Method for establishing strength model of carbon fiber reinforced composite connection joint

Abstract

The present invention provides a method for establishing a strength model of a carbon fiber reinforced composite connection joint, which relates to the technical field of strength models of composite connection joints. The method includes obtaining key parameters of the material constitutive model, conducting quasi-static tensile tests on three types of connection joints of carbon fiber reinforced composites, and obtaining the first strength model of the three types of connection joints based on the load-displacement curve; obtaining the experimental strength model based on the relationship between three curve functions; using the key parameters of the constitutive model as input parameters to establish finite element models of the three types of joints, simulating and fitting three load-displacement simulation curves, integrating the experimental strength model based on the simulation curve function relationship and preset parameters to obtain the first initial parameter strength model, and unifying the three first initial parameter strength models to obtain the initial parameter model; using the method of controlling variables to obtain the final model under the change of each influencing parameter. The beneficial effects of the present invention have important value for the engineering application of composites.

Description

Technical Field

[0001] The present invention relates to the technical field of strength models for composite material connection joints, and more particularly, to a method for establishing a strength model for a carbon fiber reinforced composite material connection joint. Background Art

[0002] Carbon fiber reinforced composite materials (CFRP) are widely used in the fields of aerospace, rail transit, automobiles, and ships due to their high specific modulus, high specific strength, and strong fatigue resistance. In recent years, with the continuous improvement of lightweight requirements, the application of CFRP has gradually shifted from non-load-bearing components to main load-bearing structures. Since CFRP cannot be connected using welding technology, its connection structures mostly appear in the forms of adhesive bonding, mechanical connection, and hybrid connection. The connection part of CFRP is the weak link of the structure, and 60% - 80% of the failures occur at the connection part, which seriously affects the service safety of CFRP. Therefore, the strength of CFRP adhesive bonding, riveting, and adhesive-riveting hybrid joints has received increasing attention in engineering applications.

[0003] CFRP adhesive joints have the advantages of uniform stress distribution, good sealing performance, sound absorption and shock absorption, light weight and high efficiency, etc., but they also have problems such as easy brittle fracture, strong instability, sensitivity of adhesives to environmental factors, and easy aging in environments such as humidity, heat, and salt spray. The failure modes of adhesive joints can be divided into four types: interfacial failure, cohesive failure, mixed failure, and adherend failure. However, there are many factors affecting the strength of CFRP adhesive joints, such as: surface treatment methods (mechanical, chemical, and electrochemical), structural dimensions (joint form, adhesive layer thickness and width), adherend materials (aluminum alloy, carbon fiber, and epoxy resin), and environment (temperature, humidity, load, fire, ultraviolet light). Under the influence of these factors, the failure mechanism of CFRP adhesive joints is intricate and the failure mode is randomly variable. At present, there is still a lack of in-depth research on the strength characteristics and influence laws of CFRP adhesive joints.

[0004] The CFRP riveted joint has a simple process, high connection efficiency and low production cost, and has good plasticity and stability. However, during the riveting process, holes need to be drilled on the surface of the CFRP plate, and there is stress concentration around the holes, which is prone to crack generation. This will weaken its mechanical properties and thus affect the strength of the joint. In addition, galvanic corrosion may occur when the fastener contacts the CFRP plate. Therefore, the research on the strength of CFRP riveted joints mainly focuses on aspects such as static failure, fatigue failure, fretting wear and galvanic corrosion of the joints. The static failure mode of CFRP riveted joints is mainly affected by geometric dimensions, material mechanical properties, riveting parameters and forming processes. The main failure modes of CFRP riveted joint fatigue are the fracture, delamination and tearing of the CFRP plate. It can be seen that the riveting area is the weak area of the structure, and stress concentration and fretting wear are the main reasons for CFRP riveted joint fatigue. To sum up, the anisotropy of CFRP materials makes the static / fatigue failure of riveted joints a non-linear coupling problem and a structural plastic deformation mechanics problem, and at the same time has characteristics such as suddenness, difficult monitoring and difficult prevention. The analysis of the strength characteristics of CFRP riveted joints helps to improve structural safety. However, the research in this area is still insufficient.

[0005] The CFRP bonded-riveted hybrid joint is a composite of bonded and riveted joints. It can reduce the stress concentration caused by pre-drilled holes through adhesives and avoid the hidden danger of brittle fracture of the adhesive. Therefore, the bonded-riveted hybrid joint combines the advantages of riveted joints and bonded joints better and has stronger load-bearing capacity. The common failure forms of CFRP bonded-riveted hybrid joints include: rivet deformation, rivet fracture, adhesive layer failure, CFRP plate compression deformation, carbon fiber peeling / tearing / fracture failure and hybrid failure. Although the CFRP bonded-riveted hybrid joint exhibits excellent connection performance, the influencing factors are also greatly increased compared with single connections, and its strength characteristics and the relationship with the strength of single joints need to be further studied in depth. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for establishing a strength model of a carbon fiber reinforced composite material connection joint to improve the above problems. To achieve the above purpose, the technical solutions adopted by the present invention are as follows:

[0007] In the first aspect, the present application provides a method for establishing a strength model of a carbon fiber reinforced composite material connection joint, including:

[0008] Obtain the key parameters of the constitutive model through the constitutive relation test of carbon fiber reinforced composite materials, and use them as the parameter input of the finite element model of carbon fiber reinforced composite materials. The key parameters of the constitutive model include the tensile, compressive and shear mechanical property parameters of carbon fiber reinforced composite materials; conduct quasi-static tensile tests on three types of connection joints of carbon fiber reinforced composite materials to obtain three load-displacement curves, and fit the three load-displacement curves to obtain the first strength models of the three connection joints; obtain the test strength model based on the relationship between the three curve functions; the quasi-static tensile tests of the three connection joints include the quasi-static tensile tests of bonded joints, riveted joints and bonded-riveted hybrid joints, and the three load-displacement curves are the load-displacement curves of bonded joints, riveted joints and bonded-riveted hybrid joints;

[0009] Take the key parameters of the constitutive model as input parameters, and establish finite element models of three types of joints of carbon fiber reinforced composite materials based on the Hashin failure criterion, the quadratic nominal stress criterion and the B-K criterion. Simulate to obtain three load-displacement simulation curves, fit the three load-displacement simulation curves to obtain the function relationship of the simulation curves, and integrate the test strength model based on the curve simulation function relationship and preset parameters to obtain the first initial parameter strength model. Unify the three first initial parameter strength models to obtain the initial parameter model, where the three finite element models are the finite element models of bonded joints, riveted joints and bonded-riveted hybrid joints respectively;

[0010] Integrate the test strength model based on the curve simulation function relationship and preset parameters to obtain the first initial parameter strength model. Unify the three first initial parameter strength models to obtain the initial parameter model, where the variable of each preset parameter in the initial parameter model is the initial value;

[0011] Adopt the control variable method to perform parameter variation processing on the initial parameter model to obtain the final model under the variation of each influencing parameter.

