J. Cent. South Univ. (2019) 26: 2717-2728

DOI: https://doi.org/10.1007/s11771-019-4208-2

Investigation on temperature field of unidirectional carbon fiber/epoxy composites during drilling process

BAO Yong-jie(鲍永杰)¹, WANG Yi-qi(王一奇)², GAO Hang(高航)²,LIU Xue-shu(刘学术)³, ZHANG Yi-ni(张旖旎)⁴

1. Engineering Training Center, Dalian University of Technology, Dalian 116024, China;

2. Key Laboratory of Ministry of Education for Precision and Non-Traditional Machining Technology, School of Mechanical Engineering, Dalian University of Technology, Dalian 116024, China;

3. School of Automotive Engineering, Dalian University of Technology, Dalian 116024, China;

4. Shenyang Liming Aero-Engine Group Corporation Ltd., Shenyang 110043, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: The phenomenon of heat accumulation and transportation in the composite materials is a very typical and critical issue during drilling process. In this study, a three-dimensional temperature field prediction model is proposed using finite difference method, based on the partly homogenization hypothesis of material, to predict temperature field in the process of drilling unidirectional carbon fiber/epoxy (C/E) composites. According to the drilling feed motion, drilling process is divided into four stages to study the temperature distributing characteristics. The results show that the temperature distribution predicted by numerical study has a good agreement with the experimental results. The temperature increases with increasing the drilling depth, and the burn phenomena is observed due to the heat accumulation, especially at the drill exit. Due to the fiber orientation, an elliptical shape of the temperature field along the direction is found for both numerical and experimental studies of C/E composites drilling process.

Key words: composite; drilling; finite difference method; temperature field; thermal analysis

Cite this article as: BAO Yong-jie, WANG Yi-qi, GAO Hang, LIU Xue-shu, ZHANG Yi-ni. Investigation on temperature field of unidirectional carbon fiber/epoxy composites during the drilling process [J]. Journal of Central South University, 2019, 26(10): 2717-2728. DOI: https://doi.org/10.1007/s11771-019-4208-2.

1 Introduction

Carbon fiber/epoxy (C/E) composites are widely used in the fields of aerospace and transportation due to their advanced properties, such as lightweight, high modulus and high strength [1-4]. Generally, composite components are assembled by adhesive bonding and/or bolt joint. Drilling is an important processing method for assembling composite components by a bolted joint. During drilling process, it may generate a lot of cutting heat that can cause obvious temperature rise and large temperature gradient in the heat affected zone (HAZ). When the temperature exceeds the glass transition temperature (Tg) of epoxy resin, strength, elastic modulus and other mechanical properties of the resin will decrease sharply, which is extremely harmful to the workpiece properties and tool life [5]. The effects of thermal damage during the curing process on material strength have been studied. CIRISCIOLI et al [6] presented a technique for determining the mechanical properties of thick composite laminates. They found that the central composite laminates which separated from the thick composite laminates showed lower shear strength, due to different curing process. ESPOSITO et al [7] pointed that overheating may decrease interlaminar shear strength of laminates. LI et al [8] reported that the multi-walled carbon nanotubes (MWCNTs) can improve the interlaminar strength and thermal conductivity, which was beneficial to thermal dissipation. The maximum drilling temperature of the MWCNTs reinforced composite was below Tg of the matrix resin and declined by 23 °C compared to traditional composites. MERINO-PEREZ et al [9] reported the influence of the material properties and cutting speed on the heat dissipation in the drilling of carbon fiber reinforced plastic (CFRP) composites using uncoated WC-Co tools. High cutting speeds yielded higher thermal gradients outside the hole edge than low cutting speeds. The heat removal is difficult when dry machining is employed. WEINERT et al [10] studied the influence of the cutting parameters on the tool temperature in the drilling of CFRP laminates by embedding thermocouples in the tool. SORRENTINO et al [11] studied the influence of cutting parameters and measured the temperature during drilling on tool and in the laminate for both carbon fiber and glass fiber reinforced plastics.

