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Materials Science and Engineering A 472 (2008) 347–352

Technical note

Effect of drilling induced damage on notched tensile and pinbearing strengths of woven GFR-epoxy composites

B. Srinivasa Rao a, R. Rudramoorthy b, S. Srinivas c, B. Nageswara Rao d,∗a Aerospace Mechanisms Group, Vikram Sarabhai Space Centre, Valiamala, Trivandrum 695547, India

b Department of Production Engineering, PSG College of Technology, Coimbatore 641004, Indiac Composites Pressurised Systems Group, Vikram Sarabhai Space Centre, Vattiyoorkavu, Trivandrum 695013, India

d Structural Analysis and Testing Group, Vikram Sarabhai Space Centre, Trivandrum 695022, India

Received 8 September 2006; received in revised form 2 March 2007; accepted 5 March 2007

bstract

Analytical and experimental studies were made to evaluate the notched tensile and pin bearing strengths of the woven glass fiber reinforcedGFR)-epoxy composite laminates. The symmetric glass/epoxy woven mat cross-ply laminates containing 16 woven mats were prepared usingand lay-up technique with 45% of volume fraction. The laminates were cut into specimens as per ASTM standards. The experiments were donet three different feed rates, spindle speeds and hole diameters to examine the influence of drilling parameters on the notched tensile strength andearing strengths of the laminates. The design of experiments and analysis of variance (ANOVA) techniques of Taguchi, were utilized to study the

tatistical influence of the drilling parameters on the extension of delamination. The fracture data of center-hole tensile specimens were correlatedsing a modification in one of the stress fracture criteria viz., the point stress criterion (PSC). The bearing strength was correlated with notchednd un-notched tensile strengths of the woven fabric composite laminates. 2007 Elsevier B.V. All rights reserved.

trengt

pd

oaeapDdcped

eywords: Drilling damage; Woven GFR-epoxy composites; Notched tensile saguchi technique

. Introduction

With the advent of composite materials and their wide usen structural applications, it has become necessary to drill holesnto the laminates to facilitate bolting or riveting to the mainoad-bearing structures. Twist drills are widely used in industryo produce holes rapidly and economically. In fact, the chiseldge of the drill point pushes aside the material at the centers it penetrates into the hole. Various drills such as saw drill,andlestick drill, core drill, etc. have been used for drilling ofomposite materials. Cutting variables in drilling process (feednd speed) have great influence on the thrust force and torquend hence on the quality of the machined holes [1–5]. Low feedsn some cases improve the surface roughness due to the reduction

f thrust force. In other cases drilling at lower feeds and highpeed leads to increased temperature generated, assisted by aow coefficient of thermal conduction and a low transition tem-

∗ Corresponding author. Tel.: +91 471 2565831; fax: +91 471 2564181.E-mail address: bnrao52@rediffmail.com (B. Nageswara Rao).

drtar

i

921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2007.03.023

h; Pin-bearing strength; Point stress criterion; Analysis of variance (ANOVA);

erature of plastics. The accumulated heat around the tool edgeestroys the matrix stability and produces fuzzy and rough cuts.

The drilling of composite laminates generates several kindsf damage, which are described at the macro level (delaminatedrea) and at the micro level (cracks, fiber-matrix debonding,tc.). Hocheng and Tsao [6,7] have presented a comprehensivenalysis of delamination in use of various drill types andredicted the critical thrust force at the onset of delamination.elamination is the most evident damage that can be generateduring the drilling of a laminate. This kind of damage islassified as peel-up delamination at the twist drill entrance andush-down delamination at the twist drill exit. Experimentalvidence shows that the degree of peel-up delaminationepends on the feed rate and on the helix angle of the twistrift. Push-down delamination is mainly affected by the feedate, by the presence of support beneath the specimen, and byhe twist drill temperature. Stress concentration, delamination

nd microcracking associated with drilled holes significantlyeduce the composites performance [8–12].