[0012] Preferably, the quasi-static tensile test of the riveted joint in the quasi-static tensile tests of the three types of joints includes five stages, namely the elastic deformation stage, the plastic deformation stage, the crack propagation stage, the competitive failure stage and the fracture failure stage. The elastic deformation stage is the stage where the load rises approximately linearly with the displacement and the rivet and the CFRP plate show elastic deformation. The plastic deformation stage is the stage where the rivet undergoes plastic deformation as the load continues to increase. The crack propagation stage is the stage where cracks appear around the rivet hole and the load rises slowly in a fluctuating manner. The competitive failure stage is the stage where the load begins to decline slowly. The fracture failure stage is the stage where the joint undergoes fracture failure.

[0013] Preferably, the finite element models of the three carbon fiber reinforced composite joints include the finite element model of the adhesive joint of carbon fiber reinforced composite, the finite element model of the riveted joint of carbon fiber reinforced composite, and the finite element model of the adhesive-riveted hybrid joint of carbon fiber reinforced composite, wherein the element types, mesh refinement, contact modes, constitutive models, input parameters, damage initiation criteria, and crack surface propagation criteria in the finite element model of the adhesive joint of carbon fiber reinforced composite and the finite element model of the riveted joint of carbon fiber reinforced composite are the same.

[0014] Preferably, the first strength models of the three connection joints are obtained by fitting the three load-displacement curves:

[0015]

[0016] wherein, F B is the tensile load of the adhesive joint, and d is the displacement;

[0017]

[0018] wherein, F R is the tensile load of the riveted joint, and d is the displacement;

[0019]

[0020] wherein, F H is the tensile load of the adhesive-riveted hybrid joint, and d is the displacement.

[0021] Preferably, the experimental strength model is obtained based on the relationship between the three curve functions, including:

[0022]

[0023] wherein, F B , F R and F H are the tensile loads of the adhesive, riveted, and adhesive-riveted hybrid joints respectively, and d is the displacement.

[0024] Preferably, the experimental strength model is integrated based on the simulation curve function relationship and preset parameters to obtain the first initial parameter strength model, including:

[0025] Set the initial values of the parameter variables;

[0026] Fit the loads and displacements of the three joints corresponding to the initial values of the parameter variables to obtain the initial parameter models of each joint, including the first initial adhesive parameter strength model, the second initial riveted parameter strength model, and the third adhesive-riveted hybrid joint strength model. The calculation formula of the first initial parameter strength model is as follows:

[0027]

[0028]

[0029] Wherein, is the first initial bonding parameter strength model, and d is the displacement;

[0030]

[0031] Equation

[0032] In, is the first initial riveting parameter strength model, and d is the displacement;

[0033] Wherein, is the first initial bonding and riveting hybrid parameter strength model, and d is the displacement.

[0034] Preferably, the obtaining of the initial parameter model includes:

[0035] Unify the first initial bonding parameter strength model, the second initial riveting parameter strength model, and the third bonding and riveting hybrid joint strength model to obtain the initial parameter model, and the calculation formula is as follows;

[0036]

[0037] Wherein, and are respectively the tensile loads of the bonding, riveting, and bonding and riveting hybrid joints in the initial parameter model, and d is the displacement.

[0038] Preferably, the parameter variation processing of the initial parameter model, wherein the parameters include the rivet diameter, the lay-up method of the carbon fiber reinforced composite material, the interlaminar friction coefficient, the adhesive layer thickness, the overlap length, the overlap width, the number and arrangement of rivets, the rivet row spacing, and the rivet column spacing.

[0039] In a second aspect, the present application further provides a model establishment device based on a carbon fiber reinforced composite material, including:

[0040] A memory for storing a computer program;

[0041] A processor for implementing the steps of the strength model establishment method of the carbon fiber reinforced composite material connection joint when executing the computer program.

[0042] In a third aspect, the present application further provides a readable storage medium, on which a computer program is stored, and when the computer program is executed by a processor, the steps of the above-mentioned strength model establishment method for a carbon fiber reinforced composite material connection joint are implemented.

[0043] The beneficial effects of the present invention are as follows:

[0044] The unified strength model of the CFRP bonded, riveted, and bonded-riveted hybrid joints proposed by the present invention comprehensively reveals the strength characteristics and parameter influence laws of the three types of joints through a method combining finite element simulation, tensile test, and numerical analysis, establishes a unified model for the strength of CFRP joints under each influencing parameter, and the correlation of the model is good, which can accurately characterize the strength of CFRP joints.

[0045] Based on the unified strength model, the present invention can obtain the strength model of the bonded-riveted hybrid joint through the strength models of the bonded and riveted joints. Similarly, if the strength model of the bonded-riveted hybrid joint is known, the strength models of the bonded and riveted joints can also be obtained, forming a complete set of CFRP joint strength evaluation methods to guide the strength check and parameter optimization of CFRP connection structures.

[0046] Other features and advantages of the present invention will be described in the subsequent description, and, in part, will be obvious from the description, or can be understood by implementing the embodiments of the present invention. The objectives and other advantages of the present invention can be realized and obtained by the structures specifically pointed out in the written description, claims, and drawings. Description of the Drawings

[0047] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can also be obtained based on these drawings without creative efforts.

