A comprehensive prediction method for delamination and intra-laminar damage during drilling of CFRP laminates

Abstract

The invention belongs to the field of carbon fiber composite material drilling processing, and specifically discloses a comprehensive prediction method for drilling delamination and intra-layer damage of CFRP laminates. The method aims at the problem that the strain rate of a single-layer plate during CFRP cutting will affect its failure destruction and damage formation, resulting in the difficulty in accurately predicting drilling delamination and intra-layer damage of CFRP laminates. The method takes into account the change of the mechanical behavior of the CFRP single-layer plate with the strain rate, defines the failure criterion and damage evolution criterion of the single-layer plate taking into account the influence of the strain rate, and simultaneously adopts Cohesive units to simulate the interlayer interface. Based on ABAQUS finite element simulation calculation software, a drilling simulation model of the CFRP laminate is established. Through numerical calculation, the comprehensive prediction of drilling delamination and intra-layer damage of the CFRP laminate is realized.

Description

Technical Field

[0001] The invention belongs to the field of drilling processing of carbon fiber composite materials, and in particular relates to a comprehensive prediction method for drilling delamination and intra-layer damage of CFRP laminates. Background Art

[0002] Carbon fiber reinforced composite materials (CFRP) have the characteristics of high specific strength, high specific stiffness, excellent wear resistance and impact resistance, and are widely used in the aerospace field. When manufacturing CFRP parts, drilling is usually required to meet the assembly and connection requirements after laying, curing and molding. However, since CFRP is composed of carbon fiber and epoxy resin at the microscopic level, the fiber strength is high and it is not easy to cut effectively, and the resin strength is low and it is easy to crack; it presents anisotropy and stacking characteristics at the macroscopic level, and the interlayer bonding strength is low. Therefore, when drilling CFRP laminates, it is often easy to cause delamination damage between single-layer plates, and damage such as fiber pull-out, burrs and tearing in any single-layer plate (intra-layer damage). These damages are not only easy to cause large assembly errors, but are also likely to seriously shorten the service life of the workpiece. Therefore, it is necessary to measure the drilling delamination and intra-layer damage of CFRP laminates under different process parameters, and then analyze the formation mechanism of damage to guide the suppression of drilling damage.

[0003] The experimental method for measuring the drilling delamination and intralaminar damage of CFRP laminates requires a large number of experiments, which is long and costly. Moreover, the quantitative characterization of some damage will destroy the processed components, resulting in inaccurate damage measurement results. The model required to predict the cutting damage of CFRP laminates using theoretical analysis is extremely complex, and the theoretical analysis is mainly applicable to predicting right-angle cutting damage, and it is difficult to accurately solve the drilling damage. However, numerical simulation can accurately characterize the complex dynamic cutting process of the drill bit on CFRP in three-dimensional space, which is convenient for intuitive observation of drilling delamination and intralaminar damage; by modifying the parameters of the numerical model, the drilling damage under different processing conditions can be predicted. Therefore, numerical simulation is the preferred method for obtaining drilling delamination and intralaminar damage of CFRP laminates. However, in order to achieve effective prediction of drilling damage of CFRP laminates, the material mechanical behavior of each CFRP single layer and interlaminar interface in the simulation model must be accurately defined, including constitutive model, failure criterion and damage evolution criterion; at the same time, the setting of model boundary conditions and constraints must reflect the actual drilling process; and the definition of mesh size and unit type must be optimized to ensure high model calculation efficiency.

[0004] In recent years, scholars have conducted extensive research on the simulation of drilling damage in CFRP laminates. In 2018, N. Feito et al. published "Experimental and numerical analysis of step drill bit performance when drilling woven CFRPs" in the journal Composite Structures. They equated each single-layer CFRP plate to anisotropic homogeneous materials, characterized the failure behavior of the single-layer plate based on the failure criterion containing four failure modes, used Cohesive units to simulate the interlayer interface, established a simulation model for step drill drilling CFRP laminates, and used this model to analyze the variation of CFRP drilling delamination damage with spindle speed and feed speed. However, the above research mainly considered the effect of the tool on the interlayer interface of CFRP during drilling, and carried out parameter analysis of delamination damage, but did not predict the intralayer damage of the workpiece, and could not obtain the influence of processing conditions on intralayer damage. In 2021, Yong Liu et al. published a paper titled "Scale-span modelling of dynamic progressive failure in drilling CFRPs using a tapered drill-reamer" in the journal Composite Structures. The paper used defined microscopic failure criteria and progressive damage evolution criteria to characterize the intralayer damage of CFRP. Based on the bilinear Cohesive unit to simulate delamination damage, the drilling simulation of CFRP laminates was carried out. The effects of spindle speed and feed rate on drilling delamination and intralayer damage were studied, which is helpful to guide damage suppression. However, existing studies have shown that the strain rate of a single-layer plate during CFRP cutting has a significant influence on its failure and damage formation, but the above model does not consider the change in the mechanical behavior of the single-layer plate material with the strain rate. Therefore, there is still a certain error in the predicted delamination and intralayer damage. Summary of the invention

[0005] The purpose of the present invention is to provide a comprehensive prediction method for drilling delamination and intra-layer damage of CFRP laminates in response to the defects of the prior art. This method aims to solve the problem that the strain rate of a single-layer plate during CFRP cutting will affect its failure and damage formation, resulting in difficulty in accurately predicting drilling delamination and intra-layer damage of CFRP laminates. This method takes into account the change in the mechanical behavior of a single-layer CFRP plate with the strain rate, defines the failure criterion and damage evolution criterion of a single-layer plate taking into account the influence of the strain rate, and simultaneously uses Cohesive units to simulate the interlayer interface. Based on the ABAQUS finite element simulation calculation software, a drilling simulation model of CFRP laminates is established. Through numerical calculation, a comprehensive prediction of drilling delamination and intra-layer damage of CFRP laminates is achieved.

