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T E C H N I C A L A R T I C L E

Measuring Delamination Severity of Glass Fiber-ReinforcedEpoxy Composites During Drilling Process

V.A. Nagarajan1, S. Sundaram2, K. Thyagarajan3, J. Selwin Rajadurai4, and T.P.D. Rajan5

1 Faculty of Mechanical Engineering, Anna University, Tirunelveli, Nagercoil, India2 Department of Manufacturing Engineering, Annamalai University, Chidambaram, India

3 Department of Mechanical Engineering, Noorul Islam College of Engineering, Kumarcoil, India

4 Department of Mechanical Engineering, Government College of Engineering, Tirunelveli, India

5 National Institute for Interdisciplinary Science and Technology, CSIR, Trivandrum, India

Keywords

Drilling, Delamination Severity, MATLAB,

Glass Fiber Epoxy Composites

Correspondence

J. Selwin Rajadurai,Department of Mechanical Engineering,

Government College of Engineering,

Tirunelveli, Tamilnadu, India

Email: [email protected]

Received: June 27, 2011; accepted:

December 3, 2011

doi:10.1111/j.1747-1567.2012.00809.x

Abstract

Glass fiber-reinforced epoxy composites are one of the potential light-

weight structural materials used in various engineering applications due

to its excellent properties. Drilling is most widely applied for fastening the

composite structures; nevertheless, the damage induced by this operation may

reduce the component performance drastically. To establish the damage level,

delamination is measured quantitatively using digital imaging techniques. In

this study, to quantify the delamination severity effectively, a new refined

delamination factor (FDR) is proposed and validated using experimental results

observed from three-point bend tests (3PT) and modified short beam shear tests.

The value of determined refined delamination factor (FDR) is more accurate

compared to the calculated conventional (FD) and adjusted (FDA) delamination

factors.

Introduction

Application of composite materials is dominating

in engineering field due to good specific strength,

stiffness, fatigue limit, light weight, and near net shape

production technique available for the processing,

molding, and curing of fiber-reinforced plastics

(FRP) to achieve the desired tolerances.1 One of

the main difficulties associated with drilling of

composite material is delamination failure. According

to Khashaba,2 delamination is one of the main

reasons for the rejection of approximately 60%

of the composite components produced in aircraft

industries. When the stresses induced in the layers of

the laminate during the drilling operation exceed theinterlaminar strength of the laminate, delamination

failure occurs. The influence of factors such as tool

geometry and machining parameters on delamination

has been studied by several researchers. Nevertheless,

few authors have approached both tool geometry

and high-speed machining (HSM) when drilling

composites, more specifically glass fiber composites.

Even though many researchers have attempted

on the effect of tool geometry and machining

parameters on delamination, only very few have

focused on the same with drilling of composite

laminates. Influence of different drill geometry on

delamination of laminates fabricated through hand

lay-up technique was investigated by Davim et al.3

The author employed a toolmakers microscope to

evaluate the damage. In that study, the influence of

vibration frequency and amplitude during drilling of

composites was considered. Arul et al.4 express that

the delamination factor as the ratio of maximum

diameter in the damaged zone to the drill diameter.

The results indicated that the damage increases withincrease in both cutting speed and feed rate. Next,

the authors employed an optical microscope coupled

with an image analyzer to study the extent of

defects caused by drilling. In this work, the authors

characterize delamination factor as a ratio of the

maximum diameters in the damaged zone to the

drill diameter. After drilling holes of small diameters,

66 Experimental Techniques 37 (2013) 66 73 2012, Society for Experimental Mechanics


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V.A. Nagarajan et al. Advanced NDT

Aoyama et al.5 concluded that the delamination is

generated along the fiber in the hole wall surface and

it propagates as the surface roughness increases. Apart

from the above, few more works were carried out in

the field of HSM, and attempts were made to obtain

relationship between various controllable parameters

and their influence on quality of drilled hole. Kao

et al.6 investigated the tribological properties of coated

drills against the holes drilled on glass fiber-reinforced

epoxy resin laminates. From the results, the authors

concluded that drills coated with 5% MoS2 Cr

have increased the life of the drill bits, two times

than that of uncoated drills. The influence of tool

point angle, spindle speed, and feed with respect to

delamination was discussed, and the delamination

was also measured digitally in the study of Compos

Rubio et al.7 From the discussion mentioned above,

delamination and surface finish are the two important

variables that need focus during drilling of composite

laminates. These two variables are influenced by otherprocess parameters such as feed rate, cutting speed,

drill geometry, tool wear, and tool material.812

The delamination failure in drilling operation can

occur either at drill bit entry, known as peel-up, or

at the exit of the bit, termed as push-out. Out of

these two delamination mechanisms associated with

drilling of FRP, push-out at the drill exit is more

severe. The key for solving the problem lies in reduc-

ing the thrust force when drilling. Optical microscopy

and scanning and digital photography are the tech-

niques employed to measure the delamination quali-

tatively. The same can be measured quantitatively as

follows. Delamination factor is one such parameter,which is used to characterize the level of damage on

the work material at the entry and exit of the drill. The

delamination factor (FD) may be calculated from the

ratio of the maximum diameter (Dmax) of the delam-

ination zone to the drill diameter (D0) as follows:13

FD =Dmax

D0(1)

