Determination of optimum parameters on delamination in drilling of GFRP composites by Taguchi method

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 17, August 2010, pp Determination of optimum parameters on delamination in drilling of GFRP composites by Taguchi method Erol Kilickap* Department of Mechanical Engineering, Dicle University, Diyarbakir, Turkey Received 26 August 2009; accepted 8 June 2010 Drilling is one of the important machining processes in hole making operations. Delamination is a vital problem during any drilling operation. It causes structural integrity reduction and poor assembly tolerance as well as potential for long-term performance deterioration. As a result, drilling of any material requires dimensional stability and interface quality. In this study, glass fibre reinforced plastic composite is selected as experimental material for investigation of cutting parameters (cutting speed, feed rate and tool geometry) affecting delamination in drilling operation. Moreover, the Taguchi method is used to determine optimal cutting parameters for damage-free drilling material. A plan of experiments, based on L 16 Taguchi design method, is performed drilling with cutting parameters in a GFRP composite. The orthogonal array, signal-tonoise (S/N) ratio and analysis of variance (ANOVA) are employed to investigate the optimal drilling parameters of GFRP composites using four different drills. The experimental results demonstrate that the feed rate is the major parameter among the controllable factors that influence the delamination. Additionally, the optimal combinations of the cutting parameters are determined. Keywords: Taguchi method, Analysis of variance, Design optimization, Drilling, Delamination, Glass fibre reinforced plastic composites The uses of glass fibre reinforced plastic (GFRP) composites in engineering applications such as automotive, aircraft and manufacture of spaceships and sea vehicles industries have been increased considerably in resent years due to their significant advantages over other materials. They provide high specific strength/stiffness, superior corrosion resistance, light weight construction, low thermal conductivity, high fatigue strength, ability to char and resistance to chemical and microbiological attacks. As a consequence of the widening range of applications of GFRC, the machining of these materials has become a very important subject for research 1-3. Drilling is a frequently practiced machining process in industry owing to the need for component assembly in mechanical pieces and structures 4. Delamination is a dramatic problem associated with drilling fibrereinforced composite materials that, in addition to reducing the structural integrity of the material, also results in poor assembly tolerance and has the potential for long-term performance deterioration 5. Therefore, a precise drilling needs to be performed to ensure the dimensional stability and interface quality 6. * ekilickap@dicle.edu.tr It is necessary to understand the relationship among the various controllable parameters and identify the important parameters that influence the quality of drilling. Mohan et al. 7 have performed and analyzed delamination in drilling process of GFRP composite material. Hocheng and Tsao in their study on drilling of composites materials with various drill bits, presents a comprehensive analysis of delamination in use of various drill types, such as saw drill, candle stick drill, core drill and step drill. In their analysis, the delamination is predicted and compared with twist drill 8. Davim et al. 9 studied the behaviour of two cemented carbide drilling (K10) with distinct geometry ( Stub Length and Brad & Spur ) when drilling glass fibre reinforced plastic (GFRP) manufactured by hand lay-up. Their study s objective was to establish a correlation between cutting parameters with the specific cutting pressure, thrust force, damage factor and surface roughness. Moreover, a number of research workers have investigated the drilling of GFRP composite materials using Taguchi design method to investigate the influence to cutting parameters on delamination. Palanikumar et al. 10 studied evaluation of delamination in drilling GFRP composites using HSS twist drill and 4-flute cutter. They developed

