A differential extrusion process for a composite material stack

Abstract

This invention belongs to the field of composite material laminate processing technology, and discloses a hole extrusion process for composite material laminates with differentiated extrusion amounts. The composite material laminate consists of n material layers. During hole extrusion, the extrusion amount of each of the n material layers is controlled to reach the optimal extrusion amount. The extrusion amount is the rate of change of the pore diameter of the connecting holes within the material layers before and after hole extrusion. The optimal extrusion amount for a single material layer is determined by first determining the preset range of the extrusion amount for that material layer, then conducting finite element simulation experiments, recording the pore periphery stress, bushing residual stress, and pore wall damage percentage of that material layer under different extrusion amounts, and taking the extrusion amount with the maximum pore periphery stress, the maximum bushing residual stress, and a pore wall damage percentage within 20% as the optimal extrusion amount for that material layer. This invention effectively reduces stress concentration in the connecting holes of composite material laminates, homogenizes the nail load distribution in multi-nail connections, and significantly improves the performance of its connection structure.

Description

Technical Field

[0001] This invention belongs to the field of composite material lamination technology, and relates to a hole extrusion process for differentiated extrusion amounts in composite material lamination. Background Technology

[0002] Composite materials, with their unique advantages such as high specific strength, high specific modulus, lightweight, and excellent corrosion resistance, have become one of the key materials for load-bearing structures in the aerospace field. In actual engineering assembly processes, these composite structural components often need to be reliably connected to metal parts such as titanium alloys and aluminum alloys. This method of connecting heterogeneous materials places higher demands on the reliability and durability of the structure. Multi-nail connections, due to their ability to distribute loads through multiple fasteners and effectively adapt to complex stress scenarios, have become the mainstream method for assembling such heterogeneous structures. By rationally arranging multiple fasteners, multi-nail connections can improve the overall strength and stability of the connected structure to a certain extent, adapting to complex engineering application requirements. However, composite materials exhibit significant anisotropy, with their mechanical properties varying significantly in different directions. Furthermore, composite materials are almost purely elastic, lacking the plastic deformation capacity of metallic materials. This makes composite materials prone to stress concentration at connection holes, and the uneven distribution of nail load is particularly pronounced in multi-nail connections. This uneven load distribution not only significantly reduces the static strength of the connected structure but also drastically shortens its fatigue life, thus becoming a weak point in structural design.

[0003] Patent application CN120963135A discloses a laminated structure and a method for processing differentiated holes in the laminated structure. This method involves performing a first reaming of the laminated structure, selecting bushings of suitable materials based on the material properties of different plates, and fitting the bushings onto a tapered mandrel according to the laminated sequence. An axial force is applied using an extrusion device to pull the mandrel, ensuring an interference fit between the bushing and the corresponding plate. Finally, a second reaming is performed to ensure consistent bushing inner diameter, achieving differentiated hole processing. However, the compatibility of the bushing material with the plate material lacks quantifiable parameter optimization standards, relying mainly on experience for selection. It also fails to provide optimization schemes for hole extrusion process parameters (such as extrusion amount, bushing thickness, and mandrel taper angle), making it difficult to improve the strength and fatigue life of the connection structure through process improvements.

[0004] Patent application CN105588759A discloses an experimental method for indirectly determining the nail load distribution ratio during the failure process of a composite multi-nail joint structure. This method alternates between ordinary titanium alloy bolts and nail load sensors, applying displacement loads and force control loads according to ASTM standards. Through repeated assembly, loading, and unloading, the nail load distribution ratio of the composite multi-nail joint structure is indirectly determined throughout its entire failure process. However, this method focuses primarily on determining nail load distribution, without addressing structural strengthening improvements related to the hole extrusion process. Furthermore, the repeated disassembly and reassembly of bolts and nail load sensors can affect the assembly accuracy of the joint structure. It also fails to address the issue of adaptability of extrusion amounts for different material layers in the hole extrusion process, thus failing to improve the root cause of uneven nail load distribution from the processing stage. Additionally, it does not consider the influence of residual stress fields after hole extrusion on the nail load transmission path, making it difficult to guide the optimized design of the hole extrusion process.

