Materials and Design 56 (2014) 379–386

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Materials and Design

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Technical Report

Synergy of fiber length and content on free vibration and dampingbehavior of natural fiber reinforced polyester composite beams

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.11.039

⇑ Corresponding author. Tel.: +91 4563 289042; fax: +91 4563 289322.E-mail address: isiva@klu.ac.in (I. Siva).

K. Senthil Kumar a, I. Siva a,⇑, P. Jeyaraj b, J.T. Winowlin Jappes c, S.C. Amico d, N. Rajini a

a Centre for Composite Materials, Department of Mechanical Engineering, Kalasalingam University 626126, Indiab Department of Mechanical Engineering, National Institute of Technology, Surathkal, Indiac Department of Mechanical Engineering, CAPE Institute of Technology, Tirunelveli 627114, Indiad Department of Materials Engineering, UFRGS, Porto Alegre/RS, Brazil

a r t i c l e i n f o a b s t r a c t

Article history:Received 18 September 2013Accepted 18 November 2013Available online 26 November 2013

This work addresses the results of experimental investigation carried out on free vibration characteristicsof short sisal fiber (SFPC) and short banana fiber (BFPC) polyester composites. Influence of fiber lengthand weight percentage on mechanical properties and free vibration characteristics are analyzed. Compos-ite beam specimen is fabricated with random fiber orientations at17 MPa compression using compres-sion molding machine. Natural frequencies and associated modal damping values of the compositelaminates were obtained by carrying out the experimental modal analysis. It is found that an increasein fiber content increases the mechanical and damping properties. For SFPC, 3 mm fiber length and50 wt% fiber content yielded better properties, whereas for BFPC, 4 mm fiber length and 50 wt% fiber con-tent was the best combination. Scanning electron microscopy was performed to study the interfacialmechanism.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

In many applications, natural fiber composites are realisticalternatives to synthetic fiber reinforced composites, since theformer fibers possess many comparatively recognitions, such aslow density, high specific strength, low cost, biodegradability,renewability, good thermal and acoustic insulating properties[1–5]. The most commonly used plant fibers for polymer reinforce-ment are sisal, jute, banana, flax, coir, ramie, kenaf, hemp, palmyra,etc., which already contribute to various engineering applicationsthat involve low load applications like cordage, sacks, fishnets,ropes, wall coverings and mats [6].

The performance of composite materials are usually based ontheir mechanical characteristics, such as tensile, flexural, compres-sion and impact properties. These characteristics are essential toestablish the material performance in various content conditions.However, natural fiber reinforced composites are not fully investi-gated for structural engineering application especially dynamicloading conditions, even though the dynamic characteristics of amaterial is closely related with its mechanical properties.

For the past decade, enormous work has been reported on thecharacterization of the static properties of banana and sisal fiberreinforced thermoset matrix composites [7–9]. Banana fiber,extracted from the banana stem, offers a range of applications

including automotive such as internal trim, seat-back trim, dash-board supports, rear shelves and exterior parts, such as transmis-sion covers. Banana fibers can be obtained in bulk quantity,without paying any additional cost-input [10] and shows advanta-ges when used in reinforcement for polymer matrix [11,12]. Josephet al. [13] described a remarkable increment of mechanical proper-ties in the composites by increasing the fiber content, and Rao et al.[14] studied the effect of fiber content on static mechanicalstrength of banana and other natural fibers reinforced polyestercomposites. However, not much can be found in relation to thevibrational analysis of banana fiber reinforced composites.

