A method for predicting sub-layer and inter-layer damage under CFRP drilling thermal force coupling

Abstract

This invention relates to a method for predicting intra-layer and inter-layer damage under the thermo-mechanical coupling effect of CFRP drilling, belonging to the field of carbon fiber composite material drilling. It involves a comprehensive prediction method for intra-layer and inter-layer damage in CFRP laminates during drilling. The aim is to address the problems of neglecting material property thermal degradation, low computational efficiency, and element distortion in existing technologies. This method analyzes the mechanical behavior of a single CFRP layer under temperature variations, defines failure criteria and damage evolution criteria considering temperature effects, and simulates the inter-layer interface using cohesive elements. Based on the ABAQUS finite element simulation software, a drilling thermo-mechanical coupling simulation model of the CFRP laminate is established. This model significantly improves computational efficiency by optimizing mesh size, adjusting the mass scaling factor, and selecting appropriate element types, while ensuring the accuracy of simulation results. By calculating this simulation model, a comprehensive prediction of CFRP laminate drilling delamination and intra-layer damage is achieved, obtaining the intra-layer and inter-layer damage conditions after drilling considering temperature effects. This method enables a comprehensive analysis of CFRP laminate drilling delamination and intra-layer damage, suitable for guiding the suppression of drilling damage.

Description

Technical Field

[0001] This invention belongs to the field of carbon fiber composite material drilling and processing, and relates to a comprehensive prediction method for intralayer and interlayer damage under the thermo-mechanical coupling effect of carbon fiber composite material drilling. Background Technology

[0002] Carbon fiber reinforced resin matrix composites (CFRPs) possess excellent properties such as high strength, lightweight, corrosion resistance, and fatigue resistance, making them highly promising for applications in aerospace, weaponry, and transportation. To meet these application requirements, CFRP components require drilling after molding to achieve assembly connections. However, CFRP drilling is a typical and complex thermo-mechanical coupling process. Within the enclosed machining area, the large amount of cutting heat generated by the drill bit and workpiece contact is difficult to dissipate. Simultaneously, the low thermal conductivity of CFRP workpieces easily leads to a rapid increase in temperature in the drilling area. More critically, the mechanical properties of the resin matrix in CFRP are highly sensitive to temperature. When the drilling temperature exceeds the glass transition temperature of the resin (170℃~200℃), the matrix material softens, and its key mechanical properties, such as modulus and strength, decrease sharply, easily initiating and exacerbating intralayer and interlayer damage during CFRP drilling. Within each single-layer CFRP, matrix softening directly weakens its support for the fibers, leading to a significant weakening of the fiber-matrix interface strength. This makes it difficult to effectively cut the fibers, resulting in frequent damage such as fiber pull-out, burrs, and intralayer tearing. Between the single-layer CFRP, the bonding performance of the resin-rich areas is severely degraded due to high temperatures, and the interlayer shear strength decreases significantly. Under the action of drilling axial forces, interlayer separation is easily induced, leading to delamination damage. Therefore, it is urgent to develop effective methods for predicting material temperature and intralayer and interlayer damage under the thermo-mechanical coupling effect of CFRP drilling, thereby revealing the temperature change law and damage formation mechanism, and proposing techniques to suppress hole-making damage.

[0003] Studying CFRP drilling temperature and damage using experimental methods is not only cumbersome due to the large number of experiments required, resulting in long cycles and high costs, but existing temperature measurement technologies (such as thermal imagers and thermocouples) also struggle to accurately measure the temperature of the entire machining area. Furthermore, experimental methods cannot directly observe the generation and propagation of damage during drilling, especially failing to visually demonstrate the complex thermo-mechanical coupling relationship between temperature changes, material softening, axial force fluctuations, and damage formation. Theoretical analytical methods for predicting CFRP drilling damage require extremely complex mathematical models, and are mostly applicable only to simple working conditions (such as right-angle cutting). When facing complex thermo-mechanical coupling dynamic processes like drilling, theoretical analytical methods also struggle to accurately solve for instantaneous temperature distribution, failing to effectively couple the temperature-induced material property degradation with the interrelationship between drilling force and damage. In contrast, numerical simulation can accurately characterize the complex dynamic drilling process between the drill bit and CFRP in three-dimensional space, effectively analyzing the impact of temperature changes on material failure behavior. By modifying numerical simulation parameters, it can also efficiently predict drilling processes under different machining conditions, making it the preferred method for analyzing this thermo-mechanical coupling problem. However, to accurately predict drilling damage of CFRP using numerical simulation, it is necessary to accurately define the mechanical and thermal behavior of CFRP and cutting tool in the model, as well as the heat generation and heat transfer mechanisms between them. In particular, it is necessary to consider the changes in material mechanical property parameters with temperature when defining the elastic constitutive model, failure criterion and damage evolution criterion of CFRP.

[0004] In recent years, scholars have conducted extensive research on CFRP drilling simulation. In 2018, N. Feito et al. published "Experimental and numerical analysis of stepdrill bit performance when drilling woven CFRPs" in *Composite Structures*, analyzing the dynamic process of stepdrill bit drilling CFRP using a finite element model. They investigated the influence of drill bit geometry and process parameters on axial force and delamination damage during CFRP drilling. However, this study focused primarily on the mechanical behavior during CFRP drilling, lacking discussion on temperature distribution and its impact on damage formation. In 2024, Yong Liu et al. published "Thermal-mechanical coupling in drilling high-performance CFRP: Scale-span modeling and experimental validation" in *Composite Structures*, developing a multi-scale thermo-mechanical coupling simulation model for CFRP drilling. This study predicted the temperature distribution, axial force, and damage in the drilling region. However, this model treats the mechanical properties of CFRP as constant values ​​and does not consider its softening characteristics as the temperature increases. On the other hand, the model's overall running time is long because multiple physical fields are solved simultaneously.