[0005] Patent application CN113722861A discloses a method for predicting the strength and failure modes of composite bolted connections. This method first determines key information such as the material and geometric parameters of the composite bolted connection structure, establishes a multi-nail three-dimensional stress analysis model to obtain the extrusion stress and bypass stress of the bolt holes, and then draws the strength envelope using a single-nail progressive damage model to predict the strength and failure modes (net cross-section tensile failure and extrusion failure) of the bolt holes. However, this method is only a failure prediction method for the connection structure and does not make technical improvements to the hole extrusion process itself. Furthermore, it does not incorporate key parameters such as the residual stress field and hole wall damage after hole extrusion, and therefore cannot quantify the impact of the hole extrusion process on bolt load distribution and failure modes.

[0006] Therefore, a hole extrusion process with differentiated extrusion amounts for composite material laminates is needed to solve the above problems, which is of great significance. Summary of the Invention

[0007] The purpose of this invention is to solve the problems existing in the prior art and to provide a hole extrusion process for differentiated extrusion amounts of composite material laminates.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A hole extrusion process for differential extrusion amount of composite material stack, wherein the composite material stack consists of n material layers, n≥2, and the extrusion amount of each of the n material layers is controlled to reach the optimal extrusion amount during hole extrusion.

[0010] The extrusion amount is the rate of change in the pore diameter of the connecting pores within the material layer before and after pore extrusion;

[0011] The method for determining the optimal extrusion amount of a single material layer is as follows: First, determine the preset range of the extrusion amount of the material layer based on the type and mechanical properties of the material. Then, conduct finite element simulation tests and record the hole periphery stress, bushing residual stress, and hole wall damage ratio of the material layer under different extrusion amounts. The extrusion amount with the maximum hole periphery stress, the maximum bushing residual stress, and the hole wall damage ratio (i.e., the percentage of the number of meshes that are determined to have damage and have been removed in the finite element simulation results relative to the total number of meshes on the hole wall surface) within 20% is taken as the optimal extrusion amount of the material layer.

[0012] As a preferred technical solution:

[0013] The hole extrusion process for differentiated extrusion amounts in composite material laminates, as described above, includes the following specific steps:

[0014] (A) Drilling and reaming connection holes on composite material laminates;

[0015] (B) Insert a mandrel into n bushings at the same time (lubrication measures can be applied to the contact interface between the bushing and the mandrel to reduce frictional resistance), and then install the n bushings into a connecting hole at the same time. The n bushings are embedded in n material layers respectively. The initial outer diameter of the n bushings is the same and slightly smaller than the initial diameter of the connecting hole to ensure that the bushings are smoothly installed into the connecting hole and to reserve a compression allowance.

[0016] The optimal extrusion amount E for the j-th material layer j The thickness t of the inner bushing of the j-th material layer j The maximum diameter D of the mandrel m Bushing outer diameter D w The following relationship must be satisfied:

[0017] ;

[0018] Where j = 1, 2, ..., n;

[0019] (C) Axial extrusion of the mandrel: An axial force is applied to the mandrel so that the largest diameter portion of the mandrel extrudes n bushings;

[0020] (D) Remove the mandrel and bore the final hole.

[0021] The specific steps of the finite element simulation test for the hole extrusion process with differentiated extrusion amounts for composite material laminates as described above are as follows:

[0022] (a) Constructing a finite element model of the perforation extrusion process based on the material constitutive model;

[0023] (b) Define the mandrel as a rigid body, and set the boundary constraints of each component (consistent with the tooling of the actual extrusion test) and the mandrel loading path according to the tooling constraints and loading method of the actual extrusion test.

[0024] (c) Define the contact relationship;

[0025] (d) Select a set of discretized extrusion amounts from the preset range, simulate the axial extrusion process of the mandrel, and output the periphery stress, bushing residual stress and hole wall damage ratio of the material layer under different extrusion amounts.

[0026] In the hole extrusion process of a composite material laminate with differentiated extrusion amount as described above, in step (a), the material layer, bushing and mandrel in the finite element model of the hole extrusion process are all selected as eight-node linear hexahedral elements, and the elements near the hole wall are meshed to improve the calculation accuracy.

[0027] In the above-described composite material laminate differential extrusion process, in step (c), the contact relationship is as follows: the friction coefficient of the contact interface between the mandrel and the bushing is set to 0.05~0.1, and the friction coefficient of the contact interface between the bushing and the material layer is set to 0.1~0.15, and a penalty contact algorithm is adopted.

[0028] The above-described composite material laminate differential extrusion process involves, after extrusion, inserting a bolt into each bushing and tightening it with a nut to form a multi-bolt connection structure.