Sisal fiber is obtained from the leaves of the Agave Sisalanaplant, being widely available in southern parts of India especiallyin Tamilnadu. Sisal fiber has several advantages like very low cost,availability, renewability, good strength, resistance to deteriora-tion in seawater, easy processing, low weight and recyclability.They also possess high specific strength, stiffness, and resistanceto stretch [15–20]. Paramasivan and Kalam [21] examined thefeasibility of developing polymer-based composites with Sisal fiberreinforcement. Mukherjee and Satyanarayana [22] have studiedthe mechanical properties of sisal fiberand reported their variationwith fiber length. McLaughlin [23] stated that the flaws or weaklinks, which are the main factor to govern strength of the sisalfibers, are irregularly spaced along the fibersurface, and conse-quently strength of the fiberdepends on its length. Joseph et al.[24] have explored the effect of interfacial adhesion on themechanical and fracture behavior of short sisal fiber reinforced

380 K. Senthil Kumar et al. / Materials and Design 56 (2014) 379–386

composites and reported that the composites revealed a growingtrend in properties with fiber content. Thomas et al. [25] comparedthe mechanical properties of sisal to other natural fibers andreported that the microfibrillar angle and lumen size of sisal iscomparatively higher, hence sisal fiber reinforced composites showhigher impact strength.

Reducing resonant amplitude of vibration is an important is-sue in machine and component design process. The resonantamplitude of vibration is significantly influenced by modal damp-ing associated with each mode of the structure. Generally, damp-ing associated with fiber reinforced composite structures is higherthan conventional metal structures due to the viscoelastic behav-ior, fiber–matrix interaction and damping due to damage [26,27].Suarez et al. [28] studied the influence of fiber length and fiberorientation on damping and stiffness of graphite/epoxy and Kev-lar/epoxy synthetic fiber reinforced polymer composite materialsreported that varying the fiber orientation has more significancein producing higher damping than varying fiber aspect ratio.Berthelot [29] measured damping parameters of unidirectionalbeams of glass fiber and Kevlar fiber composites as a functionof fiber orientation and evaluated the results using Ritz method,concluding that beam and plate damping depends on the vibra-tion modes. Khalili et al. [30] carried out free vibration analysisof sandwich beams using improved dynamic stiffness methodfocusing on the effect of parameters like density, thickness andshear modulus of the core for various boundary conditions onthe first natural frequency. Kumar and Singh [31] carried outexperiments using impulse hammer method to analyze the vibra-tion and damping characteristics of curved panel treated withconstrained viscoelastic layer. Gibson [32] investigated the useof modal vibration response measurements for the characteriza-tion of composite materials and structures. This author reportedthat modal testing by using impulsive excitation methods hasthe potential to be a fast and accurate approach not only forthe characterization of intrinsic material properties, but also forquality control and inspection. Rajini et al. [33] investigated theeffect of nanoclay addition on free vibration response usingimpact hammer method for the MMT/coconut sheath hybridpolymer composite laminates.

In all, there are many works on the use of a natural fibers asreinforcement in thermoset polymer composites. However, only afew papers report on free vibration and damping characteristicsof natural fiber reinforced composites. On this context, the presentstudy describes the effect of fiber length and contenton mechanicalstrength and dynamic characteristics of banana fiber polyestercomposite (BFPC) and sisal fiber polyester composite (SFPC) usingimpulse hammer technique. Correlation between their mechanicalproperties and dynamic characteristics was done with the help ofinterfacial observations based on microscopy analysis.

2. Experimental details

2.1. Materials

Sisal fiber and banana fiber used in this study were supplied byShiva exports, Tirunelveli, Tamilnadu, India. Unsaturated

Table 1Properties of banana and sisal fibers [25].

Fiber Cellulose(%)

Hemi-cellulose (%)

Lignin(%)

Moisturecontent (%)

Density (kg/m3)

Banana 63–34 19 5 10–11 1350Sisal 65–70 12 9.9 10 1450

isophthalic polyester resin, initiator and accelerator were suppliedby Vasivibala resins (P) Ltd., Chennai, Tamilnadu, India. Theproperties of sisal and banana fiber areshown in Table 1.

2.2. Fabrication of composites

Initially, the fibers were washed with distilled water and driedin air. The composite was produced by compression molding in ametal die of 300 � 125 � 3 mm3.The mold was cleaned and polish-ing wax applied. The dried fibers (sisal and banana, separately)were randomly arranged in the mold by hand layup. The resin(with 1.5 wt% of initiator) was poured onto the fibers and the moldwas closed and compressed for 17 MPa, allowing for resin curingfor 24 h at room temperature.Composites with variable fiberlength (3, 4 and 5 mm) and fiber weight content (30, 40 and50%) were produced.

