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DOI: 10.1177/0021998305047267

2005 39: 663Journal of Composite MaterialsAbu Bakar A. Hariharan and H. P. S. Abdul Khalil

and Impact Behavior of Oil Palm Fiber-Glass Fiber-reinforced Epoxy ResinLignocellulose-based Hybrid Bilayer Laminate Composite: Part I - Studies on Tensile

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Lignocellulose-based Hybrid BilayerLaminate Composite: Part I Studies onTensile and Impact Behavior of Oil PalmFiberGlass Fiber-reinforced Epoxy Resin

ABU BAKARA. HARIHARAN*

School of Materials and Mineral Resources

Engineering Campus, Universiti Sains Malaysia14300 Nibong Tebal, Seberang Perai Selatan, Penang, Malaysia

H. P. S. ABDUL KHALIL

School of Industrial Technology, Universiti Sains Malaysia

11800 Penang, Malaysia

(Received January 19, 2004)(Accepted June 28, 2004)

ABSTRACT: The tensile and impact behavior of the oil palm fiberglass fiber

hybrid bilayer laminate composites are studied. The fiber mats are impregnated withepoxy resin and cured at 100C for 1 h followed by post curing at 105C. Thehybridization of the oil palm fibers with glass fibers increases the tensile strength, theYoungs modulus, and also the elongation at break of the hybrid composites. Anegative hybrid effect is observed for the tensile strength and Youngs modulus whilea positive hybrid effect was observed for the elongation at break of the hybridcomposites. The impact strength of the hybrid composite increases with the additionof glass fibers. The hybrid composites which are impacted at the glass fiber layerexhibit a higher impact strength and a positive hybrid effect compared to thoseimpacted at the oil palm fiber layer. The scanning electron micrographs andphotomicrographs of tensile and impact fracture samples are taken to study thefailure mechanism and fibermatrix interface adhesion.

KEY WORDS: hybrid composites, lignocellulose-reinforced composites, laminatecomposites, oil palm fiber composites.

INTRODUCTION

IN RECENT YEARS,the usage of lignocellulosic fibers or plant fibers as a replacement for

synthetic fibers such as carbon, aramid, and glass fibers in composite materials has

*Author to whom correspondence should be addressed. E-mail: [email protected]

Journal ofCOMPOSITE MATERIALS, Vol. 39, No. 8/2005 663

0021-9983/05/08 066322 $10.00/0 DOI: 10.1177/0021998305047267 2005 Sage Publications

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left behind after the removal of oil palm fruits for the oil refining process at the oil

refineries. Based on a report by Tanaka [25], 16 million tons of empty fruit bunches

were discharged per annum from each and every oil palm refinery in the country regularly

in the year 2000. The empty fruit bunches are then used as boiler feedstock in the oil

mill and are also left to mulch and degrade as soil fertilizers in the field, while the majority

of the empty fruit bunches are not utilized, posing a serious environmental threat.

Therefore, by using the oil palm empty fruit bunch fibers, which were extracted from

the empty fruit bunches by Sabutek (M) Sdn. Bhd. (a local company in Malaysia), as

a reinforcement in composite materials, the biomass waste generated by the oil palm

industry can be reduced significantly.

In this work, the mechanical properties of oil palm fiberglass fiber hybrid bilayer

laminate composite are studied. Earlier works done on the lignocellulose fiberglass fiber

hybrid composite basically concentrated on the intermingle fiber system [1517,1921,23].

In addition, the lignocellulose-based sandwich composite system was studied by Mohan

and Kishore [26], and Clark and Ansell [27]. Since studies on a bilayer hybrid composite

based on lignocellulose fibersynthetic fiber have not been reported yet, the opportunitywas taken to evaluate and report the tensile and impact properties of the oil palm fiber

glass fiber hybrid bilayer laminate composite, in this paper. Scanning electron micrograph

and images of tensile and impact fractured samples have been taken in order to study the

fracture mechanism of the hybrid bilayer system. The flexural, moisture absorption, and

flammability properties of the hybrid bilayer composite will be reported in a future

publication.