Although many studies of thermal dissipation of composites focused on the preparation technology and machining process have been performed, it has practical value to set up a thermal model of composites to predict the temperature field before machining [6-17]. A number of models have been developed based on the finite element method (FEM). KIM et al [18] developed a three-dimensional (3D) FEM model for laser assisted machining, and a prediction method and a thermal analysis method for heat source shapes were proposed. CHENG et al [19] developed a 3D numerical difference heat flow model for predicting the temperature during laser drilling of CFRP. AI et al [20] reported a high performance 3D orthogonal woven C/C composite at high temperature modeling using the FEM. SORRENTINO et al [11] reported a two- dimensional (2D) instant temperature field modeling by FEM during drilling [11]. VOISEY et al [21] established a 2D laser drilling temperature field model of composites to explore carbon fiber expansion. WANG et al [22] and BAO et al [23] carried out a temperature-dependent experiment and developed a drilling temperature model on the diamond abrasive tool to analyze damages induced by cutting heat. CHENG et al [24] used finite difference method (FDM) to propose a laser drilling model for carbon fiber composites to predict the quality of drilled hole. LEE et al [25] used ANSYS software to study the heat conducting of C/E composites during grinding process.

Because cutting edges lie in a half-closed space and the thermal conductivity of composite material is anisotropic, the variation of the cutting heat generated during drilling composite material is an irregular and complicated process. However, the distributing characteristics of temperature field during the drilling process are an important key to understand composite materials drilling mechanism. Thus, it is necessary to reveal the variation of temperature field during drilling process.

A 3D temperature field model is proposed for predicting the variation of temperature field for drilling unidirectional carbon fiber/epoxy (UD C/E) composite materials, which is based on the FDM and partly homogenization hypothesis of materials. To verify the proposed temperature field model and investigate the characteristics of temperature distribution, drilling experiments are carried out and the temperature distribution at exit of hole is examined by using the infrared thermometer. Moreover, the effects of drilling depth and spindle speed on the variation of temperature field are studied during the drilling process.

2 Numerical modelling of temperature field

2.1 Calculation of thermophysical properties

It becomes very important to determine the thermal conductivity, when a modeling of composite material is established for analyzing thermal properties. Due to the thermal conductivity of carbon fiber is much higher than that of epoxy resin, the thermal conductivity of C/E composites is dramatically affected by the direction of carbon fibers. Therefore, the thermophysical parameters of UD C/E composites are supposed to be homogenized according to the volume of fiber and resin [5]. According to the rule of mixture, the density and specific heat capacity of carbon fiber and resin are calculated as Eqs. (1) and (2). The equivalent longitude thermal conductivity of UD C/E composite along the carbon fiber direction is calculated as Eq. (3). On the other hand, the equivalent transverse thermal conductivity of UD C/E composite perpendicular to the carbon fiber direction is calculated as Eq. (4) based on the effective-medium theory [26].

                    (1)

                     (2)

                    (3)

                        (4)

where ρ_f, c_f are the density and specific heat capacity of carbon fiber; ρ_r, c_r are the density and specific heat capacity of epoxy resin; ρ, c are the density and specific heat of UD C/E composites; k_l is the longitude thermal conductivity of UD C/E composite; k_t is the transverse thermal conductivity of UD C/E composite; k_f is the thermal conductivity of carbon fiber; k_r is the thermal conductivity of epoxy resin; and V_f is the fiber volume fraction of UD C/E composite.

Thermophysical properties of UD C/E composites are shown in Table 1. The density and specific heat capacity are calculated according to the rule of mixture.

