Mech Perfor of Short Banana&Sisel Hybrid Fiber Reinforced Polyster Composites-12.Full

7/28/2019 Mech Perfor of Short Banana&Sisel Hybrid Fiber Reinforced Polyster Composites-12.Full

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Mechanical Performance of Short Banana/SisalHybrid Fiber Reinforced Polyester Composites

M ARIES IDICULADepartment of Chemistry, Mar Thoma College, Tiruvalla, 689 103

Kerala, India

K URUVILLA JOSEPHDepartment of Chemistry, SB College, Changanacherry, Kerala, India

SABU T HOMAS *School of Chemical Sciences, Mahatma Gandhi University,Priyadarshini Hills P.O., Kottayam, 686 560 Kerala, India

ABSTRACT: Short randomly oriented banana and sisal hybrid fiber reinforced polyestercomposites, banana/polyester composites and sisal/polyester composites were fabricated at differentfiber loading say, 0.20 to 0.50 V f . Composites were prepared by varying the relative volume fractionof the two fibers at each fiber loading. When the fiber loading was increased; tensile, flexural, andimpact properties increased. Better performance was shown by composites having volume fraction,0.40 V f . Tensile strength, tensile modulus, flexural strength, and flexural modulus showed a positivehybrid effect when the volume ratio of the fiber was varied in the hybrid composites at eachfiber loading. Maximum tensile strength was observed in composites having volume ratio of banana and sisal 3 : 1. When the volume ratio of sisal was increased, the impact strength of thecomposite increased. Different layering patterns were tried at 0.40 V f , keeping the volume ratio of fibers 1 : 1. Tensile properties were slightly greater in the trilayer composite with banana as theskin material. Bilayer composites showed higher flexural and impact property. SEM studies werecarried out to evaluate fiber/matrix interactions. Experimental results were compared withtheoretical predictions.

KEY WORDS: banana, sisal, hybrid, composite.

INTRODUCTION

N ATURAL FIBERS ARE environment friendly, low priced, and sustainable naturalresources that have been in considerable demand in recent years. They havealready established a track record as simple filler material in automobile parts. A lotof research work is taking place in this field [15]. Herrera and Gonzalez [6]studied the mechanical properties of henequen fiber reinforced HDPE composites. Joshiet al. [7] reviewed that natural fiber composites are superior to glass fiber composites.

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

Journal of R EINFORCED PLASTICS AND COMPOSITES , Vol. 29, No. 1/2010 12

0731-6844/10/01 001218 $10.00/0 DOI: 10.1177/0731684408095033 SAGE Publications 2010

Los Angeles, London, New Delhi and Singapore

by T Raj on May 21, 2013 jrp.sagepub.comDownloaded from
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Recently, many studies are going on in this field [811]. A range of propertiescan be obtained by combining two or more different types of fibers in a commonmatrix. The effect of hybridization of natural fibers with glass fibers has been studiedextensively [12]. By careful selection of reinforcing fibers, the material costs can besubstantially reduced.

Nowadays, it is observed that by hybridizing two natural fibers, high performancecomposites can be produced. Paiva Junior et al. [13] used plain weave hybrid ramie-cottonfabrics as reinforcement in polyester matrix and showed the high potential of ramie fiberand weak contribution of cotton fiber as reinforcement in lignocellulosic fiber composites.Jacob and coworkers [14,15] studied the mechanical properties and cure characteristics of sisal oil palm hybrid fiber reinforced natural rubber composites.

In this study, banana fibers and sisal fibers were selected to hybridize and reinforce apolyester matrix to develop high performance and cost effective composites. The physicalproperties of natural fibers are mainly determined by chemical and physical compositions,such as structure of fibers, cellulose content, lumen size, microfibrillar angle, and degree of

polymerization. When compared to other natural fibers, sisal and banana have goodmechanical properties. The properties of the fibers are given in Table 1.