Alternatively, the ratio of the delaminated area to

the hole area may also be used. In this case, the

adjusted delamination factor (FDA) is calculated from

Eq. 2, in which the first part represents the size of

the crack contribution (conventional delamination

factor FD) and the second part represents the

damaged area contribution.

FDA = FD +AD

(Amax A0)(F2D FD) (2)

whereAD is the damaged area,Amax is the area related

to the maximum diameter of the delamination zone

(Dmax), and A0 is the area of the nominal hole, which

corresponds to D0.

Even though the delamination is estimated quan-

titatively by various researchers using either delami-

nation factor or adjusted delamination factor, in this

work it was observed that the specimen with lower

adjusted delamination factor gets failed more quickly

than the do specimens with higher adjusted delami-

nation factor. This insists the need for a revision in the

current form of adjusted delamination factor. Hence

in the revised form of delamination factor equation,

in addition to damage zone size, drill diameter and

area correspond to nominal diameter; importance was

given to severity of damages.

Experimental Procedure

Drilling experiments were conducted on a CNC

machining center with 5-kW power. Its spindle speed

range is 2002500 rpm with a resolution of 1 rpm,

and the feed range is from 5 to 200 mm/min. The lam-

inates were produced by the hand lay-up technique

and were made up of epoxy matrix reinforced with

62% weight of woven glass fiber with an orientation

of five layers of [0/45] and two layers of [0/90]

laminates. Fourteen layers of glass fiber were used

resulting in a 9.57-mm-thick laminate. Table 1 shows

the mechanical properties of composite material used

for testing.14

The sized composite laminate of 160 80 mm was

fixed on the machining center using appropriate

clamping device and back plate. Then the laminate

was drilled using a brand new 10-mm end mill cuttermade up of high-speed steel, and the detail is shown

in Fig. 1. Drilling was performed by varying spindle

speeds and feeds. A feed rate of 25150 mm/min in

steps of 25 mm/min was used in the experimental

work. The same set of feed rates was used for spindle

speeds of 1000, 1200, and 1400 rpm.

Eighteen holes were drilled for the specified

cutting parameters for a single cutting tool of an

individual size. In order to account for unforced errors

and damages induced during machining operation,

three holes of same parameter were drilled, so the

Table 1 Mechanical properties of composite material used for testing

Fiber type E-glass 21xK43 Gevetex

Matrix type LY556/DY063 epoxy

Fiber volume fraction, Vf 0.62

Longitudinal modulus, E11 (GPa) 34.41

Transverse modulus, E22 (GPa) 6.53

In-plane shear modulus, G12 (GPa) 3.43

Major Poissons ratio, 12 0.217

Experimental Techniques 37 (2013) 66 73 2012, Society for Experimental Mechanics 67


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Advanced NDT V.A. Nagarajan et al.

(a)

(b)

(c)

Figure 1 Various types of drilling cutters of 10 mm diameter,(a) router,

(b) end mill cutter, and (c) twist drill.

total number of holes required was worked out

to be 54.

Results and Discussion

The damage at the push-out was captured qualita-

tively using a Nikon 300 digital camera (Nikon India

Private Ltd., India) with ST 800 flash. The lighting

environment must be adequate enough to obtain a

good response out of the sensor, but not too excessive

to cause blooming or saturation of the sensor. Series

of fiber-optic point sources lighting were used to

minimize the effects of ambient lighting and simplifyimage processing. The line sketch for the camera setup

is shown in Fig. 2. The size of the damage zone was

measured quantitatively5,1518 using the concept of

neural network MATLAB 7.0 software. Using Eqs. 1

and 2, delamination factor (FD) and adjusted delam-

ination factor (FDA) proposed by various researchers

are calculated and shown in Table 2.

