Analysis of delamination in drilling glass fiber reinforced polyester composites

Analysis of delamination in drilling glass fiber reinforced polyester composites

Materials and Design 45 (2013) 80–87 Contents lists available at SciVerse ScienceDirect Materials and Design journal homepage: www.elsevier.com/loca...

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Materials and Design 45 (2013) 80–87

Contents lists available at SciVerse ScienceDirect

Materials and Design journal homepage: www.elsevier.com/locate/matdes

Technical Report

Analysis of delamination in drilling glass fiber reinforced polyester composites T.V. Rajamurugan a,⇑, K. Shanmugam a, K. Palanikumar b a b

Department of Manufacturing Engineering, Annamalai University, Chidambaram 608 002, India Department of Mechanical Engineering, Sri Sai Ram Institute of Technology, Chennai 600 044, India

a r t i c l e

i n f o

Article history: Received 17 May 2012 Accepted 18 August 2012 Available online 6 September 2012

a b s t r a c t Glass fiber reinforced plastic (GFRP) composite materials are finding increased application in aeronautical, automobile and structural applications. Drilling is a complex process, owing to their tendency to delaminate is used to join composite structures. In the present work, an attempt has been made to develop empirical relationships between the drilling parameters such as fiber orientation angle, tool feed rate, rotational speed and tool diameter with respect to delamination in drilling of GFR–polyester composites. The empirical relationship has been developed by using response surface methodology. The developed model can be effectively used to predict the delamination in drilling of GFRP composites within the factors and their limits are studied. The result indicated that the increase in feed rate and drill diameter increases the delamination size whereas there is no clear effect is observed for fiber orientation angle. The spindle speed shows only little effect on delamination in drilling of GFR–Polyester composites. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction GFR–polyester composites materials are finding increased applications in many engineering fields due to their high strength to weight ratio and has excellent resistance to corrosion and have good impact properties [1]. Drilling is the complex machining process due to the variation in geometrical parameters change along the cutting edge. During drilling problems such as fiber pull-out, thermal degradation, fiber/matrix debonding, matrix cracking can occur due to non-homogeneous nature of fiber reinforced plastic FRP-composites [2]. The delamination analysis in drilling of composite materials is an important concern and has to be maintained at minimal. The analysis in drilling of FRP composite material is carried out by many researchers. Campos Rubio et al. [3] have conducted experiments on GFRP composites and concluded that HSM (High Speed Machining) is suitable for drilling GFRP composites ensuring low damage levels. Hocheng et al. [4] have studied the various drill types such as saw drill, candle stick drill, core drill and step drill on thrust force and delamination in drilling GFRP composites. They have analyzed the critical thrust force at the onset of delamination and compared with twist drill. Chen and Tsao [5] have investigated the cutting performance of different coated twist drills experimentally. The experimental results indicated that the thrust force is the indicator of delamination. The drilling process is not significantly different for the various coated drills under the same drilling conditions. Mohan et al. [6] have studied the ⇑ Corresponding author. Tel.: +91 9790636943. E-mail addresses: [email protected], [email protected] (T.V. Rajamurugan). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.08.047

delamination analysis using signal to noise ratio in drilling of GFRP composites. They concluded that the use of high cutting speed and low feed favor the minimum delamination. Khashaba et al. [7] have studied the delamination analysis in drilling of chopped composites. They have observed that the delamination size decreases with decreasing the feed, whereas no clear effect of the cutting speed on the delamination size is observed. Arul et al. [8] have carried out drilling studies on GFRP composites. He has analyzed the delamination using linear elastic fracture mechanics, classical plate bending theory and the mechanics of composites. They have observed that the delamination is directly related to the thrust force during drilling, which in turn is a function of process parameters; feed, speed and the status of the cutting edge. From the above studies, it has been found that delamination in drilling is an important concern which has to be analyzed. For analyzing delamination experimental design scheme is used. Kilickap [9] conducted drilling experiments based on L16 Taguchi’s experimental design method on GFRP composites and determined the optimal condition of cutting parameters. Gaitonde et al. [10] conducted experiment to minimize the delamination at entry and exit in Medium Density Fiber(MDF) composites and also studied the interaction effects of the machining parameters. Palanikumar et al. [11] have conducted experiment by using high-speed steel-made twist drill and 4-flute cutter and have used regression and Analysis of Variance (ANOVA) for analysis. Bernasconi et al. [12] reported the variation of the fatigue strength as a function of specimen fiber orientation. Tsao and Hocheng [13] have studied the thrust force associated with the distributed peripheral moment causing the delamination in drilling of composites. Zitoune et al. [14] conducted experiments on Carbon Fiber Reinforced Plastic (CFRP)/Al stack using carbide K20 drills

