Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
Na –
4.1 Sketch of the structure of soda lime glass.
develop high thermal stresses that can induce cracking. The second important
family of glasses were developed to overcome this problem.
4.1.2 Borosilicate glasses
Borosilicate glasses have a typical composition (wt %)
80 SiO 2 , 15 B 2 O 3 , 5 Na 2 O
and a small amount of Al 2 O 3 . The coefficient of thermal expansion of these
glasses is only one third of that of soda lime glass and their resulting good
thermal shock resistance makes them suitable for cooking and chemical
glassware (Pyrex glass).
Some properties of these glasses and others are listed in Table 4.1.
Thermal shock resistance
Fracture caused by sudden changes in temperature is a problem with glasses
and ceramics. If a piece of the heated material is plunged into cold water,
the surface layer immediately attains the water temperature before there is
time for heat to flow from the interior of the specimen. The contraction
of the surface layer is thus constrained, creating a biaxial surface stress
given by: