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 DOI: 10.1177/0731684413492867

2013 32: 1370 originally published online 20 June 2013Journal of Reinforced Plastics and CompositesSJ Amith Kumar and K Sabeel Ahmed

compositesCompression behavior and energy absorption capacity of stiffened syntactic foam core sandwich

  

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Original Article

Compression behavior and energyabsorption capacity of stiffened syntacticfoam core sandwich composites

SJ Amith Kumar and K Sabeel Ahmed

Abstract

Phenolic syntactic foam sandwich composites are potential materials for aerospace and marine applications because of

their good strength to weight ratio, stiffness to weight ratio, fire resistance and better energy absorption characteristics.

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 (RIPH) structure on compression

behavior and energy absorption capacity of sandwich composites is experimentally investigated under flatwise and

edgewise loading configuration. Bare RIPH core and only syntactic foam core sandwich composites are also investigated

for comparison purpose. The results show that RIPH incorporated syntactic foam has the highest compressive proper-

ties and energy absorption capacity compared to only syntactic foam core sandwich composites. Microscopic features of

syntactic foam fragments tested under flatwise compression are also investigated to predict the failure mode.

Keywords

Syntactic foam, honeycomb structure, sandwich composite, compression behavior, energy absorption

Introduction

Sandwich composites are widely used as structuralmaterials in aerospace, marine and automobile sectorsbecause of their light weight, high strength-to-weightand stiffness-to-weight ratios, good buckling resistance,good energy-absorbing capacity and design versatility.1

Variety of core materials have been developed for sand-wich structures, such as honeycombs of various mater-ials,2–6 polymeric foams,7–11 polymeric foam filledhoneycombs,12–14 etc. Attempts have also been madeto develop sandwich composites with cores of syntacticfoams by the fusion of hollow particles (such as ceno-sphere, glass micro-balloons, epoxy hollow spheres, etc)in a matrix and characterize the same for mechanicalperformance.15–24

Gupta et al. investigated flatwise compressive prop-erties15 of cenosphere/epoxy based syntactic foam, flex-ural properties16 of glass micro-balloons/epoxy basedsyntactic foam core sandwich composites for differentradius ratios of cenospheres and micro-balloons. Theyconcluded that, the decrease in the radius ratio of ceno-spheres results in increase in the compressive strengthand modulus. The core shear stress and the skin

bending stress in three-point and four-point bendingare not affected by the radius ratio of micro-balloons.Shivakumar et al.17 developed low-cost syntactic foammade from fly ash, using a resole phenolic resin binder ata low volume percentage of about 6%. The compression,tension, shear and fracture toughness tests were con-ducted on this foam sample. They found that compres-sion strength appears to be a linear function of densitywhich in turn is a function of binder content. The com-pressive, tensile, shear and fracture toughness propertiesare found to be comparable or better than commerciallyavailable core materials. Zhang and Zhao18 manufac-tured aluminum matrix syntactic foams with low-cost porous ceramic spheres of diameters between0.25 and 4mm by pressure infiltration casting.

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: sabil_k@yahoo.com

Journal of Reinforced Plastics

and Composites

32(18) 1370–1379

! The Author(s) 2013

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DOI: 10.1177/0731684413492867

