Factors affecting comfort: human physiology and the role of ballistic clothing

Ballistic protection

Ballistic protection involves protection of body and eyes against projectiles of various shapes, sizes, and impact velocities (Adanur, 1995). Such protection is generally required for soldiers, policemen and general security personnel. Historically, ballistic protection devices were made from metals and were too heavy to wear, but textile materials now provide the same level of ballistic protection as metals but have relatively low weight and are therefore comfortable to wear. Most of the casualties during military combat or during unintended explosions are from the flying matter caused by the explosion hitting the body. It is reported that during military combat, only 19% of casualties are caused by bullets, as high as 59% of casualties are caused by fragments, and about 22% are due to other reasons. The number of casualties due to ballistic impact can be reduced 19% by wearing helmets, 40% by wearing armour and 65% by wearing armour with helmet (Scott, 2000). High-performance clothing used for ballistic protection dissipates the energy of the flying particles by stretching and breaking the yarns and transferring the energy from the impact at the crossover points of yarns (Scott, 2000).

The ballistic protection of a material depends on its ability to absorb energy locally and on the efficiency and speed of transferring the absorbed energy (Jacobs and Van Dingenen, 2001). One of the earliest materials used for ballistic protection was woven silk that was later replaced by high-modulus fibres based on aliphatic nylon 6,6 having a high degree of crystallinity and low elongation. Since the 1970s, aromatic polyamide fibres, such as Kevlar® (Du Pont) and Twaron® (Enka) and ultra-high-modulus polyethylene (UHMPE) are being used for ballistic protection (Scott, 2000).

17.7.2.1 Ballistic protection

Ballistic protection concerns apparel, vests, armors, helmets, and structural reinforcement for vehicles as well. The woven, knitted or nonwoven fabrics, laminates, and composites are used for ballistic protection. The type (knife, hand gun, assault rifle bullet, high-velocity bullet) and level of the threat are considered in design and manufacturing of ballistic protective apparel. The structure of armors may include ceramic plates, special fibers/textile structures, laminated/coated textiles, and composites depending on these parameters. In addition, blunt impact protection could be imparted to armors by including shock-absorbing materials.

Different types of body armor used for ballistic protection and different materials and structures used for body armor, the test methods used for the evaluation of ballistic performance, government regulations related to the manufacturing and use of protective clothing, and the methods of testing in several countries of the world have been described (Wang, Kanesalingam, Nayak, & Padhye, 2014).

In terms of textiles, ballistic protection can be divided into two broad categories: soft, “wearable” armor, wherein the ballistic protection is provided by the soft, flexible textile, and soft-rigid armor, wherein ballistic protection is provided by a combination of inflexible armor plates that are integrated into a high-modulus textile. Soft-rigid composite textile systems for ballistics protection typically comprise ceramic armor plates, for example, boron nitride, tungsten carbide, tungsten disulfide, aluminum nitride, and so forth, coated or contained in high-modulus organic polymers, such as para-aramids, for example, Kevlar and Twaron, or UHMW polyolefins, for example, Spectra and Dyneema (https://www.ncjrs.gov/pdffiles1/nij/247281.pdf) (Owens, 2011).

In such systems, rigid plates are designed to intercept an incoming projectile and disperse its kinetic energy over a large area, while the soft textile is used to disperse as much kinetic energy as possible and cause deformation of the round prior to it reaching the ceramic plates (Owens, 2011). Another technology that has demonstrated the ability to serve in an antiballistic capacity is a composite system consisting of a high-performance antiballistic fiber combined with a shear-thickening colloidal dispersion. Shear-thickening fluids or STFs are liquids whose viscosity increases as a function of applied stress. A mixture of cornstarch and water is the classic example of an STF. Researchers at the University of Delaware have produced antiballistic yarns possessing STFs intercalated into the fiber (Owens, 2011) (Wagner & Brady, 2009).

