A damage prediction method for a pre-laminated l-shaped composite bolted joint

Abstract

The application discloses a kind of L-shaped composite bolt connectors containing pre-preparation layered damage prediction method, belong to composite material damage prediction field.Method includes: the progressive damage constitutive model of composite material layer unit and interlaminar unit is established, and the writing of damage constitutive model is completed using the VUMAT interface of ABAQUS;Establish the finite element model of L-shaped composite laminated plate tensile condition without containing pre-preparation damage and with layered defect;Boundary conditions are established to laminated plate model, load is applied and is solved using ABAQUS / EXPLICT, obtains the load-displacement curve and damage information of L-shaped composite bolt connector structure.The application can effectively predict the influence of R angle area layered defect position and area on the strength of L-shaped composite bolt connector.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of composite material damage prediction, and particularly relates to a finite element method for progressive damage and delamination failure of an L-shaped composite material bolted joint with a pre-prepared delamination in an R-angle region. BACKGROUND

[0002] Carbon fiber composites have been widely used in aerospace, transportation and other fields due to their high specific strength, high specific modulus, corrosion resistance and designability. Composite components are mostly flat panels, but as the application scenarios extend, composite materials are gradually formed into complex curved surface structures and applied to circumferential connection structures. However, carbon fiber reinforced composites are a typical difficult-to-machine material, and their heterogeneous, anisotropic and laminated structure characteristics make them prone to fiber tearing, delamination and micro-cracks during machining. Especially for composite connection structure components with curved parts, delamination defects are inevitably introduced during the manufacturing process, which reduces the strength of the composite curved part.

[0003] Finite element simulation, as an important research method for composite materials, plays a very important role in the design and analysis of laminated plate components. The simulation results not only provide a reference for design, but also predict the strength and damage form of existing structures, saving experimental costs.

[0004] At present, although much progress has been made in the simulation research of the strength of composite flat plates and cylindrical structures, there is less research on L-shaped composite material bolted joints, especially the influence of delamination defects introduced in the R-angle region on the strength. SUMMARY

[0005] The present application aims to overcome the defects in the prior art and provide a damage prediction method for an L-shaped composite material bolted joint with a pre-prepared delamination, which can effectively predict the influence of the delamination defect position and area in the R-angle region on the strength of the L-shaped composite material bolted joint.

[0006] The specific technical solutions adopted by the present application are as follows:

[0007] The present application provides a damage prediction method for an L-shaped composite material bolted joint with a pre-prepared delamination, which is as follows:

[0008] S1: Establish a simulation finite element model; the finite element model includes an L-shaped composite material bolted joint with a pre-prepared delamination, a support plate and a bolt;

[0009] S2: Establish a composite material layer element progressive damage constitutive model;

[0010] S3: Based on the progressive damage constitutive model described in S2, the program was written using the ABAQUS-VUMAT user dynamic material subroutine module, and a progressive damage constitutive model subroutine module was established.

[0011] S4: Based on the finite element model described in S1, establish the interlayer cohesive contact properties containing prefabricated layers;

[0012] S5: Embed the progressive damage constitutive model subroutine module described in S3 into the finite element model described in S1, and combine it with the interlayer cohesive contact properties described in S4 to perform calculations and simulations on the finite element model described in S1, and further obtain the load, displacement and damage information of the L-shaped composite bolt connection structure containing prefabricated layers.

[0013] Preferably, S1 is as follows:

[0014] A mesh model of an L-shaped composite bolt connector, support plate, and bolts with prefabricated layers was created using Hypermesh. The L-shaped composite bolt connector was modeled with layup according to the required layup angle and thickness, and rigid constraints were imposed on the support plate and bolts. Then, ASSEMBLY was used to assemble the L-shaped composite bolt connector, support plate, and bolts with prefabricated layers. After that, a dynamic explicit analysis step was set, and boundary conditions were applied to the model.

