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Synthesis, processing and characterization of shear thickening fluid (STF) impregnated fabric composites
Materials Science and Engineering A 527 (2010) 2892–2899
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Synthesis, processing and characterization of shear thickening fluid (STF) impregnated fabric composites Tarig A. Hassan, Vijay K. Rangari ∗ , Shaik Jeelani Center for Advanced Materials (T-CAM), Tuskegee University, Tuskegee, AL 36088, United States
a r t i c l e
i n f o
Article history: Received 21 August 2009 Received in revised form 3 January 2010 Accepted 6 January 2010
Keywords: Shear thickening Nanoparticles Sonochemical Composites
a b s t r a c t Shear thickening is a non-Newtonian fluid behavior defined as the increase of viscosity with the increase in the applied shear rate. The shear thickening fluid (STF) is a combination of hard metal oxide particles suspended in a liquid polymer. This mixture of flowable and hard components at a particular composition, results in a material with remarkable properties. In this manuscript the shear thickening fluid (STF) was prepared by ultrasound irradiation of silica nanoparticles dispersed in liquid polyethylene glycol polymer. The as-prepared STFs have been tested for their rheological and thermal properties. Kevlar and Nylon fabrics were soaked in STF/ethanol solution to make STF/fabric composite. Knife threats and quasistatic penetration tests were performed on the neat fabrics and STF/fabric composite targets for both engineered spike and knife on areal density basis. The results showed that STF impregnated fabrics have better penetration resistance as compared to neat fabrics without affecting the fabric flexibility. This indicates that the addition of STF to the fabric have enhanced the fabric performance and can be used in liquid body armor applications. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Throughout recorded history humans have always used various kinds of materials as armors to protect their bodies from injury in the battle field and other unsafe situations, starting from the animal skins used by the caveman, the metal shields used by the Roman warriors or the body armors used nowadays, the need to protect oneself is there, and it will go on over time. Over the years many products have been invented and tried in order to improve body armor [1]. The U.S. Army Research Laboratory is developing a new technology to save soldiers’ lives. This new technology is liquid body armor. This type of body armor is light and flexible making the soldiers safer and allowing them to remain mobile and not hindering them from moving, running and aiming their weapons. The objective of the liquid body armor technology is to produce a new thin, flexible, lightweight and inexpensive material that have an equivalent or even better ballistic properties than the existing Kevlar fabric. The shear thickening fluid is a material with remarkable properties. STF is very deformable material in the ordinary conditions and flows like a liquid as long as no force is applied. However it turns
∗ Corresponding author at: Materials Science and Engineering, Center for Advanced Materials, 101 James Center, Tuskegee University, Tuskegee, AL 36088, United States. Tel.: +1 334 724 4875; fax: +1 334 727 4224/2286. E-mail address: [email protected] (V.K. Rangari). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.01.018
into a very rigid solid-like material at high shear rates that prevent the bullet from penetrating the soldier’s body. Shear thickening is a non-Newtonian flow behavior observed as an increase in viscosity with increasing shear rate or applied stress [2]. This phenomenon can occur in colloidal dispersions. In more concentrated colloidal suspensions have been shown to exhibit reversible shear thickening resulting in large, sometimes discontinuous increases in viscosity above a critical shear rate. Two main causes of reversible shear thickening have been proposed: the order–disorder transition [3–7] and the “hydrocluster” mechanism [8–13]. This transition from a flowing liquid to a solid-like material is due to the formation and percolation of shear induced transient aggregates, or “hydroclusters,” that dramatically increase the viscosity of the fluid. Support for this hydrocluster mechanism has been demonstrated experimentally through rheological, rheo-optics and flow-SANS experiments [8,14] as well as computer simulation [15]. It has been reported in the literature that shear thickening has been observed for a wide variety of suspensions such as clay–water [16], calcium carbonate–water [17], polystyrene spheres in silicon oil [18], iron particles in carbon tetrachloride [19], titanium dioxide–resin [20], silica–polypropylene glycol [21], and silica–ethylene glycol [22]. The phenomenon of shear thickening of suspensions in general has no useful applications in industrial production. Recently Wegner’s group and U.S. Army research lab developed a body armor using shear thickening fluid and Kevlar fabric [22]. These research results demonstrate that ballistic penetration resistance of Kevlar fabric is enhanced by impregnation
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Fig. 1. Steps of the fabric impregnation procedure.
of the fabric with a colloidal shear thickening fluid. Impregnated STF/fabric composites are shown to provide superior ballistic protection as compared with simple stacks of neat fabric and STF. Comparisons with fabrics impregnated with non-shear thickening fluids show that the shear thickening effect is critical to achieving enhanced performance. In the present investigation we report on the synthesis of STF using sonochemical method, which can be potentially used for the body armor applications. Many researchers have used various techniques to prepare the shear thickening fluid. Acoustic cavitations technique is one of the efficient ways to disperse nanoparticles into the liquid polymers. In this case, the application of alternating acoustic pressure above the cavitations threshold creates numerous cavities in the liquid. Some of these cavities oscillate at a frequency of the applied field (usually 20 kHz) while the gas content inside these cavities remains constant. However, some other cavities grow intensely under tensile stresses while yet another portion of these cavities, which are not completely filled with gas, start to collapse under the compression stresses of the sound wave. In the latter case, the collapsing cavity generates tiny particles of ‘debris’ and the energy of the collapsed one are transformed into pressure pulses. It is noteworthy that the formation of the ‘debris’ further facilitates the development of cavitation. It is assumed that acoustic cavitations in liquids develop according to a chain reaction. Therefore, individual cavities on real nuclei are developing so rapidly that within a few microseconds an active cavitations region is created close to the source of the ultrasound probe. The development of cavitations processes in the ultrasonically processed melt creates favorable conditions for the intensification of various physio-chemical processes. Acoustic cavitations accelerate heat and mass transfer processes such as diffusion, wetting, dissolution, dispersion, and emulsification. The objective of this study is to synthesize the shear thickening fluid in a single step reaction through high power ultrasound technique, fabricate STF/fabric composite and characterize it for stab resistance applications. 2. Experimental 2.1. Materials The materials used in this study include dry powder of spherical silica nanoparticles (15 nm in size) purchased from Nanostructured & Amorphous Materials, Inc., Los Alamos, NM. Dry silica nanopowder was selected over the colloidal silica solution to prevent water and moisture contamination in the synthesis process. Polyethylene glycol was used as the liquid polymer in the STF synthesis pro-
