Materials and Design 90 (2016) 795–803

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Materials and Design

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Layering pattern effects on vibrational behavior of coconut sheath/banana fiber hybrid composites

K. Senthil Kumar a, I. Siva a,⁎, N. Rajini a, J.T. Winowlin Jappes a, S.C. Amico b

a Centre for Composite Materials, Department of Mechanical Engineering, Kalasalingam University, 626126, Indiab DEMAT, Federal University of Rio Grande do Sul (UFRGS), Porto Alegre, RS, Brazil

⁎ Corresponding author.E-mail address: isiva@klu.ac.in (I. Siva).

http://dx.doi.org/10.1016/j.matdes.2015.11.0510264-1275/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 January 2015Received in revised form 10 November 2015Accepted 12 November 2015Available online xxxx

Two different fibers, namely short banana (B) and naturally woven coconut sheath (C), were hybridized in poly-ester matrix composites using compression molding. Various composites were produced with the same overallfiber wt.% and varying the relative wt.% of the individual fibers. Banana and coconut sheath fibers were surfacetreated using 1 N alkali solution to enhance interfacial adhesion. Static mechanical and dynamic characteristicssuch as natural frequency and damping were studied and impulse hammer technique was employed to studythe dynamic characteristics of the composites. Mechanical performance was maximized for the highest relativeamount of banana fiber in the composites. The mechanical properties were also found to vary with the layeringpattern. Irrespective of the relative wt.% of fibers and layering pattern used, alkali treatment showed a positiveeffect on the evaluated properties. The CBC layering pattern exhibited the greatest damping, indicating better en-ergy absorption capability brought by the porous structure of the coconut sheath fiber.

© 2015 Elsevier Ltd. All rights reserved.

Keywords:Hybrid compositesVibrationElastic propertiesBanana and coconut sheath fibers

1. Introduction

The use of renewable materials as reinforcement in polymer matrixcomposites is growing steadily. In particular, advantages such as acces-sibility, low cost, non-hazardous nature and positive impact on environ-ment have attracted attention of the researchers towards natural fibers[1]. The replacement of syntheticfiberswith natural ones does not occurin high loading applications. Nevertheless, medium load applications,especially in the automotive sector, can accommodate natural fiber re-inforced polymer composites more easily based on decreased fuel con-sumption. The composites are developed using various natural fiberssuch as banana, date, husk, bamboo, flax, hemp, sisal, jute, pineapple,abaca and coir [2,3].

The development of compositeswith naturalfibers expresses a lot ofchallenges. Nevertheless, there have beenmany reports, as in [4], show-ing improved mechanical properties after fiber surface treatment usedto improve interfacial adhesion between natural fibers and matrices.In general, this treatment can have an effect on morphology of thefiber surface, moisture content, mechanical strength and aspect ratio[5]. Many chemical treatments, such as silane, KMnO4, sodium lauryletc., have been reported [6]. Among them, the use of an alkali is verycommon, being able to promote fiber/matrix mechanical interlockingdue to the removal of the waxy layer from the fiber surface [7].

Hybrid composites are obtained when using more than one rein-forcement in a matrix in such a way to exploit the characteristics ofthe individual reinforcements. Their properties depend on the proper-ties of the individual fibers used, compatibility between the fibers andthe matrix, surface roughness of fibers etc. [8]. Idicula et al. [9] havestudied banana/sisal fiber reinforced polyester composites and reporteda positive hybrid effect on tensile and flexural properties. Sathishkumaret al. [10] have examined snake grass with hybrid coir/banana fibercomposites with respect to layering sequence and volume fraction.The highest tensile strength was found in snake grass/banana compos-ites andmaximum flexural strengthwas found in snake grass/coir com-posites. They concluded that the layering sequence can significantlyaffect the mechanical strength of the composites.

Reddy et al. [11] have studied kapok and sisal hybrid compositeswith respect to fabric content and volume ratio between fibers. Theyconcluded that greater sisal content resulted in better hardness andflexural properties. The effect of hybridization on mechanical andwater absorption properties of banana/sisal composites was studiedby Venkateshwaran et al. [12], who found that the increase in sisal con-tent of the composite increased the mechanical properties up to 50%.The tensile properties of sun hemp/palmyra–polyester compositeswere investigated by Dabade et al. [13]. The optimal fiber length andfiber content were reported to be 30 mm and 55%, respectively.

