Literature Review: Body Armor Overview
Literature Review: Body Armor Overview
- Body Armor: Introduction
- History
In modern times, security forces and military personnel use body armor to mitigate the likelihood of serious injury while engaged in their professional activities. However, the use of body armor is by no means a new phenomenon. From the earliest times that man developed weapons, he has also produced means of providing protection against weapons wielded by others, including the use of body armor.
Early man used comparatively primitive methods, but as civilizations developed and techniques advanced, body armor evolved. Whilst the soldiers of ancient Greece used shields fashioned of metal, wood and leather, techniques and technology improved, until in the Middle Ages those who could afford it – e.g. the Knights – had polished metal armor covering much of their body. Then in the last century with its two World Wars, various attempts were made to advance the technology of body armor, and the first so-called bullet-proof vests were designed in America in the two decades following World War I (Carothers, 1988).
- Types of Body Armor
Various techniques and materials have been developed and tried over time, seeking to develop body armor that is effective against projectiles and stabbing, yet is sufficiently light in weight to afford relatively normal movement when worn.
1.2.1 Soft Body Armor
According to Bellis (n.d.), the medieval Japanese were the first people known to have used soft body armor, which in that instance was manufactured of silk. Bellis relates how the idea was revived in America towards the end of the 19th century, when the military attempted to create silk body armor. The developers found that whilst the armor was effective against bullets travelling at speeds of up to 400 feet per second, it provided insufficient protection against higher speed bullets, such as those of a new generation of ammunition for handguns. Those bullets typically travelled at a speed at least 50 percent faster. It was that factor as well as the high cost of silk which ruled out that technique and material for soft body armor.
Bellis also reports that in World War II a so-called “flak jacket was created, using a material called ballistic nylon. However, she notes that the flak jacket – whilst effective against shrapnel – was not able to protect the wearer against the majority of bullets fired from pistols or rifles. They were also reported to be “cumbersome and bulky” to wear.
Bellis notes that in the late 1960s new fibers were discovered that facilitated the development of modern soft body armor. A research program sponsored by the National Institute of Justice identified new materials that could be used to develop and design body armor sufficiently light in weight to allow a police officer to wear while on-duty. As a consequence, new materials with excellent anti-ballistic resistance were identified.
Then a decade or so later, Kevlar ballistic fabric was discovered by the DuPont organization (originally as a substitute for the steel reinforcement of motor tires). In body armor terms, Bellis hails that discovery as “one of its most significant achievements.”
In a separate article (“Kevlar – Stephanie Kwolek”, n.d.), Bellis reports that Kevlar is “five times stronger than the same weight of steel” and has other advantages such as being very light in weight and is not susceptible to corrosion.
In a UK-based website feature by SafeGuard Armour – a manufacturer of Kevlar body armor – it is stated that: “According to comprehensive statistics, Kevlar® bullet proof vests have had a direct role in saving the lives of more than 4000 American police officers shot in the line of duty.”
1.2.2 Hard Body Armor
Whilst earlier forms of hard body armor were typically made from metal – usually steel – the weight was a problem. However, modern hard body armor is both lighter and more effective, and can be made from various materials such as ceramics or composite materials incorporating ceramics, often combined with a backing of Kevlar or a similar para-aramid material (like Kevlar) (“Materials” 2012).
1.3 Body Armor Functions
1.3.1 Ballistics Resistance
Ballistic resistance is a measure of how well the body armor absorbs the kinetic energy of a projectile (e.g. a bullet) when the projectile impacts with it. In a multi-layer armor construction, each successive layer absorbs part of that energy, with the goal of stopping the projectile completely (absorbing all of its energy) before it penetrates the final layer. A secondary effect of those multiple layers is to minimize the effects of blunt trauma, which can result in non-penetrative injuries such as bruising, bone breakage or damage to internal organs. There are two principally recognized testing standards for measuring the ballistic resistance of body armor:
- “NIJ Standard–0101.06 for Ballistic Resistance of Body Armor.” (2008). Published by U.S. Department of Justice, National Institute of Justice (NIJ);
- “HOSDB Body Armour Standards for UK Police (2007) Part 2: Ballistic Resistance; Publication No. 39/07/B.” (Croft & Longhurst, 2007), published by the UK Home Office, Scientific Development Branch.
The NIJ Standard classifies body armor by its ballistic resistance performance (against specific weapon types) into five categories, plus one additional category to allow for classifying to any specific and/or special requirements. Similarly, the HOSDB Standard also classifies the ballistic performance against eight of what it refers to as “threat groups” (weapon / ammunition types).
