COMPREHENISVE REVIEW OF BULLETPROOF VESTS

GRAPHICAL ABSTRACT

AMAN TAKSHAK

U-73892535

ABSTRACT

With the looming threat of war and other taunty conflicts ,the protection of troops from ballistic and blast has gained great importance. Materials are being developed to develop highly efficient and light weight armour for the armoured personal. Recent innovations in materials and manufacturing technology during the latter half of the 20th century has led to discovery of high strength materials like fibreglass, nylon and Kevlar that have provided body armour with significant decrease in weight without compromising body .While the demand for efficient products continue to rise keeping in mind same pre-requisites (increased protection at decreased weight) continue to be a major priority today too, it is recognized that future innovations might be increasingly difficult to achieve because the financial costs associated with developing new fibres are becoming cost-centric and the time-to-market for their commercialization remains long term .Ballistic protection has gone through numerous changes simultaneously with the advent of new weapons. Various such vests are currently available and are bullet resistant to a particular threat level. The protection is given by the material used to produce the ballistic panel. Different types of materials can be used to obtain various properties and ultimately achieving different levels of strength. While the material imparts strength, the amount of material used also affects the protection. The current prime focus in the market is to minimize weight and thickness of the vests without compromising on the safety of the personal. For instance, the possibility of using a Shear Thickening Fluid was explored in order to reduce the amount of textile fabric used. Presently ceramic based materials are being used for this operation such as silica ,aluminium oxide etc. This new research of finding lucrative materials with better hardness and correspondingly better toughness needs to be explored at the microscopic level. This challenge can be dealt with efforts on better use of ceramic materials

such as silicon carbide, boron carbide ,silicon nitride etc. leading to development of ceramic-ceramic composites.

INTRODUCTION

A composite material is defined as a material comprising of two or more chemically and/or physically distinct constituents (phases) combined on a macroscopic scale. The constituents present in the composite material retain their individual identities and properties, but together they produce a material system, the properties of which are designed to be superior to those of the constituent materials acting independently. A composite material consists of two phases one is called reinforcement and other is called matrix. These two phases are separated by distinct interfaces. The most useful properties of composites are high specific strength and specific stiffness, good corrosion resistance and good fatigue resistance. On account of these highly desirable characteristics, composites have rightfully this study will shed light on all the processes as well as design considerations and the scope of improvement associated with the processes of bullet-proof vest manufacturing .With the recent innovations associated with the discovery of nylon, Kevlar etc.. further research is aimed at strengthening the material at fibrous level by (1)improving properties in fibres through polymer chemistry (2)new technologies such as carbon nano-tube technologies aimed at improving fibrous strength. (3)spinning processes with better diameter reduction of fibres (4)phenomena of shear thickening fluid so as to reduce the amount of textile cloth used.

It is not possible to ensure an effective correlation between ballistic performance and a single characteristic or property of the material, due to the dynamic nature of the event occurring at intervals of time ranging from nanoseconds to microseconds. Thus, ballistic tests

under certain conditions are always required to determine the effectiveness of the protection systems. Hence the properties of ceramic materials are vital for high ballistic efficiency. In general, ceramics must have a fine microstructure and should be highly homogeneous .Apart from this ,high hardness, high relative density and high modulus elasticity are necessary factors which determine the durability of bulletproof vests. Ceramics also have a tendency of fluidity under high compressive loads which can be avoided by selection of appropriate ceramic material along with sound backing of substrates.in any bulletproof application the material has multiple layers where ceramic layer forms the front layer reinforced by polymer materials such as polyethylene and polypropylene etc. When a handgun bullet strikes the body armour, it is caught in a "web" of very strong fibres. These fibres absorb and disperse the impact energy that is transmitted to the vest from the bullet, causing the bullet to "mushroom." Additional energy is absorbed by each successive layer of material in the vest, until such time as the bullet has been stopped. Most anti-ballistic materials, like bullet proof vests and explosion-proof blankets are currently made of multiple layers of: • Kevlar fibres• Twaron fibres • Dyneema fibres

COMMON CRITERION USED FOR VEST MATERIAL SELECTION

No material is capable of perfectly handling ballistc impacts with full precision. Hence, it is imperative to understand the mechanism behind ballistic protection on the basis of which generalizations could be made. Common materials used are-

(1)Wave velocity-is given by the under-root ratio of modulus and density of the material. The higher the wave velocity of the material,

more is the ability to absorb ballistic impacts .For instance, wave velocity of Kevlar is 4-5 times higher than that of Nylon 6-6 resulting in much more coverage of area on the horizontal strain line of the stress-strain curve. Moreover, at such high strain rates, Nylon will most likely suffer creep.

