Original Research Open AccessEvaluation of.docx

Original Research Open Access
Evaluation of tensile, flexural and Impact strength of
natural and glass fiber reinforced hybrid composites
Ashik K P2*, Ramesh S. Sharma1 and Subhash Patil2
1Professor, Department of Mechanical Engineering RVCE,
Bengaluru, India.
2Research Scholar, Department of Mechanical Engineering
RVCE, Bengaluru, India.
*Correspondence: [email protected]
Abstract
The development of composite materials made up of natural
fibers is improving in engineering applications
such as Automotive, Marine and Aerospace, due to its
properties such as high specific strength, renewable,
non-abrasive, low cost, bio-degradability. Many researchers
have identified different natural fibers used
to substitute glass fiber, among them jute appears to be
favorable material because of its low cost, high
strength, high aspect ratio, good insulating and low thermal
conductivity. Hence the objective of this
research work was to evaluate the mechanical properties of
hybrid composites such as tensile strength,
f lexural strength and impact strength using static test methods
as per ASTM standards and Finite
Element Analysis was done to evaluate the properties of the
composite laminate and compared results of
FEA with experimental results. The composite laminates used
for the present investigation was fabricated
using hand layup technique. Incorporation of natural and glass

fiber in different stacking sequence used to
prepare hybrid laminates. The outcome of the present
investigation indicate that hybrid composite laminates
improve strength and can be used as alternate material for
automotive application and Finite Element
Analysis results proved the experimental results.
Keywords: Hybrid composites, mechanical properties, natural
fiber, finite element analysis
© 2017 Ashik K P et al; licensee Herbert Publications Ltd. This
is an Open Access article distributed under the terms of
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/3.0). This permits
unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
Introduction
The improvement of composite materials and their related
outline and assembling innovations is a standout amongst the
most imperative advances ever. Composites are the material
utilized as a part of different fields having selective mechanical
and physical properties and are created for specific variety of
application. Composite materials having a scope of preferences
over other routine materials, for example, elasticity, sway qual-
ity, flexural qualities, solidness and exhaustion attributes. As a
result of their various preferences they are generally utilized as
a part of the avionic business, business mechanical designing
applications, similar to machine segments, vehicles, burning
motors, mechanical segments like shafts, weight vessels and
flywheels, warm control and electronic bundling, railroad men-
tors and air ship structures and so on. The interest in natural
fiber
reinforced composite materials is rapidly growing both in terms
of engineering applications and research. These natural fibers
are completely renewable, environmental friendly, low cost and
bio-degradable, due to this characteristics natural fibers have

recently become attractive to researchers and scientists as an
alternative method for fibers reinforced composites. Among
natural fibers jute fiber appears to be promising material
because
it is inexpensive, high strength, high aspect ratio, good insulat-
ing and completely bio-degradable and recyclable. Consistency
of the composites were studied by Manuel Chiachio, et al., [1]
studied the basic structure of composite material and they
concluded that composite materials can be used in wide variety
of application such as automotive, aerospace and construction
applications. Michael Karus, et al., [2] has studied the demand
for natural fibers in automotive sector and interest in natural
fibers increasing day by day. Manufacturing of composite ma-
terials made by the natural fibers using compression moulding
has studied and natural fibers used in the transport segment
increasing in European market also explained. AninMemon,et
al., [3] has carried out investigation on jute reinforced poly-
mer composites. Composites fabricated using compression
moulding, when temperature increased effect of dispersion of
fiber from the mould increased. Furthermore tensile strength
decreased due to dispersion of fibers. Hence, investigation
concluded that temperature of mould effect the properties
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of composite material. Md. RashnalHossain et al., [4] studied
the properties of jute epoxy composite material because of
its properties compared synthetic fibers. In the investigation
composites were fabricated by using hand layup method and
composites were subjected to mechanical tests such as tensile
and flexural test, results concluded that tensile and flexural
strength of the composite depends on volume of fiber in the
composite material.Jansons et al., [5] evaluated the effect of
moisture and temperature as well as fatigue properties of
carbon fiber reinforced polymer composites. From the study
reveals that moisture absorption depends on the thickness of
the laminate and properties also depends on the thickness
of the composite. Kutty and Nandoet al., [6] investigated the
process parameters influences on the properties of Kevlar
and aramid composite laminates, investigation explains the
process parameters such as nip gap, friction ratio and mill
roll temperature maximum influence on the fiber sequence
and also influences the mechanical properties of composite.
Yuan et al., [7] studied the effect of modified Kevlar fibers
on the mechanical properties of wood-flour/polypropylene
composites. Study reveals that the addition of reinforcement
increases the properties of composites.Wang et al., [8] studied
static properties of synthetic fibers such as glass and Kevlar
reinforced composites from the investigation results shows
strength of the composite depends on the type of reinforce-
ment used in the composite laminate. After vast literature
survey carried on natural fibers and its propertiesits concluded

that lack of research progressed on jute fiber and its applica-
tion. Hence objective of this research work is to Evaluate the
Tensile, Flexural and Impact Strength of Natural and Glass
Fiber Reinforced Hybrid Composites.
Experimental
Material selection
In the present investigation jute fiber was procured from the
Jute Pragnya, Bengaluru, India. Glass Fiber was supplied by
Marchtech
Solution
s Bengaluru, India. The polyester resin
used in the investigation was general purpose polyester resin
fb-333 and the hardener used was Catalystmethyl ethyl ke-
tone peroxide (MEKP) and accelerator was cobalt napthenate.
Both resin and hardener procured from the commercial resin
supplier, Bengaluru, India. Figure 1 shows the Bi-Directional
jute and glass fiber mat.
Fabrication of composite laminate
Composite Laminate was fabricated by Hand layup technique.
The bi-directional jute fibre and the E-glass fibres were used
as reinforcement and Polyester was considered as matrix
material. Four laminates were prepared with different fiber
orientation as shown in Table 1. Bi woven fiber reinforce-

ments used for the laminate because these fibers gives better
strength compared to uni directional and chopped fibers Each
laminate included three layers of reinforcement in 00/900 Bi
woven fiber direction. Laminate L1 contains three layers of
Figure 1. Bi-Directional jute and glass fiber mat.
Laminate Composition
L1 100% Jute Fiber
L2 100% Glass Fiber
L3 60% Jute and 40% Glass Fiber
L4 60% Glass and 40% Jute Fiber
Table 1. Laminates and its composition.
jute fiber, laminate L2 contains three layers of glass fiber and
laminate L3 contains two layers of jute fiber and one layer of
glass fiber similarly laminate L4 contains two layers of glass
fiber and one layer of jute fiber.
The sticking of polyester resin to the surface was avoided
by spraying the release gel on the mould surface. In order to
obtain better surface finish, at both ends of themould, thin
plastic layer were placed for easy removing of laminate. As
per the mould size, E-glass fibers and woven mat jute fabrics

were cut for the reinforcement and positioned on the layer of
the mould after plastic layer. After reinforcement, polyester
resin taken in a liquid form was added carefully in appropri-
ate amount with hardener in the ratio 1:0.2:0.2 and resin was
poured onto the layer of reinforcement already positioned
on the mould. The polyester resin was evenly applied using
brush. The next layer of reinforcement was positioned on the
polyester resin with the help of roller with slight pressure on
the reinforcement-polyester layer to take out any air voids as
well as theleftover polyester resin on the mould. The process
was repeated for each layer of polyester and reinforcement,
until the necessary layers were placed. On the surface of fiber
and polyester resin plastic layer was placed, again for easy
removal of mould release gel was sprayed on the above layer
of the top mould and pressure was applied. After curing at the
temperature of 80˚C, the mould was removedand the ready
composite laminate was obtained and it was machined for
the required dimensions. Preparation of composite laminate
using hand layup technique is shown in the Figure 2.
Densities of the laminate calculated using the relation
weight of the laminate divided by area of the laminate. Densi-
ties of Laminate represented in the Table 2.

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Mechanical characterization
Tensile strength
Tensile test or tension test is a basic test in which a specimen
is subjected to uniaxial tension until the material fractures,
uniaxial tension refers to the force acting on the opposite faces
of the material in opposite direction with respect to each other
along the same axis. The material is placed between two grips
and is subjected to tensile or tension force. This force causes
the gauge length of the material to elongate and finally mate-
rial fractures. Thus, Elongation, final cross sectional area and
peak load of the material are obtained directly from tensile
test. In the present investigation,the tensile test laminates
were prepared asper ASTM: D3039(Dimension is 250×25×3
mm) and the testing was carried out using universal testing
machine Model: KIC-2-1000-C, it can withstand maximum

load of 10KN and machine was connected with a computer
and results are obtained in graphs and values are recorded
in the separate file. Figure 3 indicates the experimental set
up for tensile strengthtest.
Flexural strength
In the present investigation, laminates prepared as per ASTM:
D790 Standard.(Dimension is 127×12.7×3 mm) Flexural test
was carried in a three point flexural setup in universal test-
ing machine. Test carried to all four different compositions
of laminates. Figure 4 represent the loading setup of the
laminates for flexuraltest.
Table 2. Laminates and its composition.
Figure 2. Laminates fabricated using hand lay-up technique.
Figure 3. Experimental set up for tensile strengthtest.
Laminate Weight Wc
(in grams)
Density ρc
(g/cm3)
L1 699 1.23

L2 776 1.81
L3 727 1.62
L4 677 1.35
Figure 4. Loading arrangement of the specimens for
flexuraltest.
Finite element analysis
In the present investigation, static analysis of composites
was studied with the help of ANSYS 15. The modeling of
specimens was done in ANSYS considering element type as
SHELL 181, throughout the study. The element has four nodes
with six degrees of freedom at each node: translations in the
x, y, and z axes, and rotations about the x, y and z axes. Thus
each element has 24 degrees of freedom in total. The element
size of 2 was considered for meshing of specimen. Figure 5
represent the meshing used for the tensile and flexural test.
Using the simple rule-of-mixtures [9], elastic constants of the
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unidirectional composite were calculated.
For bi-woven fiber reinforced lamina
Where UD and WF denote unidirectional fiber and woven
fiber respectively. After calculating the elastic properties of
Figure 5. Meshing Used for Tensile Test (a) and flexural test
(b).
(1)
(2)
(3)
(4)
(5)

(6)
(7)
(8)
(9)
(10)
(11)
(12)
unidirectional lamina in the equation 1-6, bi directional lamina
elastic properties were calculated using equation 7-12 [11].
Impact strength
In the present investigationimpact testing was carried in a
charpy impact setup. Composite laminate prepared for the
test as per ASTM: D256 standard. The effect of strain rate on
fracture and ductility of the material was analyzed. Figure 6
indicate the experimental set up and loading arrangement
of the specimens for impact test.