[0029] The above-described orifice extrusion process for a composite material laminate with differentiated extrusion amounts forms a multi-nail connection structure. After this structure is formed, a static tensile test is performed on the multi-nail connection structure to obtain the maximum static tensile load.

[0030] The hole extrusion process with differentiated extrusion amount of a composite laminate as described above also employs digital image correlation (DIC) technology simultaneously during static tensile testing to evaluate the nail load distribution of the multi-nail connection structure.

[0031] This invention uses DIC (Diverterless Compressive Testing) to obtain the strain history of different regions in a multi-nail joint structure of composite materials, and then combines it with theoretical calculations to accurately obtain the nail load distribution at any given time. Existing technologies typically use bonded strain gauges for measurement, which has limited measurement points, general positional accuracy, and relatively large errors in the measured nail load distribution data. Compared with existing technologies, this invention has more measurement points, higher accuracy, and can quickly and accurately quantify the uniformity of nail load.

[0032] The above-described orifice extrusion process for a composite material laminate with differentiated extrusion amounts forms a multi-nail connection structure. After this structure is formed, a tensile-tensile fatigue test is performed on the multi-nail connection structure to obtain the average fatigue life.

[0033] Beneficial effects:

[0034] (1) This invention optimizes the load state of the connecting holes of composite materials through bushing extrusion process, effectively reduces stress concentration in the connecting holes of composite material laminates, homogenizes the nail load distribution of multi-nail connections, significantly improves the performance of its connection structure, and solves the problem of insufficient structural performance caused by uneven nail load distribution and stress concentration in multi-nail connections of composite material laminates.

[0035] (2) This invention uses finite element simulation experiments to quantitatively determine the optimal extrusion amount and matching bushing thickness of different material layers, thereby achieving precise control of bushing matching, mandrel parameters and contact stress.

[0036] (3) The present invention forms a stable residual stress field around the connecting hole through the differential hole extrusion process, and homogenizes the nail load distribution of the multi-nail connection from the processing source. There is no need to repeatedly disassemble and reassemble bolts and sensors. Furthermore, the nail load homogenization effect is accurately verified through DIC technology and load-strain ratio distribution method, taking into account both process improvement and effect quantification.

[0037] (4) This invention combines hole extrusion process with parameter optimization. It incorporates key parameters such as residual stress around the hole and the proportion of hole wall damage through finite element simulation, and provides a complete optimization scheme covering extrusion amount, bushing thickness and mandrel cone angle. It can not only achieve structural strengthening, but also verify the strength improvement effect through static tensile and fatigue tests, filling the technical gap of "prediction-improvement-verification". Attached Figure Description

[0038] Figures 1-4 Figures show the stress around the holes and the residual stress of the bushing in the CFRP layer under different extrusion rates. In each figure, a represents the stress around the holes in the CFRP layer under different extrusion rates, and b represents the residual stress of the bushing in the CFRP layer under different extrusion rates.

[0039] Figure 5 This invention illustrates the perforated stress and pore wall damage ratio of the CFRP layer under different extrusion amounts.

[0040] Figure 6 This is a schematic diagram illustrating the specific process of the hole extrusion of the present invention; in the diagram, the red arrow indicates the extrusion direction of the mandrel;

[0041] Figure 7 These are schematic diagrams of different sample structures of the present invention;

[0042] Figure 8 This is an example diagram of the static tensile and DIC testing device for the specimen of the present invention; the sprayed speckle coating in the diagram is for subsequent image recognition and strain detection;

[0043] Figure 9 This is a graph showing the static tensile test results of various samples from this invention.

[0044] Figure 10 The locations of strain measurement points for each section of the CFRP specimen of this invention.

[0045] Figure 11 The DIC strain contour plots of each specimen under a static tensile load of 15 kN are shown below.

[0046] Figure 12 This is a sector diagram showing the nail load distribution of each specimen under a static tensile load of 15 kN according to the present invention.

[0047] Figure 13 This is a schematic diagram of the tensile fatigue test of the specimen according to the present invention; in the figure, a is an example diagram of the tensile fatigue test of the CFRP / Ti stacked three-pronged connected specimen, b is a partial enlarged view of a, and c is a parallel specimen of the CFRP / Ti stacked three-pronged connected specimen.

