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Abstract. The addition of colloidal shear thickening fluids (STFs) to Kevlar fabrics has been shown to enhance the ballistic penetration resistance of the fabric, ...
Submitted to: NUMIFORM 2004. June 13-17, 2004. Columbus, OH.

The Effect of Rheological Parameters on the Ballistic Properties of Shear Thickening Fluid (STF)–Kevlar Composites Eric D. Wetzel1, Y. S. Lee2, R. G. Egres2, K. M. Kirkwood2, J. E. Kirkwood2, and N. J. Wagner2 1

U.S. Army Research Laboratory / Bldg. 4600, AMSRL-WM-MA / Aberdeen Proving Ground, MD 21005 Dept. of Chemical Engineering and Center for Composite Materials / U. of Delaware / Newark, DE 19716

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Abstract. The addition of colloidal shear thickening fluids (STFs) to Kevlar fabrics has been shown to enhance the ballistic penetration resistance of the fabric, under conditions of low velocities and small target sizes [1,2]. It is believed that this improvement in ballistic properties is related to the resistance of the STF to deformation at high strain rates, since the addition of Newtonian fluids to Kevlar fabric does not improve ballistic performance. However, the precise role of the STF, and its rheological properties, in the ballistic defeat process are not known. In this study, ballistic and rheological experiments are performed to determine the effect of fluid viscosity, particle loadings, and particle size and shape on the behavior of STF-Kevlar composites. These results will be help to determine which rheological parameters are most critical to achieving enhanced composite properties.

formation, with hydrocluster growth and collision eventually resulting in a percolated arrangement of the rigid particles across macroscopic dimensions [5,6]. This microstructural transformation leads to the bulk solid-like behavior. Upon relaxation of the applied stresses, the rigidized material typically reverts to the low strain rate, fluid-like behavior.

INTRODUCTION Soldiers in the U.S. Army wear protective vests to reduce the likelihood of injuries from ballistic and fragmentation threats. These vests are typically composed of multiple layers of woven Kevlar® fabrics, and are sometimes fronted by rigid ceramic plates. Although the design of these vests has improved considerably, improvements in vest performance are still desired. Specifically, the mobility, agility, and comfort of the soldier could be greatly improved by reducing the weight, bulk, or stiffness of the vest material.

The experiments of Lee et al. [1] and Egres et al. [2] demonstrated that the ballistic properties of Kevlar fabrics can be improved by the addition of colloidal STFs. This result allows for the design of fabric armors with higher flexibility, and less bulk, than comparable neat Kevlar constructions. However, the precise phenomenological characteristics of the STF which are responsible for the enhancement in ballistic properties are not well understood. In this paper, we perform experiments using STFs with different compositions and rheological responses, in order to further clarify the role of the impregnation medium on composite performance.

Lee et al. [1] and Egres et al. [2] have investigated the ballistic performance of Kevlar fabrics impregnated with shear thickening fluids (STFs). STFs are fluids whose viscosity increases with shear rate [3,4]. Of particular interest are discontinuous STFs, which at high shear rates transform into a material with solid-like properties. A typical example of a discontinuous STF is a stabilized suspension of rigid colloidal particles with a high loading fraction of particles. High shear rates induce hydrocluster

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followed by ethanol removal from the STF does not alter the rheology of the samples.

EXPERIMENTAL Materials Four types of polyethylene glycol (PEG) (Mn=200, 300, 400, 600, Acros Organics and Clariant) were used in this investigation. For the preparation of the shear thickening fluids used in target fabrication, stable polyethylene glycol (Mn=200) based particulate dispersions were produced. Four types of particles were used to prepare these dispersions: surface charge stabilized spherical colloidal silica (MP1040, Nissan Chemicals) and ellipsoidal (prolate) precipitated CaCO3 particles (Albaglos S, Opacarb 40A, and MD1074, Minerals Technologies, Inc.) having particle aspect ratios of approximately 2:1, 4:1 and 7:1. All particles were supplied as aqueous dispersions, requiring a centrifugation/solvent exchange procedure outlined in Lee et. al. [1] to produce the polyethylene glycol based STFs. TEM images of the particles are presented in Figure 1 and the properties of the particles are provided in Table 1. Centrifugation for solvent removal was performed using a high speed centrifuge (DuPont Instruments RC-5) at 16,000×g for two hours. Redispersion into polyethylene glycol was achieved using a rolling jar mixer. For the CaCO3 dispersions, four centrifugation/solvent exchange steps were performed to generate dispersions with less than 2% water. STF dispersion samples at various volume fractions were generated using a solution density meter (Anton Paar, Model DMA 48) under the assumption of ideal mixing.