Figure 6. Experimental set up and loading arrangement of
the specimens for impact test.
Results and discussion
Tensile properties
In the investigation tensile test was carried out by put on
tensile load on the composite laminate. Four different volume
of fibers and resin were tested. In every test, three samples
were tested to achieve average values of same composition
of laminateand results were noted. The laminate was fixed
in the fixture of the machine and load was applied and the
corresponding change in length of the specimen wasrecorded.
The load was applied on the laminate until it breaks and peak
load, ultimate tensile strengths were recorded. After testing
stress and strain curve obtained from the software it was
recorded and load v/s displacement graphs were generated.
Figure 7 represent the tensile test specimens.
Figure 8 shows the tensile strength of four different lami-
nates with different percentage of fiber and resin content. It
can be observed from Figure 8 that tensile strength of the
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laminate 2 which include 100% of glass fiber shows higher
tensile strength compared all other laminates. Laminate with
higher volume of glass fiber in the laminate 4 shows better
strength compared to laminate 1 and laminate 2 with higher
volume of jute fiber. Hence, from the volume fraction of glass
fiber higher in the laminates gives higher tensile strength.
Flexural properties
Flexural test was carried in the same universal testing machine
by put on load in between the test length of the composite.
Test carried on the four different laminate and the various
parameters of flexural testing was determined. peak load was
recorded and load v/s displacement graphs were generated.
Figure 9 shows the fractured flexural specimens.
Figure 7. Tensile testing specimen of jute/glass fibre.

Figure 8. Variation of tensile strength for different fiber
percentage of laminates.
Figure 9. Fractured Flexural tested specimen.
Figure 10. Variation of flexural strength for different fiber
percentage of laminates.
Figure 10 shows the flexural strength of four different laminates
with different percentage of fiber and resin content. It can
be observed from Figure 10 flexural strength of laminate 2
shows higher flexural strength compared to other laminates.
It can be observed that laminate with higher percentage
shows glass fiber gives better tensile strength compared to
any other combinations.
Finite element analysis result
Finite element analysis carried using ANSYS to validate
the experimental tensile and flexural results [10]. It can be
observed that experimental results shows good agreement
with the ANSYS values. It was concluded that by assuming
uniform mixture and properties of the fibers in the entire
laminate, the tensile strength of L2 laminate (279MPa) was
17.9% higher than L4 laminate (229MPa), 44.6% higher than
L3 (154.5MPa) laminate and 72% higher than L1 laminate.

In case of flexural testing, flexural strength of L2 laminate
(373MPa) was 19.3% higher than L4 laminate (301MPa),
42.09%
higher than L3 (216MPa) laminate and 68.36% higher than
L1 laminate (118MPa). The results of tensile and flexural test,
both experimental and ANSYS are tabulated in Tables 3 and 4.
The Figure 11 shows the contour plots of von mises stress
Laminates Experimental (MPa) ANSYS (MPa) % Difference
L1 78 86.31 9.6
L2 279 271.78 2.5
L3 154 156.6 1.6
L4 229 221.67 3.2
Table 3. Tensile Strength of laminates.
Table 4. Flexural Strength of laminates.
Laminates Experimental (MPa) ANSYS (MPa) % Difference
L1 118 122.6 3.7
L2 373 380.09 1.8
L3 210 212.06 1.9
L4 301 305.07 1.3

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of L1, L2, L3 and L4 laminates for Tensile and flexural test
analysis respectively.
Impact strength
Charpy Impact Test was used to determine the impact prop-
erties of the material. The effect of strain rate on fracture
and ductility of the material ws analyzed. From the Figure 12
it can be observed that laminate 2 with 100% of glass fiber
absorbed more energy compared to other laminates because
of high volume of glass fiber in the laminates. Figure 13 shows
the energy absorbed in the impact testing of the laminates.
Conclusions
The Finite Element Analysis and experimental studies on

the mechanical properties of natural and syntheticfiber was
investigated. Effect of fiber loading and orientation on me-
Figure 11. Finite element tensile test analysis of Laminates
L1 (a), L2 (b), L3 (c) and L4 (d) for von mises stress.
Figure 12. Finite element Flexural test analysis of Laminates L1
(a), L2 (b), L3 (c) and L4 (d) for von mises stress.
Figure 13. Energy absorbed by the laminates in impact testing.
chanical properties of jute and glass fiber reinforced polymer
based hybrid composites led to the following conclusions:
In tensile test, laminate L1 with 100% of jute fiber shows
decrease in strength when compared with laminate L2 with
100% of Glass fiber.
In flexural test, composite laminate L3 & L4 with Jute and
Glass fiber orientation concludesimproved strength than
laminate L1. In the same way Laminate L4 shows improved
strength than laminate L3, because volume of glass fiber was
higher in the laminate.
The combination of reinforcement such as glass fiber and

jute fiber in composite laminates improves the mechanical
strength and this makes way to the increase of the utilization
of natural fibers in various applications.
Experimental results validated using Finite Element Analysis,
results from the analysis proved the experimental results.
From the present investigation it has been observed that
the composites with natural fiber and synthetic fiber increases
mechanical strength such as tensile strength, flexural strength,
impact strength of the composites with the increase in fiber
and also strengthsignificantlyvaried by the fiber composition.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
Acknowledgement
Authors thankfully acknowledge Management, Principal
and Head of the Department, Mechanical Engineering,
R V College of Engineering for their constant support
and encouragement in carrying out this work.
Publication history

EIC: Saffa Riffat, University of Nottingham, UK.
Received: 30-Nov-2016 Final Revised: 14-Mar-2017
Accepted: 03-Apr-2017 Published: 18-Apr-2017
Authors’ contributions AKP RSS SP
Research concept and design -- ✓ --
Collection and/or assembly of data ✓ -- --
Data analysis and interpretation -- ✓ --
Writing the article ✓ -- --
Critical revision of the article -- ✓ --
Final approval of article -- ✓ --
Statistical analysis -- -- ✓
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doi: 10.7243/2052-6237-5-1

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Citation:
K P A, Sharma RS and Patil S. Evaluation of tensile,
flexural and Impact strength of natural and glass fiber
reinforced hybrid composites. Renew Bioresour. 2017;
5:1. http://dx.doi.org/10.7243/2052-6237-5-1
http://dx.doi.org/10.7243/2052-6237-5-1
http://dx.doi.org/10.1016/j.compositesb.2011.10.007
http://dx.doi.org/10.1300/J237v07n01_10
https://doi.org/10.1016/j.egypro.2013.06.819
https://doi.org/10.1016/j.proeng.2013.03.196
https://doi.org/10.1023/A:1020024007414
https://doi.org/10.1007/s11676-013-0335-z
https://doi.org/10.1016/0961-9526(95)00100-2
https://doi.org/10.1016/j.proeng.2014.12.390
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.207.5
914&rep=rep1&type=pdf
http://dx.doi.org/10.7243/2052-6237-5-
1AbstractIntroductionExperimentalMaterial
selectionFabrication of composite laminateMechanical
characterizationTensile strengthFlexural strengthFinite element
analysisImpact strengthResults and discussionTensile

propertiesFlexural propertiesFinite element analysis
resultImpact strengthConclusionsCompeting interestsAuthors’
contributionsAcknowledgementPublication historyReferences
Procedia Engineering 51 ( 2013 ) 561 – 566
1877-7058 © 2013 The Authors. Published by Elsevier Ltd.
Selection and peer-review under responsibility of Institute of
Technology, Nirma University, Ahmedabad.
doi: 10.1016/j.proeng.2013.01.079
Chemical, Civil and Mechanical Engineering Tracks of 3rd
Nirma University International Conference
on Engineering
(NUiCONE 2012)
Physical and Mechanical Properties of Bi-directional Jute Fiber
epoxy
Composites
Vivek Mishra*, Sandhyarani Biswas

Department of Mechanical Engineering, National Institute of
Technology, Rourkela 769008, India
Abstract
During last few years, the interest in using natural fibers as
reinforcement in polymers has increased dramatically. Natural
fibers are not
only strong and lightweight but also relatively very cheap. In
this research work, an investigation has been carried out to
make use of jute
fiber, a natural fiber abundantly available in India. The present
work describes the development and characterization of a new
set of
natural fiber based polymer composites consisting of
bidirectional jute fiber mat as reinforcement and epoxy resin as
matrix material. The
composites are fabricated using hand lay-up technique and are
characterized with respect to their physical and mechanical
properties.
Experiments are carried out to study the effect of fiber loading
on the physical and mechanical behavior of these composites.
Result
shows the significant effect of fiber loading on the mechanical
properties of the composites. Also, the formation of voids in the

composites is an influencing factor on the mechanical
properties.
© 2012 Published by Elsevier Ltd. Selection and/or peer-review
under responsibility of the Institute of Technology Nirma
University, Ahmedabad.
Keywords: Natural fiber; Jute fiber; Epoxy; Mechanical
Properties
Nomenclature
volume fraction percentage
W weight fraction
Greek symbols
density
Subscripts
ct composite
ex experimental
f fiber
m matrix
1. Introduction

Over the past few decades there is a rapid increase in the
demand of the fiber reinforced polymer (FRP) composites
because of the unique combination of high performance, great
versatility and processing advantages at favorable costs by
permutation and combination of different fibers and polymers
[1]. FRP composites possesses interesting properties like high
specific strength and stiffness, good fatigue performance and
damage tolerances, low thermal expansion, non-magnetic
* Corresponding author. Tel.:+91-9437779575; fax: +91-661-
2472926.
E-mail address: [email protected]
Available online at www.sciencedirect.com
© 2013 The Authors. Published by Elsevier Ltd.
Selection and peer-review under responsibility of Institute of
Technology, Nirma University, Ahmedabad.
562 Vivek Mishra and Sandhyarani Biswas / Procedia
Engineering 51 ( 2013 ) 561 – 566

properties, corrosion resistance and low energy consumption
during fabrication [2]. Fiber reinforced composites made up of
carbon, boron, glass and kevlar fibers have been accepted
widely as the materials for structural and non-structural
applications [3].
Environmental concerns are increasing day by day and the
demand of replacing the existing synthetic fibers with the
biodegradable, renewable and low cost natural fibers for
fabrication of composite materials increases. In comparison to
the
traditional reinforcing materials natural fiber such as sisal, jute,
abaca, pineapple and coir has acceptable specific strength
properties, low density, low abrasion multi-functionality, good
thermal properties, enhanced energy recovery and cause less
skin and respiratory irritation [4, 5]. Pervaiz and Sain [6]
examined the energy consumption of glass and natural fibers,
and
they found that by using vegetal fibers in place of glass fibers,
energy could be saved at a rate of 60% per ton of product.
Jute, a natural fiber in polymer composites would be suitable
for the primary structural applications, such as indoor elements
in housing, temporary outdoor applications like low-cost
housing for defence and rehabilitation and transportation. The
insulating characteristics of jute may find applications in

automotive door/ceiling panels and panel separating the engine
and
passenger compartments [7].The use of natural fiber like jute
not only help us in ecological balance but can also provide
employment to the rural people in countries like India and
Bangladesh where jute is abundantly available.
In this study bi-directional fiber mat has been used for the
preparation of the composites. The purpose of this study is to
investigate the potential utilization of jute fiber as
reinforcement in polymer matrix composites. Also, the effect of
jute fiber
content on the physical and mechanical behavior of the
composites is investigated.
2. Experimental Details
2.1. Materials and Method
Bidirectional jute fiber mat has been obtained from the local
sources as a reinforcing material. Epoxy resin and the
corresponding hardner are supplied by Ciba Geigy India Ltd.
The polymers composites are fabricated by hand lay-up
technique. Composite specimens with different fiber loading (0,
12, 24, 36 and 48 wt %) were prepared and subjected to
postcuring for 24 hours at room temperature.