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T.V. Rajamurugan et al. / Materials and Design 45 (2013) 80–87 Table 1 The layout of central composite rotatable design. S. No.

Design A(X1)

B(X2)

C(X3)

D(X4)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 2 2 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 2 2 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 2 2 0 0 0 0 0 0

A2 ðX 21 Þ

B2(X2)

C2(X3)

D2(X4)

AB(X1X2)

AC(X1X3)

AD(X1X4)

BC(X2X)

BD(X2X4)

CD(X3X4)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 4 4 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 4 4 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 4 4 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 4 4 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Table 2 Properties of fiber and resin. Fiber/ resin

Tensile modulus (E) (GPa)

Tensile strength (r) (MPa)

Density (q) (g/ cm3)

Shear modulus

Ultimate elongation (%)

E-glass Polyester

69 3000

2400 50

2.6 1.10

27 -

2

Table 3 Parameters and ranges used for drilling experiments. S. Parameter No.

Notation Unit

1 2

Spindle speed Tool feed rate

v f

3 4

Drill diameter d Fiber orientation h angle

Levels (2) (1)

rpm 500 mm/ 50 min mm 4 degrees 0

(0)

(1)

(2)

875 1250 1625 2000 112.5 175 237.5 300 6 22.5

8 10 45 67.5

achieve good quality holes. It is well known from the literature, that drilling parameters plays a major role in the hole quality. Delamination, is a major problem encountered during machining of glass fiber reinforced polyester composites and it has to be modeled. From the studies of previous literature, it has been known that there is no systematic modeling and analysis of delamination in drilling of GFR–polyester composites is carried out. In the present work, empirical relations are established between the drilling parameters and delamination. The empirical equation simulating the drilling process, is carried out by response surface methodology. The results revealed considerable information about the effect of process and tool parameters for minimizing delamination in drilling of GFRP composites.

12 90

and evaluated the influences on thrust force, torque and surface finish. They reported that quality of hole can be improved with proper selection of cutting parameters. In drilling of composite materials by automated drilling process, selection of parameters and experiments are determined by trial and error, based on handbook values, and/or manufacturers’ recommendations. The selection may not yield good quality holes in the vicinity of drilling performance. Furthermore, it leads to more men and machine time. Besides, in the Computer Numeric Control (CNC) controlled drilling process, even smaller changes in the drilling process parameters may cause unexpected quality of holes. Therefore, it is important to study the drilling parameters to

2. Response surface methodology Response-surface methodology (RSM) is one of the important technique in statistics used to determine the relationship between the effect of process parameters on the coupled responses [15–17]. In many experimental conditions, it is possible to represent independent factors in quantitative form as given in Eq. (1)and these factors can be thought of having a functional relationship or response as follows:

Y ¼ Uðx1 ; x2 ; . . . ; xk Þ  er

ð1Þ

where U is called response surface or response function. x1, x2, . . . , xk are quantitative process variables and er measures the experimental error. In the present work response-surface method is used to establish the mathematical relation between the output response ‘Y’ and the various parameters in the drilling of GFRP composites. Representing the Delamination factor by Fd, the response is a function of spindle speed (v), tool feed rate (f), drill diameter (d), fiber orientation angle (h), and it can be expressed as

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3. The third part consists of extra seven center points to give roughly equal precision for response within a circle of radius 1. Table 1 shows the coded conditions used to form the central composite rotatable design in the design of experiments. The design is formed based on the above three points. The variables are coded (0) as a center and (2) is regarded as lowest and (+2) is highest. The 31 experimental runs allowed the estimation of the linear, quadratic and two way interactive offers of the variables on the response [18]. For the convenience of recording and processing the experimental data, the upper and lower levels of the parameters are coded as +2 and 2. The coded value of any intermediate levels can be calculated by using the following expression [18,19].

