Ballistic Protection of Textile

Ballistic Protection Textile
Birendra Acharya
B.Tech in Textile
Vignan University, Vadlamudi, Andhra Pradesh, India
Email: acharyaraja3@gmail.com




INTRODUCTION
In general, composite materials are formed when two or more chemically distinct materials are combined so that a distinct interface separates the components (as opposed to alloys). Each of the constituent materials has its own specific physical properties, but the resulting composite has properties different from each material alone. It is desirable for the composite to take advantage of selected properties from each constituent. Of several types of composite materials, the category of particular interest for the ballistic protection is the continuous fiber-reinforced or fibrous composite. This type consists of one phase, which is usually much stronger (fiber), and the other phase (matrix). This combination leads to anisotropic properties which provide the capability of designing for specific characteristics such as high strength in one critical direction. 
 
Composite materials containing continuous unidirectional high-performance fibers are characterized by a high specific longitudinal stiffness and strength, which makes them especially interesting for weight saving applications. When composites are used for structural applications, usually stacked plies with different fiber orientations are used allowing for considerable stiffness and strength in more than one direction. Composite and textile armor systems are also increasingly being utilized as impact protection materials in weight critical environments. A typical application is personal protective items, a threat either being that of fragments of exploded shells or bullets of handguns. The use of composite and textile armor systems can result in a reduction in weight while maintaining the impact performance, or the increased impact performance for the given weight. Ballistic impact resistance of fiber-reinforced composites with high modulus and high strength fibers has been under extensive investigation due to specific properties of such fibers [1, 2]. 
 
The majority of the composites used in body armor systems take the form of textile fabrics or unidirectional tape of high modulus and high strength fibers embedded in a variety of matrix resins. Fibrous armor has the importance for several reasons. Since a man uses clothing in normal life, protective devices that can be incorporated into such clothing provide the most comfortable, compatible and inconspicuous method of providing such protection. The second reason fibers are important is that they provide the greatest strength and modulus properties that can be obtained from a given material. In the case of polymers, this is due, mainly, to the drawing operation which orients the molecules along the fiber axis, increasing strength and stiffness and providing a natural crack arresting mechanism. There are many natural and synthetic fibers, which are used for ballistic protection, but only two types of synthetic fibers can be regarded as high-performance – aramid and UHMWPE (Ultra High Molecular Weight Polyethylene) fibers. UHMWPE fibers, also known as HPPE (High Performance Polyethylene) fibers, are characterized with parallel molecules orientation along the fiber axis greater than 95% and a high level of crystallinity as opposed to conventional polyethylene fibers. This results in fibers with a very high strength and modulus of elasticity. In this study, we have investigated HPPE composites based on woven fabrics and unidirectional (UD) fibers under the high-speed ballistic impact. The fibers we have used were produced by the inventor of these fibers, the Dutch company DSM, and are known under the trade name Dyneema.

HISTORY
In 1538, Francesco Maria della Rovere commissioned Filippo Negroli to create a bulletproof vest. In 1561, Maximilian II, Holy Roman Emperor is recorded as testing his armor against gun-fire. Similarly, in 1590 Sir Henry Lee expected his Greenwich armor to be "pistol proof". Its actual effectiveness was controversial at the time. The etymology of "bullet" and the adjective form of "proof" in the late 16th century would suggest that the term "bulletproof" originated shortly thereafter. During the English Civil WarOliver Cromwell’s Ironside cavalry were equipped with Capeline helmets and musket-proof cuirasses which consisted of two layers of armor plate (in later studies involving X-ray a third layer was discovered which was placed in between the outer and inner layer). The outer layer was designed to absorb the bullet's energy and the thicker inner layer stopped further penetration. The armor would be left badly dented but still serviceable. One of the first recorded descriptions of soft armor use was found in me dieval Japan, with the armor having been manufactured from silk.

Simple ballistic armor was sometimes constructed by criminals. The suits were roughly made on a creek bed using a makeshift forge and a stringy-bark log as a muffled anvil. They had a mass of around 44 kg (96 lb), making the wearer a spectacular sight yet proved too unwieldy during a police raid at Glenrowan. Their armour deflected many hits with none penetrating, but eventually was of no use as the suits lacked protection for the legs and hands.