[0048] Figure 1 It is a schematic flow chart of the method for establishing the strength model of the carbon fiber reinforced composite material connection joint described in the embodiments of the present invention;

[0049] Figure 2 It is a schematic structural diagram of the model establishment device based on carbon fiber reinforced composite materials described in the embodiments of the present invention;

[0050] Figure 3 It is the load-displacement curve of the CFRP bonded joint of the method for establishing the strength model of the carbon fiber reinforced composite material connection joint described in the embodiments of the present invention;

[0051] Figure 4 It is the load-displacement curve of the CFRP riveted joint of the method for establishing the strength model of the carbon fiber reinforced composite material connection joint described in the embodiments of the present invention, where (a) is the single rivet curve and (b) is the multi-rivet curve;

[0052] Figure 5The load-displacement curve of the CFRP bonded-riveted hybrid joint for the strength model establishment method of the carbon fiber reinforced composite connection joint described in the embodiments of the present invention, where (a) is the curve of a single-rivet bonded-riveted joint and (b) is the curve of a multi-rivet bonded-riveted joint. Detailed implementation manners

[0053] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. The components of the embodiments of the present invention usually described and illustrated in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0054] It should be noted that: similar reference numerals and letters denote similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings. At the same time, in the description of the present invention, the terms "first", "second", etc. are only used for differential description and cannot be construed as indicating or implying relative importance.

[0055] Embodiment 1:

[0056] In the prior art, due to the particularity of carbon fiber composites, it is difficult to use welding technology for connection, and its connection structures mostly appear in the forms of adhesive bonding, mechanical connection and hybrid connection in applications. Different connection methods have different advantages and disadvantages. Adhesive bonding has the advantages of uniform stress distribution, good sealing performance, sound absorption and shock absorption, light weight and high efficiency, etc. However, adhesive bonding has the problems of brittle fracture, strong instability, and the adhesive is sensitive to environmental factors and is prone to aging in environments such as humidity, heat, and salt spray. The riveting connection process is simple and easy to implement, has high connection efficiency and low production cost, has good plasticity, and stronger safety and stability. However, drilling holes on the surface of the composite material is required during the riveting process, which will weaken its mechanical properties and cause local damage, thus affecting the strength of the joint, and galvanic corrosion may occur when the fastener contacts the composite material. The bonded-riveted hybrid connection is a composite application of the two connection methods, complementing each other. It can reduce the stress concentration caused by the pre-drilled holes through adhesive bonding and avoid the potential safety hazards caused by the brittle fracture of the adhesive, and the load-bearing capacity of the connection joint is enhanced. Therefore, establishing a unified strength model for CFRP adhesive bonding, riveting and bonded-riveted hybrid joints to evaluate the strength of CFRP joints has important research value and application prospects for further promoting the application of composite materials in engineering.

[0057] This embodiment provides a method for establishing a strength model of a carbon fiber reinforced composite connection joint.

[0058] See Figure 1 , which shows that this method includes step S100, step S200, step S300, and step S400.

[0059] S100. Obtain the key parameters of the constitutive model through the constitutive relationship test of carbon fiber reinforced composites, and use them as the parameter input of the finite element model of carbon fiber reinforced composites. The key parameters of the constitutive model include the tensile, compressive, and shear mechanical property parameters of carbon fiber reinforced composites; conduct quasi-static tensile tests on three types of connection joints of carbon fiber reinforced composites to obtain three load-displacement curves, and fit the three load-displacement curves to obtain the first strength models of the three connection joints; obtain the test strength model based on the relationship between the three curve functions; the quasi-static tensile tests of the three connection joints include the quasi-static tensile tests of bonded joints, riveted joints, and bonded-riveted hybrid joints, and the three load-displacement curves are the load-displacement curves of bonded joints, riveted joints, and bonded-riveted hybrid joints.

[0060] It can be understood that in this step, in order to obtain the tensile, compressive, and shear mechanical property parameters of CFRP, a CFRP constitutive test was carried out. The single-layer board of the specimen is a plain woven carbon fiber composite material, and vacuum-assisted resin infusion molding is used. As shown in Table 1, according to "GB / T 3354-2014 Test Method for Tensile Properties of Oriented Fiber Reinforced Polymer Matrix Composites", "GB / T 3856-2005 Test Method for Compressive Properties of Unidirectional Fiber Reinforced Plastic Flat Plates", and "GB / T 3355-2014 Test Method for In-Plane Shear Properties of Polymer Matrix Composites", the geometric dimensions of the specimens and the aluminum alloy reinforcement plates at both ends are set.

[0061] Table 1 Geometric Dimensions of CFRP Constitutive Test Specimens

[0062] Load type Layup scheme Number of layers L / mm <![CDATA[L0 / mm]]> W / mm H / mm B / mm Tensile <![CDATA[[0 / 90]5]]> 5 250 50 25 2 2 Shear <![CDATA[[±45]5]]> 5 250 — 25 2 2 Compression <![CDATA[[0 / 90]5]]> 5 132 60 10 2 2

[0063] It should be noted that the test was carried out on an MTS 809 pull-twist fatigue testing machine. The tensile and shear specimens were clamped with wedge-shaped fixtures, and the compressive specimens were clamped with self-guided compression fixtures. The strain was measured using DIC technology. Before the test, a matte white primer was evenly sprayed on the surface of the specimen, and after drying, a matte black speckle was evenly sprayed on the white primer. The test loading rate was 2 mm / min, and the test was stopped after loading until the specimen was completely fractured. The test results are shown in Table 2.

[0064] Table 2 Tensile, Compressive, and Shear Test Values

[0065]

[0066] The key parameters of the CFRP constitutive model obtained through experiments. Since the single-layer plate is a plain-woven carbon fiber composite material, the mechanical properties in the 0° and 90° directions are the same, and the numerical value is taken as their average.

[0067] It should be noted that based on the parameter information, quasi-static tensile tests on three types of joints of carbon fiber-reinforced composites are carried out, namely, the quasi-static tensile tests of bonded, riveted, and bonded-riveted hybrid joints.

