TiO2 nanocomposites

Materials Science and Engineering A361 (2003) 358–366

Fabrication, characterization, and dynamic behavior of polyester/TiO2 nanocomposites Victor M.F. Evora∗ , Arun Shukla Dynamic Photomechanics Laboratory, Department of Mechanical Engineering and Applied Mechanics, University of Rhode Island, Kingston, RI 02881, USA Received 10 February 2003; received in revised form 28 June 2003

Abstract Unsaturated polyester resin specimens embedded with small loadings of 36 nm average diameter TiO2 particles were fabricated using a direct ultrasonification method to study the effects of nanosized particles on nanocomposite bulk mechanical properties. The ultrasonification method employed produced nanocomposites with excellent particle dispersion as verified by transmission electron microscopy (TEM). Quasi-static fracture toughness, tension, and compression testing was carried out. The presence of the particles had the greatest effect on fracture toughness; negligible influence was observed in the remaining quasi-static properties. Scanning electron microscopy (SEM) of fracture surfaces was carried out to identify toughening mechanisms. The inadequacy of the bond between the filler and the matrix and the presence of minor particle agglomerations in specimens containing higher volume fractions of particles were believed to be responsible for a consistent decrease in property values beyond a volume fraction of 1 vol.%. Dynamic fracture toughness testing was carried out, and an increase in dynamic fracture toughness relative to quasi-static fracture toughness was observed. High strain rate testing conducted using a split Hopkinson pressure bar (SHPB) apparatus revealed a moderate stiffening effect with increasing particle volume fraction, although no marked effect was observed on the ultimate strength. © 2003 Elsevier B.V. All rights reserved. Keywords: Fracture toughness; Dynamic fracture; Nanocomposites; Split Hopkinson pressure bar; Strain rate; Particle reinforcement

1. Introduction Advancements in material performance depend on the ability to synthesize new materials that exhibit enhanced properties, such as strength, fracture toughness, impact resistance, durability, etc. Nanocomposites are ideal materials to meet this challenge, as it has been shown that they have the potential to deliver the aforementioned properties oftentimes with minimal increase in weight; a luxury not always realized with conventional composites or metals. It should be pointed out, however, that the degree of enhancement of a particular property is highly dependent on the matrix/filler type system used, the extent of filler adhesion to the matrix, and the level of dispersion of the filler throughout the matrix. Polymer resins have been used extensively as matrix materials for many high-performance components in the aero∗ Corresponding author. Present address: Naval Undersea Warfare Center Division, Newport, RI 02841, USA. Tel.: +1-401-832-8475; fax: +1-401-832-2431. E-mail address: [email protected] (V.M.F. Evora).

0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5093(03)00536-7

space, automobile, and electronics industry because of their mechanical, electrical, and chemical properties. Highly cross-linked thermosetting polymeric materials, such as the unsaturated polyester resin used as the matrix material in this study, are extremely brittle owing to their covalently bonded network structure, and thus are poor inhibitors of crack initiation and propagation. Nonetheless, researchers have been able to improve their toughness with the addition of soft, as well as rigid fillers, although it is important to note that the increase in toughness thus obtained is often accompanied by a concomitant decrease in modulus and strength [1–4]. Many theories have been proposed addressing the toughening mechanisms observed in epoxy-based composites. They range from massive crazing, shear flow, and stretching in the case of rubber-toughened epoxies to crack pinning, trapping, and blunting for rigid particle in the case of high modulus particle-filled epoxies [4–7]. In order to make a successful nanocomposite, it is very important to be able to disperse the filler material thoroughly throughout the matrix to maximize the interaction between the intermixed phases. This, however, is not an easy task, as

V.M.F. Evora, A. Shukla / Materials Science and Engineering A361 (2003) 358–366

nanoparticles tend to form agglomerates, or clusters, containing numerous nanoparticles, often negating the sought after “nano effect” [8]. Several techniques have been employed to synthesize nanocomposites: sol–gel, hot pressing of powders, and melt intercalation [9–12]. In situ polymerization is a straightforward and relatively novel method to produce nanocomposites in which the resin and filler are mixed, cast into a mold, and polymerized [5,13]. Mechanical characterization of nanocomposites has focused primarily on measurements of elastic modulus, strength, and quasi-static fracture toughness [10,14–16]. More recently though, researchers have begun to expand the scope of mechanical testing into fatigue, creep, sliding wear resistance, and polishing behavior [17–19]. Comparative studies have also been conducted on the fracture behavior and fracture toughness of micro- and nano-sized particles embedded in commercial resins [1,5,13]. The advent of any new material inevitably leads to the need to characterize its properties and understand its behavior under different loading conditions. The response of materials to high strain rates is very important, as in practical engineering applications structural materials are subjected, either by design or by accident, to a variety of loading rates. However, despite the abundance of conventional mechanical characterization of polymer-based nanocomposites, to date, there seems to be a lack of work in the area of dynamic fracture and high strain rate characterization. This study seeks to address these characterization issues.

