Plasticity in amorphous materials
silica and silicate glasses

Known since the then highly controversial observations of Taylor (1949), the plastic behavior of silicate glasses, which has received far less attention than fracture, nonetheless raises several fundamental questions, particularly in relation to the latter. This relation had already been analyzed in great detail by Marsh (1964) but the experimental tools available at the time were limited and the relevance of his work has not been really well understood. In this respect, amorphous materials are fundamentally different from crystalline materials, where plastic flow results from the movement of dislocations within the lattice. Moreover, this plastic response is a key determinant for the development of cracking in these brittle materials, and hence their surface damage. However, because of their brittleness, it is only possible to activate this plasticity at the micron scale, which means that the usual mechanical characterization methods on macroscopic samples are no longer applicable. We have tackled various aspects of this issue through original experimental approaches combined with modelling.

Taylor E. W. (1949). Plastic Deformation of Optical Glass, Nature 163 : 323.
Marsh, D. M. (1964). Plastic Flow in Glass, Proc. R. Soc. London, Ser. A 279 : 420 - 435.

Measuring the indentation stress field in silicate glasses - impact on the description of the plastic flow With very careful birefringence measurements, we have obtained the signal from residual stresses after (crack free) indentation in soda lime silica glass (SLS) and in amorphous silica (AS). We have also calculated the expected birefringence patterns (path difference and rotation) based on FEM predictions of the strain field derived from our previously validated constitutive relations. The agreement is very good overal for SLS while residual discrepancy in AS signal an underevaluation of the coupling between shear strain and densification. For more see here.

Plastic flow versus densification - what is the impact on indentation stress fields ? Over the past decades, constitutive relations for silicate glasses have become reliable enough that we can calculate indentation stress and strain fields and examine the impact of material parameters on indentation. 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. 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. For more see here.