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Design strategy for optimising weight and ballistic performance of soft body armour reinforced with shear thickening fluid
Journal Pre-proof Design strategy for optimising weight and ballistic performance of soft body armour reinforced with shear thickening fluid Mukesh Bajya, Abhijit Majumdar, Bhupendra Singh Butola, Sanjeev Kumar Verma, Debarati Bhattacharjee PII:
S1359-8368(19)34273-8
DOI:
https://doi.org/10.1016/j.compositesb.2019.107721
Reference:
JCOMB 107721
To appear in:
Composites Part B
Received Date: 21 August 2019 Revised Date:
9 December 2019
Accepted Date: 16 December 2019
Please cite this article as: Bajya M, Majumdar A, Butola BS, Verma SK, Bhattacharjee D, Design strategy for optimising weight and ballistic performance of soft body armour reinforced with shear thickening fluid, Composites Part B (2020), doi: https://doi.org/10.1016/j.compositesb.2019.107721. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Design Strategy for Optimising Weight and Ballistic Performance of Soft Body Armour Reinforced with Shear Thickening Fluid
Mukesh Bajya1, Abhijit Majumdar1*, Bhupendra Singh Butola1, Sanjeev Kumar Verma2, Debarati Bhattacharjee2 1
Department of Textile and Fibre Engineering, Indian Institute of Technology Delhi, India 110016 2
Terminal Ballistic Research Laboratory, Chandigarh India 160 030
Abstract This study presents some soft armour panel design strategies using shear thickening fluid reinforced Kevlar® fabric. The effect of size of silica nano and sub-micron particles on the ballistic performance of soft body armour panels against the small arms ammunition (velocity ~ 430 ± 15 m/s) has been analysed. Shear thickening fluids (STFs), namely STF-500 and STF-100, were prepared using silica particles of 500 nm and 100 nm sizes, respectively, and dispersing them in polyethylene glycol (PEG) 200. Kevlar® fabrics were impregnated with both the STFs. Multiple layers (20-24) of fabrics were stitched to prepare 13 soft armour panels which were evaluated for back face signature (BFS) against 9×19 mm lead core bullet. Soft armour panels comprised of fabrics impregnated with STF-500 yield lower BFS than the respective panels comprised of fabrics impregnated with STF-100. This study also reveals that STF impregnation of Kevlar® fabrics can reduce the BFS by 2.5 mm to 2.8 mm while keeping the areal density of the panel same (5 kg/m2). The areal density of soft armour panel can be reduced further by 10% (4.5 kg/m2), while keeping the BFS comparable or lower than that of a STF impregnated homogenous panel, by judiciously placing the STF impregnated fabrics at the rear side while neat fabrics are placed at the strike face of the panel. STF impregnated panels are found to stop the impacting bullet earlier than that by neat panel. Keywords: Back face signature; Shear thickening fluid; Soft body armour; High velocity impact
Corresponding author: Dr. Abhijit Majumdar Professor Department of Textile and Fibre Engineering Indian Institute of Technology Delhi New Delhi, India 110016 Email: [email protected]
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1. Introduction Body armours are of two types, namely hard and soft. Either combination of hard and soft body armours or standalone version of the former give protection against the threats from rifle ammunitions (viz. SLR, AK etc). Hard body armours are composite or hybrid structures comprising of metal, ceramic (boron carbide, silicon carbide) and/or textile reinforced composites. Soft body armours are made of various high-performance fibres like p-aramid (Kevlar®, Twaron® etc.) and Ultra High Molecular Weight Polyethylene (Spectra®, Dyneema® etc.) used in the form of woven, multiaxial fabrics, uni/multi-directional laminates, etc. In the past two decades, scientists and researchers explored shear thickening fluid (STF) which has the capability to reduce behind armour blunt trauma (BABT) or back face signature (BFS) and stop a bullet from penetrating the soft armour [1–7]. STF is a concentrated colloidal suspension whose viscosity increases drastically, when the applied shear rate exceeds the critical value. This non-Newtonian behaviour of STF makes it ideal for energy absorption applications. Shear thickening behaviour is exhibited by various concentrated suspensions such as corn-starch– water suspension, iron particles in carbon tetrachloride and silica nanoparticles in polyethylene glycol, etc. [8– 12] In normal conditions, the solid particles remain suspended in the medium and hence the fabric maintains its flexibility. Upon bullet impact, the particles form hydroclusters which helps in energy absorption by providing better structural integrity and by ensuring increased participation of secondary yarns of fabric [13–15]. One of the most important particle parameters which influences the rheological behaviour of STF is size of particle in dispersed phase. In general, it has been observed that higher the particle size, lower the critical shear rate and higher the peak viscosity [16–18]. Therefore, choice of particle size remains crucial for the design of STF reinforced soft armour panel. Numerous research works have been reported on the polyethylene glycol or PEG (dispersion medium) and silica nanoparticle (dispersed phase) based STF impregnated Kevlar® fabrics. STF impregnated Kevlar® fabrics show improvement in impact resistance. There exist two different schools of thought about the role of STF in enhancing the impact resistance of fabrics. One group of researchers believe that the improvement is due to the increase in inter-yarn friction [6, 19, 20]. In contrast, the other group of researchers attribute the enhanced performance to shear thickening mechanism [10, 16]. However, STF impregnated fabrics have mostly been explored at lower impact velocity range in which an impactor in dropped or a projectile is fired at a velocity lower than Mach 1 [4, 5, 7, 12 ,21–25]. Xu et al. [26] investigated the effect of silica particle size and loading on the stab resistance of body armour. They found that as the particle size and STF loading increase, energy absorption also increase. Majumdar and Laha [13] showed that the shear thickening effect is critical for
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achieving enhanced performance via an increase in the yarn pull-out force upon transition of STF to its solidstate. Seshagiri et al. [27] found that when Kevlar® was used with Oobleck and Silica-PEG STF, the deformation produced on a glass plate was reduced considerably as compared to that of neat Kevlar® sample. Khodadadi et al. [15, 24] reported numerical and experimental results of STF impregnated panels at impact velocity range from 40 m/s to 60 m/s. They found improvement in performance and ascribed it to frictional interaction between yarns and silica in impregnated fabrics. Park et al. [3, 5, 6] found that the impregnation of aramid fabric with STF increased the frictional force of a single yarn in the fabric, which consequently increased the modulus of the yarn. The layering sequence was found to be very important in improving the protective performance of STF impregnated hybrid multilayer panels, which influenced not only the BFS value but also the perforation ratio and bullet expansion. Some recent research papers have reported that the effectiveness of STF, in improving the impact resistance, depends upon the fabric structure [28, 29]. Arora et al. [29] found that the beneficial role of STF ceases to exist if the fabric is very tightly woven. The impregnation of STF increases the effective mass of the fabric varying from 10% to 46% [10, 12, 22, 23]. Therefore, to keep the mass of the soft armour panel within the prescribed limit, either the number of fabric layers should be reduced after STF treatment so that the overall areal density of panel remains the same or few layers of soft armour panels should be impregnated with the STF. If the latter is followed, then the STF impregnated fabric layers should be judiciously placed so that they perform effectively. This research work presents the effect of silica particle size on the ballistic performance of STF impregnated soft armour panels under two scenarios, namely keeping the number of fabric layers same and keeping the areal density of the panel same. The comparison of ballistic performance of soft armour panels when the STF impregnated fabric layers are placed at the strike face and at the rear side has also been made.
2. Experimental 2.1 Materials Kevlar® 363 2S (scoured) square plain woven fabric, having 28 ends (warp) per inch and 28 picks (weft) per inch and areal density of 200 g/m2 was sourced from DuPont, USA. It was made of multifilament Kevlar 129 yarn having linear density of 840 denier, tensile strength of 3.4 GPa, tensile modulus of 78 GPa and breaking strain of 3.3%. Polyethylene glycol of molecular weight 200 (PEG 200) and ethanol were sourced from Merck Life Science Pvt. Ltd. (India) and Changshu Hongsheng Fine Chemical Co. Ltd. (China), respectively. Silica nanoparticles (100 nm) dispersed in water (40% w/w) was obtained from Nissan Chemical Corporation
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(Japan), and silica sub-micron-particles (500 nm) in powder form was obtained from Nippon Chemical Industrial (Japan).
