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Materials Science and Engineering A 474 (2008) 390–399
A crumb rubber modified syntactic foam
Guoqiang Li a,b,∗, Manu John a
a Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803, USAb Department of Mechanical Engineering, Southern University, Baton Rouge, LA 70813, USA
Received 27 January 2007; received in revised form 5 April 2007; accepted 6 April 2007
bstract
In this study, the impact response and residual strength of a crumb rubber modified syntactic foam, which contained up to 20% by volume ofrumb rubbers, were investigated. The foam had a hybrid microstructure bridging over several length scales. It was formed by dispersing hollowlass beads and crumb rubber particles into a microfiber and nanoclay filled epoxy matrix. Sandwich beam specimens were prepared using theybrid foam as core and fiber reinforced epoxy as facings. A low velocity impact test using an instrumented drop tower impact machine wasonducted on the sandwich beams and control beams made of the foam only. Four-point bending tests were conducted on the impact damagedpecimens and control specimens without impact damage. The effect of the hybrid foam on the low velocity impact response and residual strength
as evaluated based on the test results. The stress field interaction was evaluated using a finite element analysis. It was found that the rubberizedyntactic foam possessed a higher capacity to dissipate impact energy and to retain bending strength. There was a positive composite action betweenhe hollow glass bead particles and crumb rubber particles by means of stress field interaction and reduction in stress concentration. 2007 Elsevier B.V. All rights reserved.
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issieirorrcaited
eywords: Foam; Crumb rubber; Low velocity impact; Sandwich; Residual str
. Introduction
Syntactic foams are particulate-filled composite materialsonsisting of hollow glass spheres embedded in a resin matrix.hey have been used as the core in composite sandwich struc-
ures due to their light weight and water tightness. The resinatrix usually used in manufacturing the foam materials are
poxies. They are preferred to other matrix systems due to theirigh strength and stiffness, thermal and environmental stabil-ty, and creep resistance [1]. These epoxy systems tend to formross-links when curing. This cross-linking mechanism resultsn a brittle behavior of the epoxies [2]. Owing to the brittlenessf the epoxies, the impact tolerance of the epoxy-based foam iseak and the residual strength is low. It is desired to improve the
oughness of the epoxy matrix without considerably sacrificingtrength and stiffness.
An effective way of toughening epoxies is to add rubber par-icles [3–10]. Evidently, the rubber particles can absorb morempact energy through elastic deformation of the particles, lead-
∗ Corresponding author at: Department of Mechanical Engineering, Louisianatate University, Baton Rouge, LA 70803, USA. Tel.: +1 225 578 5302;ax: +1 225 578 5924.
E-mail address: [email protected] (G. Li).
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921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2007.04.029
; Finite element analysis
ng to higher toughness of the matrix. In addition, the lowertiffness rubber particles serve as stress concentrators. Once thetress exceeds the strength of the materials, microcracks willnitiate. Accompanying the creation of microcracks, a consid-rable amount of impact energy will be consumed, resultingn higher energy absorption capacity. However, these microc-acks will not easily develop into macrocracks. The propagationf microcracks will be blunted, stopped, and arrested by theubber particles through mechanisms like rubber pinning andubber bridging-over. Therefore, the addition of rubber parti-les provides a way of absorbing impact energy; it also providesmechanism of preventing the microcracks from developing
nto macrocracks or catastrophic structural failure. In summary,he rubber particles may enhance the impact tolerance of thepoxy matrix through micro length-scale damage and elasticeformation.
Azimi et al. [11] investigated the fatigue crack propagationnd fracture toughness of a modified syntactic foam containingoth glass microballoons and reactive liquid rubber. They con-ributed the enhanced crack propagation resistance and fracture
oughness to a synergistic action between the microballoon andhe rubber modified epoxy matrix. Gupta et al. [12] investigatedhe static compressive toughness of glass microballoon basedyntactic foams containing a small amount (2% by volume) of
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G. Li, M. John / Materials Science a
rumb rubber particles. They found a significant increase in com-ressive toughness and energy absorption. However, they alsobserved about 50% decrease in Young’s modulus and about0% reduction in compressive strength. It is believed that themall amount of crumb rubber particles may only serve as inclu-ions in the foam. They may not fully display their synergy orositive composite action with microballoons. Also, as a sand-ich core, it is desired to know its behavior when subjected
o transverse bending loads and dynamic impact loads becauseransverse bending loads are more important than uniaxial com-ressive loads for composite sandwich structures and foreignbject impacts, in particular low velocity impacts, which cannote avoided during manufacturing, transportation, and installa-ion.
