A method for optimizing the extrusion strengthening process of a composite material laminated structure connecting hole

Abstract

The present application belongs to the technical field of composite material hole extrusion strengthening, and relates to a composite material laminated structure connecting hole extrusion strengthening process optimization method. First, a composite material connecting hole extrusion finite element model and a composite material laminated structure connecting load finite element model based on the progressive damage theory are established, and boundary conditions are defined. Then, based on the composite material connecting hole extrusion finite element model, the hole extrusion strengthening process under different process parameters is simulated to obtain the stress distribution around the hole and the damage state. Then, taking the minimization of the damage around the hole and the maximization of the strengthening effect as the optimization goal, based on the obtained simulation results, the mapping relationship between the process parameters and the damage is established, and the optimal process parameters are screened out. Finally, the hole extrusion strengthening process is implemented based on the optimal process parameters, and the damage condition and fatigue life of the strengthened connecting hole in the service process are verified through the composite material laminated structure connecting load finite element model. The present application improves the reliability and durability of the composite material laminated structure connection.

Description

Technical Field

[0001] This invention belongs to the field of composite material hole extrusion strengthening technology, and relates to an optimization method for the extrusion strengthening process of connecting holes in composite material laminated structures. Background Technology

[0002] Composite materials, with their high specific strength, high specific stiffness, excellent fatigue resistance, and designability, have become key materials for the main load-bearing structures of advanced aircraft. In practical applications, composite materials are often mechanically connected with metal components to form laminated structures, thereby achieving weight reduction and improving overall mechanical properties. Among these, composite / metal laminated connection structures are particularly widely used in main load-bearing parts such as wings and fuselages due to their advantages such as high load-bearing capacity, high reliability, and good resistance to galvanic corrosion.

[0003] However, with the increasing performance requirements of aircraft, the fatigue life problem of the aforementioned connection structure under complex alternating loads has become increasingly prominent. The structure contains numerous connection holes, which are prone to stress concentration under bolt compression, becoming the source of fatigue crack initiation and propagation, severely restricting the structure's safety and service life. To address this problem, hole extrusion fatigue strengthening technology has been proven to effectively improve the stress distribution at the hole edges and significantly extend fatigue life. However, due to the anisotropy and heterogeneity of composite materials, as well as the diverse types and large dimensional differences of laminated structures, key process parameters in the hole extrusion strengthening process (such as bushing thickness, extrusion amount, mandrel cone angle, etc.) are difficult to systematically optimize using traditional empirical methods.

[0004] Traditional process optimization methods heavily rely on numerous repetitive experiments to obtain process parameters under specific operating conditions, resulting in low efficiency, long cycles, and high costs. Especially for composite / metal laminated structures, there is still a lack of integrated process parameter impact assessment and optimization covering the entire process from hole extrusion strengthening to joint loading, making it impossible to effectively control initial damage while ensuring strengthening effects. For example, patent CN120963135A uses a differentiated hole extrusion method for laminated structures to strengthen composite / titanium laminated structures. Although the results show that this method can improve the fatigue life and connection performance of the three-prong connection structure in composite / titanium laminated structures, this patent does not consider the potential impact of process parameters on the strengthening effect.

[0005] Therefore, researching an optimization method for the extrusion strengthening process of connecting holes in composite laminate structures is of great significance. This method can systematically, rapidly, and quantitatively optimize the extrusion strengthening process parameters of connecting holes in composite laminate structures, enabling the scientific selection of process parameters, effective evaluation of strengthening effects, and improvement of the reliability and durability of composite laminate structure connections. Summary of the Invention

[0006] The purpose of this invention is to solve the problems existing in the prior art and to provide an optimized method for the extrusion strengthening process of connecting holes in composite material laminated structures.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] An optimized method for extrusion strengthening of connecting holes in composite laminate structures includes the following steps:

[0009] S1: Establish the finite element model of the extrusion of the joint hole in the composite material and the finite element model of the connection of the composite laminate structure under load based on the progressive damage theory, and define the boundary conditions.

[0010] S2: Based on the composite material connection hole extrusion finite element model established by S1, the hole extrusion amount, bushing thickness and mandrel cone angle are used as process parameter variables to simulate the hole extrusion strengthening process under different process parameters, and obtain the hole periphery stress distribution and damage state.

[0011] S3: With the optimization objectives of minimizing hole periphery damage and maximizing the strengthening effect, based on the simulation results obtained in S2, the optimal fixed parameters of bushing thickness and mandrel cone angle are first screened and determined. Then, with hole extrusion amount as the core optimization variable, the mapping relationship between hole extrusion amount and damage is established, and the optimal hole extrusion amount parameter is screened out, thereby achieving rapid quantitative optimization of process parameters.