Several studies [14–16] have reported on static mechanical proper-ties using various natural fibers as reinforcement in polyester matrix.However, other properties of natural fiber composites such as dielectric,

Table 1Chemical and mechanical properties of banana and coconut sheath fibers [9,17,18].

Fiber Cellulose Hemicellulose Lignin Moisture content Density Microfibrillar angle Tensile strength Young's modulus

(%) (%) (%) (%) (g/cm3) (°) (MPa) (GPa)

Banana 63–64 19 5.0 10–11 1.350 11 550 ± 6.7 3.5Coconut sheath 68 22 20.6 8.79 1.375 31 88.6 4.4

Fig. 1. Stacking sequences followed in the composites.

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thermal, dynamic mechanical and transport properties are hardly seen.Indeed, very limitedwork has been reported on the dynamic character-istics of natural fiber composites using free vibration method. Thesecharacteristics are important for structural engineering applicationssuch as machine support, construction industries and automotive com-ponents, in which existing systems would benefit by the use of lightermaterials with better dynamic-mechanical characteristics.

In this work, hybrid composites were developed using a combina-tion of alkali treated short banana and coconut sheath fibers in a polyes-ter matrix. Tensile, flexural and impact properties, along with natural

Fig. 2. Experimental setu

frequency and damping characteristics, were analyzed focusing on thestudy of the effect of fiber relative wt.% and layering pattern.

2. Experimental details

2.1. Materials

Coconut sheath was directly extracted from the outer bask of the co-conut tree and banana fiber was purchased from Shiva exports,Tirunelveli/TN, India. An unsaturated isophthalic polyester resin was

p for modal analysis.

Fig. 3. Variation of tensile strength of untreated (a) and NaOH treated (b) B/C hybrid composites.

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used,with a curing agent (methyl ethyl ketone peroxide—MEKP) and acatalyst (cobalt naphthenate), supplied by M/s Vasavibala Resins Ltd.,Chennai/TN, India. Some important characteristics of short banana andcoconut sheath fibers in untreated condition are shown in Table 1. Alkali(NaOH) was supplied by United Scientific, Madurai/TN, India, and as-received fibers were mercerized with 1 N NaOH solution for 1 hfollowed by thorough washing and drying in hot air oven at 60 °C for6 h.

2.2. Composite fabrication

Banana fibers (B) with 4 mm length and naturally woven coconutsheath (C) were laid inside the mold cavity at random orientation. As-sembles of dried fibers were pre-compressed to 10 kgf/cm2 for a dayprior to molding. Formulated resin was poured into the mold cavityfirst and then fiber sets were stacked in it with the application ofsome resin between fiber layers. Finally, the mold was closed using 50kgf/cm2 pressure and kept for 24 h at room temperature. The hybridcomposites were produced in four different layering patterns (Fig. 1),namely CBC, CCB, BCB and BBC. Pure banana (BBB) and pure coconut

Fig. 4. Tensile modulus of hybrid composites with different layering patterns.

sheath (CCC) fiber composites were also produced for comparison. Inall composites, the overall fiber weight content was kept constant. Thecured composite was taken from the mold and cut to produce speci-mens for testing. CCC, BCB, CBC, and BBB are defined as skin-core com-posites and CCB and BBC are defined as skin-eccentric composites.

2.3. Mechanical testing and modal analysis

Tensile and flexural tests were conducted in an Instron (Series-3382) testing machine according to ASTM D3039-08 and ASTM D790-10, respectively. The un-notched Charpy impact test was conducted ac-cording to ASTM D256-10. An average of five samples is reported ineach case for all tests.

Fig. 2 shows the experimental setup used for modal analysis. Com-posite samples of 200 mm × 20mm× 3mmwere used. An accelerom-eter (Kistler model 8778A500)was attachedwith the help of wax at theend of this specimen. For obtaining higher frequencies, a modally tunedimpact hammer (Kistlermodel 9722A500)with sharp hardened tipwasused. The displacement signal from the accelerometerwas recorded in aPC through data acquisition system (DAS) (DEWE 43, Dewetron Corp.,Austria) and an ICP conditioner (MSIBRACC). Two adaptors were usedto capture the output signals, one for the accelerometer signal and an-other for measuring the magnitude of the response by the impulse

Fig. 5. Untested and tensile tested specimen (untreated).

Fig. 6. Fractography of tensile tested hybrid composites: Untreated CCB (a) and NaOH treated CCB (b).

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hammer. The damping factor was obtained from the well-known tech-nique called logarithmic decrement method [19].