The microfibrillar angle and lumen size of sisal fiber is higher than of banana fiber.Hence sisal fiber reinforced composites show comparatively high impact strength.Pavithran et al. [16] reported the impact strength of unidirectionally aligned sisal fiber/polyester composites. In general, the strength of a fiber increases with increasing cellulosecontent and decreasing spiral angle with respect to the fiber axis. The cellulose content of banana and sisal fiber is almost same, but the spiral angle of banana (11 8) is much lowerthan sisal (20 8). Hence the inherent tensile properties of banana fiber will be higherthan sisal. The diameter of banana fiber is lower than that of sisal [17]. Hence the

surface area of banana fibers in unit area of the composite will be higher and hencethe stress transfer is increased in banana reinforced composite compared to that of sisalreinforced composite.

This study establishes the mechanical performance of banana/sisal reinforced polyestercomposites. Tensile properties of both fibers were determined. Tensile properties of thecomposites as a function of fiber loading and fiber composition and layering patterns wereanalyzed. Three point flexure properties of composites were also investigated. The impactstrength as a function of fiber loading, fiber composition and layering patternswere investigated. Tensile and impact fracture mechanism was studied by scanningelectron microscopy. Hybrid effect was also calculated.

Table 1. Properties of banana and sisal fiber.

Properties Banana fiber Sisal fiber

Cellulose % 6364 65Hemicellulose % 19 12Lignin % 5 9.9Moisture content % 1011 10Density (kg/m 3 ) 1350 1450Flexural modulus (GPa) 25 12.517.5Microfibrillar angle 11 8 20 8Lumen size ( mm) 5 11

Short Banana/Sisal Hybrid Fiber Reinforced Polyester Composites 13



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EXPERIMENTAL

Materials

Isopthalic polyester resin HSR 8131 obtained from M/s. Bakelite Hylam Ltd.,Hyderabad, India was used in this study. Banana and sisal fibers were obtained fromSheeba Fibers and Handicrafts, Poovancode, Tamilnadu, India. The curing agents methylethyl ketone peroxide and catalyst cobalt naphthenate used were of commercial grade andwere obtained from M/s. Sharon Enterprises, Cochin, India. Important characteristics of polyester resin are given in Table 2.

Preparation of Composites

Banana and sisal fibers were cut into 30-mm length. A hand lay-up method followed bycompression molding was adopted for composite fabrication. The curing of polyester was

done by the incorporation of 1 vol% methyl ethyl ketone peroxide. A 1% (volume percent)cobalt naphthenate was added as catalyst. Keeping the volume ratio of banana and sisal3 : 1, 1 : 1, and 1 : 3, short randomly oriented intimately mixed hybrid composites wereprepared at different fiber loading say, 0.200.50 V f . Unhybridized composites were alsoprepared at these fiber loadings. Keeping the volume ratio of banana and sisal 1 : 1 andtotal fiber loading 0.40 V f , different layering patterns such as trilayer (banana/sisal/bananaand sisal/banana/sisal), bilayer (banana/sisal) composites were prepared. Mats of choppedfibers were prepared and were impregnated in polyester resin in a mold havingdimension 150 150 3 mm. Curing was done at room temperature for 24 h under aconstant pressure of 1 MPa. Different layering patterns are schematically represented in

Figure 1(ad). In Figure 1(a), banana is the skin material and sisal is the core material andit is in the reverse order in Figure 1(b). Figure 1(c) is the bilayer composite and Figure 1(d)is the intimate mix composite.

Mechanical Testing

Mechanical testing of single banana and sisal fibers was carried out in a FIE universaltesting machine with a gauge length of 30 mm and cross head speed of 1 mm/min. Forstatistical reasons 30 samples were tested and average values of mechanical properties werecalculated. To determine the stress on the fibers, the load was converted to this parameterby measuring the fiber diameters using Leica (DMLP) polarizing light microscope.

Table 2. Typical properties of liquid resin.

