Carbon-Nanofiber-Reinforced Syntactic Foams: Compressive Properties and Strain Rate Sensitivity RONALD L. POVEDA 1 and NIKHIL GUPTA 1,2 1.—Composite Materials and Mechanics Laboratory, Department of Mechanical and Aerospace Engineer- ing, Polytechnic Institute of New York University, Brooklyn, NY 11201, USA. 2.—e-mail: [email protected]The current study is focused on exploring the possibility of reinforcing syn- tactic foams with carbon nanofibers (CNFs). Syntactic foams are hollow, particle-filled composites that are widely used in marine structures and are now finding applications in other modes of transportation due to their high stiffness-to-weight ratio. The compressive properties of syntactic foams rein- forced with CNFs are characterized over the strain rate range of 10 4 to 3,000 s 1 , which covers seven orders of magnitude. The results show that despite lower density with respect to neat epoxy, CNF/syntactic foams can have up to 7.3% and 15.5% higher quasi-static compressive strength and modulus, respectively, for the compositions that were characterized in the current study. In addition, these properties can be tailored over a wide range by means of hollow particle wall thickness and volume fraction, and CNF volume fraction. The compressive strength of CNF/syntactic foams is also shown to generally increase by up to a factor of 3.41 with increasing strain rate when quasi-static and high-strain-rate testing data are compared. Extensive microscopy of the CNF/syntactic foams is conducted to understand the failure and energy absorption mechanisms. Crack bridging by CNFs is observed in the specimens, which can delay final failure and increase the energy absorption capacity of the specimens. Deformation of CNFs is also noticed in the material microstructure. The deformation and failure mecha- nisms of nanofibers are related to the test strain rate and the structure of CNFs. INTRODUCTION Syntactic foams are lightweight composites composed of hollow particles dispersed in a matrix material. Syntactic foams are used in applications in which damage tolerance and energy absorption under compressive loading conditions are desired. Submarines extensively use syntactic foams for buoyancy and damage tolerance. The collision of the USS San Francisco nuclear submarine with an under-sea mountain in 2005 highlighted the importance of syntactic foams in submarine struc- tures. Syntactic foam modules present in the eye- brow of the submarine absorbed the impact force and kept the inner hull and nuclear reactor safe. Syntactic foams also helped the submarine in obtaining buoyancy to surface despite severe damage to ballast tanks. A combination of high hydrostatic compressive strength, low weight, and resistance to moisture absorption make syntactic foams ideal for submarine structures. The struc- ture of the Deepsea Challenger underwater vehicle used in the Mariana Trench dive in 2012 was made of syntactic foam panels, which highlighted the capabilities of syntactic foams in such severe performance conditions. Other defense applica- tions of syntactic foams include missile support plugs used in launchers, blast mitigating panels for vehicles, and the deckhouse of ships. In some of these applications, syntactic foams are used as the core material in sandwich structures with either fiber reinforced laminates or metal sheets used as skins. 1,2 With the possibility of achieving multifunction- ality in syntactic foam properties by using carbon nanofibers (CNFs), new applications can be devel- oped by tailoring a combination of strength, modulus, thermal conductivity, and electrical JOM, Vol. 66, No. 1, 2014 DOI: 10.1007/s11837-013-0791-0 Ó 2013 The Minerals, Metals & Materials Society 66 (Published online November 2, 2013)
Transcript
Carbon-Nanofiber-Reinforced Syntactic Foams: CompressiveProperties and Strain Rate Sensitivity
RONALD L. POVEDA1 and NIKHIL GUPTA1,2
1.—Composite Materials and Mechanics Laboratory, Department of Mechanical and Aerospace Engineer-ing, Polytechnic Institute of New York University,Brooklyn,NY 11201, USA. 2.—e-mail: [email protected]
The current study is focused on exploring the possibility of reinforcing syn-tactic foams with carbon nanofibers (CNFs). Syntactic foams are hollow,particle-filled composites that are widely used in marine structures and arenow finding applications in other modes of transportation due to their highstiffness-to-weight ratio. The compressive properties of syntactic foams rein-forced with CNFs are characterized over the strain rate range of 10�4 to3,000 s�1, which covers seven orders of magnitude. The results show thatdespite lower density with respect to neat epoxy, CNF/syntactic foams canhave up to 7.3% and 15.5% higher quasi-static compressive strength andmodulus, respectively, for the compositions that were characterized in thecurrent study. In addition, these properties can be tailored over a wide rangeby means of hollow particle wall thickness and volume fraction, and CNFvolume fraction. The compressive strength of CNF/syntactic foams is alsoshown to generally increase by up to a factor of 3.41 with increasing strainrate when quasi-static and high-strain-rate testing data are compared.Extensive microscopy of the CNF/syntactic foams is conducted to understandthe failure and energy absorption mechanisms. Crack bridging by CNFs isobserved in the specimens, which can delay final failure and increase theenergy absorption capacity of the specimens. Deformation of CNFs is alsonoticed in the material microstructure. The deformation and failure mecha-nisms of nanofibers are related to the test strain rate and the structure ofCNFs.