[0048] Figure 14 This is a bar chart of the tensile fatigue test results of this invention;

[0049] In the figure, 1-CFRP layer, 2-Ti alloy layer, 3-mandrel, 4-titanium alloy seamless bushing, 5-stainless steel seamless bushing, 6-CCD camera. Detailed Implementation

[0050] 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.

[0051] A hole extrusion process for differentiated extrusion amounts of a composite material laminate, wherein the composite material laminate consists of two material layers, namely a CFRP (carbon fiber reinforced resin matrix composite) layer and a Ti alloy layer, and the specific steps are as follows:

[0052] (1) Determine the optimal extrusion amount of the CFRP layer;

[0053] The method is as follows: First, determine the preset range of the extrusion amount of the CFRP layer (2%~5%), then conduct finite element simulation tests, record the hole periphery stress, bushing residual stress and hole wall damage ratio of the CFRP layer under different extrusion amounts, and take the extrusion amount with the maximum hole periphery stress, the maximum bushing residual stress and the hole wall damage ratio within 20% as the optimal extrusion amount of the CFRP layer.

[0054] The steps of finite element simulation experiments are as follows:

[0055] (a) Constructing a finite element model of the perforation extrusion process based on the material constitutive model;

[0056] In the finite element model of the hole extrusion process, the CFRP layer, bushing and mandrel are all selected as eight-node linear hexahedral elements. The mesh of the elements near the hole wall is refined to improve the calculation accuracy.

[0057] (b) Define the mandrel as a rigid body, and set the boundary constraints of each component (consistent with the tooling of the actual extrusion test) and the mandrel loading path according to the tooling constraints and loading method of the actual extrusion test.

[0058] (c) Define the contact relationship;

[0059] The contact relationship is as follows: the friction coefficient of the contact interface between the mandrel and the bushing is set to 0.05~0.1, and the friction coefficient of the contact interface between the bushing and the CFRP layer is set to 0.1~0.15, using a penalty contact algorithm;

[0060] (d) Select a set of discretized extrusion amounts (2%, 2.5%, 3%, 3.5%, 4%, 4.5%, and 5%, respectively) from a preset range to simulate the axial extrusion process of the mandrel. Output the peri-hole stress, bushing residual stress, and hole wall damage percentage of the CFRP layer under different extrusion amounts. Some results are shown below. Figures 1-5 As shown;

[0061] analyze Figures 1-5 It can be seen that when the extrusion amount is 2%, the stress around the hole is 285 MPa, and the proportion of hole wall damage is only 2.7%; when the extrusion amount increases to 3%, the stress around the hole rises to 558 MPa, and the proportion of hole surface damage increases to 16.3%; when the extrusion amount increases to 4%, the proportion of hole surface damage increases to 33.5%; when the extrusion amount increases to 5%, the proportion of hole surface damage increases to 59.7%; however, the stress on the bushing does not change significantly with the extrusion amount. Under small extrusion amounts, the stress on the bushing is relatively uniform, and when the extrusion amount increases, obvious stress concentration appears at the outlet.

[0062] Considering the above factors, when the extrusion amount of the CFRP layer is controlled at around 3%, the damage to the CFRP layer is not serious, the negative impact on its bonding performance is small, and it also has a significant peripore stress field, resulting in a better extrusion effect. Therefore, the optimal extrusion amount for the CFRP layer is 3%.

[0063] (2) Determine the optimal extrusion amount of the Ti alloy layer;

[0064] The determination method is the same as for the CFRP layer; the optimal extrusion amount for the Ti alloy layer is 5%.

[0065] (3) Hole extrusion, the specific process is as follows Figure 6 As shown;

[0066] (A) Drill and reamer three connecting holes (referred to as hole A, hole B, and hole C, with an initial diameter of 7.6 mm) on the composite material laminate.

[0067] (B) After inserting a mandrel 3 (with a maximum diameter of 6.38 mm) into two bushings (a seamless titanium alloy bushing 4 and a seamless stainless steel bushing 5) at the same time, the two bushings are simultaneously installed into a connecting hole. The two bushings are embedded in two material layers respectively (the seamless titanium alloy bushing is embedded in the CFRP layer 1, and the seamless stainless steel bushing is embedded in the Ti alloy layer 2). The initial outer diameter of the two bushings is 7.58 mm, which is slightly smaller than the initial hole diameter of the connecting hole.

[0068] The thickness of the seamless titanium alloy bushing 4 is 0.69 mm, so that the extrusion amount of the CFRP layer 1 can reach the optimal extrusion amount.