2.2. Physical and Mechanical Characterization
The theoretical density of the composites can be obtained in
terms of the weight fractions and densities of the
constituents, and is given by Eq. (1)
1
(1)
/ /
ct
f f m mW W
ct, f and m correspond to the composites, fiber
and matrix, respectively.
Water immersion technique has been used to determine the
actual density of the prepared composites experimentally.
The volume fraction of voids in composites is given by the

relation
(2)ct ex
ct
v
where ex is the experimental density of the composite
fabricated.
Hardness measurement is done using a Rockwell-hardness tester
equipped with a steel ball indenter. Tensile test is
performed as per ASTM D 3039-76 test standards using
universal testing machine Instron 1195.Three point bend test is
carried out in the same machine at a cross head speed of 10
mm/min to obtain the flexural strength and inter laminar shear
strength (ILSS). Impact strength of the composites is evaluated
by a low velocity impact tests conducted in an impact tester
as per ASTM D 256 test standards.
3. Result and Discussion
3.1. Physical and Mechanical Properties

The theoretical density, experimental density and void fraction
(in percentage) are reported in the Table 1. The presence
of the voids may affect the mechanical properties of the
composites. The void formation in the polymer composites can
Engineering 51 ( 2013 ) 561 – 566
occur due to air entrapment during the preparation of resin
system and moisture absorption during the material processing
or
storage. A higher void content in the composites shows that
resin has not thoroughly surrounded the fibers and resulting in
weaker interfacial strength which in turn reduces strength and
stiffness of composites, mutual abrasion of fiber leads to fiber
fracture and damage and crack initiation and growth due to void
coalescence[8]. From Table 1 it is found that pure epoxy
has the minimum void content, with the addition of 12 wt. %
fiber the void content increases instantly to 5.312 %. But with
the further increase in the fiber content from 12 wt. % to 48 wt.
% the void content of the specimens decreases. The
theoretical density of the composites increases as the fiber
loading increases.

Table 1. Comparison between Experimental density and
Theoretical density
BJFE: bidirectional jute fiber epoxy, BD: bidirectional
Figure 1 shows the effect of fiber loading on the hardness of
composites. It has been found that the hardness of the
composite increases with the increase in the fiber loading. In
general the fibers increase the modulus of composite which in
turn increases the hardness of fiber. This is because hardness is
a function of relative fiber volume and modulus [9]. Surface
hardness value of 40 HRB is obtained from pure epoxy
specimen. The surface hardness value increases by 77% with the
incorporation of 12 wt. % fiber in the matrix. The maximum
surface hardness value of 85.5 HRB is obtained fro m
bidirectional jute epoxy composites reinforced with 48 wt. % of
jute fiber.
Fig 1. Effect of fiber loading on hardness of composites
The variation in tensile strength and tensile modulus of

composite with increase in fiber content is shown in Fig 2. It is
clearly visible that with the increase in fiber content in the
epoxy matrix, the tensile strength and modulus also increases.
There is a proper transmission and distribution of the applied
stress by the epoxy resin resulting in higher strength. Similar
observations have been made by Bijwe [10] in case of aramid
fabric/polyethersulfone composites. The bidirectional jute
fiber composite can bear higher load before failure compared to
neat or unfilled epoxy. The tensile strength varies from 43
MPa to 110 MPa and tensile modulus from 0.15 GPa to 4.45
GPa with the fiber varies from 0 to 48 wt%.
The result obtained from the three point bend test is shown in
Fig 3. It has been found that there is a reduction in the
flexural properties of specimen with 12 wt. % fiber loading.
Similar observations have also been made by Dong and Davies
[11]. According to their study, the reduction in the flexural
properties of the composites is due to weak interfacial bonding
and existence of voids. The flexural strength and modulus of the
composites increases with the increase in the fiber loading
Designation
Composite composition
Theoretical

ct)
g/cm3
Expt.
Density
ex)
g/cm3
Void Fraction (%)
BJFE-1 Epoxy + 0 wt.-% Jute Fiber (BD) 1.150 1.147 0.261
BJFE-2 Epoxy + 12wt.-% Jute Fiber (BD) 1.170 1.108 5.312
Engineering 51 ( 2013 ) 561 – 566
after 12 wt. % fiber loading. The maximum flexural strength
and modulus of 55.8 MPa and 3.02 GPa respectively, is

obtained at 48 wt. % of fiber loading. The flexural strength and
modulus of 48 wt. % fiber loading are increased by 20 %
and 37 % in comparison to the neat epoxy. The jute fiber
inclusions enhance the load bearing capacity and ability to
withstand bending of the composites [12].
The effect of fiber loading on the inter-laminar shear strength
(ILSS) of the jute epoxy composite is shown in Fig 4. The
ILSS value decreases drastically for the composites with fiber
loading from 0 wt. % to 12 wt. %, however it increases on
further increase in fiber loading from 12 wt. % to 48 wt. %. The
maximum ILSS of 66.5 MPa is obtained at 48 wt. % fiber
loading.
Fig 2. Effect of fiber loading on tensile strength and modulus of
composites
Fig 3. Effect of fiber loading on flexural strength and modulus
of composites
Fig 4. Effect of fiber loading on inter-laminar shear strength of
composites

Engineering 51 ( 2013 ) 561 – 566
The impact strength of the bidirectional jute epoxy composites
in shown in Fig 5. The energy absorbed by the composite
due to impact load is 2.87, 3.69, 4.264, 4.59 times of pure
epoxy matrix for composites with fiber content of 12 wt. %, 24
wt.%, 36 wt.% and 48 wt.% respectively. The maximum impact
strength is of 4.875 J in the case of composite with 48 wt.%
of fiber loading. The increase in the impact strength with the
increased fiber loading may be due to the fact that more energy
will have to be used up to break the coupling between the
interlaced fiber bundles. Good adhesion between the fiber and
matrix is also responsible for the good resistance to crack
propagation during impact test. The increased fiber content will
increase the contact area between the fiber and matrix, if there
is good impregnation of fibers in the resin. At higher fiber
loading the impact transfer should be more efficient [15].

Fig 5. Effect of fiber loading on impact strength of composites
4. Conclusion
The following conclusions have been drawn from the study of
the jute epoxy composite:
1. Successful fabrication of the bidirectional jute fiber
reinforced epoxy composite has been done by the hand lay-up
technique.
2. The minimum and maximum void content are in neat epoxy
and 12 wt. % fiber loading specimens respectively. It is also
found from the study that the void content decreases with the
increase in fiber loading.
3. The hardness, tensile properties and impact strength of the
jute-epoxy composites increases with the increase in fiber
loading.
4. The properties like flexural strength and inter-laminar shear
strength are greatly influenced by the void content of the
composites. It has been found that these properties reduced
from 0 wt.% to 12 wt.% fiber loading and with the reduction in
the void content from 12 wt.% to 48 wt.% the properties are
improved.
References

[1] Zaman H. U., Khan A., Khan R. A., Huq T., Khan M. A.,
Shahruzzaman Md., Mushfequr Rahman Md., Al-Mamun Md.,
and Poddar P., 2010.
Preparation and Characterization of Jute Fabrics Reinforced
Urethane Based Thermoset Composites: Effect of UV Radiation,
Fibers and Polymers,
11(2), p. 258.
[2] Jawaid M., Abdul Khalil H.P.S., Abu Bakar A., Noorunnisa
Khanam P., 2011. Chemical resistance, void content and tensile
properties of oil
palm/jute fibre reinforced polymer hybrid composites, Materials
and Design, 32, p. 1014.
[3] Gowda T. M., Naidu A.C.B., Rajput C., 1999. Some
mechanical properties of untreated jute fabric-reinforced
polyester composites, Composites: Part
A, 30, p. 277.
[4] Huq T., Khan A., Akter T., Noor N., Dey K., Sarker B.,
Saha M.,2011. Thermo-mechanical, Degradation, and Interfacial
Properties of Jute Fiber-
reinforced PET-based Composite, DOI:
10.1177/0892705711401846.

[5] Chin C.W., Yousif B.F., 2009.Potential of kenaf fibres as
reinforcement for tribological applications, Wear, 267, p. 1550.
[6] Pervaiz M., Sain M.M., 2003. Carbon storage potential in
natural fibre composites, Resources Conservation and Recycling
39(4), p.325.
[7] Khondker O. A., Ishiaku U S., Nakai A., Hamada H., 2005.
Fabrication and Mechanical Properties of Unidirectional Jute/PP
Composites Using Jute
Yarns by Film Stacking Method, Journal of Polymers and the
Environment, 13(2) , p. 115.
[8] Boey F.Y.C., 1990. Reducing the Void Content and its
Variability in Polymeric Fibre Reinforced Composite Test
Specimens using a Vacuum
Injection Moulding Process, Polymer Testing, 9 , p. 363.
[9] Srinivasa C.V., Bharath K.N., 2011.Impact and Hardness
Properties of Areca Fibre-Epoxy Reinforced Composites,
Journal of Material Science and
Engineering 51 ( 2013 ) 561 – 566

Environment, 2(4), p. 351.
[10] Bijwe J., Awtade S., Satapathy B.K., Ghosh A., 2004.
Influence of concentration of aramid fabric on abrasive wear
performance of polyethersulfone
composites, Tribology Letters, 17 (2), p. 187.
[11] Dong C., Davies I.J., 2011.Flexural Properties of Wheat
Straw Reinforced Polyester Composites, American Journal of
Materials Science, 1(2), p. 71.
[12] Mantry S., Satapathy A., Jha A.K., Singh S.K., Patnaik A.,
2010. Processing and Characterization of Jute Epoxy
Composites Reinforced with SiC
Derived from Rice Husk, 29(18), p. 2869.
[13] Åkesson D., Skrifvars M., Seppälä J., Turunen M.,
2011.Thermoset Lactic Acid-Based Resin as a Matrix for Flax
Fibers, Journal of Applied Polymer
Science, 119, p. 3004.

