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Evaluation of the Influence of Fibre Length and Concentration on Mechanical Performance of Hemp Fibre Reinforced Polypropylene Composite

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Evaluation of the Influence of Fibre Length and Concentration on Mechanical Performance of Hemp Fibre Reinforced Polypropylene Composite

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The paper deals widi die evaluation of interfacial shear stress (ISS) between reinforcement fibre (hemp) and polypropylene matrix through single fibre pull-out method and subsequently the critical length of the composite grade hemp fibre has been determined. In the present study the average ISS value of 5.9 MPa was used to determine the critical length of hemp fibre, which was found to be 3.4 mm. The theoretical prediction of the tensilestrength and modulus of hemp-polypropylene composite by using Kelly-Tyson and Cox-Krenchel models, respectively, have been reported in the paper. Model results were validated by experimental works with different fibre lengths and volume fraction of hemp fibre in the composite. The effect of fibre length and content on the flexural strength and stiffness of the hemp-polypropylene composite has also been studied. The effect of moisture absorption on composite tensile strength was predicted by modifying the Kelly-Tyson model. The model curve was also compared with another set of experimental works done at differentmoisture contents in the composite.
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Evaluation of the Influence
of Fibre Length and
Concentration on Mechanical
Performance of Hemp Fibre
Reinforced Polypropylene
Composite
Muhammad Pervaiz a , Mohini Sain a & Arun Ghosh a
a Faculty of Forestry , University of Toronto , 33
Willcocks Street M5S 3B3, Toronto, ON, Canada
Published online: 23 Sep 2008.
To cite this article: Muhammad Pervaiz , Mohini Sain & Arun Ghosh (2006) Evaluation
of the Influence of Fibre Length and Concentration on Mechanical Performance of
Hemp Fibre Reinforced Polypropylene Composite, Journal of Natural Fibers, 2:4,
67-84, DOI: 10.1300/J395v02n04_05
To link to this article: http://dx.doi.org/10.1300/J395v02n04_05
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Evaluation of the Influence
of Fibre Length and Concentration
on Mechanical Performance
of Hemp Fibre Reinforced
Polypropylene Composite
Muhammad Pervaiz
Mohini Sain
Arun Ghosh
ABSTRACT. The paper deals with the evaluation of interfacial shear
stress (ISS) between reinforcement fibre (hemp) and polypropylene ma-
trix through single fibre pull-out method and subsequently the critical
length of the composite grade hemp fibre has been determined. In the
present study the average ISS value of 5.9 MPa was used to determine
the critical length of hemp fibre, which was found to be 3.4 mm. The the-
oretical prediction of the tensile strength and modulus of hemp-poly-
propylene composite by using Kelly-Tyson and Cox-Krenchel models,
respectively, have been reported in the paper. Model results were vali-
dated by experimental works with different fibre lengths and volume
fraction of hemp fibre in the composite. The effect of fibre length and
Muhammad Pervaiz (E-mail: muhammad.pervaiz@utoronto.ca), Mohini Sain, Pro-
fessor (E-mail: m.sain@utoronto.ca), and Arun Ghosh are affiliated with the Faculty of
Forestry, University of Toronto, 33 Willcocks Street, Toronto, ON, Canada M5S 3B3.
The present address for Arun Ghosh is: Case Western Reserve University, Macro-
molecular Science and Engineering, 210 Adelbert Road, Cleveland, OH 44106 (E-mail:
arun.ghosh1@case.edu).
The authors wish to express their sincere gratitude to the Network of Center of Ex-
cellence (Auto-21st Century) and Atofina Canada for their financial support in the pro-
ject.
Journal of Natural Fibers, Vol. 2(4) 2005
Available online at http://www.haworthpress.com/web/JNF
©2005 by The Haworth Press, Inc. All rights reserved.
doi:10.1300/J395v02n04_05 67
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content on the flexural strength and stiffness of the hemp-polypropylene
composite has also been studied. The effect of moisture absorption on
composite tensile strength was predicted by modifying the Kelly-Tyson
model. The model curve was also compared with another set of experi-
mental works done at different moisture contents in the composite. [Ar-
ticle copies available for a fee from The Haworth Document Delivery Service:
1-800-HAWORTH. E-mail address: <docdelivery@haworthpress.com> Web-
site: <http://www.HaworthPress.com> ©2005 by The Haworth Press, Inc. All
rights reserved.]