3. Results and discussion

3.1. Mechanical testing

Fig. 3a–b shows the effect of layering pattern on tensile strength ofC/B hybrid composites in untreated and alkali (NaOH) treated condi-tion. Low weight of banana fiber decreases strength of the hybrid com-posites but higher content can significantly increase the tensile strengthof the hybrids. Moreover, the skin-eccentric type (BBC) composites ex-hibited higher tensile strength, even higher than the pure banana com-posites (BBB).

In all cases, tensile strength was higher for the alkali treated condi-tion. It is also interesting to notice that strength of CBC behaves differ-ently in untreated (decreased) and alkali treated (increased)conditions comparedwith CCC composites. That is, the interfacial adhe-sionmay have a greater effect than the layering pattern. Perhaps the al-kali treatment can causemorphological changes in fiber surface that can

Fig. 7. Fractography of tensile tested hybrid composit

lead to a closer packing of fiber in addition to the change in mechanicalbonding between fibers and matrix.

The tensile properties of a hybrid composite are also influenced bythe specific strength of the fibers used [20] and, according to Table 1,tensile strength of banana is higher than that of coconut sheath fiber.For the same relative wt.% of banana fiber i.e. 70% (BBC and BCB) the se-quence BBC possess higher tensile strength. This reflects the importanceof the layering pattern on hybrid composites. It is also observed that un-treated BCB and BBB donot significantly differ in strength. However, thedifference is higher when using alkali treated fibers, justifying thechemical treatment.

Fig. 4 shows tensile modulus of the composites. All hybrid compos-ites showed enhancement in stiffness compared to the pure BBB com-posites. Moreover, the treated fibers yielded higher modulus in thehybrid composites, being BBC the highest.

Fig. 5 shows untested and tensile tested specimens, and the latter re-veals delamination failure due to the axial load. Fig. 6a–b shows the SEMimage of CCB hybrid composite in untreated and alkali treated condi-tions. Large gaps between fibers and fiber pull-out can be noticed prob-ably due to the poor fiber/matrix adhesion. An uneven fiber distribution

es: untreated BBC (a) and NaOH treated BBC (b).

Fig. 8. Effect of hybridization and fiber treatment on flexural strength: untreated (a) and NaOH treated (b) composites.

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in thematrix is also apparent. These features suggest failure of compos-ites at minimum loading.

Fig. 7a–b shows the SEM image of BBC hybrid composite with twodifferent magnifications. In the untreated condition, large extent ofvoids are seen due tofiber pullout, aswell asmatrix cracking, separationof fibers and fiber shear (Fig. 7a) indicating poor interfacial adhesion.Accumulated fibers at specific regions and bending of fibers are shownin Fig. 7b suggesting greater resistance to the applied load.

Poor wetting of banana and coconut sheath fibers with polyesterresin may lead to poor strength in untreated condition. At the sametime, a closer packing between fibers is seen in alkali treated compos-ites. In general, this treatment can remove impurities and inorganic con-taminants on the fiber surface, providing better compatibility withmatrix. Also, the easier spreading of resin around the fibers can providegreater resistance to fiber pull out due to a higher surface area. Althoughsome fiber/matrix debonding is found in Fig. 7b, it is not enough to shearout the fiber at the interface due to a better adhesion.

Pure coconut sheath fiber composite showed higher flexuralstrength than pure banana composites (see Fig. 8) and flexural strengthof the alkali treated hybrid composites was higher than the respectiveuntreated fiber ones. The highest value was observed for skin-coretype of hybrid with 30% of B treated fibers (i.e. CBC). Flexural propertiesare greatly influenced by the strength of the skin, or external layers, asdiscussed in [21]. Jawaid et al. [22] studied hybridization of wovenjute fabrics with oil palm empty fruit bunch fibers and reported that

Fig. 9. Untested and flexural tested specimen (untreated).

positioning of the jute woven as extreme layers yielded superior flex-ural strength.

An interesting finding is that untreated skin-core compositesfollowed a reverse trend compared to NaOH treated composites (Fig.8a–b). Therefore, positioning of layers, compatibility and distributionof fibers influence the properties of the composites. Further, the delam-ination mechanism differs in skin-core and skin-eccentric composites.Since two inter-laminar planes exist in the former composites, delami-nation is more likely, lowering strength of these composites.