[0015] Preferably, S2 is as follows:

[0016] The stress-strain constitutive equation for composite materials is:

[0017]

[0018] Where, σ ij Let ε be the stress tensor. ij Let be the strain tensor; i,j take values ​​of 1, 2, 3, representing the in-plane fiber direction, in-plane transverse direction, and out-of-plane direction, respectively; C is the elastic stiffness matrix, C d f is the elastic stiffness matrix after initial damage; k The failure criterion is set at 1; when it is less than 1, the material is in an elastic state; when it reaches 1, the material is damaged. The Chang-Chang criterion is used as the failure criterion formula for composite materials.

[0019]

[0020] Where k = ft, fc, mt, mc, represent the fiber tension, fiber compression, matrix tension, and matrix compression states, respectively; X T S represents the tensile strength of a single-layer plate in the fiber direction, α is the tensile-shear coupling coefficient in the fiber direction, and S... 12 X is the in-plane shear strength. CY represents the compressive strength in the fiber direction. T Y represents the transverse tensile strength. C The transverse compressive strength is given; the elastic stiffness matrix of the composite material after damage is:

[0021]

[0022] Where, d ft d fc d mt and d mc These are the damage variables characterizing the four damage modes: fiber tension, fiber compression, matrix tension, and matrix compression; E ij and ν ij These are the elastic modulus and Poisson's ratio, respectively; S mt and S mc The tensile-shear coupling and compressive-shear coupling coefficients of the composite matrix are respectively taken as 0.93; the equivalent stress and damage variables of the material are defined using a linear stiffness degradation form.

[0023]

[0024] Where, σ k,eq and δ k,eq The equivalent stress and equivalent displacement after damage, and f k Equivalent stress and equivalent displacement when G = 1 k These represent the fracture energies corresponding to the four damage modes. The final failure displacement is calculated using the following formula:

[0025]

[0026] The equivalent stress and equivalent displacement after damage are defined as follows:

[0027]

[0028] Introducing the element feature length l c To reduce the sensitivity of the finite element model to mesh size, the calculation expression is as follows:

[0029]

[0030] Where V is the unit volume and t is the thickness of a single layer of the laminate.

[0031] Preferably, S3 is as follows:

[0032] The program for the progressive damage constitutive model described in S2 was written using the VUMAT interface of ABAQUS. Material parameters, including the strength and stiffness of the composite laminate to be tested, were collected, and material property items of the composite material were established using the variable interface in the program.

[0033] Preferably, S4 is as follows:

[0034] A universal contact is adopted, and cohesive contact is applied to the contact surface set of two adjacent single layers; according to the actual delamination defect area surface set in the R-corner region of the composite laminate with delamination defects, the corresponding unit surface needs to be deleted, and these deleted unit surfaces are replaced with surface-to-surface contact.

[0035] Preferably, S5 is as follows:

[0036] The main model files established in S1 and S4 are combined with the progressive damage constitutive model subroutine module established in S3, and ABAQUS / EXPLICT is used to solve the problem to obtain the loading load, displacement, and damage information of the L-shaped composite bolt connection structure.

[0037] Compared with the prior art, the present invention has the following advantages:

[0038] A finite element model was established that comprehensively considers fiber, matrix, and interlaminar damage, enabling it to accurately predict the strength of L-shaped composite materials with delamination defects, determine the location and area of ​​the initial delamination in the R-corner region and its influence on strength, and accurately analyze the various types of damage generated under load. This model has certain guiding significance for further establishing reasonable manufacturing quality evaluation indicators for L-shaped composite materials. Attached Figure Description

[0039] Figure 1 A schematic diagram of a simulation model of an L-shaped composite bolt structure with prefabricated layers in the R-corner area;

[0040] Figure 2 This is a tensile simulation and experimental load-displacement curve of the bolt structure of the present invention without prefabricated layered L-shaped composite material;

