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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME AND TECHNOLOGY (IJMET) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) IJMET Volume 4, Issue 3, May - June (2013), pp. 136-148 © IAEME: www.iaeme.com/ijmet.asp Journal Impact Factor (2013): 5.7731 (Calculated by GISI) ©IAEME www.jifactor.com INVESTIGATION INTO THE EFFECTS OF PROCESS PARAMETERS ON DELAMINATION DURING DRILLING OF BD-CFRP COMPOSITE USING TAGUCHI DESIGN OF EXPERIMENTS AND RESPONSE SURFACE METHODOLOGY Nagaraja*1, Mervin A Herbert1, Divakara Shetty2, Raviraj Shetty2 and Murthy BRN2 *1 (Corresponding Author: Department of Mechanical Engineering, National Institute of Technology Karnataka, Srinivasa Nagar, Surathkal, Mangalore-575025, India) 2 (Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal University, Manipal-576 104, Karnataka, India) ABSTRACT Delamination is an inter-ply failure phenomenon induced by drilling and has been recognized as a major damage encountered when drilling composite laminates. The damage caused at the entry and exit of the drilled hole is characterized by delamination factor. In this study, the effects of process parameters such as spindle speed, feed rate, drill diameter and point angle on delamination during drilling of bidirectional carbon fiber reinforced polymer (BD-CFRP) composite laminate have been investigated by using Taguchi design of experiments (DOE). The results obtained are analyzed and validated using response surface methodology (RSM) and analyses of variance (ANOVA). The study reveals that drill diameter and spindle speed are the main contributing process parameters for the variation in the drilling induced delamination of BD-CFRP composite laminate. It is evident from the investigation that feed rate and point angle are least sensitive to the drilling induced delamination of BD-CFRP in high speed steel drills. The study shows that both experimental and the predicted results of delamination factor are in good agreement. Keywords: Drilling, delamination, feed rate, thrust force, torque, high speed steel, bidirectional carbon fiber reinforced polymer composite. 136 International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME 1. INTRODUCTION Carbon fiber-reinforced polymer (CFRP) composites are well recognized for their superior mechanical properties such as low weight, high strength and stiffness, excellent fatigue and corrosion resistance and low thermal expansion [1]. CFRP composite laminates find wide applications in aerospace industries, defense, ships, automobiles, machine tools, sports equipments, transportation structures, power generation, and oil and gas industries [1, 2]. Composite materials are synergistic combination of two or more micro-constituents that differ in physical form and chemical composition. The objective of having two or more constituents is to get the benefit of superior properties of all the constituents without compromising on weakness of either [3]. Although CFRP composites are produced to near- net shape, machining is often needed to fulfill the requirements related to tolerances of assembly needs. Among all machining processes, drilling is the most indispensable method for fabrication of products with composite panels. The performance of these products is mainly dependent on surface quality and dimensional accuracy of the drilled hole. The quality of drilled hole is influenced by the cutting conditions, tool material and geometry [4].The material anisotropy resulting from fiber reinforcement considerably influences the quality of the drilled hole. Hence, precise machining needs to be performed to ensure the dimensional stability and interface quality [5]. The quality of the drilled hole is also influenced by the thrust force and torque generated during drilling, which in turn is affected by the factors such as tool geometry, speed, feed etc. Higher the value of thrust force and torque higher will be the structural damage and tool wear. Therefore, many researchers have attempted to minimize the generation of the thrust force and torque by designing different types of drilling tools [6]. The drilling operation of CFRP composites has several undesirable effects such as fiber breakage, de-bonding, pull-out, stress concentration, thermal damage, micro cracking, delamination etc. [7, 8, 9]. Among the problems caused by drilling, delamination occurs mainly due to localized bending in the zone situated at the point of attack of the drill. The delamination drastically reduces assembly tolerance and strength against fatigue, thus degrading the long- term performance of composites [10, 11]. In practice, it has been found that the delamination associated with push-out is more severe than that associated with