2.2 Preparation of STF Two STFs were prepared in this study using different silica particles sizes, i.e., 100 nm and 500 nm and they were denoted as STF-100 and STF-500, respectively. In both the STFs, PEG 200 was used as a dispersing medium because of its good thermal stability (boiling point > 150 °C) and less volatility. Amount of silica particles (65 wt. %) and PEG (35 wt. %) was kept constant for both the STFs. To prepare STF-100, water dispersed silica was mixed with PEG in requisite weight proportion and then the mixture was kept inside the sonication bath for 8 hr at 80 °C to remove the water. STF-100 was prepared after the removal of water. STF500 was prepared by probe sonication mehtod in which 500 nm silica powder was mixed with ethanol and the mixture was homogenised for 5 minutes by using a homogeniser rotating at 17400 rpm. Homogenised solution of silica and ethanol was then sonicated for 5 minutes. After that, ethanol dispersed silica was mixed with PEG and homogenised for 5 minutes. Final solution of silica, PEG and ethanol was sonicated for 20 minutes. Prepared sonicated solution was then kept inside the oven at 120 °C to remove the ethanol. After removal of ethanol, STF-500 was prepared.
2.3 Impregnation of Fabrics with STF In order to impregnate the Kevlar fabric with STF, the prepared STFs were first diluted with ethanol such that w/v ratio of PEG: ethanol was 1:4. A high-speed homogeniser (17400 rpm) was used to avoid the agglomeration of silica particles and to ensure the uniform dispersion. Kevlar fabric samples of size 25 cm × 25 cm were cut and impregnated with diluted STF solution using Matthis Lab Padder, in which two padding rollers are laid horizontally. The schematic representation of impregnation process is shown in Figure 1. Padding was repeated twice at 2 bar pressure and 3 m/min delivery speed. Thereafter, the samples were kept in a hot air oven at 100 °C for 30 minutes to evaporate the ethanol. The add-on of STF for each sample was calculated using Equation (1).
Add − on% =
T −N ×100 N
(1)
where T is the weight of STF impregnated Kevlar fabric and N is the weight of corresponding neat Kevlar fabric.
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2.4 Stitching and shot location of soft armour panels The neat and STF impregnated fabric layers were stacked in a planned manner as described in the Table 1. For all the panels, ‘K’ denotes the Kevlar fabric; the number following ‘K’ corresponds to the number of fabric layers; ‘N’ and ‘S’ describe whether the fabric layers are neat or STF impregnated, respectively. ‘A’ implies alternate stacking of neat and STF impregnated fabrics, and the number preceding ‘A’ corresponds to repeating unit of alternation. For example, K10 N + K10 S 1A implies that 10 layers of neat and STF impregnated fabrics each (K10 N + K10 S) were used and 1 layer of neat and 1 layer of STF impregnated Kevlar fabrics were placed alternately (1A) and the neat fabric was at the strike face. The assembled fabric plies were stitched using JUKI LU 2810 sewing machine. The stitching pattern and shot location are shown in Figure 2. Stitching was done in diamond pattern with 5 mm stitch length and 50 mm distance between the two parallel stitch lines [30]. Five shots locations were marked on the panel and a minimum distance of 51 mm was maintained between a
shot and edge of the panel and also between two shots according to BIS 17051-2018.
3. Testing and Characterization 3.1 Rheological analysis of STF Steady state rheological properties of prepared STFs were evaluated using Anton Paar Physica MCR 51 stresscontrolled rheometer. The tests were performed using a parallel plate geometry having two parallel plates separated by a gap of 0.3 mm. Diameter of upper and lower plates is 25 mm and 50 mm, respectively. Rheological tests were conducted at three different temperatures, i.e. 15 °C, 25 °C and 35 °C, and shear rate was varied from 1 s-1 to 1000 s-1.
3.2 Scanning Electron Microscopy (SEM) Surface morphology of neat and STF impregnated fabrics was evaluated by using a ZEISS EVO 15 scanning electron microscope (SEM). The SEM samples were prepared on a double-sided carbon tape and sputter coated with gold to prevent the charge build-up on samples.
3.3 Yarn pull-out test Yarn pull-out tests were performed to study the inter-yarn friction. The tests were carried out on a universal tensile testing machine (Tinius Olsen H5KS) equipped with a specially designed lower jaw for holding the fabric sample. The sample size used for the test is shown in Figure 3 (a). First, rectangular fabric samples (12
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cm × 16 cm) were cut and then yarns were unravelled from the top so that fringes of yarns having length of 8 cm protrudes out from the fabric. During the yarn pull-out test, a yarn was gripped by the movable upper jaw and the fabric was gripped by the lower jaw without any slackness, as shown in Figure 3(b). The tests were conducted at a crosshead speed of 300 mm/min and the maximum pull-out force (N) and force-displacement curve were recorded. Five specimens of each sample were tested and then the average was calculated. 3.4 Dynamic impact testing Low-velocity dynamic impact tests were carried out for both neat and STF impregnated fabric samples having dimension 16 cm ×16 cm. The tests were conducted on a falling-dart type impact tester (CEAST, Model: Fractovis Plus) following ASTM D3763. The fabric samples were placed between two circular jaws having knurled metallic surfaces to grip the sample with a clamping force of 5 kN. The pitch and density of the knurl were 0.2 cm and 25 cm-2, respectively. The diameter of hemispherical tip of the impactor was 13 mm. The impact velocity and total impact energy were 4.5 m/s and of 200 J, respectively. The dynamic test set-up is shown in Figure 4. As the areal densities of neat and STF impregnated fabrics are different, a normalising factor is required to compare the energy absorbed irrespective of the areal density. Considering the STF add-on to be %, the normalized value of impact energy absorption by an STF impregnated sample is calculated using Equation 2
En = where
Ea (1 + x /100)
is the normalized energy (J) absorption and
(2) is the absolute energy (J) absorption.
3.5 Ballistic testing of panels and back face signature measurement Soft armour panels having different configurations were tested against the 9×19 mm lead core bullet (mass 7.45 g) with striking velocity of 430 ± 15 m/s. Soft armour panel and BFS created by a non-perforating bullet on backing material (Roma Plastilina® No.1 Grey clay) are shown in Figure 5. BFS is a critical aspect of ballistic performance evaluation as the former determines the lethality of internal injuries to vital organs during a nonperforating ballistic impact. BFS was obtained by measuring the maximum depth of indentation on clay, created by the armour panel when impacted by a non-perforating bullet. The backing material was calibrated for its consistency and viscoelasticity before conducting the experiments as per standard NIJ 0101.06. A steel ball having mass of 1.063 kg was dropped on the backing material from a height of 2 m and depth of indentation was measured. The average indentation depth of the material was kept within the range from 18.5 to18.8 mm to
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ensure minimum variation in BFS due to backing material. Two samples were tested on each calibrated backing material after that it was recalibrated. The test parameters used for high-velocity impact test is given in Table 2 and the test set-up for the same is shown in Figure 6.
4. Results and discussions 4.1 Rheological behaviour of STFs Flow curves of STF-100 and STF-500, at three different temperatures, are shown in Figure 7(a) and 7(b), respectively. The flow curves confirmed the non-Newtonian behaviour of both the STFs at all three temperatures. Further, it can be seen from the Figure 7 and Table 3 that with the increase in temperature, the critical shear rate and shear rate at maximum viscosity increases and peak viscosity decreases for both the STFs. This change in critical shear rate and viscosity is due to increased Brownian motion of particles, at elevated temperature, which requires more shear force to induce clustering of particles or shear thickening [31]. Moreover, at higher temperature, the thickness of solvation layer formed by PEG on silica particles is smaller causing higher effective distance between the particles [32]. This also necessitates higher shear force for onset of shear thickening In addition, the rheological results reveal that, as the particle size increases, critical shear rate decreases. These results are agreement with the findings of other researchers [16,18]. This trend confirms that the number of particles per unit volume of STF-100 is 125 times higher than that of STF-500. Besides, the higher specific surface charge on smaller particles cause higher intensity of inter-particle repulsive force. Therefore, higher shear force is required for the onset of shear thickening in case of STF-100. Another interesting finding is that STF-500 shows a plateau region, over a wide span of shear rate, after reaching the peak viscosity. This may be attributed to the higher inertial effect of larger particles. In contrast, STF-100 demonstrates shear tinning immediately after attaining the peak viscosity.
