Developments in design of ballistic protective systems - I

Ballistic protection systems exist in the form of vests, jackets and helmets for personal protection and also for protection of vehicles. Design of the ballistic protection system depends on the threat level of the victim, says Dr A K Rakshit and M A Hira

Mankind seeks ballistic protection since time immemorial. The earliest means of protection was from heavy and stiff metallic suit of armour. The next generation material was steel. However, in these systems comfort aspects were entirely overlooked. Over the period situations have been changed. Typical war of sword and stick is over. Today we have to provide protection from bullets, splinters, daggers etc. at varying threat levels. Combining the comfort and threat level, today’s design of ballistic armours are made of textile materials.

In the 19th century, soldiers protected themselves by wearing a couple of dozen layers of wetted fine silk shirts. With advent of synthetic fibre, ballistic protection was synonymized with nylon (polyamides) fabrics. Technological developments in high performance fibres resulted in Aramids (aromatic polyamides) that have replaced nylons for the purpose. Several other high performance fibres Ultra High Modulus Polyethylene (UHMPE), p - phenylene-2, 6-benzobisoxazole (PBO), have come to the service.

Classification of threat levels

The ballistic protective fabrics belong to the class of protective textiles in the parlance of technical textiles. Ballistic protection involves protection of the wearer’s body and eyes against projectiles and fragments of various shapes, size and impact velocity. The projectiles are a part of ammunition shot through weapons such as pistols, revolvers and rifles. The fragments originate from explosions or pellets of shotguns and are solid and diverse in shape. Thus ballistic protective garments would find applications in the form of garments like vests and jackets, helmets and vehicle reinforcement depending upon the nature and type of threat the victim is likely to be subjected to.

The threat level of the victim depends on the properties of projectile/fragment. It is important to note that a ballistic protective material, which is effective against a projectile at a certain impact, may not be effective against the same projectile if the impact speed is changed. The National Bureau of Standards’ Law Enforcement Standards Laboratory, USA, has identified six formal and one special type armour, as Type I, IIA, II, IIIA, III, IV and Special Type Armour. Each armour type provides multiple-hit or single-hit ballistic protection against a specified bullet at a specified velocity. For example, Type III provides protection against 7.62 mm AK 47/56 bullets with nominal masses of 123 gr., impacting at a velocity of 725 metres per second. Special type Armour covers any other protection level that is not covered in any one of the six armour types.

Mechanics of ballistic protection

The mechanism, by which ballistic protection is achieved, involves absorption of energy of the projectile/fragment. These projectiles / fragments have large kinetic energy decided by their mass and projected velocity. At the time of striking the target velocity of the projectile/fragment are equally important. Ballistic protective garments can dissipate the energy of the projectile in the following manner:

• Deformation of the fabric layers under impact
• Punching or cutting of the yarns
• Dissipation of impact energy from one layer to the next fabric layer, thereby gradually reducing its impact energy.

These mechanisms act differently in different fabrics and even in a given fabric different projectiles have differing effects.

For example, when a projectile hits a woven fabric a shock wave or strain wave is introduced in the fabric, which spreads through its yarns. The primarily impacted yarns interact with other yarns by means of couplings at the cross-ver points of the fabrics. The strain wave can thus be pushed over a large number of yarns. The positive effect of this mechanism is that the energy will be absorbed over a large area. The velocity of the strain wave dissipation of energy would depend upon the modulus of the constituent of the fabric, i.e., the fibres.

Therefore, the factors that influence the energy absorption characteristics of ballistic protective systems can be listed as below:

• Protective fabric design
• Properties of the constituent material
• Fabric design parameters
• Fabric density
• Impact conditions
• Projectile mass
• Projectile geometry
• Striking velocity

Design aspects

It is observed that the ballistic protective clothing is required to satisfy the following conditions.

1. Light in weight
2. High bulk
3. Comfortable to wearer
4. Facilitate body movement

Accordingly, the choice of fibre, design of fabric, fabric finish, binding resin and making-up is carefully done to suit the threat level the system is targeted to serve. Subsequent sections cover these aspects.

