flexural behavior of stiffened syntactic foam core sandwich composites

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SJ Amith Kumar and K Sabeel AhmedFlexural behavior of stiffened syntactic foam core sandwich composites

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Original Article

Flexural behavior ofstiffened syntacticfoam core sandwichcomposites

SJ Amith Kumar and K Sabeel Ahmed

Abstract

Sandwich composite with phenolic syntactic foam as a core and glass polymeric lamin-

ates as face skins are widely used in ship hull structures because of its high specific

properties and buoyant nature. In the present work, syntactic foam is prepared by

uniform mixing of cenosphere and phenolic resin in equal proportions. The effect of

stiffening the syntactic foam core by resin-impregnated paper honeycomb structure on

flexural behavior of sandwich composites is experimentally investigated under flatwise

and edgewise loading configuration. Microscopic features of fractured coupons are also

investigated to predict the failure mode. The results reveal that use of resin-

impregnated paper honeycomb structure in syntactic foam to form the core leads to

considerable improvement in the flexural properties of sandwich composites in both

flatwise and edgewise loading configurations. The span of sandwich composite has sig-

nificant role in altering the magnitudes of core shear strength, skin bending strength and

mode of failure. Coupons under edgewise configuration failed in progressive failure

mode; however, in flatwise configuration coupons failed in sudden brittle mode.

Keywords

Syntactic foam, honeycomb structure, sandwich composite, flexural behavior, failure

mode

Journal of Sandwich Structures and Materials

2014, Vol. 16(2) 195–209

! The Author(s) 2013

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DOI: 10.1177/1099636213512498

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Department of Mechanical Engineering, Jawaharlal Nehru National College of Engineering, Shimoga,

Karnataka, India

Corresponding author:

K Sabeel Ahmed, Department of Mechanical Engineering, Jawaharlal Nehru National College of

Engineering, Shimoga 577204, Karnataka, India.

Email: [email protected]

at TEXAS SOUTHERN UNIVERSITY on December 6, 2014jsm.sagepub.comDownloaded from


Introduction

Polymeric composite materials are widely used in the design and construction oflight-weight structural components in aerospace, marine and automotive sectors.Sandwich construction is one such technique that provides high specific properties[1]. Sandwich structural panel consists of two thin but stiff face skins separated by alight-weight core. The face sheets (skins) and core have excellent in-plane and out-of-plane behaviors, respectively [2]. Further, the sandwich construction must exhi-bit good mechanical response under static loading [3–7] and impact loading [8–12].Foam-cored sandwich structures are increasingly used in the aforementioned sec-tors for its low-density characteristics. Structural response of the sandwich com-posite depends mainly on the mechanical properties of the foam core material [13].The purpose of the core is limited to transmit shear stresses between the face skinsand to keep the skin distance approximately constant during the deformationunder transverse loading condition [14]. Syntactic foam is one such core materialwhich yields higher strength and stiffness properties. Syntactic foam-based sand-wich composites are presently being used in wide variety of marine and navalengineering applications [15].

In particular to ship hull, structure are subjected to sagging (where ship is sup-ported more at its ends) and hogging (where ship is supported more in its middle)conditions. Under such conditions, most of the structures of ship hull (such as keel,frame, plating, decks, bulkheads etc) are subjected to out-of-plane and in-planeloads. Sandwich composites in flatwise orientation are commonly used as structuralpanels such as roof, floor and decks, where the face sheets (skins) carry the flexuralload and the core carries the shear load. In beams, sandwich structural componentsare used in the edgewise orientation for higher stiffness. Attempts have been madeto develop sandwich composites with cores of syntactic foams by the fusion ofhollow particles in a matrix and characterize the same for mechanical performance[16–23]. Papa et al. [16] investigated in-plane and out-of-plane responses of sand-wich composites. They found significant anisotropic behavior of sandwich com-posites, which depends on the plane of loading direction. Gupta et al. studied theeffects of length to thickness ratio [17] and density variation [18] on flexuralresponse and failure mode of sandwich composites with core of hollow glassspheres/epoxy-based syntactic foam. They reported that density and length-to-thickness ratios of syntactic foam core have significant influence in alteringthe flexural properties of sandwich composites.