EXPERIMENTAL

Materials

An epoxy resin based on bisphenol A (Clear Epoxy Resin 331) and a polyamide (Epoxy

Hardener A062) were supplied by Euro Chemo-Pharma Sdn. Bhd. Benzyl alcohol was

supplied by Aldrich Company. Oil Palm Empty Fruit Bunch Fibers (OPEFB) were

obtained from Sabutek (M) Sdn. Bhd. E-glass-chopped strand mat fibers (GF) were

supplied by Euro Chemo-Pharma Sdn. Bhd.

Preparation of Random Oil Palm Fiber Mat

The empty fruit bunch fibers were washed and cleaned of impurities and dried in an

oven at 80C for 24 h [20,21]. The equilibrium moisture content of the fibers was about

11%. The dried fibers were then kept in a sealed polyethylene bag to prevent it from

reabsorbing moisture from the environment. In order to prepare a nonwoven fiber mat,

a weighed quantity of fibers were dispersed in a sieve which was placed in a tub of water.

Once the fibers were evenly dispersed and the mat was formed, the sieve was taken out

from the tub. The excess water from the mat was drained out by pressing the mat against a

flat plate. The random fiber mat was subsequently dried in an oven at 80C for 24 h. The

dried fiber mat was then compacted under pressure at 8000 psi in a compression moldfollowed by trimming of the fiber mat edges in order to obtain a uniform shaped fiber mat.

Lignocellulose-based Laminate Composite: Part I 665



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Preparation of Bilayer Laminate Hybrid Composite

The laminate composite was prepared by following a prepreg route as described by Hill

and Abdul Khalil [10]. A high-density polyethylene (HDPE) liner was used for the fiber

mat impregnation process. A schematic diagram of the laminate impregnation process is

shown in Figure 1. One end of the liner was sealed with a tape while the other end was left

open so that the resin could be poured into the liner. A cardboard was placed at the sealed

end of the liner and was connected to a vacuum pump. The cardboard was used to prevent

excess resin from entering the vacuum pump during the impregnation process. The glass

fiber (GF) and the oil palm fiber mats (EFB) were stacked (one on top of the other) and

were placed in the liner. The middle section of the liner was sealed with clips as shown in

Figure 1.

A fixed amount of resin was stirred manually for 10 min in a plastic container using a

glass rod. Table 1 shows the resin formulation used for the impregnation process. The

mixture was then warmed in an oven at 70C for about 10 min in order to further reduce

the viscosity of the resin. While vacuum was applied to the liner containing the fiber mat,the resin was poured into the open end of the liner (Figure 1). The clips in the center were

removed and the open end of the liner was closed with the clips thus allowing the resin to

flow and impregnate the mat. Once the mat was completely impregnated, the vacuum

connection was removed and the liner was cut open. The impregnated mat was then

transferred onto a 2-mm thick aluminum plate which was subsequently placed on a hot

press. Spacer bars of 10 mm thickness were placed beside the mat and the mat was

compressed at a constant pressure of 6000 psi while squeezing out the excess resin. The

laminate composite was left to cure at 100C for 1 h. An open leaky mold method was used

Resin flow

Middle section

Sealed end Cardboard Fiber matOpen end

Vacuum pump Clips HDPE liner

Figure 1. A schematic diagram of the laminate impregnation process.

Table 1. Resin formulation used for the impregration process.

Amount

Epoxy resin 100 phr

Polyamide 30 phr

Benzyl alcohol 10 wt.% of totalepoxypolyamide mixture

666 A. B. A. HARIHARAN ANDH. P. S. A. KHALIL



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in this research. Once the laminate was cured, the laminate composite was removed from

the plate followed by post curing in an oven at 105C for 30 min. Finally, the laminate

composite was cooled in a cold press under a constant pressure of 500 psi for 15 min in

order to prevent warpage of the laminate composite. Table 2 shows the formulation for the

hybrid bilayer laminate composite while Figure 2 shows the schematic diagram of the

bilayer hybrid laminate composite. The physical and mechanical properties of epoxy, oil

palm fiber, and glass fiber are given in Table 3.