Composites: Part B 42 (2011) 856–873
Contents lists available at ScienceDirect
Composites: Part B
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c
a t e / c o m p o s i t e s b
A review on the tensile properties of natural fiber reinforced
polymer composites
H. Ku ⇑ , H. Wang, N. Pattarachaiyakoop, M. Trada
Centre of Excellence in Engineered Fibre Composites and
Faculty of Engineering, University of Southern Queensland,
Australia
a r t i c l e i n f o
Article history:
Received 15 March 2010
Received in revised form 19 November 2010
Accepted 8 January 2011
Available online 15 January 2011
Keywords:
A. Polymer-matrix composites (PMCs)
B. Mechanical properties

D. Mechanical testing
E. Compression moulding
1359-8368/$ - see front matter � 2011 Elsevier Ltd. A
doi:10.1016/j.compositesb.2011.01.010
⇑ Corresponding author. Tel.: +61 31 445 2485; fax
E-mail address: [email protected] (H. Ku).
a b s t r a c t
This paper is a review on the tensile properties of natural fiber
reinforced polymer composites. Natural
fibers have recently become attractive to researchers, engineers
and scientists as an alternative reinforce-
ment for fiber reinforced polymer (FRP) composites. Due to
their low cost, fairly good mechanical prop-
erties, high specific strength, non-abrasive, eco-friendly and
bio-degradability characteristics, they are
exploited as a replacement for the conventional fiber, such as
glass, aramid and carbon. The tensile prop-
erties of natural fiber reinforce polymers (both thermoplastics
and thermosets) are mainly influenced by
the interfacial adhesion between the matrix and the fibers.
Several chemical modifications are employed
to improve the interfacial matrix–fiber bonding resulting in the
enhancement of tensile properties of the

composites. In general, the tensile strengths of the natural fiber
reinforced polymer composites increase
with fiber content, up to a maximum or optimum value, the
value will then drop. However, the Young’s
modulus of the natural fiber reinforced polymer composites
increase with increasing fiber loading. Khoa-
thane et al. [1] found that the tensile strength and Young’s
modulus of composites reinforced with
bleached hemp fibers increased incredibly with increasing fiber
loading. Mathematical modelling was
also mentioned. It was discovered that the rule of mixture
(ROM) predicted and experimental tensile
strength of different natural fibers reinforced HDPE composites
were very close to each other. Halpin–
Tsai equation was found to be the most effective equation in
predicting the Young’s modulus of compos-
ites containing different types of natural fibers.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction applicable for aerospace, leisure, construction,
sport, packaging
A fiber reinforced polymer (FRP) is a composite material con-
sisting of a polymer matrix imbedded with high-strength fibers,
such as glass, aramid and carbon [2]. Generally, polymer can be
classified into two classes, thermoplastics and thermosettings.

Thermoplastic materials currently dominate, as matrices for bio-
fibers; the most commonly used thermoplastics for this purpose
are polypropylene (PP), polyethylene, and poly vinyl chloride
(PVC); while phenolic, epoxy and polyester resins are the most
commonly used thermosetting matrices [3]. In the recent
decades,
natural fibers as an alternative reinforcement in polymer
compos-
ites have attracted the attention of many researchers and
scientists
due to their advantages over conventional glass and carbon
fibers
[4]. These natural fibers include flax, hemp, jute, sisal, kenaf,
coir,
kapok, banana, henequen and many others [5]. The various
advan-
tages of natural fibers over man-made glass and carbon fibers
are
low cost, low density, comparable specific tensile properties,
non-abrasive to the equipments, non-irritation to the skin,
reduced
energy consumption, less health risk, renewability, recyclability
and bio-degradability [3]. These composites materials are
suitably
ll rights reserved.

: +61 31 446 9556.
and automotive industries, especially for the last mentioned
appli-
cation [3,6]. However, the certain drawback of natural
fibers/poly-
mers composites is the incompatibility between the hydrophilic
natural fibers and the hydrophobic thermoplastic matrices. This
leads to undesirable properties of the composites. It is therefore
necessary to modify the fiber surface by employing chemical
mod-
ifications to improve the adhesion between fiber and matrix [3].
There are many factors that can influence the performance of
natural fiber reinforced composites. Apart from the hydrophilic
nature of fiber, the properties of the natural fiber reinforced
com-
posites can also be influenced by fiber content/amount of filler.
In
general, high fiber content is required to achieve high
performance
of the composites. Therefore, the effect of fiber content on the
properties of natural fiber reinforced composites is particularly
sig-
nificance. It is often observed that the increase in fiber loading

leads to an increase in tensile properties [7]. Another important
factor that significantly influences the properties and interfacial
characteristics of the composites is the processing parameters
used. Therefore, suitable processing techniques and parameters
must be carefully selected in order to yield the optimum
composite
products. This article aims to review the reported works on the
ef-
fects of fiber loading, chemical treatments, manufacturing
techniques and process parameters on tensile properties of
natural
fiber reinforced composites.
mailto:[email protected]
http://www.sciencedirect.com/science/journal/13598368
http://www.elsevier.com/locate/compositesb
H. Ku et al. / Composites: Part B 42 (2011) 856–873 857
2. Tensile properties
Generally, the tensile properties of composites are markedly
improved by adding fibers to a polymer matrix since fibers have

much higher strength and stiffness values than those of the
matri-
ces as shown in Tables 1–3 [3,8].
Consider the tensile strength of S-glass from Table 1, and that
of
polypropylene (PP) from Table 2 and that of polyester resin
from
Table 3, it can be found that the tensile strength of the fiber
(S-glass) is 75–150 times higher than those of the matrices (PP
and polyester resin). It can also be found that the Young’s
modulus
of the fiber (S-glass) is 80–160 times higher than those of the
matrices (PP and polyester resin) [3–8].
In general, higher fiber content is desired for the purpose of
achieving high performance of short fiber reinforced polymer
com-
posites (SFRP) [7]. It is often observed that the presence of
fiber or
other reinforcement in the polymeric matrix raises the
composite
strength and modulus [5]. Therefore, the effect of fiber content
on the tensile properties of fiber reinforced composites is of
partic-

ular interest and significance for many researchers [7].
Nonwoven mats from hemp and polypropylene fibers in various
proportions are mixed and hot pressed to make composite
materi-
als. The effect of hemp fiber content and anisotropy are
examined
on the basis of tensile properties of the resultant composite
materials. The tensile strength, with fibers in the perpendicular
direction, tended to decrease with increasing hemp fiber content
(a maximum decrease of 34% at 70% of hemp) as depicted in
Fig. 1. Whereas, the tensile strength, with fibers in the parallel
direction, showed a different trend and a maximum value was
found with increasing fiber loading. It was found that the
tensile
strength of composites with fibers in the perpendicular direction
was 20 – 40% lower than those of composites with fibers in
parallel
direction. Since the fibers lay perpendicular to the direction of
load,
they cannot act as load bearing elements in the composite
matrix
structure but become potential defects which could cause
failure.
As expected, better tensile properties are found in the

specimens
cut from the composite sheets parallel to the direction of
carding
as depicted in Fig. 1 [9].
In general, the Young’s modulus of the composite materials in-
crease with an increase in fiber content, reaching a maximum
va-
lue at 50% hemp fiber loading and then decreasing slightly at
70% hemp fiber content. The Young’s modulus was almost two
and a half times higher at 50% hemp fiber loading than at 0%
fiber
content, i.e. pure PP as depicted in Fig. 2 [9].
Fig. 3 illustrated the tensile strength of 20-mesh hardwood, 40-
mesh hardwood, flax and rice hull fibers reinforced HDPE
compos-
ites. Li et al. [5] reported that flax fiber content from 10% to
30% by
mass was mixed with high-density polyethylene (HDPE) by
extru-
sion and injection moulding to produce bio-composites. The re-
Table 1
Properties of selected natural and manmade fibers [adapted from
3, 8].

Fiber Density (g/cm3) Elongation (%)
Cotton 1.5–1.6 7.0–8.0
Jute 1.3 1.5–1.8
Flax 1.5 2.7–3.2
Hemp 1.47 2–40
Kenaf 1.45 1.6
Ramie N/A 3.6–3.8
Sisal 1.5 2.0–2.5
Coir 1.2 30
Softwood kraft pulp 1.5 4.4
E-glass 2.5 0.5
S-glass 2.5 2.8
Aramid (Std.) 1.4 3.3–3.7
Carbon (Std. PAN-based) 1.4 1.4–1.8
sults showed that increasing fiber content resulted in increasing
tensile properties initially as depicted in Fig. 3. It peaked at
20%
by volume; it then dropped. However, the elongation at break of
the composites showed the reverse trend as depicted in Fig. 4
[5].
The tensile strengths of 40-mesh hardwood fibers reinforced

HDPE composites increased gradually, and up to a maximum at
25% of fiber loading by volume, and then dropped back as
depicted
in Fig. 3 [11]. On the other hand, the tensile strengths of 20-
mesh
hardwood fibers reinforced HDPE composites reduced with
increasing fiber loading [11]. This is totally different from that
of
40-mesh hardwood fibers. The tensile strengths of rice hull
fibers
reinforced HDPE composites were shown in Fig. 3 [10]; the
behaviour of the curve was more or less the same as those found
in 20-mesh hardwood but it has a maximum tensile strength at
5% by volume of fiber content [10]. The tensile strengths
decreased
with increasing particulate loading slightly [10].
Fig. 5 showed the Young’s modulus of 20-mesh hardwood, 40-
mesh hardwood, flax and rice hull fiber reinforced HDPE
compos-
ites with varying percentage by volume of fiber loading. It can
be
found that the Young’s modulus of 20-mesh and 40-mesh
hardwood fibers reinforced HDPE composites with fiber loading
of 0–40 wt.% [11]. The value increased with increasing fiber

load-
ing. Up to 30% volume fraction of hardwood, the Young’s
moduli
of 20-mesh hardwood fiber composites were lower than their
counterparts. After 35% volume fraction of hardwood, the
Young’s
moduli of 20-mesh hardwood fiber composites were higher than
their counterparts. Fig. 5 also illustrated the Young’s modulus
of
flax fibers reinforced HDPE composites with fiber loading of 0–
40% vol. [5]. It can be found that the Young’s modulus
increased
with increasing fiber content [5]. The Young’s modulus of rice
hulls
fibers reinforced HDPE composites with fiber loading of 0–40%
vol.
was depicted in Fig. 5 [10]. The trends of all the curves for Fig.
5
were more or less the same as, i.e. the values of the Young’s
modulus increased progressively with increasing fiber loading.
However, the largest increase with increasing fiber content was
for flax fiber reinforced composites, while the least increase
was
for rice hull fiber reinforced composites.

