Analysis of thrust force and characteristics of uncut fibres at non-conventional oriented drilling of unidirectional carbon fibre-reinforced plastic (UD-CFRP) composite laminates
Norbert GEIER1*, Tibor SZALAY2, Márton TAKÁCS3 1,2,3
Budapest University of Technology and Economics, Department of Manufacturing Science and Engineering, H-1111 Budapest, Műegyetem rkp. 3., Hungary *
Corresponding author’s e-mail:
[email protected], mobile phone number: +3614362641
Abstract Carbon fibre-reinforced plastic (CFRP) is an often-used structural material in the high-tech industries, like aerospace, wind turbine, sport, automobile, robotics and military. Due to both the growing application area of composites, and the advanced construction requirements, the used thickness of the CFRP plates increases, and the necessity of drilling holes on the sides of the plates (normal II direction) become even more important. Many researchers studied the machinability of UDCFRP using numerous drilling experiments at the normal I direction. However, drilling experiments at normal II and axial directions were not published yet. The main objective of the present study is to analyse and discuss the influence of a non-conventional drilling direction on hole-quality parameters and on the thrust force. Drilling experiments were carried out in unidirectional CFRP at nonconventional drilling direction, based on central composite inscribed design. Influences of feed rate and cutting speed were analysed using response surface methodology (RSM) and analysis of variance (ANOVA) techniques. Characteristics of uncut fibres were analysed using digital image processing (DIP). The results have proved that the effect of the cutting speed is more significant when drilling UD-CFRP at the non-conventional drilling direction than at the conventional one. Furthermore, the specific feed force (kf) in the case of the non-conventional drilling direction was more than three times higher than the kf in the case of the conventional one. Keywords CFRP, Machinability, Optimisation, Thrust force, Uncut fibres Nomenclature Ac (%) – Area factor d (mm) – Tool diameter Ft (N) – Thrust force kf (N/mm2) – Specific feed force n (rev/min) – Rotational speed s* (same as the mean) – Standard deviation vc (m/min) – Cutting speed vf (mm/rev) – Feed rate x (mm) – Lengths of the longest uncut fibre α (1) – Significance level θ (°) – Fibre cutting angle ϕ (°) – Fibre orientation angle
1. Introduction Carbon fibre-reinforced plastic (CFRP) is an often-used construction composite material in the aerospace, marine, humanoid-robotics, sport, wind turbine and automobile industries due to its specific mechanical properties, damage tolerance and corrosion resistance [1–4]. In order to meet the micro and macro geometrical requirements of the CFRP parts, mechanical machining of these materials are often necessary. However, the machining behaviour of CFRP is difficult because of the nonhomogeneity, anisotropy of the material, and the intensive abrasive wear-effect of the carbon fibrereinforcements on the tool. Mechanical joining CFRP components is often preferred by rivets and bolts [5], the most common machined features are therefore through and stage holes [2, 6]. By applying improper machining technologies, several geometrical errors could occur: like delamination, fibre pull-outs, uncut fibres and matrix burnings [7]. These errors often cause loss of strength of CFRP components, and often result in additional machining operations and costs. Researchers and tool-manufacturers offers a wide range of solutions to decrease geometrical errors by applying (i) back-up support plate [8], (ii) special tool geometry [5, 9] (iii) special tool patch [10], etc. The structure of CFRP materials is highly non-homogeneous and the mechanical properties are anisotropic because of the two components of the composite: the matrix and the fibre-reinforcements. Because the mechanical properties of the CFRP are not homogeneous, the machinability properties are therefore highly effected by the cutting directions. Many researchers [11–17] use the fibre cutting angle (θ - angle between the vector of the cutting speed and the fibre orientation) as an independent variable to describe the influence of the cutting direction on the machined surface-quality. However, this one-dimensional factor is insufficient for distinguishing the difference between the possible drilling directions, as demonstrated in Fig.1.
Fig. 1 Schematic drawings of possibly drilling directions in UD-CFRP: (a) normal I direction, (b) normal II direction and (c) axial direction.