[0012] S4: Under the conditions of fixed bushing thickness and fixed mandrel cone angle determined in S3, static tensile tests and fatigue tests were carried out on composite laminate pore extrusion reinforced specimens with different pore extrusion amounts. By comparing the results with the finite element model of the composite laminate structure connection under load established in S1, the correctness of the pore extrusion amount optimization results, as well as the damage state and fatigue life of the reinforced connection pores during service, were verified.

[0013] As a preferred technical solution:

[0014] The above-described optimization method for the extrusion strengthening process of connecting holes in composite laminate structures, in step S1, establishes the finite element model of the extrusion of connecting holes in composite materials and the finite element model of the connection under load in composite laminate structures based on the ABAQUS platform.

[0015] As described above, the optimization method for the extrusion strengthening process of the connecting holes in a composite laminate structure includes, in step S1, a composite material connecting hole extrusion finite element model comprising a composite material sample, an extrusion mandrel, and a bushing. This finite element model mainly performs stress distribution analysis and damage prediction around the holes in the composite material sample. The loading process of the composite laminate structure connection mainly refers to the process in which the strengthened composite laminate sample is subjected to tensile load when connected by a single nail. The loading finite element model of the composite laminate structure connection includes a composite material laminate sample, a bushing, and a high-strength bolt.

[0016] As described above, the optimization method for the extrusion strengthening process of the connecting holes in composite laminate structures involves both the finite element model of the extrusion of the connecting holes in composite materials and the finite element model of the connection under load in composite laminate structures, which are used to predict damage by combining the Hashin failure criterion and the Linde stiffness degradation model.

[0017] Hashin's failure criteria specifically include four failure modes: fiber tensile failure, fiber compressive failure, matrix tensile failure, and matrix compressive failure.

[0018] The Hashin failure criterion serves two main purposes: firstly, it determines whether various types of damage have occurred in the composite material; secondly, once one or more types of damage have occurred, the criterion is used to calculate the load variables and, based on these load variables, to calculate the material damage value. The Hashin failure criterion specifically includes four failure modes: fiber tensile failure, fiber compressive failure, matrix tensile failure, and matrix compressive failure.

[0019] (1) The formula for determining fiber tensile failure is as follows:

[0020] ;

[0021] (2) The formula for the fiber compression failure criterion is as follows:

[0022] ;

[0023] (3) The formula for the matrix tensile failure criterion is as follows:

[0024] ;

[0025] (4) The formula for the matrix compression failure criterion is as follows:

[0026] ;

[0027] In the above four failure modes criterion formulas, subscript 1 indicates the fiber direction, subscript 2 indicates the direction perpendicular to the fiber in the fiber plane, and subscript 3 represents the direction perpendicular to both 1 and 2. σ 11 σ 22 and σ 33 These represent the normal stresses in three directions, σ and σ'. 12 σ 13 and σ 23 S represents shear stress, respectively. 12 S 13 and S 23The shear strength is defined in each direction. The stress on the composite material is incorporated into the four failure criteria mentioned above. If the calculated value is greater than or equal to 1, this type of failure is considered to have occurred.

[0028] The Linde stiffness degradation model specifically includes damage variables corresponding to four typical damage modes: fiber tensile damage variables, fiber compressive damage variables, matrix tensile damage variables, and matrix compressive damage variables. The formula is as follows: Where e represents the natural constant, d represents the damage variable, F represents the load variable, and D represents the parameter related to the material fracture energy.

[0029] (1) The formula for the fiber tensile damage variable is as follows:

[0030] ;

[0031] (2) The formula for the fiber compression damage variable is as follows:

[0032] ;

[0033] (3) The formula for the matrix tensile damage variable is as follows:

[0034] ;

[0035] (4) The formula for the matrix compression damage variable is as follows:

[0036] ;

[0037] In the damage variable formulas corresponding to the four typical damage modes mentioned above, G1 and G2 are the critical fracture energy release rates in the fiber direction and perpendicular to the fiber direction, respectively, and L c It is the characteristic length of the finite element mesh, which can eliminate mesh sensitivity in actual calculations; , , and These represent the load variables for fiber tension, fiber compression, matrix tension, and matrix compression, respectively.

[0038] The incremental step iterative approach is used to achieve the coupled update of damage evolution and stress field. The specific steps are as follows:

[0039] (1) Incremental step initialization: At the beginning of each incremental step, based on the strain results of the previous step. Apply a preset strain increment The total strain of the current increment step is obtained. .

[0040] (2) Initial stress calculation:

[0041] The constitutive expression of composite materials, represented by the equation of the generalized Hooke's law, is as follows:

[0042] ;

[0043] Where σ1, σ2, and σ3 represent three normal stress components; τ 12 τ 13 τ 23 ε1, ε2, and ε3 represent three shear stress components; γ represents three normal strain components. 12 γ 13 γ 23 It represents three shear strain components.

