Regional Conference on Engineering Mathematics, Mechanics, Manufacturing & Architecture (EM*ARC) 2007

© 2007 Noise, Vibration and Comfort Research Group

Editors: M. J. Mohd Nor, S. Abdullah, N. Jamaluddin, R. Ismail, S. Mohamed Haris and A.Arifin

STUDY ON THE DYNAMIC CHARACTERISTIC OF

COCONUT FIBRE REINFORCED COMPOSITES

I.Z.Bujang, M.K.Awang and A.E.Ismail

Department of Mechanical Engineering
Universiti Tun Hussein Onn Malaysia
86400 Batu Pahat, Johor, Malaysia

ABSTRACT

The goal of this paper is to determine the mechanical properties and dynamic
characteristics of a proposed combined polymer composite which consist of a
polyester matrix and coconut fibres (also known as coir fibres). The influence of
fibres volume on the mechanical properties and dynamic characteristic of the
composites was also evaluated. Composites with volumetric amounts of coconut
fibre up to 15% were fabricated and they were arranged in randomly oriented
discontinues form. Tensile test was carried out to determine the strength of
material, while experimental modal analysis was executed to obtain the dynamic
characteristics of the composite material. The acquired results show that the
tensile modulus changes with the fibre content. The strength of coconut fibre
reinforced composites tends to decrease with the amount of fibre which indicates
ineffective stress transfer between the fibre and matrix. The stiffness factor also
gives the same effect to the dynamic characteristic of composite where the
natural frequency decreased with the increase of coconut fibre volume. However
the damping peak was found to be increased by the incorporation of the fibre.
When higher fibre content of 10% was used, the damping peak shows the
maximum value for almost all the frequency mode. It was observed that the
effects of reinforcing polyester matrix with the coconut fibres caused the
composites to be more flexible and easily deform due to high strain values and
reduction of high resonant amplitude.

Keywords: natural coconut fibres, tensile test, experimental modal analysis,

natural frequency, damping

INTRODUCTION
M..1. Mohd Nor, S. Abdullah, N. Jamaluddin, R. Ismail, S. Mohamed Haris and A. Arifin

Natural fibres can be easily obtained in many tropical and available throughout
the world. Today these fibres are considered as environment friendly materials
owing to their biodegradability and renewable characteristics. Natural fibres such
as sisal, jute, coir, oil palm fibre have all been proved to be good reinforcement in
thermoset and thermoplastic matrices (Varma et al. 1989; Joseph et al. 1996;
Sreekala et al. 1997; Geethamma et al. 1998). Nowadays, the use of natural
fibres reinforced composites is gaining popularity in automotive, cosmetic, and
plastic lumber applications because it offers an economical and environmental
advantage over traditional inorganic reinforcements and fillers (Murali &
Mohana 2007).
In searching for such new material, a study has been made where coconut
fibre (also known as coir fibre) is compounded with composite material. Coir is
the natural fibre of the coconut husk where it is a thick and coarse but durable
fibre. It is relatively water-proof and has resistant to damage by salt water and
microbial degradation (Ray 2005). Figure 1 shows the outer husk of coconut fruit
which can be used as a source of fibre and coir pitch. Meanwhile, the
investigation of the mechanical properties and dynamic characteristics of the coir
fibre reinforced composites is vital. This is due to the fact that having a suitable
stiffness and damping coefficients of composites can be applied to the certain
applications which satisfy the needs of their characteristics such as strong, rigid,
light weight, environmental friendly materials (Shaikh et al. 2003). The example
of application of coir fibre reinforced composites is in industrial automotive
where it used to make seat cushions for Mercedes automobiles. Even though it
has advantageous properties, the coir fibre composites still have some
undesirable properties such as dimensional instability, flammability which not
suitable for high temperature application and degradability with humidity,
ultraviolet lights, acids and bases (Brahim & Cheikh 2006). Therefore, a lot of
efforts have been carried out to improve the performance of coir fibre reinforced
composites.