Figure 1. TEM images of the STF particles.

Rheological Measurements Rheological measurements for the spherical particle STFs were performed with a Physica MCR500 rheometer in stress-controlled mode, with a cone–plate geometry having a cone angle of 0.035 radian and a diameter of 25 mm. To remove the effects of sample loading, a preshear of 1 s-1 was applied for 60 s prior to further measurement. For the ellipsoidal particle STFs, stress sweeps were performed on an SR 5000 stress control rheometer (Rheometric Scientific Corp.) using a 25mm, 0.02 radian cone, following a preshear stress ramp to a stress of 300 Pa in 300 seconds. Stress sweeps of the polyethylene glycol samples were measured using a 40mm diameter, 0.02 radian cone, or a 40 mm diameter, 0.0397 radian cone. All measurements presented here were performed at 25ºC, and were reproducible.

TABLE 1. Properties of the STF particles. Particle

Length (nm) Silica PCC(2:1) 567 ± 217 PCC(4:1) 1004 ± 465 PCC(7:1) 1323 ± 667

Diameter (nm) 120± 5 328 ± 103 233 ± 71 201 ± 77

Aspect ratio L/D 1 1.72 ± 0.34 4.30 ± 1.45 6.68 ± 2.56

Plain-woven Hexcel Aramid (poly-paraphenylene terephthalamide) high performance fabric (Style 706, Kevlar KM-2, 600 denier, areal density of 180 g/m2) was used for target fabrication. To facilitate impregnation of STF into the Kevlar fabric, STF was first diluted using ethanol to reduce the surface tension and viscosity. For targets containing high loadings of STF, an equal volume of ethanol was added to the STF prior to incorporation into the target. Higher volume ratios of ethanol to STF were required to effectively impregnate the fabric at low STF loadings. The composite fabric samples were then heated to 80ºC for 20 minutes to remove the ethanol. Rheological investigations confirm that dilution with ethanol

Ballistic Measurements Ballistic experiments were conducted using a smooth bore helium gas gun with 0.22 caliber NATO standard fragment simulation projectiles (1.1 gram, chisel-pointed metal cylinder). All experiments were performed at a velocity of approximately 244 m/s, with 5.08 cm × 5.08 cm targets backed by a clay witness. The depth of penetration into the clay witness was used to calculate the percentage of the initial projectile kinetic energy which was absorbed by the target, according to the calibration procedure outlined in [1]. Higher values of target energy absorption correspond to better armor ballistic performance.

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Impact velocities were measured using time-of-flight chronograph measurements, using both light and paper triggers.

on the clay backing. The clay backing is approximately 35.6 cm × 50.8 cm (much larger than the target area). In the second set (Table 3), the targets are first backed by a support layer consisting of a single layer of Kevlar glued to a 5.08 cm diameter hoop of copper rod, with the target and support layer then placed into an aluminum frame and backed by a 5.08 × 5.08 cm block of clay. These test conditions are described in more detail in [1]. Because of the presence of the support layer, which is included in the target energy absorption value, the reported energy absorption values for the second set of experiments will be higher than those of the first set of experiments, for comparable target types. Therefore, it is not possible to directly compare data between the two sets of experiments. Also note that separate clay energy absorption calibration curves were created for each set of experiments.

TABLE 2. Target description and ballistic results for the first set of ballistic experiments. Number Target Type of Volume of Kevlar number fluid of fluid layers (mL) 1 None 0 4 2 0 6 3 0 10 4 0 14 5 0 20 6 0 28 7 STF 0.025 4 8 0.125 4 9 0.5 4 10 1 4 11 2 4 12 4 4 13 STF 0.25 4 14 0.5 8 15 0.875 14 16 1.25 20 17 PEG200 2 4 18 4 4 19 8 4 20 PEG300 2 4 21 4 4 22 8 4 23 PEG400 2 4 24 4 4 25 8 4 26 PEG600 2 4 27 4 4 28 8 4