Fibres used in ballistic protection

It is required that the component fibres should have high strength, high modulus and low elasticity. High strength and high modulus would enable absorption of projectile impact, while low elasticity would prevent indentation of the body and subsequent bursting. Apart from these basic requirements, the fibres should have high melting point, high cut resistance, low thermal and electrical conductivity, resistance to heat, water, chemicals and ultraviolet radiation.

The basic criterion of ballistic protection is spreading the projectile load over a large area. This is necessary for ballistic protection; it serves better to dissipate the energy of impact by deformation and breaking the material. Nylon, which has a high work of rupture, was thought of to be the ideal for this purpose. However, simple toughness is not just sufficient for ballistic protection. The use of Aramid came to rise due to the fact that ballistic protection is a combination of transverse velocity propagation and the tensile properties of the protective material. Presently the Ultra High Modulus Polyethylene (UHMPE) is gaining popularity because of its advantages over Aramid.

Nylon

This fibre has been traditionally used in ballistic application. However, the disadvantage of these fabrics is that they are very heavy and on wetting the ballistic performance is marred. These were suitable only for the low velocity arms and not for the modern day weapons.

Carbon fibres

These fibres also have high modulus and high tensile strength. The fibre modulus beyond 500 gpd is very beneficial towards higher energy absorption. However, this also results in higher brittleness of the fibre and reducing the ability of the fibre for absorbing strain-energy. Hence the carbon fibres become unsuitable for the ballistic end use. Also the compressive strain of the carbon fibres is low, which makes them unsuitable for the application.

Ceramic fibres

Although ceramics have high density as metals, they are suitable for ballistic applications due to their high compressive strength and hardness. Since the dynamic stress limits of ceramics are higher, sharp point of the projectiles is quickly eroded, leaving lower mass. This would greatly reduce the energy of impact, further enhancing the effectiveness of ceramics. However, inherent brittleness and lack of flexural strength of ceramics renders them incapable of being used alone as single material for the fabrication of armour.

Aramid

Offers good ballistic protection along with reduced weight and bulk of the system. It has replaced steel pot helmets and nylon vests, since Kevlar (aramid) is five times stronger than steel on an equal weight basis and significantly stronger than nylon. The energy dissipation characteristics are also better in case of Kevlar. The aramid fibre named Twaron (Akzo) is nowreplacing Kevlar in ballistic application.

Ultra High Modulus Polyethylene (UHMPE)

These fibres are produced by geal spinning are manufactured by Allied Signal and DSM in various grades with trade names Spectra and Dyneema respectively.

The Spectra fibre has three grades Spectra 900, Spectra 1000 and Spectra 2000; with increasing order of improved tensile characteristics and good chemical resistance.

The Dyneema fibre has parallel orientation of molecules and high crystalinity. This results in ultra high strength in the fibre. The Dyneema SK60 grade is used for general purpose while SK66 is meant for ballistic use. The SK76 is the latest grade of Dyneema fibre meant for the ballistic application.

UHMPE fibres have lower density as compared to the aramids and hence the longitudinal stress wave propagation velocity is very high. The aramid fibres having higher friction coefficient are more suitable for absorbing the impact of small diameter projectiles. However, in case of other projectiles the UHMPE fibres out perform the aramids.

Thermoplastic Liquid Crystal Polymer (TLCP)

Manufactured by Hoechst Cleanese under the trade name “Vectran”, this fibre has been found suitable for ballistic application because of its high strength and rigidity, low moisture absorption, negative coefficient of thermal expansion and good chemical expansion and good chemical resistance. It is reported to be five times stronger than steel and ten times that of aluminum.

p-Phenylene-2,6-Bezobisoxazole (PBO)

PBO is the latest entrant into the field of ballistic protection. The fibre has tensile modulus higher than carbon, High Performance PolyEthylene (HPPE) or aramid fibres. It also has high heat and chemical resistance. The specific gravity is higher than HPPE and aramid fibres.