2. Nanocomposite fabrication Few nanocomposite materials are commercially available and are very expensive. Furthermore, they are typically avail-

able from the manufacturer in small pallets the size of a grain of rice that must be melted and extruded into desired shapes suitable for mechanical testing. On the other hand, a general purpose unsaturated polyester (MR17090) available from Ashland Chemical Company is a castable polyester resin that can be easily cured with a simple procedure [20]. The resin is a highly cross-linked thermosetting polymer that is transparent and very brittle; an ideal candidate for the study of the effect(s) of nanoparticles on fracture behavior. Given the simplicity with which this resin can be cast and the other attributes it provides, it became the matrix material of choice for this study. TiO2 (titania) nanoparticles with 36 nm average diameter, obtained from Nanophase Technologies Corporation, were chosen as the filler component because of their spherical morphology, high modulus, and availability. 2.1. Casting procedure Spherical titania nanoparticles were added to 230 mg of polyester resin at varying volume percentages, and mechanically mixed in a glass beaker for approximately 5 min. The mixture was placed in a vacuum chamber at 28 Torr for 5 min to remove trapped air bubbles generated during the mechanical mixing process. After the deaeration process, the mixture was poured into a stainless steel beaker surrounded by an ice enclosure, and a 2 cm diameter acoustic probe from a Vibra-Cell ultrasonic processor resonant at 20 kHz was used to disperse the nanoparticles throughout the matrix. Ultrasonic energy was employed for 70 min in the pulsing mode (10 s on, 10 s off) for an effective sonication time of 35 min. Pulsing was primarily used to inhibit heat build-up in the specimen, but additionally enhanced material processing by allowing the material to settle back under the probe after each ultrasonic burst. The ice enclosure was simultaneously

Acoustic probe

Mechanical mixing

Ice basin

Vacuum chamber

Vacuum chamber

Mold

359

Cast sheet

Fig. 1. Schematic of nanocomposite fabrication procedure.

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Fig. 2. TEM micrographs depicting level of TiO2 nanoparticle dispersion within the polyester matrix. Volume fraction of particles: (a) 1 vol.%; (b) 2 vol.%; (c) 3 vol.%; (d) 4 vol.%.

used to inhibit heat build-up. After the sonication process, the catalyst methyl ethyl ketone peroxide (MEKP) and the accelerator cobalt octate were added separately to the mixture and mixed thoroughly at 0.85 and 0.03% by weight of polyester, respectively, to initiate and accelerate the polymerization process. The mixture was then very briefly deaerated at 28 Torr to remove trapped air bubbles generated during the sonication process. The brevity of the second deaeration process was necessary as polymerization commences almost immediately after the catalyst and the accelerator are added. The final mixture was poured into a rectangular mold lined with thin (0.18 mm) mylar sheets to obtain smooth flat surfaces. In order to prevent particle settlement at the bottom, the mold was rotated at 2 rpm for at least 6 h until the mixture was rigid enough so that particles could no longer migrate. The resin mixture was allowed to cure at room temperature for 72 h. After this time, the specimen was taken out from the mold and post-cured in an air-circulating oven for 4 h at 52 ◦ C followed by 5 h at 63 ◦ C. After allowing the oven to come down to room temperature, the specimens were taken out and machined to the desired shapes. The above procedure was perfected after numerous iterations of different combinations of sonication times, intervals, and resin quantities. The fabrication evolution is shown schematically in Fig. 1. 2.2. Microstructural observations Transmission electron microscopy (TEM) analysis of the cast specimens was conducted to verify the level of nanoparticle dispersion in the matrix. TEM micrographs of specimens containing 1, 2, 3, and 4 vol.% TiO2 are shown in

Fig. 2. As can be seen from these pictures, the fabrication procedure employed produced nanocomposites with excellent particle dispersion, particularly in specimens containing 1, 2, and 3 vol.% TiO2 . Though it is evident that there were fundamentally more and somewhat larger agglomerates present in the specimen containing 4 vol.% TiO2 , it is also clear that the vast majority of these agglomerates were still in the nanometer size range. Thus, the use of these specimens in the testing provided the added benefit of clarifying the effect(s) of agglomeration on this particular nanocomposite system.

