The strengths of crystalline and glassy oxides decrease with time under a constant applied load. This static fatigue is usually modeled with a power law equation between times to failure t when a sample is subjected to an applied stress і:
logt = c – «logs (5)
in which c is a constant and the stress exponent n is a measure of the susceptibility of the material to fatigue. The larger the n value the more resistant the material is to fatigue. Typical values of n for silicate glasses are 13 or lower ; for alumina an n value of about 35 was found , showing that alumina has much better fatigue resistance than most other oxides under ambient conditions.
This fatigue in oxides results from reaction with water, which can break the cation – oxygen bonds in the material; for example in alumina:
Al – O – Al + H2O = AlOH + HOAl (6)
Thus, when the ambient atmosphere is dry the fatigue failure time is long, and as the humidity increases the fatigue time decreases.