Table 1 Thermophysical properties of UD C/E composites

2.2 Analysis of heat source and heat conduction equation

A 3D numerical model was proposed based on the FDM to investigate the temperature distribution of drilling process for CFRP composites. The temperature field during drilling process is a unsteady-state and the drill should be regarded as a moving heat source. For simplification, the upper and lower surfaces of material specimen are regarded as convective boundaries while the rest four surfaces are taken as insulating. As shown in Figure 1, the heat flux load q₁ which comes from the major cutting edges and chisel edge can be considered as a moving conical heat source. Due to the side edges only play a guiding role in drilling process, the amount of heat flux load q₂ is much smaller than q₁, and it does not affect the machining ability of materials along with the heat transfer. Thus, q₂ generated from the side edges can be neglected to simplify the numerical model.

Figure 1 Temperature field model of drilling C/E composites

Thermal radiation is also ignored compared with solid heat conduction. Because q₁ increases as the linear velocity of cutting edges increases, q₁ is proportional to the radius.

The step sizes along three directions of x, y, z are set as △x, △y, △z, respectively. The time step is set as △t.

                      (5)

where N_x, N_y, N_z are the grid numbers in the three directions, respectively. N_t is the grid number of the time.

The majority of work done by the drill is converted to heat conducting to the workpiece,chip, tool and the surrounding environment in the drilling process. The average heat flux load q₀ is shown as follows [27]:

                       (6)

where η is the energy proportional coefficient; ω, v_f are the angular velocity and feed velocity of the drill; M and F_z are the torque and the thrust force; S is the conical surface area formed by the main cutting edge; η, M and F_z can be determined according to experiment settings.

The upper and lower surfaces are convective boundaries. The boundary conditions and difference forms are written as follows:

(z=0, l_z )                      (7)

                 (8)

where h is the heat transfer coefficient; k_z is thermal conductivity of the C/E composites in z-direction.

The rest four surfaces are taken to be insulating. The difference forms are written as follows:

            (9)

Initial condition of each node of the temperature field model is written as follows:

     (10)

The heat conduction equation of 3D mode can be written as follows:

    (11)

where k_x, k_y are thermal conductivities of the C/E composites in the x- and y-directions, respectively. q(x, y, z) is a heat source term, and T is temperature rise. In order to adapt the FDM to programming, the heat conduction equation and boundary conditions are written in difference forms [28, 29].

Thus, based on the above theories, a proposed 3D numerical model was used to predict the temperature field of drilling UD C/E composites by running the Microsoft Visual Basic program.

3 Experimental details

A 5.5 mm thick UD C/E composite and a cemented carbide twist drill were used to investigate the temperature field in drilling process. As shown in Figure 2(a), the diameter and vertex angle of cemented carbide twist drill were 10.0 mm and 118°, respectively. The projection length of the main cutting edge in the drill core line was 3.0 mm. As shown in Figure 2(b), the UD C/E composite consists of a 35-ply CFRP. The CFRP contains 40 vol% T-300 carbon fiber and 60 vol% tetraglycidyl 4,4’-diamino-diphenyl methane (TGDDM), which are used as the reinforcement and matrix, respectively. The Tg of TGDDM is above 265 °C. The experimental device and data collection method is illustrated in Figure 2(c). The Thermo-Vision^TM A40-M infrared thermometer (FLIR Systems Inc., USA) was used to measure temperature distribution at exit of the hole during the whole drilling process. The data of temperature distribution was used to verify the numerical model.

Considering the drill geometry and composite thickness, four particular stages of the drilling process were presented for discussing the processes of heat generation and thermal transmission. As shown in Figure 3, four stages are defined according to the feed of chisel edge at 1.0, 3.0, 5.5, and 6.5 mm respectively away from top surface of composites in thickness direction.

Composite specimens were drilled with a constant feed rate of 25 mm/min under two different spindle speeds, 3000 and 7000 r/min. At least five repeated experiments were performed for each condition. Torque and trust force were measured by Kistler 9257B. Initial room temperature was 16 °C. The drilling process was performed in natural convection environment, thus, heat transfer coefficient (h) was set as 20 W/(m²·k). Moreover, the proportional coefficient of energy (η) for improving the accuracy of proposed model is set as 17% by fitting experimental data. Drilling parameters and other relevant parameters are shown in Table 2.