3. Materials and methods Quasi-static tests consisted of fracture toughness, tension, and compression testing. Dynamic tests consisted of high strain rate compression testing using a split Hopkinson pressure bar (SHPB) apparatus and dynamic fracture testing using a modified SHPB (MSHPB) apparatus [21]. 3.1. Quasi-static experiments Specimens were fabricated using the procedure previously described. Five experiments were conducted at each volume fraction. The results were averaged and are presented with a 95% confidence interval calculated using Student’s t-distribution, as denoted by the error bars in the plots. 3.1.1. Quasi-static fracture toughness testing Quasi-static fracture toughness testing was carried out pursuant to ASTM D5045 [22]. Specimen configuration is

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W

a

B

S Fig. 3. Schematic of three-point bend specimen.

1

1/2

Fracture Toughness (MPa-m )

shown in Fig. 3. The length, width, and thickness of the specimens were 50, 12, and 6 mm, respectively. As with the testing of metals, the above ASTM standard requires that the initial crack on the specimen be sharp. Thus, after an initial notch was machined with a circular saw, a sharp razor blade was tapped into the notch to produce a sharp natural crack. Tapping experience was first gained by tapping into transparent polyester specimens, as the growth of the natural crack was clearly visible. The same tapping technique was then used on the opaque nanocomposite specimens, and postmortem microscopy analysis of the natural cracks created in these specimens validated the tapping technique. Fig. 4 shows the variation in quasi-static fracture toughness as a function of volume percentage of TiO2 nanoparticles. It is clear that the addition of the particles had a great effect on fracture toughness. Even at small loadings of 1, 2, and 3 vol.%, increases in toughness of 57, 42, and 41%, respectively, were observed when compared with the virgin polyester. At 4 vol.% though, the toughness (0.55 MPa m1/2 ) was approximately that of the polyester (0.54 MPa m1/2 ). As previously mentioned, specimens containing 1, 2, and 3 vol.% TiO2 possessed excellent particle dispersion with negligible agglomeration. On the other hand, for specimens containing 4 vol.% TiO2 , more agglomerations were present. It is believed that the initial increase in fracture toughness, followed by the precipitous decline observed at the 4 vol.% point, is directly related to the level of dispersion

0.9 0.8 0.7 0.6 0.5 0.4 -1

0

1

2

3

4

Volume% TiO2 Fig. 4. Variation of quasi-static fracture toughness as a function of volume fraction of TiO2 nanoparticles.

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of the nanoparticles within the matrix and to the weak bond which exists between the fairly inert titania particles and the polyester. Particles can inhibit crack initiation and propagation if dispersion is such that mechanisms like crack trapping and pinning are allowed to occur [23]. Crack trapping, in particular, can be promoted by ensuring that there exists a strong bond between the filler and the matrix. Although not used in this study, silane has been used for this purpose [4,24]. Agglomerations, on the other hand, can behave as crack initiation sites, lowering the fracture toughness of the composite. The 10% decrease in toughness observed between the specimen containing 1 vol.% TiO2 (0.85 MPa m1/2 ) and that of the specimens containing 2 and 3 vol.% (0.77 and 0.76 MPa m1/2 , respectively), is attributed to an increase, although minor, in agglomerations present in the 2 and 3 vol.% specimens. In other words, as the volume fraction of particles was increased beyond 1 vol.%, whatever advantage was gained up to this point began to diminish. Scanning electron microscopy (SEM) analysis of the region near crack initiation was carried out on the fractured surfaces of the specimens to identify toughening mechanisms believed to be responsible for the observed changes in quasi-static fracture toughness. Fig. 5 shows the results of the SEM analysis. It should be pointed out that energy dispersive spectroscopy (EDS) analysis was simultaneously conducted on the fractured surfaces to determine the chemical composition of various regions. In fact, EDS indicated that the white round spots shown in Fig. 5b–d were rich in Ti, thus confirming that the spots were indeed titania nanoparticles. It is evident from Fig. 5 that there is a qualitative difference in the fracture surface features of the different specimens. The fracture surface of the virgin polyester specimen is relatively smooth, indicating that minimal energy was required to fracture the specimen. On the other hand, the surfaces of the specimens embedded with nanoparticles depict rougher features such as out-of-plane flaking and thumbnail-type markings that require additional energy to be formed. As more energy was imparted from the strain energy of the deforming specimens for the creation of these features, a corresponding increase in fracture toughness was obtained vis-a-vis the neat polyester. Differences in fracture surface features among the specimens containing nanoparticles are still very pronounced. Thumbnail-type markings shown in Fig. 5c are indicative of crack pinning by the particles. Evidence of crack trapping can also be seen in this figure. The predominant fracture surface features in Fig. 5c and d are out-of-plane flaking, depicted by the crisscross pattern. The specimens represented in these two figures (2 and 3 vol.%, respectively) exhibited fracture toughness values that were lower than that exhibited by the specimen containing 1 vol.%. This indicates that the thumbnail markings are a more dominant type of toughening mechanisms than that the out-of-plane flaking.