Xi ¼

½2X  ðX max þ X min Þ X max X min

ð4Þ

2

where Xmax is the upper level of the parameter, Xmin is the lower level of the parameter and Xi is the required coded values of the parameter of any value of X from Xmin to Xmax. 3. Experimental procedure Fig. 1. Close-up view of VMC and typical drill bit used.

3.1. Specimen preparation The GFR–polyester specimen used for this investigation is prepared by using hand lay-up technique. Unsaturated polyester resin matrix reinforced with 60% weight E-glass fiber is used for preparation and the thickness is approximately 10 mm. The density of the polymer used is 1.10 g/cm3. The resin used for the fabrication of composite material is polyester and the hardner used is methyl ethyl ketone peroxide and accelerator is Kerox C-20. The specification of fiber used is E-glass with fiber density 2.6 g/cm3, fiber diameter 16 lm, and the dimension of the specimen is having length180 mm, breadth 90 mm, and height 12 mm. The properties of the fiber and resin are presented in Table 2. 3.2. Drilling tests

Fig. 2. Scheme of the delamination factor.

F d ¼ f ðv ; f ; d; hÞ

ð2Þ

The general quadratic response-surface model, used to evaluate the parametric effects is as follows:

Y ¼ b0 þ

X

bi xi þ

X

2

bii xi þ

X

bij xi xj

ð3Þ

where b0 is the coefficient for constant term and bi, bii, bij are the coefficients for linear, square and interaction terms respectively. For analyzing the drilling parameters, owing to the slightly wider ranges of the parameters, five levels, central composite, rotatable design is used. Central composite rotatable design is one of the important design methods used in RSM which is used to establish the 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 parameters considered is four. The design is subdivided into three parts as shown below [18–21]. 1. The first part consisting of 2k factorial design having 16 points (k = the number of process parameters). 2. The second part consists of extra points to form a CCD with a. The figure formed by these points is called as a star. The value of a must be 2k/4 in order to make the design rotatable. The number of star points is 8 for four machining parameters.

Delamination in drilling is an important concern, it affects the workpiece quality consequently the joint conditions in the composite structures. During drilling many factors affect the delamination. Feed rate is an important factor, which affects the quality of drilled holes [11]. The spindle speed also has some effect on delamination. Apart from the feed and speed, the drill diameter and orientation of the fiber in the composite materials also have influence on the machining of composites. Based on the literature and the previous work done on this field by authors [22,23], the independently controllable predominant machining parameters that have greater influences on the delamination in drilling GFRP composite have been identified. Detailed analysis has been carried out to for fixing the limits of the parameters. The parameters used and their ranges are presented in Table 3. The drilling experiments are conducted on GFR–polyester composites in Computer Numeric Control (CNC) Vertical Machining Center (VMC 100). The machining center has a maximum spindle speed of 5000 rpm with a table size of 1270  254 mm. The experiments have been carried out using ‘‘Brad & Spur’’ cemented carbide (K10) drill of five different diameters having Twist Length 300 , Overall Length 500 and Shank size ½0 , supplied by ‘‘wood Tech Enterprises, USA’’. The close-up view of the vertical machining center used for conducting the experiments and the typical drill used for the experimentation are presented in Fig. 1. The machining operations are carried out as per the condition given by the design matrix (Table 1) at random to avoid systematic errors. Delamina-

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T.V. Rajamurugan et al. / Materials and Design 45 (2013) 80–87 Table 4 The parameters used, their coded values, real values and output response. Exp. No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Input parameters Coded value

Output response

Real value

Spindle speed (rpm)

Tool feed rate (mm/min)

Drill diameter (mm)

Fiber orientation angle (°)