As can be seen in Fig.1, three different drilling types can be determined based on the orientations of the axis of the tool and of the fibre-reinforcement: (i) normal I (conventional) direction, where the axis of the tool is perpendicular to the axis of the fibres, and both of the axis are in the Y-Z plane, (ii) normal II direction, where the axis of the tool is also perpendicular to the axis of the fibres, but the axis are in the X-Y plane, and (iii) axial direction, where the axis of the tool is parallel to the axis of the fibres. Many researchers studied the machinability of UD-CFRP (unidirectional CFRP) using numerous drilling experiments at the normal I direction [16, 18–24]. However, drilling experiments at normal II and axial directions were not published yet, because CFRP plates are normally thin, the axis of the machined holes are therefore perpendicular to the largest surface of the CFRP plate. However, the development of high-tech technologies and construction requirements initiated the appearance of thick CFRP plates. E.g. (i) the maximum thickness of CFRP parts in aircraft structures is 25-40 millimetres [25, 26], (ii) industrial and robotics industries use thick construction CFRP plates, like SUZENA the humanoid robot (as can be seen in Fig. 2.). The importance of the present research is the industrial need for the machining of these thick CFRP plates in different directions (e.g. normal II direction of Fig.1). This paper also gives a detailed summary about the possible measuring methods regarding the evaluation of the machined structures. CFRP
CFRP
Fig. 2 SUZENA the humanoid robot has load bearing thick CFRP plates. AUTOMATICA 2016 Exhibition in Munich, Germany. Many researchers investigated the influence of machining conditions, such as cutting parameters, tool geometry, material properties, cooling systems, preliminary-manufacturing methods and other environmental features on the thrust force [6, 7, 12, 13, 17], [23, 27], on the tool wear [16, 28] and on the hole quality characterized by delamination [21, 24], fibre pull-outs, uncut fibres [12, 13], [18, 29], surface roughness [7, 13, 29] when drilling CFRP at normal I direction. This research focuses on investigating the effect of drilling direction of normal II and process parameters (cutting speed and feed rate) on thrust force and on the characteristics of uncut fibres. Merino-Pérez et al. [23] investigated the influence of cutting speed on thrust force in the case of conventional dry drilling of CFRP using uncoated tools. They and other researchers [6, 7, 13] showed that the effect of cutting speed is not significant during drilling CFRP at normal I direction. It was found that the feed rate has the most significant influence on the thrust force, followed by the number of holes and the structure of the CFRP composites [2, 7, 11]. Wang et al. [30] conducted high-speed drilling experiments in CFRP, and decreased the thrust force with the application of pre-drilled pilot holes. They suggested a technology, which could solve the problem of uncut fibres, but no results are
published yet. Li et al. [17] carried out orthogonal cutting experiments in UD-CFRP and investigated the effect of the fibre cutting angle on the chip formation mechanisms. They classified the chip formation mechanisms into four types. Furthermore, they showed that the fibres can be cut properly, when the fibre cutting angle is in the range of 90 and 180 degrees. Wang et. al [22] showed that the occurrence of uncut fibres depends on the fibre fracture mode. Therefore, it is necessary to understand the interaction of fibres with the cutting tool. They, as well as [17] pointed out that the fibre fracture mode is significantly influenced by the fibre cutting angle (θ), by the rake angle (γ) and by the cutting edge radius (rβ). In the case of θ90°+γ, the fibres slip on the rake face of the cutting tool. Furthermore, larger rε causes bendingdominated and smaller rβ causes crushing-dominated fibre fracture mode. Thus, smaller cutting edge radius is preferable when machining fibre-reinforced polymer composites. In the case of small cutting edge radius (compared with the depth of cut) five different chip formation processes can be defined, according to Sheikh-Ahmad [31]. These five types depend on the fibre cutting angle and the rake angle, as can be seen in Fig. 3.
Fig. 3 Cutting mechanisms at orthogonal machining of unidirectional fibre-reinforced composite materials (k shows the direction of the fibres) [32] In the case of positive rake angle and fibre cutting angle of 0° (Type I), the cutting mechanism is delamination- and bending-dominated, as can be seen in Fig. 3(a). The main cutting force is influenced significantly by the strength of the connection between the fibres and the matrix. However, in the case of negative rake angle (Type II), the rake face of the cutting tool compresses the fibres and causes them to buckle. Chips are usually small and discontinuous; the cutting force is usually lower than in the case of chip formation process of Type I. In the case of fibre cutting angle of 0-90° (Type III and IV), compression-induced shear and interlaminar shear fracture are dominated. Furthermore, the cutting resistance of the fibres significantly influences the main cutting force. Cutting mechanism of Type III is observable at any rake angle, as can be seen in Fig. 3(c) and (d). Furthermore, cutting edge crushes the fibres, but do not delaminate or bend them. The chip thickness is often greater than the depth of cut when the fibre cutting angle is around 135° (Type V). In this case, the cutting tool