[0069] The thickness of the stainless steel seamless bushing 5 is 0.75mm, so that the extrusion amount of the Ti alloy layer 2 can reach the optimal extrusion amount.

[0070] (C) Axial compression of the mandrel: An axial force is applied to the mandrel so that the largest diameter portion of the mandrel compresses the two bushings;

[0071] (D) Remove the mandrel and bore the final hole.

[0072] like Figure 7 As shown, after the hole extrusion, the present invention also inserts a bolt into each bushing and tightens it with a nut to form a multi-bolt connection structure, denoted as sample THE-S. For comparison, the present invention also prepared samples NTHE, THE-A, and THE-BC.

[0073] The difference between sample NTHE and sample THE-S is that its A, B, and C holes have not undergone hole extrusion.

[0074] The difference between sample THE-A and sample THE-S is that only hole A in THE-A is subjected to hole extrusion.

[0075] The difference between sample THE-BC and sample THE-S is that only holes B and C in THE-BC are subjected to hole extrusion.

[0076] Furthermore, the present invention also conducted static tensile tests and tensile-tensile fatigue tests on four types of specimens to verify the effect of the hole extrusion process on improving the static strength and fatigue life of the specimens.

[0077] like Figure 8As shown, the specific process of the static tensile test is as follows: Accurately measure the original dimensions of the specimen (such as gauge length and diameter) to calculate the cross-sectional area. The specimen is vertically clamped in the matching fixture of the MTS-C4310 universal tensile testing machine, ensuring that the specimen axis coincides with the center line of the clamps to avoid bending stress. The testing machine is set to displacement control mode and a constant tensile speed of 2 mm / min for loading. During loading, the testing machine applies a continuous and stable axial tensile force to the specimen. The data acquisition system synchronously records the force-displacement data throughout the process (the CCD camera 6 is equipped with a 50mm focal length lens to capture specimen strain information and acquire 2736×2192 pixel digital images) until the specimen breaks. When the loading force drops by more than 80% from its peak value, the specimen is considered to have completely failed, and the test automatically stops. The maximum force value recorded at this point is the maximum static tensile load.

[0078] Static tensile test results are as follows Figure 9 As shown, the maximum static tensile load of the un-perforated specimen NTHE was 45.89 kN, while the maximum static tensile loads of specimens THE-S and THE-BC were 48.78 and 49.91 kN, respectively, representing increases of 6.3% and 8.8% compared to NTHE. The static tensile properties of the specimens after perforation were significantly improved. This is because the bushing installed during perforation reduces bolt bending under load, increases the bearing area of ​​the hole wall, and reduces stress concentration. More importantly, the CFRP layer does not undergo plastic deformation, leading to uneven load distribution in the multi-nail connection structure under load. However, the bushing installed during perforation undergoes plastic deformation, which to some extent has a beneficial effect on the homogenization of the nail load, thereby improving the overall load-bearing capacity of the specimen. However, the load-bearing capacity of specimen THE-A, where only hole A underwent perforation, decreased by 4.2%, to only 43.95 kN. The decrease in load-bearing capacity of the multi-nail connection structure due to perforation of hole A is mainly due to two reasons. Firstly, the uneven distribution of bolt load in multi-bolt connections during load-bearing results in the largest tensile load being borne by hole A. Furthermore, hole extrusion requires drilling a larger diameter hole to install the bushing, leading to a smaller hole edge distance in the specimen and a partial decrease in load-bearing capacity. Secondly, the hole extrusion process is similar to an interference connection. While it increases the contact area between the bolt and the hole wall and reduces stress concentration in a single hole during load-bearing, it also affects the distribution of bolt load.

[0079] like Figure 8As shown, during the static tensile test, this invention simultaneously employs DIC (Digital Extensometer) to evaluate the load distribution of the multi-nail connection structure. The specific procedure is as follows: First, a speckle pattern is created on the specimen surface, and multiple strain measurement sections are set along the nail arrangement direction. Multiple measurement points, including the intersection points of the bolt axes, are arranged on each section. Next, while the testing machine applies a static tensile load to the specimen, a sequence of speckle images of the specimen surface is continuously captured using a CCD camera or similar equipment. Then, these images are processed using DIC software, and strain information at each section measurement point under different loads is extracted using the virtual extensometer principle. The average strain of each section is then calculated. To obtain a more accurate strain caused by pure tension, the influence of the bending component needs to be eliminated. Specifically, the strain range on the same section is statistically analyzed to obtain the bending strain, which is then subtracted from the average strain. Finally, based on the corrected average tensile strain data of each section, the actual load percentage borne by each nail in the structure is calculated using the load-strain ratio distribution method. By comparing and analyzing the nail load distribution results of specimens treated with the perforated extrusion process and those without, the effect of this process on improving the uniformity of load distribution in multi-nail connection structures, i.e., the uniformity effect of nail load distribution, can be quantitatively evaluated.

