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Effect of carbon nanofibers on tensile and compressive characteristics of hollow particle filled composites
Materials and Design 31 (2010) 1332–1337
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of carbon nanofibers on tensile and compressive characteristics of hollow particle filled composites Momchil Dimchev, Ryan Caeti, Nikhil Gupta * Composite Materials and Mechanics Laboratory, Department of Mechanical and Aerospace Engineering, Polytechnic Institute of New York University, Brooklyn, NY 11201, USA
a r t i c l e
i n f o
Article history: Received 30 June 2009 Accepted 4 September 2009 Available online 9 September 2009 Keywords: A. Polymer matrix composites B. Foams E. Mechanical properties
a b s t r a c t The effect of presence of carbon nanofibers on the tensile and compressive properties of hollow particle filled composites is studied. Such composites, called syntactic foams, are known to have high specific modulus and low moisture absorption capabilities and are finding applications as core materials in aerospace and marine sandwich structures. The results of this study show that addition of 0.25 wt.% carbon nanofibers results in improvement in tensile modulus and strength compared to similar syntactic foam compositions that did not contain nanofibers. Compressive modulus decreased and strength remained largely unchanged for most compositions. Tensile and compressive failure features are analyzed using scanning electron microscopy. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Use of hollow particles to embed porosity in a matrix material provides several advantages for aerospace and marine structural applications [1,2]. Such composites are called syntactic foams to differentiate from other foams containing open or closed cell gas porosity. Existence of a thin but stiff shell around porosity helps in obtaining low density in the composite material without a severe penalty on mechanical properties [3]. Studies have shown that such porous composites can have higher modulus than the matrix material but the strength is generally lower [3–5]. These materials are able to absorb significant amount of energy under compressive loading conditions due to the presence of porosity, which leads to large strain at a constant load level due to progressive crushing of particles [6–8]. In addition, localized damage to the material does not result in any significant increase in moisture absorption in such composites due to the morphology of the porosity. It is also noted that use of hollow particles made of glass helps in reducing the overall thermal expansion coefficient and thermal conductivity of the composite, providing better dimensional stability, which can especially benefit in marine and space applications [9,10]. Metal and polymer matrix syntactic foams are characterized in several recent studies for mechanical and tribological properties [6,7,11]. Apart from glass hollow particles, fly ash cenospheres are also widely used in fabricating syntactic foams [11–14]. High strain rate properties of these materials are also of interest for applications requiring high damage tolerance under impulsive loading conditions [13,15]. Design charts presented in some recent theoretical * Corresponding author. Tel.: +1 718 260 3080; fax: +1 718 260 3532. E-mail address: [email protected] (N. Gupta). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.09.007
studies enable selection of appropriate volume fraction and wall thickness of hollow particles to obtain a desired level of mechanical properties in syntactic foams [3]. These charts clearly show that the modulus of lightweight syntactic foams can be higher than that of the matrix material, providing significant benefit in specific modulus. A large number of published studies on hollow particle filled composites have used brittle matrix materials such as epoxy resins [8,16]. Tensile fracture of such composites at small strains is a concern [17]. In addition, the tensile strength is found to be 30–50% lower than the neat resin [17]. Under flexural loading conditions also their fracture is found to initiate on the tensile side of the specimen [18,19]. Several reinforcement mechanisms have been used to improve the tensile strength of such composites. In the first mechanism, glass or carbon fibers are added to the composite microstructure to improve the tensile strength [19–23]. Presence of fibers is found to increase the tensile fracture strength by means of crack bridging. However, compressive modulus is found to decreases due to the presence of random fibers [20]. In the second mechanism nanoclay is used to reinforce the matrix resin used in fabricating syntactic foams [24,25]. A small volume fraction of nanoclay is found to be effective in enhancing the stiffness of the composite. However, such reinforcement is not successful in enhancing the tensile fracture strength because of small length scale of nanoclay particles, leading to inadequate crack bridging effect. Mechanisms of energy dissipation and crack bridging provided by the longer aspect ratio reinforcements can be beneficial in enhancing the mechanical properties of these composites [26,27]. Use of carbon nanotubes can be promising in such applications but for large scale composites the cost of such reinforcement is prohibitive and development of appropriate large
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scale processing methods is very challenging [28,29]. Therefore, the possibility of using carbon nanofibers for enhancing the mechanical properties of syntactic foams is explored in the present study. Carbon nanofibers are considered promising in many structural composite materials for enhancement of mechanical properties without significant increase in the cost of the composite [30,31]. 2. Materials and methods 2.1. Constituent materials Epoxy resin DER 332 and hardener DEH 24, manufactured by DOW Chemical Co., are used as the matrix materials. The resin to hardener ratio is maintained at 14:1 by volume. Four types of glass hollow particles, manufactured by 3M, are used as inclusions. In all composites 0.25 vol.% carbon nanofibers are added. Higher nanofiber content is difficult to disperse and significant clustering and entanglement are observed. Carbon nanofibers, PR-19, supplied by Pyrograf Products Inc., OH, are used as the nanoscale reinforcement. The density of these fibers, provided by the manufacturer, is 1950 kg/m3. The compositions of the fabricated syntactic foams are provided in Table 1. Various properties of particles that are used in fabricating composites are provided in Table 2. The particle size distribution is measured using a sieve shaker and the true particle density is measured using a Quantachrome Ultrapycnometer 1000. The mean particle density calculated from these parameters is also provided in the table. The nomenclature for particle type in Table 2, is consistent with the manufacturer’s notations. Two digits present in the particle type are representative of nominal density of that particle type, for example, S22 particles have nominal density of 220 kg/m3.