On the other hand, the drilled specimens were

tested by the following tests to confirm and vali-

date the values of FD and FDA. American Society for

Testing and Materials (ASTM) has proposed two test

standards: (1) three-point bend test (3PT) (D2344)

involves the use of a three-point flexure specimen to

measure interlaminar shear stress of laminated com-

posites subjected to transverse loads19 and (2) the

modified short beam shear (MSBS) test (ASTM D790)

is also used to estimate the interlaminar shear stress

but in MSBS test, in between loading head and

specimen one rubber sheer and one stiffer plate alu-

minum are placed. Main purpose is to make the point

Camera Stand

Digital Camera

Series of Lights

Laminate

Specimen

Computer

Figure 2 Line sketch for the camera setup.

load as uniformly distributed load. Here, specimen

was subjected to uniformly distributed load. That is

why the specimen fails in small amplitude of loadcompared with 3PT.

Both the tests offer failure load of the specimen in

kilonewtons. Using this failure load, the interlaminar

shear stress can be computed using the following

equation:

xzmax =3pmax

4bD(3)

where, xzmax refers to maximum induced shear

stress in xz plane, in which x refers to the axial

direction of the beam and z refers to the thickness

direction with the origin coincident with the mid-

thickness plane; pmax is the failure load; b is the

width of the specimen; and D is the thickness of the

specimen.

The results of both of these tests and the computed

interlaminar shear stress are given in Table 3.

From the above validation tests, the following

observations have been made:

Average failure load estimated using 3PT for holes

drilled at 1000 rpm with a 25 mm/min feed rate is

298.21 kN.

Corresponding interlaminar shear stress is

353.14 MPa.

Average failure load estimated using MSBS test is

242.39 kN and corresponding interlaminar shear

stress is 294.27 MPa.

Average failure load estimated using 3PT for holes

drilled at 1000 rpm with a 150 mm/min feed rate is

372.40 kN.

Corresponding interlaminar shear stress is 452.17

MPa.



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Table 2 Calculated delamination factor and adjusted delamination factor for holes drilled with 10-mm end mill for different conditions

Speed (rpm) Feed (mm/min)

Maximum length of

damage, Dmax (mm)

Total damaged area,

AD (mm2)

Maximum damaged

area,Amax (mm2) FD FDA

1000 25 13.50 62.22 143.07 1.350 1.804

50 15.43 80.50 186.90 1.543 2.166

75 15.08 93.69 178.51 1.508 2.226

100 13.81 42.47 149.71 1.381 1.688125 12.62 25.82 125.02 1.262 1.446

150 13.92 69.36 152.11 1.392 1.907

1200 25 13.58 44.40 144.77 1.358 1.684

50 16.66 53.95 217.88 1.666 2.096

75 13.75 51.59 148.41 1.375 1.756

100 14.22 34.62 158.73 1.422 1.681

125 12.62 40.67 125.02 1.262 1.551

150 13.87 101.96 151.02 1.387 2.140

1400 25 14.16 62.34 157.40 1.416 1.882

50 12.61 59.63 124.82 1.261 1.685

75 14.12 37.81 156.51 1.412 1.694

100 13.00 52.34 132.67 1.300 1.677

125 14.07 49.18 155.40 1.407 1.773

150 13.59 91.05 144.98 1.359 2.027

Table 3 Failure load and interlaminar shear stress by 3PT and MSBS

tests, for holes drilled with 10-mm end mill for different conditions

Test Parameters Trail

Failure

load (kN)

Interlaminar

shear stress

(MPa)

Three-point

bend test

(3PT)

1000 rpm and

25-mm feed

1 298.42 352.54

2 299.67 354.89

3 296.56 351.99

Average 298. 21 353. 14

1000 rpm and

150-mm feed

1 373.07 452.61

2 371.65 453.78

3 372.48 450.12Average 372. 40 452. 17

Modified short

beam shear

(MSBS) test

1000 rpm and

25-mm feed

1 242.49 294.19

2 243.57 295.95

3 241.11 292.67

Average 242. 39 294. 27

1000 rpm and

150-mm feed

1 309.41 375.38

2 307.84 373.69

3 308.45 374.92

Average 308. 56 374. 66

Average failure load estimated using MSBS test

is 308.56 kN and the corresponding interlaminar

shear stress is 374.66 MPa. Holes drilled with a 25 mm/min feed rate is more

prone to failure than that with a 150 mm/min feed

rate.

The test values obtained by 3PT and MSBS tests are

not correlated with the values of FD and FDA obtained

through the MATLAB 7.0, so it is in need to make fine

tuning in the process of image, and that has been done

by the following method. It is a seven-stage process

out of which three stages are shown in Fig. 3. Here the

importance is given for the depth of damage, which

is called as severity of damage. Depending upon the

depth of damage or severity of damage, the intensity

of reflected light from the damaged zone is varied.