KEYWORDS. Interfacial shear stress, critical fibre length, hemp/poly-
propylene composite, Kelly-Tyson model, mechanical properties
INTRODUCTION
Automotive applications represent the best opportunity for natural fi-
bre reinforced thermoplastics due to some distinctive advantages over
glass fibres, like, low weight (35-40% less as compared to glass fibre),
low price, better crash absorbance and sound insulation properties.
These fibres are renewable, non-abrasive to process equipment and can
be incinerated at the end of their life cycle for energy recovery as they
possess a good deal of calorific value. Among the natural fibres, hemp,
kenaf and flax show much higher tensile strength than the other natural
fibres (Rowell et al. 2000; Bledzki et al. 1999; Voorn et al. 2001). Over
the past few years, some research has been dedicated to agrofibre rein-
forced polypropylene composites as a possible replacing material for
glass fibre mat thermoplastics (GMT) (Heijenrath et al. 1996; Peijs et al.
1998; Mieck et al. 1996). Several reports have been published on hemp
fibre reinforced polymer composites produced by sheet molding and
resin transfer molding techniques (Rouison et al. 2003; Pervaiz et al.
2003).
The interface in any fibre-matrix composite system is responsible to
transmit stress from weaker polymer matrix to stronger (higher strength)
fibres. This stress transfer efficiency of the fibre reinforced composite
largely depends on the fibre-matrix interface and mechanical properties
of fibre and polymer. Interfacial shear stress (ISS) is a generally ac-
cepted expression of physically measured value of adhesion between fi-
bre and matrix. Among various delicate methods to measure ISS, single
fibre pull-out method is the widely accepted and most elegant and reli-
able (Sydenstricker et al. 2003). In this method, a partially embedded
68 JOURNAL OF NATURAL FIBERS
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fibre in a block of matrix is pulled using a low capacity load cell. The
critical fibre length, at which maximum fibre load is achieved at the ax-
ial centre of the fibre, plays an important role in dictating the overall
stress transfer from matrix to reinforcement medium. Fu et al. (2000)
have used the single fibre pull-out method to determine critical fibre
length of glass and carbon fibres by determining ISS between these
fibres and polypropylene in single fibre composites. Many research
works have reported in literature to analyse the properties of fibre/ma-
trix interface of non-biodegradable synthetic fibre reinforced compos-
ites by using the technique of single fibre pull-out (Hui et al. 1999;
Anderson et al. 1993; Shioya and Takaku 1995; Netravali et al. 1989;
Rao and Drzal 1991; Zhandarov et al. 1992). Joseph et al. (2002) have
used the method of single fibre pull-out to determine ISS value between
phenol formaldehyde resin and banana and glass single fibre composite.
It has been become imperative to develop and utilize comprehensive
modelling techniques, especially in case of new product development
and modifying properties of existing products. The development of sta-
tistical model for the single fibre composite has proved to be very chal-
lenging. Aveston et al. (1971) have proposed the elementary statistical
model for single fibre composite; beyond this analysis many research-
ers have worked on developing the theoretical model to evaluate the
properties of single fibre composites (Henstenburg and Phoenix 1989;
Curtin 1991; Kimber and Keer 1982; Hui et al. 1996; Cox 1952). The
important parameters investigated for fibre reinforced thermoplastic
composites are fibre content and length. Most of the modelling studies
are based on short synthetic fibres (glass and carbon) and thermoplastic
matrix of polypropylene. Thomason and Vlug (1996) have studied glass
fibre thermoplastics prepared in wet-deposition method and reported
the effect of fibre length and content on tensile and flexural modulus by
comparing the experimental results with Cox model (Cox 1952). Garkhail
et al. (2000) have used flax fibre and polypropylene in film staking
method and prepared natural fibre mat thermoplastics (NMT) and com-
pared the experimental data with commercially available glass fibre mat
thermoplastics (GMT). They have also compared the data with mi-
cro-mechanical models for strength and stiffness and found good agree-
ment with prediction curves. Similar work has been reported by Oever et
al. (2000) where both scutched and hackled flax fibres have been used
in combination with polypropylene and composite samples have been
made in wet-laid process.
In the current work, for the first time hemp fibre based thermoplastics
have been used for comprehensive modelling based on fibre content and
Pervaiz, Sain, and Ghosh 69
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length after evaluating ISS and critical fibre length. The prediction of
hemp-polypropylene composite strength and stiffness by using Kelly-
Tyson and Cox-Krenchel models (Cox 1952; Oever et al. 2000; Krenchel
1964) based on fibre length and content has been reported in this paper.