bends, then crushes the fibres, as can be seen in Fig. 3(f). In the case of drilling UD-CFRP, all of the introduced chip formation types are observable according to Wang et al. [22]. Delamination is one of the most critical machining induced error in CFRP according to [33], however, this macro geometrical error is not examined in this study but the characteristics of uncut fibres. Li et al. [34] showed that fibre burrs are highly influenced by fibre orientation and fibre cutting angle. Xu et al. [12] drilled holes in UD-CFRP and investigated the exit burr defects. They observed that the uncut fibres -along the exit hole circumference- are located at fibre cutting angles between 110° and 150°. Based on the local characteristics along the hole circumference, they introduced a new indicator: the burr area (AB), which can be observed in Fig. 8(a). Researchers are using the burr area as a qualitynumber in order to minimize uncut fibres along the exit of the hole. S. Gaugel et. al [18] introduced the area of fibre-overlap, expressed by AFO=Anom-A, where Anom is the nominal hole area of the drilled hole and A is the free area of the hole (as can be see in Fig. 5). However, these quality-indicators (AB, AFO) are not (hole diameter-) specific ones, therefore a new specific indicator: the area factor (Ac) is introduced by the authors. which will be discussed in the next chapter. The main objective of the present study is a comparative machinability analysis of UD-CFRP drilled at normal I and II directions. In the present work, numerous conventional drilling experiments were carried out at normal II direction. The experimental results are analysed and discussed, and even compared with the results of a previous research work (drilling UD-CFRP at normal I direction) [7]. The rest of the paper is organised as follows: first, the experimental setup, the digital image processing and material tests are presented. Second, the thrust force and characteristics of uncut fibres are analysed. Finally, the drilling orientations (normal I and II) are compared and discussed.
2. Experimental setup and evaluation methods 2.1 Experimental setup UD-CFRP workpiece was made by hand lay-up process as follows: 55 unidirectional carbon fibre layers (344.4 g/m2) and epoxy resin matrix (FM20 resin and MH3124 hardener in ratio of 100:35 consequently) were placed on a mould and the entrapped air was removed by squeezing rollers. The workpiece was heated to a temperature of 60° in order to accelerate the curing process. Then, the specimen was cut by a waterjet machine into 110x25x6 mm pieces. Detailed material properties can be found in Section 2.3.
Fig. 4 Experimental machining setup: (a) back-up support plate (b) workpiece (UD-CFRP), (c) clamp and (d) KISTLER dynamometer The drilling experiments were carried out on a Kondia B640 three-axis CNC machine centre (12,000 rpm, 10 kW). Because of the high abrasive wear-effect of the carbon fibres, the machine centre was equipped with a NILFISK GB733 vacuum cleaner in order to clear off the small particle chips from the cutting space. An Ø11.2 (mm) TIVOLY 11400111120 uncoated twist drill with a point angle of 118° was used, without any cooling and lubricating fluid. A back-up support plate was applied in order to restrain deflection of material at the exit of the hole. A KISTLER 9257BA three components dynamometer with a sampling frequency of 6 kHz was used to measure the thrust force of the drilling process. Furthermore, the force-data were collected by KISTLER DynoWare, and analysed by Microsoft Excel and Minitab software. The images of the exit of the holes were taken by a Dino-Lite AM4013MT digital microscope (magnification: 10-70x, resolution: 1.3 Megapixel, maximum frame rate: 30 fps) and analysed using IrfanView and Wolfram Mathematica software. The experimental machining setup can be seen in Fig. 4. Drilling process-parameters were determined using the central composite inscribed design. The levels of the factors (feed rate and cutting speed) were chosen based on previous studies and suggestions by tool producers, which are listed in Table 1.
Table 1 Experimental parameters and their levels Factors
Levels -21/2 50 70
vc (m/min) - cutting speed vf (mm/min) - feed rate
-1 64.6 103.7
0 100 185
+1 135.4 266.3
+21/2 150 300
2.2 Digital Image Processing (DIP) Digital image processing is a favourable and widely-used technique to analyse geometrical damages in CFRP [18, 20, 35]. In this study, the quality of the drilled holes is characterized by a specific number, which is defined as area factor (Ac), and expressed by Eq. (1). A 100(%) Ac Anom
(1)
, where A is the clear (white area in Fig. 5(c)) and Anom is the nominal hole area of the drilled holes. Thus, according to the uncut fibres, higher Ac values indicate better-quality and lower Ac values indicate worst-quality holes. The border of these areas can be seen also in the figure. Both of them are defined in this study using the digital image segment process which replaces the pixels of the picture of the exit of the holes with a number between 0 and 1, then places the values into a matrix. Then, the matrix is analysed and processed in the following two steps: replacement of white-grey (1-0.5) data into white (step 1), then replacement of black-grey (0-0.5) data into black (step 2). The digital image segment process used in this study can be seen in Fig. 5. a)
b)
c) Anom
A
step 1
step 2
step 1 step 2 white pixels
Fig. 5 Image segment process of drilled CFRP holes: (a) Original image of drilled exit of the hole, (b) changed white-grey colour using the colour histogram, (c) changed black-grey colour using the colour histogram. The amount of the white pixels is proportional to the clear area (white) of the drilled hole (Ac)
2.3 Material properties of applied UD-CFRP The structure of the applied UD-CFRP composite is highly non-homogeneous and the mechanical properties are anisotropic, the machinability properties are therefore highly effected by the cutting directions. In this section, important material properties related to different fibre directions are discussed. A detailed mechanical testing was conducted and the following material properties were defined: (i) tensile strength (in different fibre orientations), (ii) interlaminar shear, (iii) Shore D hardness and (iv) Charpy impact strength (in different fibre orientations). The schematic drawings of the material tests can be seen in Fig.6.
