined based on the nail shear force and the load. 上 S 下 The life of the fastening hole is calculated using an engineering method based on out-of-plane bending fastening hole life analysis.

[0044] The beneficial effects of this invention are:

[0045] Based on simulation analysis, this invention establishes an engineering correction algorithm for the stress required in the engineering calculation method of fastening hole life. It realizes the engineering calculation of fastening hole life under complex loading conditions, with high calculation accuracy and high reliability of analysis results, and solves the problems that urgently need to be solved in practical engineering applications. Attached Figure Description

[0046] Figure 1 A simplified diagram of a fastener connection;

[0047] Figure 2 This is a schematic diagram of the equivalent nail load stress concentration factor curve. Detailed Implementation

[0048] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0049] A method for calculating the life of fastening holes under complex loading conditions includes the following steps:

[0050] The first step involves creating fastening holes on the geometric model of the test structure, establishing a detailed finite element model of the test structure and its surrounding structures (the surrounding structures refer to all load-bearing structures connected to the test structure), refining the mesh for the fastening holes, defining the element properties of the analysis model, the contact relationships between parts in the component model, and applying boundary loads to the boundaries of the component model to constrain rigid body displacements. The analysis conditions are then established, and a calculation file including finite element mesh, nodes, material, element properties, loads, contact, and constraint information is output.

[0051] The second step involves simplifying the fasteners into beam elements and using MPC (Multi-Point Constraints) to create layered connections with the connecting structure. Then, the material, element, and node information of the beam elements, along with the node information of the MPC connections, is synthesized into the calculation file from the first step for finite element analysis. (See diagram for simplified fastener connection.) Figure 1 , Figure 1 The left image shows the actual connection of the structure, while the right image is a simplified connection diagram.

[0052] The third step is to extract the stresses on the upper and lower surfaces of the fastening holes in the test structure from the finite element analysis results. (The positions of the points on the upper and lower surfaces of the hole are in the same direction as the principal stresses; the uppermost layer of the structure under test is called the upper surface, and the lowermost layer is called the lower surface), and the corresponding nail load shear force P. pin剪力 and the axial tensile force P of the nail pin轴向拉力 ;

[0053] The fourth step involves determining the equivalent geometric dimensions (plate width W, hole diameter D, and edge distance e) of the structure corresponding to the fastening hole life calculation method based on the structural form and load conditions of the location of the fastening hole in the assessment structure. Then, by consulting the stress concentration factor handbook, the stress concentration factor K for the bypass load of the fastening hole is obtained. TG ;

[0054] Fifth, based on the equivalent geometric dimensions of the structure (plate width W, hole diameter D, and edge distance e), the equivalent stud stress concentration factor K for the MPC connection is obtained from the equivalent stud stress concentration curve. tPinMPC ,See Figure 2 . Figure 2 To simplify the connection using MPC (Mechanical, Proportional, and Concrete) fittings, the maximum principal stress of the fastening holes was calculated using the finite element method for different plate widths (W), edge distances (e), and hole diameters (D). The stresses were then plotted after dimensionless processing. The cluster of curves in the figure represents the equivalent stress concentration factor of the MPC connection at different e / w ratios. The horizontal axis represents W / R (where R is the hole radius), and the vertical axis represents the equivalent nail load stress concentration factor K. tPinMPC (Maximum principal stress at the edge of the MPC connection hole / hole extrusion stress);

[0055] The sixth step involves using a modified algorithm and simulation analysis to calculate the various reference stresses required for the fastening holes;

[0056] The sixth step includes the following sub-steps:

[0057] 1) Calculate the upper and lower surface stresses under the actual connection condition of the fastening hole. Establish a local detailed finite element model of the test structure (the same structure as in step 1) and the solid fastener. Define the contact relationship between the solid fastener and the test structure, and apply an axial tensile force P to the solid fastener. pin轴向拉力Constraints were defined on the assessment structure, and finite element analysis was performed to obtain the stress on the upper and lower surfaces of the hole under the actual fastening connection. The locations of the stress points on the upper and lower surfaces of the hole and the directions of the principal stresses are the same as in step three.

[0058] 2) Calculate the stresses on the upper and lower surfaces of the fastening hole under the simplified MPC connection. Based on step 1), remove the solid fasteners and replace them with a simplified connection. Apply axial tensile force to the MPC, define constraints on the test structure, and perform finite element analysis to obtain the stresses on the upper and lower surfaces of the fastening hole under the simplified MPC connection. The locations of the stress points on the upper and lower surfaces of the hole and the directions of the principal stresses are the same as in step three.

[0059] 3) Based on the nail shear force extracted in step 3, the equivalent nail stress of the MPC connection is calculated as follows:

[0060]

[0061] In the formula, D is the diameter of the fastener and t is the thickness of the plate of the test structure;

[0062] 4) Based on the spiked shear force extracted in step 3, calculate the reference stress generated by the spiked shear force.

[0063]

[0064] In the formula, W is the width of the test structure and t is the thickness of the test structure.