4.2 Surface morphology of STF impregnated fabric Scanning electron micrographs of STF impregnated fabrics are shown in Figure 8. It is observed that silica nano and sub-micron particles are deposited on the surface of filaments. Magnified views of fabric surface reveals that spherical silica particles have formed small clusters on the filaments.
4.3 Yarn pull-out behaviour
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Figure 9 depicts the yarn pull-out force vs displacement curves of neat, STF-100 and STF-500 impregnated Kevlar fabrics. It is observed that the yarn pull-out force increases after the impregnation of fabrics with STF, which is due to the increase in inter-yarn friction. Presence of silica particles makes the surface of filaments rough. It is also observed that peak yarn pull-out force is higher for STF-100 impregnated fabrics (80.5 N) as compared to that of STF-500 impregnated fabrics (64.3 N). This is due to the fact that specific surface area as well as number of silica particles in STF are higher for 100 nm silica particles. Therefore, it gives better coverage on filament surface causing higher yarn pull-out force. 4.4 Dynamic impact results Figure 10 shows dynamic impact test results of neat and STF impregnated Kevlar fabrics in terms of energy absorption. The absolute energy absorption by STF-100 and STF-500 impregnated fabrics are around 19% and 42% higher, respectively, than that of the neat fabric. However, STF add-on was also higher in case of STF-500 as shown in Table 4. Though both the STFs were impregnated using same process parameters, as mentioned in Section 2.3, 100 nm silica particles came out rather easily from the soaked fabric during squeezing. On the other hand, 500 nm particles got trapped on the fabric surface due to their bigger size and higher viscosity of STF-500 at low shear rate. Absolute as well as normalised energy absorption by STF impregnated fabrics are given in Table 4. Normalized energy absorption by STF-100 and STF-500 impregnated fabrics is 8.4% and 17.8% higher, respectively, than that of neat fabric. STF impregnated fabrics are expected to absorb more energy due to two reasons. First, due to enhancement of inter-yarn friction and second, due to shear thickening effect. The energy absorption by STF500 impregnated fabric is more than that of STF-100 impregnated fabric though the inter-yarn friction, indicated by yarn pull-out force, is higher for the latter. This may be due to lower critical shear rate of STF-500 than that of STF-100. From the flow curve, depicted in Figure 7, it is observed that STF-500 gets the trigger at lower critical shear rate and remains thickened for a greater span of shear rate as compared to STF-100 due to which the impact energy absorption is more in fabrics impregnated with STF-500. Besides, the rheological results given in Table 3 shows that the critical shear rate of STF-100 is higher and it shows shear thinning behaviour before attaining critical shear rate. The statistical significance test was also carried out using one-way ANOVA which is used to compare the means of two or more samples. This technique is used to ascertain the effect of a treatment or process and whether the differences between and within the groups of samples are statistically significant or not. One-way
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ANOVA results are reported in Table 5. It is noted that F-calculated is much higher than F-critical signifying that the impact energy absorption by neat and STF impregnated samples are significantly different.
4.5 Back face signature BFS was measured for each shot using a digital depth gauge. The average BFS of five shots per panel was calculated and it is depicted in Figure 11 and Figure 12 for STF-100 and STF-500 impregnated fabrics, respectively. Figure13 shows the deformation (mushrooming) of a bullet after the ballistic test.
4.5.1 STF-100 impregnated panels The experimental results of STF-100 impregnated panels are shown in Figure 11. It is seen that for the control sample, having 24 layers of neat fabric (K24 N), the BFS is 39.2 mm. BFS reduces by 6.2 mm (from 39.2 mm to 33 mm) in case of STF impregnated panel having 24 layers of fabric (K24 S). However, areal density of the panel increases by 20% (from 5 kg/m2 to 6 kg/ m2) after the STF impregnation. As panel weight is one of the decisive criteria for soft body armour, it was attempted to keep the areal density same with that of control sample by reducing the number of STF impregnated fabric layers. To achieve this, a panel (K20 S) was made by impregnating 20 layers of fabric with STF-100. For this panel, BFS is found to be 36.4 mm, i.e. 2.8 mm lower as compared to neat panel having the same areal density (5 kg/m2). As the first few layers of fabrics in a soft armour panel acts as sacrificial layers, it was hypothesised that placing the STF impregnated fabrics at the rear side of the panel may yield the same BFS with reduction in areal density of the panel. The panel made with 20 layers of fabric in which 10 layers of STF impregnated fabric are placed at the rear side (K10 N + K10 S) shows marginal increase (from 36.4 mm to 37.2 mm) in BFS as compared to the panel that has all the 20 layers of fabric impregnated with STF-100 (K 20 S). However, there is 10% reduction in areal density in panel K10 N + K10 S (4.5 kg/m2) as compared to that of control sample (5 kg/m2). When 10 neat and 10 STF impregnated fabric layers are placed alternately (K10 N + K10 S 1A and K10 N + K10 S 2A), there is no further reduction in BFS. This implies that placing the STF impregnated fabric layers at the rear side of the panel could be an effective design strategy for soft body armour.
4.5.2 STF-500 impregnated panels Figure 12 depicts the BFS of various panels impregnated with STF-500. In general, the trend is similar with that observed in case of panels impregnated with STF-100. However, panels impregnated with STF-500 shows
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lower BFS as compared to corresponding panels impregnated with STF-100, except in one case (K 20 S) where the BFS is 36.7 mm and 36.4 mm, respectively. Therefore, to have a fair comparison, normalised improvement in BFS was calculated by using the equation 3
NIB 500 = IB 500 ×
AD 100 AD 500
(3)
where NIB 500 is the normalised improvement (%) in BFS in panels impregnated with STF-500, AD 100 and AD 500 are the areal density of panels impregnated with STF 100 and STF 500, respectively, and IB500 is the improvement (%) in BFS in panels impregnated with STF-500
Table 6 presents a fair comparison of BFS in panels impregnated with STF-100 and STF-500. The improvement in BFS for STF-100 and normalised improvement in BFS for STF-500 are shown in the last two columns. The BFS of 39.2 mm obtained with the panel having 24 layers of neat Kevlar fabrics (K 24 N) was considered as the control sample for calculating the improvement i.e. reduction in BFS. Comparing the results presented in the last two columns, it can be inferred that STF-500 produces lower BFS as compared to STF-100. Thus, increase in particle size of the dispersed phase of the STF improves the ballistic response of the constituent fabrics panels. Though the inter-yarn friction characterized by yarn pull-out force is higher in case of fabrics impregnated with STF-100, this is not translated into better impact performance. Although friction is considered to be one of the major mechanisms of energy absorption in STF impregnated fabrics, some recent studies [12, 22] have shown that shear thickening plays a more dominant role in determining the impact resistance of STF impregnated fabrics. As the critical shear rate is lower for STF-500, it gets the trigger early during bullet impact and then maintains the liquid-solid transition over a large span of shear rate, thus facilitating higher energy absorption and reduction of BFS. Like STF-100 impregnated panels, in case of STF-500 impregnated panels also, K24 S shows the minimum BFS (31.3 mm) at the cost of higher areal density (6.2 kg/m2). When areal density of the panel is compensated by reducing the number of fabric layers (K20 S), BFS is reduced by 2.5 mm (from 39.2 mm to 36.7 mm) as compared to that of control sample. When only 10 layers of fabric are impregnated with STF, out of 20 layers present in a panel, the configuration K10 N + K10 S results the minimum BFS (34.5 mm). Similar observation was made in case of STF-100 impregnated panels also. It is important to note that for both STF-100 and STF-500, when only 10 layers of fabric are impregnated with STF, the configuration K10 N + K10 S yields the minimum BFS. STF requires a response time for the liquid-solid transition. As the bullet travels through the layers of fabric, its velocity reduces. Thus, STF gets sufficient response time when the STF impregnated panels
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are placed at the rear side of a panel. Moreover, STF impregnation increases the modulus of fabric by increasing the inter-yarn friction. It has been reported by researchers [5, 6] when materials having low and high modulus are placed at the front and rear side of a panel, respectively, their elongation happens in synchronised manner and thus higher energy absorption is achieved.
