Engineering, 2011, 3, 549-554
doi:10.4236/eng.2011.34064 Published Online May 2011 (http://www.SciRP.org/journal/eng)
Delamination in Fiber Reinforced Plastics: A Finite
Element Approach
P. K. Rakesh*, V. Sharma, I. Singh, D. Kumar
Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, Roorkee, India
Received December 29, 2010; revised January 13, 2011; accepted April 19, 2011
Abstract
The fiber reinforced plastics (FRPs) are being used widely in the most diverse applications ranging from the
aerospace to the sports goods industry. Drilling in particular is important to facilitate the assembly operations
of structurally intricate composite products. The drilling of holes in FRPs leads to drilling induced damage
which is an important research area. The researchers worldwide have tried to minimize the damage by opti-
mizing the operating variables, and tool designs as well as by developing unconventional methods of hole
making. Most of the work done so far has been experimental in nature with little or no focus on numerical
simulation of the drilling behavior of FRPs. In the present research endeavor, a finite element model has
been developed to investigate the drilling induced damage of FRP laminates.
Keywords: Drilling, Delamination, Finite Element Analysis
1. Introduction
The fiber reinforced plastics offers many advantages
over traditional materials such as high strength to
weight ratios, flexibility in design, dimensional stability,
corrosion resistance [1-2]. Drilling of FRPs is often
required to ascertain the structural integrity of complex
products. Delamination is a major problem associated
with the drilling of FRPs. Apart from reducing the
structural integrity of the materials; delamination also
results in poor assembly tolerance and has the potential
to cause long term performance deterioration. During
drilling, many factors affect machinability; the impor-
tant factors are machining parameters such as cutting
speed, feed rate, and the drill diameter, as well as the
drill point geometry.
The drilling induced damage not only affects the
quality of the hole but also so metimes results in-service
performance deterioration. It has been found that drill-
ing induced damage can be reduced by modifying the
drill point geometry and optimizing the process pa-
rameters [3-5]. Drilling induced damage depends on the
cutting speed and feed rate for different drill point ge-
ometries (4-facet, 8-facet, and Jo drill), and the damage
area around the drilled hole increases with an increase
in the cutting speed/feed rate [6]. It was concluded that
the drilling-induced damage in case of the Jo drill is
mini mum as comp ared to oth er drills . Mathew et al. [7]
studied the influence of using a Trepanning tool on
thrust force and torque while drilling glass fiber rein-
forced plastic (GFRP) composites. The investigation
showed that the performance of the Trepanning tool
was superior to the conventional Twist drill. A number
of research endeavors [8-15] have been undertaken to
investigate and develop optimum tool point geometry
for drilling holes in FRPs composites, but still a lot re-
mains to be done. Failure mechanisms in composites
include four types of failure modes: fiber fracture, fiber
buckling and kinking, matrix cracking under transverse
tension and shearing, and matrix crushing under trans-
verse compression and shearing [16]. Budan and
Vijayarangan [17] studied the FE analysis of drilling
process to predict the effects of the drilling parameters
and fiber volume fraction on the surface finish, hole
quality and delamination. The failure envelope gave a
clear idea of the damage zone resulting due to the drill-
ing operation. Durao et al. [18] developed a cohesive
damage model in order to simulate the thrust force and
delamination onset during drilling of CFRP composites.
The FE model was validated with the analytical model
based on linear elastic fracture mechanics (LEFM).
Zitoune and Collomet [19] proposed a numerical FE
method to calculate the thrust force responsible for the
defect at the exit of the hole during drilling in CFRP

P. K. RAKESH ET AL. 550
composites. The numerical results provide the strong
relationship with the experimental values. Rahme et al.
[20] developed the FE model to determine the critical
thrust force for delamination using a failure mechanics
approach. The shape of the drill point geometry effects
the delamination during drilling of FRP. Bhattacharyya
and Horrigan [21] developed the FE model to analyze
the drilling behavior by using the LUSAS software. The
FE drilling was carried out using backing plate and
without backing plate. In accordance with the experi-
mental results, the FE model predicts a lower value of
the delamination load compared with that predicted by
the model which ignores the shearing action. Durao et
al. [22] studied the delamination during drilling of
CFRP laminates using the FE method. Two different
simplified drill point geometries i.e. Twist drill and
C-shape drill were considered. It was observed that the
FE model was not able to evaluate the effect of the op-
erating parameters (cutting speed and feed rate) on the
thrust force and torque. Singh et al. [23] developed a
FE model for predicting the drilling characteristics of
UD-GFRP laminates. It was concluded that the thrust
force depends upon the drill point geometry and the
feed rate, and increases with the increase in both the
point angle and the feed rate. In the present research
endeavor, a FE model has been developed using a
standard FE package (ABAQUS). The investigation
focuses on the drill point geometry as an important pa-
rameter which governs the drilling induced damage.
