Indentation cracking in silicate glasses is directed by shear flow, not by densification

Abstract Over the past decades, constitutive relations have been developed to compute the mechanical response of silicate glasses at the continuum length scale. They are now reliable enough that we can calculate indentation induced stress and strain fields and examine the impact of material parameters on indentation response, and especially hardness, pile-up and stress fields. In contrast to a presently widespread assumption in the literature, we show that (shear) flow stress is the primary determinant of these properties, and that densification plays a secondary role in the indentation response of all the silicate glasses. This result applies even for large values of the densification at saturation because of the high ratio between effective volumetric yield stress (i.e. yield pressure) and flow stress. It is well-known that, depending upon composition, silicate glasses exhibit very different sensitivities to indentation cracking, although all other standard mechanical properties remain quite similar. We point out that material damage incurred through plastic shear flow, and especially shear flow instability and localization may well control crack initiation, which would resolve the paradox. Shear flow instability and damage has not been quantitatively investigated in detail in silicate glasses as yet, neither experimentally nor theoretically. However, we believe it is key to an in depth understanding of cracking resistance in silicate glasses.