Indian J.Sci.Res. 2(1) : 173-179, 2014 ISSN : 2250-0138 (Online)

ISSN: 0976-2876(Print)

__________________________________ 1Corresponding author

OPTIMIZATION OF DRILLING PARAMETERS IN COMPOSITE SANDWICH STRUCTURES (PVC

CORE)

P. GHABEZI1a, M. KHORAN

b

a1 Young Researchers and Elites club, Science and Research Branch, Islamic Azad University, Tehran, Iran

b Lecture of Esfarayen University, Esfarayen, Iran

ABSTRACT

The objective of this work is to investigate the influence of cutting speed, feed rate, and tool diameter on the uncut fiber

and delamination damage of three types of composite sandwich structures including PVC foam and faces made of

Glass/polyester. A design of experiments (full factorial) was used to assess the importance of the drilling parameters, and

digital photography technique was used to evaluate the damages from drilling. The drilling operation was assessed based

on two introduced factors including the delamination factor (DF) and uncut fiber factor (UCFF). Analysis of the

experimental results for DF indicated that feed rate and drill diameter were the most significant and insignificant

parameters, respectively. But experimental results for UCFF showed that feed rate has greatest influence. Also, the results

revealed that the both factors increase with the increase of feed rate and there is an optimum point for cutting speed and

tool diameter in evaluation of UCFF. Whereas increasing tool diameter leads to decrease of DF and generally in variation

of cutting speed for UCFF, there is a maximum point.

KEYWORDS: Cutting Speed, Drilling, Feed Rate, Panel Sandwich, Delamination.

Sandwich panels have some advantages such as ability to

provide high bending stiffness, buckling and fatigue strength

and light weight structure. Specific properties of composite

sandwich structures are high strength to weight ratio in

comparison with other laminate composites. Drilling of

composite material is one of the most frequently used to make

holes for screws, rivets and bolts for mechanical joints in the

aerospace applications. Mechanism of drilling of composite

reinforced parts is different from that of isotropic and

homogeneous material such as metal and conventional

materials. During drilling of composites occur various

damages in the hole. Among them, fiber pull-out, fiber

breakage matrix cracking and delamination are the most

significant. Absolutely, delamination is the major damage in

the drilling process that is what happens in a laminate when its

layers begin to separate in the entrance and exit of the hole.

Delamination occurs during drilling of composite laminate by

two distinguish mechanisms: peeling up of the top layer and

pushing out in the bottom layer (Mohan et.al, 2007). However,

there is a difference between delamination in the punching of

composite sheets and sandwich panels; in a sandwich panel,

because of existing two laminates in the top and bottom faces,

there are four damaged regions (peeling up and pushing out in

both top and bottom layers) in the delamination form while

there is two regions in drilling of a composite sheet. Figure 1

shows different mechanisms of delamination in the sandwich

structures that considered in this paper.

Figure 1: different mechanism and regions of delamination in a composite sandwich panel.