Model results have been validated by experimental works with four dif-
ferent fibre lengths and volume fractions of hemp fibre in hemp-PP
composite. Detailed studies have been reported (Joseph et al. 2002;
Kelly and Tyson 1965; Callister 2000) on the environmental degrada-
tion of natural fibre composites, but no significant work has been done
on prediction of the composite strength as a function of moisture ab-
sorption. In current work, an effort has been made by modifying the
Kelly-Tyson model to predict composite strength at different levels of
moisture absorption.
THEORY
The determination of the interfacial shear stress (ISS) between fibre
and matrix is an integral part of the modelling for fibre reinforced com-
posites. The following expression has been used to get the value of criti-
cal fibre length after evaluation of ISS value.
lc=σf.d/2 τ(1)
where lcis critical fibre length, σfand dare tensile strength and diameter
of fibre, respectively, and τis ISS.
The tensile strength (σc) of the composite has been predicted by us-
ing the following modified version of the Kelly and Tyson model (Kelly
and Tyson 1965).
σc=λK[σfVf(1 lc/2l)] + σm(1 Vf) (2)
where l is fibre length, K is reinforcement efficiency factor, σmis the
tensile strength of the matrix and Vfdenotes the volume fraction of fibre
in the composite. The fitting parameter, λ, is called fibre length and ori-
entation distribution function. For the discontinuous and random fibre
composite the value of λis taken as 0.2 (Garkhail et al. 2000).
The tensile modulus (Ec) of the composite has been predicted by us-
ing Cox-Krenchel model (Cox 1952; Krenchel 1964) as represented by
the following expression:
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Ec = ηoηL(EfVf)+E
m(1 Vf) (3)
Where Efand Emare tensile modulus of reinforcement fibre and matrix,
respectively, and ηo, introduced by Krenchel is a fibre orientation factor
and has a value of 3/8 for random in-plane fibre reinforced composites.
The second parameter ηL, fibre efficiency factor introduced by Cox,
takes into account the limited efficiency of stress transfer from matrix to
fibre due to finite fibre length.
MODIFICATION OF KELLY-TYSON MODEL
The mechanism responsible for lowering of the composite strength
due to moisture uptake has been shown in Figure 1(a &b). The Figure 1a
shows that on moisture absorption the diameter of single fibre increases
(d < d*) and consequently the fibre tensile strength decreases (σf*<<
σf). The symbols d and d*denote the fibre diameter before and after
moisture absorption, respectively. The symbols σfand σf*denote the
tensile strength of the fibre before and after moisture absorption. Figure
1b shows the effect of moisture absorption on fibre/polymer composite
system. VF, VM and VW denote the volume of fibre, polymer matrix
and water present in the composite. Vfand Vf*are the volume fraction
of fibre in the composite before and after moisture absorption. The
Kelly-Tyson model (2) has been modified to take into account the vol-
ume change of reinforcement fibre in a thermoplastic composite due to
accelerated moisture aging. It is assumed that moisture absorption by
matrix is insignificant. However, there is also plasticization that occurs
in the lignin and hemicellulose that affects the fibre properties.
As the σf, tensile strength of fibre, is actually force per unit cross-sec-
tional area of fibre, therefore, above expression (2) can be written as:
σc=λK[(F/A) Vf(1 lc/2l)] + σm(1 Vf) (4)
Where F and A denote the total applied force and fibre cross sectional
area, respectively. Moreover, to take into account the whole effect of
fibres’ volume, the equation (4) can be further simplified as:
σc=λK[(F/Q) Vf(llc/2)] + σm(1 Vf) (5)
Pervaiz, Sain, and Ghosh 71
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where Q is total volume of reinforced fibres in polymer matrix. Now, as
the force is constant, the strength of fibre decreases in case of water ab-
sorption due to fibre swelling and increase in diameter.
Equation (5) has been used to get the prediction values for change in
composite strength by using these data; σf= 610 MPa, σm=34MPa.
The values of λand K are 0.2 and 3/8, respectively. Average diameter
of hemp fibre measured microscopically is 65 µm and for each incre-
mental moisture absorption, new values of fibre diameter and volume
fraction are calculated by assuming that moisture is absorbed by fibres
only.
EXPERIMENTAL
Materials
Hemp Fibre
The composite grade hemp fibre was obtained from Hempline Inc.,
Canada. The average tensile strength of hemp fibre was found to be 610
MPa at 5mm gauge length and the average diameter measured through
microscope was 65 µm.