INTRODUCTION
Syntactic foams are lightweight compositescomposed of hollow particles dispersed in a matrixmaterial. Syntactic foams are used in applicationsin which damage tolerance and energy absorptionunder compressive loading conditions are desired.Submarines extensively use syntactic foams forbuoyancy and damage tolerance. The collision ofthe USS San Francisco nuclear submarine with anunder-sea mountain in 2005 highlighted theimportance of syntactic foams in submarine struc-tures. Syntactic foam modules present in the eye-brow of the submarine absorbed the impact forceand kept the inner hull and nuclear reactor safe.Syntactic foams also helped the submarine inobtaining buoyancy to surface despite severedamage to ballast tanks. A combination of highhydrostatic compressive strength, low weight, and
resistance to moisture absorption make syntacticfoams ideal for submarine structures. The struc-ture of the Deepsea Challenger underwater vehicleused in the Mariana Trench dive in 2012 wasmade of syntactic foam panels, which highlightedthe capabilities of syntactic foams in such severeperformance conditions. Other defense applica-tions of syntactic foams include missile supportplugs used in launchers, blast mitigating panels forvehicles, and the deckhouse of ships. In some ofthese applications, syntactic foams are used as thecore material in sandwich structures with eitherfiber reinforced laminates or metal sheets used asskins.1,2
With the possibility of achieving multifunction-ality in syntactic foam properties by using carbonnanofibers (CNFs), new applications can be devel-oped by tailoring a combination of strength,modulus, thermal conductivity, and electrical
JOM, Vol. 66, No. 1, 2014
DOI: 10.1007/s11837-013-0791-0� 2013 The Minerals, Metals & Materials Society
66 (Published online November 2, 2013)
conductivity. In several defense applications,including submarine structures, ship structures andblast mitigating panels, the high-strain-rate (HSR)deformation response of syntactic foams is impor-tant. Existing studies have shown that the strain-rate sensitivity in materials may exist in severalforms, which include an increase or decrease instrength with strain rate, change in failure mecha-nisms at different strain rates, and dependence ofenergy capabilities on the strain rate.3 It is alsoobserved that the material properties may showchange in a given strain rate range and may nothave a monotonic trend from quasi-static to HSRregimes. A review article is available on the HSRproperties of syntactic foams, which specificallydiscusses the effect of compressive strain rate onpolymer matrix syntactic foams.4
Syntactic foams have been extensively studiedin recent years due to the interest in theirmechanical and thermal properties. Most of thesestudies are related to their marine applicationswhere compressive strength,5 thermal insulation,6
and moisture absorption 7 characteristics or acombination of these properties is studied.8,9 Manyof these studies are focused on understanding theeffect of hollow particle volume fraction and wallthickness on the properties of syntactic foams.10,11
Increasing interest in the dynamic properties ofsyntactic foams is also noticed in recent litera-ture.12–15 The existing studies have noted that theHSR compressive strength of syntactic foams ishigher than the quasi-static values. However,within the HSR range, the syntactic foam strengthcan increase or decrease with strain rate depend-ing on the hollow particle volume fraction andwall thickness. These studies also found that thefailure mechanisms in syntactic foams are strainrate dependent. The specimens failed by shearcracking under quasi-static compression, whilecracking in the direction of compression leading tofragmentation was observed under HSR compres-sion. This change in the primary crack directionand corresponding failure mechanisms at micro-scopic level changes the energy absorption profileduring the crack-initiation and propagation stages,which are important in designing protectiveapplications.