[0044] The values ​​of each parameter in the stiffness matrix of composite materials can be represented by engineering constants:

[0045] ;

[0046] Where E1, E2, and E3 are the elastic moduli in the three principal elastic directions, ν 12 ν 21 ν 13 ν 31 ν 23 and ν 32 It is the Poisson's ratio in all directions, G 23 G 31 and G 12 These are the in-plane shear moduli of the three planes, respectively. These are common factors. The relationship between Poisson's ratios in orthotropic anisotropic materials can be expressed as follows:

[0047] .

[0048] (3) Failure mode determination: Substitute the initial stress into the Hashin failure criterion and determine whether the four failure modes of fiber tension, fiber compression, matrix tension, and matrix compression have occurred. If the failure criterion of any mode is met, the material is determined to have been damaged in this incremental step.

[0049] (4) Stiffness degradation and stress update: If the material is determined to have a certain type of failure, the Linde stiffness degradation model is called to calculate the damage variables corresponding to the failure.

[0050] According to the progressive damage theory, the mechanical properties of composite materials will degrade to a certain extent after damage occurs, which is reflected in the decrease of engineering constants in the stiffness matrix, as shown in the following equation:

[0051] ;

[0052] However, this degradation is not a constant, but a variable related to material damage, expressed in a nonlinear form, as shown in the following equation:

[0053] ;

[0054] Where, d m d represents the damage variable of the composite matrix. f This represents the fiber damage variable in the composite material. Damage variable d m and d f The calculated result is a real number between 0 and 1. When the damage variable value is 0, it indicates that the material is undamaged; when its value is 1, it indicates that the material is completely destroyed, and the stiffness in the above formula is 0.

[0055] Then through constitutive relations Complete the stress update for the current increment step.

[0056] (5) Iterative progress: The strain, stress and damage state of the current increment step are used as the initial values ​​to enter the calculation of the next increment step until the progressive damage analysis of the entire loading process is completed.

[0057] The above-described method for optimizing the extrusion strengthening process of connecting holes in composite laminate structures involves rapid quantitative optimization of process parameters in step S3. The decision-making process does not rely on constructing additional proxy models, but is directly based on the visualized cloud map and quantitative data generated by finite element simulation.

[0058] The optimization method for the extrusion strengthening process of the connecting holes in a composite laminate structure as described above, specifically includes the following steps in step S3: First, by comparing the stress distribution cloud maps under different extrusion amounts, a bushing thickness that can generate uniform pressure and does not produce local yielding is selected; second, by comparing the damage distribution under different mandrel cone angles, a cone angle combination that can balance inlet damage and outlet strengthening effect is selected; finally, under the optimal conditions of fixed bushing thickness and mandrel cone angle, the critical extrusion amount that will not cause unacceptable initial damage when the expected strengthening effect is achieved is determined.

[0059] As described above, the optimization method for the extrusion strengthening process of the connecting holes in a composite laminate structure includes step S4, which specifically includes: conducting static tensile tests and fatigue tests on composite laminate extrusion strengthening samples with different extrusion amounts; simultaneously measuring the strain field around the holes in the composite material using digital image correlation (DIC) technology; and comparing the test results with the simulation prediction results of the finite element model of the composite laminate structure connection under load to verify the correctness of the parameter optimization results.

[0060] Beneficial effects:

[0061] (1) The present invention provides an optimization method for the extrusion strengthening process of connecting holes in composite laminate structures. Based on the finite element model of composite strengthening and connection under load established on the ABAQUS platform, the method systematically analyzes the influence of different process parameters on the stress and damage distribution around the holes, constructs the mapping relationship between process parameters and damage, and forms a closed-loop optimization process. The method aims to minimize damage and maximize strengthening effect, realizes rapid quantitative optimization of process parameters, and significantly improves the efficiency and accuracy of process optimization. By avoiding the limitation of traditional methods relying on a large number of experiments, the present invention can quickly guide the selection of hole extrusion strengthening process parameters and accurately evaluate the performance of the strengthened connecting holes in service, thereby effectively improving the reliability and durability of composite laminate structure connections.

[0062] (2) The present invention provides an optimization method for the extrusion strengthening process of connecting holes in composite laminate structures. This method can evaluate the influence of different combinations of process parameters on the stress and damage around the hole through a finite element simulation system, clarify the effect of each parameter on the initial damage of the hole wall, and then quickly screen out the optimal process parameters. This not only realizes the quantitative optimization of process parameters, but also guides the efficient implementation of the actual strengthening process, ensuring that the stress distribution of the strengthening connecting hole is reasonable and the damage is controllable during the loading process, and significantly improving the safe service performance and fatigue life of the composite laminate connecting structure. Attached Figure Description

[0063] Figure 1 Finite element model of extrusion of joint holes in composite materials and finite element model of reinforcement and connection process of joint holes in composite / titanium alloy laminates under load;

[0064] Figure 2 Flowchart for calculating the progressive damage model of composite materials;

[0065] Figure 3 Figures showing the stress distribution around the holes and the stress and strain distribution of the bushing in composite materials with different bushing thicknesses;

[0066] Figure 4 The diagram shows the damage distribution of the composite material hole wall under different bushing thicknesses.