[0068] Among them, the connected parts of the CFRP bonded joint are woven CFRP laminates, and the adhesive model is TS828 high-performance structural adhesive (acrylate structural adhesive), which has a fast curing speed, high curing strength, good toughness and peel resistance, and is suitable for bonding CFRP plates. The geometric dimensions of the specimens are designed with reference to the standard "GB / T 33334-2016 Adhesives - Test method for tensile lap-shear strength (composite to composite)". The tests of CFRP bonded, riveted, and bonded-riveted hybrid joints are still carried out on the MTS 809 pull-torsion fatigue testing machine, and the loading rate and stopping criteria are the same as those of the CFRP constitutive model test. The typical load-displacement curve of the CFRP bonded joint is as Figure 3 shown. It can be seen that the curve is approximately linear before the fracture of the bonded joint, and it fails instantly after reaching the ultimate load. The failure mode is a mixed failure (cohesive failure and adhesive interface failure exist simultaneously), that is, partial tearing and debonding occur in the bonding area, and at the same time, there is cohesive failure of the adhesive layer. When the adhesive layer fractures during the test, there is an obvious sound, which is a brittle fracture.

[0069] Among them, the quasi-static tensile test of the CFRP riveted joint is carried out: Since the composite material is a brittle material, too many holes are likely to cause damage to the CFRP plate. Therefore, in engineering, the number of rows of rivets in the CFRP riveted joint is generally controlled within 3 rows, as Figure 4 shown. In this embodiment, the CFRP riveted specimen is a single-rivet and a multi-rivet joint with two rows and two columns. The CFRP laminates are connected by a pneumatic rivet gun for blind riveting, and the rivets are stainless steel open-type flat head blind rivets. Among them, the load-displacement curve of the CFRP riveted joint is as Figure 4 (a) and (b) shown. It can be seen that the failure process of both the single-rivet and multi-rivet joints can be divided into the following 5 stages:

[0070] (1) Elastic deformation stage (O-A section): The load rises approximately linearly with the displacement. Both the rivet and the CFRP plate show elastic deformation, and the rivet bears shear force and additional bending moment (caused by the asymmetry of the joint).

[0071] (2) Plastic deformation stage (A-B section): As the load continues to increase, the rivet undergoes plastic deformation.

[0072] (3) Crack propagation stage (section B-C): Cracks begin to appear around the rivet holes, and local fractures occur in the fibers and matrix, resulting in a slow increase in the load in a fluctuating pattern.

[0073] (4) Competing failure stage (section C-D): Fibers around the rivet holes are torn extensively, the shear force and additional bending moment of the rivets reach their peaks, and a competing failure situation occurs between the rivets and the CFRP plates, causing the load to start decreasing slowly.

[0074] (5) Fracture failure stage (section D-E): Plastic deformation continues to increase, exceeding the tensile strength limit of the rivets or the CFRP plates, and the joint undergoes fracture failure.

[0075] Among them, a quasi-static tensile test of the CFRP bonded-riveted hybrid joint was carried out: The CFRP bonded-riveted hybrid joint is a composite of a riveted joint and a bonded joint, and the material and load parameters are the same as those of the previous two tests. The test load-displacement curves are as Figure 5 (a) and (b) show. It can be seen that the first half of the curve is approximately linear, so the adhesive layer is the main load-bearing part in this stage. As the load increases, the adhesive layer reaches its ultimate load and fractures before the rivets. The failure of the adhesive layer is an instantaneous brittle fracture accompanied by fiber tearing, so there is an instantaneous drop point on the curve. The second half of the curve is similar to Figure 4 (a) and (b). The CFRP bonded-riveted hybrid joint undergoes 5 stages similar to those of the pure riveted joint and finally fractures completely. Therefore, the rivets are the main load-bearing part in this stage. In summary, the bonded-riveted hybrid joint combines the advantages of the riveted joint and the bonded joint well, has a stronger load-bearing capacity, and avoids the safety hazards brought by the instantaneous fracture of the adhesive layer.

[0076] S200. Take the key parameters of the constitutive model as input parameters, establish finite element models of three types of joints of carbon fiber reinforced composites based on the Hashin failure criterion, the quadratic nominal stress criterion, and the B-K criterion, simulate to obtain three load-displacement simulation curves, fit the three load-displacement simulation curves to obtain the functional relationship of the simulation curves, and integrate the test strength model based on the curve simulation function relationship and the preset parameters to obtain the first initial parameter strength model. Unify the three first initial parameter strength models to obtain the initial parameter model, where the three finite element models are the finite element models of the bonded joint, the riveted joint, and the bonded-riveted hybrid joint respectively.

[0077] It is understandable that in this step, the C3D8R element is adopted for the CFRP plate of the adhesive joint finite element model, the Hashin failure criterion is used as the failure criterion, and the progressive damage behavior of the laminate during the tensile process is simulated by combining the secondary developed VUMAT subroutine; the adhesive layer is modeled by the three-dimensional cohesive element COH3D8R, the quadratic nominal stress criterion is used as the damage initiation criterion, and the B-K criterion is used as the damage evolution criterion. The connection between the CFRP plate and the adhesive layer is connected by co-nodal coupling. To ensure the calculation accuracy, a 0.5-mm mesh is used for the adhesive layer and the bonding area, the adhesive layer elements are divided into two layers in the thickness direction, and the mesh size of the non-bonding area is 2 mm. The size, load, and boundary conditions of the finite element model are the same as those in the experiment. One end of the specimen is fully constrained, and the other end uses a reference point to couple the actual loading surface and apply a tensile displacement.