Figure 2 Schematics of cemented carbide twist drill (a), UD C/E composites (b) and experimental device for drilling experiment (c)

Figure 3 Schematic of four particular stages of drilling process

Table 2 Parameters for simulating temperature field of drilling UD C/E composites

4 Results and discussion

4.1 Simulation of temperature field distribution

The predicted temperature fields of four stages with a constant spindle speed of 3000 r/min is shown in Figure 4. Generally, the temperature field presents an elliptical distribution along the fiber direction in the plane which is vertical to the drilling direction. With increasing the drilling depth, temperature is increasing continuously due to the cumulative effect of heat. On the long axis of the ellipse, it is found that the curve of temperature distribution is changed from single-peak (in Figures 4(a)-(c)) to double-peak (in Figure 4 (d)).

As shown in Figure 4(a), the temperature field presents an elliptical distribution along the fiber direction. The highest temperature is 45.9 °C at the center of the ellipse after drilling 1.0 mm away from the top surface. The material is mainly removed by chisel edge for this stage. The short contact time and small contact area are the reason why thrust force and torque are not high enough to cause a significant increase in temperature. In addition, the processing quality is not affected at this stage because the temperature is not high enough to cause a change in material properties. When the drill moved down and reached 3.0 mm away from the top surface, the main cutting edges fully participated in the cutting; the chips also began to accumulate; and the friction generated a lot of heat. Thus, the highest temperature rises up to about 85 °C and the area affected by the heat is enlarged as shown in Figure 4(b). Compared with the first stage, the temperature at the center of the hole increases significantly, and the heat radiation area is expanded.

Figure 4 Predicted temperature fields of four stages with a constant spindle speed of 3000 r/min when drill moved down to 1.0 mm (a), 3.0 mm (b), 5.5 mm (c) and 6.5 mm (d) away from top of composites

Figure 4(c) shows the chisel edge reaches the bottom of the material, and the highest temperature rose up to about 197.0 °C. Although it is still lower than the Tg of the epoxy resin, the matrix of UD C/E composite is softened by the large amount heat generated by cutting edges in the nearby area, which means that the mechanical properties of composite may be declined.

Figure 4(d) shows the drill moved down to 6.5 mm away from the top. The temperature of the center decreases for the chisel edge drills through the material, and the heat dissipation condition is greatly changed. The highest temperature reaches to about 208.9 °C in the area of cutting edges. In this stage, the temperature reaches to the maximum value as the main cutting edges are drilling out and less involved in cutting. The thrust force and torque decrease a lot when the main cutting edges drill out. Moreover, the chips are easily removed and they can take away a lot of heat. Thus, although the cutting process continued, the generated heat was not so much and only caused a small temperature rise. From the results, it is found that the temperature field distribution is different from the previous three stages since the maximum temperature region is no longer at the center of the hole, but appears an outward expansion to the hole edge. For the case of 3000 r/min, although the maximum temperature of the whole drilling process is lower than Tg, the large amount of heat decreases the mechanical properties of UD C/E composite. Meanwhile, the material at the exit is unable to get the support from lower layer of materials. Defects such as delamination and tearing are more likely to occur at this stage.

Conclusions can be achieved that the temperature field is different from that which would be found in a homogeneous material obviously. The temperature field shows an elliptical shape and its major axis is parallel to the fiber orientation. In addition, the cutting heat accumulates during the drilling process, and the highest temperature is near the exit.