[0067] Figure 5 Figures showing the distribution of peripore stress and hole wall damage in composite materials under different mandrel front cone angles;

[0068] Figure 6 Figures showing the distribution of peripore stress and hole wall damage in composite materials under different mandrel rear cone angles;

[0069] Figure 7 Comparison of finite element simulation and experimental results of extrusion pressure in composite material connecting holes under different extrusion amounts;

[0070] Figure 8Figures showing the distribution of stress around the holes and stress and strain of the bushing in composite materials under different extrusion rates;

[0071] Figure 9 The diagram shows the damage distribution of the pore walls of the composite material under different extrusion rates.

[0072] Figure 10 The diagram illustrates the influence of different extrusion amounts on the peripore stress and pore wall damage of the composite material during the reinforcement process.

[0073] Figure 11 This image shows the DIC strain distribution around the pores in composite laminated single-nail joint specimens during static tensile testing under different extrusion amounts. NHE represents specimens without pore expansion reinforcement (N: not, HE: hole expansion); HE-2 represents specimens with pore expansion reinforcement at an extrusion amount of 2%; HE-3 represents specimens with pore expansion reinforcement at an extrusion amount of 3%; and HE-5 represents specimens with pore expansion reinforcement at an extrusion amount of 5%.

[0074] Figure 12 The figure shows the variation of compressive strain with load percentage at 6 mm (Figure (a)) and 8 mm (Figure (b)) from the hole in the composite laminate single nail connection specimens under different extrusion amounts during static tensile process.

[0075] Figure 13 Fatigue life diagrams of composite laminate specimens under different extrusion rates;

[0076] Figure 14 The damage distribution diagram of composite laminate structures under 0.3 mm tensile displacement with different extrusion amounts is shown; where CFRP represents carbon fiber reinforced resin matrix composite. Detailed Implementation

[0077] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0078] An optimized method for strengthening the extrusion process of connecting holes in composite laminate structures, comprising the following steps:

[0079] S1: As Figure 1As shown, based on the ABAQUS platform, a composite material connection hole extrusion finite element model and a composite material laminate connection under load finite element model based on progressive damage theory were established, and boundary conditions were defined. Hexahedral eight-node solid elements were selected in the modeling, and each layup was set as an element in the thickness direction. The mesh of the composite material sample was divided into regions to increase the mesh density of the extrusion influence zone around the hole in order to improve the simulation accuracy.

[0080] like Figure 1 As shown in (a), the finite element model of the composite material connecting hole extrusion includes the composite material specimen, the extrusion mandrel, and the bushing; the constraint condition of the bottom surface of the bushing is U. z =U Rx =U Ry =0, the composite material specimen is fully constrained on both sides U x =U y =U z =U Rx =U Ry =U Rz =0; the composite material specimens in the model are set according to the actual layup. Where, U x U y U z U Rx U Ry U Rz These represent the six degrees of freedom in the Cartesian coordinate system; U x Translation along the X-axis; U y Translation along the Y-axis; U z Translation along the Z-axis; U Rx Rotation about the X-axis; U Ry Rotation about the Y-axis; U Rz : Rotation about the Z-axis.

[0081] This model primarily performs stress analysis on the bushing and composite material specimen in the finite element model, with the extrusion mandrel being set as a rigid body. The mechanical property parameters of the composite single-layer plate are shown in Table 1. Since the inner wall of the bushing is coated with lubricant during the hole extrusion test, the friction coefficient between the working section of the extrusion mandrel and the inner wall of the bushing is set to 0.05; while the composite hole wall and the outer wall of the bushing are in direct contact, the friction coefficient between the composite hole wall and the outer wall of the bushing is set to 0.15.

[0082] Table 1 Mechanical property parameters of composite single-layer plates

[0083] name <![CDATA[E 11 (GPa)]]> <![CDATA[E 22 =E 33 (GPa)]]> <![CDATA[G 12 =G 13 (GPa)]]> <![CDATA[G 23 (GPa)]]> <![CDATA[v 12 = in 13 ]]> <![CDATA[v 23 ]]> value 195 8.58 4.57 2.90 0.33 0.48 name <![CDATA[X T (MPa)]]> <![CDATA[X C (MPa)]]> <![CDATA[Y T =Z T (MPa)]]> <![CDATA[Y C =Z C (MPa)]]> <![CDATA[S 12 =S 13 ]]> <![CDATA[S 23 ]]> value 3071 1747 88 271 143 102

[0084] Among them, E 11 E 22 E 33 G represents the elastic modulus in each direction. 12G 13 G 23 Let v be the in-plane shear modulus of each plane. 12 v 13 v 23 Let X be the Poisson's ratio in each direction. T Y T Z T X represents the ultimate tensile strength in three directions. C Y C Z C S represents the ultimate compressive strength in three directions. 12 S 13 S 23 Shear strength in each direction; E in Table 1 11 E 22 The symbols are: subscript 1 indicates the fiber direction, subscript 2 indicates the direction perpendicular to the fiber in the fiber plane, and subscript 3 indicates the direction perpendicular to both 1 and 2.