Spindle speed (rpm)

Tool feed rate (mm/min)

Drill diameter (mm)

Fiber orientation angle (°)

Delamination factor

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 2 2 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 2 2 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 2 2 0 0 0 0 0 0

875 1625 875 1625 875 1625 875 1625 875 1625 875 1625 875 1625 875 1625 500 2000 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250

112.5 112.5 237.5 237.5 112.5 112.5 237.5 237.5 112.5 112.5 237.5 237.5 112.5 112.5 237.5 237.5 175 175 50 300 175 175 175 175 175 175 175 175 175 175

6 6 6 6 10 10 10 10 6 6 6 6 10 10 10 10 8 8 8 8 4 12 8 8 8 8 8 8 8 8

22.5 22.5 22.5 22.5 22.5 22.5 22.5 22.5 67.5 67.5 67.5 67.5 67.5 67.5 67.5 67.5 45 45 45 45 45 45 0 90 45 45 45 45 45 45

1.76 1.42 1.72 1.54 1.67 1.45 1.74 1.58 1.44 1.50 1.52 1.74 1.53 1.64 1.62 1.74 1.67 1.57 1.34 1.68 1.46 1.60 1.58 1.62 1.78 1.79 1.64 1.72 1.67 1.70

Table 5 Analysis of variance for response surface regression model. Source

Sum of squares

DF

Mean square

F value

Prob > F

Regression Linear Square Interaction Residual error Lack of fit Pure error Cor total

0.360178

14 4 4 6 15 10 5 29

0.025727 0.033589 0.023885 0.024065 0.002984 0.002703 0.003547

8.621977817

<0.0001

0.761983083

0.6668

0.044758 0.027025 0.017733 0.404937

tion is a damage phenomena, which occurs due to the anisotropy and brittleness of composite materials. The damage around the holes (delamination) has been measured by using tool maker microscope. The delamination factor (Fd) is determined by the ratio of the maximum diameter (Dmax) of the delamination zone to the hole diameter (D). The scheme is shown in Fig. 2. The value of delamination factor can be expressed as follows [24]:

Fd ¼

Dmax D

ð5Þ

The parameters used, their coded values, real values and output response are presented in Table 4. 4. Modeling of drilling parameters using RSM Modeling of process parameters is carried out using response surface regression analysis as discussed earlier. Representing the delamination factor of the GFR–polyester composites ‘‘Fd’’, the response function can be expressed as Fig. 3. Normal plot of residuals.

Significant

Not significant

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Fig. 4. Typical holes observed in drilling GFR–polyester composite laminates.

The adequacy of the model is checked by using the Analysis of Variance (ANOVA) technique in statistics and it is shown in Table 5. Fischer’s F-test is used to analyze the data. From the calculated results, the calculated value of the F-ratio in the developed model is more than the standard tabulated value of F-ratio at a confidence interval of 95% and hence the model is set to be adequate. Further the ‘‘Probability > F’’ is <0.0001, which indicates the developed model is highly adequate at the confidence interval selected. The lack of fit was found to be not significant, hence the developed model may be accepted. The experimental data and the predicted data by the using aforesaid model are analyzed by using residual analysis. Fig. 3 shows the normal probability plot for residuals. Normal probability plots are the plots based on the central limit theorem [26]. If the residuals are grouped together in the center-line then the values are normally distributed. In Fig. 3, all the values are distributed towards the central line and hence the model is adequate at confidence interval selected.

Fig. 5. SEM photograph of the chip.

