An experimental approach has been used to measure the threshold stress inte
nsity factor (K-th) for subcritical crack growth in brittle materials using
indentation cracks. The data show that K-th values existed in soda-lime-si
lica and soda-alumina-silica glasses that were tested in a neutral aqueous
environment. For the former glass, tests also were performed in acidic and
basic solutions, A threshold for subcritical crack growth was observed for
acidic conditions but not for alkaline conditions.