Fig. 6 The schematic drawings of the material testing (a) tensile strength (b) interlaminar shear, (c) Shore D hardness, and (d) Charpy impact strength Tensile strength (σ) test was carried out on a Zwick Z250 mechanical tensile tester based on the ISO 527-5:1997 standard (v=2 mm/min, T=23°C). Four different setup (fibre orientation angle of 0°, 30°, 60° and 90°) was applied, each test was performed five times. The measured tensile strength results are listed in Table 2, tensile stress diagrams can be found in the appendices (Fig. 15). As was expected, higher fibre orientation angle decreases tensile strength. Furthermore, the percentage difference between the σ0° and σ30° (848 %) is higher than between σ30° and σ60° (124 %) or between σ30° and σ60° (51 %). This non-linear effect of fibre orientation angle on tensile strength can be a possible explanation of the non-linear behaviour of experimental data collected through the machining processes. Interlaminar shear (τ) test was conducted on a Zwick Z020 mechanical tensile tester based on the D3846-02 standard (v=1.3 mm/min, T=23 °C, Φ=0°). The test was repeated five times, and the average was calculated as follows: τ= 19,26 ± 0,76 MPa. Interlaminar shear diagram can be found in the appendices (Fig.16). Shore D hardness (SD) was measured using a Zwick H04.3150 tester based on the ISO R 868 / DIN 53505 standard (t=3 s, T=23 °C F=44.48 N). The test was repeated five times, and the average was calculated as follows: SD= 85,5±1,9. This Shore D hardness is high in the group of composites, however, the strong wear effect is not caused by the matrix material, but the carbon fibres [13].
Table 2 Effect of fibre orientation angle on material properties of applied UD-CFRP Fibre orientation angle Material properties Φ= 0 °
Φ= 30 °
Φ= 60 °
Φ= 90 °
σ (MPa) ± s* (MPa)
723.00 ± 58,29
76.25 ± 1,94
34.02 ± 1,98
22.48 ± 2,88
C (kJ/m2) ± s* (kJ/m2)
203.18 ± 31.38
33.56 ± 3.44
24.36 ± 0.31
21.08 ± 0.55
Charpy impact (C) test was carried out on a Ceast Resil Impact Junior tester based on the MSZ EN ISO 179-1 standard (W=15 J, T=23 °C). Four different setup (fibre orientation angle of 0°, 30°, 60° and 90°) was tested, each test was performed five times. The results of Charpy test are listed in Table 2. The effect of fibre orientation angle on C is significant but not linear, as was showed at the discussion of tensile strength.
