Introduction

Personal armor is the term used to describe items that are worn or carriedto provide an individual with protection from energy. In the military andlaw-enforcement environment, this energy is principally in the form ofimpact by non penetrating projectiles or blows, blast waves from explosions,and penetrating missiles.

Types of Ballistic Protective Materials

• Hard Body Armour• Soft Body Armour

Hard body armour• Hard body armour may be hard enough so that a

bullet or other high velocity fragment is deflected on impact.

• Advances in ceramic/composite armour materials have resulted in lightweight body armour systems that provide a significant amount of protection

• The main disadvantages of the steel containing hard body armour remain heavyweight and inflexibility.

Soft Body Armour

• Soft body armour with adequate ballistic protection is preferred due to their flexibility, lightweight and comfort properties.

• .Several researches on soft body armour have investigated the behavior of fibres, yarns and fabrics during ballistic impacts

General Principles of ProtectionMechanics

The technical approaches for stopping penetrating missiles and mitigating non penetrating impacts are different. The underlying principles of minimizing the effects of energy transfer from a projectile are:

• – absorbing energy in armors by making it do work on materials before it gains access to the body—breaking materials, stretching them, or compressing them all use energy, or extend the time over which it is applied to the body;

• – redistributing the energy so that other materials or the body wall are more able (due to the reduction in pressure—force per unit area) to withstand the total energy.

Five types of energy absorption occur to fulfill the basic function of persona Body armour

• Kinetic energy of the out-of –the plane fabric movement• Stain energy accumulated in the yarn of the pyramid• Kinetic energy of the fabric moving towards the point of

initial impact• Stain energy accumulated in the original fabric plane

between the pyramid and the longitudinal wave form• Energy dissipated as heat by friction fibre/ fibre fibre

/projectile

Mechanisms of Projectile Defeat

Some projectiles can be defeated using flexible textile materials and some require a rigid structure. When any projectile of a mass m, travelling at a velocity v, impacts a target, it possesses kinetic energy

(KE = 1/2 mv2). The energy acts over a very small impact area and enables the projectile to perforate materials. The term often used to describe the ability of a projectile to defeat its target is its kinetic energy density, that is, the projectile energy per unit area at the impact site.

Mechanisms of Resistance• The yarns used in woven ballistic textiles have a high

specific strength and a high modulus. These properties mean that the fibers are particularly difficult to break. The high modulus allows the energy to be dissipated as a longitudinal stress wave, that is, along the yarn. As the projectile impacts a certain point on one of the yarns, the energy imparted will travel along the yarn. When it meets the cross-over, it divides via a number of possible mechanisms.

• It can continue along the yarn (transmission), • it can be reflected back along the yarn, or it can travel

along the crossing yarn (diversion).

Material Selection Carbon Fibres• Very high modulus and tensile strength• Energy absorption rate increases steadily

with the modulus. High modulus is needed to achieve high rate of wave propagation

• Carbon fibres being high-modulus and high tenacity fibres but having low strain-to-failure would shear readily under compression

• This factor make the carbon fibre unusable

Ultra High Modulus Polyethylene UHMPE• High damage tolerance• Fatigue resistance• Ability to fail in shear or compression without losing great

amount of tensile strength• It is the strongest and almost the stiffest commercially

available fibre• Large modulus increase with the increase rate of

deformation, particularly at elevated temp.• Due to low density the velocity of longitudinal velocity is

very high• All the above factors make the UHMPE the most sought

material for ballistic protection• But the disadvantage of this fibre is low temp. resistance

Aramids• Good dynamic energy absorption• High specific strength and modulus• Excellent thermal properties• High tenacity and moderate elongation• Kevlar provides a service life of more than

100 year• Creeps little at low loads but creeps under

relatively high stress

Ceramics

• Though ceramics have very high density values as material, they are still suitable for ballistic application

• High compressive strength and hardness• Dynamic stress level are higher• Sharp point of the projectile is quickly

eroded• Leaving less effective blunt, this would

greatly reduce the energy of impact

Influence of yarn Structure on Ballistic Protection

• Friction plays an important role in the ballistic protection. Corona treatment/ plasma etching is the current practice

• Filaments with high coefficient of friction should be combined by core spun with fibres having high tensile strength and modulus and coefficient of friction.

• Fine yarns perform better than coarser yarn.• A twist factor of 1.1 is found to be suitable for

Kevlar yarns.

Fabric Design for Ballistic Protection

• If the weave is weave is too tight or the fabric is too stiff, deflection will be restricted. Causing shear failure due to the concentration of the stress at the impact point.

• Too loose weave or a soft fabric having low yarn-to-yarn friction would allow the projectile to penetrate.

• .Generally warp with lower modulus and higher extension than weft with fabric having same stiffness in both warp and weft direction is preferred.

• For ideal ballistic protection, the non-oven felt has to have a high level of entanglement of long staple fibres with only a minimum degree of needling

Finishers for Ballistic Protection

• Lamination of the fabric has been found to increase the anchoring strength of the yarn. As consequence, the yarns hit by bullet do not pull from the fabric and coating impeded in their sideward displacement by piercing projectile.

• For UHMPE, oxygen plasma treatment reduced the delaminating during impact with a corresponding reduction in energy absorption.