[0041] Figure 3 The radius of the R-corner area of ​​this invention is 28mm at a distance of 15 layers from the upper surface. 2 Tensile simulation and experimental load-displacement curves of prefabricated layered L-shaped composite bolt structure;

[0042] Figure 4 The radius of the R-corner area of ​​this invention is 28mm at a distance of 15 layers from the upper surface. 2 Schematic diagram of delamination failure in a prefabricated, layered L-shaped composite bolt structure;

[0043] Figure 5The radius of the R-corner area of ​​this invention is 28mm at a distance of 15 layers from the upper surface. 2 A schematic diagram of matrix damage during tensile simulation of a prefabricated, layered L-shaped composite bolt structure. Detailed Implementation

[0044] The present invention will be further described and illustrated below with reference to the accompanying drawings and specific embodiments. The technical features of each embodiment of the present invention can be combined accordingly, provided that there is no mutual conflict.

[0045] It should be noted that this invention is an application of computer technology in the field of composite material damage prediction. The implementation of this invention involves the application of multiple software functional modules. The applicant believes that, after carefully reading the application documents and accurately understanding the implementation principles and objectives of this invention, and in conjunction with existing known technologies, those skilled in the art can fully utilize their software programming skills to implement this invention. All references in this application document fall within this scope, and the applicant will not list them all. According to this invention, the L-shaped composite material is not limited to a uniform design of the total number of layers and thickness; a suitable total number of layers and thickness can be selected according to actual needs. Furthermore, the composite material laminate is not particularly limited, and is preferably a carbon fiber composite material or a glass fiber composite material.

[0046] To determine the location and area of ​​the initial delamination in the R-corner region and its impact on strength, and to accurately analyze the various types of damage generated under load, this invention proposes a finite element model that comprehensively considers fiber, matrix, and interlaminar damage. This model can accurately predict the strength of L-shaped composite material connectors with delamination defects, providing guidance for further establishing reasonable manufacturing quality evaluation indicators for L-shaped composite materials. Furthermore, it establishes a mapping relationship between the location and area of ​​the initial delamination and strength.

[0047] The damage prediction method of the present invention will be described in detail below.

[0048] This invention provides a damage prediction method for L-shaped composite bolt connections with prefabricated layers, the damage prediction method specifically including the following steps:

[0049] S1: Establish a simulation finite element model. The finite element model includes a prefabricated L-shaped composite bolted connector (i.e., an L-shaped composite laminate with bolt holes), a support plate, and bolts.

[0050] As a preferred embodiment of the present invention, step S1 is specifically as follows:

[0051] A mesh model of L-shaped composite bolt connectors, support plates, and bolts with prefabricated layers was established using Hypermesh software. The L-shaped composite bolt connectors were modeled with layup according to the required layup angle and thickness, and rigid body constraints were assigned to the support plates and bolts. Then, ASSEMBLY was used to assemble the L-shaped composite bolt connectors, support plates, and bolts with prefabricated layers. After that, a dynamic explicit analysis step was set, and boundary conditions were applied to the model.

[0052] S2: Establish a progressive damage constitutive model for the intralayer unit of the composite material.

[0053] As a preferred embodiment of the present invention, step S2 is specifically as follows:

[0054] The stress-strain constitutive equation for composite materials is:

[0055]

[0056] Where, σ ij Let ε be the stress tensor. ij Let be the strain tensor; i,j take values ​​of 1, 2, 3, representing the in-plane fiber direction, in-plane transverse direction, and out-of-plane direction, respectively; C is the elastic stiffness matrix, C d f is the elastic stiffness matrix after initial damage; k The failure criterion is set at 1; when it is less than 1, the material is in an elastic state; when it reaches 1, the material is damaged. The Chang-Chang criterion is used as the failure criterion formula for composite materials.