[0065] 5) Correct the stress on the upper and lower surfaces of the fastening holes in the test structure using the following correction formula:

[0066]

[0067]

[0068] 6) Calculate the reference stress generated by the bypass load.

[0069]

[0070] 7) Calculate the reference stress generated by out-of-plane bending.

[0071]

[0072] In the formula K Bending It can be obtained from relevant empirical formulas.

[0073] 8) Calculate the reference stress on the upper and lower surfaces of the fastening hole.

[0074] S 上 =S Bypass +S Bending +S pin(7)

[0075] S 下 =S Bypass -S Bending +S pin (8)

[0076] Step 7: Based on the shear force of the nail and the reference stress S on the upper and lower surfaces of the fastening hole. 上 S 下 The life of the fastening hole is calculated using an engineering method based on out-of-plane bending fastening hole life analysis.

[0077] The above description is merely a specific embodiment of the present invention, providing a detailed description of the invention. Parts not covered herein are conventional techniques. However, the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. The scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for calculating the life of fastening holes under complex loading conditions, characterized in that: The method includes the following steps: Step 1: Establish a finite element model of the assessment structure and its surrounding structures; Step 2: Simplify the fasteners in the test structure and surrounding structures, add nail connection information, and perform finite element analysis; Step 3: Extract the upper and lower surface stresses of the fastening holes in the test structure from the finite element analysis results. , nail load shear force and the axial tensile force of the nail ; Step 4: Obtain the stress concentration factor of the bypass load in the fastening hole. ; Step 5: Obtain the equivalent stud stress concentration factor for the MPC connection. ; Step Six: Calculate the reference stress of the fastening hole, including the reference stress on the upper and lower surfaces of the fastening hole. S 上 , S 下 The calculation process is as follows: 1) Establish a finite element model of the test structure and the local details of the solid fastener. Define the contact relationship between the solid fastener and the test structure, and apply a nail-loaded axial tensile force to the solid fastener. Constraints were defined on the test structure, and finite element analysis was performed to obtain the stress on the upper and lower surfaces under the actual connection condition of the fastening holes. , The positions of the stress points on the upper and lower surfaces of the hole and the directions of the principal stresses are the same as in step three. 2) Based on 1), remove the solid fasteners and replace them with simplified connections. Apply axial tensile force to the MPC. Constraints were defined on the test structure, and finite element analysis was performed to obtain the stresses on the upper and lower surfaces of the fastening holes under the simplified MPC connection. , The positions of the stress points on the upper and lower surfaces of the hole and the directions of the principal stresses are the same as in step three. 3) Calculate the equivalent stud stress of the MPC connection. In the formula, D is the diameter of the fastener and t is the thickness of the plate of the test structure; 4) Calculate the reference stress generated by the nailed shear force. In the formula, W is the width of the test structure and t is the thickness of the test structure. 5) Correct the stress on the upper and lower surfaces of the fastening holes in the test structure using the following formula: 6) Calculate the reference stress generated by the bypass load. 7) Calculate the reference stress generated by out-of-plane bending. In the formula It can be obtained from relevant empirical formulas; 8) Calculate the reference stress on the upper and lower surfaces of the fastening hole. ； Step 7: Calculate the life of the fastening hole based on the shear force of the nail and the reference stress of the fastening hole.

2. The method according to claim 1, characterized in that: In step one, the material properties of the assessment structure and its surrounding structure are defined, as well as the contact relationships between the parts in the assessment structure and its surrounding structure. Boundary loads are applied to the model boundaries to constrain rigid body displacements; The output of the finite element model of the assessment structure and its surrounding structures includes: mesh, load, contact relationship, and constraint information.

3. The method according to claim 2, characterized in that: In step two, when the fastener is simplified, it is simplified into a beam element, and multi-point constraint MPC is used to connect the beam element and the connection structure in layers. The nail connection information includes: the material, element, node, and node information of the beam element and the MPC connection.

4. The method according to claim 3, characterized in that: In step three, the upper surface of the fastening hole refers to the upper surface of the test structure, and the lower surface refers to the lower surface of the test structure.

5. The method according to claim 4, characterized in that: In step four, based on the structural form and load conditions of the location of the fastening hole in the assessment structure, the equivalent geometric dimensions of the structure corresponding to the engineering calculation method for the life of the fastening hole are determined, and the stress concentration factor handbook is consulted to obtain the stress concentration factor of the bypass load of the fastening hole.

6. The method according to claim 5, characterized in that: In step five, the equivalent stud stress concentration factor of the MPC connection is obtained by finding the equivalent stud stress concentration curve based on the equivalent geometric dimensions of the structure. ; The equivalent nail load stress concentration curve is simplified using MPC. The maximum principal stress of the fastening hole under different plate widths, edge distances, and hole diameters is calculated using the finite element method and plotted after dimensionless processing.

7. The method according to claim 6, characterized in that: In step seven, the shear force of the nail and the reference stress on the upper and lower surfaces of the fastening hole are used as the basis. , The life of the fastening hole is calculated using an engineering method based on out-of-plane bending fastening hole life analysis.

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