2. Finite Element Approach
The experimental investigations have established a
number of theories and facts regarding drilling of FRPs
but still a lot remains to be done. The findings have
been specific to drill point geometries and the material
system used in experimentation. There is no generic
model or knowledge base which can be used to under-
stand and analyze the drilling behavior of FRPs. The
effect of drilling with three different drill point geome-
tries has been investigated. The model has been devel-
oped with the following assumptions:
1) The drill is assumed to be discrete r igid.
2) The motion of the drill is provided in the Z-trans-
lation and rotation direction only.
3) A homogenized continuum provides the theoretical
basis for the constitutive model of each lamina. Plane
stress conditions are assumed adequate to model the
constitutive behavior of lamina [15].
4) Linear elasticity is assumed, if the damage state
(state of defects) does not change. This implies linear
elastic unloading and reloading in stress-strain behavior.
All nonlinear effects of the constitutive behavior are
attr ibu ted to da mag e .
Figures 1-3 show the geometric model of the three
different drill point geometries considered in the pre-
sent investigation. The FRP laminate has been modeled
as a GFRP plate of 2 mm thickness. The laminate is a
square plate of 10 cm length and the hole to be drilled is
of 8 mm diameter. The geometric models of the three
different drills are made in ProE software and then im-
ported into ABAQUS to carry out the drilling process
simulation. The drills are assumed to be discrete rigid
and meshed with R3D4 elements. The GFRP laminate
has been modeled using four layered S8R elements,
with element size of 0.8 mm3. Geometrical portioning
has been used to enforce the meshing of each layer. The
assembly is made of the GFRP composite laminate and
the drill point geometry. The material properties of
GFRP laminate as used in the FE model has been given
in Table 1.
Where, E11 and E22 are the modulus of elasticity in
principle material directions, υ12 is the Poisson’s ratio,
G12 is the bulk modulus, Xt and Xc are the X direction
tensile and compressive allowable stresses and Yt, and
Yc are the Y direction tensile and compressive allowable
stresses.
3. Damage Prediction
Failure means that one of the stress components reaches
the yield stress, and then damage occurrences and pro-
gressive failure can be observed. Damage can progress
in different directions around the weakest element in
the model; usually “Matrix Cracking” is the first dam-
age proces s to take p lac e s in ce th e ma tr ix h as th e low es t
stress to failure. A failure criterion is needed to estab-
lish initial damage of matrix or fiber. The Hashin dam-
age initiation criterion is used for more than a single
stress component to evaluate different failure modes in
different directions [25]. These criterion consider six
different damage initiation mechanisms for fiber ten-
sion and compression, matrix tension and compression,
and interlaminar normal tensile and compressive fail-
Figure 1. (a) Trepanning model. (b) Actual Trepanning tool.

P. K. RAKESH ET AL.551
Figure 2. (a)Twist drill model. (b) Actual Twist drill.
Figure 3. (a) Jodrill model. (b) Actual Jodrill.
Table 1. Elastic properties of UD-GFRP laminate [24].
E11 E22 υ12 G12 Xt X
c Y t Y c
48
Gpa 12 GPa 0.25 6.0 GPa 1200 MPa 800 MPa 59 MPa 128 MPa
ure. Once a damage initiation criterion is satisfied, fur-
ther loading will cause degradation of material stiffness
coefficients. The reduction of the stiffness coefficients
is controlled by damage variables that might assume
values between zero (undamaged state) and one (fully
damage state) for the mode corresponding to this dam-
age variable. The evolution law of the damage variable
in the damage initiation phase is based on the fracture
energy dissipated during the damage process.
4. Methodology Used to Obtain the
Damaged Area
A lot of research has been done to characterize and
quantify the drilling induced damage. Visual examina-
tion was used initially to get an idea about the damaged
area around the drilled hole. Recently with the devel-
opment of advanced methods and techniques of imag-
ing, it is possible to quantify the damage in terms of
certain geometrical features. The digital image of the
damaged area [26] was used to quantify the delamina-
tion at the drill exit. The image processing produces
satisfactory results, allowing the observation and analy-
sis of detail from the digitalized image. Using discrete
processes, the image is positioned under a rectangular
grid, and these pixels are identified by the coordinated
pair with origin at upper left corner of the image. The
damage area is obtained through the image digitaliza-
tion and processing the picture using Image J1.42, pub-
lic domain software. In order to obtain an image with
acceptable quality, a series of parameters must be ap-
propriately selected, such as brightness intensity, noise
suppression, image enhancement and edge detection.