[0005] Among the authorized patents, the "Comprehensive Prediction Method for Delamination and Intra-Layer Damage in CFRP Laminate Drilling" published by China University of Petroleum (East China) in 2024 considers the changes in material mechanical properties with strain rate and analyzes the delamination and intra-layer damage in CFRP drilling considering strain rate. However, the aforementioned patent does not consider the influence of temperature on drilling behavior, making it difficult to characterize the coupling relationship between temperature and damage during drilling. Furthermore, the "Prediction Method for Temperature Distribution in Right-Angle Cutting of Carbon Fiber Composite Materials" published by China University of Petroleum (East China) in 2025 considers the changes in material mechanical properties with temperature and achieves the prediction of temperature distribution during right-angle cutting. However, this patent does not address the prediction of CFRP drilling damage. It is worth noting that during CFRP drilling, there is a strong bidirectional coupling effect between temperature and damage; that is, increased temperature leads to material property degradation, which in turn exacerbates the generation and propagation of damage, while the evolution of damage changes the heat conduction path and affects the temperature field distribution. This complex thermo-mechanical coupling relationship cannot be achieved by simply superimposing two independent models. Crucially, this thermo-coupling effect introduces two major challenges: reduced computational efficiency and easy distortion of elements. It is necessary to balance computational accuracy and efficiency by optimizing the mesh size in the drilling area and adjusting the model quality scaling factor. It is also necessary to suppress mesh distortion and improve model convergence by changing the element type and selecting hourglass control technology. Summary of the Invention

[0006] The purpose of this invention is to overcome the technical shortcomings of existing CFRP thermo-coupling drilling simulations, such as neglecting the thermal degradation of material properties, low model computation efficiency, and easy distortion of elements. This invention proposes a comprehensive prediction method for intra-layer and inter-layer damage under thermo-coupling effects during the drilling of carbon fiber composite materials. This method addresses the problem that the thermal degradation of the mechanical properties of single-layer CFRP materials during drilling affects their failure and damage formation. Furthermore, the thermo-coupling simulation model is complex and has a long solution time, making it difficult to efficiently and accurately predict delamination and intra-layer damage during CFRP laminate drilling. This method considers the temperature-dependent changes in the mechanical behavior of CFRP single-layer CFRP, defines single-layer CFRP failure criteria and damage evolution criteria that take temperature into account, and employs partitioned meshing, optimized element types, and mass scaling factors to significantly improve computational efficiency while maintaining accuracy. This enables efficient and accurate prediction of intra-layer and inter-layer damage during CFRP laminate drilling.

[0007] This invention employs a method for predicting intra-layer and inter-layer damage in CFRP (Chemical Fluorescent Reinforced Polymer) during drilling. The method analyzes the influence of temperature on the mechanical behavior of CFRP monolayers, defines a material mechanical property degradation equation that varies with temperature, and correlates it with material failure criteria and damage evolution criteria to characterize the performance degradation and damage process of CFRP monolayers under high drilling temperatures. Simultaneously, it defines the mechanical behavior of interlayer interfaces based on cohesive elements. Furthermore, using finite element simulation technology, a thermo-mechanical coupling simulation model for drilling CFRP multi-directional laminates is established. The geometric dimensions and boundary conditions of the workpiece and tool are set according to the drilling process of CFRP laminates. The computational efficiency of the model is improved by optimizing the mesh size and adjusting the model mass scaling factor. Appropriate element types are selected, and the interlayer viscosity coefficient is set to suppress mesh distortion. Secondly, the contact properties and force and thermal performance parameters of the tool and workpiece are defined, and the heat generation and heat transfer mechanisms and heat distribution ratios are determined. Finally, by calculating this simulation model, intra-layer and inter-layer damage in CFRP laminates during drilling is predicted. The specific steps are as follows:

[0008] Step 1: Create the outermost single-layer plate at the drill entry point as a 3D deformable solid with a planar dimension of a square with a side length L1 and a single-layer thickness of D1. Based on the different functions of different regions during the drilling process, the cutting areas involving material removal are divided into fine meshes to ensure prediction accuracy, while the non-cutting areas that only serve a clamping and fixing function are divided into sparse meshes to improve computational efficiency. Through repeated experiments and comparisons, the optimal drilling area area and mesh size configuration was finally determined while balancing accuracy and computational efficiency. All meshes were generated using structured techniques, with the element type being an eight-node linear hexahedral reduced integral thermocoupled element (C3D8RT). Hourglass control for element deletion and enhancement was implemented, and element distortion control was activated, limiting the element length ratio (the ratio of the shortest to the longest side of an element). A mesh offset technique was used to generate an interlayer interface by offsetting the thickness direction of the aforementioned single-layer plate. This interface also has a planar dimension of a square with side length L1 and a thickness D2, and its element type was defined as an eight-node three-dimensional cohesive element (COH3D8). Element deletion was allowed. To ensure that the interlayer interface retains a certain stiffness when deleted, the element was set to completely destroy when the damage variable d reaches a critical value. Finally, according to the actual layup sequence, the above mesh offset method was applied iteratively to generate subsequent single-layer plates and interlayer interfaces sequentially. The element settings for all single-layer plates and interlayer interfaces were consistent with the above settings, thus constructing a complete CFRP laminate geometric model.