3. Results and discussion Response surface methodology (RSM) was applied to analyse and optimise the machining process. A second order polynomial model was applied in this research due to the expected non-linear effect of analysed factors [3, 7, 36]. The second order RSM model applied in this study is expressed by Eq. (2). n
Y ( x1 , x2 ...xn ) b0
n1
n
bi xi
i 1
i 1
bii xii2
n
b x x ij i
j
(2)
i 1 j i 1
, where Y is the optimisation parameter, xi are the factors, b0, bi, bij and bii are the regression coefficients of the parameters and ε is a random experimental error. Based on the analysis of the mathematical model of the process, optimal conditions and effect of the process parameters can be determined. The main effects and the significant factors of the analysed process can be determined at a chosen value of significance level by means of analysis of variance [7, 37, 38]. The null-hypothesis used in this study is as follows: H0: It can be stated that the analysed factor has no significant effect on the response variable. The significance level used in this study is α=0.05. 13 drilling experiments in UD-CFRP at normal II direction were carried out based on the CCI design. The measured data - thrust force (Ft), lengths of the longest uncut fibre (x) and the area factor (Ac) were processed and analysed using Microsoft Excel, Minitab and Wolfram Mathematica software. The experimental design matrix, showing the factors, measured values and absolute values of the asolute percentage error (APE) can be seen in Table 3. The APE, expressed by equation (3) is used to give the reliability of the developed RSM models. APEi
yi Yi Yi
(3)
, where yi is the experimental value (measured parameter) and Yi is the RSM-predicted value. The average APE of the thrust force is 0.17, of the lengths of the longest uncut fibre is 0.07, and of the area factor is 0.04. The average APE of the thrust force is relatively high, because the RSM model gives an
uncertain prediction at low feed rate regions (at vf=70 mm/min). The possible reason of this difference could be the higher force caused by friction at low feed rates [7, 39]. Table 3 The CCI design matrix showing actual variables, experimental values and absolute values of the absolute percentage error (APE) between the measured and predicted values No. of test (-) 1 2 3 4 5 6 7 8 9 10 11 12 13
Factors vf (mm/min) 70.0 103.7 185.0 103.7 300.0 185.0 185.0 266.3 266.3 185.0 185.0 185.0 185.0
vc (m/min) 100.0 64.6 100.0 135.4 100.0 100.0 150.0 135.4 64.6 100.0 100.0 100.0 50.0
Experimental values Ft x (N) (mm) 63.2 54.8 122.0 70.4 148.0 46.2 104.4 56.7 260.2 30.9 274.1 61.4 229.5 61.7 266.3 57.5 375.5 53.6 239.1 62.9 266.8 62.6 270.0 64.3 370.9 71.6
Ac (%) 90.6 89.5 92.5 87.6 95.4 81.8 87.6 84.1 87.2 80.5 74.4 80.9 78.6
Absolute percentage error (APE) Ft x Ac (-) (-) (-) 0.93 0.02 0.02 0.22 0.00 0.04 0.39 0.22 0.13 0.13 0.02 0.01 0.07 0.14 0.03 0.14 0.04 0.00 0.03 0.04 0.04 0.07 0.11 0.05 0.00 0.09 0.00 0.01 0.06 0.02 0.11 0.06 0.09 0.12 0.09 0.01 0.05 0.02 0.03
3.1 Analysis of thrust force The thrust force (axial cutting force component) was measured and analysed in order to evaluate the machinability of UD-CFRP at normal II direction. According to Jia et. al [5], the analysis of thrust force is necessary in order to understand and describe the removal mechanisms at the exit of the hole. Discrete Fourier transformation (DFT) was applied to decrease the noise effect. The effect of cutting speed (vc) and feed rate (vf) on the filtered thrust force can be seen in Fig.7(a) and expressed by Eq. (4). The goodness-of-fit of the developed RSM based model can be explained by the following regression coefficients: R2=86.72 %, adjR2= 77.23 %. Ft (v f , vc ) 6 4.24v f 3.98vc 0.00641v 2f 0.0215vc2 0.00796v f vc
(4)
From the surface plot of Fig.7(a), it is clear, that the feed rate increases the thrust force. It is well known that the larger the feed rate the larger the chip-section is, which causes the increase of the cutting force [37]. The effect of the feed rate on the thrust force can be seen on the main effects plot in Fig.7(b). The ANOVA table (Table 3) shows that the feed rate has the most significant impact on the thrust force (F-value: 28.79, P-value: 0.001).
Fig. 7 Effect of cutting speed (vc) and feed rate (vf) on thrust force (Ft): (a) response surface and (b) main effects plot for Ft Drilling CFRP at higher cutting speed causes higher cutting temperature, which results in a decrease of the elastic modulus of the resin matrix, therefore the thrust force decreases in the case of drilling CFRP at higher cutting speed. The effect of the cutting speed is highly non-linear, as can be seen in the response surface and on the main effects plot in Fig.7(b). These non-linear effect of the cutting speed was observable also, when drilling UD-CFRP at normal I direction [7]. The effect of the cutting speed at drilling UD-CFRP at normal I direction is low (F-value: 0.01, P-value: 0.906) [7], but in this study the effect of the cutting speed is more considerable (F-value: 6.38, P-value: 0.039). The possible reason of this difference can be explained considering the different reinforcement directions, from the point of view of the axis of the cutting tool. Future work is needed in order to find a more detailed explanation of this difference. Based on the present research, in the case of conventional drilling UD-CFRP at normal II direction, the optimal machining process parameters for minimizing thrust force are feed rate of 70 mm/min and cutting speed of 105 m/min. Table 3 ANOVA table for thrust force (Ft) versus cutting speed (vc) and feed rate (vf) Source Model Linear vf vc Square vfvf vc vc Interaction vfvc Error Total
