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Damage Investigation in Drilling of Glass Fiber

Reinforced Plastic Composite LaminatesDr. N. Bhatnagar

a, I. Singh

a& D. Nayak

a

aDepartment of Mechanical Engineering, Indian Institute of Technology, Delhi, India

Available online: 07 Feb 2007

To cite this article: Dr. N. Bhatnagar, I. Singh & D. Nayak (2004): Damage Investigation in Drilling of Glass Fiber ReinforcPlastic Composite Laminates, Materials and Manufacturing Processes, 19:6, 995-1007

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MATERIALS AND MANUFACTURING PROCESSES

Vol. 19, No. 6, pp. 995–1007, 2004

Damage Investigation in Drilling of Glass Fiber

Reinforced Plastic Composite Laminates

N. Bhatnagar,* I. Singh, and D. Nayak

Department of Mechanical Engineering, Indian Institute of Technology,

Delhi, India

ABSTRACT

Machining of fiber reinforced plastic is a process that is substantially different

from metals. The inherent anisotropy in the material system makes the materialremoval mechanism quite complex. Drilling composite materials is necessary to

ascertain the structural integrity of intricate composite products. Drilling of

laminated structures results in extensive damage around the drilled hole. This

study is an attempt to quantify and propose mathematical models for drilling-

induced damage in terms of the cutting speed and the feed speed. Mathematical

models have also been proposed for the resulting thrust force and the torque.

Four different drill point geometries, namely 4-faceted, 8-faceted, parabolic, and

Jodrill, are investigated in this study.

Key Words: Drilling; GFRP; Damage; Drill geometry; NDT.

*Correspondence: Dr. Naresh Bhatnagar, Department of Mechanical Engineering,

Indian Institute of Technology, Delhi 110016, India; Fax: 91-11-26582053; E-mail:

[email protected].

995

DOI: 10.1081/LMMP-200034486 1042-6914 (Print); 1532-2475 (Online)

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INTRODUCTION

Unlike most engineering materials, polymeric composites are characterized by

marked anisotropy, structural inhomogenity and lack of plastic deformation

behavior. Near-net shape manufacturing capabilities of fiber reinforced plastic

(FRP) along with their high specific strength and stiffness make them attractive in

almost all fields of engineering, ranging from aerospace to consumer goods.

Additional benefits in the form of corrosion resistance, lower maintenance costs,

ease of transportation, and installation make them very useful material. Although

most of the composite products are made to a near-net shape, subsequent machining

is required for assembly purposes. Drilling is one of the most important, frequently

practiced, and unavoidable machining operation for components used in FRP

composite structures. Intricacy in structures necessitates hole generation to facilitate

assembly. The mechanism of drilling composite materials has been recognized as a

process fundamentally different from that of homogenous metal removal. Thediverse properties of the fiber and the matrix combined with fiber orientation have a

significant effect on the drilling process. During drilling, the fibers take a high

proportion of the load, which prevents uniform plastic deformation as normally

observed during chip formation in metal cutting.

Composite materials pose additional difficulties while solving the problem of

controlling the drilling process. These difficulties arise from the anisotropic nature

of the material, as determined by the stacking sequence of the laminate. This prevents

the use of empirical models for control due to the difficulty in dealing with the large

number of parameters that determine the material characteristics and quantifying

them with any certainty. Drilling-induced damage is one of the important factors that

influence the long-term performance deterioration of the composite product. The

estimation and characterization of drilling-induced damage therefore becomes

imperative for ascertaining the reliability and durability of composite structureswith drilled holes. A number of attempts correlating the damage zone to the drilling

parameters—that is, cutting speed and the feed rate—have been made. Tagliaferri

et al.[1] correlated the damage zone width to the ratio of cutting speed and feed rate.

It was found that the higher the ratio, the better the cut quality. Chen [2] proposed

a statistical model correlating the delamination factor with the average thrust force.