Ballistic tests of these composite fibers show a 250% increase in stopping power of STF-treated Kevlar fibers compared to Kevlar alone. Production of these STF-enhanced textiles for other applications has already begun by Dow Corning under the name Deflexion. Now, consider a merger of M-5 fiber with STF technology and it becomes apparent that soft antiballistic armor will soon be a reality (Owens, 2011). The protective power of typical aramid-based multilayered ballistic fabrics designed to defeat high-velocity ballistic impacts can be improved if wool is incorporated into the weave structure. Ballistic tests have shown that synthetic fabrics blended with wool can at least match the dry or wet ballistic performance of an equivalent pure Kevlar fabric when tested under National Institute of Justice (NIJ) (2014) Ballistic Standard Level III A. The inclusion of the wool can significantly improve the tear strength of pure synthetic ballistic fabrics (Sinnppoo, Arnold, & Padhye, 2010). The use in range of wool fiber and its blends can be increased and further explored for technical textiles applications. Tegris® is a thermoplastic 100% polypropylene composite for hard and soft armor applications, including personal body armor, vehicle armor, blast blankets, and a number of other armor-related applications to counter fragment, projectile, and blast threats (http://millikenmilitary.milliken.com/en-us/technologies/Pages/composites.aspx).

For lightweight stiffness, TYCOR-reinforced core materials are comprised of closed cell foam wrapped in fiberglass. When laid in a mold and infused with resin, TYCOR becomes a stiff, strong material lighter then infused balsa. TYCOR can be used in a variety of applications including bridges, boats, submarine camels, and more (http://millikenmilitary.milliken.com/en-us/products/Pages/impact-resistant-composites.aspx).

The US Army is experimenting with new, advanced composites, to improve vehicle and body armor, providing lighter and more effective protection from different threats, including bullets, fragments, IEDs, and mines. One of the most promising materials is the new high-strength M5 fiber, developed by Akzo Nobel central research labs and currently produced by Magellan Systems International. It has an extraordinary potential for use in armor systems for personnel and vehicles, flame and thermal protection, as well as in high-performance structural composites. Potential Army applications of the fiber include fragmentation vests and helmets, composites for use in conjunction with ceramic materials for small arms protection and structural composites for vehicles and aircraft. It enables the fabrication of advanced lightweight composites into hard and soft ballistic armor. M5 offers significant advantages over both steel and carbon, which is currently used for fabrication of aerospace and automotive structural parts (http://defense-update.com/products/m/m-5-fiber.htm).

M5 fiber is based on the rigid-rod polymer poly{diimidazo pyridinylene (dihydroxy) phenylene}; M5 fiber-based armor has the potential to substantially decrease the weight of body armor while enhancing or maintaining impact performance. Composite fragmentation armor systems were developed using less than optimal quality M5 fiber and tested under ballistic impact; the performance of these armor systems was exceptional. The crystal structure of M5 is different from all other high-strength fibers; the fiber not only features typical covalent bonding in the main chain direction, but it also features a hydrogen bonded network in the lateral dimensions. M5 fibers currently have an average modulus of 310 GPa, (i.e., substantially higher than 95% of the carbon fibers sold), and average tenacities currently higher than aramids (such as Kevlar or Twaron) and on a par with PBO fibers (such as Zylon), at up to 5.8 GPa. Based on these results, it is estimated that fragmentation protective armor systems based on M5 will reduce the areal density of the ballistic component of these systems by approximately 40%–60% over Kevlar KM2 fabric at the same level of protection (Cunniff & Auerbach, http://web.mit.edu/course/3/3.91/www/slides/cunniff.pdf).

The central tasks of ballistic protection are the absorption and the dissipation of energy caused by a ballistic impact. For this reason, bulletproof vests generally consist of a number of layers. Their fabrics or composite layups are made of yarns of high-performance fibers. At the impact of a bullet the material absorbs the kinetic energy—a handgun projectile travels at a speed of 400 m a second—by stretching of fibers and other stiff fibers which disperse the load over a large area throughout the material. This slows the bullet down and finally hinders it from penetrating the body. Body armor designed specifically to defeat rifle fire has to be more rigid, because those projectiles travel at speeds of around 800 m a second. Therefore, besides the layers with fibers, hard materials such as ceramics or metal plates have to be inserted. The protective plates absorb and dissipate this greater kinetic energy upon impact and also the bullet itself gets blunted.

Carbon nanotube fibers woven as a cloth or incorporated into the polymer matrix composite materials are also reported to improve the ballistic performance and enhance stiffness, strength, and toughness against the most aggressive ballistic threats.