An experimental study has been performed to analyze thenfluence of drilling parameters on the extension of the damage

348 B. Srinivasa Rao et al. / Materials Science and Engineering A 472 (2008) 347–352

Table 1Assignment of the levels to the factors (feed rate, spindle speed and drilldiameter)

Level Feed rate(mm/rev)

Factors spindlespeed (rpm)

Drill diameter(mm)

1 0.0635 500 623

asc

2

pfvtbde

tdioomardt

dfantsm

tng

N

ets

casdt(

pf

TO

T

123456789

T

0.1016 710 80.254 1000 10

nd to evaluate the notched tensile strength and pin-bearingtrength of the woven glass fiber reinforced (GFR)-epoxyomposite laminates.

. Experiments

The design of experiments using statistical approach is a sim-le and systematic approach by identifying various independentactors and levels, and conducting experiments by varying oneariable at a time. Main drawback of the statistical approach ishat no precise guidelines for the sequence of experiments toe conducted and level combination for each experiment. Theesign of experiments using Taguchi method seems to be morefficient compared to statistical methods.

Delamination associated with drilled holes further reduceshe notched tensile strength of laminates. The influence ofrilling parameters on the extension of delamination is exam-ned by conducting the number of experiments as per the designf experiments and analysis of variance (ANOVA) techniquesf Taguchi [13]. The statistical treatment of the measured data isade in two phases. The first phase is concerned with ANOVA

nd the effect of factors. The second phase is aimed on the cor-elation of delamination factor with drilling parameters. Therilled hole specimens are scanned using the AutoCAD drawingechnique [5] to measure the damage around the hole.

The design of an experiment involves: selection of indepen-ent variables (factors); selection of number of level settingsor each independent variable; selection of orthogonal array;ssignment of independent variable to each column of orthogo-

al array; conducting experiments. In the plan of experiments,hree factors (feed rate, spindle speed and drill diameter) wereelected at three levels. Table 1 gives the factors and the assign-ent of the corresponding level for which the values taken by

Fs

w

able 2rthogonal array L9 (34) of Taguchi and values of delamination factor (Fd)

est runs Feed rate,f (mm/rev)

Spindle speed,N (rpm)

Drill dD (mm

0.0635 500 60.0635 710 80.0635 1000 100.1016 500 80.1016 710 100.1016 1000 60.254 500 100.254 710 60.254 1000 8

he average value of the delamination factor, F̄d = 1.16037.

Fig. 1. Experimental setup for drilling operation.

he factors. Before selecting an orthogonal array, the minimumumber of experiments to be conducted shall be fixed, which isiven by

Taguchi = 1 + number of factors X(number of levels − 1)

= 1 + 3X(3 − 1) = 7.

The orthogonal array chosen was the L9 (34). The plan ofxperiments in Table 2 is made of nine tests (array rows) wherehe first column was assigned to feed rate, the second to thepindle speed and third to drill diameter.

Fig. 1 shows the experimental setup. Drilling operations wereonducted using a standard twist drill having 118◦ drill pointngle, 22◦ helix angle and 51◦ chisel angle. Both back and topurface plates having 12 mm hole-diameter were used duringrilling. Totally three drill bits were used and with each drill bit,hree holes were made as per the Taguchi’s design of experimentssee Table 2).

Woven glass/epoxy laminates containing 16 layers were pre-ared using hand lay-up technique with 45% fiber volumeraction (Vf). These laminates were cut into specimens (see

ig. 2) to evaluate the un-notched tensile strength, notched ten-ile strength and pin bearing strength.

Test coupons for evaluation of notched tensile strength asell as bearing strength are shown in Fig. 3. The scanned view

iameter,)

Delaminationfactor, Fd

Deviation in delaminationfactor, (Fd − F̄d)

1.138891 −0.0214791.140646 −0.0197241.142925 −0.0174451.155094 −0.0052761.158766 −0.0016041.162825 0.0024551.173070 0.0127001.182114 0.0217441.189003 0.028633

B. Srinivasa Rao et al. / Materials Science and Engineering A 472 (2008) 347–352 349

F3s

oc

F

iz

T∑AG1

F(d

‘lebtscsRafor(s

F

Ff

ig. 2. Dimensions of test specimens. (a) Tensile test specimen (ASTM D039/D 3039M); (b) tensile specimen with a central hole (ASTM D3039); (c)pecimen for bearing strength evaluation (ASTM D953).

f the drilling-induced delamination for different machineryonditions is shown in Fig. 4.