Appearance A clear pale yellow liquid

Viscosity at 25 8C (cps) Brookfield viscometer 650Specific gravity at 25 8C 1.11

Typical properties of cured unreinforced resin(specimens cured for 24 h at room temperature followed by post curing for 24 h at 80 8C

Tensile strength 33 MPaFlexural strength 70 MPaImpact strength 9 kJ/m 2

14 M. I DICULA ET AL .



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By assuming that the fibers have a near circular cross-section, the diameters wereconverted to cross-sectional area, and hence stress could be determined by dividing theload values by this constant.

Tes t specimens were cut f rom the composi te sheets having d imensions(100 15 3) mm. Tensile testing was carried out using FIE electronic universal testingmachine TNE 500 according to ASTM D 638-76. Three point flexure properties werealso tested using the same machine according to ASTM D -790. Izod impact test onunnotched specimen was determined using a pendulum impact testing machine. A 25 J

pendulum was used.

RESULTS AND DISCUSSION

Intrinsic Properties of Fibers

Tensile properties and diameters of banana and sisal fibers are given in Table 3.Tensile strength and modulus of banana fiber is higher than sisal fiber. Elongation atbreak of sisal fiber is slightly greater than banana. The diameter of banana fiber is lessthan sisal fiber.

Tensile Properties

EFFECT OF FIBER LOADING ON TENSILE PROPERTIES Figure 2 represents the stressstrain behavior of short randomly oriented intimately

mixed banana/sisal hybrid fiber reinforced polyester composites having volume ratio of banana and sisal 1 : 1 at different fiber loading. The behavior of neat polyester resin (gum)is also presented in the figure. The brittle nature of polyester is clear from the curve. Thestressstrain curves of the composites show a linear behavior at low strains followed by asignificant change in slope showing a non-linear behavior, which is maintained up to thecomplete failure of the composite. Fibers and matrix behave linearly at low strains. The

second stage of the curve leading to decrease in slope corresponds to the plasticdeformation of matrix and to micro-crack initiation in the matrix. Randomly oriented

Banana

(a) (b)

(c) (d)

Banana

SisalBanana

Sisal

Sisal

Intimate mixBanana

Sisal

Figure 1. Schematic representation of different layering patterns of hybrid composites: (a) banana/sisal/ banana, (b) sisal/banana/sisal, (c) bilayer, and (d) intimate mix.




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fibers inhibit the crack propagation and gradual debonding of the fibers from the matrixoccurs during plastic deformation. Unstable propagation of the initiated cracks throughthe matrix occurs and the strength decreases abruptly to an almost zero value. As the fiberloading increases the slope in the curves occurs at higher stress values. The slope of composite having 0.40 V f is higher, which is obvious from the figure.

Figure 3 represents the effect of fiber loading on tensile strength of banana/polyestercomposite ( B), sisal/polyester composite ( S ) and hybrid composites having volume ratio of banana and sisal 3 : 1, 1 : 1, and 1 : 3, respectively. In all cases it can be observed that tensilestrength increases with fiber loading. Prominent increase is observed up to 0.40 V f . . Whenthe volume fraction increases from 0.20 to 0.30 V f , the tensile strength increases to 18, 32,32, 33, and 20% in the composites B, B : S 3:1 , B : S 1:1 , B : S 1:3 and S ,respectively. While the volume fraction increases from 0.20 to 0.40 V f , the correspondingincrease in tensile strength is 43, 44, 48, 53, and 51%, respectively. After that, tensilestrength slightly increases only in the hybrid composite with B : S 3 : 1. At high fiberloading, fiber agglomeration results, which decrease the stress transfer between the fiber

and the matrix. This shows that 0.40 V f is the maximum allowable fiber content at whichmaximum stress transfer occurs from fiber to matrix.

Table 3. Tensile properties and diameter of banana and sisal fibers.