Table 1 Compositions of the fabricated composite materials. Particles (vol.%)
Nanofiber (vol.%)
Resin (vol.%)
Hardener (vol.%)
30 40 50
0.25 0.25 0.25
65.1 55.7 46.4
4.65 4.05 3.35
Table 2 Distribution of size and true particle density of microballoons used in the study. Particle type
Particle size distributiona (lm)
Particle volume (%)
True particle densityb (kg/m3)
S22
<25 25–63 63–90 >90
7.16 67.16 24.86 0.82
712.0 234.9 178.7 161.8
S32
<25 25–63 63–90 >90
6.05 62.07 31.23 0.64
587.1 342.0 251.5 278.5
<25 25–63 63–90 >90
7.55 41.95 48.45 2.05
678.3 400.7 312.7 300.4
<25 25–63 63–90 >90
7.06 48.71 43.26 0.97
759.8 489.6 391.1 376.1
K37
K46
a b
Obtained through sieve analysis. Measured using a pycnometer.
Mean particle density (kg/m3)
2.2. Composite fabrication method In the first step nanofibers are mixed with epoxy resin for 30 min at 650 rpm using a mechanical mixer fitted with a high shear impeller. The processing method is optimized according to the previously published studies on using this method for nanoclay dispersion in the same resin [32]. Once nanofibers are dispersed in the resin, microballoons are added in the desired quantity and mixing is continued for another 30 min. At the end of mixing slurry of uniform viscosity is obtained. Ten minutes of degassing time is allowed after mixing. At this stage hardener is added to the slurry and mixed at a slow speed using a wooden dowel to minimize any air entrapment at this stage. The slurry is transferred to aluminum molds of 230 155 12.5 mm3 size. The molds are tapped to release any entrapped air pockets and reduce the unwanted air porosity in the matrix. The composite slabs are cured in the mold for 24 h and post cured in an air convection oven at 100 °C for 3 h. Composition and density of the twelve types of composites fabricated are provided in Table 3. The composite nomenclature starts with ‘‘N” to denote the presence of nanofibers, followed by the nominal particle density, and then the particle volume fraction. Increase in volume fraction of particles of the same wall thickness decreases the composite density. Additionally, particles of higher wall thickness used in the same volume fraction lead to higher composite density. 2.3. Mechanical testing Tensile and compressive tests are conducted on the fabricated specimens. Tests are carried out on Instron 4467 machine having a 50 kN load cell. Crosshead displacement velocities of 0.5 and 1 mm/min are maintained for tensile and compressive testing, respectively. Specimen dimensions for the tensile testing are 230 155 12.5 mm3. A 25.4 mm gauge length Instron extensometer is used to obtain the strain data during testing. The compression test specimens have cubic geometry with each side of 12.5 mm. Crosshead displacement is used to calculate strain in the compression testing. Load and displacement data obtained from Instron Bluehill software is used to calculate stress and strain. Some of the fractured specimens are observed using scanning electron microscope (SEM) after sputter coating with gold. 3. Results and discussion Tensile and compressive modulus and strength results are summarized in Fig. 1. Results for plain syntactic foams of comparable
Table 3 Details of the fabricated composite specimens. Each composite type contains 0.25 vol.% carbon nanofibers. 254 Particle type
Particle volume (%)
Composite nomenclature
Composite density (kg/m3)
S22
30 40 50
N220-30 N220-40 N220-50
871.5 774.6 691.5
S32
30 40 50
N320-30 N320-40 N320-50
853.6 806.7 675.6
K37
30 40 50
N370-30 N370-40 N370-50
880.9 841.8 742.6
K46
30 40 50
N460-30 N460-40 N460-50
918.3 872.0 775.5
328
377
465
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Fig. 1. (a) Tensile modulus, (b) tensile strength, (c) compressive modulus, and (d) compressive strength of nanofiber reinforced syntactic foams.