1.3.2 Stab Resistance
Stab resistance of body armor is maximized using either fabrics that have a dense type of weave, or using armor comprising closely-spaced laminated layers, which help to negate the high impact forces characteristic of stab threats caused by pointed knives, spikes, or ice picks, etc. When the point of the weapon impacts the outer surface of the armor, its materials either deflect the threats, or tend to stretch before failing (due to their characteristics of high tensile strength combined with a high resistance to cutting or tearing) while the force of the impact force is effectively spread over a greater area of the armor. Where used, multiple layers also help dissipate the stab threat impact energy.
Just as for ballistic resistance, stab resistance is defined in recognized Standards, such as “NIJ Standard-0115.00, Stab Resistance of Personal Body Armor” (2000). This Standard classifies threats into three levels, 1, 2 and 3, where 3 is the highest threat level.
- Materials Used for Aramid Fabrics
2.1 Yarn and Fabric
An upgraded version of DuPont’s Kevlar (Kevlar XP) weighs 10 percent less and provides a reduced amount of deformation of the backface (15 percent) (“Kevlar® XP™ for Soft Armor — Comfort and Protection” n.d.). DuPont’s competitor Teijin Aramid produces Twaron CT Microfilament, which the manufacturers claim avoids the need for compromises between “ballistic protection and restricted freedom of movement.” Each fiber comprises no less than 1000 separate filaments – 50 percent more than in other yarns of aramid material. They also claim that it has “high energy absorption, high tenacity, and high modulus of elasticity, which permits rapid dispersion of the deformation waves” (“Twaron” n.d.).
2.2 Structure Types
Whilst the choice of specific materials determines to a great extent the ballistic or stab resistance of body armor, the structures also have a considerable effect on its efficacy.
2.2.1 Woven Structures
Woven structure body armor is widely used, although it is recognized that ballistic performance can be influenced by the type and structure of the weave method used. Azrin Hani et al. (2012) provide a wealth of technical detail on weave types in their paper: “Body Armor Technology: A Review of Materials, Construction Techniques and Enhancement of Ballistic Energy Absorption.”
A paper by Cunniff (1992) presents an analysis of the effects on woven fabrics of ballistic impacts, discussed from the perspective of yarns assembled into single-ply fabric structures, such as Spectra® and Kevlar® 29, with various yarn deniers and types of weave. The effects of assembling these fabric plies into the structures of body armor are compared by measuring the responses of systems employing spaced armor and systems using multiple-ply armor. The pronounced energy absorption decrease found in the Spectra and nylon systems is thought to be due to “increased transverse stresses and possible interference of the deflection characteristics of fabric plies by subsequent plies.”
Chen, Lo, Tayyar & Day (2002) investigate the mouldability of angle-interlock woven fabrics for use in technical applications. The premise is that mouldable fabrics can have practical uses in a range of applications from “composites to body armor.” Specifically, the paper investigates the potential of moulding angle-interlock fabrics which the authors state “have low shear rigidity compared to woven fabrics with other weave structures.” The investigations use assessment based on two methods: shear and deformation testing. The findings show that for a given type of yarn, the mouldability of these angle-interlock woven fabrics bears a close relationship to two parameters: density of the fabric and the numbers of weft layers.
The thicker the fabric (more weft layers), the more mouldable is the fabric.
In a 2003 paper, Chen & Tayyar discuss a technique for the manufacturing of three-dimensional (shaped) woven fabrics. Possible applications include composite materials for reinforcement of military or riot police helmets, female body armor and other uses, where a seamless construction facilitates more efficient manufacture and better protection. Seams create a weak point, so a seamless construction is a definite advantage.
Further investigation into the ballistic performance of woven aramid fabrics is provided by Kaharan, Kus & Eren (2007). The investigation results showed that fabric ply numbers and types of stitching had significant effects on the fabric’s ballistic performance.
2.2.2 Nonwoven Structures
A study by Chia-Chang, Jia-Horng and Chu-Cheng (2011) investigates the use of fabricated compound nonwoven materials used as soft body armor. The material used for the investigation comprised layers of a web made by applying low melting point polyester (LMPET) onto the unaligned fibers of HSPA6 (high strength polyamide 6). Then a compound nonwoven fabric was created by sandwiching high strength filaments of Vectran between two layers of that web. That composite was then needle-punched and thermally bonded to produce the finished composite structure. It was tested in two ways: a falling weight test and a ballistic impact test.