Near the exit of the hole, delamination damage is due to

separating the thin uncut layer (the bottom layers as shown in

Figure 1) from the remainder of the laminate. This damage

leads to weaken of the structure, so investigation on

delamination in the composite sandwich panels is a critical

issue. Liu et al. summarized an up-to-date progress in

P. GHABEZI AND M. KHORAN : OPTIMIZATION OF DRILLING PARAMETERS IN COMPOSITE SANDWICH STRUCTURES (PVC CORE)

Indian J.Sci.Res. 2(1) : 173-179, 2014 - 174 -

mechanical drilling of composite laminates including

vibration-assisted twist drilling, grinding drilling, conventional

drilling, high speed drilling, delamination, bit geometry and

materials, tool wear and thrust force (Liu et al., 2012). Singh

and Sharma presented a mathematical model to show step-by-

step procedure to capture complex drilling dynamics (Singh

and Sharma, 2013). Bogdan and Xirouchakis tested wide

range of cutting parameters and determined the cutting loads

distribution (axial and tangential) along the work-piece

thickness and tool radius by analyzing the thrust and torque

curves when drilling with 3 different drills carbon-fiber

(CFRP) and glass-fiber (GFRP) reinforced composite plates

(Bogdan and Xirouchakis, 2011). Rajmohan and Palanikumar

investigated the application of response surface methodology

and central composite design for modeling, optimization, and

analyzing burr height, thrust force and surface roughness in

the drilling of hybrid metal matrix composites (Rajmohan and

Palanikumar, 2013). Wang et al. used uncoated, diamond

coated and AlTiN coated carbide (WC–Co) drills and

investigated the wear of certain coated drills when drilling

carbon fiber reinforced composites (CFRP) (Wang et al.,

2013). In this paper different types of cores (PVC foam, Balsa

wood and corrugated-foam) in a sandwich panel is considered

under various machining conditions (cutting speed, feed rate

and tool diameter). The objective of this research is finding the

best machining condition to made a hole in all three sandwich

panels using define a dalamination factor (DF) and uncut fiber

factor (UCFF), and comparison of hole quality in them to find

best core for fabricating sandwich panels.Text of the

introduction.

EXPERIMENTS

Materials

In this study used a specific unsaturated polyester resin

suitable for VARTM process with the viscosity of 90–120 MP

as mixed with 0.01% cobalt naphthenate as accelerator and

1.25% methyl ethyl ketone peroxide as initiator. Also in this

paper used Polyvinyl chloride (PVC) foam, AIREX C70.75,

with thickness of 10mm and nominal density of 80 kg/m3, as

core in manufacturing of composite samples. Both face sheets

of the sandwich panels are made of six layers of bidirectional

(0/90) E-galss fabrics with the surface density of 200 g/m2.

Manufacturing process

All specimens were fabricated using VARTM process. Details

of this process are shown in Fig. 2.

Figure 2: detail of VARTM process; 1- vacuum pump, 2- safety tank, 3- indicator, 4- distribution layer, 5- Dacron peel ply, 6-

vacuum bag, 7- resin transfer tubes, 8- valves and joints, 9- release Wax, 10- seal paste.

Tools

The drilling process were carried out on the all of sandwich

specimens, using a Twist drill bit made of high speed steel

manufactured to DIN 338, 4, 7 and 9 mm diameter, a 118°

point angle and 43, 69 and 81mm working length respectively.

A universal milling machine DECKEL FP4M with 4-kW

spindle power, spindle speed 50 to 2500 rpm, feed speed 8-

630 mm/min (x/y/z) and Tilting table 800*460 mm was used

to perform experiments (Fig. 3).

Indian J.Sci.Res. 2(1) : 173-179, 2014 ISSN : 2250-0138 (Online)

ISSN: 0976-2876(Print)

__________________________________ 1Corresponding author

Figure 3: Universal milling machine DECKEL FP4M.

Drilling

The drilling tests were conducted on PVC foam core sandwich

samples at three spindle speeds of the drill 500, 1600 and 2500

rpm, at feed rates of 50, 200 and 400 mm/min for drill

diameter of 4, 7 and 9 mm. All tests were run without using

coolant during drilling process. For evaluation of delamination

damage around and into the holes used digital photography

technique mean a Cyber-shot™ camera with 8.1 MP

resolution and auto-focus ability. The typical drilled holes are

presented in Fig. 4.

Figure 4: Drilled holes of sandwich composite samples.

Delamination And Uncut Fiber Factors

There are different types of damage in a drilling process of

composite reinforced material such as matrix cracking, fiber

pull-out, uncut fiber and matrix, fiber breakage and

delamination. Delamination in one of the most important of

them and have two general types: peel ply and push out. In the

other hand, for both of them there are two modes of

delamination damage, one damage fiber around the hole and

the other mode is existing uncut fiber and matrix that is

appeared into the hole, so in this paper to evaluate

delamination damage in the sandwich panels used two factors;

delamination factor (DF) and uncut fiber factor (UCFF). These

factors are defined in the following form (Fig. 5).

UCFF = AI / AHOLE = AI / ΠR2 (1)

DF = AO / AHOLE = AO / ΠR2 (2)

In which, AHole is the diameter of the drill in mm2, Ai is section

area between hole circle and minimum damage zone and Ao is

the area between circle of hole and maximum of the

delamination zone in mm2.

Indian J.Sci.Res. 2(1) : 173-179, 2014 ISSN : 2250-0138 (Online)

ISSN: 0976-2876(Print)

__________________________________ 1Corresponding author

Figure 5: different sections for calculation of delamination factors.