Target Average Average Std. dev. Target areal impact energy of energy Number mass density velocity absorbed absorbed of tests (g) (g/cm2) (m/s) (%) (%) 1.88 0.073 249.5 58.96 3.99 3 2.82 0.109 216.3 70.61 4.27 3 4.70 0.182 251.5 82.07 1.41 3 6.58 0.255 234.9 86.01 1.41 2 9.40 0.364 214.0 89.50 0.72 2 13.16 0.510 242.8 92.91 0.59 2 1.92 0.074 252.9 67.96 6.82 2 2.09 0.081 237.0 72.41 2.02 2 2.70 0.105 247.5 72.33 2.09 2 3.53 0.137 250.7 78.59 1.25 2 5.17 0.200 249.5 82.07 1.48 2 8.47 0.328 213.1 83.68 1.58 2 2.29 0.089 248.1 67.70 2.15 2 4.58 0.178 248.3 82.97 0.62 2 8.02 0.311 248.9 89.18 1.22 2 11.46 0.444 224.2 91.20 1.73 2 4.12 0.160 234.2 41.94 16.31 2 6.36 0.246 215.0 59.34 6.03 2 10.84 0.420 252.7 40.88 10.97 2 4.13 0.160 246.1 46.96 1.50 2 6.38 0.247 246.8 51.05 2.37 2 10.88 0.422 247.7 51.00 10.89 2 4.14 0.160 246.9 53.18 9.84 2 6.39 0.248 243.1 60.30 5.30 2 10.90 0.422 247.2 53.60 11.73 2 4.14 0.160 249.0 49.92 4.74 2 6.39 0.248 245.6 47.63 12.41 2 10.91 0.423 247.2 49.52 8.74 2

For the first set of experiments (Table 2), four types of target architectures are compared: neat Kevlar, with increasing number of layers; 4 layers of Kevlar, with increasing volumes of added STF; a fixed ratio of 4 layers of Kevlar to 0.25 ml of STF, with increasing numbers of Kevlar layers and proportional increases in the volume of STF; and 4 layers of Kevlar, with increasing volumes of added PEG200, PEG300, PEG400, and PEG600. The STF in all of these experiments is composed of 120 nm spherical silica particles at a volume fraction of φ=46% in PEG200.

TABLE 3. Target description and ballistic results for the second set of ballistic experiments. All targets contain 4 layers of Kevlar fabric. Target number 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62

Particle Target volume Fluid Target areal Impact Energy fraction mass mass density velocity absorbed (g) (g) (g/cm2) (m/s) (%) None 0.00 1.88 0.073 243.8 76.65 Spherical 0.00 2.24 4.12 0.160 251.9 65.18 0.20 2.71 4.59 0.178 231.9 71.36 0.25 2.83 4.71 0.182 222.6 78.47 0.30 2.94 4.82 0.187 249.4 77.41 0.35 3.06 4.94 0.191 266.1 80.04 0.40 3.18 5.06 0.196 248.8 86.94 0.45 3.29 5.17 0.200 264.9 88.95 0.45 3.29 5.17 0.200 244.0 89.78 Ellipsoidal 2:1 0.10 2.53 4.41 0.171 232.2 77.27 0.20 2.91 4.79 0.186 222.0 79.58 0.25 3.12 5.00 0.194 252.0 80.70 0.30 3.22 5.10 0.198 247.3 83.91 0.35 3.36 5.24 0.203 252.8 84.49 0.40 3.57 5.45 0.211 257.2 87.38 0.45 3.67 5.55 0.215 235.8 87.11 0.50 3.95 5.83 0.226 240.5 85.21 Ellipsoidal 4:1 0.10 2.67 4.55 0.176 211.6 70.95 0.20 3.04 4.92 0.191 239.8 78.83 0.25 3.23 5.11 0.198 250.8 77.66 0.30 3.31 5.19 0.201 227.9 84.18 0.35 3.51 5.39 0.209 244.1 85.65 0.40 3.68 5.56 0.215 239.5 87.93 0.45 4.00 5.88 0.228 235.6 88.73 0.50 4.22 6.10 0.236 234.1 89.50 Ellipsoidal 7:1 0.10 2.58 4.46 0.173 253.6 66.66 0.20 2.98 4.86 0.188 250.2 66.63 0.25 3.15 5.03 0.195 266.7 68.47 0.30 3.33 5.21 0.202 246.3 84.97 0.30 3.31 5.19 0.201 243.8 80.61 0.35 3.48 5.36 0.208 238.7 82.47 0.40 3.62 5.50 0.213 209.1 85.42 0.40 3.65 5.53 0.214 242.0 86.20 0.45 3.83 5.71 0.221 242.6 84.99 Type of STF

For the second set of experiments (Table 3), 4 layers of Kevlar are impregnated with 2 ml of STF composed of PEG200 with four different types of colloidal particles: 120 nm spherical silica, 2:1 aspect ratio CaCO3, 4:1 aspect ratio CaCO3, and 7:1 aspect ratio CaCO3. The volume fraction of particles in PEG is varied systematically in each case. Note that, since the particle density is higher than the carrier fluid density, and total STF volume is held constant, the target areal density increases slightly as particle volume fraction increases. For comparison, experiments were also performed on 4 layers of neat Kevlar, and 4 layers of Kevlar impregnated with 2 ml of PEG200.