cess. The average molecular weight of the PEG used in this study is 200 g/mole. PEG is non-toxic and easy to handle in addition to that it is thermally stable and easily available in bulk quantities which make it useful for bulk production. Ethyl alcohol was used as the solvent in the sonochemical process. The evaporation temperature of ethanol is around 79 ◦ C and that make it easy to be removed from the reaction mixture via the evaporation process to prepare the rheology test sample. Both PEG and ethyl alcohol were purchased from Sigma–Aldrich Chemicals, St. Louis, MO. The Kevlar and Nylon fabric fabrics used in this study and they were purchased from DuPont Company and Performance Textile Incorporated – Duxbury, MA, respectively. 2.2. Processing 2.2.1. Synthesis of shear thickening fluid The sonochemical method was used to synthesize shear thickening fluid. In this technique known weight percentages of silica nanoparticles (40%) and polyethylene glycol (60%) were mixed with an excess amount of ethanol and irradiated with high intensity ultrasonic horn (Ti-horn, 20 kHz, 100 W/cm2 at 50% amplitude) for 5 h at 10 ◦ C; the reaction temperature was maintained by using a Thermo NESLAB chiller. 2.2.2. Fabrication of STF/fabric composite The STF/fabric composite targets were prepared for testing according to the following procedure. The resultant sonochemical reaction solution of STF/ethanol (40:60) was used for fabric impregnation. 381 mm × 381 mm layers of Kevlar and Nylon fabrics were cut and impregnated individually into STF/ethanol solution. Each sheet of Kevlar or Nylon was soaked in an aluminum container filled with STF/ethanol solution for 1 min and then squeezed using an 11.34 kg steel cylinder to get rid of the excess amount of the solution and then hanged for 48 h at room temperature to remove the solvent. Fig. 1 shows the steps of the fabric impregnation procedure. The weight of the fabric sheets was measured before and after the impregnation to ensure that the fabric layers have been equally impregnated with the STF/ethanol solution. These STF/fabrics composites were then arranged into multilayer targets, as shown in Table 1. Each target has a number of fabric layers selected to match overall target areal densities as closely as possible. The targets were positioned on multi-layer foam of backing material as specified in the NIJ standard 0115.00 (Stab Resistance of Personal Body Armor) [23]. The backing material consists of four layers of 5.8 mm thick neoprene sponge, followed by one layer of 31 mm thick polyethylene foam, backed by two 6.4 mm thick layers of rubber (all backing
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Table 1 Test targets. Label
Yarn material
Yarn denier
Yarn count (yarns/in.)
STF (wt%)
Single layer areal density (g/cm2 )
Number of layers in target
Target areal density (g/cm2 )
Kevlar STF/Kevlar Nylon STF/nylon
Kevlar Kevlar Nylon Nylon
600 600 1050 1050
34 × 34 34 × 34 23 × 21 23 × 21
0.0 28.7 0.0 25.0
0.018 0.025 0.041 0.055
15 11 6 5
0.275 0.279 0.248 0.275
Fig. 2. Dynamic stab testing target showing the backing material and the witness papers.
Table 2 Explanation of depth measurement technique. Number of witness papers penetrated
Penetration depth (mm)
0 1 2 3 4 5
0 0–5.8 5.8–11.6 11.6–17.4 17.4–23.2 >23.2
materials were obtained from PCF Foam Corp., Cincinnati, OH). Five sheets of synthetic polymer-based polyart witness papers (Arjobex Corp., Charlotte, NC) were inserted between the target and all four layers of neoprene sponge and the polyethylene foam as presented in Fig. 2. The number of penetrated witness papers indicates the depth of penetration as listed in Table 2. 2.3. Characterization The as-prepared STF samples were tested for its rheological properties using a TA Instruments Rheometer-AR2000. Testing was carried out at room temperature in a steady state flow mode and shear rate ramp of 0–125/s using a peltier plate and a cone plate of size 40 mm and 2◦ angles. The peltier plate has a temperature range of −20 to 200 ◦ C with a typical heating/cooling rate up to 20 ◦ C/min and a temperature accuracy of ± 0.1 ◦ C. Thermal gravimetric analysis (TGA) was conducted to determine the weight percentage of silica and polyethylene glycol in as-prepared STF sample. The test was done under nitrogen atmosphere using a TGA-Mettler Toledo: Model Number 581 apparatus at heating rate of 5◦ /min from room temperature to 800 ◦ C.
Transmission electron microscopy (TEM) has been performed on the silica nanoparticles and as-prepared STF sample, using a JOEL-2010 transmission electron microscope. Samples for TEM were prepared by dispersion of nanoparticles and STF in ethanol and placed a drop of solution on a copper grid (carbon coated copper grid – 200 mesh) and dried in air then used for TEM analysis. To investigate the infusion of STF in the fabric sheets, scanning electron microscopy analyses were carried out using JEOL JSM 5800 scanning electron microscope. The STF/fabric composite samples and neat fabric samples were precisely cut in to small pieces using the Checkadee cutter and placed on a double sided carbon tape and coated with gold/palladium to prevent charge buildup by the electron absorption by the specimen. Dynamic stab test was performed for neat and STF/fabric composite targets to evaluate the fabric resistance against the spike and knife threats. Tests were performed using a drop tower equipped with knife and spike impactors, based on the NIJ standard 0115.00 (stab resistance of personal body armor) [23]. As specified in the NIJ standard engineered knife S1 blade represents the performance of large commando-style blades or larger kitchen knives, S1 knife blade feature a pointed tip and a stiff backbone two cutting edges. S1 knife is 100.0 ± 0.05 mm long and 20 ± 0.1 mm wide and 2.0 ± 0.05 mm in thickness; the tip of the knife is 49.2 mm long with an angle of 23 ± 0◦ 30 . Another weapon used in this study is an engineered spike that is designed to represent a class of pointed weapons used mostly in assaults in correction facilities. It is based on the design commonly used in the “California Ice Pick” test. According to the NIJ standard, the engineered spike is 177.8 ± 1.6 mm long and 4.14 ± 0.0254 mm in diameter; the diameter start to decay for the last 76.2 ± 6.35 mm to reach 1.905 ± 0.0508 mm at the tip of the spike. The tip of the spike is 4.445 ± 0.127 mm long with a point sharp of 0.0762 mm maximum diameter. The knife and spike threats were dropped from different heights marked with letters correspond to different impact energies as presented in Table 3. Two sets of experiments were performed for four different targets (neat Kevlar target, STF/Kevlar composite target, neat Nylon target and STF/Nylon composite target) one set for the spike threat and the second set for the knife threat, the drop level was varied from letters A to I for the first set and for the second set the drop level varied from J to R. To complement the drop tower tests, quasistatic stab tests were also performed on the neat targets and STF/fabric composite
Table 3 Impact energy distribution for dynamic stab testing. No.
