Page 497 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 18.4 Techniques for Strengthening and Annealing Glass 77

Chemical Tempering. In this process, the glass is heated in a bath of molten KNO3,
KZSO4, or NaNO3, depending on the type of glass. lon exchanges then take place,
with larger atoms replacing the smaller atoms on the surface of the glass. As a result,
residual compressive stresses develop on the surface. This condition is similar to that
created by forcing a wedge between two bricks in a brick wall.
The time required for chemical tempering is about one hour longer than that for
thermal tempering. Chemical tempering may be performed at various temperatures.
At low temperatures, part distortion is minimal; therefore, complex shapes can be
treated. At elevated temperatures, there may be some distortion of the part, but the
product can then be used at higher temperatures without loss of strength.

Laminated Glass. Laminated glass, a product of another strengthening method
called laminate strengthening, it consists of two pieces of flat glass with a thin sheet
of tough plastic in between. When laminated glass is cracked, its pieces are held
together by the plastic sheet--a phenomenon commonly observed in a shattered
automobile windshield.
Traditionally, flat glass for glazing windows and doors has been strengthened
with wire netting (such as chic/een wire-with a hexagonal mesh) embedded in the
glass during its production. When a hard object strikes the surface, the glass shat-
ters, but the pieces are held together because of the toughness of the wire, which has
both strength and ductility.

Bulletproof Glass. Laminated glass has considerable ballistic impact resistance and
can prevent the full penetration of solid objects because of the presence of a tough poly-
mer film in between the two layers of glass. Bulletproofglass (used in some automobiles,
armored bank vehicles, and buildings) is a more challenging design, due to the very high
speed and energy level of the bullet and the small size and the shape of the bullet tip, rep-
resenting a small contact area and high localized stresses. Depending on the caliber of the
weapon, bullet speeds range from about 350 to 950 mfs.
Bulletproof glass (also called bullet-resistant glass) ranges in thickness from
7 to 75 mm. The thinner plates are designed for resistance to handguns, and thicker
plates are for rifles. Although there are several variations, bulletproof glass basically
consists of glass laminated with a polymer sheet (usually polycarbonate). The capac-
ity of a bulletproof glass to stop a bullet depends on (a) the type and thickness of the
glass; (b) the size, shape, weight, and speed of the bullet; and (c) the properties and
thickness of the polymer sheet.
Polycarbonate sheets commonly are used for bulletproof glass. As a material
widely used for safety helmets, windshields, and guards for machinery, polycarbon-
ate is a tough and flexible polymer. Combined with a thick glass, it can stop a bullet,
although the glass develops a circular shattered region. Proper bonding of these lay-
ers over the glass surface is also an important consideration, as there usually is more
than one round fired during such encounters. Also, in order to maintain the trans-
parency of the glass and minimize distortion, the index of refraction of the glass and
the polymer must be nearly identical.
If a polymer sheet is only on one side of the glass, it is known as a one-way
bulletproof glass. In a vehicle, the polymer layer is on the inside surface of the glass.
An external bullet will not penetrate the window, because the bullet will strike the
glass first, shattering it. The glass absorbs some of the energy of the bullet, thus
slowing it down. The remaining energy is dissipated in the polymer sheet, which
then stops the bullet. This arrangement allows someone inside the vehicle to fire
back. A bullet from inside penetrates the polymer sheet and forces the glass to break
outwards, allowing the bullet to go through. Thus, a one-way glass stops a bullet
fired from outside but allows a bullet to be fired from inside.