4.6 Number of perforated layers and perforated mass After the ballistic tests of neat and STF impregnated panels, a post-impact analysis was carried to determine number of perforated layers and perforated mass. Figures 14 and 15 depict these parameters for STF-100 and STF-500 impregnated soft armour panels, respectively. It is observed that for neat panel (K24 N), the bullet is stopped, on an average, after perforating 9-10 layers of fabrics. In contrast, STF impregnated panels stop the bullet between 6-8 layers. This implies that the number of perforated layers for neat panels is higher as compared to that of STF impregnated panels. This can be attributed to higher inter-yarn friction in STF impregnated fabrics which ensure participation of secondary yarns in energy absorption. Perforated mass (kg/m2) is calculated using Equation 4. n
P.M =
where
i
∑ i =1
ni × ADi
(4)
N
is number of perforated layers of fabric during bullet impact,
is total number of layers in the
soft armour panel and ADi is areal density of ith layer of fabric. When all the layers of fabric in a soft armour panel are identical, number of perforated layers and perforated mass will have exactly the same trend as the latter becomes a simple multiple of the former. However, as the areal densities of neat and STF impregnated fabrics are different, number of perforated layers and perforated mass are calculated separately. It is interesting to note that perforated mass is lower when both neat and STF impregnated fabrics are combined to make soft armour panel, especially in case of panels impregnated with STF-500. For homogeneous soft armour panels (K24 N, K24 S, K20 S), perforated mass ranges from 1.66 kg/m2 to 1.96 kg/m2 for STF-100 and from 1.88 kg/m2 to 1.97 kg/m2 for STF-500. In contrast, for hybrid soft armour panels comprising of neat and STF impregnated fabrics (K10 N+ K10 S, K10 S + K10 N, K10 N+ K10 S 1A, K10 N+ K10 S 2A), perforated mass ranges from 1.50 kg/m2 to 1.86 kg/m2 for STF-100 and from 1.31 kg/m2 to 1.59 kg/m2 for STF-500. This supports the hypothesis that armour panels should be designed by keeping materials having different properties in different layers. Among the four hybrid panels, K10 N + K10 S has perforated mass of 1.58 kg/m2 and 1.31 kg/m2, respectively, for STF-100 and STF-500 which is second best
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and the best in the respective group. The average number of perforated layers for this hybrid panel (K10 N + K10 S) is 7 and 5.8, respectively, for STF-100 and STF-500. These values are either very close to the minimum value or the minimum value of number of perforated layers within the respective groups. This analysis bolsters the hypothesis that STF impregnated fabrics should be placed at the rear side of the panel, in a block, for extracting the best ballistic performance. While comparing Figures 14 and 15, it is observed that, in general, the number of perforated layers and perforated mass is lower in case of panels impregnated with STF-500. The reasons behind this has already been explained in section 4.5.2. Figure 16 shows different layers of panel after the ballistic test. In case of neat panel, the bullet wedges through the fabrics by displacing the yarns laterally. On the other hand, in STF impregnated panel, the fibre rupture takes place when the bullet pierces through the fabric layers. Thus, higher amount of energy is absorbed which results in stoppage of bullet at lower values of number of perforated layers and perforated mass.
5. Conclusions Effective strategies have been developed for designing of soft body armour by using STF impregnated Kevlar fabrics. Fabrics impregnated with STF prepared by using 500 nm silica (STF-500) show higher impact energy absorption as compared to fabrics impregnated with STF prepared by using 100 nm silica (STF-100). The soft armour panels made from the former also yield lower BFS than the panel made from the latter when the number of fabric layers is equal. Use of hybrid panels comprised of neat and STF impregnated fabrics is found to be an effective strategy to reduce the weight of panel as well as to get the desired BFS. When STF impregnated panels are placed at the rear side of the panel, in a block, comparable or lower BFS is obtained as compared to the homogeneous panels where all the fabric layers are impregnated with STF. Besides, this strategy also helps to reduce the weight of soft body armour by 10%. This hybrid panel also demonstrates the capability to stop the bullet earlier as compared to other configurations of hybrid panels specially with STF-500.
Acknowledgement: The authors are grateful to Terminal Ballistic Research Laboratory (TBRL), Chandigarh, India, for the financial and technical assistance in this research work (Grant no: TBRL/ CARS/61/2014). The support of Science and Engineering Research Board (SERB), Reliance Industries and FICCI are also greatly acknowledged.
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List of Tables: Table 1: Details of soft armour panel configuration Sl. no.
Panel configuration
No. of layers
Areal density (kg/m2) 100 nm silica 500 nm silica
1
K 24 N
24
5.0
5.0
2
K 24 S
24
6.0
6.2
3
K 20 S
20
5.0
5.3
4
K10 N + K10 S
20
4.5
4.8
5
K10 S + K10 N
20
4.5
4.7
6
K10 N + K10 S 1A
20
4.7
4.8
7
K10 N+ K10 S 2A
20
4.6
4.7
Table 2: Ballistic testing parameters Parameters
Value
Distance between target and barrel
5m
Distance between weapon and velocity measurement screen
2.5 m
Number of shots per sample
5
Minimum distance on panel between two shots
51 mm
Temperature
21 ± 2.9 °C
Relative humidity
50 ± 20 %
Table 3: Rheological parameters of STF-100 and STF-500 Temperature (°C)
Critical shear rate (s-1)
15 25 35
82.3 112 132
Peak viscosity (Pa.s)
Critical shear rate (s-1)
STF-100
Peak viscosity (Pa.s) STF-500
70.2 49.6 34.6
16
11 30 47
89 43 19
Table 4: STF add-on and impact energy absorption by Kevlar® fabrics. STF
Add-on (%)
Energy absorption (J) Absolute
Normalized
STF-100
10-12
79.5
72.3
STF-500
20-22
94.4
78.6
Table 5: One-way ANOVA for impact energy absorption Sources of variation
Sum of squares
Degrees of freedom
Mean of squares
F
P-value
Fcritical
Between groups
1909.6
2
954.8
9.2
0.0037
3.88
Within groups
1236.0
12
103.0
Total
3145.6
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Table 6: Comparison of BFS in STF-100 and STF-500 impregnated panels Sl. no. Configuration Areal density of Areal density of STF-100 STF-500 impregnated panel impregnated panel (kg m-2) (kg m-2) 1 K 24 S 6 6.2 2 K 20 S 5 5.3 3 K10 N + K10 S 4.5 4.8 4 K10 S + K10 N 4.5 4.7 5 K10 N + K10 S 1A 4.7 4.8 6 K10 N+ K10 S 2A 4.6 4.7
Improvement in BFS for STF-100 (%) + 15.8 + 7.1 + 5.1 - 7.1 + 5.1 + 3.0
List of Figures:
Figure 1: Schematic representation of STF impregnation in fabric
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Normalised improvement in BFS for STF-500 (%) + 19.5 + 5.9 + 11.2 + 9.0 + 7.0 + 11.5
Figure 2: Stitched panel of Kevlar fabric
Figure 3: (a) Sample used for the yarn pull-out test and (b) schematic representation of lower jaw for yarn pull-out test
Figure 4: Dynamic impact test set-up
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Figure 5: (a) A soft armour panel (b) BFS on backing material
Figure 6: Ballistic test set-up for BFS measurement
Figure 7: Flow curves of STFs made from (a) 100 nm and (b) 500 nm silica particles
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Figure 8: Scanning electron micrograph of fabrics impregnated with STF (a) STF-100, (b) STF-100 enlarged, (c) STF-500, (d) STF-500 enlarged
Figure 9: Yarn pull-out behaviour of neat and STF impregnated fabrics
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Figure 10: Impact energy absorption by neat and STF impregnated Kevlar® fabrics
Figure 11: BFS of soft armour panels impregnated with STF-100.
