Abhijit Majumdar, Jyotirmoy Das and Rajib Bhattacharyya
Department of Textile and Fibre Engineering, Indian Institute of Technology Delhi, India
Introduction
Stab-resistant body armour given to the security personnel like police, law-enforcement officers, security guards, military troops, etc. is designed to prevent sharp-objects from penetrating. ‘Stabbing’ is based on a mechanism in which a pressure fracture occurs as immense force is applied by a tiny surface of the knife tip (1). Stabbing involves not only puncture but also severe cutting due to the sharp edge of the pointed object. Although the total energy of a stab strike may be less than that of a bullet attack, the extremely pointed
knife tip applies the force on a very small area, resulting in a high pressure and energy density at the point of impact (2). For instance, the energy density at the point of impact of a 9×19 mm bullet (mass of 8 g, velocity of ~ 400 m/s) would be approximately 10 J/mm 2 . In contrast, it would be approximately 100 J/mm 2 in the case of a knife attack (3). Thus, a mere soft armour intended for ballistic protection might not be sufficient for protection against stab attacks. In 2019, India alone reported 8931 fatal stabbing incidents (4). The prevalence of such attacks necessitates the development of stab-resistant system, which may be a standalone entity or in combination with soft armour. Stab attacks generate high loads and to defeat them, armour needs to have a certain thickness and stiffness (5-7). Since the majority of the important organs are located in the chest and upper abdomen, modern stab-resistant body armour systems often only cover these areas. In this article, the materials, test methods and working mechanism of stab-resistant armour have been elucidated.
Differences between ballistic and stab impacts
The differences between ballistic and stab impacts are presented in Table 1.
Materials used in stab-resistant armour
Stab-resistant armours are manufactured mainly from two dimensional (2D), or unidirectional (UD) laminates of high-performance fibres like nylon, para-aramids, ultra-high molecular weight polyethylene (UHMWPE). These fibres are preferred due to their high strength-to- weight ratio. At present, Kevlar®, Vectran®, Dyneema®, and Spectra® fibres and yarns occupy a lion’s share in the global market in making stab-resistant armours. Some recent studies (1-2) by this group of authors have explored the comparative performance of some high-performance yarns (Kevlar®, Spectra® and Vectran ®) in stab resistance in warp, weft and bias direction. It was noted that the stab resistance is always higher in bas direction as both warp and weft yarns are cut by the knife.
Testing of stab resistance
When evaluating the stab resistance performance, tests are conducted using two distinct methods: the quasistatic and dynamic stab testing. However, quasistatic method fails to replicate real-world stabbing conditions. This limitation is addressed by the dynamic stab test that simulates actual stabbing scenarios accurately.
Dynamic stab resistance testing is conducted in accordance with standards such as NIJ 0115.00, the Home Office Body Armour Standard 2017, or VPAM (KDIW 2004) (8, 9). The NIJ standard categorises protection into three levels, given in Table 2, based on the energy distribution observed during actual stabbing incidents. Level 1, 2 and 3 correspond to the 85 th , 90 th and 96 th percentile, respectively. Each protection level is further divided into two distinct energy levels, E1 and E2, where E2 represents over-test conditions, i.e., 50% higher energy compared to that of E1. For a given protection level, the knife should not penetrate beyond 7 mm at energy level E1, and 20 mm at energy level E2.
The penetration depth is calculated from the cut length using the conversion table prescribed by the NIJ 0115.00 standard. The NIJ standard specifies three types of impactors, based on their stabbing implementations, namely P1 knife (a single-edged knife with a pointed tip), S1 knife (a double-edged knife with a pointed tip), and a spike (a pointed tip). Figure 1 depicts P1 and S1 knives.
At lower energy level, increasing the number of fabric layers, in an armour, may help in reducing the knife penetration depth. However, at higher energy levels, if one keeps on increasing the fabric layers, the end product becomes bulky and heavy. The issue was addressed by this research group by shear thickening fluid (STF) treatment and ceramic coating on the high-performance fabrics (Indian patent applied). Figure 2 depicts the penetration depth for neat and STF treated panels at higher energy levels (24 J to 36 J). It was found that STF treated fabric panel can be lighter as well as better protective (less penetration depth) than the neat fabric panel.
filaments) perpendicular to the knife exhibit sharp cuts from the cutting edge of the knife, whereas others ruptured by the blunt edge show fibrillation and swelling of fractured region.
Future directions for stab-resistant armours
In future, researchers are expected to advance the technologies by investigating novel fabrics structures and treatments to improve stab resistance. Multi-axial fabrics or 3D fabrics could be explored for stab-resistant applications. Plasma treatments and ceramic particle additions are also attracting interest for their potential to enhance the performance of STF-treated fabrics (12). Besides, a new material known as shear thickening gel (STG) has recently emerged as a promising candidate for stab resistance applications.
Conclusions
This article presents a brief summary of stab-resistant body armour, materials used and related standard. Improvement of stab resistance by applying STF on woven fabrics is succinctly discussed. The failure of fibres during stabbing and deformation of stab are also explained. As the topic is relatively new, there are enormous scopes of research and development in terms of materials, structure and coating process. Moreover, development of universal soft armour that can give protection against stab and 9×19 mm bullet should be attempted. There is a need for industry-academia collaboration to develop light-weight stab resistant armour.
References
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