The dependence of tensile properties of micro winceyette fiber
reinforced thermoplastic corn starch composites on fiber
contents
was studied. Fig. 6 illustrated that with the increase fiber
content
from 0% to 20 wt.%, the tensile strength was approximately
trebled
to 150 MPa [12]. The increase was progressive. However, the
elon-
gation of the composites decreased with increasing fiber loading
as
depicted in Fig. 7. The elongation dropped significantly
between fi-
ber loading of 0–10% by weight; after this the decrease was
very
slightly. On the other hand, the energy at break of the
composites
decreased slightly from neat resin to 5 wt.% of fiber and
dropped
significantly from 5% to 10% by weight of fiber as depicted in
Fig. 8; after this there was a slight increase [12].
Tensile strength (MPa) Elastic modulus (GPa) Refs.
400 5.5–12.6 [6,7]
393–773 26.5 [6]

500–1500 27.6 [4]
690 70 [4]
930 53 [4]
400–938 61.4–128 [8]
511–635 9.4–22 [8]
593 4.0–6.0 [9]
1000 40 [9]
2000–3500 70 [9]
4570 86 [9]
3000–3150 63.0–67.0 [9]
4000 230–240 [9]
Table 2
Properties of typical thermoplastic polymers used in natural
fiber composite fabrication [adapted from 3, 8].
Property PPa LDPE HDPE PS Nylon 6 Nylon 6,6
Density (g/cm3) 0.899–0.920 0.910–0.925 0.94–0.96 10.4–1.06
1.12–1.14 1.13–1.15
Water absorption-24 h (%) 0.01–0.02 <0.015 0.01–0.2 0.03–
0.10 1.3–1.8 1.0–1.6
Tg (�C) �10 to �230 �125 �133 to �1000 N/A 48 80

Tm (�C) 160–176 105–116 120–140 110–1350 215 250–269
Heat deflection Temp (�C) 50–63 32–50 43–60 Max. 220 56–80
75–90
Coefficient of thermal expansion (mm/mm/�C � 105) 6.8–13.5
10 12–13 6–8 8–8.86 7.2–9
Tensile strength (MPa) 26–41.4 40–78 14.5–38 25–69 43–79
12.4–94
Elastic modulus (GPa) 0.95–1.77 0.055–0.38 0.4–1.5 4–50 2.9
2.5–3.9
Elongation (%) 15–700 90–800 2.0–130 1–2.5 20–150 35–>300
Izod impact strength (J/m) 21.4–267 >854 26.7–1068 1.1 42.7–
160 16–654
a PP = polypropylene, LDPE = low density polyethylene, HDPE
= high-density polyethylene and PS = polystyrene.
Table 3
Properties of typical thermoset polymers used in natural fiber
composites [adapted
from 3, 8].
Property Polyester
resin
Vinyl ester

resin
Epoxy
Density (g/cm3) 1.2–1.5 1.2–1.4 1.1–1.4
Elastic modulus (GPa) 2–4.5 3.1–3.8 3–6
Tensile strength (MPa) 40–90 69–83 35–100
Compressive strength (MPa) 90–250 100 100–
200
Elongation (%) 2 4–7 1–6
Cure shrinkage (%) 4–8 N/A 1–2
Water absorption
(24 [email protected] �C)
0.1–0.3 0.1 0.1–0.4
Izod impact strength (J/m) 0.15–3.2 2.5 0.3
Fig. 1. Tensile strength of polypropylene/hemp fibres with
varying percentage by
weight of fibres [adapted from 9].
Fig. 2. Young’s modulus of polypropylene/hemp fibres with
varying percentage by

weight of fibres [adapted from 9].
Fig. 3. Tensile strength of 20-mesh hardwood, 40-mesh
hardwood, flax and rice
hull fibres reinforced HDPE composites [adapted from 5, 10 and
11].
858 H. Ku et al. / Composites: Part B 42 (2011) 856–873
Fig. 9 illustrated that with the increase of fiber content from 0%
to 20 wt.%, the Young’s modulus was approximately trebled to
140 MPa [12]. From 0% to 10% by weight of fiber loading, the
Young’s modulus was steady but increased progressively after
that
[12].
Khoathane et al. [1] found that increasing the amount of
bleached hemp fiber (0–30 w/t%) resulted in the initial increase
of tensile strength of the fiber reinforced 1-
pentene/polypropylene
(PP1) copolymer composite at 5% fiber content to 30 MPa from
20 MPa for the neat resin as depicted in Fig. 10. The tensile
strength
then dropped to a low 23 MPa at 20% fiber loading [1]. After
this,
the tensile strength increased again and its value was about at

par with that of 5% fiber content when the fiber was 30% [1].
Fig. 11 illustrated the effect of fiber contents on Young’s
modulus
of bleached hemp fiber reinforced PP1 composites [1]. The
value
of the Young’s modulus increased by over twice from 1.3 GPa
(neat
resin) to 4.4 GPa (30% w/t) [1].
Long-discontinuous natural fibers of kenaf and of jute rein-
forced polypropylene (PP) composites fabricated by carding and
hot pressing process with fiber weight fraction varying from
10%
to 70% in steps of 10% were studied [13]. The experimental
results
illustrated that the tensile and modulus strength of both kenaf
and
jute fiber reinforced PP composites increased with increasing
fiber
loading and a maximum was reached before falling back at
higher
fiber weight fraction. These were illustrated in Figs. 12 and 13
[13].
From the above citations and discussions, it can be found that

the values of the tensile strength of natural fiber reinforced
com-
posites increased with increasing fiber loading up to a maximum
or optimum value before falling back. However, it is generally
true
that the values of the Young’s modulus increased progressively
with increasing fiber loading. On the other hand, some
researchers
found totally the opposite trend to the increase of composite
strength with increasing fiber content. This can be attributed to
many factors such as incompatibility between matrix and fibers,
improper manufacturing processes, fiber degradation and others.
Fig. 4. Tensile elongation of bio-composites vs. fiber mass
concentration [adapted from 5].
Fig. 5. Young’s modulus of 20-mesh hardwood, 40-mesh
hardwood, flax and rice hull fibre reinforced HDPE composites
with fibre loadings of 0–40% vol. [adapted from 5, 10
and 11].
0

20
40
60
80
100
120
140
160
0 5 10 15 20
Fiber content by weight
Te
ns
ile
s

tr
en
gt
h
(M
P
a)
Fig. 6. The effect of fiber content on the tensile strength of
micro winceyette fiber
reinforced thermoplastic corn starch composites [adapted from
12].
0
20
40
60

80
100
120
0 5 10 15 20
E
lo
ng
at
io
n
(%
)
Fig. 7. The effect of fiber content on the elongation of micro
winceyette fiber

12].
The hydrophilic nature of natural fibers is incompatible with
hydrophobic polymer matrix and has a tendency to form aggre-
gates. These hydrophilic fibers exhibit poor resistant to
moisture,
which lead to high water absorption, subsequently resulting in
poor tensile properties of the natural fiber reinforced
composites.
Moreover, fiber surfaces have waxes and other non-cellulosic
sub-
stances such as hemi-cellulose, lignin and pectin, which create
poor adhesion between matrix and fibers. Therefore, in order to
improve and develop natural fiber reinforced polymer
composites
with better tensile properties, it is necessary to increase fibers
hyp-
hobicity by introducing the natural fibers to surface chemical
mod-
ification (surface treatment). The fiber modification is
attempted to
improve fibers hydrophobic, interfacial bonding between matrix
and fiber, roughness and wettability, and also decrease moisture
absorption, leading to the enhancement of tensile properties of

the composites [13–17].
The different surface chemical modifications, such as chemical
treatments, coupling agents and graft co-polymerization, of
natural
fibers aimed at improving the tensile properties of the
composites
were performed by a number of researchers. Alkali treatment,
also
called mercerization, is one of the most popular chemical treat-
ments of natural fibers. Sodium hydroxide (NaOH) is used in
this
method to remove the hydrogen bonding in the network
structure
of the fibers cellulose, thereby increasing fibers surface
roughness
[13]. This treatment also removes certain amount of lignin, wax
and oils covering the external surface of the fibers cell wall,
0
50

100
150
200
250
0 5 10 15 20
E
ne
rg
y
B
re
ak
(N
m

)
Fig. 8. The effect of fiber contents on the energy at break of
12].
0
20
40
60
80
100
120
140
160

0 5 10 15 20
Y
ou
ng
's
M
od
ul
us
(N
/m
m
2)
Fig. 9. The effect of fiber contents on the Young’s modulus of

12].
0
5
10
15
20
25
30
35
0 5 10 15 20 25 30
Te
ns

ile
s
tr
en
gt
h
(M
P
a)
Fig. 10. The effect of fiber contents on tensile strength of
bleached hemp fiber
reinforced PP1 composites [adapted from 1].
0
0.5
1
1.5
2

2.5
3
3.5
4
4.5
0 5 10 15 20 25 30
Y
ou
ng
's
m
od
ul
us
(G

P
a)
Fig. 11. The effect of fiber contents on Young’s modulus of
bleached hemp fiber
reinforced PP1 composites [adapted from 1].
depolymerises the native cellulose structure and exposes the
short
length crystallites [14]. Acrylic acid treatment was also reported
to
be effective in modifying the natural fibers surface. A study on
flax
fibers-reinforced polyethylene bio-composites by Li et al. found
that the efficiency of such a treatment was higher than alkali
and
silane treatment [14].
The chemical coupling method is also one of the important
chemical methods, which improve the interfacial adhesion. In
this
method the fiber surface is treated with a compound that forms
a

bridge of chemical bonds between fiber and matrix. The
chemical
composition of coupling agents allows them to react with the
fiber
surface forming a bridge of chemical bonds between the fiber
and
matrix. Most researchers found these treatments were effective
and showed better interfacial bonding [13]. Among different
cou-
pling agents, maleic anhydride is the most commonly used. In
gen-
eral, the literature reports improvements in tensile strength and
elongation at break when maleic anhydride grafted matrices are
used as compatibilizers (coupling agent) [15].
Hu and Lim [18] investigated that alkali treatment significantly
improved the tensile properties of hemp fiber reinforced
polylactic
acid (PLA) compare to those untreated. Figs. 14 and 15 showed
that
the composites with 40% volume fraction of alkali treated fiber
have the best tensile properties. The tensile strength and tensile
modulus of the composites with 40% treated fiber are 54.6 MPa
and 85 GPa respectively, which are much higher than neat PLA,
especially for the tensile modulus which is more than twice of

that
of neat PLA (35 GPa).
Fuqua and Ulven reported that fiber loading of treated (alkali
and bleached) and untreated flax fiber without compatibilizer
(maleic anhydride grafted polypropylene or MAPP) in PP
compos-
ites caused inferior tensile strength (even compared with pure
PP)
[19]. However, treated fiber loading with compatibilizer
resulted in
favourable tensile strength as depicted in Fig. 16 [19]. Fig. 17
illus-
trated that the continuously increased trend of composite
modulus
can be found in all cases (untreated, bleached and treated) and
reached a maximum value at 65/5/30 (wt.% PP/MAPP/fiber
loading)
[19]. This can be argued that the introduction of alkali treatment
with 5% MAPP in the natural fiber reinforced plastic
composites
helped to improve both tensile strength and Young’s modulus of
the composites compare to those without MAPP.
Liu et al. evaluated the effects of different fiber surface modifi-

cations, 2%NaOH, 2 + 5%NaOH (Note that 2 + 5% NaOH
treatment is
a continuation treatment from 2%NaOH process and then soaked
with 5% NaOH) and coupling agent, on jute/polybutylene
succinate
(PBS) bio-composites [20]. The experiment results showed that
surface modifications could remove surface impurities,
increased
surface roughness and reduced diameter of jute fiber, subse-
quently, significantly increased the tensile strength and modulus
of the composites but decreased breaking elongation as depicted
in Figs. 18–20. It was observed that the bio-composites of jute
fi-
bers treated by 2%NaOH, 2 + 5%NaOH or coupling agent,
obviously
had their tensile properties increased when compared to those
un-
treated and yielded an optimum value at fiber content of 20
wt.%.
The results also showed that the strength and stiffness of
compos-
ites were dependent on the types of treatment. In Figs. 21 and
22,
the 100/0/0 referred to w/t% of PP (100%), MAPP (0%) and
fiber loading

(0%); while 65/5/30 referred to w/t% of PP (65%), MAPP (5%)
and fiber
loading (30%).
Li et al. [14] studied flax fiber reinforced polyethylene bio-
composites. In the study, flax fibers, containing 58 w/t% of flax
shives
were used to reinforce polyethylene (high-density polyethylene
and
linear low density polyethylene). The composites contained 10
w/t%
of fiber and processed by extrusion and injection moulding.
Five sur-
face modification methods, alkali, silane, potassium
permanganate,
acrylic acid, and sodium chlorite treatments, were employed to
im-
prove the interfacial bonding between fibers and matrix. Fig. 21
0
5

10
15
20
25
30
35
40
45
0 10 20 30 40 50 60 70
Fiber weight fraction (%)
Te
ns
ile
s

tr
en
gt
h
(M
P
a)
kenaf Jute
Fig. 12. Tensile strength of bio-composites of PP vs. fibre
weight fraction [adapted from 13].
0
200
400
600

800
1000
1200
1400
0 10 20 30 40 50 60 70
Te
ns
ile
m
od
ul
us
(M
P

a)
kenaf Jute
Fig. 13. Tensile modulus of bio-composites of PP vs. fibre
weight fraction [adapted from 13].
0
10
20
30
40
50
60
30 35 40 45 50
Fiber volume fraction (%)

Te
ns
ile
s
tr
en
gt
h
(M
P
a)
Fiber untreated Fiber alkali treated
Fig. 14. Tensile strength of treated and untreated hemp-PLA
composites vs. fibre content [adapted from 18].
(LLDPE) and Fig. 22 (HDPE) showed that the biocomposite
tensile

strengths were increased after surface modifications. Among
these
surface modification techniques, acrylic acid was found to be a
rela-
tively good method in enhancing tensile properties of both flax/
HDPE and LLDPE bio-composites [14].
Fuqua and Ulven investigated the different MAPP loading (0, 5
and 10 w/t%) effects on tensile properties of corn chaff fiber
rein-
forced polypropylene composites [19]. They also investigated
the
effect of various treatments, silane z-6011, silane z-6020 and
5 w/t% MAPP, on corn chaff fiber & distilled dried grains
(DDGS)
0
10
20
30

40
50
60
70
80
90
30 35 40 45 50
Fiber volume fraction (%)
Te
ns
ile
m
od
ul

us
(G
P
a)
Fiber untreated Fiber alkali treated
Fig. 15. Tensile modulus of treated and untreated hemp-PLA
composites vs. fibre content [adapted from 18].
0
5
10
15
20
25
30

35
100/0/0 85/0/15 80/5/15 70/0/30 65/5/30
Formulation
Te
ns
ile
s
tr
en
gt
h
(M
P
a)
Untreated fiber Bleached fiber Alkaline treated fiber

Fig. 16. Effect of coupling agent concentration on tensile
strength of PP composites with 10% w/t coir fibre [adapted from
19].
0
0.2
0.4
0.6
0.8
1
1.2
1.4
100/0/0 85/0/15 80/5/15 70/0/30 65/5/30
Formulation
Y

ou
ng
's
m
od
ul
us
(G
P
a)
Untreated fiber Bleached fiber Alkaline treated fiber
Fig. 17. Effect of coupling agent concentration on Young’s
modulus of PP composites with 10 w/t% coir fibre [adapted
from 19].

0
5
10
15
20
25
30
0 5 10 15 20 25 30
Fiber content (wt%)
Te
ns
ile
s

tr
en
gt
h
(M
P
a)
Without treatment 2% NaOH teatment 2%+5%NaOH treatment
Coupling agent treatment
Fig. 18. Effect of surface modification on tensile strength of
PBS/jute bio-composites with different fibre loading [adapted
from 20].
0
0.5
1
1.5

2
2.5
0 5 10 15 20 25 30
Fiber content (wt%)
Te
ns
ile
m
od
ul
us
(G
P
a)

Fig. 19. Effect of surface modification on tensile modulus of
PBS/jute bio-composites with different fibre loading [adapted
from 20].
0
1
2
3
4
5
6
7
0 5 10 15 20 25 30
Fiber content (wt %)

B
re
ak
in
g
el
en
ga
tio
n
(%
)
Fig. 20. Effect of surface modification on breaking elongation
of PBS/jute bio-composites with different fibre loading [adapted
from 20].

reinforced polypropylene composites [19]. It was found that 5
w/t%
MAPP yielded the optimum value for the composites in term of
tensile strength and modulus as shown in Figs. 23 and 24
respec-
tively [19]. The strength reduction observed with high MAPP
12.8
13
13.2
13.4
13.6
13.8
14
14.2
14.4
LL
D

P
E
U
-L
(u
nt
re
at
ed
bi
oc
om
po
si
te
)

N
-L
(a
lk
al
i
tre
at
ed
bi
oc
om
po
si
te
)

S
-L
(s
ila
ne
tre
at
ed
bi
oc
om
po
si
te
)
P
-L

(p
ot
as
si
um
bi
oc
om
po
si
te
)
A
-L
(a
ry

cl
ic
tre
at
ed
bi
oc
om
po
si
te
)
C
-L
(s
od
iu

m
ch
lo
rit
e
tre
at
ed
bi
oc
om
po
si
te
)
LLDPE and Biocomposite

Te
ns
ile
s
tr
eg
th
(M
P
a)
Fig. 21. Tensile strength of LLDPE and fibre-LLPDE bio-
composites after surface modifications [adapted from 14].
18.8
19
19.2
19.4
19.6
19.8

20
20.2
20.4
20.6
H
D
P
E
U
-H
(u
nt
re
at
ed

bi
oc
om
po
si
te
)
N
-H
(a
lk
al
i
tre
at
ed
bi

oc
om
po
si
te
)
S
-H
(s
ila
ne
tre
at
ed
bi
oc

om
po
si
te
)
P
-H
(p
ot
as
si
um
bi
oc
om
po
si

te
)
A
-H
(a
ry
cl
ic
tre
at
ed
bi
oc
om
po
si

te
)
C
-H
(s
od
iu
m
ch
lo
rit
e
tre
at
ed
bi
oc

om
po
si
te
)
HDPE and Biocomposites
Te
ns
ile
s
tr
en
gt
h
(M
P

a)
Fig. 22. Tensile strength of HDPE and fibre-HPDE bio-
composites after surface modifications [adapted from 14].
0
5
10
15
20
25
30
35
neat 3 wt%
corn
5 wt%

corn
7 wt%
corn
9 wt%
corn
Te
ns
ile
S
tr
en
gt
h
(M
P
a)

0 wt% MAPP
5 wt% MAPP
10 wt% MAPP
Fig. 23. Effects of MAPP loading on tensile strength of corn
chaff fibre reinforced PP composites [adapted from 19].
loading was caused by the interaction between the
compatibilizer
(MAPP) and the fiber/matrix system. The anhydride units of
MAPP
maintain loop confirmations within the composite systems,
since
they all can act with equal probability with the cellulose in the
corn fibers. Coupled with MAPP’s low average molecular
weight,
the interaction between the PP matrix and MAPP becomes domi-
nated principally by Van der Waals forces; since chain
entangle-
ment of PP and MAPP is virtually impossible. MAPP that is not
utilizes for fiber/matrix adhesion and is therefore mechanically
harmful to the composites, which leads credence to the
significant
performance variation between 5 and 10 w/t% loadings.

However,
through the use of 5 w/t% MAPP, it was found that the tensile
prop-
erties of the composites increase, especially tensile strength
com-
pared to neat resin and those untreated.
Sain et al. investigated the effect of a low-molecular weight
MAPP on tensile properties of polypropylene reinforced with
the
varieties of natural fibers such as old newsprint, kraft pulp and
hemp [20]. Figs. 25 and 26 showed that the optimum level of
the
0
0.2
0.4
0.6
0.8

1
1.2
1.4
1.6
1.8
2
neat 3 wt% corn 5 wt% corn 7 wt% corn 9 wt% corn
E
la
st
ic
M
od
ul

us
(G
P
a)
0 wt% MAPP
5 wt% MAPP
10 wt% MAPP
Fig. 24. Effects of MAPP loading on tensile modulus of corn
chaff fibre reinforced PP composites [adapted from 19].
0
10
20
30
40

50
60
0 1 2 3 4 5
Coupling Agent, wt%
Te
ns
ile
s
tr
en
gt
h
(M
P
a)

Tensile strength (MPa)
Fig. 25. Tensile strength of MAPP loaded old newsprint-filled
PP composites [adapted from 20].
3
3.05
3.1
3.15
3.2
3.25
3.3
43210
Coupling Agent, wt%
Te

ns
ile
m
od
ul
us
(G
P
a)
Tensile modulus (GPa)
Fig. 26. Tensile modulus of MAPP loaded old newsprint-filled
PP composites [adapted from 20].
coupling agent (MAPP) by weight of the old newsprint-filled PP
composites was 4% for tensile strength and 1.5% for tensile
modu-
lus respectively [20].