[0085] like Figure 1 As shown in (b), the finite element model of the composite laminate structure under load includes the composite laminate specimen (i.e., composite / titanium alloy laminate), bushing and high-strength bolts;

[0086] The constraint conditions of the nodes on the end face of the titanium alloy laminate are set to U. x =U y =U z =0, a displacement load along the x-direction is applied to the end face node of the composite plate; a preload F is applied to the high-locking bolt. p 9kN; Preload F p The calculation formula is , of which M t The torque represents the tightening torque (8 Nm), k represents the torque coefficient (0.14), and d represents the torque coefficient. t The nominal diameter of the high-strength bolt is 6.35 mm. Contact pairs are defined on the contact surfaces of the composite plate hole wall and the bushing outer wall, the high-strength bolt and the bushing inner wall, and the titanium alloy laminated plate hole wall, with friction coefficients set to 0.15 and 0.1, respectively. Simultaneously, contact pairs are defined on the contact surfaces of the high-strength bolt and the composite plate and the titanium alloy laminated plate surface, and the composite plate and the titanium alloy laminated plate surface, with friction coefficients set to 0.2 and 0.1, respectively. The stress-strain relationship between the titanium alloy laminated plate and the bushing is described by the JC constitutive model, and its calculation formula is as follows: ,in, Represents equivalent force, Represents equivalent plastic strain, Represents the equivalent plastic strain rate. A represents the reference equivalent plastic strain rate, B represents the initial yield strength, C represents the hardening modulus, and n represents the strain rate sensitivity factor.

[0087] The mechanical property parameters of each part material during the static tensile process of composite material / titanium alloy laminated hole extrusion strengthening and single nail connection are shown in Table 2.

[0088] Table 2 Mechanical property parameters of materials for each part

[0089] name Material <![CDATA[Tensile strength σ b (MPa)]]> <![CDATA[Yield strength σ s (MPa)]]> Elastic modulus E (GPa) Poisson's ratio v Titanium alloy plate Ti6Al4V 1072 880 112 0.34 mandrel W6Mo5Cr4V2 2320 2060 218 0.31 bushing Ti6Al4V 1072 880 112 0.34 High-strength bolts Ti5553 1240 1120 132 0.33

[0090] like Figure 2 The diagram shown is a flowchart of the progressive damage model calculation for composite materials, which includes the following steps:

[0091] Step 1: Perform stress analysis on the composite material to obtain the stress distribution through the constitutive relation of the composite material; the constitutive relation of the composite material is as follows: ,in It is a stress matrix containing 6 stress components. It is a strain matrix containing 6 strain components. This is the stiffness matrix of the composite material; where the six stress components represent three normal stress components (σ1, σ2, σ3) and three shear stress components (τ). 12 τ 13 τ 23 );

[0092] Both the composite material pore extrusion finite element model and the composite material laminate structure connection under load finite element model are used to predict damage by combining the Hashin failure criterion and the Linde stiffness degradation model.

[0093] Hashin's failure criteria specifically include four failure modes: fiber tensile failure, fiber compressive failure, matrix tensile failure, and matrix compressive failure.

[0094] The Linde stiffness degradation model specifically includes damage variables corresponding to four typical damage modes: fiber tensile damage variables, fiber compressive damage variables, matrix tensile damage variables, and matrix compressive damage variables. The formula is as follows: Where d represents the damage variable, F represents the load variable, and D represents the parameter related to the material fracture energy.

[0095] Step 2: Based on the current stress state of the composite material unit, determine the failure status and failure type of the current composite material through the composite material hole extrusion and connection load failure criterion model.

[0096] Step 3: If a certain type of failure occurs, the corresponding damage variables are calculated using the composite material hole extrusion finite element model and the composite material laminate structure connection load-bearing finite element model. The stiffness of the composite material is reduced, and the stress is recalculated until the boundary conditions of the composite material hole extrusion and connection load-bearing finite element model S1 are reached, so as to obtain the final stress distribution and damage state around the composite material laminate connection hole.

[0097] S2: Based on the composite material connection hole extrusion finite element model established by S1, the hole extrusion amount, bushing thickness and mandrel cone angle are used as process parameter variables to simulate the hole extrusion strengthening process under different process parameters, and obtain the hole periphery stress distribution and damage state.