Theoretical models by Lachaud et al.[3] and Zhang et al.[4] have been developed that

predict the critical thrust force beyond which delamination (damage at the exit side of

the hole) takes place. Linear elastic fracture mechanics and the classical plate bending

theory principles have been used to develop such models. These models focus on thrust

force as the principal factor that causes material damage during drilling. Application

of these models to the practical drilling operation requires an in-depth study. Drill

point geometry is another important parameter that considerably influences drilling-

induced damage. Mathew et al.[5] found that thrust force is considerably lower fortrepanning tools in comparison to the standard twist drill. Chen[2] studied the effect

of various geometry parameters (point angle, helix angle, web thickness) and

correlated them to the cutting forces. Miller[6] carried out a study on various drill point

geometries, but the damage estimation was purely qualitative in nature. This research

initiative is an attempt to quantify drilling-induced damage (delamination), and

to correlate it with the drilling parameters and the cutting forces.

996 Bhatnagar, Singh, and Nayak

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EXPERIMENTAL SETUP

Drilling of FRP composite laminates depends on the cutting speed and the feed

rate, along with tool geometry and other material parameters. In this study, these are

recognized as the prime factors that influence drilling-induced damage. Four-layered

[(0/90)/0]s glass fiber reinforced plastic (GFRP) laminates were made by a hand

layup technique. Curing was done at room temperature for about 24 h. The matrix

was epoxy with a standard grade of LY556 with hardener HY951. The fiber volume

fraction was calculated using the ignition loss method (ASTM D2584-68) and was

found to be 0.60 on an average of five samples. The specimens were cut to size of

100Â 50 mm and 4 mm thickness. As shown in Fig. 1, a CNC machine was used to

drill holes in laminates. The specimens were mounted on top of a drill dynamometer

(Kistler, Type 9272); the thrust force and torque signals from the dynamometer were

routed to the charge meters and were recorded in the IBM PC via A/D card. Solid

carbide drills (K-10) of diameter 4 mm and 8 mm were used in the experiments. Thetype of drill point geometry was one of the important factors of investigation in the

experiments. Four types of solid carbide drills namely 4-faceted, 8-faceted, parabolic

point, and Jodrill were used. All the drills selected were of substantial different

geometries and were expected to behave differently under similar drilling conditions.

Different drill point geometries are shown in Fig. 2.

Figure 1. Experimental setup.

Damage Investigation in Drilling of GFRP Composite Laminates 997

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EXPERIMENTATION

The experiments were carried out at different levels of cutting speed and feed

rates. As has been reported,[7]

carbide drills give better surface finish and more holesto failure; therefore, HSS drills were eliminated from this study. After an initial trial

run, a suitable range of cutting speed and feed was chosen for which the visual

damage around the drilled hole was minimum. The cutting speed/feed speed (V s/V f )

was defined as a nondimensional parameter, and the damage quantification was

done at different values of V s/V f . The nondimensional parameter V s/V f was varied

from 450 to 2800. The cutting forces, namely torque (T ) and thrust (F ), were

recorded using the drill dynamometer.

Quantification of Drilling-Induced Damage

Delamination, matrix burning, fiber pullout, and circularity defects are some of

the drilling-induced damage forms. Delamination is one of the most commondamage form usually associated with drilling of laminated structures. The last plies

tend to open up as the drill pierces through the laminate to generate a hole. A general

perception recognizes the thrust force as the main culprit causing delamination. A

nondestructive dye penetrant test in conjunction with digital image processing was

used to quantify the drilling-induced damage at the exit side of the hole. A

fluorescent dye, Zyglo 27-A, with a specified combination of developer was sprayed

(a) Jodrill (b) 8-faceted

(c) 4-faceted (d) Parabolic

Figure 2. Different drill point geometries under investigation.


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around the drilled hole. The laminates were left to dry for 30 min. The specimens

were observed under ultraviolet light. The fluorescent dye penetrated into the

damaged zone and gave a clear picture of the drilling-induced damage around the

hole. Digital images of the drilled holes after the nondestructive inspection were

recorded under ultraviolet light using a digital camera (Sony). Digital image

processing was then used to quantify the drilling-induced damage. Image

segmentation and thresholding principles that are based on the difference in the

gray-scale values of image pixels were used to distinguish the damaged area around

the drilled hole.