[0009] Step 2: Import the geometric model of the actual drill bit and set the rotation center of its drill tip as the reference point for subsequent application of motion loads; then, divide the drill bit into sections, generating a fine mesh in the cutting tip and cutting edge regions to ensure calculation accuracy, and generating a coarse mesh in other non-critical regions to improve calculation efficiency. All mesh elements are generated as four-node linear tetrahedral meshes using a free method, without setting element deletion; finally, add coupling relationships between the drill tip reference point and all nodes of the drill bit for subsequent extraction of axial forces.

[0010] Step 3: Define the material constitutive models of the tool and the workpiece; due to the significant differences in the material properties of the tool and the workpiece, and considering the low wear of the tool in a single drilling operation, this model does not consider tool failure and damage, but only defines its density, Young's modulus, Poisson's ratio, thermal conductivity and specific heat capacity, without setting failure initiation and damage evolution.

[0011] CFRP monolayers exhibit anisotropic elastomers on a macroscopic scale. Therefore, elastic constitutive relations are used to characterize their mechanical behavior before failure. Different mechanical property parameters are defined for the monolayer along the fiber direction, perpendicular to the fiber direction, and the thickness direction. Furthermore, to accurately simulate the thermo-mechanical coupling behavior of CFRP during drilling—that is, the influence of temperature changes on damage formation and damage changes on temperature transmission—the characteristics of its mechanical properties changing with temperature must be considered. Therefore, by systematically reviewing the mechanical property parameters of CFRP at different temperatures from relevant literature, a functional relationship between the CFRP mechanical property parameters and temperature was obtained, as shown in equation (1-5).

[0012] (1)

[0013] In the formula, the underlined parameter represents the value as a function of temperature. The temperature is real-time; the elastic modulus is equal in the direction perpendicular to the fiber and along the workpiece thickness, i.e. Its variation with temperature can be calculated using equation (2):

[0014] (2)

[0015] The shear moduli of planes 1-2, 1-3, and 2-3 are determined by equation (3), where plane 2-3 is an isotropic plane, and the subscripts 2 and 3 can be interchanged, i.e. :

[0016] (3)

[0017] The tensile strength and compressive strength along the fiber direction and perpendicular to the fiber direction can be calculated using the following formula (4):

[0018] (4)

[0019] The shear strengths of planes 1-2, 1-3, and 2-3 are shown in equation (5), where... :

[0020] (5)

[0021] Under this setting, all mechanical property parameters involved in subsequent calculations are related to the current temperature.

[0022] Considering the four typical failure modes of CFRP, the Hashin-Puck failure criterion is selected as the initial criterion for intralayer damage, as shown in equation (6-9):

[0023] Tensile failure along the fiber direction ( ≥0):

[0024] (6)

[0025] Compression failure along the fiber direction ( <0):

[0026] (7)

[0027] Tensile failure perpendicular to the fiber direction ( ≥0):

[0028] (8)

[0029] Compression failure perpendicular to the fiber direction ( <0):

[0030] (9)

[0031] In the formula, superscript , These represent tensile and compressive failures, respectively, and the subscripts are used to indicate these failures. , These represent the fiber and the matrix, respectively. 1, 2, and 3 represent the direction along the fiber, perpendicular to the fiber, and the workpiece thickness direction, respectively. , These represent the effective normal stress and shear stress, respectively. , Let i and j represent the intensities in directions 1 and 2, respectively, where i, j = 1, 2, 3. The shear strength of the composite material. , , It is the stress component that forms the fracture plane when compressing failure occurs perpendicular to the fiber direction, and its relationship with the effective stress component is shown in equation (10):

[0032] (10)

[0033] In the formula, The angle of the CFRP fracture surface is 53° in this example;

[0034] The shear strength perpendicular to the fiber direction on the fracture surface is determined by equation (11):

[0035] (11)

[0036] , The friction coefficient is calculated from the material parameters and the angle of the fracture surface.

[0037] (12)

[0038] When a CFRP monolayer plate meets any failure criterion, it is determined that damage has occurred, and then enters the damage evolution stage. During this process, the material stiffness will gradually decrease according to the linear damage evolution criterion. This evolution process is controlled by the damage factor d, which starts from 0 and increases, and the material stiffness decreases accordingly. When d approaches 1, it is considered that its stiffness has completely degraded and it has completely lost its load-bearing capacity. It is automatically deleted in the calculation. The damage factor d in different material directions can be obtained according to equation (13):

[0039] (13)

[0040] , , The strain at the onset of failure. , , The strain at final fracture is given by the following formula:

[0041] (14)

[0042] in, It is the elastic modulus. , , The values ​​represent the tensile and compressive forces in direction 1, and the fracture energy under tensile load in direction 2. Indicates the characteristic length of the cell;

[0043] The damage factor in the transverse compression failure mode is calculated based on the strain on the fracture surface:

[0044] (15)

[0045] in, The results are calculated from the strain components on the fracture surface:

[0046] (16)

[0047] It is the strain at the onset of failure, calculated by extracting the failure factor until it reaches 1. The value is obtained. Indicates the strain at final fracture:

[0048] (17)

[0049] In the formula, For fracture energy, It is the stress at the onset of failure, determined by the stress components on the fracture surface, and its calculation method is the same as... similar;

[0050] The mechanical properties of the workpiece material as a function of temperature, as well as the elastic constitutive model, failure criterion, and damage evolution criterion, are all defined through user-defined subroutines. At the same time, state variables are set to control the failure deletion of the control unit.