Figure 12: BFS of soft armour panels impregnated with STF-500
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Figure 13: A 9×19 mm bullet (a) undeformed before test, (b) deformed after test
Figure 14: Number of perforated layers and perforated mass for STF-100 impregnated panels
Figure 15: Number of perforated layers and perforated mass for STF-500 impregnated panels
22
Figure 16: Neat panel (a) front, (b) middle (c) stopping layer and STF impregnated panel (d) front, (e) middle (f) stopping layer
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CRediT author statement Mukesh Bajya: Conceptualization, Methodology, Writing- Original draft preparation. Abhijit Majumdar, Bhupendra Singh Butola: visualization, supervision, writingreviewing and editing Sanjeev Kumar Verma, Debarati Bhattacharjee: Guidance in samples testing and analysis of data
Declaration of Conflict of Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Mukesh Bajya [email protected]
Abhijit Majumdar (CORRESPONDING AUTHOR) [email protected]
Bhupendra Singh Butola [email protected]
S1359-8368(19)34273-8
DOI:
https://doi.org/10.1016/j.compositesb.2019.107721
Reference:
JCOMB 107721
To appear in:
Composites Part B
Received Date: 21 August 2019 Revised Date:
9 December 2019
Accepted Date: 16 December 2019
Please cite this article as: Bajya M, Majumdar A, Butola BS, Verma SK, Bhattacharjee D, Design strategy for optimising weight and ballistic performance of soft body armour reinforced with shear thickening fluid, Composites Part B (2020), doi: https://doi.org/10.1016/j.compositesb.2019.107721. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Design Strategy for Optimising Weight and Ballistic Performance of Soft Body Armour Reinforced with Shear Thickening Fluid
Mukesh Bajya1, Abhijit Majumdar1*, Bhupendra Singh Butola1, Sanjeev Kumar Verma2, Debarati Bhattacharjee2 1
Department of Textile and Fibre Engineering, Indian Institute of Technology Delhi, India 110016 2
Terminal Ballistic Research Laboratory, Chandigarh India 160 030
Abstract This study presents some soft armour panel design strategies using shear thickening fluid reinforced Kevlar® fabric. The effect of size of silica nano and sub-micron particles on the ballistic performance of soft body armour panels against the small arms ammunition (velocity ~ 430 ± 15 m/s) has been analysed. Shear thickening fluids (STFs), namely STF-500 and STF-100, were prepared using silica particles of 500 nm and 100 nm sizes, respectively, and dispersing them in polyethylene glycol (PEG) 200. Kevlar® fabrics were impregnated with both the STFs. Multiple layers (20-24) of fabrics were stitched to prepare 13 soft armour panels which were evaluated for back face signature (BFS) against 9×19 mm lead core bullet. Soft armour panels comprised of fabrics impregnated with STF-500 yield lower BFS than the respective panels comprised of fabrics impregnated with STF-100. This study also reveals that STF impregnation of Kevlar® fabrics can reduce the BFS by 2.5 mm to 2.8 mm while keeping the areal density of the panel same (5 kg/m2). The areal density of soft armour panel can be reduced further by 10% (4.5 kg/m2), while keeping the BFS comparable or lower than that of a STF impregnated homogenous panel, by judiciously placing the STF impregnated fabrics at the rear side while neat fabrics are placed at the strike face of the panel. STF impregnated panels are found to stop the impacting bullet earlier than that by neat panel. Keywords: Back face signature; Shear thickening fluid; Soft body armour; High velocity impact
Corresponding author: Dr. Abhijit Majumdar Professor Department of Textile and Fibre Engineering Indian Institute of Technology Delhi New Delhi, India 110016 Email: [email protected]
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1. Introduction Body armours are of two types, namely hard and soft. Either combination of hard and soft body armours or standalone version of the former give protection against the threats from rifle ammunitions (viz. SLR, AK etc). Hard body armours are composite or hybrid structures comprising of metal, ceramic (boron carbide, silicon carbide) and/or textile reinforced composites. Soft body armours are made of various high-performance fibres like p-aramid (Kevlar®, Twaron® etc.) and Ultra High Molecular Weight Polyethylene (Spectra®, Dyneema® etc.) used in the form of woven, multiaxial fabrics, uni/multi-directional laminates, etc. In the past two decades, scientists and researchers explored shear thickening fluid (STF) which has the capability to reduce behind armour blunt trauma (BABT) or back face signature (BFS) and stop a bullet from penetrating the soft armour [1–7]. STF is a concentrated colloidal suspension whose viscosity increases drastically, when the applied shear rate exceeds the critical value. This non-Newtonian behaviour of STF makes it ideal for energy absorption applications. Shear thickening behaviour is exhibited by various concentrated suspensions such as corn-starch– water suspension, iron particles in carbon tetrachloride and silica nanoparticles in polyethylene glycol, etc. [8– 12] In normal conditions, the solid particles remain suspended in the medium and hence the fabric maintains its flexibility. Upon bullet impact, the particles form hydroclusters which helps in energy absorption by providing better structural integrity and by ensuring increased participation of secondary yarns of fabric [13–15]. One of the most important particle parameters which influences the rheological behaviour of STF is size of particle in dispersed phase. In general, it has been observed that higher the particle size, lower the critical shear rate and higher the peak viscosity [16–18]. Therefore, choice of particle size remains crucial for the design of STF reinforced soft armour panel. Numerous research works have been reported on the polyethylene glycol or PEG (dispersion medium) and silica nanoparticle (dispersed phase) based STF impregnated Kevlar® fabrics. STF impregnated Kevlar® fabrics show improvement in impact resistance. There exist two different schools of thought about the role of STF in enhancing the impact resistance of fabrics. One group of researchers believe that the improvement is due to the increase in inter-yarn friction [6, 19, 20]. In contrast, the other group of researchers attribute the enhanced performance to shear thickening mechanism [10, 16]. However, STF impregnated fabrics have mostly been explored at lower impact velocity range in which an impactor in dropped or a projectile is fired at a velocity lower than Mach 1 [4, 5, 7, 12 ,21–25]. Xu et al. [26] investigated the effect of silica particle size and loading on the stab resistance of body armour. They found that as the particle size and STF loading increase, energy absorption also increase. Majumdar and Laha [13] showed that the shear thickening effect is critical for
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achieving enhanced performance via an increase in the yarn pull-out force upon transition of STF to its solidstate. Seshagiri et al. [27] found that when Kevlar® was used with Oobleck and Silica-PEG STF, the deformation produced on a glass plate was reduced considerably as compared to that of neat Kevlar® sample. Khodadadi et al. [15, 24] reported numerical and experimental results of STF impregnated panels at impact velocity range from 40 m/s to 60 m/s. They found improvement in performance and ascribed it to frictional interaction between yarns and silica in impregnated fabrics. Park et al. [3, 5, 6] found that the impregnation of aramid fabric with STF increased the frictional force of a single yarn in the fabric, which consequently increased the modulus of the yarn. The layering sequence was found to be very important in improving the protective performance of STF impregnated hybrid multilayer panels, which influenced not only the BFS value but also the perforation ratio and bullet expansion. Some recent research papers have reported that the effectiveness of STF, in improving the impact resistance, depends upon the fabric structure [28, 29]. Arora et al. [29] found that the beneficial role of STF ceases to exist if the fabric is very tightly woven. The impregnation of STF increases the effective mass of the fabric varying from 10% to 46% [10, 12, 22, 23]. Therefore, to keep the mass of the soft armour panel within the prescribed limit, either the number of fabric layers should be reduced after STF treatment so that the overall areal density of panel remains the same or few layers of soft armour panels should be impregnated with the STF. If the latter is followed, then the STF impregnated fabric layers should be judiciously placed so that they perform effectively. This research work presents the effect of silica particle size on the ballistic performance of STF impregnated soft armour panels under two scenarios, namely keeping the number of fabric layers same and keeping the areal density of the panel same. The comparison of ballistic performance of soft armour panels when the STF impregnated fabric layers are placed at the strike face and at the rear side has also been made.
2. Experimental 2.1 Materials Kevlar® 363 2S (scoured) square plain woven fabric, having 28 ends (warp) per inch and 28 picks (weft) per inch and areal density of 200 g/m2 was sourced from DuPont, USA. It was made of multifilament Kevlar 129 yarn having linear density of 840 denier, tensile strength of 3.4 GPa, tensile modulus of 78 GPa and breaking strain of 3.3%. Polyethylene glycol of molecular weight 200 (PEG 200) and ethanol were sourced from Merck Life Science Pvt. Ltd. (India) and Changshu Hongsheng Fine Chemical Co. Ltd. (China), respectively. Silica nanoparticles (100 nm) dispersed in water (40% w/w) was obtained from Nissan Chemical Corporation
3
(Japan), and silica sub-micron-particles (500 nm) in powder form was obtained from Nippon Chemical Industrial (Japan).