Herrero-Franco and Valadez-Gonzalez studied the tensile
behaviour of HDPE reinforced with continuous henequen fibers,
which were treated by the optimum concentration (0.015 wt.%)
of silane coupling agent concentration [21]. The results
indicated
that silane increased tensile strength of the composite. It was
no-
ticed, however, that none of the fiber–matrix interface improve-
ments had any significant effect on the value of Young’s
modulus
of continuous henequen fiber reinforced HDPE composites [21].
Another important factor that significantly influences the
properties and interfacial characteristics of the composites is
pro-
cessing techniques and parameters used. Common methods for
manufacturing natural fiber reinforced thermoplastic composites
are extrusion-injection moulding and compression moulding.
Tungjitpornkull and Sombatsompop researched on the
difference
in the tensile properties of E-glass fiber (GF) reinforced
0

2
4
6
8
10
12
14
16
0 2 4 6 8 10 12 14 16 18
Fiber content (% wt)
Te
ns
ile

m
od
ul
us
(G
P
a)
Compression molding Twin screw extrusion
Fig. 27. Tensile modulus of glass fibre reinforced WPVC
composites manufactured by twin screw extrusion and
compression moulding processes [adapted from 22].
wood/PVC (WPVC) composites, manufactured by twin screw
extrusion and compression moulding processes respectively
[22].
The experimental results suggested that the GF/WPVC
composites
produced from compression moulding gave better tensile
modulus

than those from their counterparts as depicted in Fig. 27. The
shear
stress in compression moulding was lower than that in twin
screw
extrusion, as a result there was less thermal degradation of PVC
molecules and less breakage of glass fiber, resulting in longer
fiber
length in the composites manufactured by compression
moulding.
The composite manufactured by compression moulding would
have higher specific density, which resulted in less void and air
and was then stronger than its counterpart [22].
The study by Siaotong et al. aimed to determine the optimum
val-
ues for fiber content by mass (0%, 12.5% and 25%), extrusion
barrel
zone temperatures (75–110-120–130-140 �C and 75–120-130–
140-150 �C) and extrusion screw speed (110 and 150 rpm) for
the
production of flax fiber reinforced polyethylene (HDPE and
LLDPE)
composites [23]. Response surface methodology was applied as
opti-
mization technique over three response variables: density

deviation
(%), tensile strength (MPa) and water absorption (% mass
increase) of
the composites. According to statistical analysis, the optimum
val-
ues that yield the highest tensile strength (17.09 MPa for
LLDPE
composite and 21.70 MPa for HDPE composite) were: fiber
content
of 6.25%, barrel zone temperatures of 75–116-126–136-146 �C
and
screw speed of 118 rpm for LLDPE composites, and fiber
content of
5%, barrel zone temperatures of 75–118-128–138-148 �C and
screw
speed of 128 rpm for HDPE composites. The optimum values of
tem-
peratures (T) were closer to the higher levels (75–120-130–140-
150 �C) because lower temperatures result in inconsistent melt
of
resin that can lead to non-uniform dispersion of the fibers in the
composites and eventually lower the tensile strength. The
optimum
values of screw speed were closer to the lower level (110 rpm).
This

was because the higher screw speed led to shorter residence
time,
non-uniform dispersion of fibers, high porosity, and
consequently,
lowers tensile strength. However, the unexpected result was the
very low optimum level of the fiber content. Theoretically, an
in-
crease of flax fibers should improve the mechanical properties
of
the composites, yet, the results of tensile strength negated this
[23].
Li et al. determined the appropriate value of injection tempera-
ture and pressure for flax fiber reinforced high-density
polyethyl-
ene bio-composites. The results showed that higher fiber
content
in composites led to higher mechanical strength [24]. Injection
temperature of lower than 192 �C was recommended for better
composite quality because at higher temperature, fiber degrada-
tion (fiber degradation temperature � 200 �C) might have oc-
curred, therefore, lead to inferior tensile properties. However,
the
injection temperature should not be lower than 160 �C in order
to ensure adequate melting of matrix. In comparison with

injection
temperature, the influence of injection pressure was not
obvious.
However, higher injection pressure is preferred to obtain better
composite tensile properties [24].
The optimum pressure was determined for the natural fibre mat
(hemp and kenaf) reinforced acrylic resin manufactured by
high-
tech vacuum compression process. Fig. 28 showed that the
maxi-
mum pressure for the composites was at 60 bars. Above this
value,
there was a decrease in tensile properties of the composites due
to
the damage of the fiber structure. The advantages of using
vacuum
technology are to allow a reduction of the press time to a
minimum
without decreasing the performance of the cured materials. In
addition, the work conditions were significantly improved when
the vacuum chamber process was used. [25].
Khondker et al. studied the processing conditions of unidirec-
tional jute yarn reinforced polypropylene composites fabricated

by film stacking methods [26]. From optical micrographs
obtained,
they suggested that there must be an optimum processing
temper-
ature for which this composite might perform better in tensile
properties. According to the optical microscopy results, they
showed that the composites moulded at a temperature of 160 �C
for 15 min and under 2.0 MPa moulding pressure, would have
the PP matrix films fused and the PP melted completely and
pene-
trated into the fiber bundles. This temperature was considered
favourably ideal for the processing of composites that used
ligno-
cellulosic fibers as reinforcement, as most lignocellulosic fibers
cannot withstand processing temperatures higher than 175 �C
for
longer duration, and hence limiting their ability to be used with
some thermoplastic resins [26].
The effect of the melting-mixing technique parameters on the
tensile properties of sisal fiber reinforced polypropylene
compos-
ites were optimised by varying the 29 through 32, mixing time
of 10 min, rotor speed of 50 rpm and a mixing temperature of
170 �C were found to be the optimum mixing conditions. For

mix-
ing times (Figs. 29 and 30), below the optimum value, the
tensile
strength and Young’s modulus were low because of ineffective
mixing and poor dispersion of the fiber in PP matrix. As the
mixing
time was increased, melting of PP resin became extensive and
re-
sulted in better fiber distribution into the matrix. When mixing
time was more than 10 min, fiber breakage and degradation
would
happen, leading to a decrease in tensile properties. For mixing
tem-
peratures (Fig. 31), the performance of short fiber composites
was
controlled directly by fiber aspect ratio, quality of dispersion
and
0
1000
2000

3000
4000
5000
6000
7000
8000
0 20 40 60 80 100 120 140 160
Pressure (bars)
Te
ns
ile
m
od
ul

us
(M
P
a)
MD CD
Fig. 28. Tensile modulus of natural fiber mat (hemp and kenaf)
reinforced acrylic composites in machine direction (MD) and
cross direction (CD) under varying pressures
[adapted from 25].
28
29
30
31
32
33

34
35
36
37
0 2 4 6 8 10 12 14
Time (min)
Te
ns
ile
s
tr
en
gt
h
(M

P
a)
Fig. 29. Tensile strength of melting mixing of PP/sisal
composites with varying mixing times; fibre content 30%, fibre
length 10 mm [adapted from 27].
28
29
30
31
32
33
34
35
36

37
38
0 2 4 6 8 10 12 14
Time (min)
Y
ou
ng
's
m
od
ul
us
(M
P
a)

Fig. 30. Tensile modulus of melting mixing of PP/sisal
composites with varying mixing times; fibre content 30%, fibre
interface between fiber and polymer. Below the optimum value,
viscosity as well as shear stress generated in the mixture was
very
high, resulting in the break down of fibers to shorter lengths
during
mixing, leading to a lower tensile strength of the composites.
On
the other hand, if mixing temperature was above the optimum,
the thermal degradation of fibers would occur, leading to the
de-
crease of tensile properties. For mixing speeds (Fig. 32), low
tensile
strength was observed at speeds lower than the optimum value
0
5

10
15
20
25
30
35
165 166 167 168 169 170 171 172 173 174 175
Temperature ( oC)
Te
ns
ile
s
tr
en

gt
h
(M
P
a)
composites with varying mixing temperatures; fibre content
30%, fibre length 10 mm [adapted from 27].
25
27
29
31
33
35

37
39
30 35 40 45 50 55 60
Rotor speed (rpm)
Te
ns
ile
s
tr
en
gt
h
(M
P
a)

composites with varying rotor speeds; fibre content 30%, fibre
due to poor dispersion of fibers in molten PP matrix. Above the
optimum rotor speed, there was a reduction in strength because
of fiber breakage at high rotor speed [27].
3. Mathematical modelling
Facca et al. exploited a micromechanical model which was a
semi-empirical modification of the rule of mixtures (ROM)
strength Eq. (10):
r1U ¼ rFU 1 �
lC
2l
� �
V F þ r�Mð1 � V FÞ; l P lC ð1Þ
The modified equation for cylindrical fibers was
r1U ¼ asiV F
l

d
þ r�Mð1 � V FÞ; l 6 lC ð2Þ
The modified equation for rectangular fibers was
r1U ¼ asiV F
l
2
� �
W þ T
WT
� �
þ r�Mð1 � V FÞ; l 6 lC ð3Þ
where r1U, a, si, r�M , l, lC, VF, d, W, T are composite tensile
strength,
the clustering parameter, interfacial shear strength, maximum
stress evaluated at the peak composite strength, fiber length,
criti-
cal fiber length, fiber volume fraction, cylindrical fiber
diameter,
rectangular fiber width, rectangular fiber thickness,

respectively.
All of the above-mentioned parameters are available from liter-
ature to predict the tensile strength of HDPE reinforced with a
vari-
ety of natural fibers (hemp, hardwood flour and rice hulls) and
synthetic (E-glass) fibers [10].
Note that, the direction of short fiber is assumed to be perfectly
aligned and fiber curvature is negligible. Also, experimental ap-
proaches are required to determine the interfacial shear strength
(si) of the fiber; either fiber pullout or fragmentation test can be
used. Figs. 33–37 showed the predicted and experimental tensile
strength of different natural fiber reinforced HDPE composites
[10]. It was found that for most cases the tensile strength of the
predicted and experimental results were at par. It can be argued
that Eqs. (1)–(3) gave a good prediction of the experimental
results
except those shown in Fig. 37, where, the experimental tensile
strength of HDPE composites reinforced with rice hulls fibers
ini-
tially increased to a maximum value of 24.88 MPa at 5 vol.% of
rice
hulls fiber; it then gradually dropped to a minimum value of
17.11 MPa at 40 vol.% of filler. On the other hand, the
predicted

tensile strength of the composites initially decreased to a mini-
mum value of 21.78 MPa at 5 vol.% of rice hulls fiber; it then
grad-
ually increased to a maximum value of 28.78 MPa at 25 vol.%
of
filler before dropping back to 24.11 MPa at 40 vol.% of filler
[10].
Facca et al. also found that the increase by weight of natural
short fibers like hemp, hardwood, rice hulls in high-density
poly-
ethylene manufactured by twin-screw brabender mixer com-
pounding and compression moulding, increased the tensile
modulus of all composites [11]. Again, in order to reduce cost
0
5
10
15