2.3. Mechanical testing

Tensile and flexural tests were carried out in an Instron (Series-3382) testing machine according to ASTM: D3039-08 and ASTM:D790-10, respectively. The unnotched Charpy impact test wasdone on an impact tester according to ASTM: D256-10. An averageof 5specimens were use din each test.

2.4. Modal analysis

Modal analysis is a technique used to study the dynamic char-acteristics of a mechanical structure. Moreover, this techniquecan describe a structure in terms of its natural characteristicswhich are the natural frequency, damping and mode shapes. Thereare two well-known methods for performing modal analysis,namely impact test and vibration shaker. From a theoretical stand-point, the measured frequency response functions (FRF) may beobtained with both, but there are usually differences due to prac-tical aspects related to data collection.

In this study, modal analysis was performed with the help of animpact hammer test. The photograph of the experimental setupand the line diagram shown in Fig. 1a and b is comprised of impacthammer with a sharp hardened tip (Kistler model 9722A500), andaccelerometer attached to the end of the rectangular compositelaminate (dimensions: 200 � 20 � 3 mm [34]) with wax. This sys-tem was used for obtaining higher frequencies and the hitting withthe impulse hammer occurred at three equally spaced places of thelaminate (referred to as 1, 2 and 3 in Fig. 1).

The displacement signal from the accelerometer was recordedin a PC using data acquisition system (DAS), (DEWE43, DewteronCorp., Austria) and ICP conditioner (MSIBRACC). Two separateadaptors were used for capturing the output signal, one for theaccelerometer signal and the other for the hammer response afterimpact with the laminate.

2.5. Damping factor

Damping is vital to the study of the dynamic characteristics offiber reinforced composites. Damping mechanisms in natural fiber

Flexural modulus(GPa)

Microfibrillarangle (�)

Tensile strength(MPa)

Young’s modulus(GPa)

2–5 11 54 3.512.5–17.5 20 68 3.8

Fig. 1. Experimental setup for free vibration analysis. Photograph (a), Line diagram (b).

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composites differ entirely from those in conventional materialsand energy dissipation depends upon factors like viscoelasticnature of matrix and/or fiber, interphase, damage and visco-plasticcharacteristics [35]. In particular, damping for materials withnatural fiber is difficult to study due to their chemical constituents,nevertheless it shows good damping characteristics due to theirinherent porous nature.

The half-power band width method was employed to find thedamping coefficient values of banana and sisal fiber reinforcedcomposites through FRF curves obtained from the FFT analyzer.In accordance, the damping values were calculated based on Equa-tion (1):

f ¼ Dx=2xn ð1Þ

where f – Damping coefficient, Dx – bandwidth, and xn – naturalfrequency.

3. Results and discussion

3.1. Mechanical properties

Table 2 displays the combined effect of fiber length and contenton the mechanical properties of the banana and sisal composites.From Table 2, an increasing trend in tensile strength can be noted

Table 2Effect of fiber length, content on mechanical properties of composites.

Sample Fiber wt% Tensile Strength (MPa) Flexural St

3 mm 4 mm 5 mm 3 mm

BFPC 30 20.0 ± 1.3 21.1 ± 1.1 24.1 ± 1.4 34.3 ± 1.240 17.7 ± 1.5 27.5 ± 1.2 23.7 ± 1.3 44.0 ± 1.150 28.6 ± 1.6 30.5 ± 1.1 21.5 ± 1.3 46.7 ± 1.3

SFPC 30 26.9 ± 1.5 26.5 ± 1.5 31.5 ± 1.2 45.2 ± 1.140 30.7 ± 1.3 25.8 ± 1.3 33.5 ± 1.2 55.4 ± 1.550 31.7 ± 1.3 20.2 ± 1.3 27.3 ± 1.3 57.7 ± 1.3

Fig. 2. SEM image of tensile fractured BFPC:

for the BFPC from 3 to 4 mm. However, this was not found from 4to 5 mm perhaps due to considerable fiber entanglement. A similareffect was seen for 50 wt%.