This image is captured by the digital camera and a set

of images are taken for training. During training, the

back ground is eliminated by ground truth technique

and certain features like primary color components

of the image under study are extracted and selected.

At the time of testing, the same features are testedwith the trained neural network, the mean square

error (MSE) is calculated, and the training is carried

out till the iteration process reaches the iteration

maximum. Finally, the results classified as heavy

damaged, medium damaged, and light damaged areas

are obtained. These three zones are colored as follows:

(1) heavily damaged area (red), (2) medium damaged

area (green), and (3) lightly damaged area (light

red). The flow chart of the proposed algorithm is

represented in Fig. 4.

Existing delamination factor or adjusted delami-

nation factor depends on either maximum length of

damage (Dmax) or maximum damaged area (Amax)

and the area of damaged zone (AD), respectively.

Various researchers calculate the value of maximum

damaged area (Amax) by considering only the maxi-

mum diameter of the delamination zone (Dmax).It can

be observed from Table 4 that the total area of damage

(AD) for hole drilled with 25 mm/min feed rate and

1000 rpm is 62.22 mm2, whereas the total area of



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(a) Captured Image

Light Red: Lightly

damaged area

(b) Segregation of

pixels depends on

its intensity

Green: Medium

damaged area

(c) Final image for

measuring of data

Red: Heavily

damaged area.

Figure 3 Steps in neural network in

MATLAB for the calculation of Dmax and

area of damage.

Figure 4 Flowchart of the proposed algorithm.

damage (AD) for hole drilled with 150 mm/min feed

rate and for the same speed is 69.36 mm2 only. The

magnitude of FDA for the hole mentioned in first case

is 1.804 and for the later is only 1.907. On the basis

of the concept proposed in the literature, it can be

decided that the hole considered in the second case

having FDA value of 1.907 is more prone to dam-

age when compared with the first case, which has

FDA value of 1.804. It means that the damages are

high in the second case when compared with the

first case.

Hence the measurement of total area of damage

(Amax), which is mainly concentrated around the

vicinity of drilled hole alone, is not sufficient to quan-

tify the delamination factor. Because the validation

tests prove that the holes drilled with a 25 mm/min

feed rate is more prone to failure than that of holes

drilled with a 150 mm/min feed rate at 1000 rpm.

Hence it is very essential to refine the formula in

Eq. 2 for the calculation of adjusted delamination

factor to correlate with the test values.

This process of refining should include the effect

of severity of damage. To refine the adjusted

delamination factor (FDA), in addition to the variables

Dmax and Amax, the severity of damage should also be

accounted for.

From Table 4, it can be observed that the heavily

damaged area (AH) for hole drilled with a 25 mm/min

feed rate at 1000 rpm is 13.03 mm2, whereas it is only

4.92 mm2 for the hole drilled with a 150 mm/min

feed rate at the same speed. Hence in the formulation

of refined delamination factor importance should

be given for the severity of damage in additionto the maximum length of damage, total damaged

area, and size of the hole. Keeping these points in

mind, it is proposed that the delamination failure

can be effectively characterized using Buckinghams

theorem.20

The Buckinghams theorem states if there are

n variables in a physical phenomenon and if these

variables contain m fundamental dimensions, then

the variables are arranged into (n m) dimensionless

terms. Each term is called term. Accordingly, as

discussed earlier, in the expression for delamination

factor, due importance should be given to severity

of damage in addition to Dmax and D0. Here,the terms (Dmax/D0) (AH/A0), (AM/A0), and (AL/A0)

were identified as dimensionless terms. The

procedure for the development of the proposed

refined delamination factor FDR can be summarized

as follows: FDR = f(Dmax, D0, A0, AH, AM, AL). This

equation can also be expressed in terms of terms as

f(1, 2, 3) = 0.



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Table 4 Split value of damaged area depends on severity for holes drilled with 10-mm end mill for different conditions and its FDR values

Speed (rpm)

Feed

(mm/min)

Maximum length of

damage, Dmax (mm)

Total damaged

area,AD (mm2)

Heavily damaged

area,AH (mm2)

Medium damaged

area,AM (mm2)