RESULTS AND DISCUSSION

Thrust and Torque Response

Figure 3 shows the thrust force signals for different drill point geometries. It is

quite evident that the variation in the drill point geometry results in a characteristic

variation in the drilling behavior of different drill points. The stepped-drill (Jodrill)

gives a characteristic plot showing the variation very clearly. The variation in the

signals for a single drill point may be attributed to the heterogeneous nature of the

Figure 3. Thrust force response of different solid carbide drill point geometries with respect

to time at cutting speed (V s)/feed speed (V f )¼ 2800.


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composite laminate. The fibers take a higher proportion of the load and thus impair

the uniform plastic deformation of the material. The drill point encounters fibers and

matrix at regular intervals, and therefore, there is a variation in the signals for a

single drill point. Generally, for all the drill points, it can be observed that the thrust

force signal increases with time and attains a maximum value at the time of complete

engagement of the drill point with the laminate and then decreases as the drilling

process is completed. The thrust force signal for the 8-faceted drill point shows a

continuous nature, whereas some aberrations are observed with the 4-faceted and the

parabolic point.

Figure 4 shows the torque response of different drill point geometries. A typical

behavior observed with the Jodrill can be attributed to the stepped shoulder that

characterizes the drill geometry for Jodrill. The signals for the 4-faceted and

8-faceted drill points show a wider spread with respect to time, whereas the parabolic

point gives a typical peak in the torque signal. Thus, it is evident that the drilling

characteristics of different drill point geometries are substantially different and can

affect the damage induced around the drilled hole as a result of the drilling process.A general perception relates the drilling-induced damage to a critical thrust force

beyond which damage takes place. Theoretical models for critical thrust force based

on the linear elastic fracture mechanics and classical plate bending theory have been

developed. These models serve as a basis for minimization of the drilling-induced

damage. In this study, the cutting forces were investigated as a function of a

nondimensional parameter cutting speed (V s)/feed speed (V f ) ratio. Figures 5 and 6

Figure 4. Torque response of different drill point geometries with respect to time at cutting

speed (V s)/feed speed (V f )¼ 1800.


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highlight the variation of the thrust force with V s/V f for four different drill point

geometries.It is observed from these figures that the thrust force value shows a continuous

decrease (for all the drill point geometries) with an increase in V s/V f for both drill

diameters investigated. It can also be seen that the 4-faceted drill point records the

maximum thrust force. The effect is quite predominant for the 8-mm diameter drill.

The parabolic point gives a better thrust force response as compared with other drill

point geometries.

[(0/90)/0] GFRP, 4 mm diameter

0

5

10

15

20

25

30

3540

0 1000 2000 3000

Cutting speed/Feed speed

T h r u s t f o r c e ( N )

4-faceted

parabolic

8-faceted

Jodrill

Figure 5. Thrust force vs. V s/V f (drill diameter 4 mm).

[(0/90)/0]s GFRP, 8 mm diameter

0

10

2030

40

50

60

70

0 2000 4000 6000


T h r u s

t f o r c e ( N )

4-faceted

parabolic

8-faceted

Jodrill

Figure 6. Thrust force vs. V s/V f (drill diameter 8 mm).


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Figure 7 shows the variation of torque with the cutting speed/feed speed ratio.

Unlike the thrust force that shows a continous decrease with increasing V s/V f ; the

torque gives local minima in the variation. The decrease in the torque values toward

the end can be attributed to the softening of the matrix due to heat build up at higher

cutting speed and lower feed speeds. Moreover, low feed speed increases the

engagement time of the drill with the laminate resulting in damage due to heat. The

thrust and torque response for different drill point geometries was mathematically

modeled using multiple linear regression. The thrust force and torque are expressed

as a function of the cutting speed (V s) and the feed speed (V f ). Different statisticalmodels are proposed.