2 266 INDIAN J ENG. MATER. SCI., AUGUST 2010 empirical models to prediction in drilling of GFRP composites using response surface regression. Davim and Reis 11 presented an approach using Taguchi s method and ANOVA to establish a correlation between cutting speed and feed rate with the delamination in a composite laminate. Tsao and Hocheng 12 investigated a prediction and evaluation of delamination factor in use of twist drill, candle stick drill and saw drill. The aim of the study was to establish a correlation between feed rate, spindle speed and drill diameter by using Taguchi s method and analysis of variance. Enemuoh et al. 13 presented an approach for development of delamination-free drilling in fibre reinforced plastic. To obtain the optimum cutting parameters, a combination of Taguchi s experimental analysis and multi-objective optimization criteria was developed. Rao et al. 14 presented a comprehensive study of delamination in use of various drill types, three different feed rate and spindle speeds. The influence of drilling parameters on the extension of delamination was examined by conducting the number of experiments as per the design of experiments and analysis of variance techniques of Taguchi. In order to understand the effects of machining parameters in the various machining, a number of researchers used Taguchi techniques The objective of this paper is to demonstrate an application of Taguchi parameter design in order to achieve low damage with a particular combination of cutting parameters in drilling of GFRP composite materials. workpiece material specimens having size of mm were cut from the plate. The drilling tests were conducted on a SX XHMT vertical drilling machine. Drills used throughout test were all 7 mm diameter with high speed steel (HSS). Drill characteristics are shown in Table 1. All tests were run without coolant at cutting speeds of 5, 10, 15 and 20 m/min and feed rates of 0.1, 0.2, 0.3 and 0.4 mm/rev. The damage around the holes as measured using a microscope, Nikon Epiphot 200 optical microscope, with 10 magnification and 1 µm resolution. Delamination and calculation of delamination factor Delamination is recognized as one of the major causes of damage when drilling of GFRP composites. The damage generated associated with drilling glass fibre reinforced composites were observed, both at the entrance and the exit during the drilling. To determine the delamination factor around the holes, the maximum diameter (D max ) in the delamination zone was measured (Fig. 1). The value of delamination factor (F d ) can be determined by Eq, (1) 9,20 : Experimental Set-up and Machining Conditions Materials and method In this study, a GFRP composite manufactured by hand lay-up, with ultimate tensile strength of 240 MPa and modulus of elasticity of 25 GPa, provided by Armaplast Inc (Turkey) was chosen as test materials. The GFRP composites have E-glass fibres and epoxy matrix. The GFRP composite was provided in the size of mm. The Fig. 1 The delamination size around the drilled hole Drill type Table 1 Drill tool types and characteristics Description HSS twist drill, 109 mm long, point angle of 118 o HSS-E TiN coated twist drill, 109mm long, point angle of 135 o HSS standard brad point drill, 105 mm long HSS steel step drill, 105 mm long, step diameter:4mm, step length:10mm, step angle of 90 o, point angle of 118 o

3 [ KILICKAP : DELAMINATION IN DRILLING OF GFRP COMPOSITES 267 Fd D max =... (1) D drill In which, D max is the maximum diameter of the delamination zone in mm and D drill is diameter of the drill in mm. Taguchi Method and Design of Experiment Taguchi method Traditional experimental design methods could be very complicate and difficult to use. Additionally, these methods could require a large number of experiments, when the number of process parameters increase 17,21. In order to minimize the number of tests required, Taguchi experimental design method, a powerful tool for designing high quality system, was developed by Taguchi. This method uses a special design of orthogonal arrays to study the entire parameter space with small number of experiments only. Taguchi recommends analyzing the mean response for each run in the inner array, and he also suggests analyzing variation using an appropriately chosen signal-to-noise ratio (S/N). These S/N ratios are derived from the quadratic loss function, and three of them (Eqs (2)-(4)) are considered to be standard and widely applicable. 2 S y Nominal is the best : = 10log 2 N s... (2) S 1 n 2 smaller is the best : = 10log yi i 1 N n =... (3) S 1 n 1 larger is the best : = 10log i= 1 2 N n yi... (4) where y is the average of observed data, s 2 is the variation of y, n is the number of observations, and y is the observed data 7,22. Regardless of category of the performance characteristics, the lower S/N ratio corresponds to a better performance. Therefore, the optimal level of the process parameters is the level with the lowest S/N value. The statistical analysis of the data was performed by analysis of variance (ANOVA) to study the contribution of the factor and interactions and to explore the effects of each process on the observed value 19,23,24. Design of experiment In this study, two machining parameters were used as control factors and each parameter was designed to have four levels, denoted 1, 2, 3 and 4 (Table 2). The experimental design was according to an L 16 (4^5) array based on Taguchi method while using the Taguchi orthogonal array would markedly reduce the number of experiments 17,19. A set of experiments designed using the Taguchi method were conducted to investigate the relation between the process parameters and delamination factor. The DESIGN EXPERT 6.07 software was used for regression and graphical analysis of the obtained data. Results and Discussion Experimental results The delamination occurs at entrance and exit during the drilling GFRP composites. Thus, a drilling test was conducted to evaluate the effect of cutting parameters and drill types on the damage at workpiece (entrance and exit sides). After measuring the maximum diameter D max in the damage around each hole, the delamination factor (F d ) is determined by using Eq. (1). Tables 3 and 4 show the computed values of the delamination factor at workpiece entrance and exit, respectively. Analysis of the S/N ratio In the Taguchi method, S/N ratio is the measure of quality characteristics and deviation from the desired Table 2 Drilling parameters and levels Symbol Drilling parameter Level 1 Level 2 Level 3 Level 4 A Cutting speed (m/min) B Feed rate (mm/rev) Table 3 Experimental design using L 16 orthogonal array and the computed values of Fd entrance Trial no. Levels of factors Drill type v f 118 o 135 o Step Brad