[0006] The technical solution of the present invention is: a comprehensive prediction method for drilling delamination and intra-layer damage of CFRP laminates, comprising the following steps:

[0007] 1) Establish the geometric model of CFRP laminate based on ABAQUS finite element simulation software;

[0008] 2) Import the geometric model of the drill bit used;

[0009] 3) Assign corresponding constitutive models and mechanical properties parameters to different materials;

[0010] 4) Create an assembly of the drill bit and the CFRP workpiece drilling process;

[0011] 5) Set dynamic explicit analysis steps for simulation analysis;

[0012] 6) Define the constraints and contact modes between the drill bit and the workpiece and between them;

[0013] 7) Define the boundary conditions of the model;

[0014] 8) Submit the task for calculation and comprehensively predict the drilling delamination and intra-layer damage of CFRP laminates.

[0015] When building the geometric model of the CFRP laminate:

[0016] First, any outermost single-layer plate of the CFRP laminate is created as a three-dimensional deformable entity, and its length and width are set to L1 and L2, and its thickness is set to D1;

[0017] Secondly, the cutting area and non-cutting area are divided into two areas on this single-layer plate, and the meshes are divided into two areas. The cutting area is divided into a fine mesh, and the non-cutting area is divided into a coarse mesh. The mesh units are all generated as hexahedrons using the structural method. The unit type is an eight-node linear hexahedron reduced integration unit, and the unit is set to be deleted;

[0018] Then, the grid offset method is used to offset a layer of interlayer interface along the thickness direction of the single-layer plate. Its length and width are also L1 and L2, and its thickness is D2. The grid size is the same as that of the single-layer plate. The unit type is set to an eight-node three-dimensional cohesive unit, and unit deletion is allowed.

[0019] According to the number of plies of the CFRP laminate, the grid offset method is continued to be used to alternately generate the remaining single-layer plates and interlayer interfaces. The unit type settings of the two are the same as those of the above-mentioned single-layer plates and interlayer interfaces, thus forming the geometric model of the CFRP laminate.

[0020] When importing the geometry of the drill used:

[0021] First, set the rotation center at the drill tip as the reference point;

[0022] Secondly, fine grids and coarse grids are divided at the drill tip and other areas of the drill bit respectively. The grid units are all generated as tetrahedrons in a free way. The unit type is a four-node linear tetrahedron unit, and unit deletion is not set.

[0023] When assigning the corresponding constitutive models and mechanical properties parameters to different materials:

[0024] Since the CFRP single-layer plate has the characteristics of anisotropy and elasticity, the elastic constitutive relationship is used to characterize the mechanical behavior of the single-layer plate before failure, and different mechanical properties are set for the single-layer plate along the fiber direction, in-plane perpendicular to the fiber direction, and thickness direction.

[0025] Since the strain rate of a single-layer plate during CFRP cutting will affect its failure and damage formation, the effect of the strain rate on the mechanical properties of the single-layer plate needs to be considered before defining the failure criterion and damage evolution criterion of the single-layer plate.

[0026] The variation curves of the strength and fracture toughness of a single-layer plate with strain rate are shown in equations (1) and (2):

[0027] (1)

[0028] (2)

[0029] In the formula, is the real-time strain rate, is the reference strain rate;

[0030] S Represents strength Y T , Y C , S 12 , S 13 and S 23 ; G C Represents fracture toughness and ; and are the strength and fracture toughness at the reference strain rate, respectively;

[0031] On this basis, the failure criteria of single-layer boards containing different failure modes are defined by combining the Hashin criterion and the Puck criterion, as shown in equations (3)-(6):

[0032] Tensile failure along the fiber direction ( s1 ≥0):

[0033] (3)

[0034] Compression failure along the fiber direction ( s 1 <0):

[0035] (4)

[0036] In-plane tensile failure perpendicular to the fiber direction ( s 2 ≥0):

[0037] (5)

[0038] In-plane compression failure perpendicular to the fiber direction ( s 2 <0):

[0039] (6)

[0040] In the formula, F is the failure variable, the subscripts f and m represent the direction along the fiber and perpendicular to the fiber, respectively, and the superscripts T and C represent the tensile and compressive effects, respectively. When the failure variable reaches 1, the material fails in the corresponding mode;

[0041] s i and is the stress in the main direction and shear direction of the material, i , j =1, 2, 3, respectively, along the fiber direction, perpendicular to the fiber direction in the plane and the thickness direction, X and Y represent the strength along the fiber direction and perpendicular to the fiber direction, respectively. S ij For single-layer boards ij In-plane shear strength;

[0042] is the shear strength perpendicular to the fiber direction on the fracture surface when the CFRP unidirectional laminate fails in compression perpendicular to the fiber direction, and is calculated according to formula (7):

[0043] (7)

[0044] in, i is the angle between the fracture surface and the thickness direction of CFRP; s n , t s , tt It is the stress component in the normal direction of the fracture surface, along the fiber direction in the fracture surface, and perpendicular to the fiber direction in the fracture surface. Its relationship with the effective stress component is as follows:

[0045] (8)

[0046] m t , m s is the friction coefficient, which can be obtained from the angle of the fracture surface and the material strength parameters based on the Mohr-Coulomb theory:

[0047] (9)

[0048] After CFRP fails, the material stiffness continues to decrease with the evolution of internal damage. By defining a linear damage evolution criterion, the damage evolution process of a single-layer plate after any mode of failure occurs is described, that is, the damage variable is defined as shown in equations (10) and (13): d Control the stiffness reduction of single-layer plate materials;