In this study, a similar rubberized syntactic foam was inves-igated. This foam was formed by dispersing hollow glass beadsmicroballoons) and a considerable amount of crumb rubber par-icles into a microfiber and nanoclay filled epoxy matrix. It wasxpected that each component would be responsible for a par-icular structural or functional property. The hollow glass beadsould serve to reduce the weight and provide a mechanism for
bsorbing impact energy by glass beads crushing and interfacialebonding; the crumb rubber particles would enhance the tough-ess; the microfiber and nanoclay would increase mechanismsr sites for energy absorption and would also serve to reinforcehe matrix if a sufficient amount of microfiber and nanoclay weresed. Here a sufficient amount of microfiber means the fiber vol-me fraction must be larger than the critical fiber volume fraction13]; while a sufficient amount of nanoclay required depends notnly on the volume fraction of nanoclays but also on the statusf the nanoclay in the polymer matrix, phase separated, inter-alated, or exfoliated. It was expected that this foam would beighter, stronger, stiffer, and tougher. It would have an increasedmpact tolerance and structural capacity.
The objective of this study was to experimentally evaluatets impact tolerance and residual strength, and understand the
echanisms for improvement in impact response and residualtrength. To this end, both low velocity impact and four-pointending tests were conducted on foam specimens and sand-ich specimens with the foam as core. Scanning electron
icroscopy (SEM) studies were performed on the impact dam-ged specimens to examine the fracture surface and also studyhe toughening mechanisms involved. A finite element analy-is was conducted to understand the stress field interaction and
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able 1hysical and mechanical properties of raw materials
aterial Flexural strength(MPa)
Flexuralmodulus (MPa)
Tensile strength(MPa)
ER 332 – – –EH 24 – – –ER 332 + DEH 24 108 2793 66anomer I.28 E – – –illed fibers – – 3448rumb rubber – – –lass beads – – –-glass 7715 fabric – – 3000
ngineering A 474 (2008) 390–399 391
eduction in stress concentration due to the presence of bothicroballoons and rubber particles.
. Specimens preparation and experimentation
.1. Raw materials
The epoxy, DER 332 and the hardener DEH 24 were obtainedrom DOW Chemical. The mixing ratio for the epoxy and theardener was 17:3 by volume. Nanomer I.28E was supplied byanocor Incorporation. The 1.6-mm long milled glass fibersere provided by Fiberglast Developments Corporation. Q-cel048 hollow glass particles were received from Potters Indus-ries Incorporation and crumb rubber GF 170 was obtainedrom Rouse Polymerics. E-glass 7715 style plain woven fabricbtained from Fiberglast Developments Corporation was usedor preparing the sandwich skins. Table 1 summarizes the physi-al and mechanical properties of the epoxy resin system and theroperties of the constituents added.
.2. Fabrication
The primary step of the fabrication process involved dis-ersion of nanoclay I.28E in epoxy DER 332. Dispersion waschieved with the help of a “Sonics Vibracell” ultrasonic probeperating at a power of 750 W obtained from Sonics and Mate-ials Incorporation. Ultrasonic mixing uses high-energy sonicaves to force intrinsic mixing of particles and matrix via sonic
avitations. The required amount of nanoclay was measured andhen mixed manually with epoxy in a beaker for 3–4 min. Theltrasonic probe was immersed in the mixture, tuned to producecoustic waves that resonate at a frequency of 20 kHz ± 50 Hznd amplitude of 40% of the maximum amplitude. Ultrasonicixing was continued for 20 min, which ensured a proper dis-
ersion, i.e., without clustering, of the nanoclay particles in thepoxy, as evidenced by the SEM observations in Figs. 11–13.urther, the 1.6 mm long milled glass microfibers were added to
he epoxy–nanoclay mixture and mixed for 3–4 min.The next step of fabrication involved premixing of crumb rub-
er particles and the hollow glass beads in the required volume
ractions. Premixed rubber and glass particles were then addedo the epoxy–nanoclay–microfiber mixture along with the hard-ner DEH 24 and mixed with a mechanical blender till a thicklurry was obtained. This slurry was then cast into a woodenUltimateelongation (%)
Viscosity (cps) Average particlesize (�m)
Density (g/cm3)
– 4000–6000 – 1.16– 19.5–22.5 – 1.074.4 900 – –– – 10–15 1.94.8 – 15.8 2.5– – 89 1.15– – 50 0.48– – – 2.54
392 G. Li, M. John / Materials Science and
Table 2Volume fractions of each batch (%)
Batch no.