RESULTS Rheological Behavior Figures 2-5 show the rheological behavior of spherical and ellipsoidal particle suspensions of various particle volume fractions. In all cases, shear thinning at low shear rates is followed by shear thickening at higher shear rates. The strength of the

Two sets of ballistic experiments were performed. In the first set (Table 2), the targets are placed directly

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shear thickening response increases as particle loading increases, with discontinuous shear thickening at the highest particle loadings. The shear rate at which shear thickening behavior is first observed decreases as particle loading increases. Also note that the particle volume fraction at which discontinuous shear thickening occurs decreases as particle aspect ratio increases.

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Viscosity (Pa · s)

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φ = 0.45

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FIGURE 4. Viscosity as a function of shear rate for 4:1 aspect ratio CaCO3 STF at various particle loadings.

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FIGURE 2. Viscosity as a function of shear rate for spherical silica STF at various particle loadings. 10

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φ = 0.31 φ = 0.30 φ = 0.28 φ = 0.25 φ = 0.20 φ = 0.15 φ = 0.10 φ = 0.05

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FIGURE 5. Viscosity as a function of shear rate for 7:1 aspect ratio CaCO3 STF at various particle loadings.

φ = 0.40 φ = 0.35 φ = 0.30 φ = 0.25 φ = 0.20 φ = 0.15 φ = 0.10 φ = 0.05 0.01

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Figure 7 shows the ballistic performance of the targets of Table 2, as a function of areal density. For neat Kevlar (targets 1-6), increasing numbers of fabric layers increases the target ballistic performance. By adding PEGs of various molecular weights (targets 1728), the ballistic performance of 4 layers of Kevlar generally decreases. For these targets, the energy absorption does not appear to depend strongly on the molecular weight of the PEG.

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-1

Shear rate (s )

FIGURE 3. Viscosity as a function of shear rate for 2:1 aspect ratio CaCO3 STF at various particle loadings.

Figure 6 shows the rheological properties of the polyethylene glycols of various molecular weights. The fluids generally exhibit Newtonian behavior, with slight shear thinning for PEG600. Fluid viscosity increases as molecular weight increases.

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Kevlar, while the performance at high particle loadings is better than that of neat Kevlar. Energy absorption does not show a clear dependence on particle aspect ratio.

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Viscosity (Pa · s)

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Neat Kevlar Kevlar + STF, fixed 4 layers of Kevlar Kelvar + STF, fixed fabric-to-STF ratio Kelvar + PEG200 Kevlar + PEG300 Kevlar + PEG400 Kevlar + PEG600

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FIGURE 6. Viscosity as a function of shear rate for PEGs of various molecular weights. 40 0

In contrast, adding increasing volumes of STF to 4 layers of Kevlar (targets 7-12) causes a noticeable increase in ballistic performance, as previously reported in [1]. The ballistic performance of the STFKevlar composite is comparable, on a per weight basis, to that of neat Kevlar. However, the number of fabric layers of the STF-Kevlar composite is fewer than the number of fabric layers in the comparable neat Kevlar targets. For example, target 11, with 4 layers of Kevlar and 2 ml of STF, absorbs as much energy as 10 layers of Kevlar. The decreased number of Kevlar layers leads directly to a decrease in overall target thickness and an increase in flexibility [1]. At the highest STF loading (target 12), 4 ml, the performance of the STF-Kevlar composite drops slightly relative to neat Kevlar with the same areal density.

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FIGURE 7. Ballistic performance of the targets of Table 2 as a function of areal density. 90

Energy absorbed (%)

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80 4 layers neat Kevlar 75 Spherical Ellipsoidal 2:1 Ellipsoidal 4:1 Ellipsoidal 7:1

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Figure 7 also compares the performance of neat Kevlar to Kevlar with a low volume addition of STF, where the Kevlar-to-STF ratio remains constant (targets 13-16). The energy absorption of these STFKevlar composite targets, on a per weight basis, is again comparable to neat Kevlar. However, the number of layers of Kevlar in the composite specimens is again reduced relative to the number of fabric layers in the neat specimens, although the reduction is not as dramatic as in the composites loaded with high volumes of STF.

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FIGURE 8. Ballistic performance of the targets of Table 3 as a function of STF particle volume fraction. The horizontal line provides a reference value for the energy absorption of 4 layers of neat Kevlar.