Drawing the circle in outer periphery touching the far-
thermost damaged point gives Amax and drawing a circle
touching inner periphery of th e hole gives the hole area
[27] as shown in Figure 4. The delamination factor (Fd)
is given in Equation (1).
Delamination factor, Fd = Amax/Ahole (1)
5. Results and Discussion
As discussed in Section 2, there is a need to develop a
generic model which when validated with experimental
findings is capable of predicting the drilling behavior of
FRPs. The input to such model would be the material
properties, the drill point geometry and the operating
parameters. In order to validate the model, the investi-
gation has been carried out to compare the effect of
three different drill point geometries on the drilling
induced damage. Figures 5-7 show the matrix damage
plot as predicted by the numerical method of FE
Analysis and is validated with the experimental results.
As it is clear from the figures, the damaged area
around the drilled hole predicted for Twist drill is larger
than that generated by the other drill point geometries
under investigation. The experimentally found damaged
area has also been compared with the numerically pre-
dicted damaged area around the drilled hole. It can be
clearly observed that the damaged area predicted by the
simulation process matches closely with the experi-final
time using different constant flow rates. The initial flow
rate (10.47 cm3/min) was calculated to maximize
Figure 4. Schematic layout of damage area (Amax) and hole
area (Ahole) [27].

P. K. RAKESH ET AL. 552
Figure 5. Matrix damage plot (Twist drill). (a) Matrix da- mage plot. (b) Experimental plot [27].
Figure 6. Matrix damage plot (Trepanning tool) (a) Matrix damage plot. (b) Experimental plot
Figure 7. Matrix damage plot (Jo drill) (a) Matrix damage plot. (b) Experimental plot [26]
mentally found damage area qualitatively.
The drilling induced damaged area has also been
quantified by using the digital image processing. The
methodology used for quantification has been discussed
in Section 4. Figure 8 gives the comparison of the dam-
age caused by the three different drill point geometries
under investigation. It is quite clear that the drilling
induced damage in the form of exit delamination (rep-
resented by the delamination factor) caused by Twist
drill is more than that caused by the other drills under
investigation. Table 2 gives the comparison between
the experimental and the numerical predictions of the
delamination factor found while drilling at 2250 RPM
and feed rate of 20 mm/min with the three different drill

P. K. RAKESH ET AL.553
Figure 8. Comparison of Delamination factor (Fd).
Table 2. Comparision of the Delamination factor (Fd) at
2250 RPM.
Types of Drill Simulation Experimental
Twist 4.43 2.86
Trepanning 3.08 2.03
Jo drill 3.79 2.53
poin t g e o me tries.
The difference in the predicted and the experimental
value s may b e attr ibuted to th e fact i t is not a lways pos-
sible to completely quantify the drilling induced dam-
age experimentally using non-destructive dye penetrant
testing and digital imaging. The FE modeling on the
other hand gives a very clear picture of the exact
amount of area around the drilled hole which has been
damaged. The important point to note is the importance
of drill point geometry in defining the drilling charac-
teristics of FRPs. The proper selection of drill point
geometry can lead to production of damage-free holes.
The results of the proposed FE model have been
compared extensively with the experimentally estab-
lished results. The numerical results of drilling behavior
of GFRP laminates while using the Twist drill and the
Trepanning tool substantiates the experimental findings
of Mathew et al. [7]. An extensive experimental valida-
tion of the model would lead to minimization of the
experimental efforts which are time consuming and cost
intensive. The FE model developed hereby presents
enormous opportunities in terms of optimizing the op-
erating variables and the drill point geometry for mak-
ing damage free holes in FRP laminates.
6. Conclusions
The major objective of the present research endeavor
was to de ve lop a fin ite el emen t mo del in ord er to in ves-
tigate the drilling behavior of FRPs. The following
conclusions can be drawn on the basis of the present
investigation :
 The drill point geometry plays a significant role in
defining the damage characteristics while drilling in
FRP laminates. A judicious selection of the drill
point geometry on the basis of work-piece material
will lead to production of damage free holes.
 The optimal drill point geometry (Twist drill) for
drilling of holes in metals is not suitable for making
holes in FRP laminates as it results in substantial
drilling induced damage around the drilled hole.
 The qualitative and the quantitative comparison of
the numerical results with the experimental findings
prove that the proposed model can be used for ex-
haustive investigation of the drilling behavior of
FRPs.
The FE model can be used to optimize the drilling
parameters (cutting speed and the feed rate) and the
drill point geometry for making damage-free holes in
FRPs.
7. Acknowledgements
The authors acknowledge the infrastructural support of
Institute Computer Center at IIT Roorkee for providing
the facility to carry out the computational wo rk.
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