1 2 3 4
Epoxy 100 40 40 40Microballoon 0 57.6 47.6 37.6Crumb rubber 0 0 10 20NM
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old and cured for 24 h at room temperature followed by posturing in an oven at 100 ◦C for 3 h. After post curing, the slabpecimen was demolded and cut to beam specimens 304.8-mmong, 50.8-mm wide and 15.2-mm thick.
Four batches of specimens were prepared. The volume frac-ion of each constituent in each batch is summarized in Table 2.bviously, Batch 1 was the control batch. By comparing Batch 2ith Batch 1, the effect of microballoon on the impact response
nd residual strength can be identified. Batch 3 was a modifica-ion of Batch 2 by replacing 10% by volume of microballoonsith crumb rubber particles. Therefore, comparing Batch 3 withatch 2, the effect of rubber incorporation can be identified.atch 4 was a further modification of Batch 3 by replacing andditional 10% by volume of microballoons with crumb rub-er particles. Therefore, from Batch 2 to Batch 4, the effect ofubber addition can be evaluated (rubber content 0%, 10%, and0% for Batch 2, Batch 3, and Batch 4, respectively).
Each batch in Batches 1 and 2 contained twelve identicalpecimens. Six of these specimens were wrapped with two layersf E-glass 7715 plain woven fabric reinforced epoxy to preparesandwich structure, while the others were left unwrapped,
o be used as core specimens. Each batch in Batches 3 and 4ontained 27 identical specimens. Twenty-one of these spec-mens were wrapped as mentioned above, while the othersere left unwrapped, to be used as core specimens. For eachatch in Batches 1 and 2, three core specimens were impactested and three were four-point bending tested; while threeandwich specimens were four-point bending tested and threeere firstly impact tested and then four-point bending tested
o determine their residual strength. For each batch in Batchesand 4, three core specimens were impact tested and threeere four-point bending tested; while three sandwich specimensere four-point bending tested and 18 were firstly impact tested
nd then four-point bending tested to determine their residualtrength.
After the fabrication process, the core specimens from eachatch were weighed. Based on the dimensions of each specimen,
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able 3ensity calculations
atch Weight (g) Volume (cm3) Density (g/cm3)
264 235.35 1.12192 235.35 0.82196 235.35 0.83208 235.35 0.88
Engineering A 474 (2008) 390–399
he volume was calculated. Thus the nominal density of eachatch of specimens could be evaluated. This was compared withhe theoretical density obtained by the rule-of-mixture’s method13].
Table 3 summarizes the density calculations for differentatches of foam core specimens. From Table 3, there is a closegreement between the nominal density and the theoretical den-ity obtained by the rule-of-mixture’s method. It is also seenhat the Batch 2 specimen has the highest reduction in den-ity compared with the Batch 1 (pure resin) specimen. This cane attributed to the highest concentration of hollow glass parti-les. On the other hand, the addition of rubber particles resultedn a slight increase in density when compared with the Batch
specimen, due to the higher density of rubber particles thanhat of microballoons. Compared with the control batch (Batch, pure resin), the reduction of density for Batch 2 was about7%, followed by about 26% for Batch 3 and about 21% foratch 4.
.3. Experiment
.3.1. Low velocity impact (LVI) testsThe LVI tests were conducted using a DynaTup 8250HV
mpact testing machine according to ASTM D2444 to inves-igate the impact resistance by measuring the initiation andropagation energies. The load–time and energy–time responsesf the specimens were measured by the instrumented featuref the DynaTup. Experiments were conducted at velocitiesf 2 m/s, 3 m/s, and 4 m/s. A hammer weight of 3.4 kg wassed.
.3.2. Four-point bending testsIn order to evaluate the inherent bending strength and resid-
al strength of the sandwich and foam core structures, four-pointending tests were performed according to ASTM C393 usingn MTS 810 machine. The cross-head speed was 4 mm/min andpan length was 254 mm. After impact testing the foam corepecimens failed completely, thereby making it impossible toonduct bending tests on them. On the other hand, the sand-ich structures after impact testing were subjected to four-pointending tests to investigate the residual strength.