The simulation was also carried out with a spindle speed of 7000 r/min at feed rate of 25 mm/min to make further discussion on the distribution of temperature field of UD C/E composites. At the moment when the chisel edge just drills through, the predicted temperature fields is shown in Figure 5. The distribution tendency shows similarity with that when the speed is 3000 r/min, but the highest predicted temperature is 267 °C with an increase of 58 °C. The increase in temperature could be explained as that the increase of material removal and friction in unit time generated more cutting heat.

It is found that the temperature increased while the drill moves down during the drilling process shown in Figure 4. Figure 6 shows that the experimental and simulation results of temperature at the central point of exit raises with time. Along with the drill moves down, the cutting heat accumulates and temperature rises up gradually. And the simulation results of heat accumulation at the central point of exit agrees with the experimental results well.

Figure 5 Predicted temperature field at exit (n=7000 r/min)

Figure 6 Experimental and simulation temperatures during drilling process at central point of exit

From the simulation results, it is found that the shape of the temperature field when drilling UD C/E composites is elliptic with the longer axis paralleling to the fiber direction. The reason might be explained as follows: the cutting heat is mainly transferred through the carbon fiber as the thermal conductivity of resin and interface between fiber and resin is much lower in comparison [30]. In other words, the anisotropic thermal conductivity performs as the key reason why the shape of temperature field is an ellipse. The low thermal conductivity of materials makes it difficult for heat conduction. Drilling is also a process when heat is accumulated, which leads to the high temperature at the exit. Mechanical properties of materials are affected, and the defects are easier to be found.

4.2 Quality of four particular moments after drilling

Figure 7 shows the morphology when the drill just reached 1.0 mm away from the top surface and the material is mainly removed by chisel edge. The temperature is still very low compared with the following stages and the quality is also acceptable.

Figure 8 shows the morphology of the hole edge, when the main cutting edges start to be fully participated in the cutting process. Compared with the first stage, more scratches start to appear, and the uplifts are found at the edge of the entrance.

Figure 7 Morphology when drill just reached 1.0 mm away from top

Figure 8 Morphology when drill just reached 3.0 mm away from top

Figure 9 shows the morphology of exit when the drill just reaches the bottom. The high- temperature induced by processing results in the melting of resin matrix, which could be a significant sign that the temperature is above the glass transition temperature of epoxy resin. As shown in Figure 9, the melting resin matrix is bonded on materials after cooling, resulting in uneven surface and color change of materials.

Figure 10 shows the morphology of exit when the drilling process is finished and the main cutting edges are totally out of processing.The high- temperature causes the changes in material properties that the burr and tearing defects appear at the exit. It is inferred that the quality of processing has decreased with temperature increasing during processing.

4.3 Verification of temperature field distribution at exit of workpiece

Figures 11(a) and (b) show the temperature field when the chisel edge just reaches the bottom. There is a single peak temperature curve on the longer axis of the ellipse indicating the highest temperature lay in the center, and the highest measured temperatures is 201.0 °C. Figures 11(c) and (d) show the temperature field distribution when the chisel edge drills through. The double- peak temperature curve on the longer axis of ellipse indicates that the major cutting edges area or nearby areas have the highest temperature, and the temperature in the center is a little bit lower. The measured temperature at this stage is 216.0 °C. The predicted highest temperatures at these two moments are 190.9 °C and 208.9 °C, respectively. The simulation errors of the two moments are 5.02% and 3.29%, respectively.

Figure 9 Morphology of exit when drill just reached bottom (5.0 mm away from top)

Figure 10 Morphology of exit when drilling process was finished (6.5 mm away from top)

The workpiece is also drilled with the spindle speed of 7000 r/min at the feed rate of 25 mm/min to make further validation of the model. The simulation and experimental results of temperature field at the moment the chisel edge just drilled through are shown in Figure 12. The highest predicted and measured temperatures are 267.0 °C and 285.0 °C, respectively, which means a simulation error of 6.32%.

