SLOW CRACK-GROWTH IN AEROGELS

The chemical susceptibility was measured by a dynamic method on aeroge ls immediately after supercritical drying and after oxidation treatmen t. Five loading speeds between 10 mu m/min and 100 mm/min were used. A fter oxidation treatment, the Weibull modulus increased, which indicat es a narrowing of the strength distribution. Toughness of the aerogels was measured by the single edge notch beam technique. 'As-prepared' a erogels did not show slow crack propagation. However, as the oxidized materials exhibit a stress corrosion phenomenon, the crack propagation rate was evaluated using the median value of strengths obtained with the highest crosshead speed. The lifetime prediction is estimated as a function of failure probability and from a proof testing experiment.