.3.3. MorphologyThe surface morphology of the tested specimens was
bserved using a Hitachi S 3600N VP SEM. This was conductedo investigate the crack pattern at the fracture surfaces and athe same time get an insight into the toughening mechanismsnvolved in the rubberized foam material.
Reduction in density (%) Theoretical density (g/cm3)
– 1.1526.79 0.7925.89 0.8521.43 0.92
G. Li, M. John / Materials Science and Engineering A 474 (2008) 390–399 393
Table 4Impact test results of core specimens
Batch no.
1 2 3 4
Initiation energy (J)Average 6.00 1.40 6.43 7.20Standard deviation 0.70 0.12 0.26 0.01
Propagation energy (J)
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Average 1.20 4.00 2.31 2.63Standard deviation 0.05 0.50 0.90 0.17
. Results and discussion
.1. Low velocity impact tests
.1.1. Core specimensThe LVI test results of the core specimens impacted at 3 m/s
re summarized in Table 4. The energy corresponding to the peakoad can be defined as the initiation energy. Propagation energyan be defined as the difference of the maximum impact energynd the initiation energy of a particular specimen [14]. Prop-gation energy gives a measurement of the amount of energybsorbed due to damage and plastic deformation. As discussedreviously [15], initiation energy does not necessarily meanross damage initiation as defined in Ref. [13]; instead, it ismeasurement of the capacity for elastic strain energy absorp-
ion. It is in this sense that the initiation energy is used in theollowing text.
From Table 4, the incorporation of microballoon, nanoclay,nd milled glass fiber (Batch 2) reduces the initiation energy andncreases the propagation energy, as compared to the pure epoxypecimen (Batch 1). The reduction in initiation energy suggestshat the capacity for the Batch 2 specimen to store elastic strainnergy is decreased; the increase in propagation energy, on thether hand, suggests that more damage is created/propagatedn the Batch 2 specimen. As a sandwich core, the combina-ion of lower initiation energy and higher propagation energys not desired because it will reduce the residual load carryingapacity of the sandwich. Therefore, Batch 2 is less ideal ascore material. The reason for this may be that the microbal-
oons in Batch 2 serve as stress concentration centers due to theismatch between the modulus of elasticity of the microbal-
oons and the epoxy matrix. The stress concentration inevitablynduces microcracks. Although milled fiber may blunt or arrest
icrocracks by fiber bridge-over, the small amount of milledbers are insufficient to contain the majority of the microcracks,
eading to more damage and increased propagation energy.Incorporation of rubber resulted in an enhancement of the
nitiation energy (Batches 3 and 4). The initiation energynhancement was about 7% and 20% for Batches 3 and 4 speci-ens, respectively, when compared with the Batch 1 specimen.hen compared with the Batch 1 specimen, Batches 3 and
specimens also had an enhancement of propagation energyy almost two-fold. This enhancement of both initiation andropagation energy values for Batches 3 and 4 specimens cane attributed to the toughening effect due to the presence of
Fig. 1. Failure mode of a Batch 1 specimen subjected to LVI test.
rumb rubber, the strengthening effect due to the introductionf microfiber and nanoclay, and the improvement in stress dis-ribution. Crumb rubber particles might help in absorbing morenergy by crack propagation and formation of new surfaces at aicro length-scale.Figs. 1 and 2 show the failure mode of a Batch 1 core speci-
en and a Batch 3 core specimen after the low velocity impactest. It is clearly seen that the Batch 1 specimen failed in a brittleashion. The specimen was shattered into pieces. For the Batch 3pecimen, the failure mode was more ductile. Only one macro-copic crack was found. Therefore, the low stiffness and highuctility rubber particles could have played the role of stressoncentrators and crack arresters. It can thus be concluded thathe rubber particles helped in preventing the microcracks fromrowing into macrocracks and catastrophic structural failure.his also validates the above argument that crumb rubber parti-les helped in absorbing more energy by crack propagation andormation of new surfaces at a micro length-scale.
As discussed previously, the Batches 3 and 4 can be treateds a modification of Batch 2 by, respectively, replacing 10%nd 20% by volume of microballoons with crumb rubber par-icles. Therefore, the effect of rubber addition on the impactesponse can be identified by comparing Batches 2–4 (0% rubberor Batch 2, 10% for Batch 3, and 20% for Batch 4). The effect ofubber volume fraction on the initiation energy and propagationnergy is visualized in Fig. 3 based on the test results in Table 4.
