Non-standard composite material bolt and design method thereof

Abstract

The application provides a non-standard composite material bolt and a design method thereof, and the method comprises the following steps: constructing a parameterized model of a bolt-nut connecting structure; based on the parameterized model, performing multi-working condition mechanical simulation on the connecting structure by using a finite element software, verifying a preset performance analysis criterion, and obtaining mechanical simulation data and performance analysis conclusions; for a fixed nominal outer diameter bolt, based on the mechanical simulation data and the analysis conclusions, performing collaborative iteration by using an optimization algorithm and the finite element software, and obtaining an optimal geometric parameter combination of the bolt; for the optimal geometric parameter combination, calculating the bolt-nut contact area and thread fatigue resistance analysis by using the finite element software, and obtaining an optimal material combination of the bolt-nut; and based on the optimal geometric parameter combination, the optimal material combination and performance verification data, obtaining a design scheme of the non-standard composite material bolt. The scheme can significantly reduce the falling risk of the composite material fastener thread.

Description

Technical Field

[0001] This invention relates to the fields of mechanical engineering and materials technology, and in particular to a non-standard composite material bolt and its design method. Background Technology

[0002] Bolts, as essential fasteners, play an irreplaceable role in various structural connections. Their design directly impacts the overall structural strength and safety. While traditional metal bolts demonstrate good strength and stability, they face challenges in areas such as lightweighting, corrosion resistance, and compatibility with composite material components, particularly in harsh environments where their performance is often insufficient. For example, when using metal bolts to assemble large composite material components, the high rigidity of the bolts and the low shear strength of the composite material can lead to localized stress concentration, delamination and matrix cracking under cyclic loading, uneven load distribution between threads due to material anisotropy, and potential corrosion.

[0003] In recent years, composite materials, due to their excellent specific strength and corrosion resistance, have been widely used in fastener manufacturing through injection molding, machining, or continuous fiber molding processes. However, their anisotropic material properties lead to fundamental differences from traditional metallic materials in static load-bearing capacity, dynamic durability, and failure modes. Currently, the industry lacks thread design data and experience specifically tailored to the characteristics of composite materials, resulting in numerous blind spots in actual design processes. Furthermore, existing standard thread design concepts are still focused on metallic systems and cannot effectively guide the optimization of thread stress area, geometric structure adjustment, and load-bearing failure mode design when directly applied to composite materials. How to increase the contact area through non-standard geometric parameter design to avoid the risk of thread peeling has become a bottleneck that urgently needs to be overcome in high-performance composite material joining technology.

[0004] Therefore, there is an urgent need to provide a non-standard composite material bolt and its design method. Summary of the Invention

[0005] This invention provides a non-standard composite material bolt and its design method, which can solve the problem of insufficient load-bearing capacity of composite material bolts when existing metal bolt design methods are directly transferred to composite material systems.

[0006] In a first aspect, the present invention provides a design method for non-standard composite material bolts, the method comprising: Construct a parametric model of the bolt-nut connection structure; wherein the parametric model includes geometric parameters, material parameters, and contact definitions; Based on the parametric model, finite element software was used to perform multi-condition mechanical simulation of the connection structure and verify the preset performance analysis criteria to obtain mechanical simulation data and performance analysis conclusions. Among them, the preset performance analysis criteria include thread multi-mode failure judgment criteria, head fillet radius to head thickness ratio influence analysis criteria, and material-ply matching performance comparison criteria. For bolts with a fixed nominal outer diameter, based on the aforementioned mechanical simulation data and analysis conclusions, the optimal combination of geometric parameters for the bolt is obtained through collaborative iteration using optimization algorithms and finite element software. For the optimal combination of geometric parameters, finite element software was used to calculate the contact area of ​​the bolt and nut and perform thread fatigue analysis to obtain the optimal material combination of the bolt and nut. Based on the optimal combination of geometric parameters, the optimal combination of materials, and the performance verification data, the design scheme for the non-standard composite material bolt is obtained. Preferably, the geometric parameters include the nominal outer diameter of the bolt, thread parameters, and bolt head parameters; the thread parameters include thread height, pitch, thread angle, tip width, and root width; the bolt head parameters include head thickness and under-head fillet radius; the material parameters include composite material grade, fiber layup direction, and winding or braiding method; and the contact is defined as surface-to-surface contact type and coefficient of friction.