1ST WORLD WAR

The first official attempts at commissioning body armor were made in 1915 by the British Army Design Committee, in particular a 'Bomber's Shield' for the use of bomber pilots who were notoriously under-protected in the air from anti-aircraft bullets and shrapnel. The Experimental Ordnance Board also reviewed potential materials for bullet and fragment proof armor, such as steel plate. A 'necklet' was successfully issued on a small scale (due to cost considerations), which protected the neck and shoulders from bullets traveling at 600 feet per second with interwoven layers of silk and cotton stiffened with resin. The Dayfield body shield entered service in 1916 and a hardened breastplate was introduced the following year.The British army medical services calculated towards the end of the War, that three quarters of all battle injuries could have been prevented if an effective armor had been issued.

The French also experimented with steel visors attached to the Adrian helmet and 'abdominal armor' designed by General Adrian. These failed to be practical, because they severely impeded the soldier's mobility. The Germans officially issued body armor in the shape of nickel and silicon armor plates that was called 'Lobster armor' from late 1916. These were similarly too heavy to be practical for the rank-and-file, but were used by static units, such as sentries and occasionally the machinegunners. An improved version, the Infantrie-Panzer, was introduced in 1918, with hooks for equipment.
Testing a bulletproof vest in Washington, D.C. September 1923
The United States developed several types of body armor, including the chrome nickel steelBrewster Body Shield, which consisted of a breastplate and a headpiece and could withstandLewis Gun bullets at 2,700 ft/s (820 m/s), but was clumsy and heavy at 40 lb (18 kg). A scaledwaistcoat of overlapping steel scales fixed to a leather lining was also designed; this armor weighed 11 lb (5.0 kg), fit close to the body, and was considered more comfortable.

2ND WORLD WAR
In 1940, the Medical Research Council in Britain proposed the use of a lightweight suit of armor for general use by infantry, and a heavier suit for troops in more dangerous positions, such as anti-aircraft and naval gun crews. By February 1941, trials had begun on body armor made of manganese steel plates. Two plates covered the front area and one plate on the lower back protected the kidneys and other vital organs. Five thousand sets were made and evaluated to almost unanimous approval - as well as providing adequate protection, the armor didn't severely impede the mobility of the soldier and were reasonably comfortable to wear. The armor was introduced in 1942 although the demand for it was later scaled down. The Canadian Army in northwestern Europe also adopted this armor for the medical personnel of the 2nd Canadian Infantry Division.

RECENT YEARS
Kevlar soft armor had its shortcomings because if "large fragments or high velocity bullets hit the vest, the energy could cause life-threatening, blunt trauma injuries in selected, vital areas. Ranger Body Armor was developed for the American military in 1991. Although it was the second modern US body armor that was able to stop rifle caliber rounds and still be light enough to be worn by infantry soldiers in the field, it still had its flaws: "it was still heavier than the concurrently issued PASGT (Personal Armor System for Ground Troops) anti-fragmentation armor worn by regular infantry.
Armor
MECHANISM
Ballistic vests use layers of very strong fibres to "catch" and deform a bullet, mushrooming it into a dish shape, and spreading its force over a larger portion of the vest fiber. The vest absorbs the energy from the deforming bullet, bringing it to a stop before it can completely penetrate the textile matrix. Some layers may be penetrated but as the bullet deforms, the energy is absorbed by a larger and larger fiber area.While a vest can prevent bullet penetration, the vest and wearer still absorb the bullet's energy. Even without penetration, modern pistol bullets contain enough energy to causeblunt force trauma under the impact point. Vest specifications will typically include both penetration resistance requirements and limits on the amount of impact energy that is delivered to the body.Vests designed for bullets offer little protection against blows from sharp implements, such as knives, arrows or ice picks, or from bullets manufactured of non-deformable materials, e.g., those containing a steel core instead of lead. This is because the impact force of these objects stays concentrated in a relatively small area, allowing them to puncture the fiber layers of most bullet-resistant fabrics.
Mechanism of ballistic vest
Textile vests may be augmented with metal (steel or titanium), ceramic or polyethylene plates that provide extra protection to vital areas. These hard armor plates have proven effective against all handgun bullets and a range of rifles. These upgraded ballistic vests have become standard in military use, as soft body armor vests are ineffective against military rifle rounds. Prison guards and police often wear vests which are designed specifically against bladed weapons and sharp objects. These vests may incorporate coated and laminated para-aramid textiles or metallic components.(16)The primary factors which influence the performance of bulletproof or protective material are strength, modulus and elongation at break, deformability of projectile and the velocity of transverse shock wave in the fiberNo armour designi is suitable for all the situations and the performance of the protective system depends on the interaction of its various components. Hence, it is important to understand the mechanism of the ballistic protection by which ballistics operate and sometimes melting and fusion at the interlacement points has alo been noted.(3 )