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Themechanical response and energy absorption of thesefoams under static and dynamic conditions were inves-tigated. They concluded that the energy absorption cap-acity of these foams is better than those with onlyaluminum foams. The effects of heat-treatment onmechanical properties of syntactic carbon foams con-taining hollow carbon microspheres in phenolic resinwere studied by Liying Zhang.19 Their results showedthat the introduction of more interval voids during car-bonization resulting in a reduction of the mechanicalproperties. Compressive and tensile characteristics ofvinyl ester matrix syntactic foams were investigated byGupta et al.20 They found that these foams have higherfailure strain under compressive loading conditionscompared to epoxy matrix syntactic foams.Compressive moduli of these foams were found to belower than that of neat vinyl ester resin, but their specificcompressive modulus was higher. Also, the tensilemodulus was found to be higher than that of the neatresin and compressive modulus of same type of syntacticfoam. Jhaver and Tippur21 presented the processing andcompression behavior of lightweight syntactic foam-filled aluminum honeycomb composites. Differentfoam-filled honeycomb composites were prepared byvarying the volume fraction of micro-balloons in thesyntactic epoxy foam while keeping the volume fractionof the metallic network the same. Th foam-filled honey-comb composites showed 26–31% and 36–39% increasein the elastic modulus and plateau stress, respectively,along the W-direction, when compared to the conven-tional syntactic foam having the same volume fraction ofhollow micro-balloons. Samsuddin et al.22 developedand characterized epoxy syntactic foam filled withepoxy hollow spheres. They suggested that this foamcould be a potential material for various engineeringapplications as it exhibited relatively high compressivestrength and modulus. Swetha and Kumar23 preparedhollow glass microsphere/epoxy based syntactic foam bystir casting process. Their experimental investigationsreveal that the compressive properties decrease withthe increase in hollow microsphere content. Taoet al.24 developed syntactic foam from ceramic micro-sphere (with different ranges of microsphere diameter)and aluminum 6082 alloy, and characterized the samefor tension, shear and compressive properties. The effectof microsphere diameter on these properties was inves-tigated. They found that the microsphere diameter hasno significant effect on tensile and shear properties,whereas the compressive strength decreases with theincrease in the size of microsphere.

A review of the literature reveals that, syntacticfoams of different materials have been successfullyused as structural materials. However, most of the stu-dies deal with the effect of change in volume fraction,density, microsphere particle size (radius ratio), etc, on

mechanical properties of syntactic foams. The benefit ofstiffening the syntactic foam by the incorporation ofresin impregnated paper honeycomb (RIPH) corestructure is not found in the literature till date. Thepresent work explores this possibility by investigatingthe compression behavior and energy absorption cap-acity of sandwich composites with core of syntacticfoam integrated with RIPH structure.

Energy absorption of foam core sandwich composites

Energy absorption capacity of foam core sandwichcomposite is usually determined under flatwise com-pression test as per ASTM C365 standard. In flatwisecompression test, compressive properties, energyabsorption capacity and crush force ratio of foamcore sandwich composites are determined by applyinga load in a direction normal to the plane of the facings.Figure 1 shows typical nature of the stress–strain plotfor foam core sandwich composites under flatwisecompression.

The total energy absorbed by the foam core sand-wich composite EA is the summation of the modulus ofresilience (area ABE) and modulus of toughness (areaBCDE). Hence,

EA ¼1

2� �yield � "yield þ �yield � "Crush: ð1Þ

However, in the case of foam cores, compressive strainat yield point is much less and is difficult to measurethan the compression strain at crushing point (i.e."yield<<"crush). Hence, the first term of equation (1)can be neglected. Thus,

EA ¼ �yield � "Crush: ð2Þ

Further, the structural effectiveness of the foam coresandwich composites is determined by crush force

A

BC

DE

σyieldσaverage

εyieldεcrush

σcrush

Strain

Stre

ss

Figure 1. Typical stress–strain plot of foam core sandwich

composite under flatwise compression.

Amith Kumar and Sabeel Ahmed 1371

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ratio, �25 given by equation (3).

� ¼Favg

Fyieldð3Þ

where Favg is the load corresponding to �average andFyield is the load corresponding to �yield (Figure 1). Itcan be seen from the equation (2) that the energyabsorption capacity of the foam core sandwich com-posites can be improved by increasing its compressionstress at yield point (�yield) and/or compression strain atthe crushing point ("crush). It has been proved thatincrease in the volume fraction of resin binder in syn-tactic foam leads to the improvement in �yield.

17,23,26

This in turn increases the density of the foam material,because of higher density of resin binder compared tothe density of cenosphere, resulting in overall increasein the weight of the sandwich composite. In the presentwork, an attempt is made to investigate the effect ofRIPH structure in syntactic foam on compressive prop-erties and energy absorption capacity of sandwich com-posites. Hence, changes in the compressive properties,energy absorption capacity and mode of failure can beattributed to RIPH structure integrated with syntacticfoam core sandwich composites. Bare RIPH core andonly syntactic foam core sandwich composites are alsoinvestigated for comparison purpose.

