Near the static fatigue limit in glass

Fracture surfaces formed near the crack growth threshold in soda-lime-silicate glass are examined by atomic force microscopy. Cracks held below the apparent crack growth threshold for 16 h alter their mode of growth. The fracture plane changes from a flat surface to one that exhibits substantial out of plane growth. The direction of crack growth changes from 3° to 5° to the original growth direction. However, the change in growth direction is not uniform along the crack front; some portions of the front propagate at +3° to +5°, while adjacent portions propagating at -3° to -5° to the original growth direction. Thus, the crack is no longer flat, but becomes wavy after the 16 h hold period. This out of plane growth may be partially explained in terms of a crack growth model developed by Chuang and Fuller, which predicts an enhancement in the corrosion rate on the flank of a crack at stresses below the stress corrosion threshold. Alternatively, the unevenness of the crack plane after the hold period may be a consequence of a modification of the fracture toughness of the glass as a consequence of leaching.