[0049] d According to the real-time strain of the material e , the strain at the onset of injury e 0 and the strain at final fracture e f The value of the damage is 0 at the beginning of the damage and 1 at the final fracture. When a unit of the single-layer plate fails, the relevant effective stress component of this unit is multiplied by (1- d ), ensuring that the stress component of the damaged unit gradually decreases to 0 as the strain increases; the stretching along the fiber direction ( ) and compression ( ) failure, and tensile stress perpendicular to the fiber direction ( ) failure are:

[0050] (10)

[0051] In the formula, e i is the strain in the main direction of the material; the strain at the onset of failure ( , and ) As shown in formula (11), the strain at the final fracture ( , and ) is calculated using formula (12):

[0052] (11)

[0053] (12)

[0054] in, E i is along i The elastic modulus in the direction, , and They are respectively the fracture toughness of the material under tension and compression along the fiber direction and under tensile load perpendicular to the fiber direction. L c is the element characteristic length; the damage variable in the compression failure mode perpendicular to the fiber direction is calculated based on the strain on the fracture surface:

[0055] (13)

[0056] In the formula, ,symbol Meaning: For any real number x , ; The strain component on the fracture surface is:

[0057] (14)

[0058] in, c ij is the strain in the shear direction of the material; the strain at the beginning of failure It is achieved by extracting the strain when the failure variable reaches 1 during the calculation process. The final strain at fracture for:

[0059] (15)

[0060] In the formula, is the fracture toughness; the stress at the onset of failure The calculation method and The calculation method is similar:

[0061] (16)

[0062] The interlayer interface of the CFRP laminate is simulated based on the zero-thickness cohesive unit. Under the action of load, the upper and lower surfaces of this unit separate and move relative to each other. When the moving distance exceeds a certain value, the unit breaks, realizing the characterization of the crack initiation and propagation between the materials on both sides of the unit.

[0063] When using the cohesive force unit, it is necessary to define the stress-strain relationship in the normal direction, the first tangential direction, and the second tangential direction. The constitutive relationship before damage occurs at the interlayer interface is characterized by equation (17):

[0064] (17)

[0065] In the formula, t is the traction stress, For strain, K represents the elastic modulus; the subscripts n, s and t represent the normal direction, the first tangential direction and the second tangential direction respectively; for a cohesive unit with zero thickness, the strain in each direction is calculated as:

[0066] (18)

[0067] in, u is the separation displacement of the upper and lower surfaces of the cohesive unit in each direction, is the constitutive thickness of the cohesive element, Unlike the geometric thickness which has a value of 0, its value is taken as 1 to ensure that the strain and separation displacement are equal;

[0068] Under the action of load, the stress at the interface between layers gradually accumulates, and the secondary nominal stress criterion is used to determine the onset of failure:

[0069] (19)

[0070] In the formula, is the failure variable of the interlayer interface, t 0 Indicates the stress in each direction at failure;

[0071] After the interlayer interface fails, its stiffness is defined as the damage variable in equation (20): Under the control of , the stress components in each direction are calculated according to formula (21):

[0072] (20)

[0073] (twenty one)

[0074] in, represents the equivalent separation displacement; , and They are the equivalent separation displacement when the cohesive unit fails, the equivalent separation displacement when it fractures, and the maximum value of the equivalent separation displacement during drilling; Represents the equivalent traction stress in each direction, excluding the stress reduction caused by failure; the final cracking of the interlayer interface is judged according to the power exponent criterion:

[0075] (twenty two)

[0076] In the formula, , and is the material constant; G n , G s and G t They are the fracture toughness of the material in three directions; , and are the critical fracture toughness in three directions respectively;

[0077] The mechanical behavior of the above CFRP single-layer plate is calculated by importing the finite element software through defining a user subroutine, and the unit deletion during the cutting process is also controlled by the state parameters defined in the subroutine;

[0078] The specific material parameters of the single-layer plate and the interface between layers are given by the defined section properties. At the same time, the material direction is defined for the single-layer plate according to the ply direction.

[0079] When defining the constraints and contact modes between the drill bit and the workpiece and between each other;

[0080] First, since tool deformation and wear are not considered in the simulation process, the drill bit is constrained as an analytical rigid body;

[0081] Secondly, the drill bit and the workpiece are defined as point-surface contact. In the contact properties, the normal contact is defined as hard contact, and the tangential contact is defined as penalty friction.

[0082] Finally, to avoid intrusion during the calculation, define general contact between all contact pairs in the model.

[0083] When defining the boundary conditions for your model:

[0084] First, define the drill feed rate and spindle speed at the drill reference point defined above;

[0085] Secondly, the degrees of freedom of the peripheral nodes in the non-cutting area of ​​the workpiece are restricted in all directions to fix the workpiece.

[0086] Compared with the prior art, the present invention has the following advantages: the present invention takes into account the change of the mechanical behavior of the single-layer plate with the strain rate during the CFRP drilling process, defines the failure criterion and damage evolution criterion of the single-layer plate taking into account the influence of the strain rate, and uses the cohesive unit to simulate the interlayer interface, and establishes a drilling simulation model of the CFRP laminate.

[0087] The mechanical behavior of different phase materials in the model fully reflects the material properties of the single-layer plate and the interface between the layers. The defined mesh unit type, tool-workpiece contact mode and boundary conditions also conform to the actual drilling process. By calculating the simulation model, a comprehensive simulation of the drilling delamination and intra-layer damage formation process of CFRP laminates was achieved, and the delamination and intra-layer damage after drilling were obtained.