Nanoscale reinforcements have been used to en-hance the properties and performance of syntacticfoams.16 Nanoclay,17,18 carbon nanotubes, 19,20 andCNFs 21,22 are the most widely used nanomaterialsfor this purpose. Long-aspect-ratio nanostructuresprovide enhancement in strength and energyabsorption due to crack-tip deflection and crack-bridging mechanisms. The low cost of CNFs is con-sidered attractive in developing materials for bulkapplications. The available review articles can beconsulted for mechanical properties of CNF-rein-forced composites.23,24 The present work is focusedon studying the HSR compressive behavior of CNF-reinforced syntactic foams.
1 wt.% CNF
2 wt.% CNF
5 wt.% CNF
10 wt.% CNF
15 vol.% GMB 30 vol.% GMB 50 vol.% GMB
Fig. 1. Illustration of compositions of CNF-reinforced syntacticfoams. Sixteen different grades of CNF/syntactic foams were fabri-cated with HGMs of two different wall thicknesses.
MATERIALS AND METHODS
Constituent Materials
Epoxy resin DER 332 and hardener DEH 24 fromDow Chemical Co. (Midland, MI) are used as thematrix resin system. Hollow-glass microspheres(HGMs) procured from 3M (St. Paul, MN), having220 kg/m3 and 460 kg/m3 nominal true particledensities are used. The mean particle sizes of theHGMs are 35 lm and 40 lm, respectively, and theactual particle size can vary from 10 lm to 90 lmwithin a given batch of HGMs.5 PR-19 XT-PS CNFs,supplied by Pyrograf Products Inc. (Cedarville, OH),are used as the nano-scale reinforcement. The den-sity of these fibers is 1,950 kg/m3 according to themanufacturer’s datasheet.
Composite Fabrication Method
The compositions of the composite materials fab-ricated in this study are illustrated in Fig. 1. In theCNF/syntactic foams fabricated for the study, theHGM content is varied as 15 vol.%, 30 vol.%, and50 vol.%. A total of 16 compositions of CNF/syntac-tic foams are fabricated using HGMs of two differ-ent wall thicknesses.
A mechanical mixer fitted with a high shearimpeller is used to mix CNFs in the epoxy resin for30 min at 650 rpm as per the process described in aprevious study.25 In the next step, HGMs are addedand the slurry is slowly mixed with a wooden dowel foran additional 15 min. Then, the hardener is mixedand the slurry is stirred for an additional 5 min. Theslurry is cast into aluminum molds and placed on ashaker for 5 min to degas the mixture. The mixture iscured at room temperature for 48 h and then post-cured in a convection oven at 100�C for 2 h. The CNF-reinforced syntactic foam composite nomenclature
follows the trend NXX-YYY-ZZ, where N representsCNFs, XX corresponds to CNF weight fraction, YYYdenotes HGM density, and ZZ refers to HGM vol.%.Cylindrical specimens of nominal dimensions of3.5 mm thickness and 7 mm diameter are preparedfor quasi-static and HSR compression testing.
Quasi-Static Compression Testing
An Instron 4469 series mechanical test system (In-stron Corporation, Norwood, MA) with Bluehill 2 soft-ware (Instron Corporation) is used for compressiontesting in the displacement control mode at a constantcrosshead displacement velocity of 1 mm/min. Lu-bricant is applied between specimens and compressionplatens to prevent specimen barreling. The compres-sive strength, elastic modulus, and absorbed elasticenergyofeachtestedspecimenaredeterminedfromtheresulting load–displacement data, where the elasticenergy is calculated as the integral of the stress–straincurve up to the compressive strength of the composite.