[0051] The interlaminar interface of CFRP laminate is simulated using zero-thickness cohesive elements. Under load, the upper and lower surfaces of such elements will separate and slip relative to each other. When the relative displacement of the interface exceeds the critical value, the element fractures, thus effectively simulating the generation and propagation process of interlaminar cracks. When using cohesive elements for modeling, the stress-displacement constitutive relations along the normal, first tangential and second tangential directions need to be defined respectively. The constitutive relation of the interlaminar interface before damage initiation is shown in Equation (18):

[0052] (18)

[0053] In the formula, t is the traction stress. For strain, K represents the elastic modulus; the subscripts n, s, and t represent the normal, first tangential, and second tangential directions, respectively; for a cohesive element with zero thickness, the strain in each direction is calculated as follows:

[0054] (19)

[0055] Where u is the separation displacement of the upper and lower surfaces of the cohesive unit in each direction. The constitutive thickness of the cohesive unit. Unlike the geometric thickness, which has a value of 0, its value is set to 1 to ensure that the strain and the separation displacement are equal.

[0056] Under external loads, stress gradually accumulates at the interlayer interfaces, and damage occurs when a critical value is reached. The initiation of this damage is determined by the second nominal stress criterion.

[0057] (20)

[0058] In the formula, For the failure variable of the interlayer interface, t 0 This indicates the stress in each direction at the time of failure;

[0059] After interlayer interface failure, its stiffness is defined by the damage variable in equation (21). Under the control of the reduction, the stress components in each direction are calculated according to equation (22):

[0060] (twenty one)

[0061] (twenty two)

[0062] in, Represents the equivalent separation displacement; , and These represent 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. The equivalent traction stress in each direction does not include stress reduction due to failure; the final cracking of the interlayer interface is determined according to the power exponent criterion.

[0063] (twenty three)

[0064] In the formula, , and G is a material constant; n G s and G t These represent the fracture toughness of the material in three directions. , and These represent the critical fracture toughness in three directions;

[0065] To differentiate the material properties of different components, multiple interface properties were created in the model, and corresponding material parameters were assigned to the respective components. The material orientation of each ply was defined for the CFRP workpiece based on the actual layup sequence. Furthermore, to improve the computational convergence of interlayer interfaces during the damage evolution stage, a viscosity coefficient was introduced when defining the material properties of the cohesive elements. .

[0066] Step 4: Construct an assembly of the drill bit and the CFRP workpiece, align the drill bit axis with the geometric center of the workpiece, and adjust its initial position along the axis to make it as close as possible to the upper surface of the workpiece while maintaining non-intrusion.

[0067] Step 5: The drilling process simulation analysis was performed using the dynamic display analysis step. After multiple runs and verifications, the optimal mass scaling factor was determined. Variables, including stress, strain, reaction force, material damage state, and temperature field, were output through the field output request manager and the historical output request manager. All output data will be used for subsequent result analysis.

[0068] Step 6: Define the contact mode and thermal behavior between the drill bit and the workpiece. First, since tool deformation and wear are not considered in the simulation, the drill bit is defined as a three-dimensional deformable solid. Second, the contact mode between the drill bit and the workpiece adopts surface-to-point contact, where normal contact is defined as hard contact and tangential contact is defined as penalty friction. To avoid contact intrusion in the simulation, a universal contact is added between all contact pairs in the model. Simultaneously, heat conduction and heat distribution are defined in the contact properties to accurately simulate frictional heat generation and subsequent heat transfer processes, fully characterizing the thermo-mechanical coupling effect during drilling.

[0069] Since the drilling speed is relatively fast and the machining space is relatively enclosed, it is assumed that the drilling process is adiabatic. Only heat conduction during the drilling process is considered, and radiation and convection heat dissipation are not considered. The three-dimensional heat conduction can be calculated using equation (24):

[0070] (twenty four)

[0071] In the formula, , , These are the three orthogonal directions of CFRP. The fiber orientation of CFRP; , , The thermal conductivity corresponds to the three directions; In order to be in Total heat production rate at any given time; For material density, Specific heat capacity;

[0072] The initial conditions can be expressed by equation (25), where the initial temperature at any point in the finite element model is... , can be represented as:

[0073] (25)

[0074] The entire model is constrained by adiabatic boundary conditions to ensure that no element on any surface of the model exchanges heat with the external environment during the entire drilling process. The constraint equations are shown in equation (26):

[0075] (26)

[0076] In the formula, Let be the heat flux density at time t on the element surface; The direction of the element's normal.

[0077] In CFRP drilling, the heat generated by the plastic strain of the material is negligible, and frictional heat is defined as the main heat source. Considering that the friction coefficient is affected by various factors such as the cutting direction of the tool and fiber, exhibiting a complex coupling relationship, a constant friction coefficient is adopted to simplify the model. As simulation parameters.