1 2 3 4 5 6 7 8 9
Spike
Knife
Letter designation
Drop height (cm)
Energy (J)
Letter designation
Drop height (cm)
Energy (J)
A B C D E F G H I
14.4 28.9 43.2 57.6 72.1 86.6 101.03 115.6 129.6
2.7 5.4 8.1 10.8 13.5 16.2 18.9 21.6 24.3
J K L M N O P Q R
14.4 28.9 43.2 57.6 72.1 86.6 101.03 115.6 129.6
2.7 5.4 8.1 10.8 13.5 16.2 18.9 21.6 24.3
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Fig. 3. Quasistatic test for (a) spike; (b) knife.
targets for both spike and knife threats. This test was conducted to investigate the response of the neat fabric targets along with the STF/fabric composite targets under slow loading rates. Knife and spike impactors were strongly attached to the upper grip of a 2.5 kN load cell of a Zwick-Roell materials testing machine, with the
target placed on top of the multi-layered backing material used in the drop tower tests and specified in the NIJ standard 0115.00 (stab resistance of personal body armor) [23]. The targets were located below the impactor and the impactor was then pushed into the target at a rate of 5 mm/min to a total depth of 30 mm. Fig. 3 shows pictures of the quasistatic test configuration. The output of the test was recorded as load versus displacement data. Flexibility tests and thickness measurements were performed for both neat fabric and STF/fabric composite samples to examine the effect of STF addition on the fabric flexibility and thickness. To measure the flexibility, neat fabric and STF/fabric were cut into 51 mm × 51 mm square layers, 2 test specimens were prepared by stacking 4 layers for the first specimen and 10 layers of second specimen of the square layers. A 20 g weight was then attached to the test specimens according to the geometry shown in Fig. 4 and the bending angle  was measured and recorded for each specimen, this procedure was reported by Lee et al. [22]. For the thickness test a digital caliper was used to measure the specimen thickness. 3. Results and discussion
Fig. 4. Flexibility test geometry.
3.1. Rheological properties Rheology test results show that as-prepared STF sample exhibits shear thickening behavior. This behavior is very much evident as
Fig. 5. Rheology graph for as prepared STF sample.
Fig. 6. TGA graph for as prepared STF sample.
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Fig. 7. (a) TEM image of silica nanopowder; (b) TEM image of as-prepared STF sample.
seen from the graph in Fig. 5. The sample viscosity changes from ∼20 Pa s at shear rate of 5/s to 410 Pa s at 12/s shear rate before reversal trend is seen. Continuous increase in sample’s viscosity at low and very narrow shear rates range clearly confirm that the sonochemical mixing before evaporation drastically improves the shear thickening effect. 3.2. Thermal gravimetric analysis (TGA) TGA test results show two weight loss regimes the first wieght loss observed at temperature around 160 ◦ C when the PEG starts to evaporate and then decomposes at around 260 ◦ C where a major weight loss regime is noticed as seen in Fig. 6. The derivative curve clearly shows the huge weight loss of the sample indicating the total decomposition of the polymer at ∼260 ◦ C and that the remaining sample residue is silica. There is a slight difference between the remaing amount of the sample containing the silica nanoparticles (approximatley 35% of the sample) and the added silica (40%) this difference can be explained as the loss of some amount of this light
feathery small size nanopowder during the evaporation process of the ethanol. 3.3. Transmission electron microscopy (TEM) Fig. 7a shows the TEM picture of the as-received silica nanoparticles. The particles are nearly spherical in shape and the size distribution is ∼15–30 nm. The TEM micrograph of as prepared STF sample (shown in Fig. 7b) clearly shows the incorporation of silica and PEG. 3.4. Scanning electron microscopy (SEM) The SEM micrographs presented in Fig. 8 show pictures of neat Kevlar and STF/Kevlar composite samples at different magnifications. Fig. 8a shows neat Kevlar fabric without any addition of STF, the SEM images of STF/Kevlar sample presented in Fig. 8b clearly show that STF is well dispersed over the entire surface on the Kevlar sheet; it can be seen in Fig. 8c that STF is incorporated between
Fig. 8. SEM images for (a) neat Kevlar fabrics; (b–d) STF/Kevlar composite.
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Fig. 9. SEM images for (a) Neat Nylon fabrics; (b–d) STF/Nylon composite.
the fibers. The high magnification SEM image presented in Fig. 8d shows that the Kevlar fabrics are completely coated with STF. The SEM micrographs for neat Nylon and STF/Nylon composite samples presented in Fig. 9a–d show similar observations. Fig. 9a represents an SEM image of neat Nylon fabric without STF. SEM image of STF/Nylon composite sample is presented in Fig. 9b, and shows that STF is spread all over the fabric. Fig. 9c and d show that STF is covering all the Nylon fibers and that confirms the efficiency of the fabric impregnation procedure to infuse STF in Kevlar and Nylon fabrics.
higher impact energy levels. On the other hand the neat Nylon target showed maximum penetration of 5 witness papers at impact energy of ∼8 J. The results for the spike threat showed a significant improvement of the stab resistance of the STF/Nylon composite target over the neat Nylon target with just one penetrated witness paper in the first impact energy level of 2.7 J. The neat Nylon target did not show any resistance for the spike stab testing with maximum of 5 witness paper penetrated for all impact energy levels. Dynamic stab test results show a significant improvement of target stab resistant and protection in STF/fabric composite targets compared to neat fabric targets for both spike and knife threats.
