Subcritical crack growth in silica aerogel

The sub-critical crack growth domain in hydrophilic silica aerogel was studied using the Double-Cleavage-Drilled-Compression test (DCDC). The effect of temperature and water vapor content on the crack growth rate is measured. The experiments are carried out between 295 and 383 K with relative air moisture ranging from 5% to 80% Rh. Like dense silica, crack growth velocities were found to decrease when increasing the temperature at a constant water vapor concentration. Surprisingly, the crack velocity also decreases when increasing water vapor content at constant temperature. Such a behavior is contrary to expected results usually encountered in glasses. An analogy with nitrogen adsorption experiments on the same aerogels is used to explain this behavior. We assume that a capillary condensation phenomenon of water vapor inside the aerogel porosity takes place, inducing internal compressive stresses.