[0007] Preferably, the parametric model for constructing the bolt-nut connection structure includes: Construct a three-dimensional geometric model of the bolt-nut connection structure based on the geometric parameters; The three-dimensional geometric model was meshed using finite element software; the bolt body was discretized using composite shell elements or solid elements, while the threaded area and contact surface were discretized using solid elements. Set the contact interface between the bolt and nut, and define the contact properties through the contact; By employing a layered approach to assign properties to the composite material and setting composite material damage criteria, as well as an analysis configuration file with three preset performance analysis criteria, a parametric model of the bolt-nut connection structure is obtained; wherein, the composite material damage criteria include the Hashin criterion or the PUCK criterion.

[0008] Preferably, the verification of the multi-mode failure judgment criteria of the screw thread using finite element software includes: performing finite element simulations based on interface shear failure mode, vertical fiber shear failure mode and mixed shear failure mode respectively, and combining them with preset composite material damage criteria to determine the stress characteristics and critical failure load of the screw thread under each failure mode.

[0009] Preferably, the method of using finite element software to verify the influence of the ratio of the lower fillet radius to the head thickness on the analysis criteria includes: simulating and analyzing the stress concentration distribution and load transfer path under different combinations of parameters of the ratio of the lower fillet radius to the head thickness, and comparing the load distribution ratio of the thread and the screw to optimize the lower fillet radius and head thickness of the bolt.

[0010] Preferably, the finite element method software is used to verify the material-layout matching performance comparison criteria, which includes: comparing the differences in mechanical properties of different composite material grades and fiber layup schemes; selecting suitable materials and fiber layup schemes based on the mechanical property comparison results; wherein, the fiber layup scheme includes layup angle, layup direction, winding or weaving method; and the mechanical properties include axial tensile strength, transverse shear strength and interlaminar fracture toughness.

[0011] Preferably, determining the optimal combination of geometric parameters for the bolt through the coordinated iteration of optimization algorithms and finite element software includes: In MATLAB software, multiple combinations of geometric parameters are generated through optimization algorithms. The finite element analysis software adjusts the mesh model of the bolt-nut connection structure based on the received combination of geometric parameters, repeats the mechanical simulation, and outputs the performance index corresponding to each combination of geometric parameters; wherein, the performance index includes load-bearing capacity, load transfer efficiency, and stress concentration factor; The MATLAB software iteratively updates the combination of geometric parameters based on the received performance indicators until the preset objective function and constraints are met, thus obtaining the optimal combination of geometric parameters for the bolt.

[0012] Preferably, the objective function includes maximizing load-bearing capacity, optimizing load transfer efficiency, minimizing thread slippage risk, and maximizing head connection strength; the constraints include processing feasibility boundaries and lower limits of strength indicators.

[0013] Preferably, finite element software is used to calculate the bolt-nut contact area and analyze the fatigue resistance of the threads, including: Identify the thread contact area, calculate the actual contact area of ​​the bolt and nut under different thread parameters, analyze the relationship between the contact area and the load-bearing capacity, and generate a contact area-load-bearing capacity correlation curve; For the combination of composite material bolts and composite material nuts, and the combination of composite material bolts and metal nuts, fatigue damage is calculated based on the pre-set fatigue load simulation cyclic stress process. The fatigue life and failure location of different combinations are compared to select the optimal material combination of bolt and nut.

[0014] Secondly, embodiments of the present invention also provide a non-standard composite material bolt, which is obtained by the design method described in any of the first aspects above.

[0015] Compared with the prior art, the present invention has at least the following beneficial effects: In this invention, a parametric model of the bolt-nut connection structure is first constructed, and multi-condition mechanical simulations are performed using finite element software to verify the preset performance analysis criteria. Based on this, the bolt geometric parameters are optimized according to the mechanical simulation data and performance analysis conclusions. Simultaneously, contact area calculations and thread fatigue analysis are conducted to determine the optimal material combination for the bolt and nut, ultimately forming a complete design scheme for non-standard composite material bolts. Unlike traditional approaches that rely on experience or simple adjustments to standard metal dimensions, this invention fully considers the lamination characteristics, anisotropy, and brittle failure modes of composite materials. By synergistically optimizing geometric parameters such as thread height, thread width, nut thickness, and head fillet radius, it can avoid nut failure under tensile and fatigue load conditions, guide the fiber in the thread area from interlaminar shear load-bearing mode to vertical fiber shear load-bearing mode, increase the load-bearing cross-sectional area, and improve the load transfer efficiency between the thread and the shank, thereby significantly reducing the risk of thread detachment in composite fasteners. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 A flowchart of a design method for non-standard composite material bolts provided by the present invention; Figure 2 A schematic diagram of the bolt head parameters of a non-standard composite material bolt provided by the present invention; Figure 3 A schematic diagram of the thread tooth parameters of a non-standard composite material bolt provided by the present invention; Figure 4 This is a schematic diagram of the failure mode structure of the thread teeth in a non-standard composite material bolt provided in Embodiment 1 of the present invention; Figure 5 This is a schematic diagram of the failure mode structure of the thread teeth in a standard composite bolt, which is provided as Comparative Example 1 of the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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 some embodiments of the present invention, but not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0019] In standard composite bolts, especially high-performance continuous fiber reinforced composite bolts, the fibers in the thread region are mostly distributed in parallel layers or wavy patterns, while the fibers in the bolt shank are mainly oriented axially. When the bolt is subjected to axial tensile loads, the fibers in the thread are prone to interlaminar shear failure and peel off from the bolt shank due to the above-mentioned orientation difference, resulting in the continuous fiber's load-bearing potential not being fully utilized, thus significantly weakening the overall tensile performance of the bolt. At the same time, the traditional fillet design of the nut head will cause significant stress concentration in this type of brittle material system, which will have an adverse effect on the uniformity of load distribution and failure mode control. Although finite element analysis models for composite material connections have made continuous progress, such as new finite element elements for rapidly predicting the load distribution of multi-bolt composite material connections, and three-dimensional finite element simulation methods that consider detailed thread geometry and assembly torque processes, existing research mainly focuses on the overall load transfer and damage prediction of the connection plate. System design methods for the geometric parameters of composite bolts, especially research that incorporates the influence of the fiber distribution orientation in the thread on mechanical failure into the design considerations, are still relatively rare.

