Damage prediction method for L-shaped composite bolt connecting piece containing prefabricated layers

Abstract

The invention discloses a damage prediction method for an L-shaped composite material bolt connecting piece containing prefabricated layers, and belongs to the field of composite material damage prediction. The method comprises the following steps: establishing a progressive damage constitutive model of an in-layer unit and an interlayer unit of a composite material, and completing writing of the damage constitutive model by using a VUMAT interface of ABAQUS; establishing a finite element model of the stretching working condition of the L-shaped composite laminated plate which does not contain the prefabricated damage and has the layering defect; boundary conditions are established for the laminated plate model, a load is applied, ABAQUS / EXPLICT is used for solving, and a load-displacement curve and damage information of the L-shaped composite material bolt connecting piece structure are obtained. The method can effectively predict the influence of the position and area of the layering defect of the R corner area on the strength of the L-shaped composite material bolt connecting piece.

Description

Technical Field

[0001] The present invention belongs to the field of composite material damage prediction, and particularly relates to a finite element method for progressive damage and delamination failure of L-shaped composite bolted joints with prefabricated delaminations in the R corner area. Background Art

[0002] Carbon fiber composite materials have been widely used in the fields of aerospace, transportation, etc. due to their high specific strength, high specific modulus, corrosion resistance and designability. Most composite components are flat plates. However, with the extension of their application scenarios, composite materials are gradually formed into complex curved surface structures and applied to circumferential connection structures. However, carbon fiber reinforced composite materials are typical difficult-to-machine materials. Their heterogeneous, anisotropic and laminated structure characteristics lead to defects such as fiber tearing, delamination, and microcracks during the machining process. Especially for composite material connection structure parts with curved surface parts, delamination defects are inevitably introduced during the manufacturing and forming process, which reduces the strength of composite material curved surface parts.

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

[0004] At present, although many progresses have been made in the strength simulation research of composite material flat plates and cylindrical structures, there is less research on L-shaped composite bolted joints, especially the lack of research on the influence of delamination defects introduced in their R corner areas on strength. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects in the prior art and provide a damage prediction method for L-shaped composite bolted joints with prefabricated delaminations, which can effectively predict the influence of the delamination defect position and area in the R corner on the strength of L-shaped composite bolted joints.

[0006] The specific technical solution adopted by the present invention is as follows:

[0007] The present invention provides a damage prediction method for L-shaped composite bolted joints with prefabricated delaminations, specifically as follows:

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

[0009] S2: Establish a constitutive model for progressive damage of in-plane elements of the composite material;

[0010] S3: Based on the progressive damage constitutive model described in S2, use the ABAQUS-VUMAT user-defined dynamic material subroutine module to complete the program writing and establish a subroutine module for the progressive damage constitutive model;

[0011] S4: Based on the finite element model described in S1, establish the cohesive contact property between layers with a prefabricated delamination;

[0012] S5: Embed the subroutine module of the progressive damage constitutive model described in S3 into the finite element model described in S1, and combine with the cohesive contact property between layers described in S4 to perform computational 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 with a prefabricated delamination.

[0013] Preferably, S1 is specifically as follows:

[0014] Based on Hypermesh, establish a mesh model of the L-shaped composite bolt connection, support plate and bolt with a prefabricated delamination; among them, the L-shaped composite bolt connection is modeled by laying layers according to the required ply angles and thicknesses, and rigid body constraints are given to the support plate and bolt; then use ASSEMBLY to assemble the L-shaped composite bolt connection, support plate and bolt with a prefabricated delamination, then set the dynamic explicit analysis step, and then apply boundary conditions to the model.

[0015] Preferably, S2 is specifically as follows:

[0016] The stress-strain constitutive equation of the composite material is:

[0017]

[0018] Among them, σ ij is the stress tensor, ε ij is 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 is the elastic stiffness matrix after initial damage; f k is the failure criterion value. 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 composite material failure criterion formula:

[0019]

[0020] Among them, k = ft, fc, mt, mc, representing fiber tension, fiber compression, matrix tension and matrix compression states respectively; X T is the tensile strength of the unidirectional laminate in the fiber direction, α is the fiber-direction tension-shear coupling coefficient, S 12 is the in-plane shear strength, X Cis the compressive strength in the fiber direction, Y T is the tensile strength in the transverse direction, Y C is the compressive strength in the transverse direction; the elastic stiffness matrix after composite material damage is:

[0021]