Fig. 2. Failure mode of a Batch 3 specimen subjected to LVI test.
394 G. Li, M. John / Materials Science and Engineering A 474 (2008) 390–399
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Table 5Impact test results of sandwich specimens
Batch no.
1 2 3 4
Initiation energy (J)Average 9.20 7.10 11.83 12.62Standard deviation 0.50 0.90 0.17 0.62
Propagation energy (J)
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Fig. 3. Effect of rubber content on impact responses.
ropagation energy when 10% by volume of rubber is incorpo-ated. Only a slight increase in initiation energy and propagationnergy is seen when further increasing the rubber content. There-ore, a rubber addition up to 20% can significantly improve thempact response of the foam core. The reason for this may behat the incorporation of rubber particles serves to improve thetress distribution by reducing the stress concentration in theoam, possibly due to the stress field interaction between the hardarticle (microballoon) and the soft particle (rubber). A finitelement modeling in Section 3.4 will be conducted to validatehis claim.
.1.2. Sandwich specimensTypical load and energy versus time traces for each Batch of
andwich specimens impacted at a velocity of 3m/s are shownn Fig. 4. The corresponding initiation energy and propagation
nergy are summarized in Table 5. Comparing Batch 2 withatch 1, the same tendency as the core specimens can be seen,.e., lower initiation energy and higher propagation energy inatch 2. This further validates the analysis that the Batch 2
Fig. 4. Load and energy vs. time for the sandwich specimens.
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Average 4.20 6.10 1.81 3.13Standard deviation 0.10 1.00 0.20 0.56
s not an ideal core for sandwich structures. The introductionf microballoon makes the composite more brittle. The smallmount of nanoclay and milled glass fiber is not sufficient todjust the stress field distribution and prevent the microcracksrom propagating into macrocracks because the nanoclays wereeither exfoliated nor intercalated and the amount of microfiberay be only slightly higher than the critical fiber volume fraction
13].It is interesting to note that the impact response is improved
hen rubber particles are incorporated. It is seen that there isn enhancement in initiation energy by about 29% and 37%or Batches 3 and 4 specimens, respectively, when comparedo the Batch 1 specimen. The propagation energy, on the otherand, is reduced by 56.9% for Batch 3 and 25.4% for Batch 4,hen compared to Batch 1. This improvement in both initiation
nergy and propagation energy suggests that the sandwich struc-ures with Batches 3 and 4 foam as core have a higher capacityo store elastic strain energy and a higher capacity to resist dam-ge creation and propagation. It is expected that the sandwichpecimens made of Batches 3 and 4 cores will have a higherapacity to retain their bending strength.
Comparing Batch 2 with Batches 3 and 4, the effect of rub-er incorporation can be identified. The same tendency as theore specimens is seen for the sandwich specimens, i.e., a smallmount of rubber addition (10% by volume, Batch 3) leads to aignificant jump in initiation energy and a considerable dropn propagation energy. Further increasing the rubber contentesults in an insignificant increase in both initiation energy andropagation energy.
Comparing Table 5 with Table 4, the effect of adding a thinayer of FRP around the foam core can also be identified. Thenhancement in initiation energy and propagation energy whenompared with the corresponding foam core specimens can bettributed to the load carrying capacity of the thin layer of FRPkin. The FRP skin carried a major portion of the impact loadnd absorbed a considerable amount of energy (Table 5), lead-ng to higher energy dissipation (initiation energy + propagationnergy) than the corresponding foam core (Table 4).
For quasi-static impact response such as in this study (largeass and low velocity impact test), the impact response can
lso be analyzed from the point of view of impact duration.
rom Fig. 4 the time required for achieving the maximum impactorce is 0.94 ms for the Batch 1 specimen, 0.88 ms for the Batchspecimen, 1.35 ms for the Batch 3 specimen, and 1.41 ms forhe Batch 4 specimen. This result suggests that the Batch 2 sand-
G. Li, M. John / Materials Science and Engineering A 474 (2008) 390–399 395
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ig. 5. Load and energy vs. time of Batch 3 specimens impacted at differentelocities.
ich is the most brittle, followed by the Batches 1 and 3; theatch 4 is the toughest. Because a longer impact duration allowslarger number of times for wave propagation and reflection
compression wave, flexure wave, and shear wave), the impactesponse with a longer impact duration is more towards quasi-tatic response. Therefore, the rubberized foam core sandwichesBatches 3 and 4) behave more towards quasi-static, involvingore materials in absorbing impact energy and thus leading to
ess damage.