[0020] Based on this, such as Figure 1 As shown in the figure, an embodiment of the present invention provides a design method for non-standard composite material bolts, the method comprising: Construct a parametric model of the bolt-nut connection structure; wherein the parametric model includes geometric parameters, material parameters, and contact definitions; Based on the parametric model, finite element software was used to perform multi-condition mechanical simulation of the connection structure and verify the preset performance analysis criteria to obtain mechanical simulation data and performance analysis conclusions. Among them, the preset performance analysis criteria include thread multi-mode failure judgment criteria, head fillet radius to head thickness ratio influence analysis criteria, and material-ply matching performance comparison criteria. For bolts with a fixed nominal outer diameter, based on the aforementioned mechanical simulation data and analysis conclusions, the optimal combination of geometric parameters for the bolt is obtained through collaborative iteration using optimization algorithms and finite element software. For the optimal combination of geometric parameters, finite element software was used to calculate the contact area of ​​the bolt and nut and perform thread fatigue analysis to obtain the optimal material combination of the bolt and nut. Based on the optimal combination of geometric parameters, the optimal combination of materials, and the performance verification data, the design scheme of the non-standard composite material bolt is obtained.

[0021] In this embodiment of the invention, a parametric model of the bolt-nut connection structure is first constructed, and multi-condition mechanical simulation is performed using finite element software to verify the preset performance analysis criteria. Based on this, the bolt geometric parameters are optimized according to the mechanical simulation data and performance analysis conclusions. Simultaneously, contact area calculation and thread fatigue analysis are conducted to determine the optimal material combination for the bolt and nut, ultimately forming a complete design scheme for non-standard composite material bolts. Unlike traditional approaches that rely on experience or simple adjustments to standard metal dimensions, this invention fully considers the lamination characteristics, anisotropy, and brittle failure modes of composite materials. By synergistically optimizing geometric parameters such as thread height, thread width, nut thickness, and head fillet radius, it can avoid nut failure under tensile and fatigue load conditions, guide the fiber in the thread area from interlaminar shear load-bearing mode to vertical fiber shear load-bearing mode, increase the load-bearing cross-sectional area, and improve the load transfer efficiency between the thread and the shank, thereby significantly reducing the risk of thread detachment in composite fasteners.

[0022] According to some preferred embodiments, such as Figure 2 As shown, the geometric parameters include the nominal outer diameter of the bolt, thread parameters, and bolt head parameters; the thread parameters include thread height, pitch, thread angle, tip width, and root width; the bolt head parameters include head thickness and under-head fillet radius; the material parameters include composite material grade, fiber layup direction, and winding or braiding method; the contact is defined as surface-to-surface contact type and coefficient of friction.

[0023] According to some preferred embodiments, the parametric model for constructing the bolt-nut connection structure includes: Construct a three-dimensional geometric model of the bolt-nut connection structure based on the geometric parameters; The three-dimensional geometric model was meshed using finite element software; the bolt body was discretized using composite shell elements or solid elements, while the threaded area and contact surface were discretized using solid elements. Set the contact interface between the bolt and nut, and define the contact properties through the contact; By employing a layered approach to assign properties to the composite material and setting composite material damage criteria, as well as an analysis configuration file with three preset performance analysis criteria, a parametric model of the bolt-nut connection structure is obtained; wherein, the composite material damage criteria include the Hashin criterion or the PUCK criterion.

