Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
4.1.5 Environment-assisted cracking
Glasses and many other oxide-based ceramics are susceptible to slow crack
growth at room temperature in the presence of water or water vapour, if there
is a surface tensile stress acting. This may lead to time-dependent failure of
the specimen and the effect can be expressed in terms of fracture mechanics,
as discussed in Chapter 2.
Essentially, water vapour at the crack tip can react with the molecules and
break the Si–O bonds by forming a hydroxide. When the crack has grown
sufficiently for the critical stress intensity to be achieved, failure takes place
– the phenomenon sometimes being referred to as ‘static fatigue’.
4.2 Glass ceramics
Glass is a Newtonian viscous solid, so it is easy to mould without introducing
voids, but its high temperature strength is essentially low. A number of glass
compositions have been identified which can be crystallized after the shaping
process is complete. A controlled heat-treatment is required, first to nucleate
and then to grow the crystals throughout the glass. The extent of crystallization
may exceed 90% by volume and small crystal sizes of