Mechanical Testings

Tensile testing was performed according to ASTM D638-76. Rectangular strips

(120 20 10mm3) were cut from the laminate composite followed by milling of the

strips into a dumbbell shape with dimensions of 120 10 10mm3. The tensile test was

conducted on a Universal Testing Machine Model 1114 at a crosshead speed of 5 mm/min

and gauge length of 60 mm. A minimum of five samples were tested and an average valuewas recorded. Unnotched Izod impact test samples with dimensions of 70 15 10mm3

were cut from the laminate composites. The testing was conducted according to ASTM

D256 on a Zwick model 5101 with a pendulum weight of 25 J. The samples were impacted

Glass fiber plies

Oil palm fiber ply

Figure 2. The schematic diagram of the hybrid bilayer laminate composite.

Table 3. The physical and mechanical properties of epoxy, oil palm fiber, and glass fiber.

Properties Epoxy Oil palm fiber Glass fiber

Density (g/cm3) 1.15 0.71.55 2.56

Tensile strength (MPa) 20 100400 17003500

Youngs modulus (GPa) 7.97 19 6672

Elongation at break (%) 7.35 818 3

Impact strength (kJ/m2) 10.89

Flexural strength (MPa) 78 Flexural modulus (GPa) 2.13

Table 2. Hybrid bilayer laminate composite formulation.

Oil palm fiberglass fiber (wt.%)

Bilayer laminate 100/0 90/10 70/30 50/50 30/70 10/90 0/100




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at the glass fiber layer and at the oil palm fiber layer. Five samples were tested (at each

layer) at room temperature and the average value was taken as the Izod impact strength.

The Izod impact strength was calculated using the formula given below :

Impact strength kJ=m

2

Impact energy J=Crosssectional area 10

3

1

Fracture Sample Analysis

A scanning electron microscope (SEM), Model Leica Cambridge S-360 was used to

study the fracture surface of the tensile and impact specimens. The specimen was coated

with a thin goldpalladium layer using Sputter Coater Polaron SC 515 to avoid electrical

charge accumulation during examination. The basic shapes of the fiber and the fiber

matrix adhesion were also studied using the SEM. Images of fractured samples from

impact test were also taken using a digital video camera JVC 3-CCD, which was connectedto a computer and was analyzed using an Image Analysis Pro software.

RESULTS AND DISCUSSION

Tensile Properties

Figure 3 shows the stressstrain diagram of the glass fiber composite, oil palm fiber

composite, and hybrid composites. From the stressstrain curve, the deformation behavior

of the composites can be well understood.

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7

Strain (%)

Stress(MPa)

GF composite90 wt% GF

10 wt% GFEFB composite

Figure 3. The tensile stressstrain behavior of glass fiber composite, oil palm fiber composite, and hybridcomposites.




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The stressstrain curve of the composites show a linear elastic behavior until about

0.16% strain followed by a deviation from linearity which is maintained until complete

failure of the composites. The nonlinear behavior of a natural fibersynthetic fiber hybrid

composite was also reported by Thwe et al. [17], Sreekala et al. [21], and Clark and Ansell

[27]. When the load is applied to a fiber-reinforced composite, the difference in stiffness

properties between the fiber and the matrix results in the development of high local stress

and strain concentrations in the matrix [28]. Therefore, in order to prevent the local stress

and strain concentrations from inducing failures in the composite, the stresses are

redistributed in the composite through plastic deformation and microcrack initiation of

the matrix [21,29]. According to Hull and Clyne [30], these phenomena result in the

nonlinear behavior of the composite and are also related to the initiation of composite

failure. The plastic deformation though is caused by the shearing of fibers in the matrix

and also by the inherent ductility of the fiber, especially the oil palm fibers [27].

Besides this, at higher strain levels, the drop in the stressstrain curves indicates

progressive failure of the fibers and propagation of cracks through the matrix while the

end of the curve indicates the ultimate strength of the composite which is due to fiberpullout and fiber fracture. Extensive fiber pullout was observed not only in the oil palm

fiber composite but also in the oil palm fiber layer of the hybrid composites while fiber

fractures were observed in the glass fiber composite and in the glass fiber layer of the

hybrid composites (Figure 4(a)(e)). The weak interfacial adhesion between the oil palm

fibers and epoxy resin leads to the pullout of fibers from the matrix while the better

adhesion between the glass fibers and epoxy matrix resulted in fiber fracture. Existence of

epoxy matrix adhered on the surface of the glass fibers indicates a perfect adhesion as

shown in the scanning electron micrographs of the tensile fractured surfaces of the

composites (Figure 4(c)).