[0078] It should be noted that, first of all, the element type, mesh refinement method, constitutive model parameters, and failure criterion of the CFRP plate of the riveted joint finite element model are the same as those of the adhesive joint finite element model. The material elastoplasticity and metal ductile failure of the rivets are considered. The ductile damage model is adopted for the rivet constitutive model, and standard contacts are set between the CFRP plates and between the rivets and the plates. The size, load, and boundary conditions of the finite element model are the same as those in the experiment. Secondly, in the failure process of the CFRP riveted joint and the load-displacement curve obtained by finite element simulation, the simulated deformation and fracture position are consistent with the test results, verifying the accuracy of the CFRP riveted joint finite element model. The load-displacement simulation curves of the single-rivet and multi-rivet joints are in good agreement with the test curves, and the relative errors of the ultimate loads are all within 5%, further verifying the accuracy of the CFRP riveted joint finite element model. Furthermore, the CFRP adhesive-riveted hybrid joint finite element model is a combination of the riveted and adhesive joint finite element models. Its element type, mesh refinement, contact method, constitutive model, input parameters, damage initiation criterion, and crack surface propagation criterion are the same as those of the riveted and adhesive joint finite element models. The size, load, and boundary conditions of the finite element model are the same as those in the experiment. In the failure process of the CFRP adhesive-riveted hybrid joint and the load-displacement curve obtained by finite element simulation, the simulated deformation and fracture position are consistent with the test results, verifying the accuracy of the CFRP adhesive-riveted hybrid joint finite element model. Although there are certain deviations between the load-displacement simulation curve of the hybrid joint and the test curve in the main load-bearing section of the adhesive layer and the main load-bearing section of the rivets, the overall trend is also in good agreement. This deviation is due to the superposition of factors such as rivet assembly error, internal defects of the adhesive layer, and incomplete bonding in the hybrid joint, which is within an acceptable range. The relative error of the ultimate load values between the test and simulation is only 3.91%, further verifying the accuracy of the CFRP adhesive-riveted hybrid joint finite element model.

[0079] It should be noted that in this step, the finite element models of three carbon fiber reinforced composite joints are unified to obtain the first model, which includes: the first half of the curve of the bonded-riveted hybrid joint is basically the same as that of the bonded joint, indicating that the adhesive layer is the main load-bearing component in the early failure stage of the bonded-riveted hybrid joint. However, the ultimate load of the bonded-riveted hybrid joint is higher, indicating that the rivets also bear a small part of the load during the main load-bearing stage of the adhesive layer. After the adhesive layer fractures, the rivets begin to play the main load-bearing role, but the load that causes the rivets to fail is lower than the ultimate load value of the riveted joint, which further proves that during the main load-bearing stage of the adhesive layer in the bonded-riveted hybrid joint, the rivets bear a small part of the load and cause certain damage to the rivets, thus reducing the ultimate load in the main load-bearing stage of the rivets. Then, it is necessary to fit the load-displacement data of three different CFRP joints to obtain the experimental strength models of each joint as (F B ,F R and F H are the tensile loads of the bonded, riveted and bonded-riveted hybrid joints respectively, and d is the displacement):

[0080]

[0081] Wherein, F B is the tensile load of the bonded joint and d is the displacement;

[0082]

[0083] Wherein, F R is the tensile load of the riveted joint and d is the displacement;

[0084]

[0085] Wherein, FH is the tensile load of the bonded-riveted hybrid joint and d is the displacement.

[0086] According to the relationships of the strength models of the above-mentioned bonded, riveted and bonded-riveted hybrid joints, an experimental unified model of the strengths of the three joints is established, and the first model is obtained as:

[0087]

[0088] It can be seen that the experimental strength models and the experimental unified model of the three joints with the same geometric parameters are all close to the experimental values, and R 2 (square of the correlation coefficient) is greater than 0.98, which proves the accuracy of the experimental unified model. Based on the experimental unified model, the strength model of the bonded-riveted hybrid joint can be obtained through the strength models of the bonded and riveted joints; similarly, if the strength model of the bonded-riveted hybrid joint is known, the strength models of the bonded and riveted joints can also be obtained.

[0089] S300. Integrate the test strength model based on the simulation curve function relationship and preset parameters to obtain the first initial parameter strength model, and unify the three first initial parameter strength models to obtain the initial parameter model, where the variable of each preset parameter in the initial parameter model is the initial value.

[0090] It can be understood that in this step, since the test unified model is affected by the joint parameters, the control variable method is adopted in this section to study the influence of rivet diameter, CFRP ply stacking pattern, interfacial friction coefficient, adhesive layer thickness, overlap length, overlap width, number and arrangement pattern of rivets, rivet row spacing and column spacing on the strength characteristics of CFRP joints, so as to establish the final unified strength model of the three joints under each influencing parameter. The initial values of the parameter variables are: rivet diameter 4mm, CFRP ply stacking pattern [0 / 90]5, interfacial friction coefficient 0.1, adhesive layer thickness 0.2mm, overlap length 50mm, overlap width 50mm, and number of rivets 1. Fit the load and displacement of the three joints when the parameter variables are at the initial values.

[0091] Integrate the first model, including: setting the initial values of the parameter variables;

[0092] Fit the load and displacement of the three joints corresponding to the initial values of the parameter variables to obtain the initial parameter models of each joint, including the first initial adhesive parameter strength model, the second initial riveting parameter strength model, and the third adhesive-riveting hybrid joint strength model. The calculation formula of the first initial parameter strength model is as follows:

[0093]

[0094] In the formula, is the first initial adhesive parameter strength model, and d is the displacement;

[0095]

[0096] In the formula, is the first initial riveting parameter strength model, and d is the displacement;

[0097]

[0098] In the formula, is the first initial adhesive-riveting hybrid parameter strength model, and d is the displacement.

[0099] Unify the first initial adhesive parameter strength model, the second initial riveting parameter strength model, and the third adhesive-riveting hybrid joint strength model to obtain the initial parameter model. The calculation formula is as follows;

[0100]

[0101] In the formula, and are the tensile loads of the bonded, riveted, and bonded-riveted hybrid joints in the initial parameter model respectively, and d is the displacement.

[0102] S400. Using the method of controlling variables, the initial parameter model is processed with parameter changes to obtain the final model under the change of each influencing parameter.

[0103] It can be understood that in this step, the parameters include the rivet diameter, the lay-up method of carbon fiber reinforced composite material, the interfacial friction coefficient between plates, the adhesive layer thickness, the overlap length, the overlap width, the number and arrangement of rivets, the rivet row spacing, and the rivet column spacing.