HSR Compression Testing
HSR compression testing was conducted using asplit-Hopkinson pressure bar (SHPB) test system.Inconel 718 alloy incident and transmitter bars areused. The density, elastic modulus, and wave velocityof Inconel 718 are 8,890 kg/m3, 205 GPa, and4,802 m/s, respectively. Strain gages attached to theincident and the transmitter bars are used to recordthe strain pulse with respect to time. The strainsignal is transmitted to a strain gage conditioner,stored by an oscilloscope, and then acquired by acomputer. The time-dependent stress rðtÞ, the straineðtÞ, and the strain rate _eðtÞ, are calculated by:26
_eðtÞ ¼ 2cberðtÞl0
(1)
rðtÞ ¼ AEetðtÞA0
(2)
eðtÞ ¼Z t
0
_eðsÞds (3)
where cb is the sound wave velocity in the bars, eiðtÞand erðtÞ are the incident and reflected strains,respectively, recorded from the incident bar, etðtÞ isthe transmitted strain recorded from the transmitterbar, A and E are the cross-sectional area and theelastic modulus of the bars, respectively, and A0 and l0
are the cross-sectional area and the length of thespecimen, respectively. The stresses on the front andthe back surface of the specimen can be calculated by:
rFðtÞ ¼AE erðtÞ þ eiðtÞð Þ
A0(4)
rBðtÞ ¼AEetðtÞ
A0(5)
Confirmation of stress equilibrium by matchingthe stress in the front and the back surface of thespecimen is required to establish the validity to theSHPB tests.
Imaging
The specimen deformation and failure at HSRcompression was captured by a Y-3 high speedcamera from IDT/Redlake (Tallahassee, FL). A S-3400N variable pressure scanning electron micro-scope (SEM) from Hitachi USA (Schaumburg, IL)was used for observation of failure features. Thespecimens were coated with a conductive layer ofgold before microscopic observations using an EMSCD050 sputter coater from Leica Microsystems(Buffalo Grove, IL).
RESULTS AND DISCUSSION
CNFs have a complex structure, which is referredto as a stacked-cup structure. Transmission electronmicrographs of a CNF are shown in Fig. 2. TheCNFs have a hollow core along the fiber length.There is a thin zone next to the core, which appearsdarker in the micrographs and has graphene sheetsoriented along the length of the fiber, similar to thestructure of carbon nanotubes. The thicker outerpart comprises turbostratic graphite layers, whichare oriented at an angle with respect to the fibercore. The graphite layers are helically woundaround the fiber core to give it a structure thatresembles stacked cups. Detailed studies on tensiletesting of single CNFs are available,27,28 which canbe useful in understanding the properties of CNF-reinforced composites. SEM images of a represen-tative specimen of N10-460-15 CNF/syntactic foamare shown in Fig. 3. CNFs are dispersed in thematrix resin in the spaces between HGMs. Thedistribution of both CNFs and HGMs is observed tobe uniform in the micrographs.
Quasi-Static Compression Test
A representative set of compressive stress–straingraphs of CNF/syntactic foams is shown in Fig. 4. Forcomparison purposes, the values of compressivestrength and modulus for neat epoxy are experi-mentally measured to be 112.4 ± 0.5 MPa and1,482.1 ± 141.4 MPa, respectively. The generaltrends of these graphs are similar to those observedfor plain syntactic foams in previous studies,5,10
where a linear elastic region is followed by a stresspeak and a stress plateau. Variation in the stress–strain curves is observed due to HGM wall thicknessand volume fraction variation, as well as CNF con-tent. The compressive strength and modulus of theCNF/syntactic foams are presented in Fig. 5. For
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220-type CNF/syntactic foams containing 15 vol.%HGMs, a general increasing trend can be observed inboth strength and modulus with increasing CNFcontent. Overall, a general increase in the compres-sive strength and modulus is observed with increas-ing HGM wall thickness across all CNF/syntacticfoam grades. Compared with the neat resin, thelargest decrease in compressive strength and modu-lus for 220-type composites is observed as 53.6% and39.9%, respectively, for N1-220-50 CNF/syntacticfoams. This CNF/syntactic foam composition has thelowest density among all composites, which is 42%lower than that of the neat resin. Therefore, for N1-220-50 foams the specific strength is lower and thespecific modulus is higher than those of neat resin.For syntactic foams containing thicker-walledHGMs, a maximum strength increase of 7.3% is ob-served for N1-460-30, and a maximum modulus in-crease of 15.5% is observed for N10-460-15 withrespect to those of the neat resin.
