Delamination in Fiber Reinforced Plastics: A Finite Element Approach

doi:10.4236/eng.2011.35064

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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

E-mail: [email protected]

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

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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