72 JOURNAL OF NATURAL FIBERS
Force
(a) Effect of fibre swelling on strength (b) Sorption and volume fraction of fibre
VF/(VM + VF) FV/(VM + VF + VW)
VfV*
f
MM
FFW
σf
dd*
σf*
σσ
ff
*<< V*<V
ff
FIGURE 1. Schematic showing of the two dominant mechanisms responsible
for lowering the composite strength on moisture absorption.
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Thermoplastic Polymer
Homopolymer grade polypropylene (H101) was supplied by Copol
International Ltd., Canada in the form of films having thickness of 100
µm. The tensile strength and secant modulus of the polypropylene film
were 34 MPa and 0.8 GPa, respectively.
Single Fibre Pull-Out Test
The single hemp fibres of similar diameter were embedded in be-
tween two films of polypropylene (PP) with dimensions of 20 20
mm2and thickness of 100 µm. First, a film of PP (20 20 mm2) was
mounted on a Dupont Mylar (polyester) sheet with slightly larger di-
mension by using general-purpose scotch tape on the edges. The se-
lected single fibre was placed on the mounted PP film and secured by
second PP film and another Mylar polyester sheet on top of it. The pur-
pose of polyester sheets was to give support to coupon and avoid stick-
ing of PP films on hot plate during compression at high temperature.
The single hemp fibre/PP composites were prepared by heating and
pressing the coupons at 200°C on a hot plate. The embedded lengths of
single fibres ranged from 3 to 10 mm.
The free length (un-embedded) part of fibre was mounted on a paper
strip to facilitate grip in tensile tester as shown in Figure 2. The testing
was done on Sintech tensile machine with 22.7 kg load cell and the test
speed was maintained at 2 mm/min. The free end of fibre, mounted on
paper strip, was fixed in the top grip of tester attached to load cell. The
maximum load and displacement data were recorded for each test. It is
imperative to have large number of tests and results for this kind of
study to offset slightly high value of standard deviation. In the present
study out of total 58 coupons, only 35 were found valid.
Measurement of Tensile and Flexural Properties of the Composites
Representative samples of composites were manufactured by film-
stacking method. First the hemp fibres of desired length and quantity
were evenly and randomly distributed in the form of layers having ap-
proximately 210 210 mm2dimension. Then fibre layers and required
number of PP films were attached alternately and put into an electric
oven for about two hours at 105°C to completely dry out the moisture
from raw materials, especially cellulose fibres. Immediately after this,
the stack of fibres and PP films was transferred to a hydraulic press,
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having 5 104kg capacity with air/water cooling arrangement. Im-
pregnation was achieved at a temperature of 200-205°C for 10 minutes
and at a pressure of 2.5 106Pascal. The press was cooled at the end of
heating/impregnation cycle to 50°C in about 4 minutes before complet-
ing the compression moulding process. The thickness of the composite
was in the range of 1.5-2 mm. To have smooth surface of sheets and pre-
vent sticking of PP/fibres on hot press plates, Dupont Mylar (polyester)
films were used at top and bottom of stacked mats. Sixteen combina-
tions (four different lengths; 10, 20, 30, 40 mm and four different volume
fractions; 0.10, 0.21, 0.33, 0.48) were used in compression moulding of
composite sheets.
The flexural properties of the samples (13 mm 150 mm) were
tested by using Zwick Universal Testing Machine (Z 100) according to
ASTM D-790 with machine speed of 5 mm/min and span width of 53
mm. The tensile testing of the samples was done by using the Zwick
UTM and the sample specimens were prepared according to ASTM
D-638 method. At least five samples of each category were tested for
tensile and flexural properties.
Water Absorption Test
The tensile specimens were immersed in distilled water at 35°C. The
specimens were then removed from time to time and weighed up to hun-
dredth of a gram after quickly wiping the surfaces and edges with tissue
74 JOURNAL OF NATURAL FIBERS
Force
Load cell
Paper strip
Fibre
Matrix
Grip
FIGURE 2. Schematic showing of single fibre pull-out testing
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paper. Same procedure was repeated till the target moisture content was
achieved approximately. Similar method for accelerated sorption stud-
ies was reported by Joseph et al. (2002). Five samples for each degree of
water absorption were treated similarly and tested on Zwick tester for
tensile strength.