FIBER USED
Most anti-ballistic materials, like bullet proof vests and explosion-proof blankets, are currently made of multiple layers of:-
  • Kevlar fibers
  • Dyneema fibers
  • Twaron fibers
KEVLAR FIBRES
Kevlar is the registered trademark for a para-aramid synthetic fiber, related to other aramids such as Nomex and Technora. Developed by Stephanie Kwolek at DuPont in 1965, this high-strength material was first commercially used in the early 1970s as a replacement for steel in racing tires. Typically it is spun into ropes or fabric sheets that can be used as such or as an ingredient in composite material components. Currently, Kevlar has many applications, ranging from bicycle tires and racing sails to body armor, because of its high tensile strength-to-weight ratio; by this measure it is 5 times stronger than steel. It is also used to make modern drumheads that withstand high impact. When used as a woven material, it is suitable for mooring lines and other underwater application A similar fiber called Twaron with roughly the same chemical structure was developed by Akzo in the 1970s; commercial production started in 1986, and Twaron is now manufactured by Teijin.(11,12).
Poly-paraphenylene terephthalamide – branded Kevlar – was invented by Polish-American chemist Stephanie Kwolek while working for DuPont, in anticipation of a gasoline shortage. In 1964, her group began searching for a new lightweight strong fiber to use for light but strong tires. The polymers she had been working with at the time, poly-p-phenylene-terephthalate and polybenzamide, formed liquid crystal while in solution, something unique to those polymers at the time.

The solution was "cloudy, opalescent upon being stirred, and of low viscosity" and usually was thrown away. However, Kwolek persuaded the technician, Charles Smullen, who ran the "spinneret", to test her solution, and was amazed to find that the fiber did not break, unlikenylon. Her supervisor and her laboratory director understood the significance of her accidental discovery and a new field of polymer chemistry quickly arose. By 1971, modern Kevlar was introduced. However, Kwolek was not very involved in developing the applications of Kevlar.(12)

STRUCTURE & PROPERTIES:
Kevlar: bold represents a monomer unit,dashed lines indicate hydrogen
When Kevlar is spun, the resulting fiber has a tensile strength of about 3,620 MPa, and a relative density of 1.44. The polymer owes its high strength to the many inter-chain bonds. These inter-molecular hydrogen bonds form between the carbonyl groups and NH centers.

Kevlar's structure consists of relatively rigid molecules which tend to form mostly planar sheet-like structures rather like silk protein.(13)
  1. When Kevlar is spun, the resulting fiber has a tensile strength of about 3620 MPa, and a relative density of 1.44.
  2. The polymer owes its high strength to the many inter-chain bonds. These inter-molecular hydrogen bonds form between the carbonyl groups and NH centers.
  3. Additional strength is derived from aromatic stacking interactions between adjacent strands. These interactions have a greater influence on Kevlar than the vander Walls interactions and chain length that typically influence the properties of other synthetic polymers and fibers such as Dyneema.
  4. The presence of salts and certain other impurities, especially calcium, could interfere with the strand interactions and caution is used to avoid inclusion in its production. Kevlar's structure consists of relatively rigid molecules which tend to form mostly planar sheet-like structures rather like silk protein
THERMAL PROPERTIES :-
Kevlar maintains its strength and resilience down to cryogenic temperatures (-196°C); indeed, it is slightly stronger at low temperatures.