[0022] wherein, d ft , d fc , d mt and d mc are damage variables representing the four damage modes of fiber tension, fiber compression, matrix tension, and matrix compression respectively; E ij and ν ij are the elastic modulus and Poisson's ratio respectively; S mt and S mc are the matrix tensile-shear coupling and compressive-shear coupling coefficients of the composite material, each taking 0.93; the equivalent stress and damage variable of the material are defined in the form of linear stiffness degradation:

[0023]

[0024] wherein, σ k,eq and δ k,eq are the equivalent stress and equivalent displacement after damage, and are the equivalent stress and equivalent displacement when f k = 1 respectively, G k is the fracture energy corresponding to the four damage modes; is the final failure displacement, which is calculated by the following formula:

[0025]

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

[0027]

[0028] The element characteristic length l c is introduced to reduce the sensitivity of the finite element model mesh size, and its calculation expression is as follows:

[0029]

[0030] where V is the element volume and t is the single-layer thickness of the laminate.

[0031] Preferably, the S3 is specifically as follows:

[0032] Use the VUMAT interface of ABAQUS to complete the programming of the progressive damage constitutive model described in S2, collect material parameters including the strength and stiffness of the composite laminate to be tested, and establish the material property items of the composite material using the variable interface in the program.

[0033] Preferably, S4 is specifically as follows:

[0034] Adopt general contact and apply cohesive contact to the contact surface sets of adjacent two single layers; according to the area set of the actual delamination defect in the R - corner area of the composite laminate with delamination defects, the corresponding unit faces need to be deleted, and these deleted unit faces are changed to face - to - face contact.

[0035] Preferably, S5 is specifically as follows:

[0036] Combine the model main file established in S1 and S4 with the progressive damage constitutive model subroutine module established in S3, and use ABAQUS / EXPLICT to solve 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 beneficial effects:

[0038] Establish a finite element model that comprehensively considers fiber, matrix damage and inter - layer damage, enabling it to accurately predict the strength of the L - shaped composite material with delamination defects, determine the influence of the position and area of the initial delamination in the R - corner area on the strength, and accurately analyze various types of damage generated during loading. It has certain guiding significance for further establishing reasonable manufacturing quality evaluation indicators for L - shaped composite materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of the simulation model of the L - shaped composite bolt structure with a pre - fabricated delamination in the R - corner area;

[0040] Figure 2 It is the tensile simulation and experimental load - displacement curve diagram of the L - shaped composite bolt structure without pre - fabricated delamination of the present invention;

[0041] Figure 3 It is for the L - shaped composite bolt structure with a pre - fabricated delamination of 28 mm at the 15th layer from the upper surface in the R - corner area of the present invention 2 Tensile simulation and experimental load - displacement curve diagram;

[0042] Figure 4 It is for the L - shaped composite bolt structure with a pre - fabricated delamination of 28 mm at the 15th layer from the upper surface in the R - corner area of the present invention 2 Schematic diagram of delamination failure of the L - shaped composite bolt structure;

[0043] Figure 5At the 15th layer from the upper surface in the R - corner area of the present invention, it contains 28 mm 2 Schematic diagram of matrix damage in the tensile simulation of a prefabricated - delaminated L - shaped composite bolt structural member. Specific implementation manners

[0044] The present invention will be further described and illustrated below in conjunction with the accompanying drawings and specific implementation manners. The technical features of each implementation manner in the present invention can be combined correspondingly on the premise that there is no conflict with each other.

[0045] It should be noted that the present invention is an application of computer technology in the field of composite material damage prediction. In the implementation process of the present invention, the application of multiple software functional modules will be involved. The applicant believes that after carefully reading the application documents and accurately understanding the implementation principle and invention purpose of the present invention, and in combination with the existing well - known technologies, those skilled in the art can fully use the software programming skills they have mastered to implement the present invention. All that is mentioned in the application documents of the present invention belongs to this category, and the applicant will not list them one by one. According to the present invention, the L - shaped composite material is not limited to the unified design of the total number of plies and thickness, and the appropriate total number of plies and thickness can be selected according to actual needs; and the composite material laminate mentioned has no special restrictions, and is preferably carbon fiber composite material or glass fiber composite material.

[0046] In order to determine the influence of the position and area of the initial delamination in the R - corner area on the strength, and accurately analyze various types of damage generated during loading, the present invention proposes a finite - element model that comprehensively considers fiber, matrix damage, and inter - layer damage, enabling it to accurately predict the strength of the L - shaped composite material connector with delamination defects, which has certain guiding significance for further establishing reasonable manufacturing quality evaluation indicators for L - shaped composite materials, and establishing a mapping relationship between the position and area of the initial delamination and the strength.