Semi-Empirical Analytical Modeling of ElasticModulus
An analytical model applicable to syntactic foamsis used to estimate the elastic moduli of CNF/syn-tactic foams.29,30 The predictions are validated withexperimental results. For the differential schemedeveloped for a syntactic foam having all HGMs ofthe same diameter and wall thickness, the variationin the elastic modulus dE due to incremental addi-tion of HGM volume fraction dU is described by
dE ¼ fE Ep; mp;E; m; g� �
EdU (6)
where Ep and mp represent the elastic modulus andthe Poisson’s ratio of HGM material, E and m rep-resent the composite elastic modulus and Poisson’sratio, and g represents the radius ratio of the HGMs(ratio of the inner radius over the outer radius).Particles of different wall thickness and diametercan be incorporated in different iterations to
Fig. 2. Transmission electron micrographs of a CNF.
Fig. 3. SEM image of a N10-460-15 CNF/syntactic foam (a) at low magnification showing HGMs and (b) at high magnification showing CNFsdispersed in the matrix resin.
Fig. 5. Comparison of quasi-static (a) compressive strength and (b) compressive modulus of CNF/syntactic foams containing CNFs ranging from1 wt.% to 10 wt.%.
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account for the real distribution of these parame-ters. In addition, the particle packing limit UL canalso be imposed on the composite by using a modi-fied form of Eq. 6:
dE
dU¼ E
1� U=UL
� /p
XNj¼1
vjfpð Þ
E Ep; mp;E; m; gj
� �þ /vf
vð ÞE E; mð Þ
" #
(7)
The functions fpð Þ
E and fvð Þ
E account for the pre-sence of HGMs and the matrix voids. The volumefraction of matrix voids can be calculated by thedifference in the experimentally measured densityof syntactic foams with the theoretically calculateddensity using the rule of mixtures. The parameter vjrepresents a scaled version of a discrete distributionfunction, which describes the polydisperse compo-sition of the particle phase in the composite. A dis-crete distribution function is defined by referring toN + 1 families of particles, where the jth inclusionfamily has radius ratio gj and the index j goes from 0to N. The HGM and matrix volume fractions aredenoted by U and Um, respectively, such thatUm + U = 1. The terms /p = Up/U and /v = Uv/Utake into account the particle and matrix void vol-ume fractions, respectively, where Up + Uv = U. The
complete derivation of the model is described insufficient detail in Ref. 29
In the theoretical calculation, Ep and mp are takenas 60 GPa and 0.21, respectively, and a value of 0.35is assigned to the Poisson’s ratio of all composites.Because this model is applicable to syntactic foams,the experimentally measured properties of CNF-reinforced epoxy resin are assigned to be the matrixof CNF/syntactic foams.31 The elastic modulus pre-dictions for the CNF/syntactic foams are listed inTables I and II.
It can be observed that most of the experimentalvalues were within ±15% agreement of the semi-empirical model. The predictions for the 220-typecomposites are shown to be in better agreementwith the experimental data than the 460-type.Among the syntactic foams showing larger varia-tions are those containing only 15 vol.% particles.One possibility for these syntactic foams in partic-ular is that the HGMs may form a higher concen-tration toward the top of the specimen duringcuring because of the density difference betweenHGMs and the matrix resin. Such behavior maylead to lower elastic modulus of syntactic foams andlarger deviation than the predicted values.
HSR Compression Testing
A typical strain–time response obtained from theSHPB setup is presented in Fig. 6a. The calculatedfront and back surface stress curves for the specimen
Table I. Prediction of elastic modulus of 220-type CNF/syntactic foam composites
are presented in Fig. 6b. A close matching betweenstresses on both surfaces confirms the validity of theresults. The corresponding strain rate–strain andstress–strain graphs are shown in Fig. 6c and d,respectively. The average value of strain rate in theHSR region is calculated and presented as the nom-inal strain rate for a given test. It should be notedthat the strain rate is recovered from the test resultsand is unique for each test. Therefore, the results forall tested specimens are analyzed and presented in-stead of average values and standard deviations.