DuPont has developed its next generation of bullet-resistant Kevlar fiber that is stronger and lighter than previous versions. Kevlar XP promises to stop 44 Magnum rounds in the first two to three layers of an 11-layer vest, according to DuPont and independent lab tests. While that is impressive, DuPont says it can do this with 10% less weight and 15% less backface deformation which directly translates into less blunt force trauma to vest wearers. While 10% less weight may not sound like much, police officers and soldiers are grateful for a lighter vest, especially when they have to wear tens or even hundreds of pounds of extra gear.

The nature of protective clothing means that there are naturally similarities between different products. For example, there are a number of similarities between bulletproof and stabproof vests, including materials and design. This may seem obvious, but even Turnout Gear shares a number of similarities with bulletproof vests, beyond that both are used to protect an individual. Even their design and development follow similar paths due to the desired end result. Kevlar is not only incredibly strong, but is lightweight and flexible. This is why it is so heavily favored in body armor manufacture. However, aramids are also capable of withstanding extreme temperatures, and will not melt or degrade at temperatures up to 800°F. This means that Kevlar has also found use in Turnout Gear, although aramid manufacturers often look to provide materials with far higher heat resistance, usually at the expense of some ballistic protection. Nevertheless, Kevlar does find uses in Turnout Gear, offering some protection against impacts and blunt trauma. Some manufacturers offer blends of materials, for example, a mixture of Kevlar and Nomex, another aramid material from DuPont that has a far higher resistance to heat. By producing a blend the material can offer the heat resistance needed by firefighters while also decreasing friction and improving co-operation between the layers of Turnout Gear.

1.5.6.3 Ballistic Composites

The ballistic potential of various high-performance fibers is compared in Fig. 27, where the sonic velocity (V_s) is plotted against the specific energy absorption (E_sp) capability of several polymeric fibers. UHMWPE fibers clearly exhibit a good balance of both these properties (Fig. 28).¹⁶⁷

 

Fig. 28. Basic ballistic performance indicators for various high-strength fibers.

Reproduced from Jacobs, M.J.N., Van Dingenen, J.L.J., 2001. Ballistic protection mechanisms in personal armour. Journal of Materials Science 36 (13), 3137–3142.

PE fiber-reinforced ballistic products contain either woven fabrics or impregnated and cross-ply unidirectional reinforcement. Commercial ballistic products reinforced with cross-ply fiber laminates include Dyneema UD-HB (Fig. 29) and SpectraShield. Whereas in composites reinforced with glass fibers or aramids mainly toughened thermoset resins are used, ballistic composites reinforced with woven fabrics are often all-PE composites. The composites are produced by stacking layers of UHMWPE fabric and low density PE films. Easy production and post forming in the final shape are distinctive advantages. Laminates consisting of cross-plied unidirectional UHMWPE fibers in an elastomeric matrix like Kraton were initially developed by Allied Signal (SpectraShield)^168,169 and later by DSM High Performance Fibers (Dyneema UD-HB) for making composites with a higher ballistic efficiency than possible with fabric reinforced composites.

 

Fig. 29. Dyneema UD is a cross-ply laminate based on a unidirectional polyethylene (PE) composite material and is available as UD-SB (soft ballistics) and UD-HB (hard ballistics) (DSM Dyneema, 2017).

Cross-plied UD-composites like SpectraShield and Dyneema UD-HB are superior for stopping ballistic projectiles because of the faster distribution of energy as a result of the fully aligned fibers (Fig. 30).

 

Fig. 30. Energy dispersion pattern of SpectraShield (left) vs. woven fabrics (right) showing a wider, less localized, energy dissipation area giving improved performance of UD-ballistics.

11.3.3 Comfort

When designing e-textiles for military applications, the comfort of soldiers should never be forgotten. However, the inherent nature of the clothing integrated with electronics to achieve desired levels of protection for soldiers may also affect the degree of comfort. The thickness, type of material used and design aspects of the clothing tend to retain body heat and perspiration inside the garment, which can all lead to heat and moisture build-up and subsequently compromise the body’s ability to maintain thermal balance, resulting in discomfort and fatigue. The maintenance of thermal balance is one of the most important aspects of apparels (Nayak et al., 2009; Das and Alagirusamy, 2010). Almost all the high-performance fibres currently used in military fabrics are synthetic, and have poor heat and moisture management capability. Furthermore, the integration of electronic components and sensors tends to make clothing bulkier and increases the overall weight. In addition, electronic components generate heat. All the above factors can lead to thermal discomfort.