peel-up [12]. Hence most of the researchers paid attention to the push-out delamination. Several investigators have studied analytically and experimentally the cases in which delamination in drilling have been correlated to the thrust force during exit of the drill. The higher thrust force induces more extensive delamination to the work piece [13].It was reported that the rejection of parts due to delamination damage during the final assembly was as high as 60% in aircraft industries [14].Chen proposed delamination factor to characterize the delamination in drilling of CFRP composite [10]. It is observed that there is very little work that has been reported on the influence of process parameters on delamination during drilling of BD-CFRP composite laminate with HSS drills, using Taguchi’s DOE, ANOVA and RSM. Hence, an attempt has been made in this work for optimization of process parameters such as spindle speed, feed rate, drill diameter and point angle on delamination in drilling of BD-CFRP composite through integration of Taguchi’s design of experiments and RSM based mathematical model. 137 International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME 2. MATERIALS AND METHODS 2.1. Preparation of Test Specimen The BD-CFRP composite specimen of 200 mm × 200 mm × 4 mm was fabricated by hand lay-up followed by compression moulding technique at room temperature. The resin content of the composite laminate is maintained around 50 weight % and post curing of the composite laminate is carried out for about 8 hours at 80°C. Bidirectional plain weave type carbon fiber of arial density of 200 g.m-2 is used as reinforcement. Figure 1 BD-CFRP composite specimen. The resin used for the preparation of composite material is Bisphenol A based epoxy resin L-12 and the hardener used is Amino K-6. The advantage of using BD-CFRP composite laminate is that it has maximum stiffness and strength in all direction. The scanned image of the test specimen is shown in Figure 1. 2.2. Experimental Method A Matsuzawa micro-hardness testing machine (Model No MMT-X7A, Japan) is used for measuring Vickers hardness of the BD-CFRP composite specimen. Tensile strength of the specimen is measured as per ASTM: D 638, using a Universal Testing Machine (Lloyd LR100 K, UK). Figure 2 Experimental set up 138 International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME The three point bending technique is used for measuring the flexural properties of the test specimen as per ASTM: D 790-10. The inter-laminar shear strength (ILSS) is investigated according to ASTM: D 2344 (short beam shear test method) by Universal Testing Machine (Instron 3366). The displacement method is used for measuring the density of the composite specimen as per ASTM: D 792-08, using an electronic balance (Mettler Toledo USA). The drilling experiments on BD-CFRP composite specimen are carried out using CNC vertical (TRIAC VMC) machining centre as shown in figure 2. The thrust force generated during drilling of BD-CFRP composite is measured by 9257BA KISTLER dynamometer. The mechanical properties of the BD-CFRP composite laminate are given in Table 1. Table 1 Mechanical Properties of BD-CFRP Composite Density Vickers Tensile Young’s Elongation Flexural Flexural Inter-laminar (g.cm-3) hardness strength modulus (%) strength modulus Shearstrength (MPa) (GPa) (MPa) (MPa) (MPa) 1.302 18.2 427.46 5.9 13.32 109.35 861.19130 20 Delamination factor (Fd) of the BD-CFRP composite laminate is estimated by using the equation: Fd = Dmax/Dnom (1) where, Dmax is the maximum delaminated diameter and Dnom is the nominal diameter of the drilled hole. 3. DESIGN OF EXPERIMENT 3.1. Taguchi Method The traditional method of experimentation, i.e. varying one parameter at a time and studying its effects is considered expensive and time consuming. Hence, the design of experiments (DOE) technique has been selected which requires minimum number of experiments to be conducted. Taguchi’s robust DOE is used to formulate the experimental layout, analyze the effect of each cutting parameters and optimize the process parameters which are least sensitive to the causes of variation.Taguchi’s approach to design of experiments is easy to adopt and apply for users with limited knowledge of statistics; hence it has gained a wide popularity in the engineering and scientific community [15]. Taguchi recommends analyzing the means and S/N ratio using conceptual approach that involves graphing the effects and visually identifying the factors that appear to be significant, without using ANOVA, thus making the analysis simple. The analysis is made using the popular software specially used for design of experiment applications known as MINITAB 15. The S/N ratio characteristics can be divided into three categories: S y Nominal is the best characteristic = 10 log 2 (2) N sy 139 International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME S 1 Smaller is the best characteristic = −10 log ( ∑ y 2 ) (3) N n S 1 1 Larger is the better characteristic = − log ∑ 2 (4) N n y Where, y the average of observed data, s2 y is the variation of y, n is the number of observations, and y is the observed data. For each type of the characteristics, with the above S/N ratio transformation, the smaller the S/N ratio the better is the result when we consider delamination factor, surface roughness, thrust force, torque and stress [16]. In this work, in order to identify the best cutting parameters and to obtain minimum delamination, S/N ratio characteristic andL27 orthogonal array are used. Table 2 indicates drilling test parameters and levels. Table 2 Level and Factors (A) (B) (C) (D) Levels Spindle Speed Feed rate Point angle Drill diameter (rpm) (mm/min) (degree) (mm) 1 1200 10 90 4 2 1500 15 104 6 3 1800 20 118 8 3.2. Response Surface Methodology Response surface methodology (RMS) is a collection of mathematical and statistical techniques that are useful for modeling and analyzing problems in which a response of interest is influenced by several variables. The main goal of RSM is to optimize the response that is influenced by various process parameters [17]. In this study, central composite design is used to develop empirical relationships between the drilling parameters. Central composite design is one of the important design methods used in RSM for establishing relationship between the parameters and responses by using the smallest possible number of experiments without losing accuracy. The number of experiments conducted in the present case is 30 and the number of drilling parameter considered is 4.The second degree response surface representing the delamination factor (Fd) can be expressed as follows: Fd = β0 + β1(A)+β2(B)+ β3(C)+ β4(D)+ β5(A2)+ β6(B2)+ β7(C2)+ β8(D2)+ β9(AB)+β10(AC)+ β11(AD)+ β12(BC)+β13(BD)+ β14(CD) (5) From the observed data for delamination factor (Fd), the response function is given as follows: Fd= 0.156332 + 0.000558847A + 0.0125514B + 0.00446767C + 0.0737923D – 2.02189E- 07 2 A – 1.47879E-04B2 – 2.39641E-05C2 – 0.00367424D2 –1.08333E-06AB + 4.76190E-07AC – 9.16667E-06AD – 1.96429E-05BC – 4.37500E-04BD + 6.25000E-05CD (6) 140 International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME 4. RESULTS AND DISCUSSION Delamination in drilling is a highly undesirable problem and has been recognized as a major defect encountered in drilling operation. Delamination not only reduces assembly tolerance, but also weakens the structural integrity of the composite materials. To have a better surface finish of the drilled holes, it is necessary to control the influence of process parameters such as spindle speed, tool feed rate, drill diameter and the point angle on delamination during drilling in industry. In this study, the drilling experiments are conducted at different cutting conditions using HSS drills and the results of delamination factor of BD- CFRP composite laminate are shown in Table 3. Table 3 Experimental and predicted results of delamination factor Experimental Predicted Trial Spindle Feed rate Point Drill Error Delamination Delamination No. speed(rpm) (mm/min) angle(degree) diameter(mm) (%) factor (Fd) factor (Fd) 1 1200 10 90 4 1.059 1.072 1.22 2 1200 10 90 6 1.132 1.127 0.44 3 1200 10 90 8 1.155 1.152 0.25 4 1200 15 104 4 1.102 1.097 0.45 5 1200 15 104 6 1.15 1.149 0.08 6 1200 15 104 8 1.168 1.172 0.34 7 1200 20 118 4 1.1 1.103 0.27 8 1200 20 118 6 1.156 1.152 0.34 9 1200 20 118 8 1.165 1.172 0.6 10 1500 10 104 4 1.074 1.083 0.83 11 1500 10 104 6 1.136 1.134 0.17 12 1500 10 104 8 1.149 1.155 0.52 13 1500 15 118 4 1.084 1.097 1.19 14 1500 15 118 6 1.147 1.146 0.08 15 1500 15 118 8 1.159 1.165 0.51 16 1500 20 90 4 1.107 1.104 0.27 17 1500 20 90 6 1.148 1.145 0.26 18 1500 20 90 8 1.152 1.156 0.34 19 1800 10 118 4 1.04 1.052 1.15 20 1800 10 118 6 1.104 1.099 0.45 21 1800 10 118 8 1.118 1.117 0.08 22 1800 15 90 4 1.065 1.058 0.65 23 1800 15 90 6 1.095 1.097 0.18 24 1800 15 90 8 1.115 1.107 0.71 25 1800 20 104 4 1.068 1.075 0.65 26 1800 20 104 6 1.11 1.111 0.09 27 1800 20 104 8 1.113 1.118 0.44 The main effect plots for Signal to Noise ratio (S/N) of delamination factor (smaller is the better) is shown in figure 3.It is observed from the plots that the drill diameter and the spindle speed are the most significant design parameters that influence the delamination as the slope of these plots are large and variation of S/N ratio is also large. The feed rate and point angle are the least contributing process parameters for push down delamination as the slope gradient is smaller. 