The delamination factor (Fd) is defined as

d = Dmax

D(1)

n which Dmax being the maximum diameter of the delaminationone and D is the hole diameter as shown in Fig. 5.

Values of the delamination factor (Fd) are presented inable 2. The average value of the delamination factor (F̄d =

Fd/number of test runs) is worked out to be 1.16037.NOVA results for the delamination factor (Fd) of twist drill inFR-woven fabric composite laminates are presented in Table 3.-Mean, 2-mean and 3-mean in Table 3 are the mean values of

wF

ig. 4. Scanned view of drilling-induced delamination for different machining coneed = 0.1016 mm/rev, N = 710rpm; (c) feed = 0.0635 mm/rev, N = 1000 rpm.

ig. 3. (a) Tensile test coupons having 8, 10 and 6 mm center-hole diameter.b) Test coupons for bearing strength evaluation having 10, 8 and 6 mm hole-iameter.

Fd’ from Table 2 of the test runs where the setting values are atevels 1, 2 and 3 for the three factors, viz., feed, speed and diam-ter. Sum of squares for ‘Fd’ value for each factor is obtainedy dividing the total of the squared sum of the deviations ofhree levels with the degree of freedom (2). The total sum of thequares from Table 3 is worked out to be 0.00394132. Percentontribution of each factor is obtained by dividing the sum ofquares of individual parameter with the total sum of squares.esults in Table 3 indicate that the most important parameterffecting the delamination factor is the feed rate (94.36%). Theeed rate shows statistical and physical significance in drillingf the laminate using twist drill. Using the multi-variable linearegression analysis, the correlation between delamination factorFd) and cutting parameters viz., feed rate (f, in mm/rev), spindlepeed (N, in rpm) in drilling is obtained as

d = 1.125814 + 0.19432f + 1.827E − 05N − 0.00076D,

(2)

ith the regression coefficient, R2 = 0.923242. The results ofd obtained from the developed expression (2) are found to be

ditions with 10 mm twist drill bit. (a) Feed = 0.254 mm/rev, N = 500 rpm; (b)

350 B. Srinivasa Rao et al. / Materials Science and Engineering A 472 (2008) 347–352

Table 3Analysis of variance (ANOVA) results for the delamination factor (Fd) of twist drill

Factors 1-Mean 2-Mean 3-Mean Sum of squares (SS) Percent (%) contribution

Feed rate 1.140821 1.158895 1.181396 0.00371894 94.36Spindle speed 1.155685 1.160509 1.164918 0.000191924 4.87Drill diameter 1.161277 1.161581 1.158254 0.0000304534 0.77

The total sum of squares = 0.00394132.

Table 4Validation of the empirical relation (2) with measured delamination factor (Fd)

Test runs Feed rate (mm/rev) Spindle speed (rpm) Drill diameter (mm) Measured, Fd Dmax (mm) Estimated, Fd Eq. (2) Relative error (%)

10 0.1016 355 10 1.145 11.45 1.144 0.08711 0.254 1000 1012 0.254 710 8

wsTd

3

wo(

nw

σ

w

K

Era

puth

σ

Here ahp is the characteristic length of the fracture model andσ0 is the un-notched tensile strength.

Fig. 5. Schematic view of the damage zone.

ithin ±1% of the measured values. To confirm further, threepecimens were drilled with different machining parameters.he results in the Table 4 show a good comparison of measuredata of Fd with the empirical relation (2).

. Notched tensile strength of laminates

Fracture behavior of drilled center-hole tensile specimensith drilling damage has been examined using a modification inne of the stress fracture criteria viz., the point stress criterionPSC).

An approximate solution in the form of a polynomial for theormal stress (σy) distribution adjacent to the hole in an infiniteidth plate under a uniform stress σ is given by [14]

y(x, 0) = 1

2σ{2 + ξ2 + 3ξ4 − (K∞

T − 3)(5 − 7ξ2)ξ6} (3)

here ξ = R/x, x ≥ R and R is the hole-radius.The stress concentration factor for the infinite width plate is

∞T = 1 +

√√√√2

(√Eyy

Exx

− νyx

)+ Eyy

Gxy

(4) Fs

1.183 11.83 1.185 −0.1691.184 9.472 1.182 0.168

xx, Eyy and Gxy are the axial, transverse and shear moduli,espectively, and νxy is the major Poisson’s ratio for the laminatend minor Poisson’s ratio is νyx = (Eyy/Exx)νxy.