Fiber Diameter ( k m) Tensile strength (MPa) Tensile modulus (GPa) Elongation at break (%)

Banana 120 5.8 550 6.8 22 34Sisal 205 4.3 350 7 20 67

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S t r e s s

( M P a

)

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20

15

10

5

00 1 2 3 4

Strain (%)5 6 7 8

Gum0.20 V f 0.30 V f 0.40 V f 0.50 V f

Figure 2. Tensile stressstrain curve of banana/sisal/polyester composites having different fiber loading, keeping the ratio of banana and sisal at 1 : 1.




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At all fiber loading, tensile strength of sisal/polyester composite is lower compared tobanana/polyester composite and hybrid composites. Composites with a volume ratio of banana and sisal 3 : 1 show higher tensile strength at all fiber loading. The inherent tensileproperties of banana fiber are higher than sisal (Table 3), meaning the reinforcing effect of banana is greater in polyester than sisal. The diameter of banana fiber is less than sisal aswell. Therefore the surface area of the fiber in unit area of the composite is higher inbanana/polyester composite than that of sisal/polyester composite, giving that physicalinteraction as well as stress-transfer in unit area is higher in the case of banana filledcomposites. As the volume fraction of banana is increased, the tensile strength of thecomposite increased and a synergism is occurred in the hybrid composites.

The tensile modulus of neat polyester is 950 MPa and elongation at break is 2.8%. Thetensile modulus and elongation at break of the composites are given in Tables 4 and 5,respectively. As fiber loading increases tensile modulus also increases. The hybridcomposites also show a synergism in tensile modulus. At 0.40 V f , the hybrid compositehaving a volume ratio of banana and sisal of 1 : 1 shows the maximum tensile modulus.The elongation at break of the composites increases with fiber loading. The addition of

cellulosic fibers makes the matrix ductile. The elongation at break shows minimum valuein the hybrid composites having a volume ratio of banana and sisal 1 : 1.

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T e n s i

l e s t r e n g

t h ( M P a

)

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30

0.0 0.1 0.2 0.3

Volume fraction of fiber

0.4 0.5 0.6

B

S

B :S =3 :1B :S =1:1B :S =1:3

Figure 3. Effect of fiber loading on tensile strength of unhybridized composites and hybrid composites havingdifferent volume ratios of fibers.

Table 4. Tensile modulus in MPa of the composites having different fiber loading and fiber ratio.

Volume fraction(V f ) 100% banana

Banana:Sisal 3 : 1

Banana:Sisal 1 : 1

Banana :Sisal 1: 3 100% Sisal

0.20 1010 1090 1347 1253 1069

0.30 1312 1469 1443 1599 11850.40 1352 1536 1615 1477 10790.50 1412 1647 1545 1540 1110




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EFFECT OF VARYING THE VOLUME RATIO OF THE FIBERS ON TENSILE PROPERTIES

The tensile stressstrain curve of hybrid composites having different volume ratios of the fibers and unhybrized composites at 0.40 V f can be seen in Figure 4. S represents100% sisal fiber reinforced composite while B represents that of banana reinforcedcomposite. The slope of B is higher than S and the slope of hybrid composite havingB : S 3 : 1 is the highest. Since the microfibrillar angle of banana fiber is less than that of sisal, the reinforcing ability of banana is greater compared to sisal in polymeric matrix.The high strength of banana/polyester composite is explained above. The effect of varyingthe relative volume fraction of fibers on the tensile strength at different fiber loading of

composites is depicted in Figure 5. Banana filled composite shows higher tensile strengthcompared to sisal at all fiber loading. As the volume fraction of banana is increased

Table 5. Elongation at break (%) of the composites having different fiber loading and fiber ratio.

Volume fraction(V f ) 100% B

Banana : Sisal 3 : 1

Banana: Sisal 1 : 1

Banana : Sisal 1 :3 100% S

0.20 4 4 3 5 60.30 5 6 5 6 70.40 6 7 6 7 70.50 7 7 7. 9 9

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50

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S t r e s s

( M P a

)

20

10

00 1 2 3 4

Strain (%)5 6 7 8 9

S B

B :S =3:1B :S =1:1B :S =1:3

Figure 4. Tensile stressstrain behavior of banana/sisal/polyester composites on varying the relativevolume fraction of the two fibers and that of banana/polyester and sisal/polyester composites, keeping thetotal V f 0.40.