Materials and methods

Materials

Materials used in the present investigation are: com-mercially available recycled Kraft paper (0.07mm

thick) supplied by M/S Vasper Eco Solutions PvtLtd., Bangalore and Nomex paper (0.05mm thick) sup-plied by M/S EI Dupont India Pvt Ltd, Mumbai areused for fabrication of honeycomb core structure.

Dry fly ash cenosphere of density 450 kg/m3 suppliedby M/S Cenosphere India Pvt Ltd, Kolkota and phen-olic resin of density 1120 kg/m3 supplied by M/S RomitResins Pvt Ltd, Raigad, Maharastra are used for thefabrication of syntactic foam (blend of cenosphere andphenolic resin).

E-glass fabric of 185 gsm and Epoxy resin LY 556with Hardener HY 951 (in the ratio 10 : 1) supplied byM/S Insulation House Bangalore are used for the fab-rication of face sheets.

Fabrication of resin-impregnated paperhoneycomb structure

The process of fabrication of RIPH structure is shownschematically in Figure 2. Nomex/Kraft paper drawnfrom the roll are first cut to the required size and thencoated with epoxy resin as per the pattern shown inFigure 2, using screen printing technique. The coatedsheets of paper are stacked one over the other in such away that the resin coated areas are displaced half-widthwith successive layers. Stacked sheets are then cured at150�C for 30min under a hot press. The block of stackthus produced is placed over a steam bath to soften thepaper, and then sliced into strips of required core thick-ness (Figure 2(a)). The strips obtained by cutting theblock are then pulled in concertina fashion to open upthe sheets giving a hexagonal-cell honeycomb structure(Figure 2(b)). The expanded structure is then given

H

LW

H

L

TPaper Roll Cut to

requiredsize

Epoxycoating

Ungluedarea

Screenprinting Stacking

of sheets Consolidation Slicing torequiredthickness

(a)

(b)

Figure 2. Process of manufacturing paper honeycomb cores.(a) Process of fabrication of paper honeycomb core and (b) Expanded

paper honeycomb core.

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multiple dips in phenolic resin and cured at 150�C toproduce phenolic RIPH structures with good structuralrigidity. Nomex and Kraft paper based RIPH corestructures shown in Figure 3 are abbreviated as Nand K respectively in further discussion.

Fabrication of syntactic foam core/sandwichcomposites

Figure 4 shows the warm press molding technique usedfor the preparation of syntactic foam core (with andwithout RIPH structure). The mold consists of twomild steel plates of size (380mm� 380mm� 18mm).The spacing between the mold plates is maintained at12.5mm using a metal frame. Mold surfaces are coatedwith silicon grease and then wrapped with aluminumfoil to ensure easy removal of foam core after curing.Syntactic foam is made by the blend of cenosphere andphenolic resin uniformly mixed in the ratio of 50 : 50 byweight. The mixture is then thoroughly packed in theinner cavity of the metal frame which is kept on thelower mold plate. For preparation of the core of syn-tactic foam with RIPH, the blend is packed in theRIPH structure kept in the metal frame. After packingthe blend, the two mold plates are firmly clamped andthen heated. The core was cured at 140–150�C for15min in between the mild steel mold plates followedby room temperature curing for at least 24 h before its

use in the fabrication of sandwich composite. Provisionis also made in the mold to allow the hot gases toescape during the process of curing. The core of syn-tactic foam with RIPH structure is termed as ‘‘stiffenedsyntactic foam core’’. Syntactic foam core, syntacticfoam with Nomex RIPH and syntactic foam withKraft RIPH cores are abbreviated as SF, SFN andSFK, respectively in the further discussion.

For fabrication of sandwich composites, glass/epoxylaminates with fiber mass fraction of about 0.5 is firstprepared by wet lay up technique and then vacuumbonded to the core to form the face sheets as shownin Figure 5. The thickness of the face sheets is main-tained at 1.5mm. The sandwich composites are curedfor 24 h at room temperature and then post cured at100� 3�C for 1 h before being used for the preparationof test coupons. The configuration of different types ofdeveloped sandwich composites is shown in Figure 6.Table 1 presents the description of various sandwichcomposites prepared.