RESULTS AND DISCUSSION
Evaluation of Interfacial Shear Stress
The composites with longer length (5-10 mm) of embedded fibres do
not show any pull-out phenomenon in the curves and all the fibres are
fractured at maximum force. However, as the embedded length is re-
duced below 4 mm, more and more curves of load/displacement with
distinctive pull-out shape are obtained. Typical curves of single fibre
pull-out and fracture are shown in Figures 3 and 4, respectively.
The pull-out curve, shown in Figure 3, is well documented (Joseph et
al. 2002; Chua and Piggot 1985) and is composed of three zones. In first
elastic region, load increases with displacement and then sudden release
of strain energy occurs as interface fails which is indicative of de-bonding
phenomenon. The second part of the curve shows actual pull-out of
de-bonded fibres with increased displacement. The final region clearly
depicts frictional forces acting on fibre during pull-out phase. On the
other hand, the typical load/displacement curve (Figure 4) of fibre break
has a sudden drop in force after elastic deformation. One more important
observation is the amount of force required for pull-out and fracture test
of the fibres. As expected, the force required for fibre fracture is higher
than pull-out, which can be explained by basic theory of critical length as
mentioned by Callister et al. (2000).
The hemp fibre, which is used in this study, has ISS value of 5.9 MPa
with standard deviation of 0.97. These values have been used for esti-
mating critical length, which is eventually used in composite strength
predicting models. The comparison is drawn between theoretical and
experimental results. It has been reported by Oever et al. (2000) that flax
fibre show average ISS value of 8 MPa in PP composite. The glass fibre
and carbon fibre show average ISS values of 15.2 and 18.2 MPa, respec-
tively, in PP composite (Fu et al. 2000).
Evaluation of Critical Fibre Length
The average value of interfacial shear stress computed from 24 single
fibre pull-out tests is 5.9 MPa with standard deviation of 0.97 MPa. The
Pervaiz, Sain, and Ghosh 75
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average values of tensile strength and diameter of single hemp fibre, as
reported earlier, are 610 MPa and 65 µm, respectively. Using all these
values in expression (1), it gives the critical length of 3.4 mm of hemp
fibre in hemp/PP composite. It has been reported that the critical length
of flax in flax/PP composite is 3.0 mm (Oever et al. 2000). The glass
fibre and carbon fibres show critical length of 0.89 mm and 0.81 mm,
respectively, in polypropylene composites (Fu et al. 2000).
Effect of Fibre Length and Content
on Mechanical Performance of the Composites
Tensile Strength and Modulus
The effects of length and volume fraction of fibre on the tensile
strength of the composites are shown in Figures 5 and 6, respectively.
76 JOURNAL OF NATURAL FIBERS
0.40
0.20
0.00
0.45 0.91 2.28 3.64
Extension (mm)
Load
kg
Energy Release Zone
Pull-out Zone
Frictional Zone-Tail
FIGURE 3. Load/displacement curve for a typical single fibre pull-out test
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Model prediction results are obtained from equation (2) by using K (fi-
bre orientation efficiency factor) = 0.375, σf(fibre strength) = 610 MPa,
lc(critical fibre length) = 3.4mm, and σm(matrix tensile strength) = 34
MPa. The figures show good agreement between model predicted and
experimental results of tensile strength of the composite with different
fibre contents and lengths of hemp fibre in hemp/polypropylene com-
posite. This is also an indirect proof of validity of the critical fibre
length and interfacial shear stress evaluated experimentally in the cur-
rent research work. However, as compared to commercially available
glass fibre mat thermoplastics (GMT) the tensile strength of hemp-based
thermoplastics is much lower (Garkhail et al. 2000). Thomason et al
(1996) have reported that the tensile strength of the GMT reaches to a
maximum value of 95 MPa with glass fibre (length, 6 mm) of weight
Pervaiz, Sain, and Ghosh 77
0.80
0.40
0.00
0.380.41 1.21
Extension (mm)
Load
kg
Break
FIGURE 4. Load/displacement curve for a typical single fibre fracture test
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78 JOURNAL OF NATURAL FIBERS
42
40
38
36
34
32
0 5 10 15 20 25 30 35 40 45
Vf: 0.48
Vf: 0.33
Vf: 0.21
Vf: 0.10
Fiber Length (mm)
Tensile Strength (MPa)
FIGURE 5. Effect of fibre length on tensile strength at different volume frac-
tions (–––– model; ------- experimental)
42
40
38
36
34
32
0 0.1 0.2 0.3 0.4 0..5 0.6
Volume fraction
Tensile Strength (MPa)
L=40mm
L=30mm
L=20mm
L=10mm
FIGURE 6. Effect of fibre content on tensile strength at different fibre lengths
(–––– model, ------- experimental)
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content of 60%. The lower tensile properties of the hemp-polypropy-
lene composite may be attributed to inherent lower strength of technical
agricultural fibres and bundles separated mechanically from natural
plants. These technical agricultural fibres have diameter of 50-100 µm
with tensile strength of 550-700 MPa compared to 2500 MPa for glass
fibres. The somewhat large scatter in strength values of hemp/PP com-
posites is probably due to the anisotropic nature of agricultural fibres.