Fig. 2 shows the fractography of the BFPC (4 mm, 50 wt%) andgood adhesion between fiber and matrix can be noticed in theSEM image. The appearance of hackles (river pattern in Fig. 2a)on the fiber/matrix interface indicates its strength. Thick interfaceresults in more shear action in the interfacial zone during loadingof the composite which lead to layer by layer fracture of the matrix.SEM image of BFPC (5 mm/50 wt%) in Fig. 2b suggests the occur-rence of agglomeration, with more pulled-out fibers and delami-nated structure, which corroborates the observed decrease instrength. The same kind of results was also observed by Idiculaet al. [25].

In SFPC, an increasing trend in tensile strength was found for3 mm long fibers in the 30 to 50 wt% order, the opposite to whathappened for 4 mm long fibers whereas a mixed trend was re-corded for 5 mm long fibers. A close and thick interface was notedin the SEM image (Fig. 3a) of the SFPC (3 mm/50 wt%) and the resinrich regions show hackles and matrix failure. Cohesive force be-tween fibers may increased with the increase in fiber length andthis may originate agglomeration in loose fibers. The same wasobserved in the SFPC with 4 mm/50 wt%. Besides, poor wetting ofthe fiber surface by the matrix was also noted via SEM (Fig. 3b).

rength (MPa) Impact Strength (kJ/m2)

4 mm 5 mm 3 mm 4 mm 5 mm

30.2 ± 1.4 43.9 ± 1.3 95.2 ± 1.2 128.2 ± 1.1 153.2 ± 1.347.6 ± 1.5 37.2 ± 1.5 152.4 ± 1.3 161.3 ± 1.5 142.3 ± 1.347.9 ± 1.5 37.7 ± 1.5 187.2 ± 1.3 215.3 ± 1.4 153.2 ± 1.553.3 ± 1.5 57.5 ± 1.2 184.3 ± 1.3 195.4 ± 1.4 227.4 ± 1.439.2 ± 1.4 53.6 ± 1.2 284.1 ± 1.2 156.1 ± 1.5 275.4 ± 1.546.4 ± 1.5 54.4 ± 1.3 414.1 ± 1.2 437.3 ± 1.5 305.1 ± 1.5

4 mm/50 wt% (a) and 5 mm/50 wt% (b).

Fig. 3. SEM image of tensile fractured SFPC: 3 mm/50 wt% (a) and 4 mm/50 wt% (b).

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Table 2 depicts the variation inflexural strength as a function offiber length and fiber content. It is observed that flexural strengthincreases with fiber content of SFPC. Maximum flexural strengthwas noted for the 3 mm/50 wt% category. Nevertheless, on observ-ing the flexural strength of the SFPC as a function of fiber content,little variation was noted for 5 mm long fibers. This could be due tothe cohesive force exerted between the adjacent long fibers.

Regarding impact strength (Table 2), it tended to increase withfiber content for any fiber length. A similar trend was recordedfor both fibers, even though SFPC reached higher energy values. Thiscould be related to the higher microfibrillar angle of the sisal fiber(Table 1), which also has larger lumen size than banana fiber, whatnaturally promotes the porous nature and increases impactstrength. Fiber length may also play a role in impact strength anddissipation of energy throughout the length of the composite maybe more effective with longer fibers. However, the effect of fiberlength (for a given wt%) for the studied composites was not clear,perhaps due to a variable content of voids weakening the system.