3.5. Dynamic stab characterization (drop tower test) Fig. 10a shows the drop tower stab performance of neat Kevlar and STF/Kevlar composite targets for the knife and spike threats. As seen in the graph for the knife threat the penetration depth increases as the impact energy increase. STF/Kevlar composite target exhibit slightly less penetration depth and better stab resistance compared to the neat Kevlar target for low values of impact energy. At impact energy around 8 J both targets reach maximum penetration depth of 5 witness papers. For the spike threat, as impact energy increases, depth of penetration into the backing material also increases. The STF/Kevlar composite target exhibits significantly better stab resistance as compared with the neat Kevlar target. The neat Kevlar target exhibits maximum penetration of 5 witness papers at all impact energy levels. In contrast, STF/Kevlar composite only penetrated through 1 witness paper at impact energy of 2.7 J and continued to resist the penetration, then showed maximum penetration of 5 witness papers at impact energy of ∼8 J. The results for neat Nylon and STF/Nylon composites stab testing for the knife and spike threats are presented in Fig. 10b. The results show the same trend as impact energy increases, depth of penetration into the backing material also increases. The STF/Nylon composite target demonstrates slightly less penetration depth than the neat Nylon target for the knife stabbing test. As seen in the graph STF/Nylon composite target shows only 2 penetrated witness papers at impact energy of 2.7 J and when the impact energy reached a value of 5.4 J 3 witness papers were penetrated, the fourth witness papers penetrated at impact energy of 8.1 J and after that all 5 witness paper were penetrated for
3.6. Quasistatic characterization for neat fabric and STF/fabric composite Fig. 11a shows the quasistatic loading results for the neat Kevlar and STF/Kevlar composite targets against the knife and spike threats. As seen in the graph STF/Kevlar composite target supports significantly higher load 530 N than the neat Kevlar target 286 N for the knife. This behavior correlates with the appearance of the targets after testing, STF/Kevlar composite target showed significantly less damage, as compared with the neat Kevlar target. There is a dramatic difference in the behavior under the quasistatic loading against the spike threat between neat Kevlar and STF/Kevlar composite targets. The neat Kevlar target supports very small load 85 N before allowing the spike to penetrate through the whole target, while the STF/Kevlar composite target supports a high load of 573 N and was never punctured. For the neat Kevlar target after spike loading, all 5 witness papers were penetrated, while one of the witness papers was penetrated for the STF/Kevlar composite target. There is a major improvement in loading resistant in STF/Kevlar composite targets compared to neat Kevlar targets for spike and knife threats in quasistatic test. Fig. 11b shows the quasistatic loading results for the neat Nylon and STF/Nylon composite targets against both of the knife and spike threats. For the knife threat, STF/Nylon composite target exhibits higher loading 406 N than the neat Nylon target 284 N. For neat Nylon target 4 witness papers were penetrated and for the STF/Nylon composite target only 2 witness papers were penetrated. Against the spike threat, similar results were observed. STF/Nylon
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Fig. 11. Quasistatic results for neat and STF/fabric composite targets.
Fig. 10. Dynamic stab test results for (a) neat Kevlar and STF/Kevlar composite; (b) neat Nylon and STF/Nylon composite.
composite target resists high load of 548 N while the neat Nylon target resists only 100 N of load. For neat Nylon target all the witness papers were penetrated and for the STF/Nylon composite target only 1 witness paper was penetrated. The quasistatic loading results show a major improvement in loading resistant in STF/fabric composite targets as compared to neat fabric targets for both the spike and knife threats. This improvement is due to the addition of STF to the fabric which lead to the enhancement of the fabric strength and properties.
This results show that the addition of STF causes no change in the thickness and flexibility of Kevlar fabrics. The results for the neat Nylon and STF/Nylon composites are slightly different compared with the Kevlar fabric results. The thickness of 4 layers of neat Nylon and STF/Nylon composite is similar, although the bending angle of 4 layers of STF impregnated Nylon is a little bigger than the bending angle of the neat 4 layers of Nylon but the difference is just one degree which indicate that the difference in flexibility due to the addition of STF is very small. The results of the 10 layers of neat Nylon and STF/Nylon composite samples flow the same trend, the neat samples are thinner and more flexible but the difference is also very small and in acceptable range.
3.7. Flexibility and thickness tests The results of flexibility and thickness test for 4 and 10 layers of neat Kevlar, 4 and 10 layers of STF/Kevlar composite, 4 and 10 layers of neat Nylon, and 4 and 10 layers of STF/Nylon composite are presented in Table 4. As seen in the table the thickness of 4 layers of neat and STF impregnated Kevlar fabrics is almost the same however the flexibility of STF impregnated Kevlar is more compared to the 4 layers of neat Kevlar with bending angle of 51◦ which indicate that there is no much difference in flexibility between the 4-layer Kevlar samples with and without impregnated STF. This is also noticeable for the 10 layers of neat Kevlar and STF/Kevlar composite, they have similar bending angle and almost the same thickness.
Table 4 Flexibility and thickness test results. Sample
Number of layers
Sample weight (g)
Sample thickness (mm)
Bending angle (◦ )
Neat Kevlar Neat Kevlar STF/Kevlar composite STF/Kevlar composite Neat Nylon Neat Nylon STF/Nylon composite STF/Nylon composite
4 10 4 10 4 10 4 10
1.90 5.02 2.25 5.63 4.34 10.86 5.17 12.92
1.03 2.52 1.08 2.70 2.34 5.88 2.83 6.84
50 13 51 13 62 32 63 34
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4. Conclusions Sonochemical method can be efficiently used to synthesize shear thickening fluid with improved rheological properties and it is developed to prepare STF/ethanol solution in a single step. The as-prepared sample viscosity changes from ∼20 Pa s at shear rate of 5/s to 410 Pa s at 12/s shear rate before reversal trend is seen. SEM studies showed that the STF was uniformly dispersed over the entire volume of Kevlar and Nylon fabrics via our fabric impregnation procedure. The neat Kevlar target supports very small load 85 N before allowing the spike to penetrate through the whole target, while the STF/Kevlar composite target supports a high load of 573 N and was never punctured. For the knife threat, STF/Nylon composite target exhibits higher loading 406 N than the neat Nylon target 284 N. The quasistatic loading results show a major improvement in loading resistant in STF/fabric composite targets as compared to neat fabric targets for both the spike and knife threats. STF/fabric composites targets exhibit better stab resistance as compared neat fabric targets, this demonstrate that the impregnation of the fabric with a shear thickening fluid prepared from silica nanoparticles have enhanced the fabric performance for body armor applications. The flexibility and thickness of STF/fabric composites are comparable to the neat fabrics. Acknowledgements The authors would like to thank the United States Army Research Office (ARO) Dr. David Stepp for funding this research. Dr.