F d ¼ f ðv ; f ; d; hÞ

ð6Þ

The model chosen was a second degree response surface expressed as follows:

5. Results and discussion

F d ¼ b0 þ b1 ðv Þ þ b2 ðf Þ þ b3 ðdÞ þ b4 ðhÞ þ b5 ðv 2 Þ þ b6 ðf 2 Þ

5.1. Delamination analysis

2

þ b7 ðd Þ þ b8 ðh2 Þ þ b9 ðv f Þ þ b10 ðv dÞ þ b11 ðv hÞ þ b12 ðfdÞ þ b13 ðf hÞ þ b14 ðdhÞ

ð7Þ

The values of the coefficients have been calculated by regression with the help of the following equations [17,25]:

X X ðYÞ  0:035714 ðX ii YÞ;

ð8Þ

X ðX i YÞ;

ð9Þ

b0 ¼ 0:142857 bi ¼ 0:041667

X X ðX ii YÞ þ 0:003720 ðX ii YÞ  0:035714 bii ¼ 0:03125 X  ðYÞ; bij ¼ 0:0625

ð10Þ

X ðX ii YÞ:

ð11Þ

The model developed based on the above is given as follows:

Delamination factor; F d ; ¼ 1:71667  0:0245833  v þ 0:0612500  f þ 0:0254167  d  0:00291667  h  0:0182292  v 2 2

 0:0457292  f 2  0:0407292  d

 0:0232292  h2 þ 0:0243750  v  f þ 0:00562500V  d þ 0:0881250V  h  6:25000E  04f  d þ 0:0143750f  h þ 0:0206250d  h

ð12Þ

Drilling of composites structures are very much needed for the industry. Drilling of composite materials is different from the conventional materials. Delamination, fuzzing, protruded fibers, incomplete removal of the fibers are some of the damages occurred in drilling. Delamination is the most critical and it leads to the reduction of bearing strength and also reduce the structural integrity of the material [11]. Fig. 4 shows the macrograph of the holes observed in drilling of GFR–polyester composites. Fig. 4a indicates the in-complete removal of fibers in the exit side and Fig. 4b shows the protruded fibers observed in drilling of GFR–polyester composites. To achieve better holes, controlling of process parameters such as spindle speed, feed rate and drill diameter are needed for the industry. The machinability of GFR–polyester composite material when drilling at different cutting conditions using Brad and Spur bit was studied. The experiments were performed and the results were obtained. Design expert software was used to derive polynomial models for Delamination factor. Fig. 5 shows the micrograph of the chip observed in drilling of GFR–polyester composites. Figure indicates the sheared and powdery fibers combined with the polyester matrix and incomplete machined fibers. Fig. 6 shows the images observed through scanning electron microscope (SEM) of the glass fiber reinforced polyester composite material work piece in drilling. The images are observed at the cut sections of the drilled holes. The micrograph in Fig. 6a cut section of the hole in which the sheared fibers and matrix materials is seen. In drilling, due to the heat generation, the matrix materials are converted into the lumped masses along with the fibers. Fig. 6b shows

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85

Fig. 7. Predicted vs. actual plot.

to the top surface of the drilled hole. In figure, the distribution of matrix materials is uniform in some places and other places the distribution is not so. It is due to the un-even distribution of fibers in manufacturing and fiber pull-out during the machining operation [17]. Fig. 6c illustrates close-up view of the distribution of fibers at right angles. Fig. 6d shows the un-even distribution of matrix materials and fibers in the hole. Due to the thrust force induced during the drilling operation, the fiber and matrix materials is pulled out and the surface is shown like in figure. The reason being, high abrasiveness of the glass fibers make the tool encounters fluctuation of forces, which leads to the peeling and fiber pullout [27]. 5.2. Effect of drilling parameters on delamination

Fig. 6. SEM photographs of the hole drilled at different tool feed rate.

the micrograph of the hole in the different direction. The figure shows the machined surface along with tiny fiber particles stick