[0078] According to the law of conservation of energy, the heat generated during the cutting process will be transferred to the workpiece, the tool, and the chips, as shown in equation (27). Ignoring the heat carried away by the chips, the proportionality coefficient of heat transferred to the workpiece during drilling is defined. ;

[0079] (27)

[0080] Step 7: Define the boundary conditions of the model; First, define the feed rate and spindle speed of the drill bit at the reference point; second, restrict all degrees of freedom of the outer nodes of the non-cutting zone of the workpiece to simulate the actual clamping state. Simultaneously, assign initial ambient temperatures to both the tool and the workpiece.

[0081] Step 8: Submit the established finite element model for calculation and solution, thereby realizing the prediction of intra-layer and inter-layer damage during CFRP drilling.

[0082] The beneficial effects of this invention are that it considers the temperature-dependent changes in the mechanical behavior of a single-layer plate during CFRP drilling, defines single-layer plate failure criteria and damage evolution criteria that take temperature into account, and establishes a thermo-mechanical coupling simulation model for CFRP laminate drilling by using cohesive elements to simulate the interlayer interface. The established model more intuitively presents the process by which changes in the temperature field during drilling affect material properties, ultimately leading to accelerated damage formation. Furthermore, by systematically optimizing mesh properties and model parameters, the solution efficiency is significantly improved while maintaining computational accuracy. The defined tool-workpiece contact mode and boundary conditions also conform to the actual drilling process. By calculating this model, the temperature distribution during drilling and the intra-layer and inter-layer damage caused by the degradation of material mechanical properties with temperature can be obtained simultaneously. This method enables comprehensive analysis of intra-layer and inter-layer damage in CFRP drilling, and is suitable for guiding drilling temperature control and hole-making damage suppression. Attached Figure Description

[0083] Figure 1 This is a schematic diagram of a drilling model for a CFRP laminate. 1-Twist drill, 2-CFRP single-layer board, 3-Interlayer interface, S-Tool rotation, F-Tool feed.

[0084] Figure 2 This diagram illustrates the material mechanical behavior of a CFRP monolayer plate in any direction. The horizontal axis represents strain, the left vertical axis represents stress, and the right vertical axis represents damage variables. Segment AB represents the elastic constitutive relation before failure, point B is the failure initiation point, and segment BC represents the damage evolution stage. ε 0 It is the strain at the onset of damage, ε f σ is the strain at final fracture. 0 It refers to the strength in the direction corresponding to the material.

[0085] Figure 3 The temperature distribution during drilling of the CFRP laminate is shown, with the highest cutting temperature at 451.7K and the average temperature in the drilling area at 388.5K.

[0086] Figure 4 The results show the predicted damage within the drilled layer of the CFRP laminate.

[0087] Figure 5 The results show the predicted delamination damage from drilling in CFRP laminates. Detailed Implementation

[0088] The specific embodiments of the present invention will now be described in detail with reference to the technical solutions and accompanying drawings.

[0089] Based on the ABAQUS / Explicit finite element simulation platform, the specific steps of the method for predicting intra-layer and inter-layer damage in CFRP laminates are as follows:

[0090] Step 1: Starting with the outermost single-layer plate at the drill entry point, create a 3D deformable solid with a square of side length L1 = 20 mm and a single-layer thickness of D1 = 0.15 mm. After multiple trials and comparisons, determine the drilling area and mesh size configuration. Based on the functional differences of each area during drilling, divide the square area with side length L2 = 5 mm at the center of the single-layer plate into the cutting zone, and the rest into the non-cutting zone. The cutting zone uses a fine mesh (0.3 mm) to ensure simulation accuracy, while the non-cutting zone uses a coarse mesh (size diverging from the center to the edge, from 0.3 mm to 1 mm) to improve computational efficiency. The thickness direction is uniformly divided into a 0.15 mm mesh. All meshes are generated using structured techniques to create hexahedral elements, type eight-node linear hexahedral reduced integral thermally coupled elements (C3D8RT), with hourglass control for element deletion and enhancement, activation of element distortion control, and a length ratio limit of 0.01. A mesh offset technique was used to generate interlayer interfaces by offsetting downwards along the thickness direction of the single-layer plate. The interlayer interfaces were also squares with side length L1 = 20 mm and geometric thickness D2 = 0 mm. The mesh size was consistent with that of the single-layer plate, and the element type was set to an eight-node three-dimensional cohesive element (COH3D8). Element deletion was allowed, and the element was considered completely destroyed when the damage variable d reached 0.98. In this example, the laminate layup sequence was 0 / 45 / 90 / -45, a total of ten layers. The above mesh offset method was used alternately to generate the remaining single-layer plates and interlayer interfaces sequentially. All mesh sizes and mesh attribute settings were consistent with those described above, thus constructing a complete CFRP laminate geometric model.

[0091] Step 2: Import the twist drill geometry model (8 mm diameter) built with SolidWorks, and set the drill tip rotation center as the reference point. Perform partitioned meshing on the drill bit: generate a fine mesh (0.6 mm size) in the drill tip and cutting edge regions, and a coarse mesh (1 mm size) in the remaining regions. Generate four-node linear tetrahedral thermo-coupled elements (C3D10MT) using a free meshing method, without setting element deletion. Establish coupling constraints between the drill tip reference point and all nodes of the drill bit for subsequent drilling force extraction.

