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 MIL-HDBK-1013/12
strength of heat strengthened glass as precompression levels vary
between 5,000 and 10,000 psi (35,000 and 70,000 kPa).
Heat strengthened glass is recommended for use in fire,
ballistic, and forced entry resistant laminates for environmental
protection.
2.2.1.3
Thermally Tempered Glass. Thermally tempered glass
(TTG) is the most readily available tempered glass on the market.
It is manufactured from annealed glass (float, polished, or
plate) by heating to a high, uniform temperature and then
applying controlled rapid cooling. Thermally tempered glass is
typically four to five times stronger than annealed glass with a
design stress of 16,000 psi (112,000 kPa).
Only tempered glass meeting the minimum fragment
specifications of American National Standards Institute (ANSI)
Z97.1-1984, American National Standard for Safety Glazing
Materials Used in Buildings - Safety Performance Specifications
and Methods of Test, or certified by the Safety Glazing Council
(SGC) is to be used for blast design purposes. Certification by
the SGC constitutes compliance with ANSI Z97.1-1984. TTG is
controlled tightly in its tempering process to obtain the highest
precompression level without incurring spontaneous breakage.
This results in a higher surface precompression level and tensile
strength with less variation than annealed or heat strengthened
glass.
The design of thermally tempered glass is currently
restricted to glass meeting both American Society for Testing and
Materials (ASTM) C1048-92, Standard Specification for Heat-
Treated Flat Glass - Kind HS, Kind FT Coated and Uncoated Glass,
and ANSI Z97.1-1984. Tempered glass meeting only C1048-92 may
possess a surface precompression of only 10,000 psi (70,000 kPa).
At this level of precompression, the fracture pattern will be
similar to annealed and semi-tempered (heat strengthened) glass
with large razor-sharp shards.
The fracture characteristics of tempered glass are
superior to those of annealed glass. Due to the high strain
energy stored by the prestressing, tempered glass will eventually
fracture into small cube-shaped fragments instead of the razor-
sharp, dagger-shaped fragments associated with the fracture
pattern of annealed glass. Breakage patterns of side and rear
windows in American automobiles are a good example of the failure
mode of TTG.
Although thermally tempered glass exhibits a relatively safe
failure mode for conventional usage, failure under blast loading
still presents a significant health hazard. Results from blast
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