[0104] It should be noted that the changes of each of the above parameters are described in detail respectively:

[0105] (1) Rivet diameter: 15 kinds of rivet diameters (Φ) are set in the range of 2 mm - 16 mm. The other parameter variables of the three joints are controlled to be consistent with the initial values, and the load-displacement curves under different rivet diameters are obtained by simulation calculation. There is little difference in the elastic deformation stage of each curve, but there is a large difference in the plastic deformation stage. When the rivet diameter increases from 2 mm to 8 mm, the ultimate load gradually increases, and the joints all fail due to rivets; after exceeding 9 mm, the ultimate load gradually decreases. Due to the too large hole diameter, the bearing capacity of the joint decreases, and the failure form of the joint changes to CFRP plate failure. Therefore, 9 mm is the critical rivet diameter that causes the competitive failure of rivets and CFRP plates, and the bearing capacity of the joint is the largest at this time. The load-displacement curves of the three joints under different rivet diameters are fitted to obtain a unified strength model, and the square of the correlation coefficient (R2) of the model is greater than 0.92, which proves good correlation.

[0106] (2) CFRP lay-up method: To study the influence of the CFRP lay-up method on the joint strength, the load-displacement curves under different CFRP lay-up methods are calculated. The lay-up method has little influence on the strength of the riveted joint, but has some influence on the bonded and bonded-riveted hybrid joints. The higher the 45° lay-up ratio, the greater the ultimate load of the bonded and bonded-riveted hybrid joints, and the stronger the bearing capacity of the joints. In addition, as the 45° lay-up ratio increases, the failure form of the CFRP plate gradually evolves from transverse fracture to fracture along the 45° direction. Therefore, the load-displacement curves of the three joints under different CFRP lay-up methods are fitted to obtain a unified strength model, and the square of the correlation coefficient (R2) of the model is greater than 0.95, which proves good correlation.

[0107] (3) Coefficient of friction between plates: Eight coefficients of friction between plates (λ) were set within the range of 0.05 - 0.4, and the load-displacement curves under different coefficients of friction were calculated. The coefficient of friction has little effect on the strength of the adhesive-rivet hybrid joint and has some minor effects on the strength of the riveted joint. The coefficient of friction is positively correlated with the ultimate load of the riveted joint. This is because the greater the frictional force between the plates, the greater the anti-slip resistance between the CFRP plates, thus improving the ultimate load-bearing capacity of the joint. The load-displacement curves of the three types of joints under different coefficients of friction between plates were fitted to obtain a unified strength model, and the squared correlation coefficient (R2) of the model was greater than 0.98, proving a good correlation.

[0108] (4) Adhesive layer thickness: The adhesive layer thickness is also an important factor affecting the load-bearing capacity of CFRP adhesive joints and adhesive-rivet hybrid joints. Seven adhesive layer thicknesses (t) were set within the range of 0.1 mm - 0.7 mm, and the load-displacement curves under different adhesive layer thicknesses were calculated. As the adhesive layer thickness increases, the ultimate loads of the adhesive joints and adhesive-rivet hybrid joints gradually decrease. This is because an overly thick adhesive layer will lead to an increase in additional bending moments and adhesive layer peel stresses. Since the adhesive layer's anti-peeling ability is poorer than its anti-shearing ability, the load-bearing capacity of the joint decreases. The load-displacement curves of the three types of joints under different adhesive layer thicknesses were fitted to obtain a unified strength model, and the squared correlation coefficient (R2) of the model was greater than 0.96, proving a good correlation.

[0109] (5) Overlap length: The overlap length is also an important factor affecting the load-bearing capacity of the three types of CFRP joints. Nine overlap lengths (l) were set within the range of 10 mm - 90 mm, and the load-displacement curves under different overlap lengths were calculated. The overlap length of the joint is positively correlated with the ultimate load. This is because the increase in the bonding area and the contact area of the CFRP plates increases the joint strength, thus improving the ultimate load-bearing capacity of the joint. The load-displacement curves of the three types of joints under different overlap lengths were fitted to obtain a strength model, and the squared correlation coefficient (R2) of the model was greater than 0.92, proving a good correlation.

[0110] (6) Overlap width: The overlap width is also an important factor affecting the load-bearing capacity of the three types of CFRP joints. Nine overlap widths (w) were set within the range of 10 mm - 90 mm, and the load-displacement curves under different overlap widths were calculated. Similar to the overlap length, the overlap width of the joint is also positively correlated with the ultimate load because the increase in the overlap width also increases the bonding area and the contact area of the CFRP plates. The load-displacement curves of the three types of joints under different overlap widths were fitted to obtain a strength model, and the squared correlation coefficient (R2) of the model was greater than 0.96, proving a good correlation.

[0111] (7) Number and arrangement of rivets: To study the influence of the number and arrangement of rivets on the strength of CFRP riveted and bonded-riveted hybrid joints, 8 kinds of rivet arrangements were set (Li is the number of horizontally arranged rivets, i = 1, 2, 3, 4; Ci is the number of vertically arranged rivets, i = 1, 2, 3, 4), and the load-displacement curves under different rivet arrangements were calculated. The F-D curves with the same number of rivets are almost the same, so the rivet arrangement has little influence on the joint strength. The number of rivets is positively correlated with the ultimate load. However, the shear resistance of a single rivet in a multi-rivet joint is lower than that in a single-rivet joint. Therefore, it is not that the more rivets, the better. When designing CFRP riveted and bonded-riveted hybrid joints, both the joint strength and the utilization rate of the rivet bearing capacity should be considered. The load-displacement curves of the three types of joints with different numbers of rivets were fitted to obtain a unified strength model, and the square of the correlation coefficient (R2) of the model is greater than 0.92, proving a good correlation.

[0112] (8) Rivet row spacing: To study the influence of row spacing on the strength of CFRP riveted and bonded-riveted hybrid joints, 6 kinds of rivet row spacings (Ls) were set in the range of 10 mm - 30 mm for the riveted and bonded-riveted hybrid joints with the L2C2 arrangement, and the load-displacement curves under different rivet row spacings were calculated. It can be seen that as the rivet row spacing increases, the ultimate load of the joint first increases and then decreases, and the joint has the strongest bearing capacity when the row spacing is 18 mm. Therefore, there is a rivet row spacing that makes the ultimate bearing capacity of the joint optimal, and this row spacing should be selected during joint design. The load-displacement curves of the three types of joints with different rivet row spacings were fitted to obtain a strength model, and the square of the correlation coefficient (R2) of the model is greater than 0.95, proving a good correlation.