[0098] S3: With the optimization objectives of minimizing hole periphery damage and maximizing the strengthening effect, based on the simulation results obtained in S2, the optimal fixed parameters of bushing thickness and mandrel cone angle are first screened and determined. Then, with hole extrusion amount as the core optimization variable, the mapping relationship between hole extrusion amount and damage is established, and the optimal hole extrusion amount parameter is screened out, thereby achieving rapid quantitative optimization of process parameters.

[0099] First, finite element simulations of the extrusion strengthening process of composite material connecting holes with bushing thicknesses of 0.4 mm, 0.7 mm, 1.0 mm, and 1.3 mm were performed using the method proposed in this invention.

[0100] like Figure 3 The figure shows the stress distribution around the holes in the composite material and the stress (i.e., the stress in the composite material) and strain distribution under different bushing thicknesses. The results show that as the bushing thickness increases, the maximum stress around the holes in the composite material gradually decreases. However, if the bushing is too thin, stress concentration will occur locally in the bushing, causing damage to the hole wall of the composite material. If the bushing is too thick, the deformation will be mainly concentrated inside, resulting in a weakening of the effective compression effect on the hole wall of the composite material, which is detrimental to the connection performance.

[0101] like Figure 4 The image shows the damage to the composite material pore walls under different bushing thicknesses. The results indicate that when the bushing thickness is 0.4 mm, bushing deformation is significant, resulting in high stress on the composite material pore walls and severe overall damage, which directly affects the bonding performance of the composite material. When the bushing thickness increases to 0.7 mm, under the same extrusion amount, bushing deformation decreases, and the stress distribution is more uniform; damage is only found in the extrusion inlet area of ​​the composite material pores. When the bushing thickness increases to 1.0 mm or more, due to limited bushing deformation, the extrusion effect on the composite material pore walls is smaller, and the damage is all below 5%.

[0102] Finite element simulations of the extrusion strengthening process of composite material connecting holes with mandrel front cone angles of 3°, 6°, and 9° and rear cone angles of 3°, 6°, and 9° were performed using the method proposed in this invention.

[0103] like Figure 5 The figure shows the stress around the hole and the damage to the hole wall of the composite material under different mandrel front cone angles. The results show that when the mandrel front cone angle is 9°, a large stress is generated, and a lot of damage occurs at the extrusion inlet of the composite material; when the mandrel front cone angle is 3°, the damage to the composite material is relatively small.

[0104] like Figure 6 The figure shows the periphery stress and pore wall damage of the composite material under different mandrel rear cone angles. The results show that the change in the mandrel rear cone angle has no significant effect on the pore wall damage of the composite material, but increasing the rear cone angle can make the extrusion effect more significant.

[0105] By comparing the stress distribution cloud maps under different bushing thicknesses, a bushing thickness that can generate uniform pressure and does not produce local yielding is selected; by comparing the damage distribution under different mandrel cone angles, a cone angle combination that can balance inlet damage and outlet strengthening effect is selected; taking into account both strengthening effect and hole wall damage, the optimal process parameters are bushing thickness of 0.7 mm, mandrel front cone angle of 3°, and mandrel rear cone angle of 9°.

[0106] Under the optimal conditions of fixed bushing thickness and mandrel cone angle, the stress around the hole and the hole wall damage cloud map under different extrusion amounts are compared to determine the critical extrusion amount that will not cause unacceptable initial damage when the expected effect is achieved.

[0107] like Figure 7 The figure shows the extrusion pressure of the composite material connection hole finite element simulation and test process under different extrusion amounts. The results show that the extrusion pressure variation trend obtained by numerical simulation is basically consistent with the test, and the maximum extrusion pressure error is within 12%. This indicates that the composite material hole extrusion and connection load finite element model established by this method can accurately simulate the stress situation of the extrusion process.

[0108] like Figure 8 The figure shows the stress distribution around the pores (i.e., composite stress) and the stress and strain distribution of the bushing under different extrusion rates. As the extrusion rate increases, the stress increases sharply, resulting in significant damage to the pore walls, mesh deletion, and stress concentration in certain areas. The stress on the bushing does not change significantly with the extrusion rate; at low extrusion rates, the stress on the bushing is relatively uniform, while at higher extrusion rates, significant stress concentration occurs at the exit. However, the bushing strain shows the opposite trend, with significantly greater strain at the inlet, indicating more pronounced deformation at the inlet.

[0109] like Figure 9The figure shows the damage to the pore walls of the composite material under different extrusion rates. The results show that the damage to the pore walls of the composite material gradually intensifies with increasing extrusion rate. When the extrusion rate is small (2%~3%), only slight damage occurs at the inlet; as the extrusion rate continues to increase (4%), the damage range extends to the entire pore wall, with fiber breakage and matrix damage occurring; when the extrusion rate is too large (5%), severe damage occurs at the outlet, including matrix cracking and fiber lamination bending fracture.

