Crack speed in ultraporous brittle amorphous material

Crack growth in porous and transparent silica aerogels has been measured wi thin the range 10 nmls-0.3 mis using optical devices. The surface nature of aerogels is hydrophobic. It may be converted in hydrophilic material by an oxidation heat treatment. For low stress intensity factor both aerogels sh ow a stress corrosion mechanism analogous to that observed in dense silica glass subjected to alkaline aqueous solutions. However, the crack-rate in o xidised silica aerogels is one hundred times higher than that observed in a s obtained hydrophobic aerogels. For higher stress intensity factor values the crack rates of both aerogels superimpose indicating that the crack igno res details of aerogel structure.