Fig. 4(a and b) shows the SEM images of the BFPC of 4 mm/30 wt% and 4 mm/50 wt%, respectively. More separated fibers canbe observed in Fig. 4a and more stranded fibers can be noted inFig. 4b. The BFPC (4 mm/30 wt%) reached 161 kJ/m2 in impactstrength, which is the least in the 4 mm family. Due to the lowerfiber content in the matrix, attributed to poor fiber distribution(Fig. 4a), hence resulting in poor energy dissipation. Whereas forBFPC (4 mm/50 wt%), more uniform fiber arrangement can benoted in the fractured region, helping in explaining the higher im-pact energy absorption (215 kJ/m2). Also, fiber deteriorationisclearer in Fig. 4b, revealing maximum energy absorbed duringimpact.

For the SFPC, higher energy absorption was shown for the 4 mmfamily. Fig. 5a shows the SEM of 4 mm/40 wt% (156 kJ/m2). The

Fig. 4. SEM image of impact fractured BFPC spe

intermediate fiber content in matrix created matrix-rich regions(Fig. 5a) in the composite; which induces more brittle-type failuredecreasing impact strength. But for SFPC (4 mm/50 wt%), morefiber pull-outs are noted (Fig. 5b) which lead to higher energy dis-sipation in the composite, reaching the highest impact strength(437 kJ/m2) among all samples. In SFPC, variation in impactstrength with respect to fiber content for 5 mm family was less sig-nificant in comparison to the other families.

Fig. 6 shows the combined effect of fiber length and content ontensile modulus of the composites. It is observed that tensile mod-ulus mostly decreases with the weight percentage. Modulus of the5 mm/50 wt% SFPC was too low perhaps due to poor wetting andweak fiber–matrix interfacial adhesion, leading to higher strainand lower modulus. The fibers and matrix may have been sepa-rately loaded causingfiber stretching (marked in Fig. 3b) and de-bonding. It can also be noticed in Fig. 6, that the behavior of theflexural modulus of the composites (BFPC & SFPC) was variable,tending to decrease with fiber content especially for longer fibers.

3.2. Natural frequency of composites

In fiber reinforced composites, the natural frequency dependson many factors such as fiber length, orientation and content,and fiber/matrix interface. So it is a complex task to find thenatural frequency for fiber reinforced composites. The first threemodes of fundamental natural frequencies of short sisal and bana-na fiber reinforced composite structures were studied.

The cantilever composite structure was applied with a constantforce to one corner of the plate with the help of a piezoelectric im-pact hammer. Force was applied at five different places and itcaused the same static deformation in the plate, as confirmed bythe same force peak value even though the oscillation rate of the

cimen: 4 mm/30 wt% (a), 4 mm/50 wt% (b).

Fig. 5. SEM image of impact fractured SFPC specimen: 4 mm/40 wt% (a), 4 mm/50 wt% (b).

Fig. 6. Effect of fiber content and fiber length on tensile and flexural modulus ofcomposite.

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force changed. The response of the composite plate was measureddue to excitation with an accelerometer attached to one corner ofthe plate. The most important measurement needed for experi-mental modal analysis is the frequency response function, i.e. theratio between output response and input excitation force. Thismeasurement is typically acquired using a dedicated instrumentsuch as an FFT (Fast Fourier Transform) analyzer. The measuredtime data is transformed from the time domain to the frequencydomain, called the frequency response function, using a Fast Fou-rier transform algorithm found in signal processing analyzer andcomputer software. There are peaks in this FRF function which oc-cur at the resonant frequencies of the system. These peaks occur at

frequencies where the time response is observed to produce max-imum response corresponding to the rate of oscillation of the inputexcitation.

The first three modes of deformation pattern that exist in thestructure are named bending (Mode 1), twisting (Mode 2) and sec-ond bending (Mode 3), also known as the mode shapes of thestructure. The first three modes of natural frequencies of bananaand sisal/polyester composites (3 mm fiber length, 30 wt%) are26.0 Hz, 177.6 Hz and 492.4 Hz, and 27.6 Hz, 186.3 Hz and537.3 Hz, respectively. Fig. 7(a–c) shows the first mode of naturalfrequency of sisal and banana polyester composites for different fi-ber wt% and length. For the 3 mm fiber length/30 wt% (Fig 7a), anotable difference in natural frequency is observed between BPFCand SFPC, suggesting that the particular fiber surface and its com-patibility with polyester can cause dissimilar fiber/matrix inter-face. Whereas, for higher fiber wt%, the natural frequency of bothcomposites merge together. Perhaps the shorter fiber length canincrease the surface to contact area between fiber and matrix aswell as fiber/matrix interface, offering better stiffness to bothcomposites. For BFPC, an increase in natural frequency with fibercontent is noted for all fiber lengths, following a more linear fash-ion than SFPC, which may be related to higher fiber content.