Comfort attributes depend on thermal regulation, physical sensation, water regulation, nature of the material (fibre and finishes), design aspects and the fit of the clothing. Although research has been done to improve the comfort attributes of synthetics, the improvements have not met the high standard of requirements for soldiers’ uniforms and armour. Hence, future research on the development of smart e-textiles for soldiers should focus on the optimisation of comfort, robustness and proper functionalities.

18.6.1 Ballistic applications

Because of their high energy absorption at break, HMPE fibers are used in applications for civil, law enforcement, and military personnel where low weight needs to be combined with high protection against mechanical threats. The mechanisms of energy absorption at ballistic speeds are important in ballistic protection. The primary factors that determine the weight needed to stop a projectile are the specific energy absorption, determined by the tenacity and elongation, and the sonic velocity of fibers, determined by the specific modulus, indicating the area of the fabric to be involved in stopping the projectile. HMPE fiber has a very high score in these two properties (Fig. 18.30; Jacobs and van Dingenen, 2001).

 

Figure 18.30. Energy absorption and sonic velocity in ballistic fibers.

The combination of high modulus and high tenacity, and the potential to improve substantially upon this, makes the ballistic potential of an HMPE fiber system beyond that of any other high performance fiber (van der Werff and Heisserer, 2016).

7.1 Introduction

Lightweight hybrid composites that can offer substantial ballistic protection to tactical ground vehicles and, in turn, to military personnel are of ever-growing interest to the US military. One of the major requirements for such a ballistic protection technology is the ability to defeat or protect against the 7.62 mm NATO M80 ball round and 7.62 mm NIJ IV or DIN C5-SF AP round threats. It is also an implicit requirement that such a technology be developed without any capital or operational cost penalties; rather, there must be associated gains in operational efficiency and tactical performance during military action. Therefore, there is a need to develop armor systems that are based on high mass efficiency ballistic-shield materials, innovative and functional hybrid designs, and reliable scaleable processing methods.

Conventional armor materials are typically made of steel, aluminum, or other hard metals. Although these metallic materials primarily perform a structural function, they provide reasonably good ballistic protection (Viechnicki et al., 1991; Aghajanian et al., 2001) at appropriate thicknesses (or areal densities). Often, this approach results in parasitic weight, which not only reduces fuel efficiency but also diminishes mobility in action. Recognizing that rapid deployment, enhanced fuel mileage, and reliable ballistic (and blast) protection are the keys to dominating future battles, new and innovative approaches involving lighter materials such as ceramics and polymers have become absolutely essential. Also recognized is the need for ceramic composite armor capable of surviving multiple hits.

5.1.4 Concluding Remarks

Traditionally, the development of advanced blast- and ballistic-protection systems is carried out almost entirely using legacy knowledge and extensive fabrication-and-test trial-and-error approaches. This approach is not only economically unattractive but is often associated with significantly longer lead times. Consequently, this purely empirical approach has gradually become complemented by the appropriate cost and time-efficient computer-aided engineering (CAE) analyses. This trend has been accelerated by the recent developments in the numerical modeling of transient nonlinear dynamics phenomena such as those accompanying blast and ballistic loading conditions. In particular, advances have enabled the coupling between Eulerian solvers (used to model gaseous detonation products and air) and Lagrangian solvers (used to represent solid components of the protection systems, as well as of the projectiles). It is well-established that the utility of the CAE analyses in the development of blast-/ballistic-protective structures is greatly affected by the availability of high-fidelity physically based dynamic material constitutive (continuum) models. The all-atom and coarse-grained results pertaining to the interaction of shock waves with the material microstructure and the resulting changes in the shock wave and the material can be highly beneficial during the construction of such material models [16].

13.4 Ballistic properties of kenaf fibers reinforced poly vinyl butyral composites and its hybrid

The synthetic composite materials play an important part in ballistic protection and provide an excellent solution in terms of strength over weight ratio but it is expensive due to the high demands for its raw materials (carbon, aramid, etc.) in nonarmor application. Although synthetic fibers have an excellent strength that might be able to substitute the traditional metals, the world welcomed the use of natural fibers in composite materials. The main applications of aramid fiber are high tension conveyor belts, ropes, cables, aircrafts, sports equipment, and protective ballistic fabrics (armor). Despite these advantages, the use of aramid fiber reinforced polymer composites has a tendency to decline because of their high initial costs, their petrochemical nature, and their adverse environmental impact (Tudu, 2009).