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Eric Wetzel, Army Research Laboratory and Dr. Norman Wagner, Department of Chemical Engineering, University of Delaware for their valuable suggestions. The help of Jessie mayo and Mohammed Y. Shaik is appreciated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Synthesis, processing and characterization of shear thickening fluid (STF) impregnated fabric composites Tarig A. Hassan, Vijay K. Rangari ∗ , Shaik Jeelani Center for Advanced Materials (T-CAM), Tuskegee University, Tuskegee, AL 36088, United States
a r t i c l e
i n f o
Article history: Received 21 August 2009 Received in revised form 3 January 2010 Accepted 6 January 2010
Keywords: Shear thickening Nanoparticles Sonochemical Composites
a b s t r a c t Shear thickening is a non-Newtonian fluid behavior defined as the increase of viscosity with the increase in the applied shear rate. The shear thickening fluid (STF) is a combination of hard metal oxide particles suspended in a liquid polymer. This mixture of flowable and hard components at a particular composition, results in a material with remarkable properties. In this manuscript the shear thickening fluid (STF) was prepared by ultrasound irradiation of silica nanoparticles dispersed in liquid polyethylene glycol polymer. The as-prepared STFs have been tested for their rheological and thermal properties. Kevlar and Nylon fabrics were soaked in STF/ethanol solution to make STF/fabric composite. Knife threats and quasistatic penetration tests were performed on the neat fabrics and STF/fabric composite targets for both engineered spike and knife on areal density basis. The results showed that STF impregnated fabrics have better penetration resistance as compared to neat fabrics without affecting the fabric flexibility. This indicates that the addition of STF to the fabric have enhanced the fabric performance and can be used in liquid body armor applications. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Throughout recorded history humans have always used various kinds of materials as armors to protect their bodies from injury in the battle field and other unsafe situations, starting from the animal skins used by the caveman, the metal shields used by the Roman warriors or the body armors used nowadays, the need to protect oneself is there, and it will go on over time. Over the years many products have been invented and tried in order to improve body armor [1]. The U.S. Army Research Laboratory is developing a new technology to save soldiers’ lives. This new technology is liquid body armor. This type of body armor is light and flexible making the soldiers safer and allowing them to remain mobile and not hindering them from moving, running and aiming their weapons. The objective of the liquid body armor technology is to produce a new thin, flexible, lightweight and inexpensive material that have an equivalent or even better ballistic properties than the existing Kevlar fabric. The shear thickening fluid is a material with remarkable properties. STF is very deformable material in the ordinary conditions and flows like a liquid as long as no force is applied. However it turns
∗ Corresponding author at: Materials Science and Engineering, Center for Advanced Materials, 101 James Center, Tuskegee University, Tuskegee, AL 36088, United States. Tel.: +1 334 724 4875; fax: +1 334 727 4224/2286. E-mail address: [email protected] (V.K. Rangari). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.01.018
into a very rigid solid-like material at high shear rates that prevent the bullet from penetrating the soldier’s body. Shear thickening is a non-Newtonian flow behavior observed as an increase in viscosity with increasing shear rate or applied stress [2]. This phenomenon can occur in colloidal dispersions. In more concentrated colloidal suspensions have been shown to exhibit reversible shear thickening resulting in large, sometimes discontinuous increases in viscosity above a critical shear rate. Two main causes of reversible shear thickening have been proposed: the order–disorder transition [3–7] and the “hydrocluster” mechanism [8–13]. This transition from a flowing liquid to a solid-like material is due to the formation and percolation of shear induced transient aggregates, or “hydroclusters,” that dramatically increase the viscosity of the fluid. Support for this hydrocluster mechanism has been demonstrated experimentally through rheological, rheo-optics and flow-SANS experiments [8,14] as well as computer simulation [15]. It has been reported in the literature that shear thickening has been observed for a wide variety of suspensions such as clay–water [16], calcium carbonate–water [17], polystyrene spheres in silicon oil [18], iron particles in carbon tetrachloride [19], titanium dioxide–resin [20], silica–polypropylene glycol [21], and silica–ethylene glycol [22]. The phenomenon of shear thickening of suspensions in general has no useful applications in industrial production. Recently Wegner’s group and U.S. Army research lab developed a body armor using shear thickening fluid and Kevlar fabric [22]. These research results demonstrate that ballistic penetration resistance of Kevlar fabric is enhanced by impregnation
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Fig. 1. Steps of the fabric impregnation procedure.
of the fabric with a colloidal shear thickening fluid. Impregnated STF/fabric composites are shown to provide superior ballistic protection as compared with simple stacks of neat fabric and STF. Comparisons with fabrics impregnated with non-shear thickening fluids show that the shear thickening effect is critical to achieving enhanced performance. In the present investigation we report on the synthesis of STF using sonochemical method, which can be potentially used for the body armor applications. Many researchers have used various techniques to prepare the shear thickening fluid. Acoustic cavitations technique is one of the efficient ways to disperse nanoparticles into the liquid polymers. In this case, the application of alternating acoustic pressure above the cavitations threshold creates numerous cavities in the liquid. Some of these cavities oscillate at a frequency of the applied field (usually 20 kHz) while the gas content inside these cavities remains constant. However, some other cavities grow intensely under tensile stresses while yet another portion of these cavities, which are not completely filled with gas, start to collapse under the compression stresses of the sound wave. In the latter case, the collapsing cavity generates tiny particles of ‘debris’ and the energy of the collapsed one are transformed into pressure pulses. It is noteworthy that the formation of the ‘debris’ further facilitates the development of cavitation. It is assumed that acoustic cavitations in liquids develop according to a chain reaction. Therefore, individual cavities on real nuclei are developing so rapidly that within a few microseconds an active cavitations region is created close to the source of the ultrasound probe. The development of cavitations processes in the ultrasonically processed melt creates favorable conditions for the intensification of various physio-chemical processes. Acoustic cavitations accelerate heat and mass transfer processes such as diffusion, wetting, dissolution, dispersion, and emulsification. The objective of this study is to synthesize the shear thickening fluid in a single step reaction through high power ultrasound technique, fabricate STF/fabric composite and characterize it for stab resistance applications. 2. Experimental 2.1. Materials The materials used in this study include dry powder of spherical silica nanoparticles (15 nm in size) purchased from Nanostructured & Amorphous Materials, Inc., Los Alamos, NM. Dry silica nanopowder was selected over the colloidal silica solution to prevent water and moisture contamination in the synthesis process. Polyethylene glycol was used as the liquid polymer in the STF synthesis pro-