Density determination

The hollow volume enclosed within the cenospheregives rise to closed cell porosity and is the characteristicof the material. The porosity that arises during manu-facturing of the core due to the entrapment of air inthe cenosphere–phenolic resin blend is the open

(a) (b)

Figure 3. Resin impregnated paper honeycomb cores. (a) Nomex (N) and (b) Kraft (K).

Heat

Heat

Top Mold Plate

Bottom Mold Plate

Metal Frame

Syntactic foam + RIPH

Mold plates areclamped usingC-clamps

Figure 4. Warm press molding process.

Amith Kumar and Sabeel Ahmed 1373

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cell porosity. The overall density of the syntactic foamdepends on these porosities. The density of syntacticfoam with and without RIPH structure is measured inaccordance with ASTM C271 standard. Three identicalcoupons of each type of core are used to measure thedensity, and the average results are reported in Table 2.It can be seen from the table that incorporationof RIPH in syntactic foam leads to marginal increasein the density due to significant reduction in the opencell porosities. Similar observation was made by Zhangand Zhao.18

Compression tests

Two types of compression tests are conducted onsandwich composites namely, flatwise (to determine

the properties under out-of-plane load) and edge-wise (to determine properties under in-plane load).ASTM C365 and C364 standards are followedfor conducting flatwise and edgewise testsrespectively. The geometry of the test coupons isshown in Figure 7.

Compression tests are carried out at room tempera-ture using UTM (Instron 3382) having a capacity of100 kN. This machine is interfaced with computerusing the data acquisition system with a softwareInstron’s Series IXTM/s. The rate of loading ismaintained at 0.5mm/min as per selected ASTM stand-ard. For each type of sandwich composites, three iden-tical coupons are tested and the average results arereported. Figure 8 shows the configuration of compres-sion tests.

Mold Plate

Metal Frame withgasket seal

Glass/epoxy face sheets + syntactic foam Vacuum Bag

To Vacuum pump

Figure 5. Vacuum bagging.

S1, S2 S3 S4,S5

RIPH

Bottom face sheet

Top face sheet

SF+RIPHSF

Figure 6. Configuration of sandwich composites.

Table 1. Designation of sandwich composites.

Sandwich

designation Core material

S1 Nomex honeycomb (N)

S2 Kraft honeycomb (K)

S3 Syntactic foam (SF)

S4 Syntactic foam with Nomex honeycomb (SFN)

S5 Syntactic foam with Kraft honeycomb (SFK)

Table 2. Density and matrix porosity of core material.

Core material

(Table1)

Density of core

material (kg/m)3Open cell

porosity (%)

N 48 –

K 54 –

SF 576 10.28

SFN 624 2.8

SFK 626 2.49

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Results and discussions

Flatwise compression test

In flatwise compression, load is mainly taken up by thecore and the contribution of skin to the compressiveproperties is very small.27 Figure 9(a) shows the com-pressive stress–strain response for S1 and S2 coupons.The initial portion of the curve is linear at lower strainindicating the elastic bending of thin cell walls of RIPHcore structure. The sudden drop in the stress–straincurve is an indication of overall distortion of the cellwalls followed by nearly constant level of stress as indi-cated by the plateau region of the stress–strain curve.At this portion, the strain increases significantly until

complete failure of the core material. Further increasein the load results in complete densification of the coreas revealed by the sharp rise in the stress–strain curve.Unlike in S1 and S2, no drop in the stress–strain curveis noticed for S3, S4 and S5 coupons. The three types ofsyntactic foam based sandwich coupons (S3, S4 and S5)showed a similar compressive response exhibiting threedistinct regions namely; linear elastic region wherestress is proportional to the strain, plateau regionwhere the stress is nearly constant and the region ofsteep increase in stress for small increase in strain.24

However, the nature of the curve in the plateauregion of S3 coupon exhibits its strain hardening char-acteristic whereas S4 and S5 coupons exhibit their plas-tic characteristics.9 The energy absorption capacity of

Load

SandwichCoupon

15.5 mm

76.5 mm

76.5 mm

Top Platen

Bottom Platen

Aluminium Cap

SandwichCoupon

Top Platen

Bottom Platen

Load Load

130 mm

15.5 mm

50 mm130 mm

(a)

(b)

Figure 7. Compression test coupons. (a) Flatwise compression and (b) Edge wise compression.

