Contact residual stress relaxation in soda-lime glass: Part II. Aspects relating to strength recovery

Strength recovery of Vickers indented soda lime glass was measured and compared after annealing at two temperatures: one below and one above Tg. The atomic force microscope was used to study the cracks. At 540 °C, no changes were observed in crack morphology either below the surface or on the surface relative to the pre-anneal state. At 630 °C, both sub-surface and surface crack morphology changes were observed. The trends in strength recovery were compared with residual stress relaxation as measured by a new method of stress estimation based on nanoindentation elastic response. At short hold times at 630 °C, and regardless of the length of hold time at 540 °C, strength recovery of only ∼30% was measured while at moderately long hold times at 630 °C, strong recovery of fracture strength, ∼132% was measured. Trends in strength recovery above Tg are shown to match those of crack tip radius instead of trends in stress relaxation across the residual stress field.