Both the upper and lower surfaces of each specimen were

scanned using digital photography technique. Three different

regions for all holes were marked as shown in Figure , and

then import photos to the Auto Cad engineering software and

by recognizing namely diameter of any hole, calculate area

section of A0 and Ai. Finally derived delamination factors

based on equations 1 and 2.

Design of Experiment (full factorial)

The drill tests for all three types of composite sandwich panels

were planned using the full factorial experiments with three

levels and three factors (

Table 1).

Table 1: Assignment of levels to the factors

LEVEL CUTTING SPEED

(RPM)

FEED RATE (MM/MIN) TOOL DIAMETER

(MM)

1 500 50 4

2 1600 200 7

3 2500 400 9

Error! Reference source not found. show the results of the uncut fiber factor (UCFF) and delamination factor (DF) for the

experimental drill tests, obtained by Eqs 1 and 2 as a function of the cutting parameters including D (mm), V (rpm) and F (mm/min).

Table 2: Experimental results for the defined factors for PVC foam sandwich panels.

TEST TOOL

DIAMETER

D (MM)

CUTTING

SPEED

V (RPM)

FEED RATE

F (MM/MIN)

DF UCFF

PVC FOAM 1 4 500 50 0.495305 0.182715

PVC FOAM 2 4 500 200 0.496896 0.185898

PVC FOAM 3 4 500 400 0.53032 0.193061

PVC FOAM 4 4 1600 50 0.456311 0.199427

PVC FOAM 5 4 1600 200 0.489734 0.205793

PVC FOAM 6 4 1600 400 0.499284 0.21216

PVC FOAM 7 4 2500 50 0.50008 0.169187

PVC FOAM 8 4 2500 200 0.506446 0.180328

PVC FOAM 9 4 2500 400 0.507242 0.18749

PVC FOAM 10 7 500 50 0.496231 0.147944

PVC FOAM 11 7 500 200 0.507404 0.157818

PVC FOAM 12 7 500 400 0.527933 0.180165

PVC FOAM 13 7 1600 50 0.459591 0.149243

PVC FOAM 14 7 1600 200 0.437764 0.176528

P. GHABEZI AND M. KHORAN : OPTIMIZATION OF DRILLING PARAMETERS IN COMPOSITE SANDWICH STRUCTURES (PVC CORE)

Indian J.Sci.Res. 2(1) : 173-179, 2014 - 177 -

PVC FOAM 15 7 1600 400 0.449197 0.176787

PVC FOAM 16 7 2500 50 0.417235 0.155479

PVC FOAM 17 7 2500 200 0.559115 0.1547

PVC FOAM 18 7 2500 400 0.564312 0.147944

PVC FOAM 19 9 500 50 0.521645 0.143132

PVC FOAM 20 9 500 200 0.512527 0.152878

PVC FOAM 21 9 500 400 0.607001 0.163567

PVC FOAM 22 9 1600 50 0.481874 0.145175

PVC FOAM 23 9 1600 200 0.486748 0.142817

PVC FOAM 24 9 1600 400 0.501052 0.164039

PVC FOAM 25 9 2500 50 0.504196 0.143289

PVC FOAM 26 9 2500 200 0.553241 0.151149

PVC FOAM 27 9 2500 400 0.579021 0.152406

EXPERIMENTAL RESULTS AND DISCUSSION

The engineers often interest to realize the values of the

optimum input process parameters to reach desirable quality

during drilling operations. This optimum condition could be

either a minimum or a maximum of the input parameters. To

produce better holes, controlling of process parameters such as

cutting speed, feed rate and drill diameter are needed for the

industrial applications. The machinability of composite panel

sandwiches made of Glass/Polyester with core of PVC foam

were studied when drilling at different cutting conditions.

In this section, the effect of drilling parameters (cutting speed,

feed rate, drill diameter and core material) on DF and UCFF in

drilling of composite materials is investigated by using the

graphs and bar diagrams.