[0092] Step 3: Define the constitutive models of the tool and the workpiece; ignore the deformation and wear of the tool, and its material parameters are shown in Table 1; the CFRP single-layer plate adopts an elastic constitutive model to describe its pre-failure behavior, and the initial material parameters are shown in Table 2. The relationship between the mechanical properties of CFRP and temperature is described by formula (1-5), which is implemented by the user-defined subroutine (VUMAT). At the same time, state variables are set to be deleted by the control unit. Under this setting, all mechanical properties involved in subsequent calculations are related to the current temperature; the Hashin-Puck criterion is used to determine the initiation of damage within the layer, and the specific formula is described by formula (6-9). The damage evolution adopts a linear stiffness reduction model, which is controlled by the damage factor d. The mechanical behavior of CFRP is shown by formula (6-17); the interlayer interface is simulated by a zero-thickness cohesive element. Its damage initiation is determined by the second nominal stress criterion, as shown by formula (20). The damage evolution is controlled by formula (21-22). The material parameters of the interlayer interface are shown in Table 3.

[0093] Table 1

[0094]

[0095] Table 2

[0096]

[0097] Table 3

[0098]

[0099] Step 4: Model assembly and initial positioning; Build an assembly of the drill bit and CFRP workpiece, align the drill bit axis with the geometric center of the workpiece, and adjust the initial position along the axis so that the tool is as close as possible to the upper surface of the workpiece while maintaining non-intrusion.

[0100] Step 5: Set the analysis step and output variables; use dynamic explicit analysis steps to simulate the drilling process, with an analysis duration of 1 second and a mass scaling factor of 100,000. In the field output and history output manager, set the output frequency to 500 steps, and the output variables include stress, strain, material damage state, temperature field, reaction force, and torque for subsequent result analysis.

[0101] Step 6: Define the contact properties between the drill bit and the workpiece; since tool deformation and wear are not considered in the simulation calculation, the drill bit is defined as a three-dimensional deformable solid; the drill bit and the workpiece adopt a surface-to-point contact method, the normal contact is set as hard contact, and the tangential contact adopts penalty friction, with a friction coefficient of... The value is set to 0.35; the thermal conductivity of the tool-workpiece contact is set to 12 mJ / (mm·s·K), which decreases as the contact distance increases. When the distance exceeds 0.3 mm, heat transfer is considered to terminate. The heat distribution coefficient is set to 0.6; finally, to avoid intrusion during the calculation process, a universal contact is defined between all contact pairs in the model.

[0102] Step 7: Apply boundary conditions to the model; apply an axial feed rate of 300 mm / min and a spindle speed of 3000 rpm at the drill reference point; constrain all degrees of freedom of the outer nodes of the non-cutting zone of the workpiece to simulate the actual clamping state; set the initial temperature of the tool and the workpiece to 300 K.

[0103] Step 8: Submit the established finite element model for calculation and solution; by running this thermo-mechanical coupling simulation, the intralayer damage and interlayer delamination behavior caused by thermo-mechanical coupling during CFRP drilling can be accurately predicted; the computational efficiency of the model is significantly improved, and the calculation can be completed within 10 days, with effective control of element distortion; the simulation results intuitively show the temperature distribution in the drilling area (see appendix). Figure 3 ), and single-layer plate damage under thermo-coupling (see appendix) Figure 4 ) and interlayer interface fracture conditions (see appendix) Figure 5 ).

[0104] Through the above steps, this invention constructs a thermo-mechanical coupling analysis model of the CFRP drilling process, providing an effective numerical simulation tool for in-depth exploration of drilling damage mechanisms and optimization of tool structure and process parameters. The simulation results are as follows: Figures 3 to 5 As shown, the predicted drilling temperature distribution, intra-layer damage, and inter-layer damage are presented respectively.