The findings were that the newly-created composite material achieved an impact indentation reduction of eight percent, coupled with the added benefits that it was cheaper to produce and more than 20 percent lighter than conventional material used for soft body armor. The research also showed that improvements in cushioning and resistance to ballistic impact corresponded to the area density of the Vectran filaments used. The research samples using 400g/m2 of Vectran filaments were found to perform as well as a 44-ply Kevlar® vest.
2.2.3 Knitted Structures
A knitted mesh in combination with synthetic materials (e.g. Kevlar) to form a composite material is a useful solution for body armor. According to KnitMesh Technologies, the knitted structure “captures knife blades, helping to reduce likelihood of serious injury” (“Anti-Vandal Knitted Mesh & Anti-Slash Knitted Mesh” n.d.).
It has been shown that it is possible to further develop knitted structures to give even better cut and/or stab resistance. One such, “manufactured from p-aramid fibers with and without inlay yarns”, has been found to have the best performance when compared with other structures of the same mass and thickness parameters (Alpyildiz, Rochery, Kurbak and Flambard, 2011). The authors note in their paper that relatively few previous studies existed regarding knitted fabrics, but that “knitted structures are more interesting than nonwoven and woven structures in terms of their higher capacity to absorb energy during impact loadings.”
The Alpyildiz et al. paper refers to the scarcity of previous studies on the stab resistance of knitted fabrics, though they refer within their paper to one such study by Flambard & Polo, published in The Journal of Advanced Materials in 2004. In that study, the authors investigated the stab performance of PPTA and PBO (polyphenylinediamine-terephtalamide and polyphenyline-2,6-benzobisoxa-zole) fibers. The trademark for PPTA is Kevlar® 29, and PBO is registered as Zylon®. The findings were that when compared with the PPTA fibers, the PBO fibers demonstrated better performance. However, the best stab resistance was obtained by using a specific combination of the two fiber types.
Ballistic resistance of knitted fabrics is investigated in an Elsevier journal paper by Limin et al. (2010), in which the authors investigate the performance of a bi-axial warp-knitted (BWK). The BWK reinforcement in this textile composite comprises cross-plied (0 and 90 degrees) “straight fiber rows (warp and weft yarns) and knitted loop yarns (tricot yarns).” In order for textile-based armor to resist ballistic impacts, linearity in the structure is important. For that reason this structure features – in both weft and warp directions – not only high tensile strength but also good response to impact due to the stress waves within the composite spreading at high velocity along those straight yarns, allowing the energy of the impact to be absorbed across a greater area of the composite.
Another technique in the knitted type of structure is the use of so-called “spacer fabrics.” “Military and Police Knitted Fabrics” (n.d.) describes a range of various knitted product types including spacer fabrics produced by Baltex, a UK-based company.
2.2.4 Other Structures
Composite structures can also be used with advantage to improve overall ballistic resistance. For example, Lin (2005) proposed the use of cushion layers on the inside of the armor to reduce the effects of blunt trauma.
Other techniques include coating the armor fabrics with a natural rubber compound to improve the ballistic resistance. Alternatively, Kevlar material can be soaked in a shear-thickening fluid, which behaves as if it is a solid when struck by (e.g.) a projectile, creating what can be called “liquid body armor” (Wilson, 2007).