[0088] This method can be used to comprehensively analyze the delamination and intra-layer damage of CFRP laminates during drilling, and is suitable for guiding the suppression of drilling damage. BRIEF DESCRIPTION OF THE DRAWINGS

[0089] Figure 1 Schematic diagram of the drilling model for CFRP laminates;

[0090] Figure 2 Schematic diagram of the material mechanical behavior of a CFRP single-layer plate in any direction;

[0091] Figure 3 The prediction results of damage in the drilling layer of CFRP laminates;

[0092] Figure 4 The prediction results of damage in the drilling layer of CFRP laminates;

[0093] In the figure: 1, twist drill, 2, CFRP single-layer plate, 3, interlayer interface, 4, first layer of laminate, 5, second layer of laminate, 6, predicted intra-layer damage, 7, third layer of laminate, 8, fourth layer of laminate, 9, first layer interlayer interface, 10, predicted delamination damage, 11, second layer interlayer interface, 12, third layer interlayer interface, S, tool rotation direction, F, tool feed direction. DETAILED DESCRIPTION

[0094] The present invention is further described below with reference to the accompanying drawings and embodiments. Example 1

[0095] A comprehensive prediction method for drilling delamination and intra-layer damage of CFRP laminates. This method analyzes the effect of strain rate on the mechanical behavior of a single CFRP plate, defines the failure criterion and damage evolution criterion of the single plate at different strain rates to characterize the material removal and intra-layer damage of the single plate during CFRP drilling, and defines the mechanical behavior of the interlayer interface based on the cohesive unit to predict CFRP drilling delamination damage.

[0096] Then, the finite element simulation technology was used to establish a drilling simulation model for CFRP laminates, in which the geometric dimensions and boundary conditions of the workpiece and tool were set according to the general drilling process of CFRP laminates. The calculation efficiency of the model was improved by optimizing the mesh size and unit type of the two, and the positive and normal contact behaviors of the workpiece-tool contact interface were defined.

[0097] Finally, the drilling delamination and intra-layer damage of CFRP laminates were comprehensively predicted by calculating the simulation model;

[0098] A comprehensive prediction method for drilling delamination and intra-layer damage of CFRP laminates is proposed. The specific steps are as follows:

[0099] Step 1: Create any outermost single-layer plate of the CFRP laminate as a three-dimensional deformable entity, set its length and width to L1 and L2, and its thickness to D1;

[0100] Cutting area and non-cutting area are divided into two areas on this single-layer plate, and meshes are divided into two areas. The cutting area is divided into fine meshes to ensure high prediction accuracy of the model, and the non-cutting area is divided into coarse meshes to improve calculation efficiency. The mesh units are all generated as hexahedrons using the structural method. The unit type is an eight-node linear hexahedron reduced integration unit, and the unit is set to be deleted;

[0101] The grid offset method is used to offset an interlayer interface along the thickness direction of the single-layer plate. Its length and width are also L1 and L2, and its thickness is D2. The grid size is the same as that of the single-layer plate. Its unit type is set to an eight-node three-dimensional cohesive unit, and unit deletion is allowed. According to the number of plies of the CFRP laminate, the grid offset method is continued to be used to alternately generate the remaining single-layer plates and interlayer interfaces. The unit type settings of the two are the same as those of the above-mentioned single-layer plates and interlayer interfaces, thereby forming a geometric model of the CFRP laminate.

[0102] Step 2: Import the geometric model of the drill bit to be used and set the rotation center at the drill tip as the reference point;

[0103] Fine grids and coarse grids are divided at the drill tip and other areas of the drill bit respectively. The grid units are all generated as tetrahedrons in a free way. The unit type is a four-node linear tetrahedron unit, and unit deletion is not set.

[0104] Step 3: Assign the corresponding constitutive model and mechanical property parameters to different materials;

[0105] CFRP single-layer plate has the characteristics of anisotropy and elasticity. Therefore, the elastic constitutive relationship is used to characterize the mechanical behavior of the single-layer plate before failure, and different mechanical properties are set for the single-layer plate along the fiber direction, in-plane perpendicular to the fiber direction, and thickness direction.

[0106] Since the strain rate of a single-layer plate during CFRP cutting will affect its failure and damage formation, the effect of the strain rate on the mechanical properties of the single-layer plate needs to be considered before defining the failure criterion and damage evolution criterion of the single-layer plate.

[0107] By consulting the literature, the curves of the strength and fracture toughness of the single-layer plate with the strain rate are shown in equations (1) and (2):

[0108] (1)

[0109] (2)

[0110] In the formula, is the real-time strain rate, is the reference strain rate; S Represents strength Y T , Y C , S 12 , S 13 and S 23 ; G C Represents fracture toughness and ; and are the strength and fracture toughness at the reference strain rate, respectively;

[0111] On this basis, the failure criteria of single-layer boards containing different failure modes are defined by combining the Hashin criterion and the Puck criterion, as shown in equations (3)-(6):

[0112] Tensile failure along the fiber direction ( s 1 ≥0):

[0113] (3)

[0114] Compression failure along the fiber direction ( s 1 <0):

[0115] (4)

[0116] In-plane tensile failure perpendicular to the fiber direction ( s 2 ≥0):

[0117] (5)

[0118] In-plane compression failure perpendicular to the fiber direction ( s 2 <0):

[0119] (6)

[0120] In the formula, F is the failure variable, the subscripts f and m represent the direction along the fiber and perpendicular to the fiber, respectively, and the superscripts T and C represent the tensile and compressive effects, respectively. When the failure variable reaches 1, the material fails in the corresponding mode; s i and is the stress in the main direction and shear direction of the material, i , j =1, 2, 3, respectively, along the fiber direction, perpendicular to the fiber direction in the plane and the thickness direction, X and Y represent the strength along the fiber direction and perpendicular to the fiber direction, respectively. S ij For single-layer boards ij In-plane shear strength; is the shear strength perpendicular to the fiber direction on the fracture surface when the CFRP unidirectional laminate fails in compression perpendicular to the fiber direction, and is calculated according to formula (7):