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Fig. 5. SEM micrographs of fracture surfaces resulting from quasi-static fracture toughness testing. Volume fraction of TiO2 nanoparticles: (a) 0 vol.%; (b) 1 vol.%; (c) 2 vol.%; (d) 3 vol.%.

3.1.2. Quasi-static tension and compression testing Quasi-static tension and compression testing were carried out in accordance with ASTM D638 and ASTM D695, respectively [25,26]. Figs. 6–8 show the variation of tensile strength, modulus, and compressive strength as a function of volume fraction of TiO2 . It is clear from these graphs that changes in properties with increased volume fraction of TiO2 were small. Nevertheless, it is interesting to note the typical profile depicted in these graphs. From 0 to 1 vol.%, there consistently seems to be an increase in property values, followed by a decrease beyond 1 vol.%. It is believed that for the composite system used in this study, the specimens containing 1 vol.% particles possess an optimum level of dis-

persion, and, hence, interparticle distance that contributes to the consistent observation of increased strength and modulus. Recall that in the fracture toughness tests, the highest values were also obtained at this exact volume fraction. The decrease in strength and modulus beyond the 1 vol.% point is believed to be a direct result of the aforementioned weak interfacial bond between the filler and the matrix and the presence of minor agglomerations in the 2 and 3 vol.% specimens vis-a-vis the 1 vol.% specimen. With properties such as the ones tested in this section, the ability of the strong bond to transfer load from the weak matrix to the rigid filler

Tensile Strength ( MPa )

55

Tensile Modulus ( GPa )

4.5

4

3.5

3

50

45

40

35

2.5 -1

0

1

2

3

Volume% TiO 2

Fig. 6. Variation of tensile modulus as a function of volume fraction of TiO2 nanoparticles.

-1

0

1

2

3

Volume% TiO2 Fig. 7. Variation of tensile strength as a function of volume fraction of TiO2 nanoparticles.

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In an effort to minimize the effects of impedance mismatch between the polyester-based nanocomposites and the bar, the latter was made from polymeric LEXAN. The projectile and bar were both made from the same material in order for a well-defined single pulse to be generated. Additionally, a thin sheet of cardboard paper was placed between the projectile and the bar to reduce high frequency components in the incident pulse. The force history at the bar/specimen interface was obtained using one-dimensional elastic wave propagation theory according to the following relation [28]:

160 155 Compressive Strength ( MPa )

363

150 145 140 135 130 125

F(t) = [εi (t) + εr (t)]EA

120 -1

0

1

2

3

Volume% TiO2

Fig. 8. Variation compressive strength as a function of volume fraction of TiO nanoparticles.