The results on compressive strength with respectto strain rate for all CNF/syntactic foams are shownin Fig. 7. The HSR compressive strength is highercompared to the corresponding quasi-static valuesfor all syntactic foams. The HSR region can dem-onstrate varying sensitivity to strain rate depend-ing on the CNF content. At high volume fractions,the entanglement of long-aspect-ratio CNFs makesit difficult to completely disperse them and obtain
the full benefit of their reinforcing ability. The in-crease in strength from quasi-static to HSR loadingrates can clearly be seen in CNF/syntactic foams inFig. 7. However, the compressive strength remainsstagnant within the HSR region. The materials arenot strain-rate sensitive within the HSR range tes-ted in this work. The highest increase in compres-sive strength for CNF/syntactic foams is seen forN2-220-30, where the strength increases by a factorof 3.41 at a strain rate of 1,830/s, when compared tothe quasi-static strength. The compressive modulusvalues for CNF/syntactic foams are shown in Fig. 8.Similar to strength, the compressive modulus atHSR is higher than the quasi-static strength forcorresponding material compositions. Within theHSR region, several grades exhibit a monotonic in-crease in modulus with strain rates. Syntactic foamscontaining 220-type HGMs show higher strain-ratesensitivity than those containing 460-type HGMsas observed in Fig. 8. The highest increase in com-
(a)
(b)
(c) (d)
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-0.5
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300 500 700 900 1100
Str
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(x
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mm
/mm
)
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Fig. 6. A representative set of HSR compression test results of a N2-220-15 specimen compressed at 2,420 s�1 strain rate: (a) strain signal fromincident and transmitted bars, (b) comparison of stress at front and back surfaces of the specimen, (c) strain rate–strain graph, and (d) stress–strain graph.
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pressive modulus is noted for N2-220-30, whichshows an increase by a factor of 3 at a strain rate of1,830/s, compared to the quasi-static compressivemodulus.
Failure Analysis
A failure sequence at HSR compression for atypical N1-220-30 specimen, obtained from thehigh-speed camera, is shown in Fig. 9. The speci-men failure appears to be primarily due to shearcracks in the beginning of failure. After the initialshear failure, the specimen compressed uniformlyand a longitudinal crack developed. Shear failurebehavior is also observed under quasi-static loadingfor syntactic foams. This figure can be compared toFig. 10, which is taken on a N1-220-50 specimen
that has a higher concentration of HGMs at thesame level of CNF content. The failure of thisspecimen is clearly due to the crack propagation inthe direction of compression. The test strain rate issufficiently high to suppress the shear failure of thisspecimen, and only a longitudinal crack is observed.The crack pattern and failure are further comparedin Fig. 11 for N1-460-30 syntactic foams at threedifferent strain rates, namely quasi-static, 670 s�1,and 1,440 s�1. This specimen has the thicker-walledHGMs. The specimens uniformly compress at quasi-static strain rate, and fragmentation is seen due toshear close to the specimen edge. The shear cracksare suppressed at higher strain rates, and thespecimens show a brittle failure with multiplecracks along the compression direction. At HSRs,the specimen failure may become less sensitive to
the presence of localized microscopic defects due torapid compression. The specimen failure is observedat lower strains but at a higher stress level. Thiskind of behavior also indicates that the quasi-staticfailure is dominated by the HGM crushing andcompaction of the specimen in the porosity that isexposed by HGM crushing, whereas the HSR failureis dominated by the matrix failure. These possibili-ties are further analyzed through scanning electronmicroscopy.
Scanning electron microscopy is conducted on twocompositions: N10-220-15 and N10-460-15. Bothspecimen types have the same volume fraction ofCNFs and HGMs; the only difference is the wallthickness of HGMs used in these syntactic foams.
The particle crushing is observed to be prominent inN10-220-15 specimens at all strain rates as ob-served in Fig. 12. In comparison, Fig. 13 showsbroken particles inside a N10-460-15 specimen, butthere is a visible lack of crushed debris in this figurethat shows quasi-static compression features. Manybroken particles are still present in their originallocation despite fracture of the particles. The HSRfailure surface of N10-460-15 specimen is shown inFig. 14a, where particle crushing does not seem tobe prominent; rather, several intact HGMs can beobserved. Some broken particles are also observedon the failure surface shown in Fig. 14b, butthe number of such particles is small in thisspecimen.
Fig. 9. High-speed camera images of N1-220-30 CNF/syntactic foams tested by SHPB taken at a speed of 6,580 frames per second at a strainrate of 1,710 s�1.