[0024] In this embodiment of the invention, firstly, based on Figure 2The geometric parameters shown construct a three-dimensional geometric model of the bolt-nut connection structure. These parameters include at least the nominal outer diameter D of the bolt, thread tooth parameters, and bolt head parameters. The thread tooth is preferably designed as a trapezoidal structure, and the thread tooth parameters include thread height h, pitch P, thread angle θ, tip width b, and root width a. The bolt head parameters may include head thickness H and lower fillet radius R. The three-dimensional geometric model is then imported into finite element analysis software for mesh generation. During mesh generation, composite shell elements or solid elements are preferentially used for discretization of the composite laminate or bolt body region, while solid elements are used for fine discretization of the thread tooth region and contact interface region to accurately capture the stress distribution characteristics of the contact area. The contact interface is simulated by defining a surface-to-surface contact relationship and setting corresponding contact attribute parameters such as the friction coefficient. Furthermore, material properties are assigned to the finite element mesh model. These properties are defined in layers to reflect the ply characteristics of the composite laminate structure, specifically including the material grade, thickness, and fiber layup orientation angle of each layer (e.g., 0°, ±90°, ±45°, or other arbitrary angles, or braided or wound forms). Simultaneously, corresponding damage criteria, such as the Hashin criterion or the Puck criterion, are embedded in the material property definitions to effectively assess the damage modes of the composite material and predict the initial cracking and propagation trends at the bolt stress interface.

[0025] It should be noted that the geometry of the nut can be external hexagon, internal hexagon, square, round or other adaptable shapes, and the present invention does not make specific limitations on this; the nominal diameter of the bolt and the dimensions of each part can also be selected according to the actual application requirements, and are not limited to specific specifications.

[0026] According to some preferred embodiments, the verification of the multi-mode failure judgment criteria of the screw thread using finite element software includes: performing finite element simulations based on interface shear failure mode, vertical fiber shear failure mode and mixed shear failure mode respectively, and combining them with preset composite material damage criteria to determine the stress characteristics and critical failure load of the screw thread under each failure mode.

[0027] According to some preferred embodiments, the analysis criteria for verifying the influence of the ratio of the lower fillet radius to the head thickness using finite element software include: simulating and analyzing the stress concentration distribution and load transfer path under different combinations of parameters for the ratio of the lower fillet radius to the head thickness, and comparing the load distribution ratio between the thread and the screw to optimize the lower fillet radius and head thickness of the bolt.

[0028] According to some preferred embodiments, the finite element method software is used to verify the material-layout matching performance comparison criteria, which includes: comparing the differences in mechanical properties of different composite material grades and fiber layup schemes; selecting suitable materials and fiber layup schemes based on the mechanical property comparison results; wherein, the fiber layup scheme includes layup angle (e.g., any angle or combination of angles from 0 to 90°), layup direction, winding or weaving method; the mechanical properties include axial tensile strength, transverse shear strength and interlaminar fracture toughness.

[0029] In this embodiment of the invention, three performance analysis criteria are preset, and finite element software is used for verification analysis. For the internal region of the thread, the coupling effect of thread geometry parameters and fiber distribution orientation on failure modes is examined. Specifically, two extreme failure states—interfacial shear failure mode and perpendicular fiber shear failure mode caused by fiber orientation—are simulated, as well as a mixed failure mode that simultaneously considers both failure mechanisms. This comprehensively evaluates the failure response characteristics of thread geometry and fiber distribution under different load conditions. For the fillet region below the nut head, the influence of fillet radius R and head thickness H parameters on stress concentration and load distribution characteristics is examined. By comparing and analyzing the load distribution states of the thread region and the screw region under different combinations of R and H parameters, the influence of these parameters on the overall load uniformity of the connection structure is obtained, so as to optimize the values ​​of R and H parameters. This study investigates the influence of material grade properties and fiber layup orientation parameters on the mechanical behavior of the connected structure. Fiber layup orientation parameters include, but are not limited to, 0° layup, 90° layup, ±45° layup, and winding / braiding layup forms. By comparing simulations of different material grades and layup parameter combinations, the study obtains the influence of each factor on the structural load-bearing capacity and failure mode, providing a basis for subsequent material selection and layup scheme design.