[0121] (7)

[0122] in, i is the angle between the fracture surface and the thickness direction of CFRP; s n , t s , t t It is the stress component in the normal direction of the fracture surface, along the fiber direction in the fracture surface, and perpendicular to the fiber direction in the fracture surface. Its relationship with the effective stress component is as follows:

[0123] (8)

[0124] m t , m s is the friction coefficient, which can be obtained from the angle of the fracture surface and the material strength parameters based on the Mohr-Coulomb theory:

[0125] (9)

[0126] After CFRP fails, the material stiffness continues to decrease with the evolution of internal damage. By defining a linear damage evolution criterion, the damage evolution process of a single-layer plate after any mode of failure occurs is described, that is, the damage variable is defined as shown in equations (10) and (13): d Control the stiffness reduction of single-layer plate materials; d According to the real-time strain of the material e , the strain at the onset of injury e 0 and the strain at final fracture e f The value of the damage is 0 at the beginning of the damage and 1 at the final fracture. When a unit of the single-layer plate fails, the relevant effective stress component of this unit is multiplied by (1- d ), ensuring that the stress component of the damaged unit gradually decreases to 0 as the strain increases; the stretching along the fiber direction ( ) and compression ( ) failure, and tensile stress perpendicular to the fiber direction ( ) failure are:

[0127] (10)

[0128] In the formula, e i is the strain in the main direction of the material; the strain at the onset of failure ( , and ) As shown in formula (11), the strain at the final fracture ( , and ) is calculated using formula (12):

[0129] (11)

[0130] (12)

[0131] in, E i is along i The elastic modulus in the direction, , and They are respectively the fracture toughness of the material under tension and compression along the fiber direction and under tensile load perpendicular to the fiber direction. L c is the element characteristic length; the damage variable in the compression failure mode perpendicular to the fiber direction is calculated based on the strain on the fracture surface:

[0132] (13)

[0133] In the formula, ,symbol Meaning: For any real number x , ; The strain component on the fracture surface is:

[0134] (14)

[0135] in, c ij is the strain in the shear direction of the material; the strain at the beginning of failure It is achieved by extracting the strain when the failure variable reaches 1 during the calculation process. The final strain at fracture for:

[0136] (15)

[0137] In the formula, is the fracture toughness; the stress at the onset of failure The calculation method and The calculation method is similar:

[0138] (16)

[0139] The interlayer interface of the CFRP laminate is simulated based on the zero-thickness cohesive unit. Under the action of load, the upper and lower surfaces of this unit separate and move relative to each other. When the moving distance exceeds a specific value, the unit breaks, realizing the characterization of the crack initiation and propagation between the materials on both sides of the unit. When the cohesive unit is used, it is necessary to define its stress-strain relationship in the normal, first tangential and second tangential directions. Among them, the constitutive relationship of the interlayer interface before damage is generated is characterized by equation (17):

[0140] (17)

[0141] In the formula, t is the traction stress, For strain, K represents the elastic modulus; the subscripts n, s and t represent the normal direction, the first tangential direction and the second tangential direction respectively; for a cohesive unit with zero thickness, the strain in each direction is calculated as:

[0142] (18)

[0143] in, u is the separation displacement of the upper and lower surfaces of the cohesive unit in each direction, is the constitutive thickness of the cohesive element, Unlike the geometric thickness which has a value of 0, its value is taken as 1 to ensure that the strain and separation displacement are equal;

[0144] Under the action of load, the stress at the interface between layers gradually accumulates, and the secondary nominal stress criterion is used to determine the onset of failure:

[0145] (19)

[0146] In the formula, is the failure variable of the interlayer interface, t 0 Indicates the stress in each direction at failure;

[0147] After the interlayer interface fails, its stiffness is defined as the damage variable in equation (20): Under the control of , the stress components in each direction are calculated according to formula (21):

[0148] (20)

[0149] (twenty one)

[0150] in, represents the equivalent separation displacement; , and They are the equivalent separation displacement when the cohesive unit fails, the equivalent separation displacement when it fractures, and the maximum value of the equivalent separation displacement during drilling; Represents the equivalent traction stress in each direction, excluding the stress reduction caused by failure; the final cracking of the interlayer interface is judged according to the power exponent criterion:

[0151] (twenty two)

[0152] In the formula, , and is the material constant; G n , G s and G t They are the fracture toughness of the material in three directions; , and are the critical fracture toughness in three directions respectively;

[0153] The mechanical behavior of the above CFRP single-layer plate is calculated by importing the finite element software through defining a user subroutine, and the unit deletion during the cutting process is also controlled by the state parameters defined in the subroutine;

[0154] The specific material parameters of the single-layer plate and the interface between layers are given by the defined section properties. At the same time, the material direction is defined for the single-layer plate according to the ply direction.

[0155] Since the wear of the drill bit is not considered, only its density and modulus need to be defined;

[0156] Step 4: Create an assembly of the drill bit and the CFRP workpiece drilling process, and place the drill bit just above the geometric center of the workpiece so that the two are as close as possible but not intrusive;

[0157] Step 5: Set the dynamic explicit analysis step to perform simulation analysis, and define the required output variables in the field output request manager and the historical output request manager respectively;

[0158] Step 6: Define the constraints and contact modes between the drill bit and the workpiece and between them;

[0159] First, since tool deformation and wear are not considered in the simulation process, the drill bit is constrained as an analytical rigid body;

[0160] Secondly, the drill bit and the workpiece are defined as point-surface contact. In the contact properties, the normal contact is defined as hard contact, and the tangential contact is defined as penalty friction.