The macro-sized agricultural fibres have very complex and porous
(void space) structure. The different parts of the fibre have different
physical properties. The fibre properties also depend on the age of the
plants and the weather at which they grow up.
The composite tensile stiffness is predicted from the model equation
(3) by using the values of ηo= 0.375, Ef= 43 GPa, and Em= 0.8 Gpa.
The model predicted and experimental results are plotted in Figures 7
and 8. It has been found from the Figure 7 that the effect of fibre length
on tensile modulus is insignificant at constant volume fraction of fibre
in the composite. However, the tensile modulus of the composite in-
creases with increasing the fibre content in the composite (Figure 8). It
has been reported that there is little effect of fibre length on composite
stiffness beyond critical fibre length and stiffness is usually a function
of fibre content in the composite. This is a good agreement between pre-
dictive and experimental data of tensile stiffness of the composite illus-
trating the effects of fibre length and content on composite stiffness as
Pervaiz, Sain, and Ghosh 79
10
8
6
4
2
0
10 20 30 40
Fibre length (mm)
Tensile modulus (GPa)
5.9 66.1
7
FIGURE 7. Effect of fibre length on the composite tensile modulus (––––
model; ------- experimental)
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shown in Figures 7 and 8, respectively. In comparison with GMTs,
hemp based NMTs have compatible modulus and due to lower density
of natural fibres, the specific modulus of NMTs may even surpass that
of GMTs. Fu et al. (2000) have reported the effect of fibre volume frac-
tion and length on composite tensile properties by using short glass and
carbon fibres in polymer matrix.
Flexural Strength and Modulus
The experimental results of flexural strength and modulus of the
hemp/polypropylene composite are shown in Figure 9. As expected
both flexural strength and modulus of the hemp/polypropylene compos-
ite increases with increasing the volume fraction of hemp fibre in the
composite. The effect of fibre length on flexural properties of the com-
posites is insignificant. The significantly lower flexural strength of
NMT compared to GMT is attributed to the inherent lower strength of
technical hemp fibres compared to glass fibres (Oever et al. 2000). It
is generally believed that the higher fibre concentrations give better
strength properties and the failure strain of composite decreases with in-
crease in fibre loadings (Bijsterbosch and Gaymans 1995; Daneault et
al. 1989; Ho et al. 1996).
80 JOURNAL OF NATURAL FIBERS
10
8
6
4
2
0
Tensile modulus (GPa)
0 0.1 0.2 0.3 0.4 0..5 0.6
Volume fraction
1.4
3
4.5
6.6
FIGURE 8. Effect of fibre content on the composite tensile modulus (––––
model; ------- experimental)
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Effect of Moisture Absorption on Composite Strength
Natural fibre composites have tendency to absorb a high amount of
water due to hydrophilic nature of reinforcing cellulose fibres and pres-
ence of void spaces inside the fibre. The high level of moisture absorp-
tion by cellulose fibres leads to degradation in mechanical performance
of composite in both accelerated as well as in aging mode. The water
molecules absorbed by the composite system mostly affects the fibres
and fibre/matrix interface. Figure 10 shows that the experimental values
of the tensile strength of the hemp/polypropylene composite do not
have a very good match with the predicted values obtained from the
modified Kelly-Tyson model (5) and a diverging trend, especially at
higher moisture content, is observed. This lower experimental tensile
strength values compared to model (5) predicted values of the compos-
ite may be attributed to the fact that fibre volume increase is not the only
phenomena, which contributes to lowering of composite strength. Fi-
bre/matrix interface degradation due to water molecules, which is
caused by osmotic pressure development at fibre surface due to leach-
ing of water-soluble substances, may be another reason. However, the
modified Kelly-Tyson model (5) can serve as a basis to further refine
the prediction of composite strength as a function of moisture absorption.
