Penetration of bulletresistant jackets and vests

One of the tasks the forensic firearms examiner is often called upon to perform is the testing and evaluation of bullet-resistant vests and jackets, generally called soft body armour.

As this aspect comes within the realms of terminal ballistics, it would be a good point to review the subject.

History. Body armour, in the form of metal plates, was widely used during the time of hand- to-hand combat with swords, knives and various bludgeoning instruments. With the advent of the crossbow and firearm, the plain steel suits were found inadequate to defeat the missiles and they rapidly became obsolete.

During World War II (WWII), ballistic nylon (a copolymer of the basic polyamide) was used against shrapnel from munitions. This was, however, of little use against bullets other than low-velocity soft-lead projectiles.

The major advance in soft body armour came with a generation of what are loosely referred to as 'super fibres' which were introduced by Du Pont. The best known of these was a para-aramid fibre called Kevlar which was originally used in fabric-braced radial tyres. It did not take long, however, for it to be realized that it could be woven into a fabric which was so strong that it could be used in bullet-resistant, soft body armour.

The Kevlar fibres were simply woven into sheets, with varying thicknesses of yarn and density of weave (called denier), to provide the particular properties required. The sheets were then assembled into ' ballistics panels' which were permanently sewn into a carrier in the form of a vest.

It is undeniable that Kevlar does produce a very effective, lightweight and flexible jacket which can be tailored to stop virtually any handgun missile. It does, however, suffer from a number of problems. Firstly, it is not stable to UV light and has to be kept inside a light-proof pouch. It is also very susceptible to attack by many household chemicals, and thirdly, if wet, it loses most of its ability to stop bullets.

A recent development in the field of soft body armour is the use of an ultrahigh molecular weight polyethylene fibre called Spectra which is produced by Allied Signal Inc. This consists of exceedingly fine-spun fibres of polyethylene. These fibres are laid, in dense mats, at 90 ° to each other then covered top and bottom with a thin sheet of polyethylene. This is then heat treated to semi-melt the fibres together or bonded with a plastic resin to form a sheet. With the thousands of bonded fibres which must be pulled from the matrix to allow the passage of a bullet, the sheets are even more efficient than Kevlar.

This material is not affected by water, nor is it affected by UV light or any chemical, and it is considerably lighter than Kevlar. If it has a disadvantage, it is that its melting point is much lower than Kevlar.

Mechanism of bullet-resistant materials. To effectively stop a bullet, the material must first deform the missile. If the surface area of the bullet is large enough and the material has sufficient resistance to the passage of the bullet, then the energy transfer to surrounding fibres can occur. A non- deformed bullet will merely push apart the weave and penetrate.

If the bullet is sufficiently soft, that is, plain lead, semi-jacketed or a thinly jacketed bullet, then the material alone will often be sufficient to cause the deformation. If, however, the bullet is heavily jacketed or of the metalpenetrating type, then some intermediate, much more rigid material will be required to deform the bullet. This generally takes the form of a hard plate which fits in front of the soft body armour.

Ballistic inserts. This is the name generally given to rigid plates which are placed in front of the soft body armour. Their purpose is to break up highvelocity, hard-jacketed and metal-penetrating missiles. Once the bullet's velocity has been reduced and its shape deformed, it will be easily stopped by the underlying Kevlar or Spectra material. These inserts are generally made from either a fused ceramic material, heat-treated aluminium, hardened steel or, more recently, titanium. These can be either solid plates or small overlapping tiles.

Case examples. Soft body armour is not infallible as the following two cases illustrate. The first involved a police officer wearing a very substantial bullet-resistant vest capable of defeating 0.357" Magnum and 9 mm PB calibre bullets. He was shot at close range with a 0.45-70 rifle which has a large soft bullet weighing 400 gr at a velocity of 1500 ft/s. Whilst the jacket was successful at defeating the bullet, it was driven into the officer's chest, killing him.

The second involved a live demonstration of a ballistic insert plate made of metal. The plate was designed to defeat an armour- piercing round, but the demonstration was merely to show how effective it was against a full magazine from a sub-machine-gun. The soldier wearing the jacket was not killed, but the fragments generated by the bullet breaking up on the plate neatly severed the lower part of his jaw.

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