[0110] like Figure 10 The figure shows the variation trends of peripore stress and pore wall damage in composite materials under different extrusion amounts. The results indicate that when the extrusion amount is 2%, the maximum peripore stress is 329 MPa, and the pore surface damage accounts for only 2.7%. When the extrusion amount increases to 3%, the maximum peripore stress rises to 596 MPa, and the pore surface damage also increases to 16.3%. When the extrusion amount continues to increase to 4% and 5%, the proportion of pore surface damage increases sharply to 33.5% and 59.7%, respectively, affecting the bonding performance of the composite material.

[0111] Finally, taking into account both the strengthening effect and the damage to the hole wall, the optimal process parameters are: hole extrusion amount of 3%, bushing thickness of 0.7 mm, mandrel front cone angle of 3°, and mandrel rear cone angle of 9°.

[0112] S4: Under the conditions of fixed bushing thickness and fixed mandrel cone angle determined in S3, static tensile tests and fatigue tests were conducted on composite laminate pore extrusion reinforced specimens with different pore extrusion amounts. At the same time, digital image correlation (DIC) technology was used to measure the strain field around the pores of the composite material. The results were compared with the results of the finite element model of the composite laminate structure connection under load established in S1, and the test results were compared with the simulation prediction results of the finite element model of the composite laminate structure connection under load to verify the correctness of the pore extrusion amount optimization results, as well as the damage state and fatigue life of the reinforced connection pores during service.

[0113] like Figure 11 The figure shows the DIC strain distribution around the pores in composite laminates and single-pronged specimens with different extrusion amounts during static tensile testing. Under the same load, the compressive strain region and the maximum compressive strain are smaller in the specimen reinforced by pore extrusion.

[0114] like Figure 12The figure shows the variation of compressive strain at 6 mm and 8 mm above the hole in composite laminated single-nail connection specimens under different extrusion amounts during static tensile testing, as a function of load percentage. At 6 mm above the hole, the maximum compressive strain of the specimen without hole extrusion is -0.42%, while the maximum compressive strains of specimens with extrusion amounts of 2%, 3%, and 5% are -0.39%, -0.35%, and -0.27%, respectively. At 8 mm above the hole, the maximum compressive strain of HE-5 and HE-3 specimens is only -0.12% and -0.18%, respectively, which is 62.5% and 43.8% lower than that of the NHE specimen. As the compressive strain increases, the bolt extrusion and shear effects on the matrix and fibers of the composite material become more pronounced, making the specimens more prone to failure.

[0115] like Figure 13 The results show the fatigue performance of composite laminate specimens under different extrusion amounts under a load of 12.9 kN. The results indicate that the hole extrusion process can effectively improve the single-pile single-shear fatigue performance of the composite laminate specimens. The single-pile single-shear fatigue life of ordinary composite laminate specimens is 5.7 × 10⁻⁶ kN. 4 When the extrusion amount is 2%, the fatigue life is 11.6 × 10⁻⁶. 4 The improvement was limited, with a 104% increase. This was because the extrusion amount was relatively small, and the extrusion process did not generate significant residual stress on the bushing. Furthermore, the need to install a bushing during the hole extrusion process led to an enlarged initial pore size, reducing the overall performance of the composite material and offsetting some of the fatigue performance improvement from hole extrusion. When the extrusion amount reached 3%, the fatigue life of the sample reached 14.8 × 10⁻⁶. 4 The fatigue life of the specimens under this extrusion amount showed the most significant improvement, being 2.6 times that of the specimens without perforation-strengthened extrusion. When the extrusion amount exceeded 4%, the fatigue life decreased. This can be attributed to the increased initial damage caused by the extrusion process, which easily leads to premature fatigue failure of the specimens.

[0116] like Figure 14 The image shows the damage distribution of composite laminate structures under a tensile displacement of 0.3 mm with different extrusion amounts. The results indicate that the NHE specimen (ordinary specimen) exhibits significant uneven damage distribution; failure is most pronounced on the inner side of the CFRP regardless of fiber or matrix compression. The composite material with an extrusion amount of 3% shows a more uniform stress distribution on the pore walls, less initial damage, and a maximum stress of 749 MPa, only 76% of that of the ordinary specimen. The bending degree of the high-strength bolts is also reduced. Although the composite specimen with an extrusion amount of 5% shows more initial damage caused by pore extrusion, its stress distribution is significantly better than that of the ordinary specimen. In summary, the system verifies the correctness of selecting the 3% extrusion amount.