Fig. 9 shows the untested and flexural tested composite samples,and double-sided delamination is observed in the tested specimen.Flexural modulus of the composites is shown in Fig. 10. All hybridsshowed higher stiffness compared to the pure composites. When ba-nana fibers are used as skin layer and coconut sheath as core, highermodulus was obtained for alkali treated BCB. It is also seen that skin-core and skin-eccentric patterns influencedmodulus of the composites.

In fiber-reinforced composites, impact damage causes fiber pull-out,fiber-matrix de-bonding and matrix fracture, being the former mecha-nism dominant for impact strength. Impact strength of the C/B fiber hy-brid composites are shown in Fig. 11. Coconut sheath fiber compositesshowed higher impact strength than pure banana fiber, i.e. debonding

Fig. 10. Flexural modulus of hybrid composites with different layering patterns.

Fig. 11. Effect of hybridization and fiber treatment on impact strength: untreated composites (a) and NaOH treated (b) composites.

Fig. 12. Untested and impact tested specimen (NaOH treated).

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between coconut sheath fiber and the polyester matrix required moreenergy.

The displacement between banana fiber andmatrix at the fiber endsdue to shearing action can cause matrix cracking and increase crack

Fig. 13. Fractography of impact tested hybrid compo

propagation rate. Besides, the fiber properties themselves are also partlyresponsible, for instance, the higher microfibrillar angle of coconutsheath fiber (31°) compared to banana fiber (11°) is expected to helpincreasing impact strength.

Impact strength decreased for CBC in comparisonwith CCB compos-ite, for the same wt.% of banana fiber. Hence layering pattern also influ-ences impact strength and this may be related to the compatibilitybetween the fibers. The lower impact strength of CBC composites alsosuggests inter-laminar delamination between coconut sheath and ba-nana fibers. Impact strength increases when banana content increasedfrom 30 to 70% (BCB and BBC), decreasing after that (from 70% to100%) perhaps due to the greater possibility of intermingled cohesiveadhesion between homogeneous fibers (Fig. 11a). Furthermore, impactstrength of NaOH treated composites (Fig. 11b) was generally higherthan that of the untreated composite possibly because this treatmentremoves substances such as lignin, pectin and hemicelluloses from thefiber surface, making it rougher and better anchored to the matrix.Fig. 12 shows the untested and impact-tested composite specimen,and complete debonding of the composite is observed with fiber pull-outs.

Fig. 13a shows the SEM of CBC hybrid composite showing few voidsand minimum fiber pullout. The composite with lower stiffness can

sites: untreated CBC (a) and untreated BCB (b).

Fig. 14. Fractography of impact tested hybrid composites: NaOH treated CBC (a) and NaOH treated BCB (b).

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provide higher impact strength due to its increased energy absorptioncapability. Similarly, Fig. 13b shows the coconut fiber at the core be-tween the skins of banana fiber and more fiber pull-out. Fig. 14a–bshows SEM of similar composites (CBC and BCB), but with alkali treatedfibers. Fig. 14a shows closer bonding between fibers and matrix for CBCin comparison with the untreated condition. The more tightly bondedfibers increase the probability of brittle failure due to the presence ofmatrix rich regions shown in Fig. 14a. Cleavage matrix failure also ap-pears in this figure. The interfacial gap between coconut sheath and ba-nana fiber was found to be smaller for the alkali treated BCB composite(Fig. 14b) than for the untreated BCB (Fig. 13b).

3.2. Vibrational characteristics of hybrid composites

Tables 2–3 show natural frequency and damping obtained for thepure and hybrid composites. The fundamental mode shapes of hybridcomposites provide insight into the selection of materials for suitablestructural applications. The combination of short banana fibers withthe coconut sheath mat enhanced natural frequency of the hybrid com-posite irrespective of the layering pattern. In addition, the magnitude ofthe natural frequency of the hybrids is significantly influenced bylayering pattern. Maximum natural frequency is found for skin-coretype of composites (CBC) as indicated by the FRF curve shown inFig. 15. These results again indicate that stiffness in hybrid compositesis not only influenced by thefibers but also depends on the layering pat-tern [23]. The results also indicate that the skin-core type of hybrid pro-duces higher natural frequency than skin-eccentric, as indicated inTable 2, due to the inter-laminar separation.

Table 2Vibrational characteristics of the untreated-fiber composites.