FIGURE 1 The outer husk of the coconut

This paper addresses the characterization and performance of natural fibre

reinforced composite by analyzing the effect of fibre volume (%) on the
composite mechanical properties and its dynamic characteristics. The composites

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Study On The Dynamic Characteristic Of Coconut Fibre Reinforced Composites

were obtained by compounding polyester matrix and coir fibres in a batch mixer
to obtain a randomly oriented discontinue form. The chose of polyester as a
matrix is based on economic interest because it offers a very cheap resin,
available with good mechanical properties and used in many applications such as
transport, marine and sport.

RELATED WORKS

Natural fibres are environment friendly materials and have proved to be a

competitor for glass fibre/polyester in terms of strength performance and cost
(Baiardo et al. 2004; Brahim & Cheikh 2006; Idicula et al. 2006). Combination
of natural fibre reinforced polyester composites has been demonstrated to be an
effective method to design materials suited for various requirement. Earlier
studies by Brahmakumar et al. (2005) proved that the coir fibres can be used as
effective reinforcement and bonded in polyester matrix. These fibres were
hybridized with the matrix to get a better mechanical performance. In the studies
on mechanical performance and properties of short fibre reinforced polymer
composites, Maries Idicula et al. (2006) have shown that both fibre length
distribution and fibre orientation distribution play very important role in
determining the mechanical properties. Sapuan et al. (2003) believed that
mechanical properties of the natural fibre composites depend on several factors
such as the stress-strain behaviours of fibre and matrix phases, the phase volume
fractions and the distribution and orientation of the fibre or fillers relative to one
another. He also found that the natural fibre composites demonstrate somewhat
linear behaviour and sharp fracture.
Shaikh et al. (2003) indicated that the volume fraction of the natural fibre has
a crucial effect on the composite strength where the strength of the composite
raises linearly with the increase of volume fraction. However, different types of
natural fibre give a different effect to the composites structure and some of the
natural fibres can give an opposite effect to the strength of composite. Brahim
(2006) had pointed out that the longitudinal modulus and the longitudinal stress
increase with the rise of the volume fraction in fibres. This is obvious since the
mechanical properties of the fibres are bigger than those of the polyester matrix.
In the other hand, the strain decreases slightly from 2.7 to 2.3 when the fraction
volume in fibres increases from 0% to 21% and then rises again to reach 3.1 for
Vf = 44%. However, the effect on dynamic characteristics of the composite was
still not known. Therefore this problem has been considered in the study since the
dynamic behaviour of composite structures is very important. Huang (2001) had
carried out a micromechanical approach for investigating the dynamic response
of laminated composite plates composed of randomly oriented fibres. Bledzki &
Zhang (2001) had also investigated the dynamic mechanical behaviour of jute
fibres reinforced epoxy foams.
For measuring the damping values, Gade & Herlufsen (1990) had compared
between the Digital Filter (DF) techniques and the Discrete/Fast Fourier
Transform (DFT/FFT) techniques by using vibration decay measurements or
bandwidth determination of measured modal resonances. In the DF technique, the

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M..1. Mohd Nor, S. Abdullah, N. Jamaluddin, R. Ismail, S. Mohamed Haris and A. Arifin

damping is estimated from the decay of the free vibration response due to an
impact excitation and the advantage of this technique is very fast and doesn't
have limitations in dealing with very light damped systems. But, due to the poor
resolution of DF analyzer it is not well suited for bandwidth determination of
measured modal resonances. For the FFT techniques the damping was measured
using free vibration decay, curve fit of frequency response function measured
using impact excitation and decay of impulse response function using pseudo
random excitation with a shaker. The vibration decay method does not have any
limitation with regard to low damping, but for high damping values, the
limitation comes from the limited transfer rate of spectra, which depends on
system measurement. Normally, the dynamic mechanical test method was
employed to determine the structure relevant stiffness and damping
characteristics for various applications in engineering. For the composite
structure, the damping property can be obtained from the natural damping of its
element (Photana et al. 2003). This can be represented from the formula:

(1)

where tan 8C is damping values of the composite, tan 5f is damping values of the
fibre, tan 8m is damping values of the polymer, Vf is volume fraction of the fibre
and Vm is volume fraction of the matrix.