2.2 Preparation of STF Two STFs were prepared in this study using different silica particles sizes, i.e., 100 nm and 500 nm and they were denoted as STF-100 and STF-500, respectively. In both the STFs, PEG 200 was used as a dispersing medium because of its good thermal stability (boiling point > 150 °C) and less volatility. Amount of silica particles (65 wt. %) and PEG (35 wt. %) was kept constant for both the STFs. To prepare STF-100, water dispersed silica was mixed with PEG in requisite weight proportion and then the mixture was kept inside the sonication bath for 8 hr at 80 °C to remove the water. STF-100 was prepared after the removal of water. STF500 was prepared by probe sonication mehtod in which 500 nm silica powder was mixed with ethanol and the mixture was homogenised for 5 minutes by using a homogeniser rotating at 17400 rpm. Homogenised solution of silica and ethanol was then sonicated for 5 minutes. After that, ethanol dispersed silica was mixed with PEG and homogenised for 5 minutes. Final solution of silica, PEG and ethanol was sonicated for 20 minutes. Prepared sonicated solution was then kept inside the oven at 120 °C to remove the ethanol. After removal of ethanol, STF-500 was prepared.
2.3 Impregnation of Fabrics with STF In order to impregnate the Kevlar fabric with STF, the prepared STFs were first diluted with ethanol such that w/v ratio of PEG: ethanol was 1:4. A high-speed homogeniser (17400 rpm) was used to avoid the agglomeration of silica particles and to ensure the uniform dispersion. Kevlar fabric samples of size 25 cm × 25 cm were cut and impregnated with diluted STF solution using Matthis Lab Padder, in which two padding rollers are laid horizontally. The schematic representation of impregnation process is shown in Figure 1. Padding was repeated twice at 2 bar pressure and 3 m/min delivery speed. Thereafter, the samples were kept in a hot air oven at 100 °C for 30 minutes to evaporate the ethanol. The add-on of STF for each sample was calculated using Equation (1).
Add − on% =
T −N ×100 N
(1)
where T is the weight of STF impregnated Kevlar fabric and N is the weight of corresponding neat Kevlar fabric.
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2.4 Stitching and shot location of soft armour panels The neat and STF impregnated fabric layers were stacked in a planned manner as described in the Table 1. For all the panels, ‘K’ denotes the Kevlar fabric; the number following ‘K’ corresponds to the number of fabric layers; ‘N’ and ‘S’ describe whether the fabric layers are neat or STF impregnated, respectively. ‘A’ implies alternate stacking of neat and STF impregnated fabrics, and the number preceding ‘A’ corresponds to repeating unit of alternation. For example, K10 N + K10 S 1A implies that 10 layers of neat and STF impregnated fabrics each (K10 N + K10 S) were used and 1 layer of neat and 1 layer of STF impregnated Kevlar fabrics were placed alternately (1A) and the neat fabric was at the strike face. The assembled fabric plies were stitched using JUKI LU 2810 sewing machine. The stitching pattern and shot location are shown in Figure 2. Stitching was done in diamond pattern with 5 mm stitch length and 50 mm distance between the two parallel stitch lines [30]. Five shots locations were marked on the panel and a minimum distance of 51 mm was maintained between a
shot and edge of the panel and also between two shots according to BIS 17051-2018.
3. Testing and Characterization 3.1 Rheological analysis of STF Steady state rheological properties of prepared STFs were evaluated using Anton Paar Physica MCR 51 stresscontrolled rheometer. The tests were performed using a parallel plate geometry having two parallel plates separated by a gap of 0.3 mm. Diameter of upper and lower plates is 25 mm and 50 mm, respectively. Rheological tests were conducted at three different temperatures, i.e. 15 °C, 25 °C and 35 °C, and shear rate was varied from 1 s-1 to 1000 s-1.
3.2 Scanning Electron Microscopy (SEM) Surface morphology of neat and STF impregnated fabrics was evaluated by using a ZEISS EVO 15 scanning electron microscope (SEM). The SEM samples were prepared on a double-sided carbon tape and sputter coated with gold to prevent the charge build-up on samples.
3.3 Yarn pull-out test Yarn pull-out tests were performed to study the inter-yarn friction. The tests were carried out on a universal tensile testing machine (Tinius Olsen H5KS) equipped with a specially designed lower jaw for holding the fabric sample. The sample size used for the test is shown in Figure 3 (a). First, rectangular fabric samples (12
5
cm × 16 cm) were cut and then yarns were unravelled from the top so that fringes of yarns having length of 8 cm protrudes out from the fabric. During the yarn pull-out test, a yarn was gripped by the movable upper jaw and the fabric was gripped by the lower jaw without any slackness, as shown in Figure 3(b). The tests were conducted at a crosshead speed of 300 mm/min and the maximum pull-out force (N) and force-displacement curve were recorded. Five specimens of each sample were tested and then the average was calculated. 3.4 Dynamic impact testing Low-velocity dynamic impact tests were carried out for both neat and STF impregnated fabric samples having dimension 16 cm ×16 cm. The tests were conducted on a falling-dart type impact tester (CEAST, Model: Fractovis Plus) following ASTM D3763. The fabric samples were placed between two circular jaws having knurled metallic surfaces to grip the sample with a clamping force of 5 kN. The pitch and density of the knurl were 0.2 cm and 25 cm-2, respectively. The diameter of hemispherical tip of the impactor was 13 mm. The impact velocity and total impact energy were 4.5 m/s and of 200 J, respectively. The dynamic test set-up is shown in Figure 4. As the areal densities of neat and STF impregnated fabrics are different, a normalising factor is required to compare the energy absorbed irrespective of the areal density. Considering the STF add-on to be %, the normalized value of impact energy absorption by an STF impregnated sample is calculated using Equation 2
En = where
Ea (1 + x /100)
is the normalized energy (J) absorption and
(2) is the absolute energy (J) absorption.
3.5 Ballistic testing of panels and back face signature measurement Soft armour panels having different configurations were tested against the 9×19 mm lead core bullet (mass 7.45 g) with striking velocity of 430 ± 15 m/s. Soft armour panel and BFS created by a non-perforating bullet on backing material (Roma Plastilina® No.1 Grey clay) are shown in Figure 5. BFS is a critical aspect of ballistic performance evaluation as the former determines the lethality of internal injuries to vital organs during a nonperforating ballistic impact. BFS was obtained by measuring the maximum depth of indentation on clay, created by the armour panel when impacted by a non-perforating bullet. The backing material was calibrated for its consistency and viscoelasticity before conducting the experiments as per standard NIJ 0101.06. A steel ball having mass of 1.063 kg was dropped on the backing material from a height of 2 m and depth of indentation was measured. The average indentation depth of the material was kept within the range from 18.5 to18.8 mm to
6
ensure minimum variation in BFS due to backing material. Two samples were tested on each calibrated backing material after that it was recalibrated. The test parameters used for high-velocity impact test is given in Table 2 and the test set-up for the same is shown in Figure 6.
4. Results and discussions 4.1 Rheological behaviour of STFs Flow curves of STF-100 and STF-500, at three different temperatures, are shown in Figure 7(a) and 7(b), respectively. The flow curves confirmed the non-Newtonian behaviour of both the STFs at all three temperatures. Further, it can be seen from the Figure 7 and Table 3 that with the increase in temperature, the critical shear rate and shear rate at maximum viscosity increases and peak viscosity decreases for both the STFs. This change in critical shear rate and viscosity is due to increased Brownian motion of particles, at elevated temperature, which requires more shear force to induce clustering of particles or shear thickening [31]. Moreover, at higher temperature, the thickness of solvation layer formed by PEG on silica particles is smaller causing higher effective distance between the particles [32]. This also necessitates higher shear force for onset of shear thickening In addition, the rheological results reveal that, as the particle size increases, critical shear rate decreases. These results are agreement with the findings of other researchers [16,18]. This trend confirms that the number of particles per unit volume of STF-100 is 125 times higher than that of STF-500. Besides, the higher specific surface charge on smaller particles cause higher intensity of inter-particle repulsive force. Therefore, higher shear force is required for the onset of shear thickening in case of STF-100. Another interesting finding is that STF-500 shows a plateau region, over a wide span of shear rate, after reaching the peak viscosity. This may be attributed to the higher inertial effect of larger particles. In contrast, STF-100 demonstrates shear tinning immediately after attaining the peak viscosity.
4.2 Surface morphology of STF impregnated fabric Scanning electron micrographs of STF impregnated fabrics are shown in Figure 8. It is observed that silica nano and sub-micron particles are deposited on the surface of filaments. Magnified views of fabric surface reveals that spherical silica particles have formed small clusters on the filaments.