cess. The average molecular weight of the PEG used in this study is 200 g/mole. PEG is non-toxic and easy to handle in addition to that it is thermally stable and easily available in bulk quantities which make it useful for bulk production. Ethyl alcohol was used as the solvent in the sonochemical process. The evaporation temperature of ethanol is around 79 ◦ C and that make it easy to be removed from the reaction mixture via the evaporation process to prepare the rheology test sample. Both PEG and ethyl alcohol were purchased from Sigma–Aldrich Chemicals, St. Louis, MO. The Kevlar and Nylon fabric fabrics used in this study and they were purchased from DuPont Company and Performance Textile Incorporated – Duxbury, MA, respectively. 2.2. Processing 2.2.1. Synthesis of shear thickening fluid The sonochemical method was used to synthesize shear thickening fluid. In this technique known weight percentages of silica nanoparticles (40%) and polyethylene glycol (60%) were mixed with an excess amount of ethanol and irradiated with high intensity ultrasonic horn (Ti-horn, 20 kHz, 100 W/cm2 at 50% amplitude) for 5 h at 10 ◦ C; the reaction temperature was maintained by using a Thermo NESLAB chiller. 2.2.2. Fabrication of STF/fabric composite The STF/fabric composite targets were prepared for testing according to the following procedure. The resultant sonochemical reaction solution of STF/ethanol (40:60) was used for fabric impregnation. 381 mm × 381 mm layers of Kevlar and Nylon fabrics were cut and impregnated individually into STF/ethanol solution. Each sheet of Kevlar or Nylon was soaked in an aluminum container filled with STF/ethanol solution for 1 min and then squeezed using an 11.34 kg steel cylinder to get rid of the excess amount of the solution and then hanged for 48 h at room temperature to remove the solvent. Fig. 1 shows the steps of the fabric impregnation procedure. The weight of the fabric sheets was measured before and after the impregnation to ensure that the fabric layers have been equally impregnated with the STF/ethanol solution. These STF/fabrics composites were then arranged into multilayer targets, as shown in Table 1. Each target has a number of fabric layers selected to match overall target areal densities as closely as possible. The targets were positioned on multi-layer foam of backing material as specified in the NIJ standard 0115.00 (Stab Resistance of Personal Body Armor) [23]. The backing material consists of four layers of 5.8 mm thick neoprene sponge, followed by one layer of 31 mm thick polyethylene foam, backed by two 6.4 mm thick layers of rubber (all backing
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Table 1 Test targets. Label
Yarn material
Yarn denier
Yarn count (yarns/in.)
STF (wt%)
Single layer areal density (g/cm2 )
Number of layers in target
Target areal density (g/cm2 )
Kevlar STF/Kevlar Nylon STF/nylon
Kevlar Kevlar Nylon Nylon
600 600 1050 1050
34 × 34 34 × 34 23 × 21 23 × 21
0.0 28.7 0.0 25.0
0.018 0.025 0.041 0.055
15 11 6 5
0.275 0.279 0.248 0.275
Fig. 2. Dynamic stab testing target showing the backing material and the witness papers.
Table 2 Explanation of depth measurement technique. Number of witness papers penetrated
Penetration depth (mm)
0 1 2 3 4 5
0 0–5.8 5.8–11.6 11.6–17.4 17.4–23.2 >23.2
materials were obtained from PCF Foam Corp., Cincinnati, OH). Five sheets of synthetic polymer-based polyart witness papers (Arjobex Corp., Charlotte, NC) were inserted between the target and all four layers of neoprene sponge and the polyethylene foam as presented in Fig. 2. The number of penetrated witness papers indicates the depth of penetration as listed in Table 2. 2.3. Characterization The as-prepared STF samples were tested for its rheological properties using a TA Instruments Rheometer-AR2000. Testing was carried out at room temperature in a steady state flow mode and shear rate ramp of 0–125/s using a peltier plate and a cone plate of size 40 mm and 2◦ angles. The peltier plate has a temperature range of −20 to 200 ◦ C with a typical heating/cooling rate up to 20 ◦ C/min and a temperature accuracy of ± 0.1 ◦ C. Thermal gravimetric analysis (TGA) was conducted to determine the weight percentage of silica and polyethylene glycol in as-prepared STF sample. The test was done under nitrogen atmosphere using a TGA-Mettler Toledo: Model Number 581 apparatus at heating rate of 5◦ /min from room temperature to 800 ◦ C.
Transmission electron microscopy (TEM) has been performed on the silica nanoparticles and as-prepared STF sample, using a JOEL-2010 transmission electron microscope. Samples for TEM were prepared by dispersion of nanoparticles and STF in ethanol and placed a drop of solution on a copper grid (carbon coated copper grid – 200 mesh) and dried in air then used for TEM analysis. To investigate the infusion of STF in the fabric sheets, scanning electron microscopy analyses were carried out using JEOL JSM 5800 scanning electron microscope. The STF/fabric composite samples and neat fabric samples were precisely cut in to small pieces using the Checkadee cutter and placed on a double sided carbon tape and coated with gold/palladium to prevent charge buildup by the electron absorption by the specimen. Dynamic stab test was performed for neat and STF/fabric composite targets to evaluate the fabric resistance against the spike and knife threats. Tests were performed using a drop tower equipped with knife and spike impactors, based on the NIJ standard 0115.00 (stab resistance of personal body armor) [23]. As specified in the NIJ standard engineered knife S1 blade represents the performance of large commando-style blades or larger kitchen knives, S1 knife blade feature a pointed tip and a stiff backbone two cutting edges. S1 knife is 100.0 ± 0.05 mm long and 20 ± 0.1 mm wide and 2.0 ± 0.05 mm in thickness; the tip of the knife is 49.2 mm long with an angle of 23 ± 0◦ 30 . Another weapon used in this study is an engineered spike that is designed to represent a class of pointed weapons used mostly in assaults in correction facilities. It is based on the design commonly used in the “California Ice Pick” test. According to the NIJ standard, the engineered spike is 177.8 ± 1.6 mm long and 4.14 ± 0.0254 mm in diameter; the diameter start to decay for the last 76.2 ± 6.35 mm to reach 1.905 ± 0.0508 mm at the tip of the spike. The tip of the spike is 4.445 ± 0.127 mm long with a point sharp of 0.0762 mm maximum diameter. The knife and spike threats were dropped from different heights marked with letters correspond to different impact energies as presented in Table 3. Two sets of experiments were performed for four different targets (neat Kevlar target, STF/Kevlar composite target, neat Nylon target and STF/Nylon composite target) one set for the spike threat and the second set for the knife threat, the drop level was varied from letters A to I for the first set and for the second set the drop level varied from J to R. To complement the drop tower tests, quasistatic stab tests were also performed on the neat targets and STF/fabric composite
Table 3 Impact energy distribution for dynamic stab testing. No.