[0161] Finally, to avoid intrusion during the calculation, a general contact is defined between all contact pairs in the model;

[0162] Step 7: Define the boundary conditions of the model; first, define the feed rate and spindle speed of the drill at the drill reference point defined above; second, restrict the degrees of freedom of all directions of the peripheral nodes of the non-cutting area of ​​the workpiece to fix the workpiece;

[0163] Step 8: Submit the task for calculation to comprehensively predict drilling delamination and intra-laminar damage of CFRP laminates.

[0164] The present invention considers the change of mechanical behavior of single-layer plate with strain rate during CFRP drilling, defines the failure criterion and damage evolution criterion of single-layer plate considering the influence of strain rate, and uses cohesive unit to simulate the interlayer interface 3, and establishes a drilling simulation model of CFRP laminated plate.

[0165] The mechanical behavior of different phase materials in the model fully reflects the material properties of the single-layer plate and the interlayer interface 3. The defined mesh unit type, tool-workpiece contact mode and boundary conditions also conform to the actual drilling process. By calculating the simulation model, a comprehensive simulation of the drilling delamination and intralayer damage formation process of CFRP laminates was achieved, and the delamination and intralayer damage after drilling were obtained.

[0166] This method can be used to comprehensively analyze the delamination and intra-layer damage of CFRP laminates during drilling, and is suitable for guiding the suppression of drilling damage. Example 2

[0167] This embodiment is based on a comprehensive prediction method for drilling delamination and intra-layer damage of a CFRP laminate in the above embodiment, and uses ABAQUS finite element simulation software to perform comprehensive prediction. The specific steps are as follows:

[0168] Step 1: Attach Figure 1 The top single-layer plate of the CFRP laminate is created as a three-dimensional deformable entity, and its length and width are set to L1=L2=20 mm, and its thickness is set to D1=0.25 mm;

[0169] The cutting area and non-cutting area are divided on this single-layer board. The cutting area is a square area in the center of the single-layer board, and its size is 16 mm × 16 mm.

[0170] The cutting area and non-cutting area are meshed separately. The cutting area is meshed with a fine mesh of about 0.25 mm to ensure high prediction accuracy of the model, and the non-cutting area is meshed with a coarse mesh of about 0.5 mm to improve calculation efficiency. The mesh units are all generated as hexahedrons using the structural method. The unit type is an eight-node linear hexahedron reduced integration unit, and the unit is set to be deleted.

[0171] Using the grid offset method, a layer of interlayer interface is offset downward along the thickness direction of the single-layer plate. Its length and width are also L1=L2=20 mm, and its thickness is D2=0 mm. The grid size is the same as that of the single-layer plate. The unit type is set to an eight-node three-dimensional cohesive unit, and unit deletion is allowed.

[0172] The ply order of the laminate simulated in this embodiment is 0 / 45 / 90 / -45, including four single-layer plates, so the grid offset method is continued to be used to alternately generate the remaining three single-layer plates and two interlayer interfaces, and the unit types of the single-layer plates and the interlayer interface 3 are set to be the same as above, thereby forming a geometric model of the CFRP laminate;

[0173] Step 2: Import the geometric model of the drill bit to be used and set the rotation center at the drill tip as the reference point;

[0174] In this example, the drill bit used is a twist drill 1 with a diameter of 8 mm and a drill point angle of 90°. The tool geometry model is established by the modeling software Autodesk Inventor;

[0175] A fine grid of about 0.4 mm is divided in the drill tip area of ​​the drill bit, and a coarse grid of about 2 mm is divided in the rest of the area. The grid units are all generated as tetrahedrons in a free way, and the unit type is a four-node linear tetrahedron unit. No unit deletion is set;

[0176] Step 3: Assign the corresponding constitutive model and mechanical performance parameters to different materials; the mechanical behavior of the material in any direction of the CFRP single-layer plate 2 is as shown in the attached figure. Figure 2 As shown;

[0177] In the figure, the horizontal axis is strain, the vertical axis on the left is stress, and the vertical axis on the right is damage variable. AB The section is the elastic constitutive relation before failure. B Point is the failure start point, BC The first stage is the damage evolution stage. e 0 is the strain at the onset of injury, e f is the strain at final fracture, s 0 is the strength in the corresponding material direction; the material mechanical behavior of the single-layer plate is quantitatively calculated by equations (1)-(16), and its material mechanical performance parameters at the reference strain rate are shown in Table 1 below;

[0178]

[0179] The material mechanical behavior of the interlayer interface 3 is quantitatively calculated by equations (17)-(22), and the specific material parameters are shown in Table 2 below;

[0180] .

[0181] Step 4: Create an assembly of the drill bit and the CFRP workpiece during the drilling process. Place the drill bit directly above the geometric center of the workpiece, making them as close as possible but not intrusive. Figure 1 As shown;

[0182] Step 5: Set the dynamic explicit analysis step for simulation analysis, and the analysis time is 1 s;

[0183] Set the number of outputs to 500 in the field output request manager and the historical output request manager, and increase the output quantities such as cutting force and state variables.

[0184] Step 6: Define the constraints and contact modes between the drill bit and the workpiece and between them;

[0185] First, since tool deformation and wear are not considered in the simulation process, the drill bit is constrained as an analytical rigid body;

[0186] Secondly, the drill bit and the workpiece are defined as point-surface contact. In the contact properties, the normal contact is defined as hard contact, the tangential contact is defined as penalty friction, and the friction coefficient is 0.3.