.2. Impact behavior at different velocities
The impact behavior of the rubberized sandwich structuresas performed by varying the impact velocities. Batch 3 (10%
ubber) and Batch 4 (20% rubber) specimens were subjectedo LVI tests with three different velocities of 2 m/s, 3 m/s, andm/s and a hammer weight of 3.4 kg. Figs. 5 and 6 show the
oad and energy versus time behavior for the Batch 3 specimensnd Batch 4 specimens impacted at velocities of 2 m/s, 3 m/s,nd 4 m/s, respectively. The initiation energy and propagationnergy for both Batches 3 and 4 specimens are given in Table 6.
able 6mpact test results of Batches 3 and 4 sandwich specimens at different velocitiesnd a hammer weight of 3.4 kg
Batch no.
3 4
2 m/s 3 m/s 4 m/s 2 m/s 3 m/s 4 m/s
nitiation energy (J)Average 5.91 11.83 14.38 5.95 12.62 18.68Standard deviation 0.07 0.17 0.85 0.11 0.62 0.78
ropagation energy (J)Average 0.61 1.81 6.67 0.26 3.13 4.46Standard deviation 0.05 0.20 0.84 0.10 0.56 0.72
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ig. 6. Load and energy vs. time of Batch 4 specimens impacted at differentelocities.
Two observations can be made. (1) Both initiation energy andropagation energy increase as the impact velocity increases.his is understandable because the incident energy (available
mpact energy) increases as the impact velocity increases. Usu-lly, both the initiation energy and propagation energy haveimits. Once those limits are reached, the initiation energy andropagation energy cannot be further increased. The remainingncident energy will be carried by the projectile by perforatinghe target, instead of rebounding from the target. This suggestshat the incident energy used in this study is not high enougho perforate the target. This is exactly the case. No perforations found for any specimens. (2) The Batch 4 specimens have aigher initiation energy and lower propagation energy than theatch 3 specimens, except for the propagation energy when the
mpact velocity is 3 m/s, where it shows a lower propagationnergy for the Batch 3 specimens. This observation suggestshat, in general, the Batch 4 specimens have a higher capac-ty to store elastic strain energy and resist damage creation andropagation, probably due to its higher rubber concentration.
.3. Four-point bending test results
Table 7 summarizes the peak bending loads for various spec-mens. From Table 7, the peak-bending load was the highestor the neat core specimen (Batch 1). The peak-bending loadecreased drastically with the addition of microballoon to theeat resin (Batch 2 foam). This could be attributed to the stressoncentrations created by hollow glass-beads. Comparing Batchwith Batch 3, it is seen that replacing a portion of microbal-
oons by rubber particles further reduced the bending strength.
omparing the Batch 3 foam with the Batch 4 foam, however, its found that there is a rebound in bending strength. The reasonor this may be due to the stress field interaction, which will beurther discussed in Section 3.4.
396 G. Li, M. John / Materials Science and Engineering A 474 (2008) 390–399
Table 7Peak bending load (N)
Batch no. Foam core Sandwich Impacted sandwich
Average Standard deviation Average Standard deviation Average Standard deviation
1 3062 74 6270 232 3446 346234
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into the epoxy matrix decreased the peak-bending load of thefoam (Batch 2 foam); replacing a portion of microballoons withrubber particles (Batch 3 foam) further reduced the peak bend-
500 80 4842382 15 6236474 4 5731
It is seen that adding a thin layer of fiber reinforced epoxy skino the core significantly increased the peak-bending load of theoam core. The peak-bending load for the sandwich structuresas the highest for the Batch 1 sandwich specimen, followedy Batches 3, 4 and 2. This is understandable because the Batchcore has the highest bending strength. There was a sudden
ecrease in the peak load from Batch 1 to Batch 2, followedy an increase from Batch 2 to Batch 3. The fact that the peak-ending load of the Batch 3 sandwich is higher than that of theatch 2 sandwich suggests that the strength of the sandwicheam not only depends on the strength of the core and the skin,ut also depends on if they can work collaboratively. The Batchfoam core is much more brittle than the Batch 3. The Batchfoam core may not be compatible with the skin in terms of
tiffness and ultimate strain. The same can be said to explain therop of peak bending load for the Batch 4 sandwich, althoughhe Batch 4 foam core had a higher bending strength than theatch 3 foam core.