4.3 Yarn pull-out behaviour
7
Figure 9 depicts the yarn pull-out force vs displacement curves of neat, STF-100 and STF-500 impregnated Kevlar fabrics. It is observed that the yarn pull-out force increases after the impregnation of fabrics with STF, which is due to the increase in inter-yarn friction. Presence of silica particles makes the surface of filaments rough. It is also observed that peak yarn pull-out force is higher for STF-100 impregnated fabrics (80.5 N) as compared to that of STF-500 impregnated fabrics (64.3 N). This is due to the fact that specific surface area as well as number of silica particles in STF are higher for 100 nm silica particles. Therefore, it gives better coverage on filament surface causing higher yarn pull-out force. 4.4 Dynamic impact results Figure 10 shows dynamic impact test results of neat and STF impregnated Kevlar fabrics in terms of energy absorption. The absolute energy absorption by STF-100 and STF-500 impregnated fabrics are around 19% and 42% higher, respectively, than that of the neat fabric. However, STF add-on was also higher in case of STF-500 as shown in Table 4. Though both the STFs were impregnated using same process parameters, as mentioned in Section 2.3, 100 nm silica particles came out rather easily from the soaked fabric during squeezing. On the other hand, 500 nm particles got trapped on the fabric surface due to their bigger size and higher viscosity of STF-500 at low shear rate. Absolute as well as normalised energy absorption by STF impregnated fabrics are given in Table 4. Normalized energy absorption by STF-100 and STF-500 impregnated fabrics is 8.4% and 17.8% higher, respectively, than that of neat fabric. STF impregnated fabrics are expected to absorb more energy due to two reasons. First, due to enhancement of inter-yarn friction and second, due to shear thickening effect. The energy absorption by STF500 impregnated fabric is more than that of STF-100 impregnated fabric though the inter-yarn friction, indicated by yarn pull-out force, is higher for the latter. This may be due to lower critical shear rate of STF-500 than that of STF-100. From the flow curve, depicted in Figure 7, it is observed that STF-500 gets the trigger at lower critical shear rate and remains thickened for a greater span of shear rate as compared to STF-100 due to which the impact energy absorption is more in fabrics impregnated with STF-500. Besides, the rheological results given in Table 3 shows that the critical shear rate of STF-100 is higher and it shows shear thinning behaviour before attaining critical shear rate. The statistical significance test was also carried out using one-way ANOVA which is used to compare the means of two or more samples. This technique is used to ascertain the effect of a treatment or process and whether the differences between and within the groups of samples are statistically significant or not. One-way
8
ANOVA results are reported in Table 5. It is noted that F-calculated is much higher than F-critical signifying that the impact energy absorption by neat and STF impregnated samples are significantly different.
4.5 Back face signature BFS was measured for each shot using a digital depth gauge. The average BFS of five shots per panel was calculated and it is depicted in Figure 11 and Figure 12 for STF-100 and STF-500 impregnated fabrics, respectively. Figure13 shows the deformation (mushrooming) of a bullet after the ballistic test.
4.5.1 STF-100 impregnated panels The experimental results of STF-100 impregnated panels are shown in Figure 11. It is seen that for the control sample, having 24 layers of neat fabric (K24 N), the BFS is 39.2 mm. BFS reduces by 6.2 mm (from 39.2 mm to 33 mm) in case of STF impregnated panel having 24 layers of fabric (K24 S). However, areal density of the panel increases by 20% (from 5 kg/m2 to 6 kg/ m2) after the STF impregnation. As panel weight is one of the decisive criteria for soft body armour, it was attempted to keep the areal density same with that of control sample by reducing the number of STF impregnated fabric layers. To achieve this, a panel (K20 S) was made by impregnating 20 layers of fabric with STF-100. For this panel, BFS is found to be 36.4 mm, i.e. 2.8 mm lower as compared to neat panel having the same areal density (5 kg/m2). As the first few layers of fabrics in a soft armour panel acts as sacrificial layers, it was hypothesised that placing the STF impregnated fabrics at the rear side of the panel may yield the same BFS with reduction in areal density of the panel. The panel made with 20 layers of fabric in which 10 layers of STF impregnated fabric are placed at the rear side (K10 N + K10 S) shows marginal increase (from 36.4 mm to 37.2 mm) in BFS as compared to the panel that has all the 20 layers of fabric impregnated with STF-100 (K 20 S). However, there is 10% reduction in areal density in panel K10 N + K10 S (4.5 kg/m2) as compared to that of control sample (5 kg/m2). When 10 neat and 10 STF impregnated fabric layers are placed alternately (K10 N + K10 S 1A and K10 N + K10 S 2A), there is no further reduction in BFS. This implies that placing the STF impregnated fabric layers at the rear side of the panel could be an effective design strategy for soft body armour.
4.5.2 STF-500 impregnated panels Figure 12 depicts the BFS of various panels impregnated with STF-500. In general, the trend is similar with that observed in case of panels impregnated with STF-100. However, panels impregnated with STF-500 shows
9
lower BFS as compared to corresponding panels impregnated with STF-100, except in one case (K 20 S) where the BFS is 36.7 mm and 36.4 mm, respectively. Therefore, to have a fair comparison, normalised improvement in BFS was calculated by using the equation 3
NIB 500 = IB 500 ×
AD 100 AD 500
(3)
where NIB 500 is the normalised improvement (%) in BFS in panels impregnated with STF-500, AD 100 and AD 500 are the areal density of panels impregnated with STF 100 and STF 500, respectively, and IB500 is the improvement (%) in BFS in panels impregnated with STF-500
Table 6 presents a fair comparison of BFS in panels impregnated with STF-100 and STF-500. The improvement in BFS for STF-100 and normalised improvement in BFS for STF-500 are shown in the last two columns. The BFS of 39.2 mm obtained with the panel having 24 layers of neat Kevlar fabrics (K 24 N) was considered as the control sample for calculating the improvement i.e. reduction in BFS. Comparing the results presented in the last two columns, it can be inferred that STF-500 produces lower BFS as compared to STF-100. Thus, increase in particle size of the dispersed phase of the STF improves the ballistic response of the constituent fabrics panels. Though the inter-yarn friction characterized by yarn pull-out force is higher in case of fabrics impregnated with STF-100, this is not translated into better impact performance. Although friction is considered to be one of the major mechanisms of energy absorption in STF impregnated fabrics, some recent studies [12, 22] have shown that shear thickening plays a more dominant role in determining the impact resistance of STF impregnated fabrics. As the critical shear rate is lower for STF-500, it gets the trigger early during bullet impact and then maintains the liquid-solid transition over a large span of shear rate, thus facilitating higher energy absorption and reduction of BFS. Like STF-100 impregnated panels, in case of STF-500 impregnated panels also, K24 S shows the minimum BFS (31.3 mm) at the cost of higher areal density (6.2 kg/m2). When areal density of the panel is compensated by reducing the number of fabric layers (K20 S), BFS is reduced by 2.5 mm (from 39.2 mm to 36.7 mm) as compared to that of control sample. When only 10 layers of fabric are impregnated with STF, out of 20 layers present in a panel, the configuration K10 N + K10 S results the minimum BFS (34.5 mm). Similar observation was made in case of STF-100 impregnated panels also. It is important to note that for both STF-100 and STF-500, when only 10 layers of fabric are impregnated with STF, the configuration K10 N + K10 S yields the minimum BFS. STF requires a response time for the liquid-solid transition. As the bullet travels through the layers of fabric, its velocity reduces. Thus, STF gets sufficient response time when the STF impregnated panels
10
are placed at the rear side of a panel. Moreover, STF impregnation increases the modulus of fabric by increasing the inter-yarn friction. It has been reported by researchers [5, 6] when materials having low and high modulus are placed at the front and rear side of a panel, respectively, their elongation happens in synchronised manner and thus higher energy absorption is achieved.