141 International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME Figure 3 Main effect plots for SN ratios of delamination factor Figure 4 Delamination damage in drilling of BD-CFRP composite laminate for drill diameter of 6 mm, spindle speed of 1800 rpm, feed rate of 15 mm/min and point angle of 90° It is evident from the main effect plots of delamination factor (figure 3) that the optimum parametric conditions for minimum push down delamination in drilling of BD- CFRP composite are obtained for drill diameter of 4 mm, feed rate of 10 mm/min, spindle speed of 1800 rpm , and point angle of 90°. Since the point angle does not have significant influence on delamination, it can be set at the convenient value to get better surface finish.The response table 4 for S/N ratio of delamination factor also indicates that drill diameter is the dominant factor which influences the delamination in drilling of BD-CFRP composite laminate followed by spindle speed. The figure 4 shows the image of drilling induced delamination obtained by using HP high resolution scanner (4800dpi). 142 International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME Table 4 Responses for S/N ratio (smaller is the better) of delamination factor Spindle Speed Feed rate Point angle Drill diameter Level (rpm) (mm/min) (degree) (mm) 1 -1.0718 -0.8810 -0.9353 -0.6480 2 -1.0465 -0.9845 -0.9720 -1.0669 3 -0.7619 -1.0147 -0.9729 -1.1654 Delta 0.3099 0.1336 0.0375 0.5174 Rank 2 3 4 1 Table 5 Analysis of variance for S/N ratios of delamination factor (Fd) Source DF Seq SS Adj SS Adj MS F P P (%) ( A) Spindle 2 0.53293 0.53293 0.266467 80.05 0.000 26.270 Speed(rpm) (B) Feed 2 0.08843 0.08843 0.044214 13.28 0.006 4.358 rate(mm/min) (C) Point 2 0.00827 0.00827 0.004134 1.24 0.354 0.406 angle(degree) (D)Drill diameter 2 1.35843 1.35843 0.679213 204.04 0.000 66.962 (mm) A*D 4 0.01410 0.01410 0.003526 1.06 0.451 0.347 B*D 4 0.05636 0.05636 0.014090 4.23 0.058 1.388 C*D 4 0.01075 0.01075 0.002688 0.81 0.563 0.265 Residual Error 6 0.01997 0.01997 0.003329 Total 26 2.08924 100 The analysis of variance (ANOVA) for S/N ratio of delamination factor is carried out for a significance level of α = 0.05, i.e. for a confidence level of 95%.The P values in the ANOVA table 5 is the realized significance levels , associated with Fischer’s F test for each source of variation. The sources with P values less than 0.05 are considered to have statistically significant contribution to the performance measures. It can be seen from Table 5 that drill diameter has the highest contribution (P = 66.96%), followed by spindle speed (P = 26.27%). The interaction effects of process parameters on delamination factor during drilling of BD-CFRP composite have no statistical and physical significance as shown in the Table 5.The investigation reveals that ANOVA results of delamination are in good agreement with the conceptual S/N ratio approach used for data analyses. 143 International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME The observed results of drilling induced delamination of BD-CFRP composite for HSS drills, obtained from Taguchi DOE are validated and analyzed by using RMS model (Eqn.6). It is evident from the Table 3 that the error between the experimental and the predicted results of delamination factor is less than 2.5%, indicating that there is a good agreement between the observed results of delamination factor and the predicted results of delamination factor as per RSM model. The results obtained from Taguchi DOE can also be verified by drawing a comparison plot as shown in figure 5. The figure indicates a very close relationship between the experimental results and predicted results of delamination factor. Figure 5 Comparison of experimental and predicted values of delamination factor. The adequacy of RSM model has been tested through ANOVA method at 5% significance level, i.e., for a level of confidence of 95% [17]. Result of ANOVA for the response function delamination factor (Fd) is presented in Table 6. The sum of squares is usually performed into contributions from regression model and residual error. Mean square is the ratio of sum of squares to the degrees of freedom and F-ratio is the ratio of mean square of regression model to the mean square of residual error. From the analysis of Table 5, it is apparent that, the calculated value of F-ratio of the developed model (26.38) is greater than the F-table value (F 0.05, 14, 15 =2.46) and hence the second degree response function model developed is quiet adequate. The delamination tendency of BD-CFRP composite laminate is analyzed by generating 3D response surface plots and the corresponding contour plots. Figure 6 shows the interaction effects of feed rate and the spindle speed on delamination factor with drill