4 [ 268 INDIAN J ENG. MATER. SCI., AUGUST 2010 value. The mean S/N ratio for each level of the cutting parameters and drill types is summarized and called the L 16 experiments is also. Entrance After computed values of the delamination factor (Table 3), S/N ratios for each experiment of L 16 (4^5) was calculated by applying the Eq. (3). Table 5 gives the S/N ratio for the workpiece entrance. The objective of using the S/N ratio as a performance measurement is to develop products and processes insensitive to noise factor 24. Thus, by utilizing computed values of the delamination factor (Table 3) Table 4 Experimental design using L 16 orthogonal array and the computed values of Fd exit Trial no. Levels of factors Drill type v f 118 o 135 o Step Brad Trial no. Table 5 S/N response table for the delamination factor at entrance Levels of factors Drill type v f 118 o 135 o Step Brad and S/N ratios (Table 5), average effect response value (Table 6) and average S/N response ratios (Table 7), respectively, were calculated for each drill types. The S/N ratios response graph for delamination at workpiece entrance is shown in Figs 2 and 3. Regardless of category of the quality characteristic, a greater S/N ratio corresponds to a better performance. The level of a factor with the highest signal-to-noise ratio is the optimum level 24. Therefore, the optimal parameter combination level identified for the present investigation in the drilling process is cutting speed at level 1 (A1) and feed rate at level 1 (B1) for all the drill types(* optimum level). From Fig. 2, it can be observed that raise of cutting speed increases the delamination factor in drilling of GFRP composites by all drill tools. The increase of feed rate increases the delamination factor for all drill tools. Exit After calculating values of the delamination factor (Table 4), the next step is to compute the S/N ratios. S/N ratios for each experiment were calculated by applying the Eq. (3). Table 8 gives the S/N ratio for the workpiece exit. By using computed values of the delamination factor (Table 4) and S/N ratios (Table 8), average effect response value (Table 9) and average S/N response ratios (Table 10), respectively, were calculated for each drill types. The S/N ratios response graph for delamination at workpiece exit is shown in Figs 4 and 5. Based on the results of the S/N ratio and mean (Figs 4 and 5), the optimal cutting parameters for Fd Exit were obtained 5 m/min cutting speed (Level 1) and 0.1 mm/rev feed rate (Level 1) for all drills (*Optimum level). It can be observed that decrease of Level Table 6 Average effect response table for the Fd entrance 118 o 135 o Step Brad v f v f v f v f Table 7 Average S/N response table for delamination factor 118 o 135 o Step Brad Level v f v f v f v f * * * * * * * * *Optimum level

5 [ KILICKAP : DELAMINATION IN DRILLING OF GFRP COMPOSITES 269 Fig. 2 Effect of cutting speed on delamination factor at entrance (οs/n Mean) Fig. 3 Effect of feed rate on delamination factor at entrance (οs/n Mean) cutting speed reduces the delamination factor in drilling of GFRP composites by each drill. The increase of feed rate increases the delamination factor for all drill tools. Analysis of variance (ANOVA) Entrance The relative importance of the cutting parameters with respect to the delamination factor Table 8 S/N response table for the delamination factor at exit Trial no. Level Levels of factors Drill type v f 118 o 135 o Step Brad Table 9 Average effect response table for the Fd exit 118 o 135 o Step Brad v f v f v f v f Level Table 10 Average S/N response table for delamination factor 118 o 135 o Step Brad v f v f v f v f * * * * * * * *Optimum level was investigated to determine more accurately the optimum combinations of the cutting parameters by using ANOVA. The analysis of variance (ANOVA) was applied to study the effect of cutting parameters on the delamination factor. Tables give ANOVA results for delamination factor in drilling GFRP composites. Analysis of variance was carried out for all drill types in Tables Feed rate was found to be the major factor affecting the delamination factor at workpiece entrance in all experimental results. It can be observed from the tables that cutting speed also affects delamination factor at workpiece entrance. From Tables 11-14, values of prob > F less than indicate drilling parameter terms are