In the ballistic composite, the matrix restricts lateral motion of the fibers, giving rise to more energy being absorbed by composites leads to break the fiber. Consequently, less moving yarns may create a higher interply friction, and can act as a buffer against impact resulting in improved ballistic performance (Lim et al., 2012). Nevertheless, the composite may be stiffer and limit fiber extension if the level of fiber-matrix adhesion is too high. A stiffer composite cannot absorb more energy or disperse the energy efficiently; failure initiates with cracking of the matrix because of excess stress concentration.

Despite this growing interest in the natural hybrid composite field, only scarce attention has been devoted to the high-velocity impact behavior of these classes of hybrids. Different methods of analysis have been used for ballistic impact performance, depending on the type of response desired for the particular threat designing. Experiments were performed under bullets (National Institute of Justice (NIJ) tests) and fragments (V₅₀ tests) conditions to study the effect of hybridization on the ballistic resistance of hybrid-laminated composites. The methods consist of residual velocity, V₅₀ ballistic limit, penetration depth, and instrumented techniques. For residual velocity testing, the specimen is completely perforated. The general method for characterizing a material’s ballistic limit is to perform a V₅₀ ballistic test, the velocity at which there is an equal probability of a partial (target was not defeated) or a complete perforation (target was defeated) for the given armor and threat. The NIJ methods are used to determine minimum performance requirements for ballistic resistant protective materials levels. It is typically used for residual strength testing in which penetration resistance is not required. According to military specification MIL-STD-662 F, the test consists of taking a certain number of shots where the projectile penetrates the specimen and that same number of shots where no penetration occurs. This type of testing has been widely used by government agencies and armor manufactures for acceptance testing and material performance rating.

The ballistic experiments were conducted in an indoor firing range at the Weapon Technology Laboratory, Science and Technology Research Institute for Defence, Malaysian Ministry of Defence (STRIDE). All armor materials are subjected to standardized test such National Institute of Justice (United States) in order to be certified as safe-worthy armor materials in Malaysia. By using a powder gun, two types of bullets were fired; 9 mm, 8.0 g full metal jacket bullets and. 22 caliber (diameter of 7.62 mm) fragment simulating projectiles. These tests were performed on flat panels with partial lateral support positioned at 5 m forward from the muzzle of the test barrel to produce impacts of 90 degrees obliquity, as illustrated in Fig. 13.7. The targets were rigidly clamped between rectangular steel frames and perpendicular to the line of flight of the bullet at the point of impact. Both two chronographs and Doppler radar antenna combined with a computer were used to measure the projectile velocity; one chronograph is positioned at 2 m in front of the target and another behind it. Projectiles, which pass through the panel, are considered to be a complete penetration, while the others are defined as being partial penetrations, following the United States Department of Justice’s NIJ. The impact striking velocities (V_s) and residual velocity (V_r) of the projectiles were recorded, while the ballistic limit (V₅₀) was calculated. These types of bullets were shot according to the recorded speed in the NIJ standards.

 

Figure 13.7. Actual setup for ballistic impact test.

The ballistic limit velocity (V₅₀) was estimated using experimental data on the basis of whether the projectile penetrates the hybrid composite completely or partially, as shown in Table 13.5. It is the most common assessment tool to determine the ballistic performance of a material; however, the accuracy of the estimation increases with increasing number of ballistic tests (Boccaccini et al., 2005). Fig. 13.8 shows a plot between the initial velocity and the residual velocity for the hybrid-laminated composites. An increase in initial velocity results in the increase in the residual velocity (which is zero up to certain initial value) for all the hybrids.

Table 13.5. Ballistic resistance results

Specimen descriptions	Sample code	V50 (m/s)	Thickness (mm)
11 Aramid/8 kenaf	H5	496.8	13.1
9 Aramid/10 kenaf	H6	477.5	14.3
19 Kenaf	Kf	417.8	17

 

Figure 13.8. Residual velocities as a function of impact velocities.