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in the composite, the tensile strength increased. When banana is 67 vol% in the hybridcomposites, the increase in tensile strength compared to sisal/polyester composites is 17,28, 12, and 17% at 0.20, 0.30, 0.40, and 0.50 V f , respectively. Since the elongation at breakof sisal fiber is greater than that of banana, the highest stress carried out by banana fiber istransferred to sisal without failure of the matrix. This results in synergistic strengthening of

the fibers. Dispersion of fibers will be higher in a hybrid composite compared to that of unhybridized composite [18]. It was reported earlier that the criterion for optimumadhesion between matrix and reinforcing fibers is based on maximizing the wetting tension[19]. It is shown that the maximum wetting tension criterion best fulfils two importantrequirements for a strong interface. First, the physical interactions at the molecular levelbetween the resin and the fibers must be maximized, and second, the liquid resin mustspontaneously wet the fiber surface in order to minimize the flow density at the interface.As the dispersion increases, wetting tension as well as physical adhesion between the fiberand matrix increases. The higher interaction in the case of banana-filled composites can beexplained. The surface area of the fiber in unit area of the composite is higher in banana/polyester composite than that of sisal/polyester composite because the diameter of bananafiber is less than that of sisal fiber. Hence physical interaction as well as stress-transferin unit area is higher in the case of banana-filled composites.

Tensile modulus and elongation at break of the above composites can be seen inTables 4 and 5 respectively. Modulus of hybrid composites is higher than single systems.It is an indication of positive hybrid effect. Due to the higher inherent elongation at breakof sisal fibers compared to banana, the elongation at break of sisal fiber composite ishigher compared to banana at all fiber loading. Elongation at break is found to be lowerin composites with volume ratio of banana and sisal 1 : 1.

EFFECT OF LAYERING PATTERN ON TENSILE PROPERTIES

The tensile stressstrain curve of hybrid composites of different layering patternshaving total volume fraction 0.40 V f and volume ratio of banana and sisal 1 : 1 is depicted

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l e s t r e n g

t h ( M

P a

)

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0 20 40Volume percentage of banana fiber

60 80 100

0.20 V f 0.30 V f 0.40 V f 0.50 V f

Figure 5. Effect of varying the relative volume fraction of banana and sisal on the tensile strength of banana/ sisal hybrid fiber reinforced polyester composites at different fiber loading.




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in Figure 6. B/S /B represents the stress-strain curve of the composite in which banana isthe skin material and S /B/S is that in which sisal is the skin material. In S /B/S , the plasticdeformation occurs at a lower stress level while in B/S /B, plastic deformation occurs at ahigher stress level. The tensile strength, tensile modulus, and elongation at break of theabove composites are given in Table 6. The tensile strength was observed to be higherwhen banana was used as the skin material and sisal as core material. The tensile strengthwill be higher when the high strength material is used as the skin, which is the main load-bearing component in tensile measurements. The tensile strength of the intimate mixcomposite is slightly lower than that of the composite having banana as the skin materialand sisal as the core. Better stress transfer occurs in intimately mixed composites. In S /B/S ,the value is slightly lower because the low strength sisal fiber is used as the skin material.

In bilayer, the tensile strength is again the same. The tensile modulus is found to be highestfor the intimate mix composite, slightly lower in B/S /B and lowest for S /B/S .

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50

40

S t r e s s

( M P a

)

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20

10

00 1 2 3

Strain (%)4 5 6 7

BilayerS / B / S B / S / B Intimate

Figure 6. Tensile stressstrain behavior of composites having different layering patternvolume ratio of banana and sisal at 1 : 1 at a total fiber loading of 0.40 V f .

Table 6. Mechanical properties of composites having differentlayering patterns of fibers (total V f 0.40, banana : sisal 1:1).