At higher temperatures the tensile strength is immediately reduced by about 10-20%, and after some hours the strength progressively reduces further.

For example at 160°C about 10% reduction in strength occurs after 500 hours. At 260°C 50% strength reduction occurs after 70 hours.

DYNEEMA FIBER
UHMWPE (Ultra-high-molecular-weight polyethylene) is a type of polyolefin. It is made up of extremely long chains of polyethylene, which all align in the same direction. It derives its strength largely from the length of each individual molecule (chain). Van der Waals bonds between the molecules are relatively weak for each atom of overlap between the molecules, but because the molecules are very long, large overlaps can exist, adding up to the ability to carry larger shear forces from molecule to molecule. Each chain is bonded to the others with so many van der Waals bonds that the whole of the inter-molecule strength is high.
In this way, large tensile loads are not limited as much by the comparative weakness of each van der Waals bond.When formed to fibers, the polymer chains can attain a parallel orientation greater than 95% and a level of crystallinity from 39% to 75%. In contrast,Kevlar derives its strength from strong bonding between relatively short molecules.

PROPERTIES:-
Dyneema has the highest level value of the specific strength among commercialized organic strong fibers.. A 10mm diameter rope of Dyneema can bear up to a 20ton(theoretical value)-weight load.As indicated by its chemical formula -(CH2-CH2)n- Dyneema is formed from carbon (C) and hydrogen (H). Consequently, even if Dyneema is burned all that remains is water (H2O) and carbon dioxide (CO2) and no harmful substances are released. Following are the some of the properties of Dyneema fiber:-
  1. Dyneema is the only super-fiber with a density below 1.0 (the fiber can float on water).
  2. Dyneema is a suitable material for protective applications or sport composites applications.
  3. Dyneema exhibits excellent flexibility and excellent abrasion resistance.
  4. Dyneema has excellent light stability, chemical resistance in a wide pH range and highly crystallized structure .
  5. Excellent Chemical Stability (treat with 23 × 2000 hrs)
  6. Dyneema has a feature that it expands by lowering the temperature.
  7. Dyneema® has excellent vibration damping characteristics.
  8. Fundamentally a form of polyethylene, Dyneema possesses the same chemical properties, making it an outstanding insulator.
TWARON FIBER
Twaron is Teijin Aramid’s flagship para-aramid, a high-performance man-made fiber. Offering well-balanced performance in terms of mechanical properties, chemical resistance and thermal stability, it is widely recognized in many industries as an extremely valuable component with excellent durability.Our experience of more than 30 years, not only guarantees a technically mature product, it is also the basis for developments – often in close cooperation with our customers to tailor Twaron to the specific requirements in various applications.
Structure of Twaron fiber
Twaron is suitable for a virtually unlimited range of challenging applications, including ballistic protection, heat and cut protection, the oil and gas industry, the automotive industry and optical fiber cables to name just a few of its many uses.
PROPERTIES:-
  1. Twaron combines the following characteristics, which distinguishes it from other synthetic fibers:
  2. High strength (excellent strength-to-weight properties)
  3. High modulus
  4. High dimensional stability
  5. Excellent heat, cut and chemical resistance
  6. No melting point
  7. Low flammability
  8. Non-con1ductivity
DESIGNING OF PROTECTIVE FIBER
Principally, two classes of materials, namely fibrous materials and ceramics, have emerged having great potential in the designing of the ballistic protective garments / composites.

Depending on the mode of their application, ballistic protective clothing can be broadly divided into soft armor made from textile material and composite laminate armor or hard armor.

SOFT ARMOR:-
Soft armor is constructed from multiple layers of woven fabric without a resin binder, sewn together with meander or crosswise seam.The layers are sewn together with high tenacity aramid sewing threads which seem to perform better if they are close together due to the fact that these sewing threads themselves participate in the energy absorption.It has been observed that very high stitch frequencies of the order of 10 stitches / cm² can reduce the fabric yarn tenacity by up to 40%. (3,4 )It also been proved that for best ballistic protection, resin content should be 20-25 wt%. ( 3,4 )Some ballistic material which is fire resistant, smoke resistant, non-toxic, nonconductive , self supporting and stiff.