Fig. 2. Tensile failure features of N220-50 composites: (a) overview of fracture surface, and (b) nanofibers dispersed in the matrix on the fracture surface.
compositions, taken from [8], are also included in this figure for comparison. Tensile modulus shows about 10–20% increase for most foam compositions due to the presence of nanofibers. Tensile strength, which is usually a concern in syntactic foams, shows 20–50% increase for most composites due to the presence of nanofibers. The trends in tensile properties with respect to the hollow particle wall thickness and volume fraction are similar with and without nanofiber reinforcement. Consistent with the previous studies on fibrous reinforcement [20], inclusion of nanofibers results in reduction in compressive modulus of composites. However, the compressive strength remains nearly the same for most compositions. It is also noted that the compressive modulus is 20–30% lower than the tensile modulus for most composites. If the particle size and density data presented in Table 2, are analyzed, then it is noted that the fraction of particles having larger
diameter has lower density. Such lower density particles have thinner walls and usually crush at lower load levels. Only about 7% particles in all types have very small size and high density, which are unlikely to fracture in the initial state of any kind of loading. Under the tensile loading conditions, most particles do not fracture as shown in Fig. 2a, which is obtained on the tensile fracture surface of a specimen. The composite fracture mode is dominated by the deformation and fracture of the matrix resin. In such a case it is probable that only the largest and weakest microballoons break. As per Table 2, the volume fraction of such particles is very small, less than 1% in most particle types, and their effect in the fracture process is not very significant. Fig. 2b shows presence of nanofibers on the fracture surface of the composite, which indicates that the enhancement in the tensile strength is related to
M. Dimchev et al. / Materials and Design 31 (2010) 1332–1337
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Fig. 3. Compressive failure features of N220-50 composites at two different magnification levels.
Fig. 4. Energy absorption in (a) tensile and (b) compressive testing of nanofiber reinforced composites.
the strengthening of the matrix material by nanofibers. The tensile strength and modulus of vapor grown carbon nanofibers are reported as 2.9 GPa and 240 GPa, respectively, in the published studies [33]. These properties are significantly higher than the corresponding properties of the matrix material presented in Fig. 1. Therefore, the presence of nanofibers in the epoxy resin leads to enhancement in the tensile properties of the matrix resin, which has been observed in previous studies [34]. Increased tensile strength and modulus of the matrix resin are reflected as enhancements in the tensile properties of syntactic foams. Presence of carbon nanofibers is also found to enhance the tensile properties of other polymeric resins, such as polypropylene [35] and polystyrene [36]. The nanofibers appear to be wet with the epoxy resin and dispersed in the matrix in Fig. 2b, indicating their role in enhancing the tensile strength of the composite. Under compressive loading conditions, particles are the primary load bearing phase in the composite. In this case crushing of particles is prominent, as observed in Fig. 3, for the same composition as in Fig. 2. Crushing of weaker particles leaves a void in the matrix, which behaves as gas porosity and results in lower modulus. Due to the possibility of particle crushing, the term modulus cannot be applied to compressive test results in a strict sense in such composite materials. Rather, it refers to the slope of the apparent linear region of the stress–strain graphs. This slope is affected by the upper range of particle size, which have the lower density, and present possibility of fracture in early compression stages. It can also be assumed as the value of modulus
at an applied pre-stress, where some particles have already fractured. These observations are also affected by the particle–matrix interfacial strength. Perfect bonding between particles and matrix will lead to significantly higher stress transfer compared to the practical case where interfacial debonding is also observed under tensile loading. In this study no surface treatment is given to the particles in order to have the same particle–matrix interfacial strength so that a direct comparison of the effect of particle wall thickness and volume fraction on the composite properties can be carried out. Features of specimens fractured under compression, shown in Fig. 3, are qualitatively similar to those for the plain syntactic foams observed in previous studies [37]. Extensive particle crushing is observed under compression in these specimens. The specimen fracture is dominated by particle crushing, with only a small role played by the matrix and nanofibers. Therefore, presence of nanofibers does not lead to any enhancement in the compressive properties such composites. Energy absorption in composites with respect to composition and loading condition is presented in Fig. 4. A comparison of energy absorption under tensile and compressive loading conditions illustrates that the significantly high failure strain leads to higher energy absorption under compression. The primary mechanism of energy absorption is particle crushing under compression and matrix deformation and fracture under tensile loading. Fig. 5 shows a N220-50 specimen tested under tensile loading condition, where matrix deformation and fracture marks can be observed,
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Fig. 5. Significant deformation of matrix before tensile fracture in the presence of nanofibers.