In the first part of this work, the physical and mechanical properties of
coconut fibres used to reinforce the studied composite material were present. An
experimental investigation was carried out to study the effect of coir fibres
volume (%) on the strength of composite and the results were discussed in this
paper. In the second part, the dynamic test was then performed to describe the
effect of fibres content and the relationship of mechanical properties on the
dynamic characteristics of the developed material.

EXPERIMENTAL WORKS

Basically three main tasks were carried out to achieve the objectives of study.
The first task was the preparation of composite material by combining the
polyester and coconut coir. Then it was continued by performing the tensile test
and lastly the experimental modal analysis (EMA) was carried out to determine
the dynamic characteristics of the studied composite. Figure 2 shows the whole
processes of the study.

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Study On The Dynamic Characteristic Of Coconut Fibre Reinforced Composites

FIGURE 2 The flow chart of the study

Material

The studied composite material is made of polyester matrix reinforced with

coconut fibres which were arranged in discontinuous randomly oriented
configuration. Basically, the coir fibres obtained from the coconut husk which
was abstracted from coconut fruit. After they had been abstracted, the coir fibres
will be dried at 70°C to 80°C using drying oven. In order to avoid degradation
factor, the coir fibres need to go through the treatment process. This process

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M..1. Mohd Nor, S. Abdullah, N. Jamaluddin, R. Ismail, S. Mohamed Haris and A. Arifin

consists of immersing the coir fibres into 5% Natrium Hydroxide (NaOH)

solution for 24 hours to remove the first layer of coconut coir fibres. After that,
the obtained fibres were washed abundantly with water to remove the NaOH
before they dried again in furnace at 70°C to 80°C for next 24 hours. The coir
fibres were then soaked into 5% of silane and 95% of methanol solution for 4
hour and dried at 70°C for next 24 hours curing time. After the drying process
finished, the coconut fibres was inserted into the cutting machine to cut into
smaller pieces. This form is called whickers which its length is less than about 10
mm. The advantage of whickers is that they can easily pour into the mixture of
coconut fibres and polyester in ASTM D638 Type 1 mould (Turtle 2004). The
physical properties of coir fibres are shown in Table 1.

TABLE 1 Physical properties of coir fibres

Mechanical Properties Coconut Coir Fibre
Density (g/cm 3 ) 1.2
Elongation at break (%) 30
Tensile strength (MPa) 175
Young modulus (GPa) 4-6
Water absorption (%) 130-180

The usage of polyester resin as a matrix was chosen because it is the standard
economic resin commonly used, preferred material in industry and besides, it
yields highly rigid products with a low heat resistance property. The polyester
resin was prepared by mixing polyester of density 1.28 g/cm 3 with hardener
3554B of density 1.05 g/cm 3 at weight ratio 100: 1. The mechanical properties of
polyester resin are given in Table 2.

TABLE 2 Mechanical properties of polyester resin

Properties Polyester resin
Density (g/cm 3 ) 1.2 - 1 . 5
Young modulus (GPa) 2 - 4.5
Tensile strength (MPa) 40 - 9 0
Compressive strength (MPa) 90 - 2 5 0
Tensile elongation at break (%) 2
Water absorption 24h at 20 °C 0.1 - 0 . 3

Mould preparation

For tensile testing purpose the mould used for coconut fibres composite is made
from mild steel which followed the ASTM D638 Type 1 standard as shown as in
Figure 3. The mould was fabricated in machining lab using Advanced Machining
Lab machine. In addition, for the modal testing, the mould used for coconut coir
fibres composite is made from stainless steel square shape which length and
width of 210 mm as shown as in Figure 4.

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Study On The Dynamic Characteristic Of Coconut Fibre Reinforced Composites

FIGURE 3 ASTM D638 Type 1 FIGURE 4 The mould for plate

Preparation of Composites

Composites having different fibres content were prepared by varying the fibres
volume from 5% to 15%. In the first process of preparing the composite, a release
agent was used to clean and dry the mould before the polyester can be laid up on
the mould. The polyester was then mixed uniformly with the coconut fibres by
using a special brush in the mixed container. The mixture was poured carefully
into the moulds and flattened appropriately by using the roller before being dried
for 24 hours. After the composites were fully dried, they were separated off from
the moulds.