is crucial. If such a bond does not exist or is weak, the load is borne primarily by the weak matrix causing the material to fail prematurely. 3.2. Dynamic experiments 3.2.1. Dynamic three-point bend testing using a modified SHPB apparatus Experiments were carried out to investigate the effect of volume fraction of TiO2 nanoparticles on dynamic fracture toughness. Specimens were once again fabricated using the procedure previously described and were machined in accordance with ASTM D-5045 [22]. Specimen configuration is shown in Fig. 3. The length, width, and thickness of the specimens were 130, 50, and 12 mm, respectively. An instrumented modified SHPB apparatus was used to load a three-point bend specimen to dynamic failure as shown in Fig. 9. A brief description of the testing mechanism is presented here. Theoretical details of the test methodology can be found elsewhere [21,27]. A cylindrical projectile was propelled down the barrel of a gas gun by means of compressed air. Upon impact with the cylindrical bar, a compressive pulse was generated, which traveled down the length of the bar to the bar/specimen interface. Upon reaching the specimen, the impact force of the pulse was transmitted to the specimen and this ultimately loaded the crack tip. Incident and reflected strain-pulse histories were obtained from two diametrically opposed strain gages located at the midpoint of the bar. Incident bar

where F is the force, εi and εr the incident and transmitted strain pulses, E the dynamic Young’s modulus, and A is the cross-sectional area of the bar. Since the time to fracture was allowed to be sufficiently long, inertia effects could be neglected, and the following mode I stress intensity factor relation [29] could be used to calculate the dynamic fracture toughness: F(t)
a  KI (t) = √ f (2) W B W where KI is the stress intensity factor, B the specimen thickness, W the specimen width, a the initial crack length, and f(a/W) is a geometric factor. The dynamic fracture toughness (KID ) then corresponded to the value of stress intensity factor at the time of crack initiation, i.e. KID = KI (tinitial )

(3)

Photoelastic analysis using high-speed photography was initially used on transparent polyester specimens to ascertain that the crack initiation time thus obtained coincided with the peak force obtained using the MSHPB technique. As well, this analysis served to validate the value of KID calculated using Eq. (3). Fig. 10a shows the dynamic stress intensity factor profile as a function of time for one of the polyester specimens tested. Fig. 10b shows the specimen’s corresponding photoelastic fringe pattern 7 ␮s before crack initiation. We note that there was good agreement between the measurements of KID using the two techniques. Fig. 11 shows the variation in dynamic fracture toughness as a function of volume percentage of TiO2 nanoparticles. Once again, the error bars represent a 95% confidence interval resulting from the average of five tests conducted at each volume fraction. Quasi-static fracture toughness results are also included in this figure for comparison. It should be 3-point bend specimen

Projectile

Strain gage

(1)

Rigid block

Fig. 9. Schematic of the modified split Hopkinson pressure bar apparatus.

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Fig. 10. (a) Dynamic stress intensity factor profile of a polyester specimen. (b) Photoelastic fringe pattern in the polyester specimen of (a), 7 ␮s before crack initiation. KID value calculated using photoelastic analysis.

pointed out that in all these experiments the projectile ve˙ was approximately locity was such that the loading rate, K, 6.9 MPa m1/2 ms−1 . It is interesting to note that the dynamic results exhibit a profile similar to that of the static results. The dynamic profile corresponds approximately to the static profile shifted upward. In the dynamic case, however, we note the absence of the precipitous drop in fracture toughness with the 4 vol.% specimens. In this case, the 3 and 4 vol.% specimens have essentially the same toughness (0.79 and 0.78 MPa m1/2 , respectively). Once again, the specimen with the highest dynamic fracture toughness value was the one containing 1 vol.% TiO2 (0.95 MPa m1/2 ). It is believed that increases in dynamic fracture toughness observed in the nanocomposites were primarily due to the rate dependency of the material. The nanocomposites consistently have higher dynamic fracture toughness values (28, 20, 7, and 5% for 1, 2, 3, and 4 vol.%, respectively) when compared with the dynamic toughness of the virgin polyester.

Fracture Toughness ( MPa-m1/2 )

1.1 䊉 Quasi-static

1

䊊

0.9

Dynamic

0.8 0.7 0.6 0.5 0.4 -1

0

1

2

3

4

Volume% TiO 2 Fig. 11. Comparison between quasi-static and dynamic fracture toughness values as a function of volume fraction of TiO2 nanoparticles.