1 2 3 4 5 6 7 8 9
Spike
Knife
Letter designation
Drop height (cm)
Energy (J)
Letter designation
Drop height (cm)
Energy (J)
A B C D E F G H I
14.4 28.9 43.2 57.6 72.1 86.6 101.03 115.6 129.6
2.7 5.4 8.1 10.8 13.5 16.2 18.9 21.6 24.3
J K L M N O P Q R
14.4 28.9 43.2 57.6 72.1 86.6 101.03 115.6 129.6
2.7 5.4 8.1 10.8 13.5 16.2 18.9 21.6 24.3
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Fig. 3. Quasistatic test for (a) spike; (b) knife.
targets for both spike and knife threats. This test was conducted to investigate the response of the neat fabric targets along with the STF/fabric composite targets under slow loading rates. Knife and spike impactors were strongly attached to the upper grip of a 2.5 kN load cell of a Zwick-Roell materials testing machine, with the
target placed on top of the multi-layered backing material used in the drop tower tests and specified in the NIJ standard 0115.00 (stab resistance of personal body armor) [23]. The targets were located below the impactor and the impactor was then pushed into the target at a rate of 5 mm/min to a total depth of 30 mm. Fig. 3 shows pictures of the quasistatic test configuration. The output of the test was recorded as load versus displacement data. Flexibility tests and thickness measurements were performed for both neat fabric and STF/fabric composite samples to examine the effect of STF addition on the fabric flexibility and thickness. To measure the flexibility, neat fabric and STF/fabric were cut into 51 mm × 51 mm square layers, 2 test specimens were prepared by stacking 4 layers for the first specimen and 10 layers of second specimen of the square layers. A 20 g weight was then attached to the test specimens according to the geometry shown in Fig. 4 and the bending angle  was measured and recorded for each specimen, this procedure was reported by Lee et al. [22]. For the thickness test a digital caliper was used to measure the specimen thickness. 3. Results and discussion
Fig. 4. Flexibility test geometry.
3.1. Rheological properties Rheology test results show that as-prepared STF sample exhibits shear thickening behavior. This behavior is very much evident as
Fig. 5. Rheology graph for as prepared STF sample.
Fig. 6. TGA graph for as prepared STF sample.
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Fig. 7. (a) TEM image of silica nanopowder; (b) TEM image of as-prepared STF sample.
seen from the graph in Fig. 5. The sample viscosity changes from ∼20 Pa s at shear rate of 5/s to 410 Pa s at 12/s shear rate before reversal trend is seen. Continuous increase in sample’s viscosity at low and very narrow shear rates range clearly confirm that the sonochemical mixing before evaporation drastically improves the shear thickening effect. 3.2. Thermal gravimetric analysis (TGA) TGA test results show two weight loss regimes the first wieght loss observed at temperature around 160 ◦ C when the PEG starts to evaporate and then decomposes at around 260 ◦ C where a major weight loss regime is noticed as seen in Fig. 6. The derivative curve clearly shows the huge weight loss of the sample indicating the total decomposition of the polymer at ∼260 ◦ C and that the remaining sample residue is silica. There is a slight difference between the remaing amount of the sample containing the silica nanoparticles (approximatley 35% of the sample) and the added silica (40%) this difference can be explained as the loss of some amount of this light
feathery small size nanopowder during the evaporation process of the ethanol. 3.3. Transmission electron microscopy (TEM) Fig. 7a shows the TEM picture of the as-received silica nanoparticles. The particles are nearly spherical in shape and the size distribution is ∼15–30 nm. The TEM micrograph of as prepared STF sample (shown in Fig. 7b) clearly shows the incorporation of silica and PEG. 3.4. Scanning electron microscopy (SEM) The SEM micrographs presented in Fig. 8 show pictures of neat Kevlar and STF/Kevlar composite samples at different magnifications. Fig. 8a shows neat Kevlar fabric without any addition of STF, the SEM images of STF/Kevlar sample presented in Fig. 8b clearly show that STF is well dispersed over the entire surface on the Kevlar sheet; it can be seen in Fig. 8c that STF is incorporated between
Fig. 8. SEM images for (a) neat Kevlar fabrics; (b–d) STF/Kevlar composite.
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Fig. 9. SEM images for (a) Neat Nylon fabrics; (b–d) STF/Nylon composite.
the fibers. The high magnification SEM image presented in Fig. 8d shows that the Kevlar fabrics are completely coated with STF. The SEM micrographs for neat Nylon and STF/Nylon composite samples presented in Fig. 9a–d show similar observations. Fig. 9a represents an SEM image of neat Nylon fabric without STF. SEM image of STF/Nylon composite sample is presented in Fig. 9b, and shows that STF is spread all over the fabric. Fig. 9c and d show that STF is covering all the Nylon fibers and that confirms the efficiency of the fabric impregnation procedure to infuse STF in Kevlar and Nylon fabrics.
higher impact energy levels. On the other hand the neat Nylon target showed maximum penetration of 5 witness papers at impact energy of ∼8 J. The results for the spike threat showed a significant improvement of the stab resistance of the STF/Nylon composite target over the neat Nylon target with just one penetrated witness paper in the first impact energy level of 2.7 J. The neat Nylon target did not show any resistance for the spike stab testing with maximum of 5 witness paper penetrated for all impact energy levels. Dynamic stab test results show a significant improvement of target stab resistant and protection in STF/fabric composite targets compared to neat fabric targets for both spike and knife threats.