Woldensenbet and Sankella [19] found that the incorporation of nanoclay insyntactic foam results in improved flexural load-bearing capacity of long beamsandwich structure and degrades the flexural property of the short beam sandwichstructure. Ferreira et al. [20] studied the effects of hollow glass microsphere andshort fibre reinforcements on the flexural stiffness, compressive strength, fracturetoughness and absorbed impact energy of epoxy-based syntactic foam-based com-posites. They found that the addition of glass fiber yields slight improvement inflexure stiffness and fracture toughness, but significantly increases the absorbedimpact energy. In contrast, the addition of a small percentage of carbon fibers

196 Journal of Sandwich Structures and Materials 16(2)



results in highest improvement in fracture toughness and flexure stiffness whencompared with unreinforced syntactic foam. Doddamani et al. [21] experimentallyinvestigated that the effects of core thickness (TC) to sandwich thickness (TS) ratioand fly ash weight fractions on the specific bending modulus and strength of sand-wich composites. They observed that specific strength increases with an increase inweight fraction of filler (fly ash) while decreases with increase in TC/TS ratio.Specific modulus decreases with an increase in filler content and increases withTC/TS ratio. Ku [22] found that flexural properties of ceramic hollowspheres reinforced phenolic resin decreases with the increase in the percentage ofceramic hollow spheres in phenolic resin. Islam and Kim [23] developed novelsandwich composites made from syntactic foam core, paper skins and starch asadhesive for interface between core and skins. They concluded that the skin paperhas contributed to increase up to 40% in estimated flexural strength over syntacticfoams.

A review of the literature reveals that syntactic foams have been successfullyused as core material for structural sandwich composite. Most of these studiesinvestigated the effect of variation in the hollow particle weight fractions, density,hollow particle size (radius ratio) and incorporation of short fibers, nano fillers etcon flexural properties of syntactic foams. The benefit of stiffening the syntacticfoam by the incorporation of resin-impregnated paper honeycomb (RIPH) corestructure is not found in the literature till date. The present work explores thispossibility by investigating the flexure behavior of sandwich composites with coreof syntactic foam integrated with RIPH structure.

Materials and methods

Materials

The materials used in the present investigation are commercially available recycledkraft paper (0.07mm thick) supplied by M/S Vasper Eco Solutions Pvt. Ltd.,Bangalore, and Nomex paper (0.05mm thick) supplied by M/S EI Dupont IndiaPvt Ltd, Mumbai, used for fabrication of honeycomb core structure.

Dry fly ash cenosphere of density 450 kg/m3 with mean diameter of 150 mm and awall thickness about 15 mm (�10% of the mean diameter) supplied by M/SCenosphere India Pvt Ltd, Kolkota, and phenolic resin of density 1120 kg/m3

supplied by M/S Romit Resins Pvt Ltd, Raigad, Maharashtra, are used for thefabrication of syntactic foam (blend of cenosphere and phenolic resin).

E-glass fabric of 185 gsm and Epoxy resin LY 556 with Hardener HY 951 (in theratio 10 : 1) supplied by M/S Insulation House, Bangalore, are used for the fabri-cation of face sheets (skins).

Fabrication of cores

RIPH shown in Figure 1 is first fabricated using screen printing technique [24].

Kumar and Ahmed 197



Figure 2 shows the warm press-molding technique used for the preparation ofsyntactic foam core (with and with out RIPH structure). The mold consists of twomild steel plates of size (380� 380� 18) mm and a frame in between. The spacingbetween the mold plates is maintained at 12.5mm using a metal frame. Moldsurfaces are coated with silicon grease and then wrapped with aluminium foil toensure easy removal of foam core after curing. Syntactic foam is made by the blendof cenosphere and phenolic resin uniformly mixed in the ratio of 50 : 50 by weight.The mixture is then thoroughly packed in the inner cavity of the metal frame whichis kept on the lower mold plate. For preparation of the stiffened syntactic core (i.e.core of syntactic foam incorporated with RIPH structure), the blend is packed inthe RIPH structure kept in the metal frame. After packing the blend, the moldplates are firmly clamped and then heated to cure the syntactic foam compositionfor up to 15 minutes at a temperature of 140–150�C.

Fabrication of sandwich composites

For the preparation of sandwich composite, face sheets (skins) from E-glass/epoxycomposites with fiber mass fraction of about 0.5 are prepared by wet lay-up tech-nique. Silicon grease is first applied on the surfaces of the mold to facilitate easyremoval of the composite after curing. Epoxy resin system (epoxy +10% by weight

Heat

Heat

Top Mold Plate

Bottom Mold Plate

Metal Frame

Syntactic foam + RIPH

Mold plates areclamped usingC-clamps

Figure 2. Warm press molding process. RIPH: resin-impregnated paper honeycomb.