[0030] According to some preferred embodiments, determining the optimal combination of geometric parameters for the bolt through the coordinated iteration of optimization algorithms and finite element software includes: In MATLAB software, multiple combinations of geometric parameters are generated through optimization algorithms. The finite element analysis software adjusts the mesh model of the bolt-nut connection structure based on the received combination of geometric parameters, repeats the mechanical simulation, and outputs the performance index corresponding to each combination of geometric parameters; wherein, the performance index includes load-bearing capacity, load transfer efficiency, and stress concentration factor; The MATLAB software iteratively updates the combination of geometric parameters based on the received performance indicators until the preset objective function and constraints are met, thus obtaining the optimal combination of geometric parameters for the bolt.

[0031] According to some preferred embodiments, the objective function includes maximizing load-bearing capacity, optimizing load transfer efficiency, minimizing thread anti-loosening risk, and maximizing head connection strength; the constraints include processing feasibility boundaries and lower limits of strength indicators.

[0032] In this embodiment of the invention, for composite material bolts with a fixed nominal outer diameter, based on the aforementioned mechanical simulation data and performance analysis conclusions, geometric parameter optimization is performed using a collaborative iterative approach combining MATLAB software and finite element analysis software. Specifically, MATLAB software calls a preset optimization algorithm (e.g., genetic algorithm or particle swarm optimization algorithm) to generate multiple sets of geometric parameter combinations. These combinations include at least thread parameters (tooth height h, tooth profile angle θ, tooth tip width b, tooth root width a, pitch P) and head parameters (head thickness H, head fillet radius R). The finite element analysis software receives each set of geometric parameters, automatically updates the mesh model, and repeatedly executes the mechanical simulation, outputting performance indicators corresponding to each parameter combination. These performance indicators include load-bearing capacity, load transfer efficiency, and stress concentration factor. MATLAB software filters the parameter combinations based on the received performance indicators, eliminating inferior combinations and iteratively generating new parameter combinations until the preset objective function and constraints are met, thereby obtaining the optimal geometric parameter combination.

[0033] In the above optimization process, the thread height h is appropriately increased to increase the thread bearing area and improve the load-bearing capacity of the thread region; the thread width and root width W are increased to increase the interface bearing area between the bolt and nut, improving load transfer efficiency; the ratio of tip width b to root width a and the ratio of pitch P to bolt nominal outer diameter D are optimized to improve the load transfer efficiency per unit length, reduce the risk of thread slippage, and enhance the connection strength and stability between the thread and the bolt center position; non-standard thread angles θ (e.g., 60°, 75°, 90°, etc.) are used to adjust the shear load distribution on the thread meshing surface, optimizing the load distribution ratio between each thread; the lower fillet radius R and head thickness H of the bolt head are increased to alleviate stress concentration in the head transition area, improve the bearing strength of the connection between the head and the shank, and further improve the overall mechanical properties of the bolt. Through the synergistic optimization of the above geometric parameters, the occurrence of interlaminar failure of composite bolts can be effectively suppressed, significantly improving their overall load-bearing capacity and connection reliability.

[0034] According to some preferred embodiments, finite element software is used to calculate the bolt-nut contact area and analyze the thread fatigue resistance, including: Identify the thread contact area, calculate the actual contact area of ​​the bolt and nut under different thread parameters, analyze the relationship between the contact area and the load-bearing capacity, and generate a contact area-load-bearing capacity correlation curve; For the combination of composite material bolts and composite material nuts, and the combination of composite material bolts and metal nuts, fatigue damage is calculated based on the pre-set fatigue load simulation cyclic stress process. The fatigue life and failure location of different combinations are compared to select the optimal material combination of bolt and nut.

[0035] In this embodiment of the invention, after determining the optimal combination of geometric parameters, the contact stability and fatigue reliability of the connection structure are further verified. First, using the finite element method, the actual contact area of ​​the bolt and nut thread meshing region is calculated for the bolt structure obtained by the optimal combination of geometric parameters. Based on the calculation results, a contact area-load capacity correlation curve is generated to analyze the influence of contact area changes on the load capacity of the connection structure, thereby verifying that the connection interface can still maintain sufficient contact stability under different stress environments, ensuring connection reliability. Next, fatigue analysis of the threaded connection structure is performed. For different material and structural combinations, such as composite material bolts and composite material nuts, and composite material bolts and metal nuts, corresponding finite element models are established. Based on a preset fatigue simulation cyclic stress process, the fatigue damage of each combination is calculated, and the fatigue life and failure location distribution of different combinations are compared and analyzed. Based on this, the material combination scheme with the optimal fatigue reliability is selected, thereby designing a non-standard threaded connection structure with high fatigue resistance. Finally, the aforementioned results are summarized, including the optimal combination of geometric parameters, the preferred scheme of material layup, the optimal combination of materials, non-standard bolt-nut geometric model data that can be directly used for processing and manufacturing, and various performance verification indicators including load-bearing capacity, load transfer efficiency, fatigue life and connection stability, which constitute a complete design scheme for composite material non-standard threaded connection structure.