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

[0048] The present invention provides a damage prediction method for an L - shaped composite bolt connector with prefabricated delamination. The damage prediction method specifically includes the following steps:

[0049] S1: Establish a simulation finite - element model. Among them, the finite - element model includes an L - shaped composite bolt connector with prefabricated delamination (i.e., a composite material laminate with bolt holes in an L - shape), a support plate, and a bolt.

[0050] As a relatively preferred embodiment of the present invention, the specific steps of S1 are as follows:

[0051] Based on the Hypermesh software, a mesh model of an L-shaped composite bolt connection, a support plate, and a bolt with a prefabricated delamination is established; among them, the L-shaped composite bolt connection is modeled by laying layers according to the required laying angle and thickness, and rigid body constraints are given to the support plate and the bolt; then, the L-shaped composite bolt connection, the support plate, and the bolt with a prefabricated delamination are assembled using ASSEMBLY, and then a dynamic explicit analysis step is set, and then boundary conditions are applied to the model.

[0052] S2: Establish a progressive damage constitutive model for composite material in-layer elements.

[0053] As a preferred embodiment of the present invention, the specific steps of S2 are as follows:

[0054] The stress-strain constitutive equation of the composite material is:

[0055]

[0056] Among them, σ ij is the stress tensor, ε ij is the strain tensor; i, j take values of 1, 2, 3, representing the in-plane fiber direction, the in-plane transverse direction, and the out-of-plane direction respectively; C is the elastic stiffness matrix, and C d is the elastic stiffness matrix after initial damage; f k is the failure discrimination value. When it is less than 1, the material is in an elastic state, and when it reaches 1, the material is damaged; the Chang-Chang criterion is used as the composite material failure discrimination formula:

[0057]

[0058] Among them, k = ft, fc, mt, mc, representing the fiber tension, fiber compression, matrix tension, and matrix compression states respectively; X T is the tensile strength of the unidirectional laminate in the fiber direction, α is the fiber direction tension-shear coupling coefficient, S 12 is the in-plane shear strength, X C is the compressive strength in the fiber direction, Y T is the transverse tensile strength, Y C is the transverse compressive strength; the elastic stiffness matrix of the composite material after damage is:

[0059]

[0060] Among them, d ft , d fc , d mt and d mc are the damage variables representing the four damage modes of fiber tension, fiber compression, matrix tension, and matrix compression respectively; E ij and ν ijare the elastic modulus and Poisson's ratio respectively; S mt and S mc are the tensile-shear coupling and compressive-shear coupling coefficients of the composite matrix respectively, both taking 0.93; The equivalent stress and damage variable of the material are defined in the form of linear stiffness degradation:

[0061]

[0062] where, σ k,eq and δ k,eq are the equivalent stress and equivalent displacement after damage, and are the equivalent stress and equivalent displacement when f k = 1 respectively, and G k is the fracture energy corresponding to four damage modes; is the final failure displacement, which is calculated by the following formula:

[0063]

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

[0065]

[0066] The element characteristic length l c is introduced to reduce the mesh size sensitivity of the finite element model, and its calculation expression is as follows:

[0067]

[0068] where V is the element volume and t is the single-layer thickness of the laminated plate.

[0069] S3: Based on the progressive damage constitutive model obtained in S2, use the ABAQUS-VUMAT user-defined dynamic material subroutine module to complete the program writing and establish the progressive damage constitutive model subroutine module.

[0070] As a preferred embodiment of the present invention, the specific steps of S3 are as follows:

[0071] Use the VUMAT interface of ABAQUS to complete the program writing of the progressive damage constitutive model described in S2, collect the material parameters including the strength and stiffness of the composite laminated plate to be tested, and establish the material property items of the composite material using the variable interface in the program.

[0072] S4: Based on the finite element model obtained in S1, establish the cohesive contact property between layers with a prefabricated delamination.

[0073] As a preferred embodiment of the present invention, the specific steps of S4 are as follows:

[0074] Adopt general contact and apply cohesive contact to the contact surface sets of two adjacent single layers; according to the area set of the actual delamination defect in the R - corner area of the composite laminate with delamination defects, the corresponding unit faces need to be deleted, and these deleted unit faces are changed to face - to - face contact.