diameter (8 mm) and point angle (118°) are held constant. Table 6 ANOVA for response function of the delamination factor (Fd) Source DF Seq SS Adj MS F P Regression 14 0.034670 0.002476 26.38 0.000 Residual Error 15 0.001314 0.000094 Total 29 0.035984 144 International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME (a) (b) Figure 6 Interaction effects of feed rate and spindle speed on delamination factor for drill diameter of 8 mm and point angle of118° (a) Response surface plot (b) Contour plot It is observed from the response surface plot of figure 6(a) that the increase of feed rate increases the drilling induced delamination due to the increase of thrust force in drilling [18]. From the contour plot of figure 6(b), it is clear that with feed rate kept at low value, minimum delamination can be achieved with higher spindle speed during drilling of BD- CFRP composite laminate using HSS drills. This is due to the reason that the increase in spindle speed increases the temperature produced in drilling of composites, which softens the matrix material and shearing, intern the delamination is reduced. The result presented is correlated with the result presented by Palanikumar et al [19]. Figure 7(a & b) illustrates the interaction effect of feed rate and drill diameter on delamination with spindle speed (1800 rpm) and point angle (118°) are held constant. It is evident from the figures that increase in drill diameter increases the delamination damage during drilling of composite materials. The reason is that the increase of drill diameter increases the contact area of the hole produced which increases the thrust force during drilling of composites.The increase in thrust force leads to the increase of drilling induced delamination [20, 21]. The results reveal that the increase in feed rate and drill diameter increases the delamination factor and vice-versa. The influence of drill diameter and point angle on delamination with feed rate (20 mm/min) and spindle speed (1800 rpm) are held constant as illustrated in figure 8 (a & b). It is noticed from the figure that the minimum delamination in drilling of BD-CFRP composite is observed at lower drill diameter and minimum point angle. It is also noticed from the figure 8 that the delamination factor of BD-CFRP composite is least sensitive to the variation in point angle.From the analyses of the figures, it is concluded that the maximum spindle speed, minimum drill diameter, minimum feed rate and minimum point angle are preferred to reduce the drilling induced delamination of BD-CFRP composite laminate. 145 International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME (a) (b) Figure 7 Interaction effects of feed rate and drill diameter on delamination factor for spindle speed of 1800 rpm and point angle 118° (a) Contour plot (b) Response surface plot (a) (b) Figure 8 Interaction effects of point angle and drill diameter on delamination factor for feed rate of 20 mm/min and spindle speed of 1800 rpm (a) Contour plot (b) Response surface plot 5. CONCLUSIONS Based on the experimental results, the following conclusions can be drawn in drilling of BD-CFRP composite laminate using HSS drills: 1. The model generated by means of the design software (MINITAB 15) package shows the influence of the process parameters on delamination. 2. The results reveal that the drill diameter is the most influencing design parameter on delamination followed by spindle speed. 3. The interaction plots reveal that the minimum delamination damage is obtained at higher spindle speed, lower drill diameter and lower feed rate. 146 International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME 4. The investigation reveals that there is a perfect correlation between the experimental results of delamination factor and the predicted results of delamination factor as per RMS. 5. The results show that the drilling induced delamination is not influenced the variation in point angle. 6. The results of ANOVA for S/N ratio for delamination factor are in good agreement with the responses obtained from S/N ratio of Taguchi analysis. 6. ACKNOWLEDGEMENTS The authors are very grateful to the department of Mechanical Engineering, Manipal Institute of Technology, Manipal University, Manipal for the support rendered for conducting the drilling experiments. 7. REFERENCES [1] Guu Y.H., Hocheng H, Tai N and Liu S.Y, Effect of electric discharge machining on the characteristics of carbon fiber reinforced composites, Journal of Material Science 36, 2011, pp 2037-2043. [2] Arul S, Vijayaraghavan L, Malhotra S.K, Krishnamurthy R, The effect of vibration drilling on hole quality in polymeric composites, Int. J. Mach. Tools Manuf. 46, 2006, pp 252-259. 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