It may be said that UHMP fibre is third generation material of lighter weight as compared to the second generation p-Aramid fibres being used. Ballistic panels made with UHMPE are proven performers in multiple-hit, angle shot, and high velocity situations - meeting today’s threat head on. However, the temperature sensitivity of UHMPE fibre in comparison with Aramid (Kevlar) fibres is doubted. In this regard, study has been carried out by Allied Signal for this purpose. In experimentation, five identical panels of each of the armour systems were heated to five test temperatures ranging from 70øF to 230øF (21øC - 110øC) for one hour. Certain characteristics were revealed:

• Spectra fibre is sensitive to this temperature range and experiences loss in ballistic protection maximum to the extent of 4.92 per cent.
• Kevlar fibre remains unaffected ballistic performance.
• However, despite the slight loss in ballistic performance, Spectra armour system is far more superior in ballistic performance to Kevlar armour system.

Fabrication for ballistic protection

The important design considerations for ballistic garments are the selection of ballistic resistant materials, required degree of protection (projectile or bullet type, caliber, impact velocity), final weight of the uniform, wearer comfort and ease of movement. Each fibre and fabric type has unique ballistic properties. Depending on specific threat to be addressed, composite ballistic material can improve the performance of the protective system. Coated ballistic fabrics may also be used. Some manufacturers use a non-ballistic layer like steel or ceramics to increase the bling ballistic panels into a single unit such as bias stitching, around the edge of the panel, tack-stitching at several locations, biaxial stitching at several locations, biaxial stitching, or quilting the entire panel’.

In general, the ballistic protective systems can be categorised into two classes:

1. Hard armour systems

Composite laminate or hard armour consists of multi-layered fabric combined with resin binder. Another class of hard armour uses plates made out of ceramics, fibre reinforced plastics of about 10 mm thickness. The main function of such a vest is to reduce the shock effect on the body by absorbing energy of impact partially or completely. The bullet also deforms upon impact with reduction in kinetic energy, hence can easily be stopped by subsequent layers.

Selection of resin binder is important in the case of hard armour. The resins that have higher ductility perform better as regards energy absorption, tear initiation and propagation than brittle resins. Hence vinyl esters are more preferred to thermosetting epoxy resin. Evidence suggests that for the best ballistic performance, the resin content should be 20-25 per cent on weight of the fabric. A number of products and patents have been published on hard armour.

2. Soft armour systems

The soft armour is constructed from multiple layers of fabric without a meander or crosswise seam. Depending upon the calibre to be stopped and the yarn count, the number of fabric layers in making a bullet proof vest varies from 10 to 50 and weighs around 3 kgs. The layers are sewn together with high tenacity aramid yarns, which also contribute to the energy of the ballistic protective fabrics.

Fabric design for ballistic protection

The ballistic response of the fabric appears to be closely related to the ballistic response of the single yarns. Fabric construction particulars have a major role in the final ballistic protection.

The fabrics used for ballistic protection may be woven, knitted or non-woven. Each of the fabric design technique has its own merits and demerits.

1. Weaving

Woven textiles are most commonly used for ballistic protection. Their advantage is ease of development and suitability for the purpose. Majority of the woven fabrics for ballistic protection are woven from heavier denier yarns with minimum twist (spun twist). Fabric design are generally plain or basket. Plain weave gives maximum interlacements per unit area. Hence it is the most suitable for ballistic protection. But, other weaves like basket, satin, crowfeet, etc are also used depending upon the application.

Fabric density in woven designs is another important criteria. For instance, if the weave is too tight, or fabric is too stiff, deflection will be restricted, causing shear failure due to the concentration of stress at the impact point. While too loose a weave or a soft fabric, having low yarn to yarn friction, would allow the projectile to penetrate easily by pushing the yarn aside or giving too much deflection which can cause serious injuries to the wearer. Normally 20-25 layers of plain woven fabrics are required to provide adequate ballistic protection.

To be continued

(The authors are with SASMIRA, Mumbai)