4-Faceted drill point

F ¼ 75:488þ 2EÀ 06 ðV s=V f Þ2 À 0:0192 ðV s=V f Þ R2 ¼ 0:9995

T ¼ 19:572À 7EÀ 09 ðV s=V f Þ3 þ 3EÀ 05ðV s=V f Þ

2 À 0:04 ðV s=V f Þ R2 ¼ 0:9467


F ¼ 69:66þ2EÀ06 ðV s=V f Þ2 À0:0192 ðV s=V f Þ R2 ¼ 0:9919

T ¼ 21:322À 8EÀ09 ðV s=V f Þ3 þ 4EÀ05 ðV s=V f Þ

2 À 0:0481 ðV s=V f Þ R2 ¼ 0:8474

Parabolic drill point

F ¼ 48:135þ9EÀ07 ðV s=V f Þ2 À0:01 ðV s=V f Þ R2 ¼ 0:9995

T ¼ 19:223À 6EÀ09 ðV s=V f Þ3 þ 3EÀ05 ðV s=V f Þ

2 À 0:0394 ðV s=V f Þ R2 ¼ 0:7432

Jodrill

F ¼ 44:612þ 4EÀ 07 ðV s=V f Þ2 À 0:0061 ðV s=V f Þ R2 ¼ 0:8957

T ¼ 7:963À 4EÀ 10 ðV s=V f Þ3 þ 4EÀ 06ðV s=V f Þ

2 À 0:0061 ðV s=V f Þ R2 ¼ 0:9463

[(0/90)/0]s GFRP

0

2

4

6

8

10

12

14

0 500 1000 1500 2000 2500 3000


T o r q u e ( N - c m )

4-faceted

parabolic

8-faceted

Jodrill

Figure 7. Torque vs. V s/V f .


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The values of the regression coefficients clearly indicate that the mathematical

models fit the experimental data closely.

Drilling-Induced Damage: A Qualitative Analysis

Figure 8 shows a drilling-induced damage zone around the drilled hole for

two different stacking sequences. It is quite clear from the qualitative analysis that

the damage zone in unidirectional (UD)-GFRP laminates is elliptical in shape with

the major axis of the ellipse approximately being in the direction of the fibers. The

damage zone in the case of [(0/90)/0]s laminates is clearly much smaller than that for

UD-GFRP laminates, and is spread around the periphery of the drilled hole. Theelement of directionality in the damage zone thus found in the UD-GFRP laminates

is avoided by placing woven fabric layers at the top and at the bottom of the

unidirectional laminates.

Drilling-Induced Damage: A Quantitative Analysis

The methodology to quantify drilling-induced damage has already been

discussed. Figure 9 gives the variation of damage area/hole area (Da/H a) with

V s/V f . It is observed that Da/H a show local minima for values of V s/V f , ranging from

750 to 1500. The trends or the variations for different drill point geometries are

qualitatively the same, but differ on the quantitative scale. The values for Da/H a

show a constant increase with increasing V s/V f after showing local minima for alldrill point geometries. The thrust force (Figs. 5 and 6) decreases with increasing

V s/V f , whereas the torque (Fig. 7) shows an increase with increasing values of V s/V f after exhibiting local minima such as the Da/H a ratio.

Theoretical models developed earlier predicted critical thrust force responsible

for the damage and ignored the effect of torque completely. The results of this work

thus establish and necessitate the study of torque as an important parameter that

Figure 8. Drilling-induced damage: (a) [(0/90)/0]s laminate, (b) UD-GFRP laminate (8 mm

diameter 8-faceted solid carbide drill point at V s/V f ¼2800).


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influences damage around the drilled hole. It is important to conclude that higher

cutting speeds result in substantial material damage around the drilled hole.The

values for Da/H a are less for the 8-faceted drill point and the Jodrill. The 4-faceted

and the parabolic point result in higher values for Da/H a and subsequently larger

damage zones. It also becomes clear that the drill point geometry has a significant

effect on drilling-induced damage. The 8-faceted and Jodrill are thus the best-suited

drill point geometries for drilling GFRP laminates.