[0117] The method proposed in this invention enables rapid quantitative optimization of extrusion strengthening process parameters for composite laminate joint holes, based on finite element simulation and parameter mapping. This method systematically analyzes the influence of key process parameters such as bushing thickness, mandrel cone angle, and hole extrusion amount on stress distribution and damage behavior, guiding the selection of optimal parameter combinations with the goals of minimizing damage and maximizing strengthening effect. This not only avoids the high cost and long cycle of traditional experimental methods but also accurately predicts the performance of the strengthened structure in service, providing an efficient and reliable optimization method for the extrusion strengthening process of composite laminate joint holes, significantly improving the overall durability and safety reliability of the joint structure.

Claims

1. An optimized method for extrusion strengthening of connecting holes in composite laminate structures, characterized in that... Includes the following steps: S1: Establish the finite element model of the extrusion of the joint hole in the composite material and the finite element model of the connection of the composite laminate structure under load based on the progressive damage theory, and define the boundary conditions. S2: Based on the composite material connection hole extrusion finite element model established by S1, the hole extrusion amount, bushing thickness and mandrel cone angle are used as process parameter variables to simulate the hole extrusion strengthening process under different process parameters, and obtain the hole periphery stress distribution and damage state. S3: With the optimization objectives of minimizing hole periphery damage and maximizing the strengthening effect, based on the simulation results obtained in S2, the optimal fixed parameters of bushing thickness and mandrel cone angle are first screened and determined. Then, with hole extrusion amount as the core optimization variable, the mapping relationship between hole extrusion amount and damage is established, and the optimal hole extrusion amount parameter is screened out, thereby achieving rapid quantitative optimization of process parameters. S4: Under the conditions of fixed bushing thickness and fixed mandrel cone angle determined in S3, static tensile tests and fatigue tests were carried out on composite laminate pore extrusion reinforced specimens with different pore extrusion amounts. By comparing the results with the finite element model of the composite laminate structure connection under load established in S1, the correctness of the pore extrusion amount optimization results, as well as the damage state and fatigue life of the reinforced connection pores during service, were verified.

2. The method for optimizing the extrusion strengthening process of connecting holes in a composite laminate structure according to claim 1, characterized in that, In step S1, the extrusion finite element model of the composite material connection hole and the load-bearing finite element model of the composite material laminate structure connection were established based on the ABAQUS platform.

3. The method for optimizing the extrusion strengthening process of connecting holes in a composite laminate structure according to claim 2, characterized in that, In step S1, the finite element model of the composite material connection hole extrusion includes the composite material specimen, the extrusion mandrel, and the bushing; the finite element model of the composite material laminate structure connection under load includes the composite material laminate specimen, the bushing, and the high-locking bolt.

4. The method for optimizing the extrusion strengthening process of connecting holes in a composite laminate structure according to claim 3, characterized in that, Both the extrusion finite element model of composite material connection holes and the load-bearing finite element model of composite material laminated structure connection are used to predict damage by combining the Hashin failure criterion and the Linde stiffness degradation model. Hashin's failure criteria specifically include four failure modes: fiber tensile failure, fiber compressive failure, matrix tensile failure, and matrix compressive failure. The Linde stiffness degradation model specifically includes damage variables corresponding to four typical damage modes: fiber tensile damage variables, fiber compressive damage variables, matrix tensile damage variables, and matrix compressive damage variables. The formula is as follows: Where d represents the damage variable, F represents the load variable, and D represents the parameter related to the material fracture energy.

5. The method for optimizing the extrusion strengthening process of connecting holes in a composite laminate structure according to claim 4, characterized in that, The rapid quantitative optimization of process parameters in step S3 does not rely on building additional proxy models, but is directly based on the visualized cloud map and quantitative data generated by finite element simulation.

6. The method for optimizing the extrusion strengthening process of connecting holes in a composite material laminate structure according to claim 5, characterized in that, The process of selecting the optimal orifice extrusion amount in step S3 specifically includes: First, by comparing the stress distribution cloud maps under different orifice extrusion amounts, selecting a bushing thickness that can generate uniform pressure and does not produce local yielding; Second, by comparing the damage distribution under different mandrel cone angles, selecting a cone angle combination that can balance inlet damage and outlet strengthening effect; Finally, under the optimal conditions of fixed bushing thickness and mandrel cone angle, determining the critical orifice extrusion amount that will not cause unacceptable initial damage when achieving the expected strengthening effect.

7. The method for optimizing the extrusion strengthening process of connecting holes in a composite laminate structure according to claim 6, characterized in that, Step S4 specifically includes: conducting static tensile tests and fatigue tests on composite laminate pore extrusion reinforced specimens with different pore extrusion amounts, simultaneously measuring the strain field around the pores of the composite material using digital image correlation technology, and comparing the test results with the simulation prediction results of the finite element model of the composite laminate structure connection under load to verify the correctness of the parameter optimization results.

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