6 [ 270 INDIAN J ENG. MATER. SCI., AUGUST 2010 Fig. 4 Effect of cutting speed on delamination factor at exit (οs/n Mean) Fig. 5 Effect of feed rate on delamination factor at exit (οs/n Mean) Table 11 Table ANOVA results for the delamination factor for drill point angle 118 o Source of variance SDQ a DF b MS c F value p-value Contribution % Model < Significant Cutting speed E Feed rate < Error 3.225E E Total S.D C.V R Pred. R Mean 1.34 PRESS Adj.R Adeq. Precision Table 12 Table ANOVA results for the delamination factor for drill point angle 135 o Source of variance SDQ DF MS F Value Prop>F Contribution % Model < Significant Cutting speed < Feed rate < Error E Total S.D C.V R Pred. R Mean 1.41 PRESS Adj.R Adeq. Precision [ Table 13 Table ANOVA results for the delamination factor for step drill Source of variance SDQ a DF b MS c F Value Prop>F Contribution % Model < Significant Cutting speed 8.292E E Feed rate < Error 4.252E E Total S.D C.V R Pred. R Mean 1.18 PRESS Adj.R Adeq. Precision

7 KILICKAP : DELAMINATION IN DRILLING OF GFRP COMPOSITES 271 important. Therefore, A and B (cutting speed and feed [ rate) are significant drilling patameter terms for all drills. Figures 6 and 7 show normal probability plots of the residuals. Figures 6 and 7 revealed that the residuals generally fall on a straight line implying that the errors are distributed normally. This result was also mentioned by various researchers 18,25. Exit Tables show the results of the ANOVA with delamination factor at exit in drilling GFRP composite, for the four drills. Analysis of variance was carried out for all drill types in Tables 15, 17 and 18. Feed rate was found to be the major factor affecting the delamination factor at workpiece entrance in all experimental results. It can be observed from the tables that cutting speed also affects delamination factor at workpiece entrance. It was noticed that cutting speed was influential parameter affecting the delamination factor at exit 84.71% for drill point 135 o in Table 16. It was also observed that feed rate has an effect on delamination factor at workpiece exit %. The values of Prob > F in Tables for drilling patameter terms are less than which indicate that drilling patameter terms are noteworthy, which is desirable. Therefore, A and B (cutting speed and feed rate) are significant drilling patameter terms. Table 14 Table ANOVA results for the delamination factor for brad drill Source of variance SDQ a DF b MS c F Value Prop>F Contribution % Model < Significant Cutting speed Feed rate < Error 4.664E E Total S.D C.V R Pred. R Mean 1.30 PRESS Adj.R Adeq. Precision Fig. 6 Normal probability plot of residuals for delamination factor at entrance

8 272 INDIAN J ENG. MATER. SCI., AUGUST 2010 Fig. 7 Normal probability plot of residuals for delamination factor at exit Table 15 Table ANOVA results for the delamination factor for drill point angle 118 o Source of variance SDQ a DF b MS c F Value Prop>F Contribution % Model Significant Cutting speed Feed rate Error E Total S.D C.V R Pred. R Mean 1.60 PRESS Adj.R Adeq. Precision Table 16 Table ANOVA results for the delamination factor for drill point angle 135 o Source of variance SDQ DF MS F Value Prop>F Contribution % Model < Significant Cutting speed < Feed rate Error E Total S.D C.V R Pred. R Mean 1.53 PRESS Adj.R Adeq. Precision