[0075] Specifically, when delamination starts, it satisfies:

[0076]

[0077] Where σ n is the inter - layer normal stress, σ s is the inter - layer shear stress, T and S are the inter - layer tensile fracture strength and shear fracture strength respectively; the mixed - mode damage initiation displacement (the starting amount of stiffness softening) is:

[0078]

[0079] Where and are the initial separation displacements of pure tension and pure shear fracture initiation, EN and ET are the inter - layer tensile and shear stiffnesses; after delamination damage is initiated, the final failure mixed - mode displacement follows a power function:

[0080]

[0081] Where G IC and G IIC are the inter - layer tensile and shear fracture energies, and α is a coefficient.

[0082] S5: Embed the progressive damage constitutive model sub - routine module obtained in S3 into the finite element model obtained in S1, and combine the inter - layer cohesive contact properties in S4 to perform computational simulation on the finite element model obtained in S1, and further obtain the load, displacement, and damage information of the L - shaped composite bolt - connected structure with pre - fabricated delamination.

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

[0084] Combine the main model file established by S1 and S4 with the progressive damage constitutive model subroutine module established by S3, and use ABAQUS / EXPLICT to solve to obtain the loading load, displacement, and damage information of the L-shaped composite bolt connection structure. According to the load-displacement curve, the ultimate strength prediction values of the complete composite laminate and the composite laminate with delamination defects are obtained respectively. By comparing the ultimate strength prediction values of the composite laminate with delamination defects and the complete composite laminate model, the influence law of different delamination area areas or positions on the strength of the composite laminate is obtained; the damage nephogram can mark the damage position in the model and is used to predict the failure form and failure position of the complete composite laminate and the composite laminate with delamination defects respectively.

[0085] Example

[0086] Establish a tensile finite element model including a carbon fiber / epoxy resin L-shaped composite bolt connection, a support plate, and a bolt in ABAQUS / CAE, as Figure 1 shown, the ply sequence is [0, 60, -60] 10 , the width is 40 mm, the thickness is 5.85 mm, and the density is ρ = 1600 kg / m 3 , and the material parameters are shown in Table 1. All in-plane elements are simulated using the reduced integration three-dimensional solid element C3D8R. To prevent zero energy modes, the hourglass control is in the relaxed stiffness mode; the diameter of the lower bolt hole is 10 mm and the closest distance from the edge to the R corner area edge is 8.8 mm. The bolt is a rigid body model and the element type is R3D4. The lower bolt fixes the L-shaped composite laminate to the support plate, and the loading speed of the upper bolt is 10 mm / s. The contact surface between the bolt and the composite plate hole is a bonded contact. Compare the calculation results of the prefabricated delamination and non-prefabricated delamination models applied at 28 mm 2 at the 15th layer from the upper surface in the R corner area.

[0087] Table 1 Mechanical properties of the laminate

[0088]

[0089] Use ABAQUS / EXPLICT to calculate and simulate the tensile loading process of the L-shaped composite bolt connection. Use the user subroutine VUMAT to first determine whether the material point enters the damage stage. If it enters the damage stage, calculate the damage variable according to the damage evolution criterion to obtain the nominal stress.

[0090] Figure 2 And Figure 3 are respectively the comparison diagrams of the tensile simulation and experimental load-displacement curves of the present invention without prefabricated delamination and with 28 mm 2 prefabricated delamination at the 15th layer from the upper surface in the R corner area; it can be seen that the simulation results are in good agreement with the experimental values. Figure 4It is a delamination failure distribution diagram of an L-shaped composite material with a prefabricated delamination. It can be seen that delamination first occurs on the upper side of the R corner area. With further loading, the delamination gradually spreads to the middle and lower areas of the R corner; Figure 5 It is the matrix tensile damage and matrix compressive damage of an L-shaped composite material bolt connection with a prefabricated delamination at final failure. It can be seen that the matrix tensile damage is mainly distributed on the upper and lower surfaces where the L-shaped composite plate contacts the bolt and the significant delamination area of the R corner area, and the matrix compressive damage is mainly distributed around the upper surface where the L-shaped composite plate contacts the lower bolt. The present invention can accurately capture the progressive damage failure characteristics of an L-shaped composite material bolt connection with a prefabricated delamination defect.

[0091] Based on the ABAQUS software, the present invention develops user subroutines. The proposed prefabricated delamination method and progressive damage constitutive model can accurately predict the progressive failure process of an L-shaped composite material bolt connection with a prefabricated damaged delamination in the R corner area under tensile conditions, providing technical support for deeply clarifying the damage failure characteristics of L-shaped composite material structures and improving the lightweight strength design level.