which represent the energy absorption mechanism during tensile fracture of the composite. 4. Conclusions Effect of presence of carbon nanofibers on tensile and compressive properties of hollow particle filled composites is experimentally characterized. The nanofiber volume fraction is maintained constant at 0.25% level in all composite compositions. Microscopic studies show that the nanofibers are wet by the matrix resin. Hollow particles of four different wall thicknesses are used in three volume fractions to obtain twelve types of composites. Results show that the tensile properties are enhanced by the presence of nanofibers because matrix fracture plays a greater role in deformation and fracture of the composite under tensile loading conditions compared to the compressive loading conditions. Tensile fracture surface shows broken nanofibers, whereas microballoon crushing is prominent in compressive fracture. Acknowledgments The research is supported by the National Science Foundation Grant # CMMI-0726723 through the Materials Design & Surface Engineering and the Mechanics & Structure of Materials programs. The authors thank 3M for providing microballoons and technical information related to them. MAE Department at NYU-Poly is acknowledged for the support provided. DucAnh An is acknowledged for conducting particle size and density analysis. References [1] Ishai O, Hiel C, Luft M. Long-term hygrothermal effects on damage tolerance of hybrid composite sandwich panels. Composites 1995;26(1):47–55. [2] Grosjean F, Bouchonneau N, Choqueuse D, Sauvant-Moynot V. Comprehensive analyses of syntactic foam behavior in deepwater environment. J Mater Sci 2009;44(6):1462–8. [3] Porfiri M, Gupta N. Effect of volume fraction and wall thickness on the elastic properties of hollow particle filled composites. Composites Part B 2009;40(2):166–73. [4] Karthikeyan CS, Sankaran S, Kishore. Elastic behavior of plain and fiberreinforced syntactic foams under compression. Mater Lett 2004;58(6):995–9. [5] Kulkarni SM, Kishore. Studies on fly ash-filled epoxy-cast slabs under compression. J Appl Polym Sci 2002;84(13):2404–10. [6] Tao XF, Zhao YY. Compressive behavior of Al matrix syntactic foams toughened with Al particles. Scripta Mater 2009;61(5):461–4. [7] Tao XF, Zhang LP, Zhao YY. Al matrix syntactic foam fabricated with bimodal ceramic microspheres. Mater Des 2009;30(7):2732–6. [8] Gupta N, Woldesenbet E, Mensah P. Compression properties of syntactic foams: effect of cenosphere radius ratio and specimen aspect ratio. Composites Part A 2004;35(1):103–11.
[9] Rohatgi PK, Gupta N, Alaraj S. Thermal expansion of aluminum-fly ash cenosphere composites synthesized by pressure infiltration techniques. J Compos Mater 2006;40(13):1163–74. [10] Porfiri M, Nguyen N, Gupta N. Thermal conductivity of multiphase particulate composite materials. J Mater Sci 2009;44(6):1540–50. [11] Mondal DP, Das S, Jha N. Dry sliding wear behavior of aluminum syntactic foam. Mater Des 2009;30(7):2563–8. [12] Dadkar N, Tomar BS, Satapathy BK. Evaluation of fly ash-filled and aramid fiber reinforced hybrid polymer matrix composites (PMC) for friction braking applications. Mater Des 2009;30(10):4369–76. [13] Mondal DP, Goel MD, Das S. Effect of strain rate and relative density on compressive deformation behavior of closed cell aluminum–fly ash composite foam. Mater Des 2009;30(4):1268–74. [14] Mu Y, Yao G, Luo H. The dependence of damping property of fly ash reinforced closed cell aluminum alloy foams on strain amplitude. Mater Des, in press. doi:10.1016/j.matdes.2009.07.050. [15] Viot P, Shankar K, Bernard D. Effect of strain rate and density on dynamic behavior of syntactic foam. Compos Struct 2008;86(4):314–27. [16] Bardella L, Genna F. Elastic design of syntactic foamed sandwiches obtained by filling of three-dimensional sandwich-fabric panels. Int J Solids Struct 2001;38(2):307–33. [17] Gupta N, Nagorny R. Tensile properties of glass microballoon–epoxy resin syntactic foams. J Appl Polym Sci 2006;102(2):1254–61. [18] Wouterson EM, Boey FYC, Hu X, Wong SC. Specific properties and fracture toughness of syntactic foam: effect of foam microstructures. Compos Sci Technol 2005;65(11–12):1840–50. [19] Karthikeyan CS, Sankaran S, Kishore. Investigation of bending modulus of fiber-reinforced syntactic foams for sandwich and structural applications. Polym Adv Technol 2007;18(3):254–6. [20] Gupta N, Karthikeyan CS, Sankaran S, Kishore. Correlation of processing methodology to the physical and mechanical properties of syntactic foams with and without fibers. Mater Charact 1999;43(4):271–7. [21] Karthikeyan CS, Sankaran S, Kishore. Influence of chopped strand fibers on the flexural behavior of a syntactic foam core system. Polym Int 2000;49(2):158–62. [22] Karthikeyan CS, Kishore, Sankaran S. Comparison of compressive properties of fiber-free and fiber-bearing syntactic foams. J Reinf Plast Compos 2000;19(9):732–42. [23] Huang Y-J, Vaikhanski L, Nutt SR. 3D long fiber-reinforced syntactic foam based on hollow polymeric microspheres. Composites