Tensile Strength

The tensile strength of the oil palm fiberglass fiber hybrid bilayer laminate composite

as a function of glass fiber loading is shown in Figure 5. As observed from the graph, the

tensile strength of the oil palm fiber composite, which is about 24 MPa, is very much

inferior compared to the glass fiber-reinforced composite which has a tensile strength of

111 MPa.

This is mainly due to the nature of oil palm fibers, which are irregular in shape and size

as seen in the scanning electron micrograph in Figure 6. Furthermore, oil palm fibers also

exist in the form of fiber bundles as shown in Figure 7 [22]. The fiber bundles are

composed of several individual fibers, which are bundled together by a strong pectin

interphase [31].

According to Oksman et al. [2], the load distribution in fiber bundles is not homo-

geneous because the individual fibers are not loaded uniformly as some individual fibers

are not loaded at all. Therefore, the oil palm fibers are unable to support the stress

transferred from the epoxy matrix successfully. Furthermore, the poor adhesion between

the epoxy matrix and the oil palm fibers, which is evident by the extensive fiber pullout of

the oil palm fibers as shown in Figure 4(a), leads to a weak interfacial bond, resulting in an

inefficient stress transfer between the epoxy matrix and the oil palm fibers. As a result, the

oil palm fiber composite fails at a lower load compared to the glass fiber-reinforcedcomposite.




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However, with the addition of glass fibers into the oil palm fiber composite, the tensile

strength of the hybrid composites increased significantly as seen from the graph in Figure 5.

A similar trend was reported by Kalaprasad et al. [16,32], Pavithran et al. [20], and Mishra

et al. [15] with the addition of glass fibers into a natural fiber composite. At 0.8 volume

fraction of glass fibers, the tensile strength of the hybrid composite increased by about

(a)

(c)

(b)

Fiber pull out

Matrix cracking

Fiber pull out

Matrix cracking

Fiber fracture

Epoxy matrix

Figure 4. Scanning electron micrographs of the tensile-fractured surface of the oil palm fiber-reinforced

composite. The clean surface of oil palm fibers indicates weak adhesion between the fibers and matrix:

(a) magnification 50and (b) magnification 500; (c) Scanning electron micrograph of the tensile-fractured

surface of the glass fiber-reinforced composite. Epoxy matrix adhered on the surface of glass fibers indicates a

good adhesion between the fibers and matrix (magnification 500); (d) Scanning electron micrograph of the

tensile-fractured surface of the hybrid composite at 90 wt.% loading of glass fibers. The fracture surface of an

oil palm fiber layer (magnification 500); (e) Scanning electron micrograph of the tensile-fractured surface of

the hybrid composite at 90 wt.% loading of glass fibers. The fracture surface of a glass fiber layer(magnification 500).




10/23

263% and exhibited a tensile strength of 87 MPa, which is comparable to the tensile

strength of the glass fiber-reinforced composite.

In a hybrid composite, the mechanical properties are governed by the fiber content,

fiber length, fiber orientation, arrangement of individual fibers, extent of intermingling of

the fibers, and the interfacial adhesion between the fiber and matrix [21,29]. The tensile

failure of a hybrid composite though, is mainly dependent on the breaking strain and

modulus of the individual reinforcing fibers [15,21,33]. Glass fibers are low elongationfibers with a high modulus whereas oil palm fibers are high elongation fibers with a low

Matrix cracking

Fiber pull-out

Matrix cracking

Fiber fracture

(d)

(e)

Figure 4. Continued.




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modulus (Table 1). When the oil palm fiberglass fiber hybrid composite is subjected to a

tensile load, the glass fibers and the oil palm fibers are uniformly strained and a strain level

is reached corresponding to the failure strain of the glass fibers (smallest failure strain).A further increase in the strain level results in an early failure of glass fiber plies.