Mechanical testing

Tensile testing is the most common mechanical testing for determining the
physical properties of materials such as strength, ductility, toughness, elastic
modulus, and strain hardening. The tests consist of applying a constant strain on
the fibres and measure the load. Universal Testing Machine was used with strain
speed of 10 mm/min. The distance between clips was defined of 115 mm. Four
specimens were prepared in the study for each percentage of coir fibres in order
to get more accurate results.

Dynamic testing

Dynamic testing, sometimes called modal testing or experimental modal analysis

is a method used to extract model parameters such as natural frequency, damping
value and mode shape from the structure experimentally. The Frequency
Response Function (FRF) is a fundamental measurement produced by the testing
where the displacement, velocity, or acceleration response of a structure can be
measured. In the preparation of sample for modal testing, the composite plate
having the dimension 210 x 200 x 2 mm was prepared. The plate was divided
into 25 grid points as shown as in Figure 5 where at these points, Frequency
Response Functions (FRF) were measured in the range of 0-2000 Hz to identify
the modal characteristics. This 25 grid points were chosen to give adequate
spatial resolution to describe the global structural mode shapes.

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M..1. Mohd Nor, S. Abdullah, N. Jamaluddin, R. Ismail, S. Mohamed Haris and A. A r i f i n

FIGURE 5 Measurement locations

For the excitation purposes, basically there are two methods that can be used
which are impact hammer excitation method and shaker excitation method. In
this case, the impact hammer excitation method was chosen to determine the
modal parameter of composite structure. Figure 6 shows a typical experimental
setup for impact hammer test. The specimen was placed on a sponge to form a
free-free boundary condition. The accelerometer was mounted onto the specimen
at point 1 using wax so that the accelerometer would have the same vibration.
The modal parameters were then calculated using the Multi-Degree of Freedom
(MDOF) method. Some manipulation was done to obtain the resonance peaks in
the plot.

1- Computer
2- Data analyzer
3- Accelerometer
4- Specimen
2 5- Impact hammer

FIGURE 6 A typical experimental set-up for impact hammer test

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Study On The Dynamic Characteristic Of Coconut Fibre Reinforced Composites

RESULT AND DISCUSSION

Mechanical properties

The mechanical properties of coir fibres reinforced composites are expected to

depend on the content or volume fraction of the fibres in the composite (Murali &
Mohana 2007). Even a small change in the physical nature of fibres for a given
volume content of fibres may result in distinguished changes in the overall
mechanical properties of composites. Therefore the influence of fibres content on
mechanical properties of coir fibres reinforced composites was investigated.
Table 3 shows the mechanical properties of coir fibres reinforced composites
with fibres volume changing from 5 to 15%.

TABLE 3 Mechanical properties of composites with different coir fibres volume

Fiber content Tensile Failure Young

Strength Modulus
(vol%) Strain(%)
(MPa) (MPa)
5 24.8 3.9 633
10 21.9 4.8 461.4
15 19.8 6.1 318.8

Obviously, there is a good wetting between the fibre and matrix and a strong
interface is created which is led to a strong bonding. However, high percentage of
coir fibre will result in poor wetting between the coir fibre and polyester matrix.
It was found lead to the less area of fibre being bonded by the matrix which can
cause weak interface and thus lead to weak bonding. The composite will become
more easy to deform and flexible towards the increase of fibres content. Figure 7
shows the effect of fibres volume on the tensile strength of the composite. It
indicates that the tensile strength of composites decrease with increasing fibres
volume. This agrees with the conclusion of earlier work by Murali and Mohana
2007 that coir fibres do not enhance the tensile strength of composite. This result
reflects the lack of interfacial adhesion between matrix and fibres which behave
like voids in the continuous phase. However this behaviour make the structure
become more flexible. Figure 8 indicates that the coir fibre reinforced composites
experience ductile fractures which increase with the increment of the fibres
volume. The failure strain increases slightly from 3.9% to 4.8% when the volume
percentages in fibres increases from 5% to 10% and then rises again to reach 6.1
for fibres percentage of 15%. It can be notified that the evolution of the
composite failure strain with increasing of fibres volume is very significant since
the strain at break of the coir fibres and the polyester resin are too distant.

