One of the disadvantages of shear thickening fluid (STF) is high concentration. For better immersion of fibers, at first STF is diluted in ethanol, and then the fibers are left therein for a specified period in order to impregnate all fibers with the fluid. After this stage, to eliminate the ethanol in the sample, it is heated at a temperature range of 60–80°C. The composite’s sensitivity to the impact will be reduced through ethanol combining with the fluid and its elimination. Furthermore, by means of vacuum infusion processing method, without use of ethanol (or reducing its portion) and heating, lighter composite could be obtained. The fluid is distributed to the required extent among the fibers and the additional fluid that causes weight increase is discharged through vacuum. In addition, the bubbles among the fibers and fluid are removed by vacuum. Specimens with 3 and 5 layers have been tested. By comparing the nanocomposites and neat specimens, experimental results show higher ballistic limit in all of the nanocomposite specimens. Results show that the ballistic limit is highest in the VIP method.
The damper is one of the most basic equipments whose importance of use has been understood by man. The new dampers are provided by many reinforced fibers in a full-density network. Their mechanism is such that by relying on their network, structures disperse and attract the projectile energy in a wide area. In 1997, Bazhenov studied the effect of water on the ballistic behavior of 20 layers of woven fabric with dimensions of 200×150 mm. The layers were impacted by a spherical head projectile, and the results indicated that the projectile could not pass through the dry layers, but the wet layers were perforated by the impact and allowed the projectile to pass through them. Also, according to the observations, the fiber pullout area in the wet layers is lower than that in the dry layers. The fibers under impact in the wet layers do not tear, showing that water has not reduced the resistance of the fibers. The slippage between projectile and pulled out fibers causes perforation of dry layers. Bazhenov concluded that the presence of water reduces the friction between the projectile and fibers and consequently reduces the textile efficiency . In 2003, Cheeseman and Bogetti reviewed the factors that are effective parameters of the ballistic behavior of textile. These factors include the material properties, projectile geometry, impact speed, number of layers, boundary conditions, and friction. The authors explained the physical mechanism of each factor in detail .
In a similar test by Dong and Sun , the pullout behavior of five types of woven Kevlar was studied. These textiles were different regarding fiber size, fiber type, weight, and thickness. The summary indicated that pullout force has a direct relationship with the woven fibers’ properties under the impact. The woven fibers with higher pullout force have better properties in the impact tests .
Lee and Wagner , Lee et al. , Wetzel et al. , and Egres et al.  have studied shear thickening fluid (STF)/fabric composites for several years. These studies reported the ballistic performance of composite materials composed of Kevlar fabric impregnated with a colloidal STF (micrometer-sized silica particles dispersed in ethylene glycol). The impregnated Kevlar fabric yields a flexible yet penetration-resistant composite material. Ballistic penetration measurements have demonstrated a significant improvement due to addition of STF to the fabric without any loss in material flexibility. Such enhancement in the performance has been attributed to the increase of yarn pullout force upon transition of the STF to its rigid state during impact.
Furthermore, STF-impregnated Kevlar has vastly superior stab resistance in addition to flexible ballistic protection, which addresses a key failure mechanism of current Kevlar based on body armor. Although these preliminary studies clearly establish the viability of the STF/fabric composite as a future flexible body armor system, the entire scope of particle-polymer interaction along with the complexities associated with fabric impregnation must still be addressed before an optimal, lightweight STF/fabric system can be developed. A case in point is the use of nanometer-sized particles in place of the current micrometer-sized particles.
Because of the gain in surface area with nanoparticles, it is believed that the surface energy available at the particle polymer interface will be more, which should consequently develop the improved bonding with the surrounding matrix and the fiber, as previously shown by Nathaniel et al. , Mahfuz et al. , and Rodgers et al. .
STFs were generated by dispersing commercially available nano silica particles (11–14 nm, Degussa, Germany) in polyethylene glycol (molecular weight 200, Merck, Germany) with a weight fraction of approximately 40%. To impregnate the woven fibers in the STF fluid, the fluid should be diluted first. Therefore ethanol 99% could be used.
Rheological characterization of this STF confirmed discontinuous shear thickening at a shear rate of approximately 20 s-1. One type of Kevlar fabric with yarn denier of 600 and areal density of 0.27 g/cm2 was tested. A scanning electron microscopy (SEM) image of silica nanoparticles is shown in Figure 1.