Figure 1. Resin-impregnated paper honeycomb (RIPH) cores. (a) Nomex (N) and

(b) Kraft (K).




hardener)-impregnated E-glass fabric layers are laid down on the surface of themold one over the other until the desired thickness of the skin (1.5mm) is reached.A roller is used to achieve uniform distribution of resin system throughout the layersurface. Thereafter, already prepared syntactic foam core is placed on the layersurface. This is then followed by laying resin-impregnated E-glass fabric on to thecore surface till the desired thickness of the skin (1.5mm) is reached. Finally, theentire configuration of the sandwich composite is enclosed with vacuum bag tothe mold as shown in Figure 3. Vacuum is then created to consolidate and cure thesandwich composite configuration under a vacuum pressure of 22mm of Hg usinga vacuum pump of capacity 100LPM for up to 2 hours. The sandwich panel is thenremoved from the mold and further cured at room temperature for at least 24hours and then post-cured at 100� 3�C for 1 hour before use. The thickness ofthe entire sandwich panel is maintained uniform at 15.5mm. Figure 4 representsthe configuration of different types of developed sandwich composites. Table 1presents the description of various sandwich composites prepared.

Three-point bending test

Three-point bending (TPB) tests are conducted as per ASTM C 393 standard, thegeometry of the test coupons shown in Figure 5 are detailed in Table 2. To evaluatethe effect of span on flexure properties of sandwich composites, coupons with two

SF

Bottom face skin

Top face skin

SFKSFN

Figure 4. Configuration of sandwich composites. SF: syntactic foam; SFN: syntactic foam with

resin-impregnated nomex honeycomb; SFK: syntactic foam with resin-impregnated kraft

honeycomb.

Mold Plate

Metal Frame withgasket seal

Glass/epoxy face sheets + syntactic foam Vacuum Bag

To Vacuum pump

Figure 3. Vacuum bag molding process.

Kumar and Ahmed 199



L

L

t t

B

P

tf

C

T

S

S

(a)

L

L t

P

B

S

T

tf

t C

S

(b)

Figure 5. Geometry of three-point bending test coupons. (a) Flatwise (b) Edge wise.

Table 2. Geometry of sandwich composite for three-point bending (TPB) test.

#

Configuration

TPB

Total

length (LT) mm

Span

(LS) mm

Width

(b) mm

Depth

(d) mm

Long beam Flatwise 300 200 40 15.5

Edgewise 300 200 15.5 40

Short beam Flatwise 100 62 25.4 15.5

Edgewise 100 62 15.5 25.4

Table 1. Designation of sandwich composites.

Sandwich designation Core material

SF Syntactic foam

SFN Syntactic foam with resin-impregnated nomex honeycomb

SFK Syntactic foam with resin-impregnated kraft honeycomb




different spans are considered for the test. Tests are carried out at room tempera-ture using universal testing machine (Instron 3382) having a capacity of 100 kN.This machine is interfaced with computer using the data acquisition system with asoftware Instron’s Series IXTM/s. The rate of loading was maintained at 0.5mm/min. Figure 6 shows the configuration of TPB test.

Under flatwise TPB, load-carrying capacity of the sandwich composite panelsare usually determined by applying transverse load in a direction normal to theplane of the skins (Figure 6(a)). The mode of failure under flatwise TPB test isdetermined from skin bending stress (s) using equation (1) and core shear stress (�)using equation (2) [25].

� ¼PLS

2tf tS þ tCð Þbð1Þ

� ¼P

tS þ tCð Þbð2Þ

where, P is the peak load, LS is the span, tS is the thickness of the sandwich coupon,tC is the core thickness, tf is the skin thickness and b is the width of the sandwichcomposite.

Figure 6. Three-point bending test configuration. (a) Flatwise and (b) Edgewise.

Kumar and Ahmed 201



However, edgewise TPB test involves the determination of the load-carryingcapacity of the sandwich coupons by applying a transverse load in a directionparallel to the plane of sandwich skins (Figure 6(b)). In both flatwise and edgewisetests, flexural strength (�f) and modulus (Ef) can be calculated using equations (3)and (4), respectively.

�f ¼3PLs

2bd2ð3Þ

Ef ¼L3sm

4bd3ð4Þ

where ‘‘m’’ is the slope of the initial portion of the load-deflection curve and is ameasure of the stiffness of the material, b is the width and d is the depth of the testcoupon (b¼B, d¼ ts for flatwise test and b¼ ts and d¼B for edgewise test).

Results and discussion

Flatwise TPB test

Figures 7 and 8 depict the flexural behavior and mode of failure of sandwichcoupons under TPB test in flatwise loading configuration. The load versus deflec-tion plots of TPB test reveal that all sandwich coupons behaved like linear elasticbrittle materials [15], with minimal plastic deformation before fracture.