DF 5 2 1 1 2 1 1 1 1 7 12
Adj SS Adj MS 95573 19115 73546 36773 60202 60202 13344 13344 19930 9965 12515 12515 5010 5010 2097 2097 2097 2097 14640 2091 110213
F-Value P-Value 9.14 0.006 17.58 0.002 28.79 0.001 6.38 0.039 4.76 0.049 5.98 0.044 2.40 0.166 1.00 0.350 1.00 0.350
3.2 Analysis of hole quality The quality of the drilled holes is described in this study by the length of the longest uncut fibres (x) at the exit of the holes, and by the area factor (Ac). The segmented images of the exit of the holes in UDCFRP can be seen in Fig.9. It can be observed from the images that the burr area (where the uncut fibres occur) is observable at the fibre cutting angle of 110°-160°. Xu et al. [12] have observed the same phenomena, when drilling high-strength multi-directional (MD) CFRP with PCD twist drill. The
possible reason of the position of the burr area is, that the fibre fracture mode is bending-dominated at these fibre cutting angles [22], the cutting edge can not cut the fibres properly, because they buckle. As can be seen in Fig. 9, the hole made by the highest feed rate (5th test) has less uncut fibres that the others. However, Xu et al. [12] presented the opposite, when drilling MD-CFRP at normal I direction: the burr area grows with the feed rate. Due to the direction of the drilling (normal II), the cutting edge gets in contact with the fibre reinforcements in a different way (as can be seen in Fig. 1 and Fig. 12), therefore the burr area has more separated uncut fibre-groups, as illustrated in Fig. 8(b). The separated fibre-groups in the burr area can be controlled possibly not just by the cutting process parameters, but especially more by the ratio of the matrix and reinforcement, when pre-manufacturing the workpiece. This phenomenon is described more deeply in chapter 3.3 discussion.
Fig. 8 Burr area when drilling CFRP at (a) normal I [12] and at (b) normal II direction (k shows the direction of the fibres)
Test Nr. 1
Test Nr. 2
Test Nr. 3
Test Nr. 4
Test Nr. 5
Test Nr. 6
Test Nr. 7
Test Nr. 8
Test Nr. 9
Test Nr. 10
Test Nr. 11
Test Nr. 12
Test Nr. 13 Fig. 9 Segmented images of the hole exits in UD-CFRP The effect of cutting speed (vc) and feed rate (vf) on the longest uncut fibres (x) can be seen in Fig.10(a) and expressed by Eq. (5). The measured values can be seen in Table 2. The goodness-of-fit of the developed RSM based model can be explained by regression coefficients of R2=76.98 % and adjR2= 60.54 %. x(v f , vc ) 111.7 0.169v f 1.133vc 0.001076v 2f 0.00383vc2 0.00152v f vc
(5)
Fig. 10 Effect of cutting speed (vc) and feed rate (vf) on uncut fibres (x): (a) response surface and (b) main effects plot for x The effect of the feed rate on the longest uncut fibres can be seen on the main effects plot in Fig.10(b). The data of ANOVA table (Table 4) prove that the feed rate has the most significant impact on the uncut fibres (F-value: 6.96, P-value: 0.034), followed by the cutting speed (F-value: 1.58, P-value: 0.249). The effect of the cutting speed is also highly non-linear, as was observed at the optimisation parameter of thrust force. The interaction terms of uncut fibres are more significant (F-value: 1.73, Pvalue: 0.230) than in the case of the thrust force, therefore (i) the feed rate decreases the uncut fibres at low cutting speed (ii) the cutting speed increases the uncut fibres, after a local minimum point. Other researchers [12, 20, 35] observed that the feed rate increases the number of uncut fibres and the rate of
the push-down delamination. However, they drilled CFRP at normal I direction. Future work is needed in order to find the reason of these differences. Based on the present research, in the case of conventional drilling UD-CFRP at normal II direction, the optimal machining process parameters for minimizing uncut fibres are feed rate of 300 mm/min and cutting speed of 88 m/min. Table 4 ANOVA table for uncut fibres (x) versus cutting speed (vc) and feed rate (vf) Source Model Linear vf vc Square vfvf vc v c Interaction vfvc Error Total
DF Adj SS Adj MS 5 1040.03 208.01 2 379.39 189.69 1 309.13 309.13 1 70.26 70.26 2 583.93 291.97 1 352.26 352.26 1 159.84 159.84 1 76.71 76.71 1 76.71 76.71 7 311.02 44.43 12 1351.05
F-Value P-Value 4.68 0.034 4.27 0.061 6.96 0.034 1.58 0.249 6.57 0.025 7.93 0.026 3.60 0.100 1.73 0.230 1.73 0.230
Table 5 ANOVA table for the area factor (Ac) versus cutting speed (vc) and feed rate (vf) Source Model Linear vf vc Square vfvf vc v c Interaction vfvc Error Total
DF 5 2 1 1 2 1 1 1 1 7 12
Adj SS Adj MS 201.322 40.264 7.677 3.838 0.142 0.142 7.535 7.535 193.287 96.643 192.268 192.268 0.652 0.652 0.358 0.358 0.358 0.358 232.197 33.171 433.518
F-Value P-Value 1.21 0.393 0.12 0.892 0.00 0.950 0.23 0.648 2.91 0.120 5.80 0.047 0.02 0.892 0.01 0.920 0.01 0.920
The data of ANOVA table (Table 5) identified that the cutting speed (vc) and the feed rate (vf) does not have any statistical (α=0.05) influence on the area factor (Ac). F-values of the independent variables have significantly lower values than the critical F-value: Fcrit(0.05,1,11)=4.8443, the nullhypotheses are therefore fail to reject. ANOVA results proved that vf2 has a significant influence (Fvalue: 5.80, P-value: 0.047) on the response variable (Ac), so the effect of vf on the Ac is non-linear, as can be seen in the main effects plot in Fig. 11.