Properties Bilayer S/B/S B/S/B Intimate mix

Tensile strength (MPa) 54 54 58 57Tensile modulus (MPa) 1302 1290 1476 1615Elongation at break (%) 6 6.2 5.4 6.5Flexural strength (MPa) 65 61 63 62Flexural modulus (MPa) 2991 2846 2916 2842Impact strength (kJ/m 2 ) 43 37 36 36




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Flexural Properties

EFFECT OF FIBER LOADING ON FLEXURAL PROPERTIES By the application of flexural force, the upper and lower surface of the specimen under

three-point bending load is subjected to compression and tension and axi-symmetric planeis subjected to shear stress. So there are two failure modes in the materials; bending andshear failure. The specimen fails when bending or shear stress reaches the correspondingcritical value. The modes of failure of the composites under three-point bending can beobtained from the force deflection curves [20]. Figure 7 is the flexural stressstrain graphof the intimately mixed hybrid composites having a ratio of banana and sisal 1 : 1 atdifferent fiber loading. As loading increases, the plastic deformation occurs at higherflexural stress in composites having volume fraction up to 0.40 and then at a lower level.The stiffness of the composite shows its highest value at 0.40 V f , which is understood fromthe stressstrain curve. By increasing the fiber loading from 0.20 to 0.40 V f , the flexuralstrength is found to increase by 32%. The effect of flexural strength on fiber loading of thehybrid composites having volume ratio of banana and sisal 3 : 1, 1 : 1, 1 : 3, andunhybridized composites can be observed in Figure 8. As fiber loading increases, flexuralstrength also increases up to 0.40 V f and then decreases. Up to 0.40 V f , the fiber/matrixinteraction is improved and on further loading, the fiber-to-fiber contact increases andfiber agglomeration results which lead to a decrease in stress transfer between the matrix.The flexural modulus of the above composites can be observed in Table 7. As fiber loadingincreased, flexural modulus also increased. It was reported that the flexural strength of

short banana fiber reinforced polyester composites is lower than that of neat polyester [21].The same trend is also observed here.

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F l e x u r a

l s t r e s s

( M P a

)

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20

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00 1 2

Flexural strain (%)3 4 5

0.20 V f 0.30 V

f 0.40 V f 0.50 V f

Figure 7. Flexural stressstrain behavior of the hybrid composites at different fiber loading, keeping thevolume ratio of banana and sisal at 1 : 1.




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EFFECT OF VARYING THE RELATIVE VOLUME FRACTION OF THE FIBERS ON FLEXURAL PROPERTIES

Figure 9 shows the flexural stressstrain curve of the intimately mixed hybridcomposites having different volume ratios of fibers, banana/polyester composite,and sisal/polyester composite at 0.40 V f . The slope of the curve increases and theplastic deformation occur at higher stress level in hybrid composites compared tounhybridized ones. Maximum stress can be seen in composite with ratio of bananaand sisal of 1 : 1.

Figure 10 delineates the effect of varying the volume ratio of banana and sisalon flexural strength in the hybrid composites at different fiber loading. Flexural strengthof the hybrid composites is higher than unhybridized composites at each fiber loading.A positive hybrid effect is observed. At all fiber loading, flexural strength is maximumwhen the relative volume ratio of banana and sisal in the composite is 1 : 1. As explainedearlier, higher compatibility as well as dispersion in hybrid composites is achieved,which lead to a better stress transfer ability in composites. Flexural modulus was also

analyzed and can be observed in Table 7. A positive hybrid effect is also observed in theflexural modulus.

Table 7. Flexural modulus(MPa) of composites having different fiber loading and fiber ratio.

Volume fraction(V f ) 100% B B : S 3 : 1 B : S 1 : 1 B : S 1 : 3 100% S

0.20 2310 2515 2247 2236 22260.30 2350 2380 2376 2395 23230.40 2723 2981 2842 2661 27370.50 2825 2958 2950 2980 2882

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l s t r e n g

t h ( M P a

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0.2 0.3Volume fraction of fiber

0.4 0.5

B

S

B :S =3:1B :S =1:1B :S =1:3

Figure 8. Effect of fiber loading on flexural strength of banana/sisal/polyester composites at different volume ratios of fibers, banana/polyester composites, and sisal/polyester composites.