Under flatwise TPB, load increases linearly with deflection until the peak load.Since the skins are perfectly bonded to the core, they are linearly strained togetherupon application of load until peak load is reached. Sudden drop in the load-deflection curve at the end of the linear elastic region in Figure 7 is an indicationof the initiation and propagation of crack in the skin on the tensile side of the

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7 8

Deflection, mm

Loa

d, k

N

SF

SFN

SFK

(a)

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1 1.5 2 2.5

Deflection, mm

Loa

d, k

N

SF

SFN

SFK

(b)

Figure 7. Load-deflection plots of sandwich composites coupons under flatwise TPB. SF: syn-

tactic foam; SFN: syntactic foam with resin-impregnated nomex honeycomb; SFK: syntactic

foam with resin-impregnated kraft honeycomb. (a) Span¼ 200 mm and (b) Span¼ 62 mm.




sandwich coupon. Crack developed on the tension side results in the bending fail-ure of skin (Figure 8(a)). It then propagates towards the compression side of thesandwich composite, leading to shear failure of the core (Figure 8(b)). This can beseen by a plateau region after peak load and/or sudden drop in the load-deflectionplot. At this region, deflection increases with no/little increase in the load indicatingthe loss of stiffness of the sandwich coupon. Figure 9 shows the effect of stiffeningthe syntactic foam on the skin bending and core shear stress of sandwich compositecoupons with two different spans.

It can be seen that both the skin bending and core shear stresses are higher forstiffened core sandwich composites. However, the skin bending stress decreaseswhereas the core shear stress increases with the decrease in the span. This indicatesthat the resistance offered by the skin to bending decreases with the decrease in thespan, whereas the resistance offered by the core to bending increases with the

Figure 8. Mode of failure under flatwise TPB. (a) Skin bending failure and (b) Core shear

failure.

Kumar and Ahmed 203



decrease in the span. For both the span lengths, skin offers greater resistance and isthe main load-bearing member as can be noticed from higher values of skin bend-ing stress compared to core shear stress. However, the overall failure of the couponis due to the combination of both tensile failure of the skin and shear failure of thecore for all types of sandwich composites under consideration. Figure 10 shows theeffect of stiffening the syntactic foam on the flexural strength and modulus ofsandwich composites.

It can be seen from the figure that both flexural strength and modulus are higherfor stiffened core sandwich composites when compared to bare syntactic foam corecomposites. This increase in the flexural properties is due to the greater resistanceoffered by the confined state of syntactic foam in resin-impregnated honeycombstructure to bending load and deflection. Among all the cases, sandwich compositesyntactic foam with resin-impregnated kraft honeycomb (SFK) depicted consider-ably higher flexural properties. Figure 10 also shows the effect of span on flatwiseflexural properties of sandwich composites. Both flexural strength and modulusincreases with the increase in the span of sandwich coupons. Stiffened syntacticfoam cored sandwich coupons exhibited 30% increase in the flexural properties atboth 200mm and 62mm span when compared to only syntactic foam core

0

10

20

30

40

50

60

70

80

SF SFN SFK

Sandwich coupons

Fle

xura

l str

engt

h, M

Pa

Span 200 mm

Span 62 mm

(a)

0

1

2

3

4

5

6

7

8

SF SFN SFK

Sandwich coupons

Fle

xura

l mod

ulus

, GP

a

Span 200 mm

Span 62 mm

(b)

Figure 10. Flexural strength and modulus of sandwich composites under flatwise TPB.

0

20

40

60

80

100

120

140

160

SF SFN SFK

Sandwich coupons

Ski

n be

ndin

g st

ress

, MP

a Span 200 mm

Span 62 mm

(a)

0

1

2

3

4

5

6

SF SFN SFK

Sandwich coupons

Cor

e sh

ear

stre

ss, M

Pa

Span 200 mm

Span 62 mm

(b)

Figure 9. Skin bending and core shear stresses for sandwich composites.




sandwich coupons. All coupons under flatwise TPB failed in brittle mode due toshear failure of the core, tensile failure of the skins and/or interfacial debonding ofskins from the core. Similar observation was made by Gupta and Woldesenbet [18]and Islam and Kim [23].

Edgewise TPB test

Figure 11 illustrates the flexural behavior of sandwich coupons under edgewiseTPB test. In edgewise TPB configurations, all sandwich coupons behaved likelinear elastic brittle materials. The trend in the load-deflection plots is similar forall types of sandwich coupons. The high failure load of sandwich coupons is due tothe presence of the vertical skins on either side of the core, which increases the load-carrying capacity. Unlike flatwise TPB test, the initiation of crack that takes placeon tension side constitutes both skin and core. The propagation of crack is resistedby both the skin and the core simultaneously leading to the progressive failure ofsandwich composites as can be seen in Figure 12. The plateau region in the load-deflection plots indicates the loss of stiffness in the material.