[0057]

[0058] Where k = ft, fc, mt, mc, represent the fiber tension, fiber compression, matrix tension, and matrix compression states, respectively; X T S represents the tensile strength of a single-layer plate in the fiber direction, α is the tensile-shear coupling coefficient in the fiber direction, and S... 12 X is the in-plane shear strength. C Y represents the compressive strength in the fiber direction. T Y represents the transverse tensile strength. C The transverse compressive strength is given; the elastic stiffness matrix of the composite material after damage is:

[0059]

[0060] Where, d ft d fc d mt and d mc These are the damage variables characterizing the four damage modes: fiber tension, fiber compression, matrix tension, and matrix compression; E ij and ν ijThese are the elastic modulus and Poisson's ratio, respectively; S mt and S mc The tensile-shear coupling and compressive-shear coupling coefficients of the composite matrix are respectively taken as 0.93; the equivalent stress and damage variables of the material are defined using a linear stiffness degradation form.

[0061]

[0062] Where, σ k,eq and δ k,eq The equivalent stress and equivalent displacement after damage, and f k Equivalent stress and equivalent displacement when G = 1 k These represent the fracture energies corresponding to the four damage modes. The final failure displacement is calculated using the following formula:

[0063]

[0064] The equivalent stress and equivalent displacement after damage are defined as follows:

[0065]

[0066] Introducing the element feature length l c To reduce the sensitivity of the finite element model to mesh size, the calculation expression is as follows:

[0067]

[0068] Where V is the unit volume and t is the thickness of a single layer of the laminate.

[0069] S3: Based on the progressive damage constitutive model obtained in S2, the program was written using the ABAQUS-VUMAT user dynamic material subroutine module to establish the progressive damage constitutive model subroutine module.

[0070] As a preferred embodiment of the present invention, step S3 is specifically as follows:

[0071] The program for the progressive damage constitutive model described in S2 was written using the VUMAT interface of ABAQUS. Material parameters, including the strength and stiffness of the composite laminate to be tested, were collected, and material property items of the composite material were established using the variable interface in the program.

[0072] S4: Based on the finite element model obtained in S1, establish the interlayer cohesive contact properties with prefabricated layers.

[0073] As a preferred embodiment of the present invention, step S4 is specifically as follows:

[0074] A universal contact is adopted, and cohesive contact is applied to the contact surface set of two adjacent single layers; according to the actual delamination defect area surface set in the R-corner region of the composite laminate with delamination defects, the corresponding unit surface needs to be deleted, and these deleted unit surfaces are replaced with surface-to-surface contact.

[0075] Specifically, the following conditions must be met when the layering begins:

[0076]

[0077] Where σ n For interlayer normal stress, σ s The interlaminar shear stress is given by T and S, respectively, representing the interlaminar tensile fracture strength and shear fracture strength. The mixed-mode damage initiation displacement (stiffness softening initiation amount) is:

[0078]

[0079] in and For pure tensile and pure shear fracture initiation, EN and ET represent the interlaminar tensile and shear stiffness, respectively; after delamination damage initiation, the final hybrid mode displacement at failure follows a power function:

[0080]

[0081] Among them G IC and G IIC Let represent the interlaminar tensile and shear fracture energies, and α be a coefficient.

[0082] S5: Embed the progressive damage constitutive model subroutine module obtained in S3 into the finite element model obtained in S1, and combine it with the interlayer cohesive contact properties obtained in S4 to perform calculation and simulation on the finite element model obtained in S1, and further obtain the load, displacement and damage information of the L-shaped composite bolt connection structure with prefabricated layers.

[0083] As a preferred embodiment of the present invention, step S5 is specifically as follows:

[0084] The main model files established in S1 and S4 are combined with the progressive damage constitutive model subroutine module established in S3, and solved using ABAQUS / EXPLICT to obtain the loading, displacement, and damage information of the L-shaped composite bolt connection structure. Based on the load-displacement curves, the predicted ultimate strength values ​​of the complete composite laminate and the composite laminate with delamination defects are obtained respectively. By comparing the predicted ultimate strength values ​​of the composite laminate with delamination defects and the complete composite laminate, the influence of different delamination areas or locations on the strength of the composite laminate is obtained. Damage contour maps can mark the damage locations in the model and are used to predict the failure modes and failure locations of the complete composite laminate and the composite laminate with delamination defects respectively.