[0187] Finally, to avoid intrusion during the calculation, a general contact is defined between all contact pairs in the model;

[0188] Step 7: Define the boundary conditions of the model; first, define the drill feed rate as 350 mm / min and the spindle speed as 3000 rpm at the drill reference point defined above;

[0189] Secondly, the degrees of freedom of the nodes at the periphery of the non-cutting area of ​​the workpiece are restricted in all directions to fix the workpiece;

[0190] Step 8: Submit the task for calculation to achieve a comprehensive prediction of drilling delamination and intra-layer damage in CFRP laminates.

[0191] The final simulation effect can be seen in Figure 3 and Figure 4 ,in Figure 3 , showing the first ply of the laminate 4, the second ply of the laminate 5, the predicted intra-ply damage 6, the third ply of the laminate 7, and the fourth ply of the laminate 8;

[0192] Figure 4 The first interlayer interface 9, predicted delamination damage 10, the second interlayer interface 11, and the third interlayer interface 12 are shown;

[0193] It can be clearly and intuitively seen from the figure that the CFRP laminate produced serious intra-layer damage and delamination damage after drilling, and the delamination area was larger than the intra-layer damage area.

[0194] This method comprehensively predicts the delamination and intra-layer damage of CFRP laminates during drilling. This method can be used to achieve a comprehensive analysis of the delamination and intra-layer damage of CFRP laminates during drilling, and can effectively guide the suppression of delamination and intra-layer damage.

[0195] The present invention is not limited to the above-mentioned embodiments. Various changes can be made within the knowledge scope of those skilled in the art without departing from the spirit of the present invention. The changed contents still fall within the protection scope of the present invention.

Claims

1. A comprehensive prediction method for drilling delamination and intra-layer damage of CFRP laminates, characterized by: The following steps are involved: 1) Establish the geometric model of CFRP laminate based on ABAQUS finite element simulation software; 2) Import the geometric model of the drill bit used; 3) Assign corresponding constitutive models and mechanical properties parameters to different materials; 4) Create an assembly of the drill bit and the CFRP workpiece drilling process; 5) Set dynamic explicit analysis steps for simulation analysis; 6) Define the constraints and contact modes between the drill bit and the workpiece and between them; 7) Define the boundary conditions of the model; 8) Submit the task for calculation and comprehensively predict the drilling delamination and intra-layer damage of CFRP laminates; When assigning the corresponding constitutive models and mechanical properties parameters to different materials: Since the CFRP single-layer plate has the characteristics of anisotropy and elasticity, the elastic constitutive relationship is used to characterize the mechanical behavior of the single-layer plate before failure, and different mechanical properties are set for the single-layer plate along the fiber direction, in-plane perpendicular to the fiber direction, and thickness direction. Since the strain rate of a single-layer plate during CFRP cutting will affect its failure and damage formation, the effect of the strain rate on the mechanical properties of the single-layer plate needs to be considered before defining the failure criterion and damage evolution criterion of the single-layer plate. The variation curves of the strength and fracture toughness of a single-layer plate with strain rate are shown in equations (1) and (2): (1) (2) In the formula, is the real-time strain rate, is the reference strain rate; S Represents strength Y T , Y C , S 12 , S 13 and S 23 ; G C Represents fracture toughness and ; and are the strength and fracture toughness at the reference strain rate, respectively; On this basis, the failure criteria of single-layer boards containing different failure modes are defined by combining the Hashin criterion and the Puck criterion, as shown in equations (3)-(6): Tensile failure along the fiber direction ( σ 1≥0): (3) Compression failure along the fiber direction ( σ 1<0): (4) In-plane tensile failure perpendicular to the fiber direction ( σ 2≥0): (5) In-plane compression failure perpendicular to the fiber direction ( σ 2<0): (6) In the formula, F is the failure variable, the subscripts f and m represent the direction along the fiber and perpendicular to the fiber, respectively, and the superscripts T and C represent the tensile and compressive effects, respectively. When the failure variable reaches 1, the material fails in the corresponding mode; σ i and is the stress in the main direction and shear direction of the material, i , j =1, 2, 3, respectively, along the fiber direction, perpendicular to the fiber direction in the plane and the thickness direction, X and Y represent the strength along the fiber direction and perpendicular to the fiber direction, respectively. S ij For single-layer boards ij In-plane shear strength; is the shear strength perpendicular to the fiber direction on the fracture surface when the CFRP unidirectional laminate fails in compression perpendicular to the fiber direction, and is calculated according to formula (7): (7) in, θ is the angle between the fracture surface and the thickness direction of CFRP; σ n , τ s , τ t It is the stress component in the normal direction of the fracture surface, along the fiber direction in the fracture surface, and perpendicular to the fiber direction in the fracture surface. Its relationship with the effective stress component is as follows: (8) μ t , μ s is the friction coefficient, which can be obtained from the angle of the fracture surface and the material strength parameters based on the Mohr-Coulomb theory: (9) After CFRP fails, the material stiffness continues to decrease with the evolution of internal damage. By defining a linear damage evolution criterion, the damage evolution process of a single-layer plate after any mode of failure occurs is described, that is, the damage variable is defined as shown in equations (10) and (13): d Control the stiffness reduction of single-layer plate materials; d According to the real-time strain of the material ε , the strain at the onset of injury ε 0 and the strain at final fracture ε f The value of the damage is 0 at the beginning of the damage and 1 at the final fracture. When a unit of the single-layer plate fails, the relevant effective stress component of this unit is multiplied by (1- d ), ensuring that the stress component of the damaged unit gradually decreases to 0 as the strain increases; the stretching along the fiber direction ( ) and compression ( ) failure, and tensile stress perpendicular to the fiber direction ( ) failure are: (10) In the formula, ε i is the strain in the main direction of the material; the strain at the onset of failure ( , and ) As shown in formula (11), the strain at the final fracture ( , and ) is calculated using formula (12): (11) (12) in, E i is along i The elastic modulus in the direction, , and They are respectively the fracture toughness of the material under tension and compression along the fiber direction and under tensile load perpendicular to the fiber direction. L c is the element characteristic length; the damage variable in the compression failure mode perpendicular to the fiber direction is calculated based on the strain on the fracture surface: (13) In the formula, ,symbol Meaning: For any real number x , ; The strain component on the fracture surface is: (14) in, γ ij is the strain in the shear direction of the material; the strain at the beginning of failure It is achieved by extracting the strain when the failure variable reaches 1 during the calculation process. The final strain at fracture for: (15) In the formula, is the fracture toughness; the stress at the onset of failure The calculation method and The calculation method is similar: (16) The interlayer interface of the CFRP laminate is simulated based on the zero-thickness cohesive unit. Under the action of load, the upper and lower surfaces of this unit separate and move relative to each other. When the moving distance exceeds a certain value, the unit breaks, realizing the characterization of the crack initiation and propagation between the materials on both sides of the unit. When using the cohesive force unit, it is necessary to define the stress-strain relationship in the normal direction, the first tangential direction, and the second tangential direction. The constitutive relationship before damage occurs at the interlayer interface is characterized by equation (17): (17) In the formula, t is the traction stress, For strain, K represents the elastic modulus; the subscripts n, s and t represent the normal direction, the first tangential direction and the second tangential direction respectively; for a cohesive unit with zero thickness, the strain in each direction is calculated as: (18) in, u is the separation displacement of the upper and lower surfaces of the cohesive unit in each direction, is the constitutive thickness of the cohesive element, Unlike the geometric thickness which has a value of 0, its value is taken as 1 to ensure that the strain and separation displacement are equal; Under the action of load, the stress at the interface between layers gradually accumulates, and the secondary nominal stress criterion is used to determine the onset of failure: (19) In the formula, is the failure variable of the interlayer interface, t 0 Indicates the stress in each direction at failure; After the interlayer interface fails, its stiffness is defined as the damage variable in equation (20): The stress components in each direction are calculated according to formula (21): (20) (21) in, represents the equivalent separation displacement; , and They are the equivalent separation displacement when the cohesive unit fails, the equivalent separation displacement when it fractures, and the maximum value of the equivalent separation displacement during drilling; Represents the equivalent traction stress in each direction, excluding the stress reduction caused by failure; the final cracking of the interlayer interface is judged according to the power exponent criterion: (22) In the formula, , and is the material constant; G n , G s and G t They are the fracture toughness of the material in three directions; , and are the critical fracture toughness in three directions respectively; The mechanical behavior of the above CFRP single-layer plate is calculated by importing the finite element software through defining a user subroutine, and the unit deletion during the cutting process is also controlled by the state parameters defined in the subroutine; The specific material parameters of the single-layer plate and the interface between layers are given by the defined section properties. At the same time, the material direction is defined for the single-layer plate according to the ply direction.