From Table 7, a low velocity impact (3 m/s) significantlyeduced the load carrying capacity of the sandwich specimens.he reduction was about 45% for the Batch 1 specimen. This was
ollowed by a reduction in peak bending load of about 23% forhe Batch 4 specimen and 19% for the Batch 2 specimen. Batch
specimens had the lowest reduction of about 15%. This is ingreement with the impact test results in Table 5. From Table 5,he Batch 3 sandwich has the lowest propagation energy, sug-esting smallest damage by impact. This may lead to the highest
apacity to retain its bending strength.Fig. 7 shows the load–deflection curves corresponding to theesidual peak load for different batches of sandwich specimens.
Fig. 7. Load vs. deflection of sandwich specimens after impact damage.
i
549 3947 28112 5282 79202 4427 3
t is seen that the Batch 3 specimen possessed the highest resid-al peak-bending load. This was followed by Batches 4, 2 and. It is noteworthy that the slope of the load deflection curvesor Batches 3 and 4 specimens is very close to the Batch 2 spec-men. This suggests that the addition of rubber particle into theonventional foam (Batch 2) only slightly reduced the stiffnessf the foam. Again, this occurred probably due to the stress fieldnteraction between the rubber particles and the hollow glasseads. It is noticed that the deflection corresponding to the peakoad is the highest for the Batch 4 specimen, followed by theatches 3 and 2. The smallest deflection is found for the Batch. This once again confirms that the addition of rubber particlesields a foam material with higher ductility.
The typical failure mode of impact damaged sandwich beamsuring four-point bending tests started from the impact damagedrea in the center of the specimen (top surface). The crack thentarted to propagate with the increase in load and resulted in theRP skin failure in the shear direction. This was followed by aomplete failure of the sandwich (tensile failure at the bottomurface) as visualized in Fig. 8.
.4. Finite element analysis
From Table 7, it is found that the addition of microballoons
ng load. However, as more rubber particles were added, the
Fig. 8. Failure mode of the sandwich.
G. Li, M. John / Materials Science and Engineering A 474 (2008) 390–399 397
ls for
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tcT(1scoThe addition of rubber particle into the epoxy matrix (Case 2)resulted in similar consequence to Case 1, with higher stress val-ues. This means that adding rubber particles will cause a higherreduction in strength than adding microballoons. This is vali-
Table 8Maximum stress for each stress component and the first principal stress (MPa)
Fig. 9. Composite mode
eak-bending load rebounded (Batch 4 foam). It is suggestedhat this may be due to the stress field interaction between the
icroballoons and rubber particles. To validate this argument,finite element analysis was conducted. The analysis was con-ucted using the ANSYS software package. In this analysis, thepoxy matrix was taken as a thin square piece with a side lengthf 500 �m. Three cases were considered. In Case 1, a circularicroballoon with a diameter of 50 �m was embedded in the
enter of the epoxy square (see Fig. 9(a)). In Case 2, a circularrumb rubber particle with a diameter of 89 �m was embed-ed in the center of the epoxy matrix (see Fig. 9(b)). In Case 3,oth the microballoon and the rubber particle, tangential to eachther, were embedded in the center of the matrix (see Fig. 9(c)).he composite structure was treated as a plane-stress body. Thelement type used to mesh each body was Plane 42. The num-er of elements was 2619 for Case 1, 2586 for Case 2, and 2691or Case 3. The Young’s modulus and Poisson’s ratio used were
.1 GPa and 0.35 for the epoxy, 72.0 GPa and 0.20 for the glassicroballoon, and 5 MPa and 0.45 for the rubber particle. Aniformly distributed tensile load, 1.0 MPa, was applied in the-direction (see Fig. 9).
C
finite element analysis.
The maximum stresses for each component of σx, σy, τxy, andhe first principal stress P1 are summarized in Table 8. The prin-ipal stress distribution for each case is shown in Fig. 10. Fromable 8 and Fig. 10, the following observations can be made:1) the addition of the microballoon into the epoxy matrix (Case) induced two additional stress components σx and τxy, andtress concentration for the component σy. The additional stressomponents and stress concentration resulted in the reductionf the strength of the foam; see Table 7 for the Batch 2 foam. (2)
ase σx σy τxy P1
1 0.579 1.323 0.856 1.3232 1.138 2.575 0.962 2.5753 0.581 1.000 0.867 1.100
398 G. Li, M. John / Materials Science and
Fig. 10. Principal stress distributions for the three cases.