[0085] Example

[0086] A tensile finite element model of a carbon fiber / epoxy resin L-shaped composite bolted connector, a support plate, and bolts was created in ABAQUS / CAE, as follows: Figure 1 As shown, the layer sequence is [0, 60, -60]. 10 It has a width of 40mm, a thickness of 5.85mm, and a density of ρ = 1600kg / m³. 3 Material parameters are shown in Table 1. All in-plane elements were simulated using reduced integral 3D solid elements C3D8R. To prevent zero-energy modes, the hourglass control was set to relaxed stiffness mode. The lower bolt hole diameter was 10mm, and its edge was 8.8mm from the nearest edge of the R-corner region. The bolt was a rigid body model with R3D4 element type. The lower bolt fixed the L-shaped composite laminate to the support plate, and the upper bolt was loaded at a speed of 10mm / s. The contact surface between the bolt and the hole in the composite plate was in bonded contact. A 28mm thick layer was applied at the R-corner region, 15 layers from the upper surface. 2 The results of the prefabricated layered model and the non-prefabricated layered model were calculated and compared respectively.

[0087] Table 1 Mechanical Properties of Laminates

[0088]

[0089] The tensile loading process of L-shaped composite bolted connections is simulated using ABAQUS / EXPLICT. The user subroutine VUMAT is used to first determine whether the material points have entered the damage stage. If they have entered the damage stage, the damage variables are calculated according to the damage evolution criterion to obtain the nominal stress.

[0090] Figure 2 and Figure 3 The present invention does not include pre-layered and R-corner areas, with a distance of 28mm from the upper surface at 15 layers. 2 A comparison of the simulated and experimental load-displacement curves of prefabricated layered tensile testing shows that the simulation results are in accurate agreement with the experimental values. Figure 4The diagram shows the delamination failure distribution of an L-shaped composite material with pre-delamination. It can be seen that delamination occurs first on the upper side of the R-corner region, and with further loading, the delamination gradually extends to the middle and lower regions of the R-corner. Figure 5 This invention describes the matrix tensile and compressive damage of an L-shaped composite bolted connector with pre-delamination at final failure. The tensile damage is mainly distributed on the upper and lower surfaces of the L-shaped composite plate in contact with the bolt, and in the significantly delamination area at the radius (R-corner). The compressive damage is mainly distributed around the upper surface of the L-shaped composite plate in contact with the lower bolt. This invention can accurately capture the progressive damage failure characteristics of L-shaped composite bolted connectors with pre-delamination defects.

[0091] This invention develops user subroutines based on ABAQUS software. The proposed prefabricated delamination method and progressive damage constitutive model can accurately predict the progressive failure process of L-shaped composite bolted connections with prefabricated damage delamination in the R-corner region under tensile conditions. This provides technical support for further elucidating the damage failure characteristics of L-shaped composite structures and improving the level of lightweight strength design.

[0092] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Therefore, all technical solutions obtained through equivalent substitution or transformation fall within the protection scope of the present invention.