(2)Crystalline nature-fine crystalline structure gives rise to high modulus and enhance the ability of the material to have higher wave velocity and sustain under higher temperature.

(3)Diameter of fibre- at smaller diameter, the coefficient of friction is high and hence energy absorption is more effective.

(4) Balance in compressive properties- properties such as fibre modulus are necessary to achieve strength and high wave propagation rate but due this will only happen at the cost of elongation and ultimately breakage. Thus higher compressive properties lower the strain energy absorption rate and simultaneously makes material prone to brittleness.

(5)Nature of yarn structure-yarn structure plays a pivotal role in the fibre properties. For instance ,if the yarn structure is too drawn then there is a tendency to suddenly slip at the time of a sudden ballistic impact. Therefore, innovations are being made to add certain procedures such as Corona treatment and plasma etching is applied. All these processes are slow but are effective procedures of enhancing fibre properties without altering too much in the basic structure.

DESIGN CRITERIA FOR VARIOUS PARTS OF THE VEST

The main idea behind an effective body armour is the comfort, especially at high temperatures ,mobility during combat and load distribution associated with the ballistic impact on the armour. This

presents the challenge pertaining to keeping an adequate balance between all the factors discussed before also keeping in mind flexibility, cost and protection against environmental exposure. Nature of the danger also varies when it comes to protection against any hostile harm. For instance, in order to deal with stab wounds, textiles need specific yarn mobility within the weave is needed but dense weaves which can curb sharp pointed attacks can lead to premature failure when it comes to bullet attacks.

At the microscopic level, the armours were manufactured using layers of woven fabrics which were stitched together but now for better protection but now they include laminates stacked with non-woven as well as combination of non- woven and woven laminates. Also ,the laminates used are all unidirectional produced in the forms of very thin sheets with a comparatively more mobile polymer resin such as Kraton used for better binding of laminates. Before applying heat and pressure, polyethylene films are also added to the layers to provide protection.

POLYMER

For the most effective polymer, Many hours of research are applied by manufacturers including detailed studies involving the molecular structure of the polymer and its building blocks. These characterstics depend heavily on spatial arrangement and nature of chemical bonds and the polymerization processes that incur to obtain the final product.

FIBRE

There are various terminologies associated with the fibre that ultimately decide on the type of fibre to be used for operation. Fibre diameter and its elastic modulus are one of the main determinants in this regard .Fibre diameter is often smaller in diameter than an average human hair. Tensile strength of a fibre is termed as “tenacity having units of force per denier where denier is the linear density of the fibre. Tenacity generally increases with decreasing fibre diameter. The other main factor termed as the elastic modulus is obtained from conducting tests on fibre by finding out its tensile strength; it is computed as the initial slope of the tensile stress-strain curve. Many polymer fibres show visco-elastic behaviour to the extent that tenacity and elastic modulus are often found out to be increasing with enhanced stain rates. Elongation at break is another terminology to characterize a fibre and basically is the amount of stretch that a fibre experiences during a tensile test at failure. Elongation is computed as a percent of the initial tested length. More properties helpful for weight- sensitive applications are specific strength and specific modulus which is a measure of the strength and modulus values divided by the linear density i.e. denier Specific strength can also be termed as “breaking length,” which is equivalent to the length of fibre required to break when hanging vertically under the effect of its own weight. Specific gravity, another important factor is the ratio of the density of the material used to the density of water. Fibres are considered to be completely buoyant if their specific gravities are less than one. Dyneema and Kevlar are currently the two man fibres used and both are made from the polymerization of extremely long chain of polyethylene producing fine crystalline structures which are high in strength. The fibres also have weak inter molecular Vander Vaal but Kevlar is this case has Hydrogen bonds with a very short molecular length compared to Dyneema. Therefore ,Kevlar has lower strength than the Dyneema. Both fibres have higher tensile strength to weight ratio. The strength of the kevlar is five times higher than steel on an equal weight basis but the

strength of Dyneema is fifteen times as stronger than steel. Between Kevlar and dyneema, the latter has 40% more strength than compared to Kevlar leading to fewer materials required for same operation.

Courtesy spectra shield

YARNS

Yarns are manufactured by segregating numerous bundles of continuous fibres and are measured on the millimetre scale .Yarns ,when spun properly as discussed in the later stage can show exemplary improvement in properties. The number of fibres within a yarn are referred to as “filament count.” Yarns constructed of continuous filaments often align the fibres in a straight configuration or in a slightly twisted helix. The helical fibre

arrangement results from the addition of twist. Twist is a mechanism that can on a macro level increase the tensile strength of yarns by decreasing the movement of individual fibres and henceforth improve the overall handling of the fibre during weaving at micro level. Twist is measured by the number of turns per unit length of yarn. Yarns are often categorized by denier rather than filament count with many woven body armours made of deniers between 100-500.