Fig. 10. High-speed camera images of N1-220-50 CNF/syntactic foams tested by SHPB taken at a speed of 6,580 frames per second at a strainrate of 1,530 s�1.
Fig. 11. N1-460-30 CNF/syntactic foam specimens subjected to (a) quasi-static, and HSR compression at rates of (b) 670 s�1 and (c) 1,440 s�1.
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The role of CNFs in the failure mechanisms ofreinforced syntactic foams is further examined athigher magnification. The microscopic examinationreveals the evidence of crack bridging by CNFs as
observed in Fig. 15. From these micrographs, theinterfacial bonding between CNFs and the matrixappears to be strong, which likely resulted in elon-gation and failure of CNFs after the matrix had
Fig. 12. SEM micrographs of N10-220-15 CNF/syntactic foam compressed under (a) quasi-static and (b) HSR conditions at a strain rate of1,700/s.
Fig. 13. SEM micrographs of N10-460-15 CNF/syntactic foam failed under quasi-static conditions.
Fig. 14. SEM micrographs of N10-460-15 CNF/syntactic foam failed under a HSR compression at strain rate of 1,960 s�1.
Fig. 15. Numerous CNFs can be seen in a crack bridging the two sides in N10-460-15 CNF/syntactic foam failed under a HSR compression atstrain rate of 1,960 s�1.
Sliding of cups with respect to each other
(a(a)(b)
Fig. 16. (a) Several CNFs in specimens subjected to quasi-static compression appear to be deformed throughout their entire length. The imageis taken on N10-460-15 CNF/syntactic foam. (b) Possible sliding of graphite layers will provide this type of structure.
Fig. 17. SEM images of N10-220-15 CNF/syntactic foam specimen failed under a HSR compressive rate of 1,700 s�1. (a) Fibers are sheared,without evidence of deformation of the entire fiber and (b) some fibers also appear to have a protrusion at the center of the cross section.
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failed. It is noted in quasi-static compression speci-mens that a large number of CNFs shows defor-mation marks along their entire length as seen inFig. 16a. The stacked-cup structure that is gener-ated by the helically folded graphite layers(Fig. 16b) can be stretched due to weak p–p bondsbetween the layers. The stretching and unfolding ofthe layers can provide this kind of structure inCNFs. The HSR failure of N10-220-15 CNF/syn-tactic foam is shown in Fig. 17. Several fibers ap-pear to have failed by localized elongation andshearing. The deformation marks are not observedalong the entire fiber lengths in this figure. Some ofthe fibers also seem to have a protrusion at thecenter of their cross section. It is likely that theturbostratic graphite layers shear first under theHSR loading, and then the load is taken by the thininner graphene layers that are oriented along thefiber length and are stronger. Elongation of graph-ene layers before failure leads to the protrusion thatis observed at the fiber cross section.
CONCLUSIONS
In the current work, CNF-reinforced syntacticfoams were studied for quasi-static and HSR com-pression. The conclusions can be summarized asfollows:
� The compressive strength and modulus of CNF/syntactic foams increased by as much as a factorof 3.4 and 3, respectively, at HSRs compared tothe quasi-static values.
� The compressive strength and modulus of CNF/syntactic foams under quasi-static testing wasshown to be 7.3% and 15.5% higher with respectto neat epoxy.
A difference in the failure mechanism was notedbased on the specimen composition and strain rate.Evidence of a crack-bridging mechanism was foundin the CNF-reinforced specimens. It was also ob-served that the nanofibers deform along their entirelength under quasi-static compression, while local-ized shear failure and deformation of inner graph-ene layers were discovered in HSR compressivefailure. Knowledge of HSR mechanical propertiesand failure mechanisms can help in designing saferstructures.
ACKNOWLEDGMENTS
This work is also supported by the Office of NavalResearch through Grant N00014-10-1-0988 andNational Science Foundation GK-12 Fellows grant0741714. The authors thank 3M, MN for providingglass microballoons and relevant technical infor-mation. The authors thank Dr. Dung D. Luong,
Gleb Dorogokupets, Sriniket Achar, and AndresDonoso for help in material fabrication, specimenpreparation, and testing. Drs. Deepam Maurya andShashank Priya are thanked for providing TEMobservations.
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