Part A 2006;37(3):488–96. [24] Maharsia RR, Jerro HD. Enhancing tensile strength and toughness in syntactic foams through nanoclay reinforcement. Mater Sci Eng, A 2007;454– 455:416–22. [25] Gupta N, Maharsia R. Enhancement of energy absorption in syntactic foams by nanoclay incorporation for sandwich core applications. Appl Compos Mater 2005;12(3–4):247–61. [26] Suhr J, Koratkar N. Energy dissipation in carbon nanotube composites: a review. J Mater Sci 2008;43(13):4370–82. [27] Ajayan P, Suhr J, Koratkar N. Utilizing interfaces in carbon nanotube reinforced polymer composites for structural damping. J Mater Sci 2006;41(23):7824–9. [28] Esawi AMK, Farag MM. Carbon nanotube reinforced composites: Potential and current challenges. Mater Des 2007;28(9):2394–401. [29] Paradise M, Goswami T. Carbon nanotubes – production and industrial applications. Mater Des 2007;28(5):1477–89. [30] Hou X, Yang X, Zhang L, Waclawik E, Wu S. Stretching-Induced crystallinity and orientation to improve the mechanical properties of electrospun PAN nanocomposites. Mater Des, in press. doi:10.1016/j.matdes.2009.01.051. [31] Yördem OS, Papila M, Menceloglu YZ. Effects of electrospinning parameters on polyacrylonitrile nanofiber diameter: an investigation by response surface methodology. Mater Des 2008;29(1):34–44.
M. Dimchev et al. / Materials and Design 31 (2010) 1332–1337 [32] Gupta N, Lin T, Shapiro M. Clay–epoxy nanocomposites: processing and properties. Jom-J Min Met Mater S 2007;59(3):61–5. [33] Tibbetts GG, Lake ML, Strong KL, Rice BP. A review of the fabrication and properties of vapor-grown carbon nanofiber/polymer composites. Compos Sci Technol 2007;67(7–8):1709–18. [34] Zhou Y, Pervin F, Jeelani S, Mallick PK. Improvement in mechanical properties of carbon fabric-epoxy composite using carbon nanofibers. J Mater Process Technol 2008;198(1–3):445–53.
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[35] Kumar S, Doshi H, Srinivasarao M, Park JO, Schiraldi DA. Fibers from polypropylene/nano carbon fiber composites. Polymer 2002;43(5):1701–3. [36] Lee H, Mall S, He P, Shi D, Narasimhadevara S, Yun Y-H, Shanov V, Schulz MJ. Characterization of carbon nanotube/nanofiber-reinforced polymer composites using an instrumented indentation technique. Composites Part B 2007;38(1):58–65. [37] Gupta N, Woldesenbet E, Kishore. Compressive fracture features of syntactic foams-microscopic examination. J Mater Sci 2002;37(15):3199–209.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of carbon nanofibers on tensile and compressive characteristics of hollow particle filled composites Momchil Dimchev, Ryan Caeti, Nikhil Gupta * Composite Materials and Mechanics Laboratory, Department of Mechanical and Aerospace Engineering, Polytechnic Institute of New York University, Brooklyn, NY 11201, USA
a r t i c l e
i n f o
Article history: Received 30 June 2009 Accepted 4 September 2009 Available online 9 September 2009 Keywords: A. Polymer matrix composites B. Foams E. Mechanical properties
a b s t r a c t The effect of presence of carbon nanofibers on the tensile and compressive properties of hollow particle filled composites is studied. Such composites, called syntactic foams, are known to have high specific modulus and low moisture absorption capabilities and are finding applications as core materials in aerospace and marine sandwich structures. The results of this study show that addition of 0.25 wt.% carbon nanofibers results in improvement in tensile modulus and strength compared to similar syntactic foam compositions that did not contain nanofibers. Compressive modulus decreased and strength remained largely unchanged for most compositions. Tensile and compressive failure features are analyzed using scanning electron microscopy. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Use of hollow particles to embed porosity in a matrix material provides several advantages for aerospace and marine structural applications [1,2]. Such composites are called syntactic foams to differentiate from other foams containing open or closed cell gas porosity. Existence of a thin but stiff shell around porosity helps in obtaining low density in the composite material without a severe penalty on mechanical properties [3]. Studies have shown that such porous composites can have higher modulus than the matrix material but the strength is generally lower [3–5]. These materials are able to absorb significant amount of energy under compressive loading conditions due to the presence of porosity, which leads to large strain at a constant load level due to progressive crushing of particles [6–8]. In addition, localized damage to the material does not result in any significant increase in moisture absorption in such composites due to the morphology of the porosity. It is also noted that use of hollow particles made of glass helps in reducing the overall thermal expansion coefficient and thermal conductivity of the composite, providing better dimensional stability, which can especially benefit in marine and space applications [9,10]. Metal and polymer matrix syntactic foams are characterized in several recent studies for mechanical and tribological properties [6,7,11]. Apart from glass hollow particles, fly ash cenospheres are also widely used in fabricating syntactic foams [11–14]. High strain rate properties of these materials are also of interest for applications requiring high damage tolerance under impulsive loading conditions [13,15]. Design charts presented in some recent theoretical * Corresponding author. Tel.: +1 718 260 3080; fax: +1 718 260 3532. E-mail address: [email protected] (N. Gupta). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.09.007