DISCUSSION To couple the rheological and ballistic results, we will first estimate the shear rates encountered in the STF during the ballistic process. A characteristic shear rate can be estimated by normalizing the impact velocity, ~244 m/s, by some characteristic length scale. A conservative length scale, the projectile diameter (5.6 mm), provides a lower bound shear rate

Figure 8 shows the ballistic performance of the targets of Table 3, as a function of STF particle volume fraction. Increasing volume fraction of particles leads to an increase in target energy absorption. In general, the ballistic performance at very low particle loadings is less than that of neat

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of 4 × 104 s-1. An upper bound shear rate could be defined based on the gap at the junction between crossed yarns, since yarn uncrimping and pull-out are expected to occur during the ballistic impact process [7]. Although it is difficult to estimate the size of this vanishingly small gap, using an estimate of 10 µm (the approximate filament diameter) results in an upper bound shear rate of 2 × 107 s-1. These shear rates, if accurate, are well above the shear thickening transition points observed in the rheological behavior of the STFs in Figures 2-5.

occurs in these systems (Figures 2 and 5). Further experiments are required to confirm these observations. It is also worth noting that the areal density of the targets in Table 3 increases as particle volume fraction increases. This increase in target mass could improve target efficiency simply through momentum effects. However, other experiments (not shown) in which total target mass is kept constant (by decreasing total STF volume as particle loading increases) showed comparable increases in target energy absorption with particle volume fraction.

The first set of ballistic experiments clearly show that STF addition provides an enhancement to the ballistic properties of Kevlar fabric. The mechanism of this improvement is not precisely known. One likely explanation is that the STF provides coupling and load transfer on a filament-filament, yarn-yarn, or ply-ply level. These interactions modify the fabric's response, and may allow the Kevlar yarns to be loaded more efficiently than without the STF presence. A slightly different explanation is that the STF absorbs energy itself, due to viscous dissipation in the fluid. This viscous dissipation would occur as the fluid is sheared, either directly by the projectile or by the relative motion of fabric elements.

CONCLUSIONS The ballistic experiments and rheological measurements show that increasing shear thickening response corresponds to increased ballistic performance in STF-Kevlar composites. However, further experiments are required to carefully segregate the effects of viscous dissipation versus frictional dissipation, and to identify the precise influence of the STF on the response of the stacked, woven fabric architecture. Yarn pull-out experiments such as those performed in Kirkwood et al. [7], utilizing STFimpregnated fabrics, will help to isolate these effects.

The addition of neat PEG, even with relatively high viscosities, generally decreases the ballistic performance of the Kevlar fabric. Note that the viscosity of the PEG600 system is comparable to that of some of the high aspect ratio particle suspensions prior to their shear thickening transition. These results suggest that the very high viscosities of the transitioned STFs, or some other property of the transitioned state, is required in order to provide the desired energy absorption effects.

REFERENCES 1. Lee, Y.S., Wetzel, E.D., and Wagner, N.J. J. Mat. Sci. 38, 2825-2833 (2003). 2. Egres, R.G., Lee, Y.S., Kirkwood, J.E., Kirkwood, K.M., Wetzel, E.D., and Wagner, N.J. "Novel flexible body armor utilizing shear-thickening fluid (STF) composites." Proceedings of 14th International Conference on Composite Materials. San Diego, CA. July 14 - 18, 2003.

The systematic increase in energy absorption as particle loading increases, coupled with the increasing shear thickening response observed for these systems, could be a direct indication of the importance of shear thickening in STF-Kevlar composites. An alternate interpretation, however, could be that the improvements in energy absorption are due simply to increased frictional effects brought about by the increased surface coverage of nanoparticles on the yarn and filament surfaces. However, it is interesting to note that the 7:1 aspect ratio STFs and the spherical particle STFs undergo a sudden jump in ballistic performance at particular particle loadings. From Figure 8, these critical transition points are between 25-30% for the 7:1 system, and between 40-45% for the spherical system. These transitional volume fractions approximately correspond to the volume fractions at which discontinuous shear thickening

3. Hoffman, R.L. J. Colloid Interface Sci. 46, 491 (1974). 4. Barnes, H.A. J. Rheol. 33, 329 (1989). 5. Bossis, G. and Brady, J.F. J. Chem. Phys. 91, 1866 (1989). 6. Bender, J.W. and Wagner, N.J. J. Colloid Interface Sci. 172, 171 (1995). 7. Kirkwood, K.M., Kirkwood, J.E., Lee, Y.S., Egres, R.G., Wetzel, E.D., and Wagner, N.J. "Yarn pull-out as a mechanism for dissipation of ballistic impact energy in Kevlar KM-2 fabric." Textile Research Journal. To appear (2004).

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