Sandwich Coupons

(a) (b)

Figure 8. Compression test configuration. (a) Flatwise compression and (b) Edge wise Compression.

Amith Kumar and Sabeel Ahmed 1375

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the foam is significantly influenced by the stress–strainbehavior and density of the foam core.28 Area underthe linear region of the stress–strain plot represents theenergy required for the onset of crack in the core,whereas the area under plateau region represents theenergy required in propagating the cracks in the coreleading to progressive collapse of the core material asnoticed in Figure 10. The absorption of energy is ter-minated at the end of the plateau region where the coreis completely crushed. No distinct crack point is noticedin the flatwise compression of all types of syntactic

foams based sandwich composites.15 It can be seenfrom Figure 10 that the propagation of the crack iscontinuous in the case of S3 but intermittent in thecase of S4 and S5 due to the presence of RIPHstructure.

The results of the compression test are summarizedin Table 3. The compressive modulus, which is themeasure of the stiffness of the material, is obtainedfrom the slope of the initial portion of the stress–strain curves. It is clear from the Table 3 that thebare RIPH core (without foam) sandwich composites

0

0.5

1

1.5

2

2.5

0 0.2 0.4 0.6 0.8 1

Compression Strain

Com

pres

sion

Str

ess,

MPa

S1

S2

00

2468

101214161820

0.05 0.1 0.15 0.2 0.25 0.3

Compression Strain

Com

pres

sion

Str

ess,

MPa

S3

S4

S5

(a) (b)

Figure 9. Stress–strain plots under flatwise compression. (a) S1. S2 and (b) S3, S4, S5.

Intermittent cracks within RIPH cells Continuous propagation of crack

(a) (b)

Figure 10. Fracture of coupons under flatwise compression.

Table 3. Summary of the results of flatwise compression test.

Sandwich

coupons

Compression strength

at yield (�yield) (MPa)

Crushing compression

strength (�crush) (MPa)

Compressive

modulus (MPa)

Energy absorption

per unit volume (EA)

(MPa)

Crush force

ratio (�)

S1 1.35 2.08 49.7 – –

S2 1.49 2.24 50.65 – –

S3 7.09 16.81 1326.47 1.87 0.98

S4 11.64 17.98 2465.58 3.02 0.99

S5 12.42 17.63 2473.39 3.33 0.98

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depicted very low compressive properties indicatingtheir limitation for use in structural applications. Inmarine applications, the stability and the performanceof the ships are enhanced by using materials of highstiffness and strength.29 It can be seen from the tablethat incorporation of RIPH in syntactic foam (S4, S5)leads to considerable improvement in compressionbehavior and energy absorption capacity when com-pared to foam sandwich composites (S3). This may bedue to the fact that the RIPH structure prevents thesyntactic foam from crushing or disintegration, whilethe syntactic foam provides constraints for the RIPHstructure from lateral deformation under flatwise com-pression load. The confined state of the syntactic foamby the walls of RIPH structure inherently stiffens thecore material. This influences the enhancement of thecompressive properties and energy absorption capacityat low level of stress promising its applications inmarine structures. Similar behavior was observed insyntactic foam-filled aluminum honeycombs by Rahuland Tippur.21 It is found that, the rise in the compres-sive stress (stress at yield) of S4 and S5 when comparedto S3 sandwich coupons is 64.17% and 75.2%, respect-ively. Also, the rise in the compressive modulus of S4and S5 when compared to S3 is 85.87% and 86.46%,respectively. Hence, the increase in the compressiveproperties of sandwich coupons S4 and S5 is may bedue to the confined state of the syntactic foam in RIPHstructure and also from the fact that the reduction inopen cell porosity18 (Table 2). The energy absorption ofsyntactic foam and RIPH integrated syntactic foamcore are established by the area under the stress–strain plot in accordance with equation (2). The crushforce ratio calculated using equation (3) is almost unityfor S3, S4 and S5, indicating their failure in a stable andprogressive manner, satisfying the ideal condition of anenergy absorber.25