0

20

40

60

80

100

120

140

0 0.2 0.4 0.6 0.8 1

Relative volume fraction (Vf) of glass fiber

Tensilestrength(MPa)

Rule of mixture

Figure 5. The effect of glass fiber loading on the tensile strength of the oil palm fiberglass fiber hybrid bilayer

laminate composite.

Figure 6. The scanning electron micrograph of the cross section of the oil palm fiber composite. Arrows

indicate irregular size and shape of the oil palm fibers (magnification 101).




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Sreekala et al. [21] and Mishra et al. [15] who worked on oil palm fiberglass fiber

hybrid and sisalglass fiber hybrid composites respectively, cited that once the glass fibers

fail, the sudden transfer of load to the weak natural fibers would result in the failure of the

natural fibers eventually leading to a catastrophic failure of the hybrid composites.

Nevertheless, in this study which is a hybrid bilayer system, the load from the failed

glass fiber plies is not directly transferred to the oil palm fibers. The failed glass fiber plies

though, are able to continue to carry the load in the laminate and are also capable of

undergoing multiple failures throughout the loading process. This may be due to the

presence of the strong interlaminar bond, which enables the adjoining oil palm fiber ply to

restrain and localize the failure of the glass fiber plies [34]. As the failed glass fiber plies are

still able to carry the load, the oil palm fibers can effectively transfer the load from the

glass fibers without failing catastrophically. As the volume fraction of glass fiber increases

in the hybrid composites, the number of glass fiber plies also increases (Table 4), thus they

are able to withstand a higher load while redistributing a lesser load to the oil palm fibers

resulting in an improved tensile strength of the hybrid composites with the addition of

glass fibers. The increase in the tensile strength of the hybrid composites is also due to the

higher tensile strength of glass fiber than the oil palm fiber (Table 3) [32]. Therefore, it is

well understood that the hybridization of the oil palm fiber composites with glass fibers

enhances the tensile strength of the hybrid composite.

Figure 5 also shows a negative hybrid effect exhibited by the oil palm fiberglass fiber

hybrid bilayer laminate composites. Marom et al. [35] defined a positive or negative hybrideffect as a positive or negative deviation from the rule of mixture behavior. The condition

Oil palm fiber

bundle

Individual fibers

Figure 7. Scanning electron micrograph of the cross section of the oil palm fiber bundle in epoxy matrix

(magnification 201).

Table 4. Relationship between the relative volume fraction of glass fibers and the numberof glass fiber plies in the bilayer hybrid laminate composite.

Relative volume fraction of glass fibers 0 0.1 0.2 0.4 0.5 0.8 1

Number of plies of glass fibers 0 2 3 5 7 9 10




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for positive or negative hybrid effect is mainly influenced by the relative volume fractions

of the individual fibers, the construction of the layers in the hybrid, and the loading

configuration (e.g., translaminar or interlaminar). The rule of mixture predictions is based

upon the weighted average of the characteristic properties of the individual composites[35,36]. Wagner et al. [36] later stressed that the weighting is proportional to the volume

fraction of the constituents without taking into consideration the internal geometry of the

composite. Furthermore, the rule of mixture predictions as explained by Sreekala et al. [21]

expects a complete intermingling of both the individual fibers in the matrix. Therefore, the

negative deviation from the rule of mixture predictions shown by the bilayer hybrid

laminate composite may be due to the presence of distinct and segregated layers of the oil

palm fibers and glass fibers. This is evident from the cross section of the hybrid composite

shown in Figure 8.

Elongation at Break

Figure 9 shows the effect of elongation at break with the addition of glass fibers in oil

palmglass fiber hybrid bilayer laminate composite. Referring to the graph in the figure, it

is noted that the elongation at break of the oil palm fiber composite is slightly lower than

the glass fiber-reinforced composite.

As the oil palm fibers are high elongation fibers compared to the low elongation glass

fibers, one would expect the oil palm fiber composite to have a higher elongation at break

than the glass fiber composite. However, owing to the low strength nature of the oil palm

fibers and its inability to withstand the load transferred from the epoxy matrix (as

explained in the previous section), the oil palm fiber composite fails catastrophically evenbefore reaching its actual extensible strain.