2. A comprehensive prediction method for drilling delamination and intra-layer damage of CFRP laminates according to claim 1, characterized in that: When building the geometric model of the CFRP laminate: First, any outermost single-layer plate of the CFRP laminate is created as a three-dimensional deformable entity, and its length and width are set to L1 and L2, and its thickness is set to D1; Secondly, the cutting area and non-cutting area are divided into two areas on this single-layer plate, and the meshes are divided into two areas. The cutting area is divided into a fine mesh, and the non-cutting area is divided into a coarse mesh. The mesh units are all generated as hexahedrons using the structural method. The unit type is an eight-node linear hexahedron reduced integration unit, and the unit is set to be deleted; Then, the grid offset method is used to offset a layer of interlayer interface along the thickness direction of the single-layer plate. Its length and width are also L1 and L2, and its thickness is D2. The grid size is the same as that of the single-layer plate. The unit type is set to an eight-node three-dimensional cohesive unit, and unit deletion is allowed. According to the number of plies of the CFRP laminate, the grid offset method is continued to be used to alternately generate the remaining single-layer plates and interlayer interfaces. The unit type settings of the two are the same as those of the above-mentioned single-layer plates and interlayer interfaces, thus forming the geometric model of the CFRP laminate.

3. A comprehensive prediction method for drilling delamination and intra-layer damage of CFRP laminates according to claim 1, characterized in that: When importing the geometry of the drill used: First, set the rotation center at the drill tip as the reference point; Secondly, fine grids and coarse grids are divided at the drill tip and other areas of the drill bit respectively. The grid units are all generated as tetrahedrons in a free way. The unit type is a four-node linear tetrahedron unit, and unit deletion is not set.

4. A comprehensive prediction method for drilling delamination and intra-layer damage of CFRP laminates according to claim 1, characterized in that: When defining the constraints and contact modes between the drill bit and the workpiece and between each other; First, since tool deformation and wear are not considered in the simulation process, the drill bit is constrained as an analytical rigid body; Secondly, the drill bit and the workpiece are defined as point-surface contact. In the contact properties, the normal contact is defined as hard contact, and the tangential contact is defined as penalty friction. Finally, to avoid intrusion during the calculation, define general contact between all contact pairs in the model.

5. The comprehensive prediction method for drilling delamination and intra-layer damage of CFRP laminates according to claim 1, characterized in that: When defining the boundary conditions for your model: First, define the drill feed rate and spindle speed at the drill reference point defined above; Secondly, the degrees of freedom of the peripheral nodes in the non-cutting area of ​​the workpiece are restricted in all directions to fix the workpiece.

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