When Kevlar is spun into a yarn, the final manufactured fibre has a tensile strength of about 3600 MPa. The polymer owes its high strength to the many inter-chain bonds and inter-molecular hydrogen bonds formed between the carbonyl and NH groups. Also, additional strength is taken from the stacking interactions between adjacent strands. These interactions will have a greater influence on Kevlar than the already present van der Waals interactions and chain length that are mainly responsible for changing properties of other synthetic polymers and fibres. With the presence of salts and certain other impurities,especially calcium interference with the strand interactions could increase and caution is used to avoid inclusion in its production.

WOVEN FABRICS

A single layer of woven fabric has uniform length and width on the very small orders of dimension. For a plain weave, yarns are interlaced between two types of woven arrangements called “warp” and “weft,” which are perpendicular to each other The geometrical arrangement of woven fabrics is key to the efficient working of the

vest. Woven fabrics also known as “crimped fabrics” as their yarns of one direction are often bent and coinciding with the yarns in the other direction. Warp yarns run parallel to the edges of the fibre.

The weft yarns run across the fabric width. These undulations are referred to as crimp. Common failures associated with weaving are blowing, local yarn rupture, fibrillation, yarn pull out and bowing.

MANUFACTURING PROCESS

Initially, all polymers used for this operation go through a polymerization process. For instance, is synthesized in solution from the monomers 1,4- phenylene-diamine (para-phenylenediamine) and terephthaloyl chloride in a condensation reaction yielding hydrochloric acid as a by product

Fibre manufacturing is done through the spinning process which can be a gel, dry, wet and melt related process. In this process ,a polymer solution.The steps in the manufacturing of life vests are similar to those employed in the garment, varying in some specifics for instance raw materials and, more importantly, safety specifications. The operations generally employ a cut-fit-trim approach in order to effectively start and end the process effectively.Now-a-days in an automated CNC machines as many as 100 life vests may be manufactured simultaneously in an automated manufacturing processes.

Spinning is the process of extruding the polymer solution through tubes uniform in cross-section and small in diameter termed as spinnerets. Spinnerets produce manmade or synthetic fibres involving dies are in close resemblance with showerheads. The polymers (and solvents, if present) are forced through holes in the spinnerets.

After the exit of polymer from the spinneret, the polymers are solidified, forming fibres which have uniform and consistent diameters and cross-sectional shapes with nearly unlimited lengths. The fibres are then stretched and drawn to rollers. Stretching further enhances the fibre tensile strength and toughness properties by altering the molecular chains.

Spinning processes may vary with the type of polymer used. For instance, thermoplastic polymers are manufactured via melt spinning, and thermoset polymers require dissolution in a solvent. Kevlar and Dyneema and fibres are produced by gel spinning; nylon (polyamide), Vectran (liquid crystal polyester), and PET (polyethylene terephthalate) fibres are formed by melt spinning; and PBI (poly benzimidazole) fibres are made by dry spinning.

Additional processes following the spinning process are also applied to fibres including sizing. Sizing are surface treatment agents applied to the fibres in order to-

(1)improve final performance of the product (2)for better handling reduce abrasion(3) exercise restraint over moisture absorption (4) protection of the fibres from environmental factors.

(5) increase compatibility for bonding with matrix materials in fibre-reinforced composites.

Followed by spinning,

Kevlar is further machined by manufacturer by sending the spinned cloth to sizable rolls and is unrolled on large sized cutting tables. The fabric is first unrolled onto a cutting table that must be long enough to allow several panels to be cut out at a time

The process then involves a cut sheet which is then placed on the layers of cloth. Now-a-days, for maximum saving and utility of the material, manufacturers use computer graphics.

Using a hand-held machine that performs like a jigsaw and is similar to that on the end of a pizza cutter, a worker cuts around the cut sheets to form panels, which are then placed in a precise way as stacks

Sewing the panels

Now-a-days due to prior cutting and stacking of the different layers, vests generally do not require sewing, as its panels are usually just cut and stacked in layers that go into tight fitting pouches in the vest, a bulletproof vest made from Kevlar can be either quilt-stitched or box-stitched.

-Quilt-stitching forms small structures of cloth which is prior to this operation are seperated by stitching, whereas box stitching forms a large single box in the middle of the vest. Quilt-stitching though is a time consuming and labour dependent process but provides a rigid panel that is hard to mobilize from high exposure areas.

-Box-stitching, on the other hand, is fast and easy and allows complete free movement of the vest.

Finishing

Panels are used to sew the shells on the shells for the panels are sewn together on the same factory floor using standard industrial sewing machines and typical techniques for sewing. The panels are then slipped inside the shells, and the accessories such as the strap are sewn on.