[0036] This invention also provides a non-standard composite material bolt, which is obtained by using the design method of the non-standard composite material bolt described in any of the above embodiments.

[0037] In this embodiment of the invention, to enable the use of the non-standard composite material bolt, nuts, washers, and clamps with the same thread design can also be machined. The materials can be metals such as aluminum alloy, titanium alloy, and steel, or composite materials such as carbon fiber composites, glass fiber composites, and basalt fiber composites. If performance permits, injection-molded polymer materials can also be used. Preferably, materials matching the bolt thread performance are selected to form a complete usage solution including washers, clamps, and preload matching. This not only provides a single bolt design but also forms a complete non-standard fastening system solution, achieving highly reliable connections under various working conditions.

[0038] This invention also provides a preparation method corresponding to the above-mentioned non-standard thread design, which mainly includes the following steps: Step 1: Material Preparation. Based on the optimal combination of geometric parameters and material layup scheme determined by the aforementioned design method, a suitable composite material system is selected for blank preparation. The composite material consists of reinforcing fibers and a matrix resin. The reinforcing fibers can be any one of carbon fiber (e.g., one or more of T300, T700, T800, T1000, T1100, or higher grades), glass fiber, or basalt fiber. The matrix resin can be any one of thermoplastic or thermosetting resins such as polypropylene, polyethylene, polyamide, polycarbonate, polyethylene terephthalate, polyetheretherketone, polyaryletheretherketone, polyetherketoneketone, polyetherimide, polyimide, polyphenylene sulfide, epoxy resin, unsaturated polyester, vinyl ester resin, bismaleimide resin, or phenolic resin.

[0039] When using compression molding, the blank can be prepared in one of the following ways: pre-processing the composite material into a round bar blank with a diameter and length adapted to the target bolt size; or stacking multilayer composite prepregs in a predetermined layup sequence, pressing them into composite laminates under vacuum or high pressure, and then machining them into round bar blanks of the corresponding size; or winding unidirectional composite prepregs into round bar blanks according to a predetermined fiber orientation, wherein the angle between the fiber direction and the axis of the bar can be set to 0°, ±45°, 90° or other arbitrary angles.

[0040] Step Two: Molding. The molding process can be either compression molding or injection molding. When using compression molding, the mold is first closed and heated to the molding temperature. The molding temperature range for thermosetting resin systems is generally 60℃ to 200℃, while for thermoplastic resin systems it is generally within 50℃ above its melting point Tm or within 100℃ above its glass transition temperature Tg. After reaching the set temperature, the blank prepared in step one is placed into the mold cavity, and a molding pressure of 0.1MPa to 300MPa is applied. The mold temperature is adjusted according to the curing or solidification process requirements of the material used. After the material has fully cured or solidified, the mold is opened and the molded product is removed.

[0041] When injection molding is used, the composite material melt containing short-cut fibers is injected into a mold with a pre-set threaded cavity, and the product is obtained after holding pressure and cooling.

[0042] Step 3: Post-processing. Necessary post-processing is performed on the formed threaded areas, including but not limited to grinding, finishing, and surface coating, to improve thread surface quality, dimensional accuracy, and corrosion resistance, ultimately obtaining non-standard composite material bolts that meet design requirements.

[0043] To more clearly illustrate the technical solution and advantages of the present invention, the following describes in detail a non-standard composite material bolt and its design method through several embodiments.

[0044] Example 1: Non-standard bolts made of thermosetting carbon fiber epoxy composite material (1) Based on the optimal combination of geometric parameters and material layup scheme determined by the aforementioned design method, select a suitable composite material system for blank preparation; Raw material preparation: Select UIN10000 carbon fiber prepreg from Guangwei Composites, roll the carbon fiber prepreg into solid rods with ±45° layup, control the porosity of the rods to <2%, the fiber direction to be consistent with the axial direction of the rods, the diameter to be 5.8 mm, and the length to be 100 mm; The optimal combination of geometric parameters is as follows: design bolt outer diameter D=7.9mm, thread height h=1.0mm, thread pitch P=1.5mm, tooth tip width b: tooth root width a=2, thread angle θ=75°; design bolt head thickness H=8mm, fillet radius R=2.5mm. (2) Bolt preparation: First, close the mold and heat it to the molding temperature of 180℃. After reaching the set temperature, put the blank prepared in step (1) into the mold cavity, press it at 180℃ for 80MPa and cure it for 2h. Open the mold and take out the molded product. Grind and trim the threaded part after molding. Use a 3mm thick carbon fiber composite plate to process a washer with the same inner diameter and rounded corner. Use stainless steel to machine nuts with the same thread size and shape for matching testing. Finally, obtain the non-standard composite bolt finished product that meets the design requirements.