Lightly damaged

area,AL (mm2) FDR

1000 25 13.50 62.22 13.03 23.02 26.17 1.709

50 15.43 80.50 9.37 36.96 34.17 1.917

75 15.08 93.69 7.06 34.68 51.95 1.818

100 13.81 42.47 6.30 10.75 25.42 1.539125 12.62 25.82 2.44 8.02 15.36 1.325

150 13.92 69.36 4.92 35.34 29.10 1.650

1200 25 13.58 44.40 3.70 12.98 27.72 1.463

50 16.66 53.95 13.02 22.20 18.73 2.019

75 13.75 51.59 6.73 22.20 22.66 1.586

100 14.22 34.62 4.04 12.66 17.92 1.533

125 12.62 40.67 2.31 12.06 26.30 1.333

150 13.87 101.96 6.91 57.29 37.76 1.929

1400 25 14.16 62.34 7.81 20.99 33.54 1.647

50 12.61 59.63 3.91 23.04 32.68 1.414

75 14.12 37.81 4.14 12.29 21.38 1.524

100 13.00 52.34 2.81 16.72 32.81 1.399

125 14.07 49.18 4.66 12.01 32.51 1.532

150 13.59 91.05 5.50 36.36 49.19 1.646

The terms are expressed as

1 =Dmax

D0. X

AH

A0

a1. Y

AM

A0

b1. Z

AL

A0

c1

Similarly,

2 =Dmax

D0. X

AH

A0

a2. Y

AM

A0

b2. Z

AL

A0

c2

and

3 =Dmax

D0. XAH

A0

a3

. YAMA0

b3

. ZALA0

c3

.

Solving the above and the refined delamination

factor FDR can be expressed as

FDR =Dmax

D0+ 1.783

AH

A0

+ 0.7156

AM

A0

2

+ 0.03692

AL

A0

3(4)

Calculated values of FDR for the holes drilled by

10-mm end mill with required variables for the

calculation are shown in Table 4.

To validate the refined delamination factor, addi-

tionally, six laminate specimens were drilled using

brand new high-speed steel tools namely, twist drill

and router (shown in Fig. 1) with 10 mm each by

the tools at the spindle speeds of 1000, 1200, and

1400 rpm with the feed rates of 25, 50, 75, 100, 125,

and 150 mm/min, with three trails, that is, 108 holes

were drilled and one set of drilled specimen is shown

in Fig. 5 and the calculated FD, FDA, and FDR values

Twist Drill End Mill Router

Figure 5 Photographic view of drilled holes with 10 mm in size.

are shown in Table 5, which are scattered due to

anisotropic nature of composite materials. The capa-

bility of the refined delamination factor to predict the

interlaminar failure is validated for these holes with

reference to the experimental values.

Conclusions

In this study, it was found that delamination is a

main cause of failure in laminated composite material

during drilling. It was evident from the earlier

discussion that refined delamination factor (FDR)

quantifies the delamination failure very effectively

when compared with the conventional (FD) and

adjusted (FDA) delamination factors, which were

explained in the literature. This is because of the fact



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Table 5 Calculated FD, FDA and their FDR values for holes drilled with 10-mm twist drill and router for different conditions

Twist drill Router

Speed (rpm) Feed (mm/min) FD FDA FDR FD FDA FDR

1000 25 1.110 1.455 1.250 1.252 1.623 1.397

50 1.199 1.494 1.219 1.313 1.692 1.461

75 1.288 1.533 1.187 1.374 1.762 1.526

100 1.377 1.572 1.155 1.436 1.831 1.591

125 1.465 1.610 1.123 1.497 1.901 1.655

150 1.554 1.789 1.145 1.558 1.970 1.720

1200 25 1.199 1.590 1.456 1.248 1.658 1.423

50 1.212 1.601 1.391 1.277 1.698 1.487

75 1.376 1.668 1.393 1.370 1.797 1.552

100 1.465 1.707 1.361 1.431 1.867 1.617

125 1.554 1.746 1.288 1.493 1.936 1.681

150 1.642 1.784 1.297 1.554 2.006 1.746

1400 25 1.287 1.725 1.566 1.244 1.694 1.449

50 1.376 1.764 1.568 1.305 1.764 1.513

75 1.465 1.803 1.578 1.366 1.833 1.578

100 1.554 1.877 1.567 1.427 1.902 1.643

125 1.642 1.881 1.499 1.488 1.972 1.707

150 1.631 1.920 1.503 1.449 1.921 1.672

that the refined delamination factor (FDR) accounts

for the severity of damage. The exactness of FDR is

validated with the help of standard test methods for

delamination failure proposed in ASTM D2344 and

ASTM D790.

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Notations and Constantsxzmax Interlaminar shear stress

A0 Nominal hole area

AD Damage area

AH Heavily damaged area

AM Medium damaged area

Amax Maximum damaged area

AL Lightly damaged area

D Thickness of the specimen

D0 Drill diameter

Dmax Maximum diameter

FD Delamination factor

FDA Adjusted delamination factor

FDR Refined delamination factor

B Width of the specimen

pmax Maximum failure load


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