In drilling of GFR polyester composites, delamination takes place on the periphery of the holes due to the thrust force, which induced on the plate through the drill bit. This is due to the inhomogeneous nature of the GFRP, i.e. soft polyester matrix and hard E-glass fiber [27]. It is a serious problem for the quality and control department, which lead to the rejection of the product. Engineers often wish to determine the values of the input process parameters at which the responses reach their optimum. The optimum could be either a minimum or a maximum of a particular function in terms of the input parameters. Among the other optimization technique, RSM found to be good. Response surface methodology (RSM) is a collection of mathematical and statistical technique useful for analyzing problems, in which several independent variables influence a dependent variable or response and the aim is to optimize the response [18].The experimental results and predicted results obtained from the model is plotted in Fig. 7. The figure indicates the close relationship between the experimental results and predicted results, hence the model is said to be adequate at 95% of confidence level and this model can be used for the prediction of delamination factor in drilling of GFR–polyester composites. Fig. 8a–f presents three-dimensional response surface plots (without considering 2 and +2 level) for the response. Delamination factor obtained from the regression model and the optimum delamination factor is exhibited by the apex of the response surface. Fig. 8a shows the effect of spindle speed and feed rate on delamination factor in drilling of Glass fiber reinforced polymers. The figure indicates that the increase of spindle speed slightly reduces the delamination factor in drilling of composites. The reason is, the increase of spindle speed increase the temperature produced in dril-

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Fig. 8. Response graphs.

ling of composites, which softens the matrix material and shearing, inturn the delamination is reduced. The result presented is correlated with the results presented by Palanikumar et al. [28], in which glass fiber reinforced epoxy composite is used as workpiece material. The increase of feed rate increases the delamination factor due to the increase of thrust force in drilling. The result pre-

sented is almost similar to the result presented by [11]. Fig 8b shows the effect of spindle speed and drill diameter in drilling of composites. The figure shows almost the same trend for spindle speed as explained previously. The increase of drill diameter increases the delamination factor in drilling of composites materials. The reason is, the increase of drill diameter increases the

T.V. Rajamurugan et al. / Materials and Design 45 (2013) 80–87

contact area of the hole produced which increases the thrust force in drilling of composites [23]. The increase in thrust force leads to the increase of delamination [29]. Fig 8c shows the effect of spindle speed and fiber orientation angle in drilling of GFR–polyester composites. The variation of fiber orientation angle does not shows the proper trend. The variation almost increases the delamination factor in drilling of GFR–polyester composites. Fig. 8d shows the interaction between the drill diameter and feed rate in drilling of GFRP composites. The increase of feed rate and drill diameter increases the delamination factor and vice-versa. The interaction effect between the feed rate and fiber orientation angle is presented in Fig. 8e. The result indicates the increase of feed rate can increase the delamination factor in drilling, which is more than that observed in case of fiber orientation angle. The effect of fiber orientation angle and drill diameter in drilling of GFRP composites is presented in Fig. 8f. The figure indicates almost the same trend as discussed earlier. In the previous study by the authors [22], they have considered thickness of the specimen instead of fiber orientation angle. Even in this case, feed rate is the dominant factor which influences the delamination in drilling of composites. From the figures, it is concluded that the maximum spindle speed, minimum feed rate, probably minimum drill diameter and moderate fiber orientation angle are preferred. Among the factors studied, feed rate is the highly influential parameter, which affects the hole quality. This finding is close to the findings presented by Tsao and Hocheng [30].

6. Conclusion The experiments are designed through the experimental design method and executed in a vertical machining center. The tool used for the experimentation is Brad and spur drill bit. Second-order polynomial model is developed using the response delamination factor. The four important input variables considered for the present research study is spindle speed, tool feed rate, drill diameter, and fiber orientation angle. The influences of all machining parameters on delamination factor have been analyzed based on the developed mathematical model. The following conclusions are drawn based on this study. 1. The results indicated that the delamination factor increases with the increase of feed rate. 2. The delamination factor decreases slightly with the increase of spindle speed. 3. The variation of fiber orientation angle does not show a general trend. Normally the increase of fiber orientation angle almost increases the delamination factor in drilling of GFR–polyester composites. 4. The increase of drill diameter increases the delamination factor in drilling composites materials due to the increase of thrust force. Instead of using bigger holes similar smaller holes may be used. 5. The experimental results indicate that proper selection of cutting parameters improves the performance in drilling. 6. Mathematical relations are established between the cutting parameters and response delamination factor using response surface method. The models are found to be fit at 95% confidence level within the levels considered. 7. Tool feed rate is the most influential parameter which influences the delamination factor in drilling composite materials followed by drill diameter. 8. By increasing the number of factors and levels, the results may be improved further.

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