The maximum natural frequency was noticed for 4 mm fiberlength/50 wt% for banana/polyester composites. Indeed, the in-crease in stiffness of the composites impacts its natural frequency.It had been previously seen in Fig. 6, that flexural modulus ofbanana fiber composites increased with fiber content. However,variation in fiber length did not significantly influence the naturalfrequency of the composites.

On the other hand, for SFPC, only 30 wt% and 50 wt% fiber con-tent increased the natural frequency for all fiber lengths. In the40 wt% case, a decrease in natural frequency occurred for all fiberlengths. For low fiber content (30 wt%), an increase in naturalfrequency was observed for SFPCs compared to BFPCs for all fiberlengths, suggesting that apart from the fiber/matrix interface, fiberrigidity also plays an important role indetermining naturalfrequency of the composite, and the more rigid fiber (i.e. sisal –see Table 1) lead to higher natural frequency.

Further, it is observed for SFPCs from Fig. 7(a–c) that, for 40 wt%fiber content, the natural frequency reduces with length. It couldbe due to more extensive agglomeration that causes weak fiber–matrix interface, decreasing Young’s modulus and also naturalfrequency. Table 3 shows the measured natural frequency of thevarious composites (Modes 2, 3) and it can be seen that the highernatural frequency for sisal is found for 5 mm, 50 wt% in all threemodes of vibration. The natural frequency of the first three modesfor sisal/polyester composites (5 mm/50 wt%) are 31.7 Hz,209.8 Hz and 591.4 Hz.

Fig. 7. Effect of fiber length, content and type on natural frequency of composites (Mode 1) for different fiber lengths: 3 mm (a), 4 mm (b) and 5 mm (c).

Table 3Effect of fiber length, content and type on natural frequency of composite (Modes 2, 3), in Hz.

Composites/mode 3 mm 4 mm 5 mm

30 wt% 40 wt% 50 wt% 30 wt% 40 wt% 50 wt% 30 wt% 40 wt% 50 wt%

BFPC Mode 2 177.6 183.1 189.8 166.8 187.6 216.1 175.2 190.0 199.4Mode 3 492.4 529.4 562.3 470.7 523.7 569.7 515.1 545.1 536.6

SFPC Mode 2 186.3 180.7 206.4 171.1 161.0 229.3 184.3 183.2 209.8Mode 3 537.3 543.7 576.5 476.3 459.3 535.7 409.7 542.1 591.4

Fig. 8. Frequency response function (FRF) of SFPC (3 mm, 50 wt%).

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3.3. Damping factor of composites

The indicated portion of the sub-graph shown in Fig. 8 is usedfor calculating the damping values using the half-power bandwidth method [22]. This figure shows how the half power pointsare obtained, i.e. by finding the intersections of line from the givenFrequency response function (FRF) curve. Qmax is the maximumamplitude obtained from the resonant peak of the curve, Dx isthe difference between the frequency values of band width whichis cut by the horizontal line taken Qmax/

p2 value below the ampli-

tude of peak value. The half power points are used to determine thedamping ratio (Dx = x1 �x2) [34].

Fig. 9(a–c) shows the first mode of damping of the various sisaland banana polyester composites. For a constant fiber length, twokinds of damping trends were observed with the increase in con-tent of banana and sisal fiber, for the former damping decreasedand for the latter, it increased. In general, higher resin contentshould lead to higher damping due to its viscoelastic nature [35].This coincides with the trend obtained from the BFPCs, however,

Fig. 9. Effect of fiber length, content and type on damping of composite (Mode 1)for 3 mm(a), 4 mm (b) and 5 mm (c) fiber length.