Testing

Bulletproof vests are tested in both wet and dry conditions. This is done because the fibres perform differently under wet conditions. Vest during testing conditions is placed and wrapped around a dummy made of clay with properties similar to that of a human flesh. A firearm with required weapon calibre and bullet size is then shot at a velocity suitable for the classification of the vest. This method of shooting forms a wide triangle of bullet holes. The vest is then turned upside down and shot the same way, this time making a narrow triangle of bullet hole

Detailed process of vest fibre manufacturing

ONGOING INNOVATIONS IN VEST TECHNOLOGY

There are various ongoing processes in order to improve the efficacy of vests. Majority of these processes aim at altering the microscopic properties of the fibre in order to increase its strength by either adding material or by altering the textile processes in order to change the weaving pattern. The areas of research currently are-

(1)Use of shear-thickening fluid-shear thickening fluid in its normal condition behaves as a liquid when under no pressure. After encountering the high speed of a ballistic bullet, the fluid behaves like a solid under high mechanical stress or shear only in a few milliseconds.

The fluid basically used for this processes is a colloid made of tiny particles which repel each other under normal conditions but under mechanical shear or stress they overcome their repulsive forces and

end up together. After the removal of mechanical stress the forces dissipate and they seperate again. The fluid used for this operation is either silica ,which is a component of sand and quartz or polyethylene glycol which is a simple polymer.

Now-a-days, MAGNETORHEOLOGICAL fluid are also being used which comprise of any type of oil with suitable viscosity .It also consists of Iron which comprises of 20-40% of the volumetric weight of the fluid. Its special characterstics is that under the influence of magnetic field, the MR fluid aligns itself along the field lines.

(2)Carbon nano-tubes-currently bullet proof vests work on the principle where multiple layers of polymer are stacked together and

their ultimate goal is to absorb the energy of the projectile .If we consider polymer matrix composites ,the fibre is unable to absorb easily due to restrictions resulting from surrounding resin.

In order to deal with this shortcoming of PMC composites, carbon nanotubes are a viable alternative possibly due to its strain at yield point i.e. 16% and correspondingly its elastic modulus. Due to its high specific gravity, it can deal with projectiles at almost 4000 m\s and henceforth are capable of storing high magnitudes of energy moreover providing protection against sudden trauma attacks on the personal by improved puncture and penetration resistance. Currently models are being developed where a single layer of single walled carbon nanotubes into the matix along with any epoxy resin.

Carbon nano tubes have also incorporated increased fracture toughness in the vests. Recent studies have shown that incorporation of CNT IN silica or alumina can enhance the fracture toughness of the ceramic by over 90%.Their are various ways in which CNT can be incorporated in the whole setup of a vest

(3)MULTIDIRECTIONAL WEAVING-In order to have an effective working vest, manufacturers used and stressed on a single mode of strength but in practice there may be multiple categories, for strength.

The concept of woven fabrics was the first process to be introduced in the vest industry followed by unidirectional weaving which provides higher strength. The problem associated with unidirectional fabrics is that forces can only be applied parallel to the fibres and not at perpendicular or at other angles which will possibly result in ripping apart of the fabrics.

(4)Fibre materials-just like nano-composites, fibre materials can also act as strong reinforcement along with fibres. Hybrid materials can be classified based on the possible interactions connecting the inorganic and organic species.

Class 1 hybrid materials are those that show weak interactions between the two phases, such as van der walls forces and weak electrostatic forces.

Class 2 hybrid materials resemble strong chemical bonding such as covalent bonds.

PERFORMANCE EFFICIENCY METHODS TO CALCULATE THE OVERALL VEST EFFICIENCY

(1).Upon impact, longitudinal wave propagation and deflection occurs and hence wave propagation velocity “V” which is speed of sound in the material is-

C=(modulus/density)1/2

(2)Transverse wave speed-when the initial impact wave reaches the end of the fibre, it is reflected back as a tensile wave leading to flow of materials afterwards and leading to formation of tensile strain. However if the impact velocity is high enough, the fibres might not

have sufficient time to respond and ends up breaking. The lowest velocity for which it fails is called critical velocity.

V=C(e/1+e)1/2

e=individual fibre strain

(3)Wave amplitude or Impedance-during impact various strain waves are observed to be travelling between fibres and matrix. The amplitude of these waves is calculated by-

I=p*C

p=density of fabric

C=wave speed

(4)Depth of penetration-it is the depth to which the bullet has nserted upon impact.

P=L(p1/p2)

p1=density of projectile

p2=density of the target material L=length of projectile

Stress and transverse waves acting on a single fibre

CONCLUSION

REFERENCES

1.www.howstuffworks.com

2.www.acadmia.edu

3.