Verification results show that the simulation errors are below 10%, which is identified that the accuracy of simulation model is acceptable. The measured highest temperature is higher and the high temperature zone is more concentrated compared with the predicted one. The reasons could be explained as follows: the thermal resistance between the fiber and the resin is neglected during calculation, so the actual heat conduction coefficient should be lower than the calculated parameters. The heat flux load coming from the chisel edge is underestimated, leading to the outward expansion of the predicted temperature field.

The exit surface morphologies of drilled UD C/E composite at the 3rd stage with different spindle speeds are shown in Figure 13. In Figure 13(a), although the HAZ could not be easily recognized by the color changing, the delamination area is easily found, which indicates that the accumulated heat reduces the interlaminar strength. In Figure 13(b), it is clearly observed the HAZ from the color changing caused by the higher heat accumulation. However, it is found that the delamination area in Figure 13(b) is much smaller than that in Figure 13(a). This is due to that the high spindle speed reduced the thrust force dramatically. Thus, even the temperature for the case with a spindle speed of n=7000 r/min is higher than that with a spindle speed of n=3000 r/min, HAZ is easily found with smaller delamination area.

Figure 11 Temperature field distribution at exit:

Figure 12 Temperature field distribution at exit:

Figure 13 Exit surface morphologies of drilled UD C/E composite at 3rd stage with a spindle speed of (a) n=3000 r/min and (b) n=7000 r/min

5 Conclusions

Temperature field of UD C/E composite materials during drilling process have been investigated by numerical and experimental studies. The whole drilling process is divided into four stages according to drilling feed motion for the investigation. The major results obtained from this study are summarized as follows:

1) The temperature distribution predicted by numerical study has a good agreement with the experimental results. Compared with experimental results, simulation errors are 3.29%-6.32%, which are acceptable (less than 10%).

2) From the experimental results, it is found that temperature increased with increasing the drilling depth, which is caused by the heat accumulation during the drilling process. When the highest temperature is higher than the glass transition temperature of resin matrix, the burn phenomena is observed, especially at the drill exit. It indicates that the high temperature is detrimental to the machining quality.

3) Due to the large difference of heat conductivity coefficient between reinforcements and matrix in the composite materials, the temperature field of drilling C/E composites is different from the homogeneous material obviously. It is found that the temperature field shows an elliptical shape along the fiber direction, whose major axis is parallel to the fiber orientation.

4) Under a constant feed speed, although increasing the spindle speed could cause torque and force decreasing, the temperature increased by increasing the spindle speed. When the accumulated heat achieves a certain level, it is harmful to machining quality.

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(Edited by ZHENG Yu-tong)

中文导读

单向碳纤维/环氧树脂复合材料钻削过程温度场研究

摘要：复合材料中热积累和传导是钻削过程中一个非常典型和关键的问题。本文采用有限差分法，基于材料热物参数均匀化假设，建立了单向碳纤维/环氧树脂(C/E)复合材料钻孔温度场的三维预测模型，并将钻削过程分为四个阶段来研究温度分布特征。结果表明，数值仿真的温度分布结果与实验结果吻合程度较高。温度随着钻孔深度的增加而逐渐升高，并且由于热量的积累，在钻削出口处观察到热损伤现象。由于纤维单向排布，C/E复合材料钻削过程温度场分布的数值和实验结果均呈现椭圆形。

关键词：复合材料；钻削；有限差分法；温度场；热分析

Foundation item: Projects(51475073, 51605076, 51875079) supported by the National Natural Science Foundation of China; Project (2017YFB1301701) supported by the National Key Research and Development Program of China

Received date: 2018-07-31; Accepted date: 2018-11-27

Corresponding author: WANG Yi-qi, PhD, Associate Professor; Tel: +86-411-84707929; E-mail: wangyiqi@dlut.edu.cn; ORCID: 0000-0001-8505-0530; GAO Hang, PhD, Professor; Tel: +86-411-84706138; E-mail: gaohang@dlut.edu.cn; ORCID: 0000-0001-8848-9872