The flexural properties of various sandwich composites are compared inFigure 13. The decrease in the flexural strength and modulus of sandwich compos-ites when compared to flatwise test indicates the effect of orientation of the sand-wich coupons. Further, it can be seen that the flexural properties of sandwichcomposites are significantly improved with the incorporation of RIPH structurein syntactic foam. Figure 13 also explains the effect of span on the flexural proper-ties. The flexural strength of short-span coupon is found to be higher than that oflong-span coupons, and vice versa in the case of flexural modulus. Thismay be due to the effect of geometric parameter (depth) of the sandwich coupons(Table 2).

The increase in the flexural strength and modulus of stiffened syntactic foamcore sandwich coupons at 200mm span is 25% and 30%, respectively, when

0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3 4 5 6 7

Deflection, mm

Loa

d, k

N

SF

SFN

SFK

(a)

0

0.5

1

1.5

2

2.53

3.5

4

4.5

5

0 0.5 1 1.5 2 2.5 3 3.5

Deflection, mm

Loa

d, k

N

SF

SFN

SFK

(b)

Figure 11. Load-deflection plots of sandwich composites coupons under edgewise TPB. SF:

syntactic foam; SFN: syntactic foam with resin-impregnated nomex honeycomb; SFK: syntactic

foam with resin-impregnated kraft honeycomb. (a) Span¼ 200 mm and (b) Span¼ 62 mm.

Kumar and Ahmed 205



compared to bare syntactic foam cored sandwich composite, whereas both proper-ties are found to be increased by 20% at 62mm span.

Microscopy

SEM study is carried out to investigate the mode of fracture of sandwich coupons.The fractured surfaces of selected samples are examined using JEOL JSM-840Asoftware controlled scanning electron microscope. The instrument is operated at10 kV and 5 kV. Samples for examination are obtained by cutting sections about 3to 4mm in length just below the fractured zone. The fractured surfaces of thesamples are sputter-coated with a thin layer of gold to minimize the chargingproblem using JEOL sputter ion coater.

Figure 14 shows the micrograph of a failed coupon, taken normal to the tensionsurface, at two different magnification levels. Broken fibers from skin and inter-facial delamination of skin from the core can be seen in Figure 14(a). The fractureof core can be observed in micrograph shown in Figure 14(b), taken at a highermagnification level. Core cracking and cenosphere debris are seen to be the dom-inant failure modes in the core.

Figure 12. Mode of failure under edgewise TPB.

0

5

10

15

20

25

30

35

40

45

SF SFN SFK

Sandwich coupons

Fle

xura

l str

engt

h, M

Pa

Span 200 mm

Span 62 mm

(a)

0

0.5

1

1.5

2

2.5

3

3.5

SF SFN SFK

Sandwich coupons

Fle

xura

l mod

ulus

, GP

a

Span 200 mm

Span 62 mm

(b)

Figure 13. Flexural strength and modulus of sandwich composites under edgewise TPB.




Conclusion

Stiffened syntactic foam core sandwich composites are developed using RIPHstructure in syntactic foam as core and GFRP laminates as face sheets (skins).The developed composites are tested for bending in both flatwise and edgewise

Figure 14. Fractured features on tension side.

Kumar and Ahmed 207



configuration. Following important conclusions are drawn from thisinvestigation.

1. Stiffening the core with RIPH structure has significant effect on skin bendingand core shear stresses.

2. For all types of sandwich composites, skin bending stresses are higher for higherspan and core shear stresses are higher for lower span.

3. Use of RIPH structure in syntactic foam to form the core leads to significantimprovement in the flexural properties of sandwich composites in both flatwiseand edgewise loading configurations due to stiffening effect of RIPH structure.

4. The effect of span on flexural properties reveal that under flatwise loading con-figuration, both the flexural strength and modulus of sandwich composites arefound to be high for higher span. However, under edgewise loading configur-ation, flexural strength is found to be high for short span, whereas flexuralmodulus is found to be high for long-span coupons.

5. Fiber splitting on tension side, skin-core delaminations and cenosphere debrisformation are the predominant modes of failure under flatwise test, whereas fiberbreakage and core cracking are significant modes of failure under edgewise test.

6. The remarkable improvement in the flexural properties by the incorporation ofRIPH structure in syntactic foam-cored sandwich composites makes it a poten-tial material for structural panels and beams application.

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flexural behavior of stiffened syntactic foam core sandwich composites

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