Pervaiz, Sain, and Ghosh 81
70
60
50
40
30
20
10
0
7
6
5
4
3
2
1
0
Flexural strength
Flexural modulus
0.1 0.21 0.33 0.48
32
44.4
53.1
65.2
1.5
3.1
4.3
6.2
Fiber volume fraction
Flexural strength (MPa)
Flexural modulus (GPa)
FIGURE 9. Experimental flexural strength and modulus of hemp/PP composite
as a function of fibre content
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CONCLUSIONS
1. The average value of interfacial shear stress of hemp/polypropy-
lene composite is 5.9 MPa and the corresponding critical length of
hemp fibre in polypropylene composite is 3.4 mm. This value of
critical fibre length is much lower as compared to synthetic glass
and carbon fibres in the polypropylene composites.
2. The experimental values of tensile strength and modulus of the
hemp/polypropylene composite are more or less same with theo-
retical values predicted from the Kelly-Tyson and Cox-Krenchel
models. As expected the tensile and flexural strength of the
hemp-polypropylene composite increases significantly with in-
creasing the hemp fibre content in the composite. The fibre length
has marginal effect on the composite stiffness at a constant vol-
ume fraction of fibre in the composite.
3. The results show that the tensile strength of the hemp-polypropy-
lene composite decreases on absorption of moisture. This experi-
mental decrease in tensile strength is greater than the values
predicted from the modified Kelly-Tyson model, which may be
attributed to the leaching of water soluble-substances from the
composite at higher moisture content.
82 JOURNAL OF NATURAL FIBERS
42
39
36
33
30
36.7
34.9
33.6
31.8
0 5 10 15 20 25 30
Moisture Content (%)
Tensile strength (MPa)
FIGURE 10. Effect of moisture absorption on hemp/PP composite tensile
strength
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REFERENCES
Anderson J., and Tamuzs V., “Fiber and interface strength distribution studies with sin-
gle-fiber composite test,” Composite Science and Technology, 48 (1993) 57.
Aveston J., Cooper G.A., and Kelly A., “Single and multiple fracture: The properties of
fiber composites.” In: The properties of fibre composites. Conference Proceedings,
National Physical Laboratory, Teddington. Guildford: IPC Science and Technol-
ogy Press, 1971. pp. 15-26.
Bijsterbosch H., and Gaymans R.J., “Polyamide 6-long glass fibre injection moldings,”
Polymer Composite, 16 (1995) 363.
Bledzki A.K., and Gassan J., “Composites reinforced with cellulose based fibres,”
Progress in Polymer Science, 24 (1999) 221.
Callister W. D. Jr., “Materials science and engineering-An introduction,” Fifth Edition,
John Wiley & Sons Inc, 2000.
Chua P.S., and Piggot M.R., “The glass fibre-polymer interface: Theoretical consider-
ation for single fibre pull-out tests,” Composite Science and Technology, 22 (1985)
33.
Cox H.L., “The elasticity and strength of paper and other fibrous materials,” British J
Appl Phys, 3 (1952) 72.
Curtin W.A., “Exact theory of fibre fragmentation in a single-filament composite,” J
Mater Sci, 26 (1991) 5239.
Daneault J., Vu-Khanh T., and Foster B., “Tensile properties of injection molded long
fibre thermoplastic composites,” Polymer Composite, 10 (1989) 313.
Fu S.Y., Lauke B., Mader E., Yue C.Y., and Hu X., “Tensile Properties of short
glass-fibre and short carbon-fibre-reinforced polypropylene composites,” Compos-
ites Part A, 31 (2000) 1117.
Garkhail S.K., Heijenrath R.W.H., and Peijs T., “Mechanical Properties of natural fi-
ber mat reinforced thermoplastics based on flax fibers and polypropylene,” Applied
Composite Materials, 7 (2000) 351.
Heijenrath R., and Peijs T., “Natural-fibre-mat-reinforced thermoplastic composites
based on flax fibres and polypropylene,” Adv Comp Let, 5(3) (1996) 81.
Henstenburg R.B., and Phoenix S.L., “Interfacial shear strength studies using the sin-
gle-filament-composite test. Part II: A probability model and Monte Carlo simula-
tion,” Polymer Composites, 10 (1989)389.
Ho K.C., Hwang J.R., and Doong J.L., “Tensile properties of short glass fibre rein-
forced polycarbonate,” Polymer Composite, 4 (1996) 563.
Hui C.Y., Phoenix S.L., and Kogan L., “Analysis of fragmentation in the single-fila-
ment composite: Roles of fiber strength distributions and exclusion zone models,” J
Mech Phys Solids, 44 (1996) 1715.