[0080] On the CFRP specimen, the strain measurement locations are as follows: Figure 10 As shown. The average strain of each section is obtained by measuring at points at different cross-sections, as shown in formula (1). The cross-section is defined as a parallel cross-section perpendicular to the loading direction. Since the contact force at the hole edge is not uniformly distributed along the thickness direction, bending will occur, affecting the measurement results of the nail load distribution. Therefore, it is necessary to measure the strain at different cross-sections to correct the influence of bending. The measurement points need to be selected at any cross-section between the two holes. The distance between each cross-section and the hole should be consistent, and the selected points should start from the axis and the number should not be less than 4. For the hole closest to the loading position, it is required to measure at least one point at the intersection of its cross-section and the axis, and at least one point on each side of the axis.

[0081]

[0082] In the formula, ε1-ε 11 for Figure 10 The strain values ​​measured at positions 1-11 in the middle, This represents the average strain measured at the corresponding cross section, where i represents different cross sections.

[0083] The corrected average tensile strain is calculated using formula (2):

[0084]

[0085] In the formula, This represents the average strain caused by tension at the corresponding cross section. This represents the average strain measured at the corresponding cross section. This represents the absolute value of the CFRP surface strain caused by the bending moment at the corresponding cross section.

[0086] Substitute into equation (3) to calculate the nail load distribution in different specimens:

[0087]

[0088] In the formula, η A η B η C These represent the percentage of bolt load for bolts A (the bolt passing through hole A), B (the bolt passing through hole B), and C (the bolt passing through hole C), respectively.

[0089] DIC analysis was performed on the four specimens, and strain contour plots were plotted for each specimen. Figure 11 The DIC strain contour plots of each specimen under a static tensile load of 15 kN are shown. Figure 12 The screw load distribution of each specimen under a static tensile load of 15 kN is shown. The screw load distribution in the NTHE specimen is not uniform; screw A bears the largest load, accounting for 42.2%, while screws B and C account for 25.5% and 32.3%, respectively. After hole compression in the THE-A specimen, the load borne by screw A increases further, reaching 45.2%. Although hole compression increases the contact area between the bolt and the hole wall, the assembly clearance of other non-critical screws is larger than the hole after hole compression reinforcement, which also affects the screw load distribution, thus increasing the screw load proportion of screw A. This is why the maximum static tensile load of the THE-A specimen is lower than that of the NTHE specimen. The screw load distribution in the THE-BC and THE-S specimens is more uniform after hole compression, and the load on screw A decreases, allowing them to withstand greater loads.

[0090] like Figure 13As shown, the specific process of the tensile fatigue test is as follows: Tensile fatigue testing is performed on the specimens using fatigue testing equipment (such as the MTS-370.10 electro-hydraulic servo fatigue testing machine). Reasonable stress levels, stress ratios, loading frequencies, and loading waveforms are set. Multiple parallel specimens are set for each test, and the average fatigue life is taken as the evaluation index to analyze the inhibitory effect of the hole extrusion process on fatigue failure. Specifically, the tensile fatigue test is conducted at room temperature using the MTS-370.10 electro-hydraulic servo fatigue testing machine. This machine has an actuator stroke of ±150mm, a displacement resolution of 0.01mm, a maximum dynamic test force of 100kN, a loading accuracy of ±1N in force control mode, and a loading frequency range of 0.01~50Hz. In the fatigue test, the stress level q is selected as 65% and 80% of the maximum static tensile load, the loading frequency is 5Hz, the stress ratio R is 0.1, and the loading waveform is a sine wave. Five parallel tests are performed for each parameter, and the average value represents the fatigue life under that parameter.