studies enable selection of appropriate volume fraction and wall thickness of hollow particles to obtain a desired level of mechanical properties in syntactic foams [3]. These charts clearly show that the modulus of lightweight syntactic foams can be higher than that of the matrix material, providing significant benefit in specific modulus. A large number of published studies on hollow particle filled composites have used brittle matrix materials such as epoxy resins [8,16]. Tensile fracture of such composites at small strains is a concern [17]. In addition, the tensile strength is found to be 30–50% lower than the neat resin [17]. Under flexural loading conditions also their fracture is found to initiate on the tensile side of the specimen [18,19]. Several reinforcement mechanisms have been used to improve the tensile strength of such composites. In the first mechanism, glass or carbon fibers are added to the composite microstructure to improve the tensile strength [19–23]. Presence of fibers is found to increase the tensile fracture strength by means of crack bridging. However, compressive modulus is found to decreases due to the presence of random fibers [20]. In the second mechanism nanoclay is used to reinforce the matrix resin used in fabricating syntactic foams [24,25]. A small volume fraction of nanoclay is found to be effective in enhancing the stiffness of the composite. However, such reinforcement is not successful in enhancing the tensile fracture strength because of small length scale of nanoclay particles, leading to inadequate crack bridging effect. Mechanisms of energy dissipation and crack bridging provided by the longer aspect ratio reinforcements can be beneficial in enhancing the mechanical properties of these composites [26,27]. Use of carbon nanotubes can be promising in such applications but for large scale composites the cost of such reinforcement is prohibitive and development of appropriate large
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scale processing methods is very challenging [28,29]. Therefore, the possibility of using carbon nanofibers for enhancing the mechanical properties of syntactic foams is explored in the present study. Carbon nanofibers are considered promising in many structural composite materials for enhancement of mechanical properties without significant increase in the cost of the composite [30,31]. 2. Materials and methods 2.1. Constituent materials Epoxy resin DER 332 and hardener DEH 24, manufactured by DOW Chemical Co., are used as the matrix materials. The resin to hardener ratio is maintained at 14:1 by volume. Four types of glass hollow particles, manufactured by 3M, are used as inclusions. In all composites 0.25 vol.% carbon nanofibers are added. Higher nanofiber content is difficult to disperse and significant clustering and entanglement are observed. Carbon nanofibers, PR-19, supplied by Pyrograf Products Inc., OH, are used as the nanoscale reinforcement. The density of these fibers, provided by the manufacturer, is 1950 kg/m3. The compositions of the fabricated syntactic foams are provided in Table 1. Various properties of particles that are used in fabricating composites are provided in Table 2. The particle size distribution is measured using a sieve shaker and the true particle density is measured using a Quantachrome Ultrapycnometer 1000. The mean particle density calculated from these parameters is also provided in the table. The nomenclature for particle type in Table 2, is consistent with the manufacturer’s notations. Two digits present in the particle type are representative of nominal density of that particle type, for example, S22 particles have nominal density of 220 kg/m3.
Table 1 Compositions of the fabricated composite materials. Particles (vol.%)
Nanofiber (vol.%)
Resin (vol.%)
Hardener (vol.%)
30 40 50
0.25 0.25 0.25
65.1 55.7 46.4
4.65 4.05 3.35
Table 2 Distribution of size and true particle density of microballoons used in the study. Particle type
Particle size distributiona (lm)
Particle volume (%)
True particle densityb (kg/m3)
S22
<25 25–63 63–90 >90
7.16 67.16 24.86 0.82
712.0 234.9 178.7 161.8
S32
<25 25–63 63–90 >90
6.05 62.07 31.23 0.64
587.1 342.0 251.5 278.5
<25 25–63 63–90 >90
7.55 41.95 48.45 2.05
678.3 400.7 312.7 300.4
<25 25–63 63–90 >90
7.06 48.71 43.26 0.97
759.8 489.6 391.1 376.1
K37
K46
a b
Obtained through sieve analysis. Measured using a pycnometer.