Fig. 11 Main effects plot for Ac (%) The highly non-linear effect of vf can be explained by the following: (i) higher feed rate results in a higher chip section, which causes higher cutting force and higher friction according to [7]. Furthermore, higher friction causes higher cutting temperature. The thermal conductivity of epoxy resin matrix is low (compared with metals), so higher cutting temperature decreases the stability of matrix material, the fibres can therefore separate from the matrix more easily (fibres are not crushed but bended). This can be a possible reason that the higher the feed rate lower the Ac. (ii) However, in the case of higher feed rates (vf >185) Ac is likely to be increased due to the higher cutting temperature, which decreases the stability of the fibres, too, so they can be crushed more effectively than in the case of low feed rates. In the future, additional experiments are required to analyse the effect of vf in more detail (full factorial test, cutting temperature measurement, etc.). Based on this study, in the case of conventional drilling UD-CFRP at normal II direction, the machining process is optimal (from the point of view for maximizing the area factor) when both feed rate and cutting speed are at maximum.
3.3 Discussion The specific feed force (kf) is a key parameter showing the specific resistance of the material to the axial cutting force (thrust force, as defined in [40]), as expressed by kf=Ft (fz d)-1, where Ft is the thrust force, fz is the feed per tooth and d is the tool diameter. Results of previous drilling experiments [7] in UD-CFRP at conventional direction (normal I) were applied to calculate the specific feed force, andto make a comparison with the results of this study (Fig. 12.)
Fig. 12 Specific feed force vs drilling directions in UD-CFRP It can be stated that the difference of the specific feed force values is significant. the specific feed force belonging to the normal II direction is more than three times higher than the specific feed force that belongs to the normal I direction. The reason of these differences can be found in the different geometric composition of the fibres and the geometry of the cutting edge, as can be seen in Fig. 13. Fig. 13(a) demonstrates, the cutting tool edge meets just one group of reinforcement fibres (on Fig. 13 (a): 1. group) at a certain hole depth. However, the cutting tool edge get in contact with more group of fibres (on Fig. 13 (b): 7.,6.,5.,4… groups) in the case of drilling at normal II direction, which can be observed in Fig. 13 (b). As was mentioned before, (i) the distribution of uncut fibres in the burr area is balanced [7, 12] when drilling at normal I direction, and (ii) the burr area contains separated uncut fibre groups, when drilling at normal II direction (as illustrated in Fig. 8.). These can be explained by the fact that at the exit of the hole (considering the same fibre cutting angle) there is a bigger space between the fibre groups (rowing) when drilling at normal II direction, than the space between the fibres, when drilling at normal I direction, as can be seen in Fig. 13. It is clear from the model (Fig. 13.) that decreasing the distance between the fibre groups (rowing) causes balanced distribution of uncut fibres in the burr area, too, when drilling at normal II direction.