9 [ [ KILICKAP : DELAMINATION IN DRILLING OF GFRP COMPOSITES 273 Table 17 Table ANOVA results for the delamination factor for step drill Source of variance SDQ a DF b MS c F Value Prop>F Contribution % Model E < Significant Cutting speed E Feed rate E < Error 2.006E E Total S.D C.V R Pred. R Mean 1.12 PRESS 6.341E-003 Adj.R Adeq. Precision Table 18 Table ANOVA results for the delamination factor for brad drill Source of variance SDQ a DF b MS c F Value Prop>F Contribution % Model < Significant Cutting speed Feed rate < Error E Total S.D C.V R Pred. R Mean 1.70 PRESS 0.26 Adj.R Adeq. Precision From the results of Taguchi method and ANOVA, the best combination levels of the cutting speed and feed rate was obtained at low levels of cutting parameters. The minimum exit delamination is observed at low level of feed rate (0.1 mm/rev) and cutting speed (5 m/min). Conclusions The application of Taguchi optimization for investigating the effect of cutting parameters (cutting speed and feed rate) on delamination factor during drilling of GFRP composite has been presented in this paper. The following conclusions can be drawn from this study: The analysis of experimental results is carried out using Taguchi s orthogonal array and analysis of variance. The level of the best of the cutting parameters and drill types on the damage is determined by using ANOVA. Taguchi design can present a systematic procedure that can successfully and efficiently identify the minimum damage in the process control of drilling machines. The results of ANOVA revealed that feed rate is the dominant cutting parameter, which has greater influence on the delamination factor for four drills. The feed rate is cutting parameters that have the physical as well as statistical influence on the delamination at entrance and exit in GFRP composite. The damage increases with both cutting parameters, which means that the composite damage is bigger for higher cutting speed and feed rate. For achieving minimal delamination on the GFRP composite, always lower feed rates and cutting speeds are preferred. Based on the S/N, optimal parameters for the minimum entrance damage are the cutting speed at Level 1(5 m/min) and the feed rate at Level 1 (0.1 mm/rev). Similarly, the optimum cutting parameters for minimum exit damage are cutting speed at Level 1(5 m/min) and the feed rate at Level 1 (0.1 mm/rev). The step drill produces less delamination the GFRP composites (entrance and exit) than three drills, i.e., the delamination factor at entrance and exit is smaller. References 1 Wallenberger F T, Watson J C & Li H, Glass fibers, edited by Miracle D B & Donaldson S L (ASM Handbook Volume: 21 Composites, USA) Blaga A, cbd205_e. html (2005) Pihtili H & Tosun N, Compos Sci Technol, 62 (2002) Davim J P & Reis P, Compos Struct, 59 (2003) Sonbaty I E, Khashaba U A & Machaly T, Compos Struct, 63 (2004) Mohan N S, Ramachandra A & Kulkarni S M, Compos Struct, 71 (2005) Mohan N S, Kulkarni S M & Ramachandra A, J Mater Process Technol, 186 (2007) Hocheng H & Tsao C C, J Mater Process Technol, 140 (2003) Davim J P, Reis P & Antonio C C, Compos Sci Technol, 64 (2004)

10 274 INDIAN J ENG. MATER. SCI., AUGUST Palanikumar K, Prakash S & Shanmugam K, Mater Manuf Process, 23 (2008) Davim J P & Reis P, Mater & Des, 24 (2003) Tsao C C & Hocheng H, Int J Mach Tools Manuf, 44 (2004) Enemuof E U, El-Gizawy A S & Okafor A C, Int J Mach Tools Manuf, 41 (2001) Rao B S, Rudramoorthy R, Srinivas S & Nageswara Rao B, Mater Sci Eng: A, 472 (2008) Hascalik A & Caydas U, Int J Adv Manuf Technol, 38 (2008) Tosun N, Cogun C & Tosun G, J Mater Process Technol, 152 (2004) Bagci E & Ozcelik B, Int J Adv Manuf Technol, 29 (2006) Noordin M Y, Vankatesh V C, Sharif S, Elting S & Abdullah A, J Mater Process Technol, 145 (2004) Savaskan M, Taptik Y & Urgen M, Itu dergisi/d Eng, 3 (2004) Arul S, Vijayaraghavan L, Malhotra S K & Krishnamurthy R, Int J Mach Tools Manuf, 46 (2006) Rosa J L, Robin A, Silva M B, Baldan C A & Peres M P, J Mater Process Technol, 209 (2009) Montgomery D C, Design and Analysis of Experiments, (Arizona State University, New York) Petropoulos G, Ntziantzias I & Anghel C, A predictive model of cutting force in turning using Taguchi and response surface techniques, 1 st IC-EpsMsO, Athens, Yang W H & Tarng Y S, J Mater Process Technol, 84 (1998) Davidson M J, Balasubramanian K & Tagore G R N, J Mater Process Technol, 202 (2008)