[0092] The above-described embodiments are only a preferred solution of the present invention, but they are not intended to limit the present invention. Those of ordinary skill in the relevant technical field can also make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all technical solutions obtained by adopting equivalent replacement or equivalent transformation methods fall within the protection scope of the present invention.

Claims

1. A damage prediction method for an L-shaped composite bolt connection with prefabricated layers, characterized in that: The details are as follows: S1: Establishing 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 the progressive damage constitutive model of composite material layer elements; S3: Based on the progressive damage constitutive model described in S2, the ABAQUS-VUMAT user dynamic material subroutine module is used to complete the program writing and establish the progressive damage constitutive model subroutine module; S4: Based on the finite element model described in S1, establish the interlayer cohesive contact properties including prefabricated delamination; S5: The progressive damage constitutive model subroutine module described in S3 is embedded into the finite element model described in S1, and combined with the interlayer cohesive contact properties described in S4, the finite element model described in S1 is simulated to further obtain the load, displacement and damage information of the L-shaped composite bolt connection structure containing prefabricated layers.

2. A damage prediction method for an L-shaped composite bolt connection with prefabricated layers according to claim 1, characterized in that: The S1 is specifically as follows: A mesh model of an L-shaped composite bolt connector with prefabricated layers, a support plate and bolts is established based on Hypermesh. The L-shaped composite bolt connector is modeled according to the required ply angle and thickness, and rigid body constraints are given to the support plate and bolts. ASSEMBLY is then used to assemble the L-shaped composite bolt connector with prefabricated layers, the support plate and bolts, and then a dynamic explicit analysis step is set, and boundary conditions are applied to the model.

3. The damage prediction method for an L-shaped composite bolt connection with prefabricated layers according to claim 1, characterized in that: The S2 is specifically as follows: The stress-strain constitutive equation of the composite material is: Among them, σ ij is the stress tensor, ε ij is the strain tensor; i,j are 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 is the elastic stiffness matrix after initial damage; f k is the failure judgment value. 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 judgment formula for composite materials: Where, k = ft, fc, mt, mc, representing fiber tension, fiber compression, matrix tension and matrix compression states respectively; X T is the tensile strength of the single-layer plate in the fiber direction, α is the tension-shear coupling coefficient in the fiber direction, S 12 is the in-plane shear strength, X C is the compressive strength in the fiber direction, Y T is the transverse tensile strength, Y C is the lateral compressive strength; the elastic stiffness matrix of the composite material after damage is: Among them, d ft , d fc , d mt and mc are the damage variables representing the four damage modes of fiber tension, fiber compression, matrix tension and matrix compression; E ij and ν ij are elastic modulus and Poisson’s ratio respectively; S mt and S mc are the tensile-shear coupling and compressive-shear coupling coefficients of the composite matrix, each of which is 0.93; the equivalent stress and damage variable of the material are defined in the form of linear stiffness degradation: Among them, σ k,eq and δ k,eq is the equivalent stress and equivalent displacement after damage, and f k =1, equivalent stress and equivalent displacement, G k is the fracture energy corresponding to the four damage modes; is the final failure displacement, calculated by the following formula: The equivalent stress and equivalent displacement after damage are defined as follows: Introducing unit characteristic length l c In order to reduce the sensitivity of the finite element model 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.

4. A damage prediction method for an L-shaped composite bolt connection with prefabricated layers according to claim 1, characterized in that: The S3 is as follows: The VUMAT interface of ABAQUS is used to complete the programming of the progressive damage constitutive model described in S2, and the material parameters including the strength and stiffness of the composite bolt connection to be tested are collected, and the material property items of the composite material are established using the variable interface in the program.

5. The damage prediction method for an L-shaped composite bolt connection with prefabricated layers according to claim 1, characterized in that: The S4 is specifically as follows: Universal contact is adopted, and cohesive contact is applied to the contact surface set of two adjacent single layers; the corresponding unit surfaces need to be deleted according to the surface set of the area where the actual delamination defects in the R corner area of ​​the composite laminate with delamination defects are located, and these deleted unit surfaces are replaced with surface-to-surface contact.

6. The damage prediction method for an L-shaped composite bolt connection with prefabricated layers according to claim 1, characterized in that: The S5 is specifically as follows: The model master file established in S1 and S4 is 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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