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EFFECT OF LAYERING PATTERN ON FLEXURAL STRENGTH OF THE COMPOSITES

The flexural stressstrain curve of trilayer composites such as banana/sisal/banana ( B/S /B),sisal/banana/sisal ( S /B/S ), bilayer composite (banana/sisal), and an intimate mix

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l s t r e s s

( M P a

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Flexural strain (%)3 4 5

S B B :S =3:1B :S =1:1B

:S

=1:3

Figure 9. Flexural stressstrain behavior of banana/sisal/polyester composites on varying the relative volumefraction of the two fibers and that of unhybridized composites at a total volume fraction of 0.40.

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l s t r e n g

t h ( M P a

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60 80 100

0.20 V f 0.30 V f 0.40 V f 0.50 V f

Figure 10. Effect of varying the relative volume fraction of banana and sisal on flexural strength banana/sisal/ polyester composites at different fiber loading.




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of neat polyester is only 9 kJ/m 2 . By the incorporation of 0.20 V f of sisal fiber the impactstrength increased to 93% and for 0.40 V f of sisal it increased to 375%. By incorporating0.40 V f of banana fiber the impact strength increased to 305%. The high impact strengthof sisal fiber is due to the high porous nature of it due to the higher lumen size and highspiral angle. In the case of hybrid composites maximum strength is obtained where

B : S 1 : 3. i.e., 356% at 0.40 V f . The main disadvantage of thermoset moldings is highshrinkage during curing, high brittle behavior and surface cracking. But on addingcellulose fiber, these drawbacks are almost eliminated. At lower fiber loading, fibers arefound embedded in the matrix and hence fiber breakage and fiber pull out occurs on theapplication of a sudden force. The fiber crowding leads to easy debonding at high loading,which increases the impact resistance. Since cellulose fibers are more porous; when loadingincreases, the impact strength also increases.

EFFECT OF VARYING THE VOLUME RATIO OF FIBERS ON IMPACT STRENGTH

The effect of varying the relative volume fraction of fibers on the impact strength of thehybrid composites at different fiber loading is depicted in Figure 13. Impact strength of banana/polyester and sisal/polyester composites can also be seen. The high impactstrength of sisal/polyester composites can be explained. Natural fiber reinforced plasticswith fibers having a high microfibrillar angle indicated a higher composite fracture-toughness than those with small spiral angles. It was reported that composites with sisalfibers (spiral angle 20 8) show good impact properties [17]. The microfibrillar angle of banana fiber is 11 8, which has lower fracture toughness compared to sisal fiber. Again thelumen size of sisal fiber is greater than that of banana fiber (Table 1) which increases theporous nature of the fiber as well as the impact strength. It is found that on increasingthe relative volume fraction of sisal in the banana/sisal hybrid fiber reinforced polyester

composite, the impact strength increased. Upon hybridization negative hybrid effectis observed on impact properties in all fiber loading. Better compatibility of the fiber,

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t h ( k J / m

m 2 )

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0.0 0.1 0.2Volume fraction of fiber0.3 0.4 0.5

B

S

B :S =3:1B :S =1:1B :S =1:3

Figure 12. Effect of fiber loading on impact strength of banana/sisal/polyester composites at different volume ratio of fibers, banana/polyester composites, and sisal/polyester composites.




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as already mentioned, will decrease the impact strength due to the least possibility for fiberpull-out in the hybrid composites.