Hui C.Y., Shia D., and Berglund L.A., “Estimation of interfacial shear strength: An ap-
plication of a new statistical theory for single fiber composite test,” Composite Sci-
ence and Technology, 59 (1999) 2037.
Joseph P.V., Rabello M.S., Mattoso L.H.C., Joseph K., and Thomas S., “Environmen-
tal effects on the degradation behaviour of sisal fibre reinforced polypropylene
composites,” Composites Science and Technology, 62 (2002) 1357.
Pervaiz, Sain, and Ghosh 83
Downloaded by [University of Toronto Libraries] at 14:48 25 June 2014
Kelly A., and Tyson W.R., “Tensile properties of fibre-reinforced metals: Copper/
tungsten and copper/molybdenum,” J Mech Phys Solids, 13 (1965) 329.
Kimber A.C., and Keer J.G., “On the theoretical average crack spacing in brittle matrix
composites containing continuous aligned fibre,” J Mater Sci Lett, 1 (1982) 353.
Krenchel H., “Fibre reinforcement–Theoretical and practical investigations of the elas-
ticity and strength of fibre-reinforced materials,” Thesis, Akademisk Forlag, Co-
penhagen, 1964.
Mieck K.–P., Lützekendorf R., and Reussmann T., “Needle-punched hybrid non-
wovens of flax and PP fibres–Textile semiproducts for manufacturing of fibre com-
posites,” Polymer Composites, 17(6) (1996) 873.
Netravali A.N., Henstenburg R.B., Phoenix S.L., and Schwartz P., “Inter-facial shear
strength studies using the single-filament-composite test. Part I: Experiments on
graphite fibers in epoxy,” Polymer Composites, 10 (1989) 226.
Oever M.J.A., Bos H.L., and Kemenade M.J.J.M., “Influence of the physical structure
of flax fibres on the mechanical properties of flax fibre reinforced polypropylene
composites,” Applied Composite Materials, 7 (2000) 387.
Peijs T., Garkhail S., Heijenrath R., Oever M. Van den, and Bos H., “Thermoplastic
composites based on flax fibres and polypropylene: Influence of fibre length and fi-
bre volume fraction on mechanical properties,” Macomol Symp, 127 (1998) 193.
Pervaiz M., and Sain M., “Carbon storage potential of natural fibre composites for au-
tomotive parts manufacturing,” Resources, Conservation and Recycling, 1-6/16
(2003).
Rao V., and Drzal L.T., “The dependence of interfacial shear strength on matrix and
interphase properties,” Polymer Composites, 12 (1991)48.
Rouison D., Sain M., and Couturier M., “Resin transfer molding of natural fibre rein-
forced plastic. 1. Kinetic study of an unsaturated polyester resin containing an in-
hibitor and various promoters,” J Appl Polym Sci, 89 (2003) 2553.
Rowell R.M., Han J., and Rowell J., “Characterization and factors effecting fibre prop-
erties,” Natural Polymers and Agrofibers Composites, (2000) 115-134.
Shioya M., and Takaku A., “Estimation of fiber and interfacial shear strength by using
a single-fiber composite,” Composite Science and Technology, 55 (1995) 33.
Sydenstricker H.D., Mochnaz S., and Amico S.C., “Pull-out and other evaluations in
sisal-reinforced polyester biocomposites,” Polymer Testing, 22 (2003) 375.
Thomason J.L., and Vlug M.A., “Influence of fibre length and concentration on the
properties of glass fibre–reinforced polypropylene: 1. Tensile and flexural modulus,”
Composites: Part A, 27A (1996) 477.
Thomason J.L., Vlug M.A., Schipper G., and Krikor H.G.L.T. “Influence of fibre
length and concentration on the properties of glass fibre-reinforced polypropylene:
Part 3: Strength and strain at failure,” Composites Part A: 27A (1996) 1075.
Voorn B.V., Smit H.H.G., Sinke R.J., and de Klerk B., “Natural fiber reinforced sheet
molding compound,” Composites: Part A, 32 (2001) 1271.
Zhandarov S.F., Pisanova E.V., and Dovgyalo V.A., “Measurement of fiber-matrix ad-
hesion by testing single-fiber composites,” Mekha-nika Kompozitnykh Materialov,
3 (1992) 384.
RECEIVED: December 16, 2004
REVISED: February 22, 2005
ACCEPTED: April 26, 2005
84 JOURNAL OF NATURAL FIBERS
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