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Engineering A 474 (2008) 390–399
ated by Table 7, Batch 3 foam. It is noted that although bothhe microballoon and rubber particle induced stress concentra-ion, the nature of the concentration is different. From Fig. 10,he principal stress concentrated at the microballoon (stiffer par-icle) and the surrounding matrix in the y-direction; while therincipal stress concentrated at the surrounding matrix only andas in the x-direction when the rubber particle (softer particle)as involved. (3) The stress field surrounding the microballoon
nd the rubber particle interacted with each other and alteredhe magnitude and the nature of the stress distribution and stressoncentration (Case 3). A noticeable feature is that the princi-al stress concentration is shifted from surrounding the particleo the edge (see Fig. 10(c)). From Table 8, the magnitude ofhe stress concentration with the presence of both particles ismaller than with only one particle. The reduction in stress con-entration may lead to an increase in strength. This is validatedy Table 7, Batch 4 foam. It is seen that the peak-bending loadf the Batch 4 foam is higher than that of the Batch 3 foam. Thisuggests that in the Batch 4 foam, there may be a sufficient num-er of rubber particles to pair with the microballoons, reducinghe overall stress concentration in the composite. This may behy a peak bending load increase was observed from Batch 3
o Batch 4. It is envisioned that further increasing the rubberontent may reduce the strength again because there may note a sufficient number of microballoons to pair with the rubberarticles. The composite strength may be controlled by rubbers.herefore, there may be an optimal rubber and/or microballoonontent that helps in improving the strength by reducing the over-ll stress concentration. More refined study is needed to find theptimal content.
.5. Scanning electron microscopy studies
Fig. 11 is a high resolution SEM image of the cracked surfacef a Batch 3 specimen. The crack propagation direction is indi-ated by the arrow. It is seen that the crack propagation is stoppedater on by the crushing of the hollow glass bead at the tip of therack (see Fig. 12). Therefore, there are mechanisms to preventhe microcracks from propagating into macrocracks. It is found
hat several mechanisms exist for absorbing impact energy inicro length-scale such as microballoon crushing (Fig. 12), fiberull-out, and rubber/matrix interfacial debonding (Fig. 13). Theffect of the small amount of nanoclays on the energy absorption
Fig. 11. SEM image of the cracked surface of a Batch 3 specimen.
G. Li, M. John / Materials Science and E
Fig. 12. Closer view of the crack of the Batch 3 specimen propagation hindranceby glass bead crushing.
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ig. 13. SEM image of the cracked surface of a Batch 3 specimen showing fiberull out.
annot be identified. Intercalated or exfoliated nanoclays mayisplay their positive effect.
. Conclusion
A rubberized syntactic foam containing organophilic silicateayers (nanoclay), milled glass fibers and a secondary and ter-iary phase of rubber and hollow glass beads, respectively, wasabricated and tested. From the test results and analysis, theollowing conclusions are obtained:
The addition of microballoons into the epoxy matrix weak-ens the capacity for absorbing elastic strain energy and thecapacity for resisting damage creation and propagation.The crumb rubber particles and the microballoons have a pos-itive composite action. Replacing a portion of microballoons
by crumb rubbers increases the initiation energy and decreasesthe propagation energy. It also increases the flexure capacity,in particular when the rubberized foams serve as sandwichcores.[
[
ngineering A 474 (2008) 390–399 399
Within the rubber content considered in this study, the impacttolerance increases as the rubber content increases.The rubbers prevent the microcracks from propagating intomacrocracks due to rubber pining. The microballoons absorbimpact energy by wall crushing. The microfibers help inenergy absorption through fiber pull-out. The effect of thesmall amount of nanoclay on the energy absorption cannot beidentified. Intercalated or exfoliated nanoclays may displaytheir positive effect.The finite element analysis shows that there is a stress fieldinteraction between the soft particles (crumb rubber) and hardparticles (microballoons). The co-existence of stiffer particlesand softer particles help in adjusting the stress distribution andreducing the stress concentration.A more refined study is needed to find the optimal rubber andmicroballoon contents.
cknowledgements
This study is based upon work supported by the U.S. Armyesearch Office under grant number W911NF-05-1-0510. Theuthors would also like to acknowledge Mr. Venkata Chakkaandeep from the Department of Mechanical Engineering atouisiana State University for his help in preparing and testing
he rubberized foam core samples.
eferences
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