Do plastic zones form at crack tips in silicate glasses?

Abstract

Abstract A number of recent studies claim that silicate glasses fracture by the formation, growth and coalescence of cavities at crack tips, in the same way as metals, but at a much smaller scale. Evidence for cavity formation comes from the examination of side surfaces of fracture mechanics specimens, at the point where the crack tip intersects the free surface. Such measurements exhibit small depressions in regions that are supposedly located in front of moving crack tips. These depressions were interpreted as cavities. In this paper, we summarize experimental results obtained using an atomic force microscope to characterize the fracture surfaces. The experimental results demonstrate an absence of residual damage on fracture surfaces that could be interpreted as cavity formation. We also observe cracks moving in glass and show that the features reported as cavities actually occur behind and not in front of the moving crack. A simulation of an atomic force microscope probe passing over the emerging tip of a crack in glass suggests that the features identified as cavities are in fact due to the roughness of the specimen surface. Our results support the view that cracks in glass propagate by brittle fracture. We find no evidence for nanoscale ductility in silicate glasses.