[0113] (9) Rivet column spacing: The column spacing is also an important factor affecting the bearing capacity of CFRP riveted and bonded-riveted hybrid joints. 6 kinds of rivet column spacings (Cs) were set in the range of 10 mm - 30 mm, and the load-displacement curves under different rivet column spacings were calculated. As the rivet column spacing increases, the ultimate load of the joint first increases and then decreases, and the joint has the strongest bearing capacity when the column spacing is 18 mm. Therefore, there is a rivet column spacing that makes the bearing capacity of the joint optimal, and this column spacing should be selected during joint design. The load-displacement curves of the three types of joints with different rivet column spacings were fitted to obtain a strength model, and the square of the correlation coefficient (R2) of the model is greater than 0.96, proving a good correlation.

[0114] Example 2:

[0115] Corresponding to the above method embodiments, in this embodiment, a model establishment device based on carbon fiber reinforced composite materials is further provided. A model establishment device based on carbon fiber reinforced composite materials described below can be correspondingly referred to with a strength model establishment method of a carbon fiber reinforced composite material connection joint described above.

[0116] Figure 2 FIG. is a block diagram of a model establishment device 800 based on carbon fiber reinforced composite materials shown according to an exemplary embodiment. As Figure 2 shown, the model establishment device 800 based on carbon fiber reinforced composite materials includes: a processor 801 and a memory 802. The model establishment device 800 based on carbon fiber reinforced composite materials further includes one or more of a multimedia component 803, an I / O interface 804, and a communication component 805.

[0117] Among them, the processor 801 is used to control the overall operation of the carbon fiber reinforced composite material-based model establishment device 800 to complete all or part of the steps in the above-mentioned strength model establishment method for carbon fiber reinforced composite material connection joints. The memory 802 is used to store various types of data to support the operation of the carbon fiber reinforced composite material-based model establishment device 800. These data may include, for example, instructions for any application or method operating on the carbon fiber reinforced composite material-based model establishment device 800, as well as application-related data, such as contact data, received and sent messages, pictures, audio, video, and so on. The memory 802 can be implemented by any type of volatile or non-volatile storage device or a combination thereof, such as Static Random Access Memory (SRAM), Electrically Erasable Programmable Read-Only Memory (EEPROM), Erasable Programmable Read-Only Memory (EPROM), Programmable Read-Only Memory (PROM), Read-Only Memory (ROM), magnetic memory, flash memory, magnetic disk or optical disk. The multimedia component 803 may include a screen and an audio component. Among them, the screen can be, for example, a touch screen, and the audio component is used to output and / or input audio signals. For example, the audio component may include a microphone, and the microphone is used to receive external audio signals. The received audio signals can be further stored in the memory 802 or sent through the communication component 805. The audio component also includes at least one speaker for outputting audio signals. The I / O interface 804 provides an interface between the processor 801 and other interface modules, and the above-mentioned other interface modules can be a keyboard, a mouse or buttons, etc. These buttons can be virtual buttons or physical buttons. The communication component 805 is used for wired or wireless communication between the carbon fiber reinforced composite material-based model establishment device 800 and other devices. Wireless communication, such as Wi-Fi, Bluetooth, Near Field Communication (NFC), 2G, 3G or 4G, or a combination of one or more of them. Therefore, the corresponding communication component 805 may include: a Wi-Fi module, a Bluetooth module or an NFC module.

[0118] In an exemplary embodiment, the model establishment device 800 based on carbon fiber reinforced composite materials can be implemented by one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), controllers, microcontrollers, microprocessors or other electronic components, and is used to execute the above-mentioned method for establishing the strength model of the carbon fiber reinforced composite material connection joint.

[0119] In another exemplary embodiment, a computer-readable storage medium including program instructions is further provided. When the program instructions are executed by a processor, the steps of the above-mentioned method for establishing the strength model of the carbon fiber reinforced composite material connection joint are implemented. For example, the computer-readable storage medium can be the above-mentioned memory 802 including program instructions, and the above-mentioned program instructions can be executed by the processor 801 of the model establishment device 800 based on carbon fiber reinforced composite materials to complete the above-mentioned method for establishing the strength model of the carbon fiber reinforced composite material connection joint.

[0120] Embodiment 3:

[0121] Corresponding to the above method embodiment, in this embodiment, a readable storage medium is further provided. A readable storage medium described below can be correspondingly referred to with a method for establishing the strength model of a carbon fiber reinforced composite material connection joint described above.

[0122] A computer program is stored on the readable storage medium. When the computer program is executed by a processor, the steps of the method for establishing the strength model of the carbon fiber reinforced composite material connection joint in the above method embodiment are implemented.

[0123] The readable storage medium can specifically be various readable storage media such as a USB flash drive, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk or an optical disc that can store program codes.

[0124] In summary, the present invention proposes a unified strength model for CFRP bonded, riveted, and bonded-riveted hybrid joints. First, the key parameters of the CFRP constitutive model are obtained through constitutive relationship tests, and the load-displacement curves are obtained through quasi-static tests of three types of CFRP joints. Then, finite element models of the three types of CFRP joints are established based on the Hashin failure criterion, the secondary nominal stress criterion, and the B-K criterion. The accuracy of the finite element models is verified by comparing the experimental and simulation results. Finally, based on the finite element models and the experimental strength models, the strength characteristics and parameter influence laws of the three types of joints are comprehensively revealed, and a unified strength model for the three types of joints under each influencing parameter is established, providing a convenient and effective tool for optimizing the parameters of CFRP joints, preventing joint failures, and ensuring the service safety of CFRP connection structures.

