SUBCRITICAL CRACK VELOCITY IN SILICA AEROGELS

Crack velocity in aerogels was measured using the double cleavage dril led compression technique. Crack length was followed using an optical equipment which allows measurement velocity from 10(-10) m/s to 10(-2) m/s. Stress intensity factor: K-I, was calculated using a polynomial relationship which expresses K-I as a function of stress and crack len gth. Crack velocity evolves as a function of the stress intensity fact or and shows that silica aerogels exhibit a stress corrosion effect. T he velocity curve depends on the structure of the aerogel at the momen t of the experiment. For a given K-I, curves were shifted to higher ve locities when aerogel was stressed under atmosphere moisture. The diff erent behaviours are explained on the microstructure scale according t o previous molecular orbital computations of the silica structure invo lving rings opening. The aerogel which is macroscopically originally h ydrophobic does not totally impede water molecules from reaching the c rack tip. Stress corrosion is estimated from the chemical susceptibili ty factor, n, whose value evolves from 35 (like silica glass) to 13, ( C) 1998 Elsevier Science B.V. All rights reserved.