The PSC [15] postulates the final fracture of a notched com-osite laminate occurs if the stress in a certain zone exceeds then-notched strength (σ0). It assumes that failure occurs whenhe normal stress σy = σ0 over some distance ahp away from theole (see Fig. 6), i.e.,

y(x, 0) = σ0 at x = R + ahp (5)

ig. 6. Characteristic length (ahp) in a center-hole wide tensile panel (pointtress criterion).

ence

e

H

η

ht(

σ

wi

α

β

ws

icutttnuvsη

a

i

K

tdtomft

[

K

Tstttltpvt

l

a

i

η

nnsf(nr

σ

4

b(ehttK

vmr

B. Srinivasa Rao et al. / Materials Sci

Applying the PSC to Eq. (3), the notched tensile strengthquation is obtained as

σ0

σ∞N

= 1

2{2 + η2 + 3η4 − (K∞

T − 3)(5 − 7η2)η6} (6)

ere

= R

R + ahp(7)

The notched tensile strength (σ∞N ) of the infinite width plate

aving a circular hole is obtained, by multiplying the notchedensile strength (σN) of finite width plate with a correction factorKT/K∞

T ), i.e.,

∞N = KT

K∞T

σN (8)

here stress concentration factor (KT) for the finite width plates [16]

K∞T

KT= α + 1

2(K∞

T − 3)(1 − β)β3 (9)

= 3(1 − (D/W))

2 + (1 − (D/W))3 (10)

= 1

2(√

9 − 8α − 1) (11)

here D = 2R, is the hole-diameter and W is the width of thepecimen.

Details are given below on the evaluation of the character-stic length (ahp) from the notched strength data of the drilledenter-hole tensile specimens. The properties of the material aresed in Eq. (4) to obtain the stress concentration factor K∞

T forhe infinite width composite plate. The stress concentration fac-or, KT for the finite width plate is obtained from Eq. (9) usinghe value of the hole-diameter to the width ratio and K∞

T . Theotched tensile strength, σ∞

N for the infinite width plate is eval-ated from Eq. (8) utilizing the notched tensile strength (σN)alue of the finite width plate. The values of σ∞

N and σ0 are sub-tituted in Eq. (6) and obtained the minimum positive value of(say, η0). The characteristic length (ahp) is evaluated from

hp = R

(1

η0− 1

)(12)

The critical stress intensity factor (KQ∞) for a given materials evaluated by

Q∞ = σ0√

2πahp (13)

It is noted from the notched strength data that the characteris-ic length increases with the notch size and the notched strengthecreases with increase in the notch size. To establish a rela-ion between the characteristic length and the notched strength

f the wide center notch tensile specimens, modifications areade in the present study by relating the critical stress intensity

actor (KQ∞) and the notched strength (σ∞N ) through two frac-

ure parameters KF and m as being followed in metallic materials

tssf

and Engineering A 472 (2008) 347–352 351

17–24]:

Q∞ = KF

{1 − m

(σ∞

N

σ0

)}(14)

he parameters KF and m in Eq. (14) are determined by a least-quare curve fit to the data of KQ∞, σ∞

N and σ0. It is assumedhat 0 ≤ m ≤ 1. Whenever m is found to be greater than unity,he parameter m has to be truncated to 1 by suitably modifyinghe parameter KF with the fracture data. If m is found to beess than zero, the parameter m has to be truncated to zero andhe average of KQ∞ values from the fracture data yields thearameter KF. For determination of KF and m, minimum of twoalues of notched tensile strength in addition to the un-notchedensile strength value are required.

