Design strategy for optimising weight and ballistic performance of soft body armour reinforced with shear thickening fluid

Abstract

This study presents some soft armour panel design strategies using shear thickening fluid (STF) 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. Two STFs, namely STF-500 and STF-100, were prepared by dispersing silica particles of 500 nm and 100 nm diameter, respectively, 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 comprising of fabrics impregnated with STF-500 yielded lower BFS than the respective panels comprising of fabrics impregnated with STF-100. This study also revealed that STF impregnation of Kevlar® fabrics can reduce the BFS by 2.5 mm–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 to or lower than that of an 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 were found to stop the impacting bullet earlier than neat panel.

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 can give protection against the threats from rifle ammunitions (e.g. SLR, AK, etc.). Hard body armours are composite or hybrid structures comprising of metals, ceramics (boron carbide, silicon carbide) and/or textile reinforced composites. On the other hand, 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 fabrics, multiaxial fabrics, uni/multi-directional laminates, etc. In the past two decades, scientists and researchers have 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], [2], [3], [4], [5], [6], [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], [9], [10], [11], [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], [14], [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], [17], [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 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], [22], [23], [24], [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 increases. Majumdar and Laha [13] showed that the shear thickening effect is critical for achieving enhanced performance via an increase in the yarn pull-out force upon transition of STF to its solid-state. Seshagiri et al. [27] found that when Kevlar® was used with Oobleck and silica-PEG STF, the deformation produced on a glass plate reduced considerably as compared to that of neat Kevlar® sample. Khodadadi et al. [15,24] reported numerical and experimental results of STF impregnated panels for a range of impact velocity from 40 m/s to 60 m/s. They found improvement in performance over neat panels 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 increased the effective mass of the fabric by 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.