References:
(Alpyildiz, Tuba, Rochery, Maryline, Kurbak, Arif, and Flambard, Xavier. (2011). “Stab and cut resistance of knitted structures: a comparative study.” Textile Research Journal 2011 81: 205. Available at: http://trj.sagepub.com/content/81/2/205
Azrin Hani A., R., Roslan A., Mariatti J., and Maziah M. (2012) “Body Armor Technology: A Review of Materials, Construction Techniques and Enhancement of Ballistic Energy Absorption.” Advanced Materials Research Vols. 488-489 (2012) pp 806-812. Available at: http://trj.sagepub.com/content/81/2/205
Bellis, Mary. (n.d.). “History of Body Armor and Bullet Proof Vests.” About.com Inventors. Available at: http://inventors.about.com/od/bstartinventions/a/Body_Armor.htm
Bellis, Mary. (n.d.). “Kevlar – Stephanie Kwolek.” About.com Inventors. Available at: http://inventors.about.com/library/inventors/blkevlar.htm
Carothers, James, P. (1988). “Body Armor . . . A Historical Perspective.” USMC CSC. Available at: http://www.globalsecurity.org/military/library/report/1988/CJ2.htm
Chen, X., Lo, W.-Y., Tayyar, A. E. & Day, R., J. (2002). “Mouldability of Angle-Interlock Woven Fabrics for Technical Applications.” Textile Research Journal 2002 72: 195-200. Available at: http://trj.sagepub.com/content/72/3/195
Chen, Xiaogang and Tayyar, Ayse Ebru. (2003). “Engineering, Manufacturing, and Measuring 3D Domed Woven Fabrics.” Textile Research Journal 2003 73(5): 375-380. Available at: http://trj.sagepub.com/content/73/5/375
Croft, John & Longhurst, Daniel. (2007). “HOSDB Body Armour Standards for UK Police (2007) Part 1: General Requirements; Publication No. 39/07/A.” BSST GmbH. Available at: http://www.bsst.de/content/PDF/39-07-A_-_HOSDB_Body_Armour1.pdf
Croft, John & Longhurst, Daniel. (2007). “HOSDB Body Armour Standards for UK Police (2007) Part 2: Ballistic Resistance; Publication No. 39/07/B.” BSST GmbH. Available at: http://www.bsst.de/content/PDF/39-07-B_-_HOSDB_Body_Armour1.pdf
Cunniff, Philip, M. (1992). “An Analysis of the System Effects in Woven Fabrics under Ballistic Impact.” Textile Research Journal 1992 62: 495-509. Available at: http://trj.sagepub.com/content/62/9/495
Flambard, X and Polo, J. (2004). “Stab Resistance of Multi-Layers Knitted Structures (Comparison Between Para-Aramid and PBO Fibers).” The Journal of Advanced Materials, Volume 36, Issue 1, Jan. 2004, pp.30-35. Available at: The National Research Council of Canada: www.cnrc.ca/icist
Kaharan, Mehmet, Kus, Abdil, & Eren, Recep. (2007). “An investigation into ballistic performance and energy absorption capabilities of woven aramid fabrics.” International Journal of Impact Engineering 35 (2008) 499–510. Available at: www.sciencedirect.com
“Kevlar® XP™ for Soft Armor — Comfort and Protection.” (n.d.). DuPont. Available at: http://www.dupont.com/products-and-services/fabrics-fibers-nonwovens/fibers/brands/kevlar/products/kevlar-xp-soft-armor.html
Limin, Jin, Hong, Hu, Baozhong, Sun & Bohong, Gu. (Mar. 2010). “A simplified microstructure model of bi-axial warp-knitted composite for ballistic impact simulation.” Elsevier Ltd.: Composites Journal, Part B 41 (2010) 337-353. Available at: www.elsevier.com/locate/compositesb
Lin, Chia-Chang, Lin, Jia-Horng, and Chang, Chun-Cheng. (2011). “Fabrication of Compound Nonwoven Materials for Soft Body Armor.” Journal of Forensic Sciences, 2011 (pp.1-6). Available at: onlinelibrary.wiley.com
Lin, Jia-Horng. (2005). “Novel Compound Cushion Layer for Reinforcement of Ballistic Resistance.” Textile Research Journal May 2005 vol. 75 no. 5 431-436. Available (Abstract) at: http://trj.sagepub.com/content/75/5/431.short
“Materials.” (2012). SafeGuard Armor. Available at: http://www.safeguardarmor.com/articles/body-armor-materials/
“Military and Police Knitted Fabrics.” (n.d.). Baltex. Available at: http://www.baltex.co.uk/Military/
“NIJ Standard–0101.06 for Ballistic Resistance of Body Armor.” (2008). The U.S. Department of Justice, National Institute of Justice (NIJ). Available at: https://www.ncjrs.gov/pdffiles1/nij/223054.pdf
“NIJ Standard-0115.00, Stab Resistance of Personal Body Armor.” (2000). The U.S. Department of Justice, National Institute of Justice (NIJ). Available at: https://www.ncjrs.gov/pdffiles1/nij/183652.pdf
“SafeGuard Armour.” (2012). SafeGuard Armour. Available at: http://www.safeguardarmour.co.uk/?gclid=CJT0pv7Nob0CFUcTwwodfbkAJQ
“Twaron.” (n.d.). Envostar. Available at: http://www.envostar.com/materials.htm
Wilson, Tracy, V. (February 2007). “How Liquid Body Armor Works.” HowStuffWorks.com. Available at: http://science.howstuffworks.com/liquid-body-armor.htm
