Ultra-low-expansion glass (ULE) is used in a number of optical applications requiring superior optical properties with low thermal expansion. Some of these applications require extensive grinding and polishing operations. It is thus instructive to understand the mechanical strength as related to element lifetime. An allowable stress can be calculated for this material based on modulus of rupture data; however, this does not take into account the problem of delayed failure. It is well known that many glass materials undergo delayed failure, due to stress corrosion, which can significantly shorten the lifetime of the article. Fortunately, a theory based on fracture mechanics has been developing enabling lifetime predictions to be made for brittle materials susceptible to delayed failure.