Glass fibers

Oil palm fibers

Figure 8. Scanning electron micrograph of the cross section of the hybrid composite at 0.8 volume fraction of

glass fibers. The distinct layering of the glass fibers and oil palm fibers can be observed in the cross section of

the hybrid composite (magnification 17).




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As observed from the graph, the hybrid composites exhibited a positive elongation at

break effect with the addition of glass fibers. The positive hybrid effect of the elongation at

break in hybrid composites was also observed by Hayashi [37], Bunsell and Harris [38],Zweben [33], Sreekala et al. [21], and Krestsis [39]. Zweben [33] concluded that in a hybrid

composite, the addition of high elongation fibers with low elongation fibers often

increased the elongation at break of the hybrid composite than the composite made from

low elongation fibers.

On the other hand, in this bilayer hybrid laminate system, with the addition of glass

fibers the elongation at break of the hybrid composites increased beyond the elongation at

break of the individual composites. This exceptional behavior of the hybrid composites is

due to the existence of a load sharing mechanism between the glass fiber plies and the oil

palm fiber ply. This is because the failed glass fiber plies are able to continue to carry the

load while redistributing the remaining load to the oil palm fiber ply. As the number of

glass fiber plies increases with the increase in the volume fraction of glass fibers, they are

able to withstand a higher applied load while redistributing a lesser load to oil palm fibers.

Thus the oil palm fibers do not fail catastrophically and are able to reach its actual

extensible strain successfully with the addition of glass fibers. As a result, the oil palm

fibers are able to restrain the crack propagation upon fracture of the glass fibers leading to

an increase in the strain level required to propagate the fiber breakage. Zweben [33],

Sreekala et al. [21], Jones and Di Benedetto [40], and Peijs et al. [41], concluded that in a

hybrid fiber composite, fibers with a low modulus and high elongation are able to stop and

deflect the crack at a micromechanical level. Furthermore, the fiber cell arrangement in a

cellulosic fiber is able to divert the crack route by blunting it. This is because when the

crack approaches the fiber cells, it surrounds the cells unable to propagate, and it finallystops [42]. Therefore, the existence of a synergistic effect between the glass fibers and oil

0

2

4

6

8

10

0 0.2 0.4 0.6 0.8 1

Relative volume fraction(Vf) of glass fiber

Elongationatbreak(%

)

Rule of mixture

Figure 9. The effect of glass fiber loading on the elongation at break of the oil palm fiberglass fiber hybrid

bilayer laminate composite.




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palm fibers and the load sharing mechanism between the glass fiber plies and oil palm fiber

ply enhance the elongation at break of the hybrid bilayer composites.

Youngs Modulus

The incorporation of glass fibers into the oil palm fiber composite increased the stiffness

of the hybrid composites as seen in Figure 10. This behavior agrees well with the work

done by Pavithran et al. [20] and Kalaprasad et al. [16,32] who worked on coirglass fiber

hybrid and sisalglass hybrid composites, respectively. Owing to the weak interfacial

adhesion between the oil palm fibers and epoxy matrix and the weak nature of the oil palm

fibers, the oil palm fiber composite is unable to withstand the applied load transferred

from the epoxy matrix resulting in an inferior stiffness property of oil palm fiber

composite compared to the glass fiber composite [43].

The enhancement in the stiffness of the hybrid composites with the addition of glass

fibers is attributed to the higher tensile modulus of glass fibers which is about 6672 GPathan that of oil palm fiber which has a tensile modulus of about 19 GPa. Furthermore,

the addition of glass fibers into the oil palm fiber composites increases the load bearing

capability of the hybrid composites resulting in an improved stiffness. This is due to the

efficient stress transfer between the glass fiber plies and oil palm fiber ply, which enables

the hybrid composites to carry a higher tensile load [18,44].