Edgewise compression test

Unlike flatwise compression, the load under edgewisecompression is mainly taken up by the skins due totheir high compressive strength and stiffness than thatof core material.27 The edgewise compression test isconducted for syntactic foam based composites only,as the bare RIPH core does not sustain the edgewisecompression load. Since the skins are perfectly bondedto the core, they are linearly strained together uponapplication of the load until peak stress is reached asshown in Figure 11. At the end of the linear elasticregion of S3 coupons, crack initiation takes place inthe core and a sudden drop in the curve can be seenas shown in Figure 12(a). This is then followed by aplateau region representing the propagation of crack inthe core. The sudden drop in the stress–strain curve at

the end of the plateau region is an indication of com-plete failure of the coupons by vertical splitting of thecore. Similar observation was made by Gupta et al.27

for epoxy based syntactic foam. In the case of S4 andS5, at the end of the linear region, delamination

0

5

10

15

20

25

0 0.005 0.01 0.015 0.02 0.025 0.03

Compression Strain

Com

pres

sion

Str

ess,

MPa

S3

S4

S5

Figure 11. Stress–strain plots under edgewise compression.

Crack Propagation

Delamination Skin failure

(a)

(b)

Figure 12. Fracture of coupons under edgewise compression.

(a) S3 and (b) S4, S5.

Table 4. Results of edgewise compression test.

Sandwich

coupons

Compression strength

(MPa)

Compressive modulus

(MPa)

S3 19.67 1771.84

S4 23.58 1971.57

S5 23.45 1973.69

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between the core and the skin is observed (Figure 12(b))and the load is transferred to the core. This behavior isindicated by the drop in the stress–strain curves. In thisregion, the generation of cracks in the core is restrictedby the presence of RIPH structure and the failure ismainly due to skin failure in terms of matrix crackingand face wrinkling. It can be seen that the compressivebehavior for S4 and S5 sandwich coupons are almostthe same. The results of the edgewise compression testare presented in Table 4. The compressive stress andmodulus of S4 and S5 coupons are higher than that ofS3 by 19 and 11%, respectively.

Microscopic features of syntactic foamcore

Flatwise compression test coupons are subjected tosevere crushing ensuing in the separation of syntacticfoam fragments from the core. Fractured features ofsyntactic foam in various localized fracture modes arestudied from the microscopic observation of syntacticfoam tested under flatwise compression. Micrographsshown in Figure 13 exhibit the fractured features ofsyntactic foam fragments at different levels of magnifi-cation. The micrograph in Figure 13(a) reveals the pres-ence of matrix porosity and failure of the syntacticfoam by shear cracks. The micrograph shown inFigure 13(b) shows the extensive formation of debrisof cenosphere and phenolic matrix.

Conclusions

Stiffened syntactic foam core sandwich composites aredeveloped using RIPH structure in syntactic foam ascore and GFRP laminates as face sheets. The developedcomposites are tested for both flatwise and edgewisecompression. The major load bearing member in flat-wise compression is the core, whereas in the edgewisecompression, it is the skin. Following important con-clusions are drawn from this investigation.

1. Bare RIPH core (without foam) sandwich compos-ites exhibit very low compressive properties com-pared to syntactic foam based sandwich composites.

2. For all types of syntactic foam based sandwich com-posites, the strain corresponding to the peak stress inedgewise compression and yield stress in flatwisecompression is about 2.5%.

3. The use of RIPH structure in syntactic foam to formthe core leads to significant improvement in the com-pressive properties of sandwich composites in bothflatwise and edgewise loading configurations due tostiffening effect of RIPH structure.

4. The syntactic foam core composites demonstratehigher compressive properties under edgewise com-pression; whereas the RIPH integrated foam corecomposites demonstrate higher compressive proper-ties under flatwise compression.

5. Incorporation of RIPH in syntactic foam results inenhancement of energy absorption capacity of sand-wich composites by more than 60%.

6. Features like shear cracks, debris formation etc, arefound to be the predominant modes of failure insyntactic foam.

7. The remarkable advantage of RIPH in cenosphere/phenolic syntactic foam (as core) such as increase incompressive strength, compressive modulus, energyabsorption etc, makes it a potential material forstructural applications.

Funding

This research received no specific grant from any fundingagency in the public, commercial, or not-for-profit sectors.

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