tests reveal that upon fracture, TTG fragments may be propelled
in cohesive clumps that only fragment upon impact into smaller
rock-salt-type fragments. Even if the tempered glass breaks up
initially into small fragments, the blast overpressure can propel
the fragments at a high enough velocity to constitute a severe
Thermally tempered glass up to 1/2 inch (12 mm) thick
can be easily purchased. Five-eighths and 3/4-inch (16-mm and
19-mm) thick TTG are also readily available. However, since
demand is small for these thicknesses, they are typically two to
two and one-half times as expensive as 1/2-inch (12-mm) TTG.
Only limited quantities of 1-inch (25-mm) thick thermally
tempered glass are available worldwide at premium prices.
Chemically Treated Glass. Glass can be tempered
chemically by a bath process where ions are exchanged between the
bath and the glass. Denser packing of the molecules close to the
surface of the glass plate produces a layer of compression.
Precompression levels between 10,000 and 45,000 psi (70,000 kPa
and 315,000 kPa) are reported by industry.
As the molecular packing and precompression layer is
usually not deeper than 0.012 inch (0.30 mm) from each surface
(compared with 20 to 22 percent of overall thickness from each
surface for thermally tempered glass), less strain energy is
stored in the glass plate and consequently the fracture pattern
resembles that of annealed glass.
Most, if not all, commercially available, chemically
tempered glass for architectural purposes is manufactured from a
soda-lime base. A common problem with soda-lime based chemically
tempered glass is that the precompression depth or case depth is
thin enough (0.001 inch (0.025 mm)) to be pierced by surface
flaws (often 0.004 inch (0.10 mm) deep). If this occurs, the
surface flaws will penetrate the inner tensile layer. The
resulting stress concentration about the tip of the flaw in the
tensile regime of the cross-section will often induce premature
failure. Chemically strengthened glass is generally produced
only in thicknesses up to 3/8 inch (10 mm).
As with heat strengthened glass, chemically
strengthened glass provides environmental benefits when used as
outboard plies for annealed glass or polycarbonate laminates.
Laminated to an outboard layer of polycarbonate, chemically
treated glass can increase the ballistic and fire resistance of
polycarbonate and provide protection against environmental decay.
Soda-lime based chemically treated glass is not
recommended for use in attack resistant windows unless it is a