Table 4Effect of fiber length, content and type on damping of composite (Modes 2 and 3).

Composites/mode 3 mm 4 mm 5 mm

30 wt% 40 wt% 50 wt% 30 wt% 40 wt% 50 wt% 30 wt% 40 wt% 50 wt%

BFPC Mode 2 0.00546 0.07438 0.1020 0.02854 0.10114 0.1216 0.01792 0.0013 0.0133Mode 3 0.00540 0.00087 0.0016 0.01203 0.00839 0.00258 0.00087 0.0001 0.0003

SFPC Mode 2 0.00186 0.07386 0.2405 0.02215 0.0072 0.11127 0.00279 0.1077 0.0437Mode 3 0.00122 0.00166 0.0023 0.00060 0.00057 0.00471 0.01162 0.00061 0.00277

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it was the opposite for SFPC. From Fig 9(b and c), it can be inferredthat apart from the fiber content, the interface thickness and inter-face stiffness also play an important role in the damping mecha-nism [35]. For the same fiber content (30 wt%), the banana fiberreinforced composite shows higher damping than sisal in all fiberlengths. Owing to the smaller diameter of banana fiber (from 100to 300 lm [36,22]), a thicker interface could appear, causing higherdamping than sisal, as reported by Bledzki et al. [37]. The greatersurface contact area of the sisal fiber due to its lower aspect ratiocan create a stronger interface and make the composites stiffer.

At 40 wt% fiber content, damping value has decreased for banana fi-ber composites except for 4 mm. This may have happened due to aweak interphase for 4 mm with 40 wt% of banana fiber reinforced com-posites, which was confirmed from their modulus (4 mm, 40 wt%)shown in Fig. 9. But, for 5 mm, 40 wt% of banana fiber composites,damping value abruptly decreases in relation to sisal fiber composites.While the fiber content increased from 40 to 50 wt% of BFPCs, dampingvalue decreased linearly, except for 5 mm. Conversely, in the case ofSFPCs, the damping value increased linearly, except for 5 mm.

Table. 4 shows the calculated damping of the various compos-ites (Modes 2, 3) and it can be seen that a similar trend of dampingvalue of mode 1 was observed in mode 2 and 3.

From Figs. 9(a–c), it is clearly noted a transition of damping at40 wt% of fiber content in both banana and sisal fiber reinforcedcomposites. In the case of banana, the 40 wt% provides betterdamping with 4 mm fiber length, whereas for sisal, higherdamping was observed for 40 wt% of fiber content with 5 mm fiberlength, perhaps due to the difference in inherent surface fibermorphology.

4. Conclusions

Based on the above experiments following conclusions weremade:

� The short sisal and banana fiber reinforced polyester com-posites were fabricated using compression molding processfor varying fiber length and content.

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� Experimental investigations showed that the dispersioncapability of SFPC offers good interfacial bonding henceexhibits better mechanical strength and free vibrationproperties compared to BFPC.

� The higher fiber content (50 wt%) offers the significantimprovement on mechanical strength as well as free vibra-tion properties compared to fiber length for both the com-posites. It could be due to the variation of interlaminarshear at fiber ends.

� The optimum fiber length and content of BFPC wasobserved as 4 mm/50 wt% and for SFPC it was 3 mm/50 wt%.

� The maximum increase in natural frequency was found in4 mm/50 wt% of BFPC and for SFPC at 5 mm/50 wt% in allthe three modes of vibration. In addition, 4 mm/50 wt% ofBFPC and 3 mm/50 wt% of SFPC offered higher dampingand it could be due to the inherent porous cross sectionof natural fibers.

Acknowledgements

The authors wish to thank the Center for Composite Materials,Department of Mechanical Engineering Kalasalingam Universityfor their kind permission to carry out the preparation and testingof the composites and wish to thank the Department of Scienceand Technology-India for funding through SR/FTP/ETA-64/2012.

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