Fig. 13.9 shows the ballistic properties of kenaf/aramid hybrid composites in terms of ballistic limit velocity (V₅₀) compared to kenaf/PVB composites. According to two ballistics test NIJ standards, Type II, IIA, III, IIIA, and the V₅₀ requirement of the US military specification, were calculated. Fig. 13.10 shows the ballistic limit (V₅₀)-volume fraction curves of kenaf and its hybrids. The kenaf volume fraction and aramid volume fraction have a significant effect on the ballistic limit velocity.

 

Figure 13.9. Ballistic limit (V₅₀) of all composites.

 

Figure 13.10. Ballistic limit (V₅₀)–fiber volume fraction curves of kenaf and its hybrids.

13.7.1 Diversity and selection

Most of the chapters in this volume relate to specific functions from ballistic protection to transportation. In contrast to this, ropes and cordage are found in a wide variety of human activities from mooring oil rigs to tying up a parcel. Ropes and cords have an amazing diversity of uses as one-dimensional tension textiles. Inevitably, not all have been mentioned. Rescue ropes for many situations, sewing up wounds, ski tow ropes, tethers for space walks, the list could go on and on, and readers will think of others. However, the descriptions of the wide range of applications should provide analogous advice on the ropes and cords that could be used for applications that have not been specifically covered.

The last paragraph of the previous section shows how, even in what seem commonplace activities, it is necessary to understand what is needed to fit the application and to make the right choice of rope. If wrong choices are made, accidents happen. It may be the wrong rope, a manufacturing fault, an installation fault, or a failure to use a rope in a proper way. There may be damage, injuries, or death that can lead to litigation. Experts then have to decide who, if anyone, may be to blame. Some breaks may be inevitable. An example is the Japanese tsunami of 2011. A great many ships broke loose from their moorings and finished up on land. There are probably no ropes that could have withstood this force, certainly none that a shipowner would consider using.

16.3.2 Ballistic protection

Fibrous materials have been widely used for developing products that primarily aim at ballistic protection. Fig. 16.12 illustrates examples of performance characteristics and related attributes for fibrous products used for ballistic protection. The idea is that these products should be able to absorb large amounts of energy due to their high tenacity, high modulus of elasticity, and low density [18–23]. The most common fibrous product used for ballistic protection is bulletproof vests. This product is also one of the oldest components of protection used by human over the years. Indeed, throughout recorded history, humans have used various types of materials as body armor to protect themselves from external objects and injuries in combat situations. The first protective clothing and shields were made from animal skins. With the invention of firearms around 1500, other materials including wood and metal shields were also used for protection. Although these materials were effective in their protective aspects, they were too heavy and impractical for high physical actions, fast movement, and battle maneuvering. This has resulted in the development of softer body armors. One of the first recorded instances of the use of soft body armor was by the medieval Japanese, who used armor manufactured from silk. It was not until the late 19th century that the first use of soft body armor in the United States was recorded. At that time, the military explored the possibility of using soft body armor manufactured from silk. The project even attracted congressional attention after the assassination of President William McKinley in 1901. While the garments were shown to be effective against low-velocity bullets (i.e., traveling at 400 feet per second or less), they did not offer protection against the new generation of handgun ammunition being introduced at that time (ammunition that traveled at velocities of more than 600 feet per second). This deficiency associated with the prohibitive cost of silk made the concept unacceptable.

 

Fig. 16.12. Performance characteristics and related attributes of fibrous products used for ballistic protection.

World War II was a turning point in the development of body armor with the introduction of the “flak jacket” made from ballistic nylon. The flak jacket was very cumbersome and bulky. It provided protection primarily from ammunition fragments but was ineffective against most pistol and rifle threats. By the late 1960s, new fibers were discovered that made their ways to today’s modern generation of body armors. The invention of Kevlar by DuPont in the 1970s was another significant turning point in the development of body armors (e.g., Kevlar 29). Ironically, the fabric was originally intended to replace steel belting in vehicle tires. In 1988, DuPont introduced the second generation of Kevlar fiber, known as Kevlar 129, which offered increased ballistic protection capabilities against high-energy rounds such as the 9-mm FMJ. In 1995, Kevlar Correctional was introduced, which provided puncture-resistant technology to both law-enforcement and correctional officers against puncture-type threats.