Claims

1. A method for predicting intra-layer and inter-layer damage under the thermo-mechanical coupling effect of CFRP drilling, characterized in that, This method analyzes the influence of temperature on the mechanical behavior of CFRP monolayers, defines the material mechanical property degradation equation as a function of temperature, and correlates it with material failure criteria and damage evolution criteria to characterize the performance degradation and damage process of CFRP monolayers under high drilling temperatures. Simultaneously, the mechanical behavior of interlayer interfaces is defined based on cohesive elements. Furthermore, a thermo-mechanical coupling simulation model of CFRP multidirectional laminate drilling is established using finite element simulation technology. The geometric dimensions and boundary conditions of the workpiece and tool are set according to the drilling process of CFRP laminates. The computational efficiency of the model is improved by optimizing the mesh size and adjusting the model mass scaling factor. Appropriate element types are selected, and the interlayer viscosity coefficient is set to suppress mesh distortion. Next, the contact properties and force and thermal performance parameters of the tool and workpiece are defined, and the heat generation and heat transfer mechanisms and heat distribution ratios are determined. Finally, by calculating this simulation model, the intralayer and interlayer damage of CFRP laminates during drilling is predicted. The specific steps are as follows: Step 1: Create the outermost single-layer plate at the drill entry point as a 3D deformable solid with a square of side length L1 and a single-layer thickness of D1. Based on the different functions of different regions during the drilling process, the cutting areas involving material removal are divided into fine meshes to ensure prediction accuracy, while the non-cutting areas that only serve a clamping and fixing function are divided into sparse meshes to improve computational efficiency. Through repeated experiments and comparisons, the optimal drilling area area and mesh size configuration were finally determined while balancing accuracy and computational efficiency. All meshes were generated using structured technology, with the element type being an eight-node linear hexahedral reduced integral thermocoupled element (C3D8RT). Hourglass control functions for element deletion and enhancement were set up, and the element distortion control function was activated. The element length ratio (the ratio of the shortest side to the longest side of the element) is limited. A mesh offset technique is used to generate an interlayer interface by offsetting from the thickness direction of the aforementioned single-layer plate. This interface is also a square with side length L1 and thickness D2, and its element type is defined as an eight-node three-dimensional cohesive element (COH3D8). Element deletion is allowed. To ensure that a certain stiffness is retained when the interlayer interface is deleted, it is set that the element completely fails when the damage variable d reaches a critical value. Finally, according to the actual layup sequence, the above mesh offset method is applied iteratively to generate subsequent single-layer plates and interlayer interfaces sequentially. The element settings for all single-layer plates and interlayer interfaces are consistent with the above settings, thereby constructing a complete CFRP laminate geometric model. Step 2: Import the geometric model of the actual drill bit and set the rotation center of its drill tip as the reference point for subsequent application of motion loads; Subsequently, the drill bit mesh is divided into zones. Fine meshes are generated in the cutting tip and cutting edge regions to ensure calculation accuracy, while coarse meshes are generated in other non-critical regions to improve calculation efficiency. All mesh elements are generated as four-node linear tetrahedral meshes in a free manner, without setting element deletion. Finally, the drill tip reference point is coupled to all nodes of the drill bit for subsequent axial force extraction. Step 3: Define the material constitutive model of the tool and the workpiece; Since there are significant differences in the material properties of the tool and the workpiece, and considering that the tool wear is low in a single drilling operation, this model does not consider tool failure and damage, but only defines its density, Young's modulus, Poisson's ratio, thermal conductivity and specific heat capacity, and does not set failure initiation and damage evolution. CFRP monolayers exhibit anisotropic elastomers on a macroscopic scale. Therefore, elastic constitutive relations are used to characterize their mechanical behavior before failure. Different mechanical property parameters are defined for the monolayer along the fiber direction, perpendicular to the fiber direction, and thickness direction. Furthermore, to accurately simulate the thermo-mechanical coupling behavior of CFRP during drilling—that is, the influence of temperature changes on damage formation and damage changes on temperature transmission—the characteristics of its mechanical properties changing with temperature must be considered. Therefore, by systematically reviewing the mechanical property parameters of CFRP at different temperatures from relevant literature, a functional relationship between the CFRP mechanical property parameters and temperature was obtained, as shown in Equation (1-5). (1) In the formula, the underlined parameter represents the value as a function of temperature. The temperature is real-time; the elastic modulus is equal in the direction perpendicular to the fiber and along the workpiece thickness, i.e. Its variation with temperature can be calculated using equation (2): (2) The shear moduli of planes 1-2, 1-3, and 2-3 are determined by equation (3), where plane 2-3 is an isotropic plane, and the subscripts 2 and 3 can be interchanged, i.e. : (3) The tensile strength and compressive strength along the fiber direction and perpendicular to the fiber direction can be calculated using the following formula (4): (4) The shear strengths of planes 1-2, 1-3, and 2-3 are shown in equation (5), where... : (5) Under this setting, all mechanical property parameters involved in subsequent calculations are related to the current temperature; Considering the four typical failure modes of CFRP, the Hashin-Puck failure criterion is selected as the initial criterion for intralayer damage, as shown in equation (6-9): Tensile failure along the fiber direction ( ≥0): (6) Compression failure along the fiber direction ( <0): (7) Tensile failure perpendicular to the fiber direction ( ≥0): (8) Compression failure perpendicular to the fiber direction ( <0): (9) In the formula, superscript , These represent tensile and compressive failures, respectively, and the subscripts are used to indicate these failures. , These represent the fiber and the matrix, respectively. 1, 2, and 3 represent the direction along the fiber, perpendicular to the fiber, and the workpiece thickness direction, respectively. , These represent the effective normal stress and shear stress, respectively. , Let i and j represent the intensities in directions 1 and 2, respectively, where i, j = 1, 2, 3. The shear strength of the composite material. , , It is the stress component that forms the fracture plane when compressing failure occurs perpendicular to the fiber direction, and its relationship with the effective stress component is shown in equation (10): (10) In the formula, The angle of the CFRP fracture surface is 53° in this example; The shear strength perpendicular to the fiber direction on the fracture surface is determined by equation (11): (11) , The friction coefficient is calculated from the material parameters and the angle of the fracture surface. (12) When a CFRP monolayer plate meets any failure criterion, it is determined that damage has occurred, and then it enters the damage evolution stage. During this process, the material stiffness will be progressively reduced according to the linear damage evolution criterion. This evolution process is controlled by the damage factor d. d starts from 0 and increases, and the material stiffness decreases accordingly. When d approaches 1, it is considered that its stiffness has completely degraded and it has completely lost its