The hybrid composite exhibited a large deviation from the rule of mixture behavior as

observed from the graph in Figure 10. This may be due to the distinct layering of the oil

palm fibers and glass fibers in the epoxy matrix. The effect of layering on the negative

hybrid effect of the Youngs modulus was also observed by Sreekala et al. [21] in oil palm

0.0

0.2 0.4 0.6 0.8 10

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


Young

'smodulus(GPa)

Rule of mixture

Figure 10. The effect of glass fiber loading on the stiffness of the oil palm fiberglass fiber hybrid bilayercomposite.




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fibersglass fiber-reinforced phenolformaldehyde hybrid composites. As the fibers are

not homogenously dispersed in the matrix, the fibers are unable to effectively restrict

the mobility and deformability of the matrix resulting in a negative hybrid effect of the

Youngs modulus. Furthermore, the Youngs modulus of the hybrid composites would

follow the rule of mixture if the fibers act in the same direction with the applied stress and

also if the interphase of the fibermatrix is perfect.

Impact Properties

Figure 11 shows the effect of glass fiber loading on the impact strength of the hybrid

composite. Based on the graph, it is noted that the oil palm fiber composite has a lower

impact strength (18 kJ/m2) than the glass fiber composite (107 kJ/m2).

According to Joseph et al. [8], the impact properties of a fiber-reinforced composite is

influenced by the nature of the constituent materials, interface properties, construction

and geometry of the composite, and also the test conditions. The mode of fracture of theoil palm fiber composite and the glass fiber composite is shown in Figures 12 and 13. As

the interfacial bonding between the oil palm fibers and the epoxy matrix is weak, fiber

pullout would be expected in the composite [10]. However, scanning electron micrograph

observation of the fractured surface of the oil palm fiber composite showed fiber fracture

as the predominant failure mechanism (Figure 14). As the oil palm fiber composite is

subjected to a high speed impact load, the sudden stress transferred from the matrix to the

fiber exceeds the fiber strength resulting in the fracture of the oil palm fibers at the crack

plane without any fiber pullout. Owing to its low strength nature, irregular cross section,

and the presence of fiber bundles, the oil palm fibers are unable to withstand the high

0

20

40

60

80

100

120

140

0 0.1 0.2 0.4 0.5 0.8 1


Impacts

trength(KJ/m2)

Oil palm fiber layer

Glass fiber layer

Rule of mixture

Figure 11. The effect of glass fiber loading on the impact strength of the oil palm fiberglass fiber hybridbilayer composite.




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transverse load and therefore fractures before reaching its fracture strain. Furthermore, as

the oil palm fibers used are long, they would have flaws distributed along its length. The

flaws would act as stress concentrators resulting in the fracture of the fibers when the load

is transferred onto the fibers [45].

The glass fiber-reinforced composite exhibited extensive delamination between the glass

fiber plies as shown in Figure 13. Glass fibers are capable of absorbing high impact energy

and are also resistant to propagation of microcracks [19,27]. Therefore, as the crack

propagates through a ply in a laminate and reaches the adjacent ply it gets arrested andbranches off and propagates at the fibermatrix interface parallel to the plane of the plies.

Impact load direction

Figure 12. Photomicrograph of the impact fracture sample of the oil palm fiber composite.

Impact load direction

Delamination

Figure 13. Photomicrograph of the impact fracture sample of the glass fiber composite.




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The crack branching produces a large surface area resulting in an increase in the fracture

energy [34].

Agarwal and Broutman [34] also cited that fiber breakages only account for a small

portion of the total energy absorbed in a composite. Park and Jang [46] later stressed that

compared to fiber fracture, delamination failure significantly increases the impact energy

absorption characteristic of the composite. As a result, the glass fiber composite exhibiteda higher impact strength than the oil palm fiber composite.

The hybridization of the glass fibers with the oil palm fiber composite increased the

impact strength of the bilayer hybrid composites significantly as shown in the graph in

Figure 11. At 0.1 volume fraction of glass fibers, the impact strength of the bilayer hybrid

composite increased about 24% when impacted at the oil palm fiber layer while an

increase of 110% was noted when the hybrid was impacted at the glass fiber layer. This is

mainly due to the superior damage tolerance capability and efficient crack resisting

characteristics of the glass fibers compared to the oil palm fibers. Furthermore, with the

increasing number of glass fiber plies as the volume fraction of glass fiber increases in

the hybrid composites, additional impact absorption energy occurs between the glass

fiber plies through extensive delamination, hence increasing the impact resistance of

the hybrid composites. Figure 15(a)(d) shows the impact fractured samples of the hybrid

composites.