The basic idea of body armor is a simple one. It is based on catching bullet that strikes body armor in a “web” of very strong fibrous assembly [22]. This assembly should absorb and disperse the impact energy that is transmitted to the vest from the bullet, causing the bullet to deform or “mushroom.” Additional energy is absorbed by each successive layer of material in the vest, until such time as the bullet is stopped. This principle requires a large area of the garment to be involved in preventing the bullet from penetrating to the body. Unfortunately and despite the great progress of development, no structure exists that will prevent penetration of all ballistic objects, at the same time being wearable and under all situations.

Typical bulletproof vests are made from multiple layers of woven fabric, with the degree of protection is increased as the number of fabric layers increase. These layers are assembled into a “ballistic panel,” which is then inserted into the “carrier,” which is constructed of conventional garment fabrics such as nylon or cotton. The ballistic panel may be permanently sewn into the carrier or may be removable [23]. Although the overall finished product looks relatively simple in construction, the ballistic panel can be very complex. Even the manner in which the ballistic panels are assembled into a single unit can differ from one product to another. In some cases, the multiple layers are bias stitched around the entire edge of the panel; in others, the layers are tack stitched together at several locations. Some manufacturers assemble the fabrics with a number of rows of vertical or horizontal stitching; some may even quilt the entire ballistic panel. No evidence exists that stitching impairs the ballistic-resistant properties of a panel. Instead, stitching tends to improve the overall performance, especially in cases of blunt trauma, depending upon the type of fabric used.

Plain woven fabric is more suitable for body armors. Neoprene coating or resination is also commonly used [18]. Needle-punched nonwoven fabrics are also used for ballistic protection. These are typically made from high-performance polyolefin fibers such as Dyneema polyethylene. The benefits of using nonwoven structures for these applications stem from their ability to provide protection against sharp fragments by absorbing projectile energy by deformation rather than fiber breakage as is the case with woven fabrics. When needle-punched nonwovens are used for ballistic protection, the felt structure should have very low mass per unit area. However, as the mass increases, woven structures become more superior to nonwoven felts [18]. Nonwoven felts should also be designed in such a way that a high degree of entanglement of long staple fibers is achieved at a minimum degree of needling since excessive needling can produce too much fiber alignment through the structure, which aids the projectile penetration.

In situations where high levels of protection (e.g., rifle fire) are required, body armor of either semirigid or rigid construction should be used. These are typically multilayer fibrous systems incorporating hard materials such as ceramics and metals. The heavy weight and high bulkiness of these body armors prevent their use in routine applications (e.g., by uniformed patrol officers or normal military operations) and restrict their use to tactical situations where it is worn externally for short periods of time when confronted with higher-level threats.

The development of more effective body armors is unlikely to cease as a result of the continuing development of weapons of increasing powers. As indicated earlier, the key aspect of development is the fibrous component from which body armors are made. The newest addition to the Kevlar line is Kevlar Protera, which DuPont made available in 1996. This is believed to be a high-performance fabric that allows lighter weight, more flexibility, and greater ballistic protection in a vest design due to the molecular structure of the fiber. Another development is the spectra fiber, manufactured by the former AlliedSignal, which is an ultrahigh-strength polyethylene fiber used to make Spectra Shield composite. This basically consists of two unidirectional layers of spectra fiber, arranged to cross each other at 0- and 90-degree angles and held in place by a flexible resin. Both the fiber and resin layers are sealed between two thin sheets of polyethylene film, which is similar in appearance to plastic food wrap. According to AlliedSignal, the resulting nonwoven fabric is incredibly strong and lightweight and has excellent ballistic protection capabilities. Spectra Shield is made in a variety of styles for use in both concealable and hard armor applications. Another product, also developed by the former AlliedSignal, uses the Shield Technology process to manufacture a shield composite called Gold Shield. This is made from aramid fibers instead of the Spectra fiber. Gold Shield is typically made in three types: Gold Shield LCR and GoldFlex, which are used in concealable body armor, and Gold Shield PCR, which is used in the manufacture of hard armor, such as plates and helmets.

Akzo Nobel has also developed various forms of its aramid fiber Twaron for body armor. This fiber uses more than 1000 fine spun single filaments that act as an energy sponge, absorbing a bullet’s impact and quickly dissipating its energy through engaged and adjacent fibers. The use of many filaments is believed to disperse an impact more quickly and allow maximum energy absorption at minimum weights while enhancing comfort and flexibility.