Interpretation of Effects at the Static Fatigue Limit of Soda-Lime-Silicate Glass
Theo Fett, J -. Guin, Sheldon M. Wiederhorn
Crack growth behavior in soda-lime-silicate glass in the vicinity of the static fatigue limit, and observations of the crack shape obtained by atomic force microscopy are rationalized are rationalized by a fracture mechanics model of the crack tip, in which a stressed layer is built up on the crack surface as a consequence of ion exchange at the crack tip. This model extends the one presented earlier by Bunker and Michalske. Ion exchange between hydronium (H3O+) in the solution and sodium (Na+) ions in the glass give rise to compressive stresses at the tip of cracks in soda-lime glasses. These compressive stresses are responsible for the occurrence of a fatigue limit in glass; for the fact that crack tips remain sharp at the fatigue limit even though the walls of the crack are corroded by the basic solutions that form as a consequence of ion exchange; for the fact that crack tip bifurcation is often observed when cracks are held at the fatigue limit for a while and then restarted at higher loads; and for the fact that a delay time to restart the crack is often observed after the crack is held under load at the static fatigue limit. Most of the predictions are in quantitative agreement with experimental observations on crack growth and crack tip structure for soda-lime-silicate glass. The prediction of the time required to restart the crack is, however, only qualitative, as experimental data report a sharp peak centered at the fatigue limit in the plot of restart time versus hold stress intensity factor, whereas the model gives a broad maximum on such a plot. Clearly, further development of the model will be needed for a better representation of the experimental data.