load-bearing capacity. It is automatically deleted in the calculation. The damage factor d in different material directions can be obtained according to equation (13): (13) , , The strain at the onset of failure. , , The strain at final fracture is given by the following formula: (14) in, It is the elastic modulus. , , The values ​​represent the tensile and compressive forces in direction 1, and the fracture energy under tensile load in direction 2. Indicates the characteristic length of the cell; The damage factor in the transverse compression failure mode is calculated based on the strain on the fracture surface: (15) in, The results are calculated from the strain components on the fracture surface: (16) It is the strain at the onset of failure, calculated by extracting the failure factor until it reaches 1. The value is obtained. Indicates the strain at final fracture: (17) In the formula, For fracture energy, It is the stress at the onset of failure, determined by the stress components on the fracture surface, and its calculation method is the same as... similar; The mechanical properties of the workpiece material as a function of temperature, as well as the elastic constitutive model, failure criterion, and damage evolution criterion, are all defined through user-defined subroutines. At the same time, state variables are set to control the failure deletion of the control unit. The interlaminar interface of CFRP laminate is simulated using zero-thickness cohesive elements. Under load, the upper and lower surfaces of such elements will separate and slip relative to each other. When the relative displacement of the interface exceeds the critical value, the element will fracture, thus effectively simulating the generation and propagation process of interlaminar cracks. When using cohesive elements for modeling, the stress-displacement constitutive relations along the normal, first tangential and second tangential directions need to be defined respectively. The constitutive relation of the interlaminar interface before damage initiation is shown in Equation (18): (18) In the formula, t is the traction stress. For strain, K represents the elastic modulus; the subscripts n, s, and t represent the normal, first tangential, and second tangential directions, respectively; for a cohesive element with zero thickness, the strain in each direction is calculated as follows: (19) Where u is the separation displacement of the upper and lower surfaces of the cohesive unit in each direction. The constitutive thickness of the cohesive unit. Unlike the geometric thickness, which has a value of 0, its value is set to 1 to ensure that the strain and the separation displacement are equal. Under external loads, stress gradually accumulates at the interlayer interfaces, and damage occurs when a critical value is reached. The initiation of this damage is determined by the second nominal stress criterion. (20) In the formula, For the failure variable of the interlayer interface, t 0 This indicates the stress in each direction at the time of failure; After interlayer interface failure, its stiffness is defined by the damage variable in equation (21). Under the control of the reduction, the stress components in each direction are calculated according to equation (22): (21) (22) in, Represents the equivalent separation displacement; , and These represent 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. The equivalent traction stress in each direction does not include stress reduction due to failure; the final cracking of the interlayer interface is determined according to the power exponent criterion. (23) In the formula, , and G is a material constant; n G s and G t These represent the fracture toughness of the material in three directions. , and These represent the critical fracture toughness in three directions; To differentiate the material properties of different components, multiple interface properties were created in the model, and corresponding material parameters were assigned to the corresponding components. The material orientation of each ply was defined for the CFRP workpiece based on the actual layup sequence. Furthermore, to improve the computational convergence of interlayer interfaces during the damage evolution stage, a viscosity coefficient was introduced when defining the material properties of the cohesive elements. ; Step 4: Construct an assembly of the drill bit and the CFRP workpiece, align the drill bit axis with the geometric center of the workpiece, and adjust its initial position along the axis to make it as close as possible to the upper surface of the workpiece while maintaining non-intrusion. Step 5: The drilling process simulation analysis was performed using the dynamic display analysis step. After multiple runs and verifications, the optimal mass scaling factor was determined. Variables, including stress, strain, reaction force, material damage state, and temperature field, were output through the field output request manager and the historical output request manager. All output data will be used for subsequent result analysis. Step 6: Define the contact mode and thermal behavior between the drill bit and the workpiece. First, since tool deformation and wear are not considered in the simulation calculation, the drill bit is defined as a three-dimensional deformable solid. Second, the contact mode between the drill bit and the workpiece adopts surface-point contact, where normal contact is defined as hard contact and tangential contact is defined as penalty friction. To avoid contact intrusion in the simulation calculation, a universal contact is added between all contact pairs in the model. At the same time, heat conduction and heat distribution are defined in the contact properties to accurately simulate frictional heat generation and subsequent heat transfer processes, and fully characterize the thermo-mechanical coupling effect in the drilling process. Since the drilling speed is relatively fast and the machining space is relatively enclosed, it is assumed that the drilling process is adiabatic. Only heat conduction during the drilling process is considered, and radiation and convection heat dissipation are not considered. The three-dimensional heat conduction can be calculated using equation (24): (24) In the formula, , , These are the three orthogonal directions of CFRP. The fiber orientation of CFRP; , , The thermal conductivity corresponds to the three directions; In order to be in Total heat production rate at any given time; For material density, Specific heat capacity; The initial conditions can be expressed by equation (25), where the initial temperature at any point in the finite element model is... , can be represented as: (25) The entire model is constrained by adiabatic boundary conditions to ensure that each element on each surface of the model does not exchange heat with the external environment during the entire drilling process. The constraint equation is shown in equation (26): (26) In the formula, Let be the heat flux density at time t on the element surface; The direction of the element's normal. In CFRP drilling, the heat generated by the plastic strain of the material is negligible, and frictional heat is defined as the main heat source. Considering that the friction coefficient is affected by various factors such as the cutting direction of the tool and fiber, exhibiting a complex coupling relationship, a constant friction coefficient is adopted to simplify the model. As simulation parameters; According to the law of conservation of energy, the heat generated during the cutting process will be transferred to the workpiece, the tool, and the chips, as shown in equation (27). Ignoring the heat carried away by the chips, the proportionality coefficient of heat transferred to the workpiece during drilling is defined. ; (27) Step 7: Define the boundary conditions of the model; First, define the feed rate and spindle speed of the drill bit at the reference point; Second, restrict all degrees of freedom of the outer nodes of the non-cutting zone of the workpiece to simulate the actual clamping state; At the same time, assign initial ambient temperature to the tool and the workpiece respectively. Step 8: Submit the established finite element model for calculation and solution, thereby realizing the prediction of intra-layer and inter-layer damage during CFRP drilling.

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