20
25
30
35
40
45
0 0.1 0.2 0.3 0.4 0.5
Volume fraction of hemp fibers
Te
ns
ile
s
tr
en

gt
h
(M
P
a)
Exp. composite tensile strength Predicted composite tensile
strength
Fig. 33. Predicted and experimental tensile strengths of HDPE
composite reinforced with hemp fibers between fiber loadings
of 10–60 wt.% [adapted from 10].
0
10
20
30
40

50
60
0 0.05 0.1 0.15 0.2 0.25 0.3
Volume fraction of E-glass fibers
Te
ns
ile
s
tr
en
gt
h
(M
P
a)

Experimental composite tensile strength Predicted composite
tensile strength
composites reinforced with E-glass fibers between fiber
loadings of 10–60 wt.% [adapted from 10].
0
5
10
15
20
25
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Volume fraction of 20-mesh hardwood fibers
Te
ns

ile
s
tr
en
gt
h
(M
P
a)
tensile strength
composites reinforced with 20-mesh hardwood fibers between
fiber loadings of 10–60 wt.% [adapted from 10].
and time consuming experiments, the experimental results of the
tensile modulus of the composites were compared with the theo-

retical values obtained from various mathematical models
shown
in Eqs. (4)–(9):
(1) Rule of mixture (ROM) [11]:
E ¼ EF V F þ EM V M ð4Þ
where EF, VF, EM and VM are the moduli and volume fractions
of the
fiber and matrix respectively.
(2) Inverse/transverse rule of mixtures (IROM) [11]:
E ¼
EF EM
V M EF þ V F EM
ð5Þ
where EF, VF, EM and VM are the moduli and volume fractions
of the
fiber and matrix respectively.
0
5

10
15
20
25
30
35
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Volume fraction of 40-mesh hardwood fibers
Te
ns
ile
s
tr
en

gt
h
(M
P
a)
tensile strength
composite reinforced with 40-mesh hardwood fibers between
fiber loadings of 10–60 wt.% [adapted from 10].
0
5
10
15
20

25
30
35
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Volume fraction of rice hulls fibers
Te
ns
il
st
re
ng
th
(M
P
a)

tensile strength
Fig. 37. Predicted and experimental tensile strength of HDPE
composites reinforced with rice hulls fibers between fiber
loadings of 10–60 wt.% [adapted from 10].
(3) Halpin–Tsai Eq. (11):
E ¼ EM
1 þ ngV F
1 � gV F
� �
ð6Þ
In Eq. (6) the parameter g is given as:
g ¼
ðEF=EMÞ� 1
ðEF=EMÞþ n
ð7Þ
where n in Eqs. 6 and 7 is a shape fitting parameter to fit
Halpin–
Tsai equation to experimental data. The significance of the

parame-
ter n is that it takes into consideration the packing arrangement
and
the geometry of the reinforcing fibers.
A variety of empirical equations for n are available in the liter-
ature, and they depend on the shape of the particle and on the
modulus that is being predicted. If the tensile modulus in the
prin-
cipal fiber direction is desired, and the fibers are rectangular or
cir-
cular in shape, then n is given by the following equations:
n ¼ 2
L
T
� �
or n ¼ 2
L
D
� �
ð8Þ

where L is the length of a fiber in the one-direction and T or D
is the
thickness or diameter of the fiber.
(4) Shear-lag theory:
E ¼ EF 1 �
tanhðgL2Þ
ðgL2Þ
!
V F þ EM V M ð9Þ
the parameter g for shear-lag analysis is available on the
literature
[10].
E ¼
3
8
E1 þ
5
8

E2 ð10Þ
where E is the elastic modulus of the composite. E1 and E2 are
the
elastic moduli of randomly oriented fiber reinforced composites
gi-
ven by Halpin–Tsai equations (Eq. (12)) [14]:
Ei ¼ EM
1 þ nigiV F
1 � gi V F
� �
; gi ¼
ðEF=EMÞ� 1
ðEF=EMÞþ ni
; ð11Þ
where ni = 2(lf/df) for i = 1 or ni = 0.5 for i = 2
Figs. 38–42 showed the Young’s modulus of natural fibers
(hemp, hardwood, rice hulls and E-glass) reinforced high-
density
polyethylene composites containing different types of natural fi-
bers at different volume fraction of the fibers. It was found that,
Halpin–Tsai model was the most accurate amongst others to

pre-
dict tensile modulus of natural fiber reinforced thermoplastics
used in the study made by Facca et al. [11].
Lee et al. found that the tensile moduli of the kenaf or jute rein-
forced PP composites increased with increasing fiber contents
up
to 40% fiber weight fraction. Furthermore, the study employed
Tsai
and Pagano’s model (Eq. (11)) in predicting the tensile modulus
of
randomly oriented long-discontinued fiber reinforced
composites.
It was found that the model predictions agreed well with experi-
mental results for the volume fraction of less than 30–40% by
weight of kenaf and jute respectively, where the void content
were
not high as illustrated in Figs. 43 and 44 respectively [13].
0
5

10
15
20
25
0 0.05 0.1 0.15 0.2 0.25 0.3
Volume fraction of E-glass fibers
Y
ou
ng
's
m
od
ul
us
(G

P
a)
Experimental Tensile Strength (MPa) Rule of Mixture
Inverse Rule of Mixture Halpin Tsai
Nairn shear-lag Mendels et al.
Fig. 38. Young’s modulus of HDPE composites containing E-
glass fibers [adapted from 11].
0
2
4
6
8
10
12

14
16
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Volume fraction of hardwood A fibers
Y
ou
ng
's
m
od
ul
us
(G
P
a)

Fig. 39. Young’s modulus of HDPE composites containing
hardwood A [adapted from 11].
0
5
10
15
20
25
30
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Volume fraction of hemp fibers

Y
ou
ng
's
m
od
ul
us
(G
P
a)
Fig. 40. Young’s modulus of HDPE composites containing hemp

fibers [adapted from 11].
4. Discussions and conclusions
The scientific world is facing a serious problem of developing
new and advanced technologies and methods to treat solid
wastes,
particularly non-naturally-reversible polymers. The processes to
decompose those wastes are actually not cost-effective and will
subsequently produce harmful chemicals. Owing to the above
ground, reinforcing polymers with natural fibers is the way to
go.
In this paper, most of the natural fibers mentioned were
plant-based but it should be noted that animal fibers like cocoon
0
2
4
6
8
10

12
14
16
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
Volume fraction of hardwood B fibers
Y
ou
ng
's
m
od
ul
us
(G
P
a)

Fig. 41. Young’s modulus of HDPE composites containing
hardwood B [adapted from 11].
0
2
4
6
8
10
12
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Volume fraction of rice hulls

Y
ou
ng
's
m
od
ul
us
(G
P
a)
Fig. 42. Young’s modulus of HDPE composites containing rice

hulls [adapted from 11].
0
500
1000
1500
2000
2500
3000
0 10 20 30 40 50 60 70
Te
ns
ile
m

od
ul
us
(M
P
a)
kenaf Prediction
Fig. 43. Predicted and experimental result of tensile modulus of
kenaf reinforced PP composites [adapted from 13].
silkworm silk, chicken feather and spider silk have also been
used
and the trend should go on. Those fibers, both animal- and
plant-based have provided useful solutions for new materials
development, in the field of material science and engineering.
Natural fibers are indeed renewable resources that can be grown
and made within a short period of time, in which the supply can
be unlimited as compared with traditional glass and carbon
fibers
for making advanced composites. However, for some recyclable

polymers, their overall energy consumption during collecting,
recycling, refining and remoulding processes have to be
considered
to ensure the damage of the natural cycle would be kept as
minimal.
On top of it, Natural fibers are low cost, recyclable, low density
and eco-friendly material. Their tensile properties are very good
0
0.2
0.4
0.6
0.8
1
1.2

1.4
1.6
1.8
0 10 20 30 40 50 60 70
Te
ns
ile
m
od
ul
us
(G
P
a)

Jute Prediction
Fig. 44. Predicted and experimental result of tensile modulus of
jute reinforced PP composites [adapted from 13].
and can be used to replace the conventional fibers such as glass,
carbon in reinforcing plastic materials. A major drawback of
using
natural fibers as reinforcement in plastics is the incompatibility,
resulting in poor adhesion between natural fibers and matrix
resins,
subsequently lead to low tensile properties. In order to improve
fi-
ber–matrix interfacial bonding and enhance tensile properties of
the composites, novel processing techniques, chemical and
physical
modification methods are developed. Also, it is obviously clear
that
the strength and stiffness of the natural fiber polymer
composites
is strongly dependent on fiber loading. The tensile strength and
modulus increase with increasing fiber weight ratio up to a
certain
amount. If the fiber weight ratio increases below optimum

value,
load is distributed to more fibers, which are well bonded with
resin
matrix resulting in better tensile properties. Further increment
in fi-
ber weight ratio has resulted in decreased tensile strength as de-
scribed in the main text. Mathematical models were also found
to
be an effective tool to predict the tensile properties of natural
fiber
reinforced composites.
Finally, it can be found that the main weakness to predict the
tensile properties of plant-based natural fiber composites by
mod-
elling was giving too optimistic values like results in Figs. 33–
37.
The modelling has to be improved to allow improvements in the
prediction of tensile properties of composites reinforced with
both
plant- and animal-based fibers.
References
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reinforced 1-pentene/
polypropylene copolymer: the effect of fiber loading on the
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[3] Malkapuram R, Kumar V, Yuvraj SN. Recent development
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reinforced polypropylene composites. J Reinf Plast Compos
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review. Adv Polym
Technol 1999;18:351–63.
[5] Li X, Tabil LG, Panigrahi S, Crerar WJ. The influence of
fiber content on
properties of injection molded flax fiber-HDPE biocomposites.
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[6] Wambua P, Ivens J, Verpoest I. Natural fibres: can they
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Enhancement of
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PP/EPDM/Flax Fibre
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Polym Sci
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