THE EFFECT OF A LAYERING PATTERN OF FIBERS IMPACT ON STRENGTH OF THE COMPOSITES

Impact resistance of a composite is the measure of total energy dissipated in the materialbefore final failure occurs. Composite fracture toughness is affected by interlaminar andinterfacial strength parameters. The interlaminar splitting and interfacial debonding act torelieve the stress concentrations at the crack tip. Thus high composite fracture toughnessand high composite interlaminar properties are incompatible. The impact strength of thehybrid composites having a ratio of banana and sisal of 1 : 1 at 0.40 V f with differentlayering patterns can be observed in Table 6. Impact strength of the bilayer composite iscomparatively high, because sisal that has high fracture toughness compared to banana ispresent on one side of the composite. Intimate mix and B/S /B have almost the same impactstrength, which is much lower than bilayer composite. As explained earlier the high tensilestrength offered to them due to better stress transfer from fiber to matrix is the reason forthe low impact strength.

Hybrid Effect

The hybrid reinforcing effect of the two fibers has been theoretically calculated.The law of additive rule of hybrid mixtures was used to calculate the hybrid effect.

The rule is given by:

X H X 1V 1 X 2V 2 1

where X H is a characteristic property of the hybrid composite, X 1 and X 2 are characteristicproperties of individual composites, and V 1 and V 2 are the volume fractions of the

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0.20 V f 0.30 V f 0.40 V f 0.50 V f


60 80 100

Figure 13. Effect of varying the relative volume fraction of banana and sisal on impact strength of banana/ sisal/polyester composites at different fiber loading.




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reinforcements in hybrid composites. Figure 14 compares the experimental tensile strengthof the hybrid composites having total volume fraction of 0.40 V f with theoreticalpredictions. A positive hybrid effect in tensile strength can be observed. Theoreticalprediction expects complete intermingling of both the fibers within the matrix. Figure 15compares the experimental tensile modulus at 0.40 V f with theoretical predictions.A positive hybrid effect can also be observed in this case.

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52

500 20 40

Volume percentage of banana fiber (%)60 80 100

0.40 V f (theoretical)0.40 V f (experimental)

Figure 14. Experimental tensile strength and theoretical predictions in intimately mixed hybrid composites having different fiber volume ratio (V f 0.40).

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1400

T e n s i

l e m o d u

l u s

( M P a

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1200

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800

0 20 40Volume percentage of banana fiber (%)

60 80 100

0.40 V f (theoretical)0.40 V f (experimental)

Figure 15. Experimental results and theoretical predictions of tensile modulus in intimately mixed hybrid composites when the relative volume fraction of the two fibers is varied at a total fiber loadingof 0.40 V f .




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CONCLUSIONS

Mechanical performance of short randomly oriented banana/polyester, sisal/polyester,and hybrid/polyester composites was studied. Composites were prepared at 0.20, 0.30,0.40, and 0.50 V f by varying the relative volume fraction of the two fibers. In allcomposites mechanical properties increases with fiber loading. Composites having 0.40 V f showed better performance. Impact strength was found to be maximum in sisal/polyestercomposites at all fiber loading. High tensile strength was obtained for composites havingvolume ratio of banana and sisal 3 : 1. As the ratio of banana is increased in the hybridcomposite, the tensile strength is increased, while the ratio of sisal is increased, the impactstrength is increased. Tensile and flexural properties show a positive hybrid effect, whileimpact performance showed a negative hybrid effect. Keeping the volume fraction at0.40 and volume ratio of banana and sisal at 1 : 1, different layering patterns such astrilayer (banana/sisal/banana and sisal/banana/sisal), and bilayer (banana/sisal) compo-sites were also studied. Flexural and impact properties were higher in bilayer composites.

Tensile strength was maximum in banana/sisal/banana composite. Experimental andtheoretical tensile properties were compared. Finally it is revealed that banana/sisalhybrid fiber reinforced polyester composites results in a positive hybrid effect in tensileand flexural properties.

ACKNOWLEDGMENTS

The authors acknowledge the financial support of the University Grants Commission.We are also grateful to Kerala State Council for Science, Technology, and EnvironmentDepartment for their financial grant.

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Mech Perfor of Short Banana&Sisel Hybrid Fiber Reinforced Polyster Composites-12.Full