Figure 10 (a and b) gives a comparison of Da/H a for two different stackingsequences. It is observed that the laminate with woven fabric layers at the top and

the bottom results in a smaller damage zone around the drilled hole as compared

with the UD-GFRP laminate. It is therefore advantageous to alter the stacking

sequence by placing woven laminate or homogenous laminate as the top and bottom

plies to minimize the drilling-induced damage in GFRP laminates.

Regression modeling was done to propose mathematical models for the drilling-

induced damage zone. A number of fits were tried to model the damage zone.

Polynomial regression models gave a good fit to the experimental data. Because the

different drill point geometries were substantially different with no common feature

or attribute, drill point geometry was not considered as a modeling parameter, and

geometry-specific models were proposed. Mathematical models for different drill

point geometries are:


Da=H a ¼ 2:4714þ 3E À 07 ðV s=V f Þ2 À 0:0007 ðV s=V f Þ R2 ¼ 0:9141


Da=H a¼2:9335À5EÀ10ðV s=V f Þ3þ2EÀ06ðV s=V f Þ

2À0:0031ðV s=V f ÞR2¼0:9467

[(0/90)/0]s GFRP

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.83

0 500 1000 1500 2000 2500 3000


D a m a g e a r e a / H o l e a r e a

4-faceted

parabolic

8-faceted

Jodrill

Figure 9. Variation of damage area (Da)/hole area (H a) with cutting speed (V s)/feed speed

(V f ) for four different drill point geometries.


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Parabolic point

Da=H a¼2:9829À4EÀ10 ðV s=V f Þ3þ2EÀ06 ðV s=V f Þ

2À0:0027ðV s=V f ÞR2¼0:9601

Jodrill

Da=H a ¼ 2:8308À3EÀ10 ðV s=V f Þ3þ2EÀ06 ðV s=V f Þ

2À0:0024 ðV s=V f ÞR2 ¼ 0:92

The mathematical models directly relate operating variables that are the cutting

speed and the feed speed to the induced damage. The operating variables in a drilling

process can be used as control variables to minimize the damage around the drilled

hole. A model correlating the damage zone to the cutting forces will necessitate

installation of expensive instrumentation to first record the forces and then correlatethem to the damage zone. Thus, the models proposed here are useful in estimating

beforehand the amount of damage that can develop during drilling and thereby help

in specifying a quality criterion. For example, a manufacturer for acceptance of

laminates with a drilled hole might specify the Da/H a value of 2 as a quality criterion.

The models may also help to optimize the operating variables for minimizing

drilling-induced damage.

Comparison for stacking sequence (4-faceted)

1

1.5

2

2.5

3

0 1000 2000 3000


D a m a g e a r e a /

H o l e a r e a UD-GFRP

[(0/90)/0]s

GFRP

(a)

Comparison for stacking sequence (8-faceted)

1

1.5

2

2.5

3

0 1000 2000 3000


D a m a g e a r e a /

H o l e a r e a UD-GFRP

[(0/90)/0]s

GFRP

(b)

Figure 10. Comparison for stacking sequence: (a) 4-faceted, (b) 8-faceted drill point.


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CONCLUSION

This research endeavor was undertaken to quantify drilling-induced damage and

to correlate it with operating variables. Four different drill point geometries were

investigated, and the following conclusions are drawn:

1. Higher thrust force values are recorded for a 4-faceted drill point, which is

comparable to the standard twist drill geometry, and therefore, it is not

recommended for drilling laminated GFRP composite materials.

2. The thrust and the torque response is relatively better for a parabolic point,

but the drilling-induced damage shows a maximum value; therefore, it is

concluded that there is no direct relation between the cutting forces and the

damage for this drill.