From Eqs. (13) and (14), one can obtain the characteristicength:

hp = 1

2π

(KF

σ0

)2{1 − m

(σ∞

N

σ0

)}2

(15)

The non-dimensional parameter, η in Eq. (6) can be expressedn the form:

= R

R + ahp= R

R + (1/2π)(KF/σ0)2{1 − m(σ∞N /σ0)}2

(16)

Using the Eqs. (16) in (6) and eliminating η, one can obtain aonlinear equation for the fracture strength, σ∞

N . The resultingonlinear equation is solved using Newton-Raphson iterativecheme to obtain the notched strength of the infinite width plateor the specified hole-diameter. The notched tensile strengthσN) of the finite width plate is obtained by multiplying theotched tensile strength (σ∞

N ) of infinite width plate by a cor-ection factor, i.e.,

N = K∞T

KTσ∞

N . (17)

. Results and discussion

Mechanical properties, notched tensile strength as well asearing strength properties of the woven glass fiber reinforcedGFR)-epoxy cross-ply laminates are presented in Table 5. Asxpected, the notched strength decreases with increase in theole-diameter. Considering the average enhanced diameters ofhe drilled holes and the respective notched strength values,he fracture parameters KF and m in Eq. (14) determined are:

F = 67.93 MPa√

m and m = 1. Notched tensile strength (gross)alues evaluated in Table 5 for the drilled central hole speci-ens are found to be reasonably in good agreement with the test

esults.To conduct the bearing test, special fixtures are required and

he task is involved when compare to tensile test. There is pos-ibility of correlating the bearing strength with notched tensiletrength [10]. The average notched tensile and bearing strengthsor the drilled 6.8 and 10 mm diameter hole specimens were

352 B. Srinivasa Rao et al. / Materials Science and Engineering A 472 (2008) 347–352

Table 5Mechanical properties, notched tensile strength as well as bearing strength properties of the Woven glass fiber reinforced (GFR)-epoxy cross-ply laminates

Test runs Test data Dmax (mm) Characteristiclength, ahp (mm)

Notched tensilestrength, σN (MPa)

Predictions Relativeerror (%)

Notched tensilestrength, σN (MPa)

Bearing strength,σB (MPa)

Relativeerror (%)

Bearing strength,σB (MPa) Eq. (18)

1 152.63 173.05 6.85 1.72 142.52 6.6 167.87 2.92 145.61 154.16 9.16 2.03 131.85 9.4 154.05 0.03 128.07 153.54 11.48 2.29 121.52 5.1 140.67 8.34 122.36 152.08 9.18 2.03 131.76 −7.6 153.94 −1.25 123.68 153.33 11.50 2.29 121.43 1.8 140.56 8.36 145.17 165.55 6.95 1.74 142.04 2.1 167.25 −1.07 119.73 150.83 11.76 2.32 120.27 −0.4 139.06 7.88 125.26 158.88 7.10 1.76 141.32 −12.8 166.32 −4.69 119.29 145.83 9.49 2.07 130.37 −9.2 152.14 −4.3

10 124.38 142.83 11.44 2.29 121.69 2.2 144.38 −1.011

E

c

w

isa

5

qvtbicdtwc

R

[[[

[

[[[[[

[

[

[

[

[403 (2005) 125–133.

1 105.70 120.00 11.85 2.332 131.58 160.50 9.456 2.07

xx = Eyy = 20.59 GPa; νxy = 0.1124; σ0 = 226.3 MPa; W = 38 mm.

onsidered and obtained a relation between them asσB

σ0= 1.2951

σN

σ0− 0.0738, (18)

ith the regression coefficient, R2 = 0.9998.By substituting the predicted notched tensile strength values

n the Eq. (18), bearing strength values are estimated. Table 5hows a good comparison between predicted bearing strengthnd experimental values.

. Conclusions

The damage induced during drilling of GFRP laminates isuantified in terms of delamination factor (Fd). Analysis ofariance (ANOVA) approach of Tauguchi is used to identifyhe most dominating factor on damage. Feed rate is found toe more contributing factor on drilling damage. For minimiz-ng drilling damage, low feed rates are preferred in drilling ofomposite laminates. The delamination factor is correlated withrilling parameters (feed rate, spindle speed and drill diame-er). Notched tensile strength of center-hole specimens predictedell with test results using a modification in the point stress

riterion.

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