The bilayer hybrid composites, which were impacted at the glass fiber layer, exhibited a

positive hybrid effect and also a higher impact strength than those impacted at the oil

palm fiber layer. The impact strength of the hybrid composite at 0.8 volume fraction of

glass fibers is comparable to the impact strength of the glass fiber composite.

Park and Jang [46], who worked on the impact performance of aramid fiber

polyethylene fiber hybrid composite, concluded that in a laminated fiber composite, the

position and volume ratio of each individual fiber in the hybrid composite determine

the impact strength of the composite. When the bilayer hybrid composite was impacted atthe glass fiber layer, the glass fibers were able to resist the high impact load and were also

Fiber fracture

Figure 14. Scanning electron micrograph of the impact-fractured surface of the oil palm fiber composite





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able to absorb a significant amount of impact energy through extensive delamination

between the glass fiber plies as shown in Figure 15(a) and (c). Thus the energy needed to

initiate and propagate the crack increases. Moreover, the delamination at the glass fiber

oil palm fiber layer interface further contributes to the additional impact energy

absorption characteristic of the hybrid composite [46,47]. The absorbed impact energy is

then dissipated to the overall laminate through the fiber breakages in the oil palm fiber ply

(Figure 16(a) and (b)).

However, when the bilayer hybrid composite was impacted at the oil palm fiber layer,

the weak oil palm fibers were unable to withstand and absorb the high impact load

resulting in fiber breakages as shown in Figure 15(b) and (d). The initiated crack

then easily propagates from the impacted surface to the back surface of the hybrid

composite. A further propagation of the crack is restricted by the glass fiber plies. Theglass fiber plies at the back surface of the laminate are unable to delaminate completely

(a) Impacted at glass fiber layer (b) Impacted at oil palm fiber layer

(c) Impacted at glass fiber layer (d) Impacted at oil palm fiber layer

Glass fiber

Oil palm fiberGlass fiber

Oil palm fiber

Delamination

Glass fiber

Oil palm fiber

Glass fiber Oil palm fiber

Restricted delamination

Figure 15. Photomicrographs of impact-fractured samples of hybrid bilayer composites: (a) and (b) 0.1

volume fraction of glass fibers; (c) and (d) 0.8 volume fraction of glass fibers. Dotted arrow indicates the

impact load direction.




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(as clearly seen in Figure 15(d)) owing to the restraint of the adjacent oil palm fiber ply.

Hence, the applied impact load is not dispersed effectively into the overall laminate and

the stresses are localized in the laminate leading to a low impact strength of the hybrid

composite [48].

CONCLUSIONS

From this research it can be concluded that :

. Hybridization of oil palm fibers with glass fibers had improved the tensile and impactproperties of the oil palm fiber composite.

(a)

(b)

Fiber fracture

Fiber fracture

Figure 16. Scanning electron micrographs of the impact fracture surface of the oil palm fiber layer when

impacted at the glass fiber layer. (a) 0.1 volume fraction of glass fiber and (b) 0.8 volume fraction of glass fiber





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. Addition of a high volume fraction of glass fibers of about 0.8 enhanced the tensile and

impact properties of the hybrid bilayer laminate composites.

. A negative hybrid effect was observed for the tensile strength and tensile modulus of the

hybrid composite. However, a positive hybrid effect was observed for the elongation at

break of the hybrid composite.

. Glass fibers when placed at the front surface (impacted surface) of the hybrid bilayer

laminate composites offered a better impact resistance compared to oil palm fibers at

the front surface. A positive hybrid effect was also observed when the hybrid

composites were impacted at the glass fiber surface.

The oil palm fiberglass fiber hybrid bilayer composite offered encouraging and

comparable mechanical properties when compared to the glass fiber-reinforced compo-

sites. Therefore, through hybridization, the oil palm fiber composite may find applications

as structural materials where higher strength and cost considerations are important

factors.

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