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M..1. Mohd Nor, S. Abdullah, N. Jamaluddin, R. Ismail, S. Mohamed Haris and A. Arifin

FIGURE 7 Tensile strength of composite FIGURE 8 Failure strain of composite

By the incorporation of coir fibres, the Modulus Young, E value of

composites increases up to 633 MPa for a fibres volume of 5% but on further
increasing the fibres content, the value decreases. Figure 9 shows that Modulus
Young, E steadily decreases with increasing fibres content which indicated lesser
contribution of the fibres towards the static mechanical properties of composites.
The minimum value of Modulus Young was obtained at a fibres volume of 15%
which specify ineffective stress transfer between the coir fibres and polyester
matrix.

FIGURE 9 Young modulus of each percentage of coir fibre

Dynamic characteristics

The natural frequency of the each percentage of coir fibres can be determined
from the plot of FRFs as shown in Figure 10. The value of the natural frequencies
can be obtained by taking the frequencies corresponding to the resonance peaks.
The range of frequency was setting up to 2500 Hz due to the usage of plastic tip.
The metal tip was not using for excitation because it has the tendency to create

194
double or multiple impacts when knocked onto the specimen even though it has a
bigger range of frequency (Ewins 1984).

FIGURE 10 Superimposed FRF for composite plate

(a) Natural frequency

Table 4 shows the result of natural frequency of coir fibre reinforced composites
for different fibre volume. Based on the data, there are inconsistent natural
frequencies for each percentage of coir fibre. This is true since the modes or
resonances are inherent properties of the structure. Resonances are determined by
the material properties and the boundary conditions of the structure (Bishop
1979). Therefore if the material properties of the structure change, its modes will
change.

TABLE 4 Natural frequencies of coir fibres reinforced composites

Natural freauencv mz)
Mode Fibre Content (vol. %)

Figure I I illustrates the graph of different natural frequency versus percentage

of coir fibres for all the frequency modes. Generally it indicates that the
M . . 1 . Mohd Nor, S. Abdullah, N. Jamaluddin, R. Ismail, S. Mohamed Haris and A. Arifin

composite with 5% volume of coir fibres shows the maximum value of natural
frequencies for the whole mode followed by 10% and 15% volume of coir fibres.
The composite with the 10% volume of coir fibres shows a slightly higher
frequency compared to 15% volume of coir fibres only for the first five mode
frequency. Somehow for the higher mode, it founds that the composite with 15%
coir fibres volume prove to have a higher value. Based on the frequency's
theoretical formula, the natural frequency of the structure depends on the stiffness
and the mass of the structure. Therefore, an increasing of the stiffness will
influence the natural frequency which increased the value. While for the mass, an
increment of the mass value will reduce the natural frequency of the structure. As
we know, each material that has been studied has its own density, hence a
reinforcement of the natural fibres will affect the mass of the structure where any
additional density value gives an increasing to the mass of the structure (Iglesias
2000). However in this case, the mass of the composites were setting almost the
same for all the three percentage of coir fibres. From this result, it can be
conclude that the composite with the low fibre volume is much stiffer which
shows lesser contribution of the coir fibres towards the stiffness of the material.
This also related to Modulus Young of the structure since the stiffness value
always depend on Modulus Young. Thus, an increase of Modulus Young has
been found to increase the natural frequency value indicating structure with high
value of the Modulus Young and tensile strength is stiffer and linear proportional
to the natural frequency value. From the tensile test, the results showed that 5%
of coir fibre composite had a good strength and this identified the results taken
from the experimental modal analysis was agreed with the theoretical formulation
of the tensile strength of studied composite.