4.6 Number of perforated layers and perforated mass After the ballistic tests of neat and STF impregnated panels, a post-impact analysis was carried to determine number of perforated layers and perforated mass. Figures 14 and 15 depict these parameters for STF-100 and STF-500 impregnated soft armour panels, respectively. It is observed that for neat panel (K24 N), the bullet is stopped, on an average, after perforating 9-10 layers of fabrics. In contrast, STF impregnated panels stop the bullet between 6-8 layers. This implies that the number of perforated layers for neat panels is higher as compared to that of STF impregnated panels. This can be attributed to higher inter-yarn friction in STF impregnated fabrics which ensure participation of secondary yarns in energy absorption. Perforated mass (kg/m2) is calculated using Equation 4. n
P.M =
where
i
∑ i =1
ni × ADi
(4)
N
is number of perforated layers of fabric during bullet impact,
is total number of layers in the
soft armour panel and ADi is areal density of ith layer of fabric. When all the layers of fabric in a soft armour panel are identical, number of perforated layers and perforated mass will have exactly the same trend as the latter becomes a simple multiple of the former. However, as the areal densities of neat and STF impregnated fabrics are different, number of perforated layers and perforated mass are calculated separately. It is interesting to note that perforated mass is lower when both neat and STF impregnated fabrics are combined to make soft armour panel, especially in case of panels impregnated with STF-500. For homogeneous soft armour panels (K24 N, K24 S, K20 S), perforated mass ranges from 1.66 kg/m2 to 1.96 kg/m2 for STF-100 and from 1.88 kg/m2 to 1.97 kg/m2 for STF-500. In contrast, for hybrid soft armour panels comprising of neat and STF impregnated fabrics (K10 N+ K10 S, K10 S + K10 N, K10 N+ K10 S 1A, K10 N+ K10 S 2A), perforated mass ranges from 1.50 kg/m2 to 1.86 kg/m2 for STF-100 and from 1.31 kg/m2 to 1.59 kg/m2 for STF-500. This supports the hypothesis that armour panels should be designed by keeping materials having different properties in different layers. Among the four hybrid panels, K10 N + K10 S has perforated mass of 1.58 kg/m2 and 1.31 kg/m2, respectively, for STF-100 and STF-500 which is second best
11
and the best in the respective group. The average number of perforated layers for this hybrid panel (K10 N + K10 S) is 7 and 5.8, respectively, for STF-100 and STF-500. These values are either very close to the minimum value or the minimum value of number of perforated layers within the respective groups. This analysis bolsters the hypothesis that STF impregnated fabrics should be placed at the rear side of the panel, in a block, for extracting the best ballistic performance. While comparing Figures 14 and 15, it is observed that, in general, the number of perforated layers and perforated mass is lower in case of panels impregnated with STF-500. The reasons behind this has already been explained in section 4.5.2. Figure 16 shows different layers of panel after the ballistic test. In case of neat panel, the bullet wedges through the fabrics by displacing the yarns laterally. On the other hand, in STF impregnated panel, the fibre rupture takes place when the bullet pierces through the fabric layers. Thus, higher amount of energy is absorbed which results in stoppage of bullet at lower values of number of perforated layers and perforated mass.
5. Conclusions Effective strategies have been developed for designing of soft body armour by using STF impregnated Kevlar fabrics. Fabrics impregnated with STF prepared by using 500 nm silica (STF-500) show higher impact energy absorption as compared to fabrics impregnated with STF prepared by using 100 nm silica (STF-100). The soft armour panels made from the former also yield lower BFS than the panel made from the latter when the number of fabric layers is equal. Use of hybrid panels comprised of neat and STF impregnated fabrics is found to be an effective strategy to reduce the weight of panel as well as to get the desired BFS. When STF impregnated panels are placed at the rear side of the panel, in a block, comparable or lower BFS is obtained as compared to the homogeneous panels where all the fabric layers are impregnated with STF. Besides, this strategy also helps to reduce the weight of soft body armour by 10%. This hybrid panel also demonstrates the capability to stop the bullet earlier as compared to other configurations of hybrid panels specially with STF-500.
Acknowledgement: The authors are grateful to Terminal Ballistic Research Laboratory (TBRL), Chandigarh, India, for the financial and technical assistance in this research work (Grant no: TBRL/ CARS/61/2014). The support of Science and Engineering Research Board (SERB), Reliance Industries and FICCI are also greatly acknowledged.
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List of Tables: Table 1: Details of soft armour panel configuration Sl. no.
Panel configuration
No. of layers
Areal density (kg/m2) 100 nm silica 500 nm silica
1
K 24 N
24
5.0
5.0
2
K 24 S
24
6.0
6.2
3
K 20 S
20
5.0
5.3
4
K10 N + K10 S
20
4.5
4.8
5
K10 S + K10 N
20
4.5
4.7
6
K10 N + K10 S 1A
20
4.7
4.8
7
K10 N+ K10 S 2A
20
4.6
4.7
Table 2: Ballistic testing parameters Parameters
Value
Distance between target and barrel
5m
Distance between weapon and velocity measurement screen
2.5 m
Number of shots per sample
5
Minimum distance on panel between two shots
51 mm
Temperature
21 ± 2.9 °C
Relative humidity
50 ± 20 %
Table 3: Rheological parameters of STF-100 and STF-500 Temperature (°C)
Critical shear rate (s-1)
15 25 35
82.3 112 132
Peak viscosity (Pa.s)
Critical shear rate (s-1)
STF-100
Peak viscosity (Pa.s) STF-500
70.2 49.6 34.6
16
11 30 47
89 43 19
Table 4: STF add-on and impact energy absorption by Kevlar® fabrics. STF
Add-on (%)
Energy absorption (J) Absolute
Normalized
STF-100
10-12
79.5
72.3
STF-500
20-22
94.4
78.6
Table 5: One-way ANOVA for impact energy absorption Sources of variation
Sum of squares
Degrees of freedom
Mean of squares
F
P-value
Fcritical
Between groups
1909.6
2
954.8
9.2
0.0037
3.88
Within groups
1236.0
12
103.0
Total
3145.6
14
Table 6: Comparison of BFS in STF-100 and STF-500 impregnated panels Sl. no. Configuration Areal density of Areal density of STF-100 STF-500 impregnated panel impregnated panel (kg m-2) (kg m-2) 1 K 24 S 6 6.2 2 K 20 S 5 5.3 3 K10 N + K10 S 4.5 4.8 4 K10 S + K10 N 4.5 4.7 5 K10 N + K10 S 1A 4.7 4.8 6 K10 N+ K10 S 2A 4.6 4.7
Improvement in BFS for STF-100 (%) + 15.8 + 7.1 + 5.1 - 7.1 + 5.1 + 3.0
List of Figures:
Figure 1: Schematic representation of STF impregnation in fabric
17
Normalised improvement in BFS for STF-500 (%) + 19.5 + 5.9 + 11.2 + 9.0 + 7.0 + 11.5
Figure 2: Stitched panel of Kevlar fabric
Figure 3: (a) Sample used for the yarn pull-out test and (b) schematic representation of lower jaw for yarn pull-out test
Figure 4: Dynamic impact test set-up
18
Figure 5: (a) A soft armour panel (b) BFS on backing material
Figure 6: Ballistic test set-up for BFS measurement
Figure 7: Flow curves of STFs made from (a) 100 nm and (b) 500 nm silica particles
19
Figure 8: Scanning electron micrograph of fabrics impregnated with STF (a) STF-100, (b) STF-100 enlarged, (c) STF-500, (d) STF-500 enlarged
Figure 9: Yarn pull-out behaviour of neat and STF impregnated fabrics
20
Figure 10: Impact energy absorption by neat and STF impregnated Kevlar® fabrics
Figure 11: BFS of soft armour panels impregnated with STF-100.
Figure 12: BFS of soft armour panels impregnated with STF-500
21
Figure 13: A 9×19 mm bullet (a) undeformed before test, (b) deformed after test
Figure 14: Number of perforated layers and perforated mass for STF-100 impregnated panels
Figure 15: Number of perforated layers and perforated mass for STF-500 impregnated panels
22
Figure 16: Neat panel (a) front, (b) middle (c) stopping layer and STF impregnated panel (d) front, (e) middle (f) stopping layer
23
CRediT author statement Mukesh Bajya: Conceptualization, Methodology, Writing- Original draft preparation. Abhijit Majumdar, Bhupendra Singh Butola: visualization, supervision, writingreviewing and editing Sanjeev Kumar Verma, Debarati Bhattacharjee: Guidance in samples testing and analysis of data
Declaration of Conflict of Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Mukesh Bajya [email protected]
Abhijit Majumdar (CORRESPONDING AUTHOR) [email protected]
Bhupendra Singh Butola [email protected]