HARD ARMOR:-
Composite laminate armor or hard armor consists of multilayered fabrics combined together with a resin binder.Another class of hard armor uses armor plates made out of ceramics and fiber reinforced plastics ( FRP ) of about 10 mm thickness.Bulletproof vests are generally so tailored that they protect the body all round, the front of the vest may be stronger than the back assuming that a shot will generally come from the front.In a US patent, coppage disclosed a bulletproof dress shirt that is adjustable to accommodated wearers of different sizes.It is made from standard dress shirt fabric and has inner layers on the front and back panels that are made from bicomponent materials to draw perspiration away from the wearer.(4)Finally, it should be born in mind that no single ballistic protective garment can provide complete protection against all types of projectiles.It is a compromise between various factors like the extent of protection required, cost, weight and comfort.

TESTING AND STANDARDS
The first NIJ standard was published in 1972, and underwent four revisions through the turn of the 21st century. The fifth revision to the NIJ ballistic standard, following the Forest Hills shooting, was released in draft form in 2007, to be finalized and published in 2008. The standard was very heavily revised, more so than all the other revisions combined. Among other things, it called for environmental testing and water submersion, in order to subject test armors to simulated conditions of heat, moisture, and mechanical damage. Earlier versions of the standard provided no testing for environmental conditions, and only a spray of water vice submersion. The revised standard also addressed the number of samples tested for certification, vest sizing, and made changes to the armor classifications and threat velocities. As shown in Chart 1, the 5 types of armor vary by bullet caliber and velocity. One of the major changes in the NIJ standard was to eliminate one of the classes of armor that was no longer considered effective protection. Additionally, three of the five remaining classes were changed in either bullet type or velocity or both, in order to expose the armor to velocities and rounds that are increasingly likely to be encountered in the line of duty. Lastly, a category was added to the ballistic tables to allow accurate testing and grading of used armor. Due to the immediate testing that followed Forest Hills, it was obvious that heat, moisture, light, and mechanical agitation reduced the effectiveness of Nylon -based armor. Earlier versions of the NIJ standard assumed that armor was new, which was the mode of testing. No thought had ever been given to the degradation of armor over time. The revised standard had specifications for mechanical agitation, humidity levels, water immersion, and temperature.
REFERENCES
  1. http://www.researchgate.net/publication/279187784_Mechanics_of_Ballistic_Resistance_of_Textile_Assemblies_and_its_Measurement
  2. INDIAN JORNAL OF FIBER & TEXTILE RESEARCH, VOL:- 22 , 1997, PP- 274-291,
  3. International Journal of Scientific & Engineering Research, Volume 4, Issue 5, May-2013, ISSN 2229-5518 IJSER
  4. http://www.dupont.com/products-and-services/fabrics-fibers-nonwovens/fibers/articles/kevlar-properties.html
  5. www.slideshare.net/vijaysagar1238/kevlar-properties-and-applications
  6. www.christinedemerchant.com/aramid_characteristics.html
  7. www.toyobo-global.com
  8. www.marlowropes.com/technical/physical-properties.html
  9. Reaugh, J.E. et al. “ Impact Studies of Five Ceramic Materials and Pyrex.” International Journal of Impact Engineering 23, 1999: 771-782.
  10. http://www.teijinaramid.com/wp-content/uploads/2012/02/1090308_Twaron-productbrochurefinal_051.pdf
  11. Tatsuya Hongū, Glyn O. Phillips, New Fibers, Ellis Horwood, 1990, p. 22
  12. J. K. Fink, Handbook of Engineering and Specialty Thermoplastics: Polyolefins and Styrenics, Scrivener Publishing, 2010, p. 35
  13. Kevlar KM2 Technical Description. dupont.com. Retrieved on 2012-05-26.
  14. Quintanilla, J. (1990). "Microstructure and properties of random heterogeneous materials : a review of theoretical results". Polymer engineering and science 39: 559–585.
  15. Michael C. Petty, Molecular electronics: from principles to practice, John Wiley & Sons, 2007, p. 310
  16. https://en.wikipedia.org/wiki/Bulletproof_vest> overview
  17. http://www.strategicstandards.com/files/2008/BodyArmor08.pdf 
 

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