To make the STF, polymer and nanoparticles were mixed together slowly. After the addition of a small amount of nanoparticles to the polymer by a mechanical blender, the force that is required to mix the materials is created (see Figure 2A). For better mixing, several blades are used. Each blade creates a different flow with different speed in the fluid. This stage is continued until all nanoparticles are distributed in the polymer. By means of an ultrasonic homogenizer, the nanoparticles are distributed in the polymer completely. Ultrasonic homogenizer is the most effective tool in distribution of non-homogeneous mixtures. This method includes an alternative sonic pressure that makes a hole in the liquid. In this study, a suitable ultrasonic homogenizer (Bandlin 3200, Germany) is used to disperse the nanoparticles and prevent agglomeration (Figure 2C). The vacuum instrument for vacuum infusion processing (VIP) method is shown in Figure 2B.
Figure 3 shows the rheology of STF with ethanol and without ethanol. This figure shows that the combination of ethanol with the fluid and its elimination reduces the STF’s sensitivity to the shear rate. In specimens with ethanol, critical shear rate is increased and final viscosity is decreased.
To fabricate the STF-fabric composites, the STF was first diluted in ethanol at an ethanol/STF volume ratio of 3:1. Individual fabric layers, each measuring 25 cm×25 cm, were then soaked in the solution for 1 min, squeezed to remove excess fluid, and dried at 80°C for 20 min as shown in Figure 4. Then three and five dried layers were stacked on top of each other. The STF weight additions reported for each target represent an average value over all of the target layers.
In the above nanocomposite fabrication methods, some problems and defects have been observed such as decreasing of the thickening properties after dilution. By using the VIP method, laminate has been made without dilatation.
One of disadvantages of STFs is the high concentration of fluid. For better immersion of fibers, at first, STF is diluted in ethanol, and then the fibers are put therein for a specified period in order to impregnate all the fibers with the fluid. After this stage, to eliminate the ethanol, the sample is heated at a temperature range of 60–80°C. The composite’s sensitivity to the impact is reduced through combining the ethanol with the fluid and its elimination. Furthermore, by means of the VIP method, without using the ethanol (or reducing its portion), and heating, lighter composite could be obtained. The fluid is distributed to the required extent among the fibers and the additional fluid that causes weight increase is discharged through vacuum. The bubbles among the fibers and fluid are also removed by vacuum.
As shown in Figure 5, fibers at first are sealed and connected to the fluid tank from one side and to the vacuum pump from other side. Upon turning the pump on, due to the created vacuum the fluid is introduced from the opposite side into the fibers. The infusion is continued until all of the fibers are impregnated with the liquid. The input tubes from the tank and output to the pump are exhibited in Figure 5. Three types of specimens are made using two methods (Figure 6).
The gas gun is an instrument used to provide high-impact velocity tests at different speeds. The ballistic tests of this research were applied with a gas gun available in the composite laboratory of the Department of Mechanical Engineering, Amirkabir University of Technology, Tehran, Iran (Figure 7).
Sandpaper was used on both sides of the specimens to fix them within the framework. Figure 8 shows the sandpaper used. The projectile used in the gas gun is shown in Figure 9 and its characteristics are shown in Table 1.
|ρ||E (MPa)||ν||Diameter (mm)|
By comparing the nanocomposites and neat specimens, experimental results show higher ballistic limit in all of the nanocomposite specimens. Results show that the ballistic limit is highest in the VIP method. Handmade specimens, neat specimens and specimens that have been made by VIP method on the fixture after impact test by gas gun are shown in Figure 10. In the neat specimens, fibers are pulled out. Specimens that have been made by the VIP method have lower weight but require more time for making. Experimental results are in Table 2.
|Type of specimen||Ballistic limit (m/s)|
|Three layers||Five layers|
Although the STF fabrication route is bypassed, the stab resistance performance of the silica-polyethylene glycol-Kevlar composite shows a tremendous increase compared to that of neat Kevlar. These improvements are believed to be due to the preferred ultrasonic cavitation route over mechanical mixing and, second, the advantage of employing nanometer- over micrometer-sized particles. Ultrasonic cavitations have accelerated and intensified properties for diffusion, dissolution, dispersion, and emulsification. By means of the VIP method, without use of ethanol (or reducing its portion), and heating, lighter composite could be obtained because the fluid is distributed to the required extent among the fibers and the additional fluid that causes weight increase is discharged through vacuum. In addition, the bubbles among the fibers and fluid are removed by vacuum. Deviation from the mean value in the VIP method than the handmade method is more. This approach reduces the repeatability. STF had more significant effect on the five layers, that could be because of further amount of STF, but specimens are in higher weight. Amount of STF are directly related to the roll pressure.
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