Mean particle density (kg/m3)
2.2. Composite fabrication method In the first step nanofibers are mixed with epoxy resin for 30 min at 650 rpm using a mechanical mixer fitted with a high shear impeller. The processing method is optimized according to the previously published studies on using this method for nanoclay dispersion in the same resin [32]. Once nanofibers are dispersed in the resin, microballoons are added in the desired quantity and mixing is continued for another 30 min. At the end of mixing slurry of uniform viscosity is obtained. Ten minutes of degassing time is allowed after mixing. At this stage hardener is added to the slurry and mixed at a slow speed using a wooden dowel to minimize any air entrapment at this stage. The slurry is transferred to aluminum molds of 230 155 12.5 mm3 size. The molds are tapped to release any entrapped air pockets and reduce the unwanted air porosity in the matrix. The composite slabs are cured in the mold for 24 h and post cured in an air convection oven at 100 °C for 3 h. Composition and density of the twelve types of composites fabricated are provided in Table 3. The composite nomenclature starts with ‘‘N” to denote the presence of nanofibers, followed by the nominal particle density, and then the particle volume fraction. Increase in volume fraction of particles of the same wall thickness decreases the composite density. Additionally, particles of higher wall thickness used in the same volume fraction lead to higher composite density. 2.3. Mechanical testing Tensile and compressive tests are conducted on the fabricated specimens. Tests are carried out on Instron 4467 machine having a 50 kN load cell. Crosshead displacement velocities of 0.5 and 1 mm/min are maintained for tensile and compressive testing, respectively. Specimen dimensions for the tensile testing are 230 155 12.5 mm3. A 25.4 mm gauge length Instron extensometer is used to obtain the strain data during testing. The compression test specimens have cubic geometry with each side of 12.5 mm. Crosshead displacement is used to calculate strain in the compression testing. Load and displacement data obtained from Instron Bluehill software is used to calculate stress and strain. Some of the fractured specimens are observed using scanning electron microscope (SEM) after sputter coating with gold. 3. Results and discussion Tensile and compressive modulus and strength results are summarized in Fig. 1. Results for plain syntactic foams of comparable
Table 3 Details of the fabricated composite specimens. Each composite type contains 0.25 vol.% carbon nanofibers. 254 Particle type
Particle volume (%)
Composite nomenclature
Composite density (kg/m3)
S22
30 40 50
N220-30 N220-40 N220-50
871.5 774.6 691.5
S32
30 40 50
N320-30 N320-40 N320-50
853.6 806.7 675.6
K37
30 40 50
N370-30 N370-40 N370-50
880.9 841.8 742.6
K46
30 40 50
N460-30 N460-40 N460-50
918.3 872.0 775.5
328
377
465
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Fig. 1. (a) Tensile modulus, (b) tensile strength, (c) compressive modulus, and (d) compressive strength of nanofiber reinforced syntactic foams.
Fig. 2. Tensile failure features of N220-50 composites: (a) overview of fracture surface, and (b) nanofibers dispersed in the matrix on the fracture surface.
compositions, taken from [8], are also included in this figure for comparison. Tensile modulus shows about 10–20% increase for most foam compositions due to the presence of nanofibers. Tensile strength, which is usually a concern in syntactic foams, shows 20–50% increase for most composites due to the presence of nanofibers. The trends in tensile properties with respect to the hollow particle wall thickness and volume fraction are similar with and without nanofiber reinforcement. Consistent with the previous studies on fibrous reinforcement [20], inclusion of nanofibers results in reduction in compressive modulus of composites. However, the compressive strength remains nearly the same for most compositions. It is also noted that the compressive modulus is 20–30% lower than the tensile modulus for most composites. If the particle size and density data presented in Table 2, are analyzed, then it is noted that the fraction of particles having larger
diameter has lower density. Such lower density particles have thinner walls and usually crush at lower load levels. Only about 7% particles in all types have very small size and high density, which are unlikely to fracture in the initial state of any kind of loading. Under the tensile loading conditions, most particles do not fracture as shown in Fig. 2a, which is obtained on the tensile fracture surface of a specimen. The composite fracture mode is dominated by the deformation and fracture of the matrix resin. In such a case it is probable that only the largest and weakest microballoons break. As per Table 2, the volume fraction of such particles is very small, less than 1% in most particle types, and their effect in the fracture process is not very significant. Fig. 2b shows presence of nanofibers on the fracture surface of the composite, which indicates that the enhancement in the tensile strength is related to
M. Dimchev et al. / Materials and Design 31 (2010) 1332–1337
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Fig. 3. Compressive failure features of N220-50 composites at two different magnification levels.