Fig. 6 show the effect of drill diameter on DF and UCFF for

sandwich panel samples (in the horizontal axis, first and

second numbers show cutting speed (rpm) and feed rate

(mm/min), respectively). The figure indicate that the increase

of drill diameter lead to a little decrease in the delamination

factor but for D=9mm this factor is increasing. In the other

hand, high diameter decreases the uncut fiber factor in drilling

of sandwich composites, generally. So there is an optimum

drill diameter for machining of these types of composite

sandwich panels. The load on the tool and contact between the

tool and composite sample are rising by increase of drill

diameter, but load to area ratio (pressure) decrease, which

decrease the damage of laminates including fiber and matrix,

hence the uncut fiber factor is reduced.

A B

Figure 6: Effect of drilling diameter on DF and UCFF in sandwich samples.

Effects of cutting speed (500, 1600 and 2500 rpm) and feed

rate (50, 200, 400 mm/min) on DF and UCFF are shown in the

Fig. 7. From the Fig. 7, it is observed that effect of feed rate is

more significant than cutting speed. It seems that cutting speed

has an insignificant effect on the both defined delamination

and uncut fiber factor. But according to the results, it can be

derived that there are optimum and maximum points for DF

and UCFF by increasing cutting speed, respectively.

Generally, the DF in the cutting speed at level 1 (1600 rpm) is

lowest, whereas about UCFF has an inverse behavior. First,

increasing cutting speed, material removal will be ease

because of softening of the matrix phase. The delamination

slightly increases at higher drill speeds because the drilling

time decreases and the transverse vibration of the tool

increase. Whereas, the delamination in the drilling of laminate

composites has a tendency to decrease with increasing spindle

speed (Sardinas et al., 2006).

P. GHABEZI AND M. KHORAN : OPTIMIZATION OF DRILLING PARAMETERS IN COMPOSITE SANDWICH STRUCTURES (PVC CORE)

Indian J.Sci.Res. 2(1) : 173-179, 2014 - 178 -

The figure show that feed rate 50mm/min is the best option to

obtain a hole with low clearance. The feed rate is the

significant parameter affecting the UCFF and push down

delamination (DF).

Figure 7: Effects of cutting speed and feed rate on DF and UCFF.

CONCLUSION

The experimental investigation is carried out to analyze two

criterions of delamination and uncut fiber factors in the

drilling of PVC foam composite sandwich panels. The effect

of cutting speed, feed rate and tool diameter on the hole

quality is analyzed. Fig. 8 show the mean values of DF and

UCFF versus different cutting parameters for PVC core

materials.

Indian J.Sci.Res. 2(1) : 173-179, 2014 ISSN : 2250-0138 (Online)

ISSN: 0976-2876(Print)

__________________________________ 1Corresponding author

Figure 8: mean values of UCFF and DF for PVC Foam sandwich specimens.

According to the experimental data, the following conclusions

are drawn:

1. Based on the experimental results, feed rate is the

factor that has the greatest influence on the DF, followed by

cutting speed and tool diameter.

2. Based on the experimental results feed rate is the

factor that has the greatest influence on the UCFF, followed

by tool diameter and cutting speed.

3. The results indicated that the DF and UCFF increase

with the increase of feed rate.

4. The results revealed that there is an optimum point

for drill diameter and cutting speed parameters for DF (level

2).

5. UCFF decreases with the increase of drill diameter.

6. The results showed that there is a maximum point

(level 2) for cutting speed in evaluation of UCFF.

7. By increasing the number of levels and factors, the

results may be improved further.

8. Best quality of hole based on the delamination factor

was given in condition: feed rate=50 mm/min (level 1), cutting

speed=1600 rpm (level 2) and drill diameter=7mm (level 2).

9. Best quality of hole based on the uncut fiber factor

was given in condition: feed rate=50 mm/min (level 1), cutting

speed=2500 rpm (level 3) and drill diameter=9mm (level 3).

10. Worst quality of hole based on the delamination

factor was given in condition: feed rate=400 mm/min (level 3),

cutting speed=500 rpm (level 1) and drill diameter=9mm

(level 3).

11. Worst quality of hole based on the uncut fiber factor

was given in condition: feed rate=400 mm/min (level 3),

cutting speed=1600 rpm (level 3) and drill diameter=4mm

(level 1).

12. Generally, the delamination factor is more than uncut

fiber factor for composite sandwich structures.

ACKNOWLEDGEMENTS

This paper carried out with finance help Esfarayen

university.

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