[0125] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

[0126] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or replacements within the technical scope disclosed by the present invention, and all should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A method for establishing a strength model of a carbon fiber reinforced composite material connection joint, characterized in that: include: The key parameters of the constitutive model are obtained through the constitutive relationship test of carbon fiber reinforced composite materials, and are used as the parameter input of the finite element model of carbon fiber reinforced composite materials, wherein the key parameters of the constitutive model include the tensile, compressive and shear mechanical performance parameters of the carbon fiber reinforced composite materials; quasi-static tensile tests on three types of connection joints of carbon fiber reinforced composite materials are carried out to obtain three load-displacement curves, and the first strength models of the three types of connection joints are obtained by fitting the three load-displacement curves; the test strength model is obtained based on the relationship between the three curve functions; the quasi-static tensile tests on the three types of connection joints include quasi-static tensile tests of adhesive bonding, riveting and adhesive-riveted hybrid joints, and the three load-displacement curves are the load-displacement curve of adhesive joints, the load-displacement curve of riveted joints and the load-displacement curve of adhesive-riveted hybrid joints; The key parameters of the constitutive model are used as input parameters. Finite element models of three types of joints of carbon fiber reinforced composite materials are established based on the Hashin failure criterion, the quadratic nominal stress criterion and the BK criterion. Three load-displacement simulation curves are obtained by simulation. The three load-displacement simulation curves are fitted to obtain the simulation curve function relationship. The experimental strength model is integrated based on the curve simulation function relationship and preset parameters to obtain the first initial parameter strength model. The three first initial parameter strength models are unified to obtain the initial parameter model, where the three finite element models are the finite element models of adhesive joints, riveted joints and adhesive-riveted hybrid joints respectively. The test strength model is integrated based on the simulation curve function relationship and the preset parameters to obtain the first initial parameter strength model, and the three first initial parameter strength models are unified to obtain the initial parameter model, wherein the variables of each preset parameter in the initial parameter model are all initial values; The control variable method is used to process the initial parameter model with parameter changes to obtain the final model under each influencing parameter change.

2. The method for establishing a strength model of a carbon fiber reinforced composite material connection joint according to claim 1, characterized in that: The quasi-static tensile test of the riveted joint in the quasi-static tensile test of the three joints includes five stages, namely, the elastic deformation stage, the plastic deformation stage, the crack extension stage, the competitive failure stage and the fracture failure stage. The elastic deformation stage is a stage in which the load increases approximately linearly with the displacement, and the rivet and the CFRP plate undergo elastic deformation. The plastic deformation stage is a stage in which the rivet undergoes plastic deformation as the load continues to increase. The crack extension stage is a stage in which cracks appear around the rivet hole and the load rises slowly in a fluctuating manner. The competitive failure stage is a stage in which the load begins to decrease slowly. The fracture failure stage is a stage in which the joint fails by fracture.

3. The method for establishing a strength model of a carbon fiber reinforced composite material connection joint according to claim 1, characterized in that: The finite element models of the three carbon fiber reinforced composite material joints include a finite element model of a glued joint of a carbon fiber reinforced composite material, a finite element model of a riveted joint of a carbon fiber reinforced composite material, and a finite element model of a glue-riveted hybrid joint of a carbon fiber reinforced composite material, wherein the unit type, mesh refinement, contact mode, constitutive model, input parameters, damage initiation criterion, and crack surface extension criterion in the finite element model of the glued joint of the carbon fiber reinforced composite material and the finite element model of the riveted joint of the carbon fiber reinforced composite material are the same.

4. The method for establishing a strength model of a carbon fiber reinforced composite material connection joint according to claim 1, characterized in that: The first strength models of the three connection joints are obtained by fitting the three load-displacement curves: In the formula, F B is the tensile load of the adhesive joint, and d is the displacement; In the formula, F R The tensile load of the riveted joint, d is the displacement; In the formula, F H is the tensile load of the glue-riveted hybrid joint, and d is the displacement.

5. The method for establishing a strength model of a carbon fiber reinforced composite material connection joint according to claim 1, characterized in that: The test strength model is obtained based on the relationship between the three curve functions: In the formula, F B , F R and F H are the tensile loads of adhesively bonded, riveted and adhesive-riveted mixed joints respectively, and d is the displacement.

6. The method for establishing a strength model of a carbon fiber reinforced composite material connection joint according to claim 1, characterized in that: The test strength model is integrated based on the simulation curve function relationship and the preset parameters to obtain a first initial parameter strength model, which includes: Set the initial value of the parameter variable; The loads and displacements of the three joints corresponding to the initial values ​​of the parameter variables are fitted to obtain the initial parameter models of each joint, including the first initial adhesive parameter strength model, the second initial riveting parameter strength model and the third adhesive-riveted hybrid joint strength model. The calculation formula of the first initial parameter strength model is as follows: In the formula, is the first initial bonding parameter strength model, d is the displacement; In the formula, is the first initial riveting parameter strength model, d is the displacement; In the formula, is the first initial glue-riveted mixed parameter strength model, and d is the displacement.

7. The method for establishing a strength model of a carbon fiber reinforced composite material connection joint according to claim 6, characterized in that: The initial parameter model is obtained, which includes: The first initial bonding parameter strength model, the second initial riveting parameter strength model and the third adhesive-riveted hybrid joint strength model are unified to obtain an initial parameter model, and the calculation formula is as follows: In the formula, and are the tensile loads of adhesive, riveted and adhesive-riveted hybrid joints in the initial parameter model, and d is the displacement.

8. The method for establishing a strength model of a carbon fiber reinforced composite material connection joint according to claim 1, characterized in that: The initial parameter model is subjected to parameter change processing, wherein the parameters include rivet diameter, laying method of carbon fiber reinforced composite material, friction coefficient between plates, adhesive layer thickness, overlap length, overlap width, rivet quantity and arrangement method, rivet row spacing and rivet column spacing.

9. A model building device based on carbon fiber reinforced composite materials, characterized in that: include: Memory for storing computer programs; A processor, configured to implement the method for establishing a strength model of a carbon fiber reinforced composite material connection joint as claimed in any one of claims 1 to 8 when executing the computer program.

10. A readable storage medium, characterized in that: The readable storage medium stores a computer program, and when the computer program is executed by a processor, the method for establishing a strength model of a carbon fiber reinforced composite material connection joint according to any one of claims 1 to 8 is implemented.

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