Fig. 13 Schematic drawings of drilling directions in UD-FRP at (b) normal I and (c) normal II directions. Based on the chip formation mechanisms (explained in the introduction), dominant chip formations can be associated to the different fibre cutting angles when drilling UD-CFRP. As can be seen in Fig. 14(a), Type I, III, IV and V fibre cutting type chip formations dominate, when drilling UD-CFRP at the conventional direction. During one rotation of the twist drill, the cutting edge continuously touches the fibre-reinforcements, as can be seen in Fig.13(a). The reviewed chip formation types therefore satisfactorily describe the cutting mechanisms. However, in the case of drilling UD-CFRP at the normal II direction, cutting of matrix has a more significant effect on the cutting mechanisms, because in certain cases the cutting edge gets in contact only with the matrix, as can be seen in Fig.13(b). Due to this unique phenomenon, the chip formation is more influenced by the matrix material when drilling at normal II direction. Fig.14(b) model shows that the matrix plays an important role at the chip formation. With this model, the higher specific cutting force can be explained in the following way. It is well known that interrupted machining causes higher mechanical strain, therefore the specific cutting force will possibly be higher in the case of drilling UD-CFRP at normal II direction. Furthermore, there is a difference in resistance of matrix material and fibre reinforcement against cutting, which causes considerable vibrations affecting hole damages and change in thrust forces. However, the verification of these statements requires further investigation.
Fig. 14 Dominant chip formation types vs fibre cutting angle in the case of (a) normal I and (b) normal II drilling directions (k shows the direction of the fibres) Future directions regarding the present research work are as follows: (i) a huge number of holes in the aircraft industries have to be machined, where the speed is one of the key factors influencing the manufacturing performance. The future goal is to increase machining performance by increasing the feed rate. However, it is also important to increase quality of the holes with using more special cutting tools. (ii) Push-down delamination and uncut fibres at the exit of the hole can be decreased by applying back-up support plate (as was used in this study, too). However, in the case of industrial applications, usually this plate cannot be applied due to the huge elements needed to be machined. Future investigation is required in order to find other solutions for the industry supporting the back layers of CFRPs. (iii) Drilling process monitoring and diagnostics are suggested in order to ensure the high machining quality. Cutting force measurement has already been suggested by many researchers in order to monitor drilling process (tool wear), but the implementation in industrial environment raises many problems. Based on the present study a future research work regarding the characteristics of uncut fibres is planned ensuring monitoring and diagnostics of the hole making process.
5. Conclusions In the present study, machining experiments were carried out in unidirectional CFRP at normal II direction (where the axis of the tool is perpendicular to the axis of the unidirectional fibres, and the axis are in the plane of the UD-CFRP plate), using a twist drill in order to analyse and compare the effect of different drilling directions. According to the present study, the following conclusions can be drawn:
The results of ANOVA have proved that the effect of the cutting speed is more considerable when drilling UD-CFRP at normal II direction than in the case of conventional (normal I) direction. It was also obtained that higher feed rate increases the thrust force.
It was found that the hole machined by the highest feed rate (300 mm/min) has less uncut fibres than the others do. Furthermore, it was concluded that the feed rate decreases the uncut fibres at low cutting speeds (50-100 m/min).
Optimal machining process parameters for (i) minimizing thrust force are feed rate of 70 mm/min and cutting speed of 105 m/min; (ii) for minimizing uncut fibres are feed rate of 300 mm/min and cutting speed of 88 m/min; and (iii) for maximizing the area factor, are on maximum of both process parameters.
It was shown that the burr area has separated uncut fibre-groups when drilling UD-CFRP at normal II direction, because the cutting edge gets in contact with separated fibre groups in the case of drilling at normal II direction.
Concerning UD-CFRP, the specific feed force belonging to the normal II direction is more than three times higher than the value that belongs to the normal I direction. The reason of these differences can be found in the different geometric composition of the fibre groups and in the geometry of the cutting edge.
In the future, additional experiments and more detailed analysis are required on the following topics: (i) influence of cutting speed on the cutting force in the case of different drilling directions. It is necessary to prove the fact that the effect of vc is larger in the case of normal II than at normal I direction. (ii) In order to explain more detail the non-linear effect of vf and the higher specific cutting force (normal II), full factorial drilling experiments in different drilling directions are planned with cutting temperature and tool vibration measurements.
Acknowledgement The authors would like to acknowledge the support provided by the CEEPUS III HR 0108 project. This research was partly supported by the EU H2020-WIDESPREAD-01-20162017-TeamingPhase2-739592 project “Centre of Excellence in Production Informatics and Control” (EPIC). This work was partly supported by the Higher Education Excellence Program of the Ministry of Human Capacities in the frame of Nanotechnology and Material Science research area of Budapest University of Technology and Economics (BME FIKP-NANO). Furthermore, the authors acknowledge to prof. Gyula MÁTYÁSI, Norbert FORINTOS and to András TŐKE for their participation in the experimental work.
Appendices
Fig. 15 Tensile stress of applied UD-CFRP in different fibre orientations: (a) ϕ=0°, (b) ϕ=30°, (c) ϕ=60° and (d) ϕ=90°
Fig. 16 Interlaminar shear of applied UD-CFRP, ϕ=0°
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