Claims

1. A method for predicting damage in L-shaped composite bolt connections with prefabricated layers, characterized in that, Specifically as follows: S1: Establish a simulation finite element model; the finite element model includes a prefabricated layered L-shaped composite bolt connector, a support plate, and bolts; S2: Establish a progressive damage constitutive model for intralayer elements in composite materials; S3: Based on the progressive damage constitutive model described in S2, the program was written using the ABAQUS-VUMAT user dynamic material subroutine module, and a progressive damage constitutive model subroutine module was established. S4: Based on the finite element model described in S1, establish the interlayer cohesive contact properties containing prefabricated layers; S5: Embed the progressive damage constitutive model subroutine module described in S3 into the finite element model described in S1, and combine it with the interlayer cohesive contact properties described in S4 to perform calculation and simulation on the finite element model described in S1, and further obtain the load, displacement and damage information of the L-shaped composite bolt connection structure containing prefabricated layers. S2 is specifically as follows: The stress-strain constitutive equation for composite materials is: ； in, For stress tensor, is the strain tensor; i,j takes values ​​of 1, 2, 3, representing the in-plane fiber direction, the in-plane transverse direction, and the out-of-plane direction, respectively. Here is the elastic stiffness matrix. This is the elastic stiffness matrix after initial damage; The failure criterion is set at 1; when it is less than 1, the material is in an elastic state; when it reaches 1, the material is damaged. The Chang-Chang criterion is used as the failure criterion formula for composite materials. ； in, , respectively representing fiber stretching, fiber compression, matrix stretching, and matrix compression states; The tensile strength of a single-layer plate in the fiber direction. The fiber-direction tensile-shear coupling coefficient is... In-plane shear strength, The compressive strength is in the fiber direction. For transverse tensile strength, The transverse compressive strength is given; the elastic stiffness matrix of the composite material after damage is: ； ； in, , , as well as These are the damage variables that characterize the four damage modes: fiber tension, fiber compression, matrix tension, and matrix compression. and These are the elastic modulus and Poisson's ratio, respectively. and The tensile-shear coupling and compressive-shear coupling coefficients of the composite matrix are respectively taken as 0.93; the equivalent stress and damage variables of the material are defined using a linear stiffness degradation form. ； in, and The equivalent stress and equivalent displacement after damage, and They are respectively Equivalent stress and equivalent displacement at time, These are the fracture energies corresponding to the four damage modes; The final failure displacement is calculated using the following formula: ； The equivalent stress and equivalent displacement after damage are defined as follows: ； Introducing the characteristic length of the unit To reduce the sensitivity of the finite element model to mesh size, the calculation expression is as follows: ； Where V is the unit volume and t is the thickness of a single layer of the laminate.

2. The damage prediction method for L-shaped composite bolt connections with prefabricated layers according to claim 1, characterized in that, S1 is specifically as follows: A mesh model of L-shaped composite bolt connectors, support plates, and bolts with prefabricated layers was created using Hypermesh. The L-shaped composite bolt connectors were modeled with layup according to the required layup angle and thickness, and rigid body constraints were assigned to the support plates and bolts. Then, ASSEMBLY was used to assemble the L-shaped composite bolt connectors, support plates, and bolts with prefabricated layers. After that, a dynamic explicit analysis step was set, and boundary conditions were applied to the model.

3. The damage prediction method for L-shaped composite bolt connections with prefabricated layers according to claim 1, characterized in that, S3 is specifically as follows: The program for the progressive damage constitutive model described in S2 was written using the VUMAT interface of ABAQUS. Material parameters, including the strength and stiffness of the composite bolt connection to be tested, were collected, and material property items of the composite material were established using the variable interface in the program.

4. The damage prediction method for L-shaped composite bolt connections with prefabricated layers according to claim 1, characterized in that, S4 is specifically as follows: A universal contact is adopted, and cohesive contact is applied to the contact surface set of two adjacent single layers; according to the actual delamination defect area surface set in the R-corner region of the composite laminate with delamination defects, the corresponding unit surface needs to be deleted, and these deleted unit surfaces are replaced with surface-to-surface contact.

5. The damage prediction method for L-shaped composite bolt connections with prefabricated layers according to claim 1, characterized in that, S5 is specifically as follows: The main model files established in S1 and S4 are combined with the progressive damage constitutive model subroutine module established in S3, and ABAQUS / EXPLICT is used to solve the problem to obtain the loading load, displacement, and damage information of the L-shaped composite bolt connection structure.

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