Layer pattern Natural frequency (Hz) Damping

Mode 1 Mode 2 Mode 3 Mode 1 Mode 2 Mode 3

BBB 31.33 216.16 569.66 0.0908 0.1216 0.0026CCC 21.92 177.37 335.86 0.0823 0.0452 0.0424CBC 35.40 261.23 726.32 0.2210 0.0298 0.0107CCB 30.52 230.7 664.1 0.2345 0.03103 0.01078BBC 28.08 213.62 617.68 0.2776 0.0364 0.0126BCB 30.52 238.04 687.26 0.2535 0.0325 0.0112

Mode 2 and Mode 3 followed a similar trend regarding natural fre-quency for different layering patterns of composites. Therefore, anyhigher mode shape is mainly dependent on the fundamental modeshape of the structure. Generally, stiffness is dependent on the Young'smodulus, density and area moment of inertia of a structure rather thanthe testing condition. In hybrid systems, these parameters are influ-enced by the chemical treatments of fibers, addition of coupling agents,layering pattern and fiber content.

Reduction in fiber diameter is expected to take place after chemicaltreatment due to the removal of constituents of the outer cells. Thiscould influence fiber compressibility, making the fibrous reinforcementmore packed for the same fiber weight content. Nevertheless, Table 3shows no significant changes in natural frequency after alkali treatment.This may be due to a possibly lower fiber stiffness after chemical treat-ment [24]. The overall results indicate that the dynamic characteristicsof the composites are mostly influenced by layering pattern ratherthan chemical treatment of the fibers.

Fig. 16a–b shows the time domain of the composite during testingby impulse hammer technique. Successive amplitudes were obtainedfrom Fig. 16b in order to find the damping coefficients tabulated inTables 2–3. It can be seen from Table 2 that higher damping is observedfor banana fiber composite in comparison to the coconut sheath com-posite. The large availability of fiber/matrix interface in short fiber com-posites can lead to greater energy dissipation, due to a considerableprobability of slippage at the interfaces. Accordingly, the coconut sheathcomposites are observed to produce minimum damping value. More-over, the change in layering pattern using two different fibers can alsoinfluence damping.

Table 3Vibrational characteristics of the alkali-treated fiber composites.

Layer pattern Natural frequency (Hz) Damping

Mode 1 Mode 2 Mode 3 Mode 1 Mode 2 Mode 3

BBB 30.52 233.15 688.48 0.1371 0.018 0.0071CCC 30.86 197.37 374.97 0.0841 0.0293 0.0191CBC 29.30 225.83 676.27 0.1590 0.0206 0.0068CCB 28.08 214.84 625.12 0.2695 0.0352 0.0121BBC 28.08 214.84 632.32 0.1581 0.0206 0.0070BCB 30.51 239.26 718.99 0.1259 0.0161 0.00535

Fig. 15. Frequency Response Function (FRF) of untreated CBC hybrid composite.

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It is also interesting to point out from Table 3 that a decrease indamping is observed due to smaller energy dissipation. At the sametime, elasticmodulus of these composites is higher. Fiber agglomerationafter chemical treatment may perhaps lead to smaller energy dissipa-tion. However, interfacial adhesion between fiber and matrix is im-proved due to higher surface contact area between them. The poorerfiber distribution due to the cohesion between fiber surfaces may pro-duce reduced damping.

Fig. 16. (a) Vibrational response time domain and (b) ma

4. Conclusions

The present work describes the effect of layering pattern and alkalitreatment on static mechanical and dynamic characteristics of coconutsheath/banana fiber reinforced hybrid polyester laminates. There wasnot much difference in mechanical properties between pure coconutsheath and banana composites, except flexural strength which washigher for coconut sheath in untreated and alkali treated conditions.

gnified view of vibrational response time free decay.

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The skin-eccentric type of composite exhibited higher tensile and flex-ural properties whereas skin-core composites resulted in good impactstrength in untreated condition. After alkali treatment, higher flexuralstrength was found for skin-core composites (CBC type). Among theskin-eccentric composites, the one with higher relative wt.% of bananafiber (BBC) yielded higher tensile and flexural strength in bothconditions.

A significant influence in natural frequency was found in both typesof hybrid composites due to changes in stiffness brought by the variablelayering pattern. Damping values were higher for all hybrid compositesin alkali treated fiber condition possibly due to greater energy dissipa-tion and restricted molecular mobility at the interface.

Acknowledgments

The authors wish to thank the Center for Composite Materials,Kalasalingam University for their kind permission to carry out thiswork. We also acknowledge the BJT/CNPq and CNPq/DST (Brazil/India) joint projects.

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