[0045] Test results: Finite element analysis and experimental testing revealed that, as Figure 4 and Figure 5 As shown, compared with the standard M6 thread in Comparative Example 1, the thread prepared in Example 1 can effectively increase the contact area between the thread and the bolt core and the thread root width W, changing the failure mode of the thread under tensile force from sliding fall-off to shear fall-off, thus improving the thread strength. The final bolt tensile strength reaches 350MPa, and the load-bearing capacity is increased by more than 30%. It also shows excellent durability in 10Hz, 200MPa tensile fatigue test (refer to GB / T13682-92 axial load fatigue test method for threaded fasteners), with no pull-out failure occurring after more than 60,000 tests.

[0046] Example 2: Glass fiber polypropylene composite bolts (1) Based on the optimal combination of geometric parameters and material layup scheme determined by the aforementioned design method, select a suitable composite material system for blank preparation; specifically, raw material preparation: select composite material pultrusion molding process to prepare bar blanks with pure 0° direction and fiber content of 70±5%; size design: design bolt outer diameter D=5.9mm, thread height h=0.75mm, pitch P=1.5mm, b:a=2, thread angle θ=90°, bolt head thickness H=6mm, and fillet radius R=2mm.

[0047] (2) Bolt preparation: First, close the mold and heat it to the molding temperature of 200°C. After reaching the set temperature, put the blank prepared in step (1) into the mold cavity, control the molding pressure to 20MPa, and then cool the mold to 80°C. After the material is fully cured, open the mold and take out the molded product. Grind and repair the threaded part after molding to finally obtain the non-standard composite material bolt product that meets the design requirements.

[0048] Test results: The matching special washer and test fixture have rounded corner clamps / wafers, which can match the rounded corners of the bolt during the test, thereby further reducing stress concentration in the bolt head and threaded part. After tensile testing, it was found that the tensile strength of this fastener is 200MPa higher than that of the standard M6 bolt of the same material (i.e. the bolt in Comparative Example 1).

[0049] Comparative Example 1: Thermoplastic carbon fiber external hexagonal M6 bolt (standard bolt) (1) Based on traditional methods, determine the combination of geometric parameters and material layup scheme, and select a suitable composite material system for billet preparation.

[0050] Specifically, the dimensions of a standard M6 bolt are as follows: nominal outer diameter D = 5.9 mm, thread height h = 0.5 mm, pitch P = 1 mm, inner diameter d = 4.9 mm, thread angle θ = 60°, bolt head thickness H = 5 mm, and head fillet radius R = 0.5 mm. Raw material preparation: Select carbon fiber prepreg with T800 carbon fiber (Zhongfu Shenying SY55) as the reinforcing fiber and polyetheretherketone (Jilin Zhongyan 330) as the matrix. The resin matrix content is controlled at about 35%. Multi-layer unidirectional fibers are wound and laid along the prepreg in one direction. The prepreg is hot-pressed at 360℃ under vacuum to prepare a round bar with a diameter of 6mm and a length of 1m, and then set aside for use. (2) Bolt preparation: First, close the mold and heat it to the molding temperature of 380°C. After reaching the set temperature, put the blank prepared in step (1) into the mold cavity, control the molding pressure to 10MPa, and then cool the mold to 180°C. After the material is fully solidified, open the mold and take out the molded product. Grind and repair the threaded part after molding to finally obtain the standard composite material bolt product that meets the design requirements.