3.5. Dynamic stab characterization (drop tower test) Fig. 10a shows the drop tower stab performance of neat Kevlar and STF/Kevlar composite targets for the knife and spike threats. As seen in the graph for the knife threat the penetration depth increases as the impact energy increase. STF/Kevlar composite target exhibit slightly less penetration depth and better stab resistance compared to the neat Kevlar target for low values of impact energy. At impact energy around 8 J both targets reach maximum penetration depth of 5 witness papers. For the spike threat, as impact energy increases, depth of penetration into the backing material also increases. The STF/Kevlar composite target exhibits significantly better stab resistance as compared with the neat Kevlar target. The neat Kevlar target exhibits maximum penetration of 5 witness papers at all impact energy levels. In contrast, STF/Kevlar composite only penetrated through 1 witness paper at impact energy of 2.7 J and continued to resist the penetration, then showed maximum penetration of 5 witness papers at impact energy of ∼8 J. The results for neat Nylon and STF/Nylon composites stab testing for the knife and spike threats are presented in Fig. 10b. The results show the same trend as impact energy increases, depth of penetration into the backing material also increases. The STF/Nylon composite target demonstrates slightly less penetration depth than the neat Nylon target for the knife stabbing test. As seen in the graph STF/Nylon composite target shows only 2 penetrated witness papers at impact energy of 2.7 J and when the impact energy reached a value of 5.4 J 3 witness papers were penetrated, the fourth witness papers penetrated at impact energy of 8.1 J and after that all 5 witness paper were penetrated for
3.6. Quasistatic characterization for neat fabric and STF/fabric composite Fig. 11a shows the quasistatic loading results for the neat Kevlar and STF/Kevlar composite targets against the knife and spike threats. As seen in the graph STF/Kevlar composite target supports significantly higher load 530 N than the neat Kevlar target 286 N for the knife. This behavior correlates with the appearance of the targets after testing, STF/Kevlar composite target showed significantly less damage, as compared with the neat Kevlar target. There is a dramatic difference in the behavior under the quasistatic loading against the spike threat between neat Kevlar and STF/Kevlar composite targets. The neat Kevlar target supports very small load 85 N before allowing the spike to penetrate through the whole target, while the STF/Kevlar composite target supports a high load of 573 N and was never punctured. For the neat Kevlar target after spike loading, all 5 witness papers were penetrated, while one of the witness papers was penetrated for the STF/Kevlar composite target. There is a major improvement in loading resistant in STF/Kevlar composite targets compared to neat Kevlar targets for spike and knife threats in quasistatic test. Fig. 11b shows the quasistatic loading results for the neat Nylon and STF/Nylon composite targets against both of the knife and spike threats. For the knife threat, STF/Nylon composite target exhibits higher loading 406 N than the neat Nylon target 284 N. For neat Nylon target 4 witness papers were penetrated and for the STF/Nylon composite target only 2 witness papers were penetrated. Against the spike threat, similar results were observed. STF/Nylon
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Fig. 11. Quasistatic results for neat and STF/fabric composite targets.
Fig. 10. Dynamic stab test results for (a) neat Kevlar and STF/Kevlar composite; (b) neat Nylon and STF/Nylon composite.
composite target resists high load of 548 N while the neat Nylon target resists only 100 N of load. For neat Nylon target all the witness papers were penetrated and for the STF/Nylon composite target only 1 witness paper was penetrated. The quasistatic loading results show a major improvement in loading resistant in STF/fabric composite targets as compared to neat fabric targets for both the spike and knife threats. This improvement is due to the addition of STF to the fabric which lead to the enhancement of the fabric strength and properties.
This results show that the addition of STF causes no change in the thickness and flexibility of Kevlar fabrics. The results for the neat Nylon and STF/Nylon composites are slightly different compared with the Kevlar fabric results. The thickness of 4 layers of neat Nylon and STF/Nylon composite is similar, although the bending angle of 4 layers of STF impregnated Nylon is a little bigger than the bending angle of the neat 4 layers of Nylon but the difference is just one degree which indicate that the difference in flexibility due to the addition of STF is very small. The results of the 10 layers of neat Nylon and STF/Nylon composite samples flow the same trend, the neat samples are thinner and more flexible but the difference is also very small and in acceptable range.
3.7. Flexibility and thickness tests The results of flexibility and thickness test for 4 and 10 layers of neat Kevlar, 4 and 10 layers of STF/Kevlar composite, 4 and 10 layers of neat Nylon, and 4 and 10 layers of STF/Nylon composite are presented in Table 4. As seen in the table the thickness of 4 layers of neat and STF impregnated Kevlar fabrics is almost the same however the flexibility of STF impregnated Kevlar is more compared to the 4 layers of neat Kevlar with bending angle of 51◦ which indicate that there is no much difference in flexibility between the 4-layer Kevlar samples with and without impregnated STF. This is also noticeable for the 10 layers of neat Kevlar and STF/Kevlar composite, they have similar bending angle and almost the same thickness.
Table 4 Flexibility and thickness test results. Sample
Number of layers
Sample weight (g)
Sample thickness (mm)
Bending angle (◦ )
Neat Kevlar Neat Kevlar STF/Kevlar composite STF/Kevlar composite Neat Nylon Neat Nylon STF/Nylon composite STF/Nylon composite
4 10 4 10 4 10 4 10
1.90 5.02 2.25 5.63 4.34 10.86 5.17 12.92
1.03 2.52 1.08 2.70 2.34 5.88 2.83 6.84
50 13 51 13 62 32 63 34
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4. Conclusions Sonochemical method can be efficiently used to synthesize shear thickening fluid with improved rheological properties and it is developed to prepare STF/ethanol solution in a single step. The as-prepared sample viscosity changes from ∼20 Pa s at shear rate of 5/s to 410 Pa s at 12/s shear rate before reversal trend is seen. SEM studies showed that the STF was uniformly dispersed over the entire volume of Kevlar and Nylon fabrics via our fabric impregnation procedure. The neat Kevlar target supports very small load 85 N before allowing the spike to penetrate through the whole target, while the STF/Kevlar composite target supports a high load of 573 N and was never punctured. For the knife threat, STF/Nylon composite target exhibits higher loading 406 N than the neat Nylon target 284 N. The quasistatic loading results show a major improvement in loading resistant in STF/fabric composite targets as compared to neat fabric targets for both the spike and knife threats. STF/fabric composites targets exhibit better stab resistance as compared neat fabric targets, this demonstrate that the impregnation of the fabric with a shear thickening fluid prepared from silica nanoparticles have enhanced the fabric performance for body armor applications. The flexibility and thickness of STF/fabric composites are comparable to the neat fabrics. Acknowledgements The authors would like to thank the United States Army Research Office (ARO) Dr. David Stepp for funding this research. Dr.
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Eric Wetzel, Army Research Laboratory and Dr. Norman Wagner, Department of Chemical Engineering, University of Delaware for their valuable suggestions. The help of Jessie mayo and Mohammed Y. Shaik is appreciated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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