2500

2000

o 1500 5% Coir Fibre

» 10% Coir Fibre
9 1000 — 1 5 % Coir Fibre

500

0 »""i , , , , , , , ,
1 2 3 4 5 6 7 8 9 1 0
Modes Shape

FIGURE 11 Natural frequencies of each percentage of coir fibres

196
Study On The Dynamic Characteristic Of Coconut Fibre Reinforced Composites

(b) Damping ratio

Based on the theoretical formulation for the damping ratio, the stiffness, mass
and damping peaks can give an effect to the damping ratio value. Figure 12
shows the effect of fibres volume percentage on damping ratio for all the modes.
By the incorporation of coir fibres, it appears that the damping ratio of composite
is increasing only for the first five modes. However for next higher modes, the
results of damping ratio are found inconsistent. In all cases, the peaks of damping
ratio for each percentage of coir fibres composite was found to decrease when the
modes increase. The composite with the volume of 15% of coir fibre shows the
high damping ratios. These values are agreed with the theoretical formulation
since any decrement of the stiffness and the mass will give an increasing value of
the damping ratio (Avitable 2001). By adding the coir fibre obviously gives the
structure vibrating in less oscillatory motion. Therefore, it gives advantage to the
structure in reducing the high resonant amplitude.

Modes Shape

FIGURE 12 Damping ratio of each percentage of coir fibre

(c) Mode shape

Figure 13 shows the first four modes shape of coir fibre reinforced composite
plate with varying the percentage of coir fibre volume. The contour represents the
residue magnitude value where the red colour stands for the largest residue
magnitude and blue colour stands for the minimum residue magnitude. For all the
cases, the plate only experiences the global vibration where the whole structure is
vibrating. The first four mode shapes of each percentage of coir fibres were
observed which can be identified from 20-350 Hz. It was found that the first and
second mode shape of the structure experienced the torsion mode. While for the
third and fourth mode shapes of the structure were observed as bending mode.

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Regional Conference on Engineering Mathematics, Mechanics, Manufacturing & Architecture (EM1 ARC) 2007
© 2007 Noise, Vibration and Comfort Research Group
Editors: M. J. MohdNor, S. Abdullah, N. Jamaluddin, R. Ismail, S. Mohamed Haris andA.Arifin

Mode 5% Coir Fibre Volume 10% Coir Fibre Volume 15% Coir Fibre Volume Description of Mode Shape

Torsion

Torsion

Bending

Bending
Study On The Dynamic Characteristic Of Coconut Fibre Reinforced Composites

FIGURE 13 Mode shape of composite plate reinforced coir fibre

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Regional Conference on Engineering Mathematics, Mechanics, Manufacturing & Architecture (EM1 ARC) 2007
© 2007 Noise, Vibration and Comfort Research Group
Editors: M../. MohdNor, S. Abdullah, N. Jamaluddin, R. Ismail, S. Mohamed Haris and A.Arifm

CONCLUSIONS

The research was carried out to investigate the static and dynamic mechanical
analysis of randomly oriented mixed coir fibres reinforced polyester composite.
The effect of coir fibres volume on mechanical properties and dynamic
characteristic of composite were studied. The results were found that the
mechanical properties have a strong association with the dynamic characteristic.
Both of the properties are greatly dependent on the volume percentage of fibres.
In general, the composite having a coir fibres volume of 5% showed the best
result. The tensile strength and Modulus Young were found to be decreased with
incorporation of coir fibres which again points to the ineffective stress transfer
between fibres and polyester resin. However the increase of coir fibres will make
the composite tend to have low stiffness and ductility. Dynamic characteristics
such as natural frequency of the composite can be predicted by analyzing the
mechanical properties. The tensile strength of composite was found to be linearly
proportional to the natural frequency. Moreover, the damping ratio was found to
be increased by incorporation of coir fibres which giving an advantage to the
structure in reducing the high resonant amplitude.

ACKNOWLEDGEMENT

The authors wish to acknowledge the support given by the Government of

Malaysia for fundamental research grant (vot. 0223) and the Vibration and
Materials Lab, Faculty of Mechanical and Manufacturing Engineering of
Universiti Tun Hussein Onn Malaysia for providing facilities for this research.
Also, the authors would like to thank Mr. Iskandar for the assist in completing the
research project.

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