3.2.2. High strain rate compression testing using an SHPB apparatus The SHPB technique is the most widely used experimental test method to study material behavior under conditions of uniform deformation between 102 and 104 s−1 . Very basic principles of the testing methodology are presented here. Additional theoretical details on the technique may be found elsewhere [30,31]. Refer to Fig. 12 for a schematic of the SHPB apparatus. A cylindrical specimen is sandwiched between the incident and transmitter bars. The bars are instrumented with diametrically opposed strain gages mounted at stations A and B to record the strain–time history of the pulses. A projectile propelled from the end of a barrel of a gas gun impacts the incident bar and a compressive pulse is generated. Upon reaching the incident bar/specimen interface, the compressive pulse is partially transmitted through the specimen and partially reflected into the incident bar. Using one-dimensional wave propagation theory, it can be shown that the amplitude of the reflected and transmitted pulses is proportional to the strain rate and stress in the deforming specimen, respectively [30]. The reflected pulse is integrated to obtain the strain, and combined with the transmitted pulse, the specimen dynamic stress–strain curve is produced. The setup used in this study consisted of an aluminum projectile and bars. Specimen dimensions (10 mm in diameter × 3 mm in thickness) were chosen so that inertia effects were kept to a minimum. The gas gun was charged to 0.21 MPa, which produced 2000 s−1 strain rate experiments. Three specimens were tested at each volume fraction. The dynamic constitutive behavior of the specimens tested is shown in Fig. 13. It is clear from this figure that the addition of nanoparticles had no marked effect on the dynamic strength. As well, there was no appreciable difference in the failure modes. The strengths obtained were 186, 185, and 185 MPa for 0, 1, and 3 vol.% specimens, respectively.

V.M.F. Evora, A. Shukla / Materials Science and Engineering A361 (2003) 358–366

Incident bar

Specimen

365

Transmitter bar

Projectile

Strain gage station A

Strain gage station B

Fig. 12. Schematic of the split Hopkinson pressure bar apparatus.

200

True Stress ( MPa)

180

Strain rate = 2000 s -1

160 140 120

3 Vol% TiO2

100 80

1 Vol% TiO2

60 40

Polyester

20 0 0

1

2

3

4

5

6

True Strain ( % ) Fig. 13. Dynamic constitutive behavior of nanocomposites at different volume fractions of TiO2 nanoparticles.

On the other hand, the nanocomposites showed a modest stiffening behavior, as seen from the initial linear portion of the graph.

4. Conclusions A fabrication technique using direct ultrasonification has been devised that produces polyester/TiO2 nanocomposites with excellent particle dispersion throughout the matrix, as confirmed by TEM observations. A subsequent study of the effects of TiO2 nanoparticles on quasi-static and dynamic bulk mechanical properties has been conducted. The results can be summarized as follows: 1. The presence of the particles has a significant effect on quasi-static fracture toughness. Even at volume fractions as low as 1, 2, and 3 vol.%, increases in toughness of 57, 42, and 41%, respectively, are observed vis-a-vis the neat polyester. The slight decrease in toughness beyond 1 vol.% is attributed to a slight increase in agglomerations beyond this point. 2. Fracture surface features such as thumbnail markings and out-of-plane flaking, as observed by SEM, indicate that toughening mechanisms such as crack pinning and crack trapping are characteristic of this nanocomposite system; the former being the dominant type. 3. Changes in quasi-static material properties in tension and compression with increasing volume fraction of nanoparticles are small due to the weak interfacial bond between the matrix and the filler.

4. Dynamic fracture toughness is higher than quasi-static fracture toughness for all volume fractions tested (0, 1, 2, 3, and 4%). The specimen containing 1 vol.% TiO2 nanoparticles has the highest value of dynamic fracture toughness (0.95 MPa m1/2 ). 5. High strain rate (2000 s−1 ) compression testing using an SHPB apparatus shows that the addition of nanoparticles contributes to a moderate stiffening behavior, although no marked effect is observed on ultimate strength (186, 185, and 185 MPa for 0, 1, and 3 vol.% specimens, respectively).

Acknowledgements The authors wish to thank Prof. Raman P. Singh from SUNY, Stony Brook and Prof. Vikas Prakash from Case Western Reserve University for their valuable suggestions during several discussions regarding this research effort. Victor M.F. Evora would like to acknowledge the financial support of the Naval Undersea Warfare Center ILIR Program. References [1] M. Hussain, A. Nakahira, S. Nishijima, K. Niihara, Mater. Lett. 27 (1996) 21. [2] A.J. Kinloch, S.J. Shaw, D.A. Tod, D.L. Hunston, Polymer 24 (1983) 1341. [3] A.C. Moloney, H.H. Kausch, H.R. Stieger, J. Mater. Sci. 19 (1984) 1125.

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