Fig. 4. Energy absorption in (a) tensile and (b) compressive testing of nanofiber reinforced composites.
the strengthening of the matrix material by nanofibers. The tensile strength and modulus of vapor grown carbon nanofibers are reported as 2.9 GPa and 240 GPa, respectively, in the published studies [33]. These properties are significantly higher than the corresponding properties of the matrix material presented in Fig. 1. Therefore, the presence of nanofibers in the epoxy resin leads to enhancement in the tensile properties of the matrix resin, which has been observed in previous studies [34]. Increased tensile strength and modulus of the matrix resin are reflected as enhancements in the tensile properties of syntactic foams. Presence of carbon nanofibers is also found to enhance the tensile properties of other polymeric resins, such as polypropylene [35] and polystyrene [36]. The nanofibers appear to be wet with the epoxy resin and dispersed in the matrix in Fig. 2b, indicating their role in enhancing the tensile strength of the composite. Under compressive loading conditions, particles are the primary load bearing phase in the composite. In this case crushing of particles is prominent, as observed in Fig. 3, for the same composition as in Fig. 2. Crushing of weaker particles leaves a void in the matrix, which behaves as gas porosity and results in lower modulus. Due to the possibility of particle crushing, the term modulus cannot be applied to compressive test results in a strict sense in such composite materials. Rather, it refers to the slope of the apparent linear region of the stress–strain graphs. This slope is affected by the upper range of particle size, which have the lower density, and present possibility of fracture in early compression stages. It can also be assumed as the value of modulus
at an applied pre-stress, where some particles have already fractured. These observations are also affected by the particle–matrix interfacial strength. Perfect bonding between particles and matrix will lead to significantly higher stress transfer compared to the practical case where interfacial debonding is also observed under tensile loading. In this study no surface treatment is given to the particles in order to have the same particle–matrix interfacial strength so that a direct comparison of the effect of particle wall thickness and volume fraction on the composite properties can be carried out. Features of specimens fractured under compression, shown in Fig. 3, are qualitatively similar to those for the plain syntactic foams observed in previous studies [37]. Extensive particle crushing is observed under compression in these specimens. The specimen fracture is dominated by particle crushing, with only a small role played by the matrix and nanofibers. Therefore, presence of nanofibers does not lead to any enhancement in the compressive properties such composites. Energy absorption in composites with respect to composition and loading condition is presented in Fig. 4. A comparison of energy absorption under tensile and compressive loading conditions illustrates that the significantly high failure strain leads to higher energy absorption under compression. The primary mechanism of energy absorption is particle crushing under compression and matrix deformation and fracture under tensile loading. Fig. 5 shows a N220-50 specimen tested under tensile loading condition, where matrix deformation and fracture marks can be observed,
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Fig. 5. Significant deformation of matrix before tensile fracture in the presence of nanofibers.
which represent the energy absorption mechanism during tensile fracture of the composite. 4. Conclusions Effect of presence of carbon nanofibers on tensile and compressive properties of hollow particle filled composites is experimentally characterized. The nanofiber volume fraction is maintained constant at 0.25% level in all composite compositions. Microscopic studies show that the nanofibers are wet by the matrix resin. Hollow particles of four different wall thicknesses are used in three volume fractions to obtain twelve types of composites. Results show that the tensile properties are enhanced by the presence of nanofibers because matrix fracture plays a greater role in deformation and fracture of the composite under tensile loading conditions compared to the compressive loading conditions. Tensile fracture surface shows broken nanofibers, whereas microballoon crushing is prominent in compressive fracture. Acknowledgments The research is supported by the National Science Foundation Grant # CMMI-0726723 through the Materials Design & Surface Engineering and the Mechanics & Structure of Materials programs. The authors thank 3M for providing microballoons and technical information related to them. MAE Department at NYU-Poly is acknowledged for the support provided. DucAnh An is acknowledged for conducting particle size and density analysis. References [1] Ishai O, Hiel C, Luft M. Long-term hygrothermal effects on damage tolerance of hybrid composite sandwich panels. Composites 1995;26(1):47–55. [2] Grosjean F, Bouchonneau N, Choqueuse D, Sauvant-Moynot V. Comprehensive analyses of syntactic foam behavior in deepwater environment. J Mater Sci 2009;44(6):1462–8. [3] Porfiri M, Gupta N. Effect of volume fraction and wall thickness on the elastic properties of hollow particle filled composites. Composites Part B 2009;40(2):166–73. [4] Karthikeyan CS, Sankaran S, Kishore. Elastic behavior of plain and fiberreinforced syntactic foams under compression. Mater Lett 2004;58(6):995–9. [5] Kulkarni SM, Kishore. Studies on fly ash-filled epoxy-cast slabs under compression. J Appl Polym Sci 2002;84(13):2404–10. [6] Tao XF, Zhao YY. Compressive behavior of Al matrix syntactic foams toughened with Al particles. Scripta Mater 2009;61(5):461–4. [7] Tao XF, Zhang LP, Zhao YY. Al matrix syntactic foam fabricated with bimodal ceramic microspheres. Mater Des 2009;30(7):2732–6. [8] Gupta N, Woldesenbet E, Mensah P. Compression properties of syntactic foams: effect of cenosphere radius ratio and specimen aspect ratio. Composites Part A 2004;35(1):103–11.
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