3. Contrary to general belief, maximum damage was not found at the

conditions where maximum thrust force was recorded. It is thereforeconcluded that the theoretical models for predicting critical thrust force must

be applied with caution and with certain limitations. The torque also plays

a significant role in influencing the drilling-induced damage, and models

incorporating both the thrust force and the torque should be developed.

4. Drilling-induced damage in the case of [(0/90)/0]s GFRP laminates was

spread around the hole, whereas the damage zone in case of UD-GFRP lami-

nates shows an elliptical nature with major axis along the direction of fibers.

5. Drilling-induced damage depends on cutting speed/feed speed ratio resulting

in a local minima between 900 and 1500 for different drill point geometries,

and this damage area around the drilled hole increases with an increase in

cutting speed/feed speed ratio.

6. The drilling-induced damage in case of the 8-faceted drill point and the

Jodrill is minimum. A modified geometry incorporating the best features of these two geometries can be developed for drilling GFRP composite

laminates, which requires future design and innovation.

7. The drilling-induced damage zone in the case of [(0/90)/0]s GFRP laminates

is smaller as compared with UD-GFRP laminates. It is advisable to use

woven layers or homogeneous mats at the top and the bottom to ensure a

good quality hole with minimum entry and exit damage.

8. Mathematical models for thrust, torque, and drilling-induced damage have

been proposed for different drill point geometries. The best-fit models are

selected to model the experimental data. The models are domain specific,

limiting operating conditions and the workpiece material investigated in this

study. A more generic damage criterion incorporating the material property

as one of the variables can be developed in the future.

ACKNOWLEDGMENT

The authors are thankful to Ministry of Human Resource and Development

(MHRD), Govt. of India, for providing financial assistance to pursue the research

under the project ‘‘Machinability Index for FRP Composite Materials.’’


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REFERENCES

1. Tagliaferri, V.; Caprino, G.; Diterlizzi, A. Effect of drilling parameters on the

finish and mechanical properties of GFRP composites. Int. J. Machine Tool

Manuf. 1990, 30 (1), 77–84.

2. Chen, W.C. Some experimental investigations in the drilling of carbon fiber

reinforced plastic composite laminates. Int. J. Machine Tools Manuf. 1997,

37 (8), 1097–1108.

3. Lachaud, F.; Piquet, R.; Collombet, F.; Surien, L. Drilling of composite

structures. Comp. Struct. 2001, 52, 511–516.

4. Zhang, L.-B.; Wang, L.-J.; Liu, X.-Y. A mechanical model for predicting critical

thrust forces in drilling composite laminates. Proc. Instn. Mech. Engrs. IMechE

2001, 215 (B), 135–146.

5. Mathew, J.; Ramakrishnan, N.; Naik, N.K. Trepanning on uni-directional

composites. Composites. Part A 1999, 30, 951–959.6. Miller, J.A. Drilling graphite/epoxy at Lockheed. Am. Machinist Automated

Manuf. 1987, October, 70–71.

7. Ramulu, M.; Branson, T.; Kim, D. A Study on drilling of composite and

titanium stacks. Comp. Struct. 2001, 54, 67–77.

Received October 1, 2003

Accepted December 18, 2003


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http://s100.copyright.com/AppDispatchServlet?authorPreorderIndicator=N&pdfSource=Keyword&publication=AMP&title=Damage+Investigation+in+Drilling+of+Glass+Fiber+Reinforced+Plastic+Composite+Laminates&offerIDValue=18&volumeNum=19&startPage=995&isn=1042-6914&chapterNum=&publicationDate=&endPage=1007&contentID=10.1081%2FAMP-200034486&issueNum=6&colorPagesNum=0&pdfStampDate=10%2F29%2F2004+21%3A15%3A56&publisherName=dekker&orderBeanReset=true&author=N.+Bhatnagar%2C+I.+Singh%2C+D.+Nayak&mac=oSMagfKjkag2kwa1iEAP6Q--



http://www.publishers.org/conference/copyguide.cfm

http://www.copyright.gov/fls/fl102.html

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AMP-200034486