[0051] Tensile testing (referencing GB / T 3098.1-2010 Mechanical Properties of Fasteners - Bolts, Screws and Studs) revealed that the bolt in Comparative Example 1 had a tensile strength of 240 MPa. Tensile failure was mainly manifested as thread pull-out, fiber delamination, and fracture at the lower fillet of the nut head. This indicates that for continuous fiber reinforced composite bolts, the trapezoidal thread in Example 1 is more beneficial for improving their strength. The trapezoidal thread has a larger contact area between the root and the thread shank, making the thread in Example 1 more secure and less prone to falling off. During tension, the force can be further transmitted inward, causing the middle fiber portion to fracture, thereby improving the overall strength of the bolt.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A design method for non-standard composite material bolts, characterized in that, include: Construct a parametric model of the bolt-nut connection structure; wherein the parametric model includes geometric parameters, material parameters, and contact definitions; Based on the parametric model, finite element software was used to perform multi-condition mechanical simulation of the connection structure and verify the preset performance analysis criteria to obtain mechanical simulation data and performance analysis conclusions. Among them, the preset performance analysis criteria include thread multi-mode failure judgment criteria, head fillet radius to head thickness ratio influence analysis criteria, and material-ply matching performance comparison criteria. For bolts with a fixed nominal outer diameter, based on the aforementioned mechanical simulation data and analysis conclusions, the optimal combination of geometric parameters for the bolt is obtained through collaborative iteration of optimization algorithms and finite element software. For the optimal combination of geometric parameters, finite element software was used to calculate the contact area of ​​the bolt and nut and perform thread fatigue analysis to obtain the optimal material combination of the bolt and nut. The bolt-nut contact area and thread fatigue resistance were calculated using finite element software, including: Identify the thread contact area, calculate the actual contact area of ​​the bolt and nut under different thread parameters, analyze the relationship between the contact area and the load-bearing capacity, and generate a contact area-load-bearing capacity correlation curve; For the combination of composite material bolts and composite material nuts, and the combination of composite material bolts and metal nuts, fatigue damage is calculated based on the pre-set fatigue load simulation cyclic stress process. The fatigue life and failure location of different combinations are compared to select the optimal material combination of bolt and nut. Based on the optimal combination of geometric parameters, the optimal combination of materials, and the performance verification data, the design scheme of the non-standard composite material bolt is obtained.

2. The design method according to claim 1, characterized in that, The geometric parameters include the nominal outer diameter of the bolt, thread parameters, and bolt head parameters; the thread parameters include thread height, pitch, thread angle, tooth tip width, and thread root width; the bolt head parameters include head thickness and head fillet radius. The material parameters include the composite material grade, fiber layup direction, and winding or weaving method; The contact is defined as the surface-to-surface contact type and the coefficient of friction.

3. The design method according to claim 1, characterized in that, The parametric model for constructing the bolt-nut connection structure includes: Construct a three-dimensional geometric model of the bolt-nut connection structure based on the geometric parameters; The three-dimensional geometric model was meshed using finite element software; the bolt body was discretized using composite shell elements or solid elements, while the threaded area and contact surface were discretized using solid elements. Set the contact interface between the bolt and nut, and define the contact properties through the contact; By employing a layered approach to assign properties to the composite material and setting composite material damage criteria, as well as an analysis configuration file with three preset performance analysis criteria, a parametric model of the bolt-nut connection structure is obtained; wherein, the composite material damage criteria include the Hashin criterion or the PUCK criterion.

4. The design method according to claim 1, characterized in that, The method for verifying multi-mode failure criteria for screw threads using finite element software includes: Finite element simulations were performed based on interface shear failure mode, vertical fiber shear failure mode and mixed shear failure mode respectively. Combined with the preset composite material damage criteria, the stress characteristics and critical failure load of the screw thread under each failure mode were determined.

5. The design method according to claim 1, characterized in that, The criteria for analyzing the influence of the ratio of the lower corner radius to the head thickness using finite element software include: The stress concentration distribution and load transfer path under different combinations of head fillet radius and head thickness ratio parameters are simulated and analyzed. The load distribution ratio of the thread and the screw is compared to optimize the head fillet radius and head thickness of the bolt.

6. The design method according to claim 1, characterized in that, The criteria for verifying material-layup matching performance using finite element software include: The mechanical properties of different composite material grades and fiber layup schemes are compared; suitable materials and fiber layup schemes are selected based on the mechanical property comparison results; wherein, the fiber layup scheme includes layup angle, layup direction, winding or weaving method; the mechanical properties include axial tensile strength, transverse shear strength and interlaminar fracture toughness.

7. The design method according to claim 1, characterized in that, The process of determining the optimal combination of geometric parameters for the bolt through the collaborative iteration of optimization algorithms and finite element software includes: In MATLAB software, multiple combinations of geometric parameters are generated through optimization algorithms. The finite element analysis software adjusts the mesh model of the bolt-nut connection structure based on the received combination of geometric parameters, repeats the mechanical simulation, and outputs the performance index corresponding to each combination of geometric parameters; wherein, the performance index includes load-bearing capacity, load transfer efficiency, and stress concentration factor; The MATLAB software iteratively updates the combination of geometric parameters based on the received performance indicators until the preset objective function and constraints are met, thus obtaining the optimal combination of geometric parameters for the bolt.

8. The design method according to claim 7, characterized in that, The objective functions include maximizing load-bearing capacity, optimizing load transfer efficiency, minimizing thread slippage risk, and maximizing head connection strength; the constraints include processing feasibility boundaries and lower limits of strength indicators.

9. A non-standard composite material bolt, characterized in that, The design method described in any one of claims 1 to 8 is used.

Images