[0091] like Figure 14 As shown, the test results demonstrate that the hole extrusion process can effectively improve the tensile fatigue performance of multi-nail joint structures. For example, under a load of 36.7 kN (80% of the maximum static tensile load), the fatigue life of the NTHE specimen without hole extrusion is 1.22 × 10⁻⁶ kN. 4 The lifespan of the THE-S sample after pore extrusion was 1.75 × 10⁻⁶. 4 The lifespan increased by approximately 43% when the load was reduced to 29.8 kN (65% of the maximum static tensile load); when the load was reduced to 29.8 kN (65% of the maximum static tensile load), the lifespan of the specimen increased by approximately 67%, indicating that the strengthening effect caused by pore extrusion became more pronounced as the load decreased.

Claims

1. A perforated extrusion process for differentiated extrusion amounts in a composite material laminate, wherein the composite material laminate consists of n material layers, n≥2, characterized in that, During hole extrusion, the extrusion amount of each of the n material layers is controlled to reach the optimal extrusion amount. The extrusion amount is the rate of change in the pore diameter of the connecting pores within the material layer before and after pore extrusion; The method for determining the optimal extrusion amount of a single material layer is as follows: first, determine the preset range of the extrusion amount of the material layer, then conduct finite element simulation tests, record the hole periphery stress, bushing residual stress and hole wall damage ratio of the material layer under different extrusion amounts, and take the extrusion amount with the maximum hole periphery stress, the maximum bushing residual stress and the hole wall damage ratio within 20% as the optimal extrusion amount of the material layer.

2. The orifice extrusion process for differentiated extrusion amounts of composite material laminates according to claim 1, characterized in that, The specific steps of hole extrusion are as follows: (A) Machining connection holes on composite material laminates; (B) After inserting a mandrel into n bushings at the same time, the n bushings are simultaneously installed into a connecting hole. The n bushings are embedded in n material layers respectively, and the initial outer diameter of the n bushings is the same. The optimal extrusion amount E for the j-th material layer j The thickness t of the inner bushing of the j-th material layer j The maximum diameter D of the mandrel m Bushing outer diameter D w The following relationship must be satisfied: ； Where j = 1, 2, ..., n; (C) Axial extrusion of the mandrel: An axial force is applied to the mandrel so that the largest diameter portion of the mandrel extrudes n bushings; (D) Remove the mandrel and bore the final hole.

3. The orifice extrusion process for differentiated extrusion amounts of composite material laminates according to claim 2, characterized in that, The specific steps of the finite element simulation experiment are as follows: (a) Constructing a finite element model of the perforation extrusion process based on the material constitutive model; (b) Define the mandrel as a rigid body, and set the boundary constraints of each component and the loading path of the mandrel according to the tooling constraints and loading method of the actual extrusion test; (c) Define the contact relationship; (d) Select a set of discretized extrusion amounts from the preset range, simulate the axial extrusion process of the mandrel, and output the periphery stress, bushing residual stress and hole wall damage ratio of the material layer under different extrusion amounts.

4. The orifice extrusion process for differentiated extrusion amounts of composite material laminates according to claim 3, characterized in that, In step (a), in the finite element model of the hole extrusion process, the material layer, bushing and mandrel are all selected as eight-node linear hexahedral elements, and the elements near the hole wall are meshed.

5. The orifice extrusion process for differentiated extrusion amounts of composite material laminates according to claim 3, characterized in that, In step (c), the contact relationship is as follows: the friction coefficient of the contact interface between the mandrel and the bushing is set to 0.05~0.1, and the friction coefficient of the contact interface between the bushing and the material layer is set to 0.1~0.15, and the penalty contact algorithm is adopted.

6. The orifice extrusion process for differentiated extrusion amounts of composite material laminates according to claim 1, characterized in that, After the holes are extruded, a bolt is inserted into each bushing and tightened with a nut to form a multi-bolt connection structure.

7. The orifice extrusion process for differentiated extrusion amounts of composite material laminates according to claim 6, characterized in that, After forming the multi-nail connection structure, a static tensile test was conducted on the multi-nail connection structure to obtain the maximum static tensile load.

8. The orifice extrusion process for differentiated extrusion amounts of composite material laminates according to claim 7, characterized in that, During the static tensile test, digital image correlation technology was also used simultaneously to evaluate the nail load distribution of the multi-nail connection structure.

9. The orifice extrusion process for differentiated extrusion amounts of composite material laminates according to claim 6, characterized in that, After forming the multi-pronged connection structure, a tensile-tensile fatigue test was conducted on the multi-pronged connection structure to obtain the average fatigue life.

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