Crack initiation in silicate glasses – plastic flow, shear bands and damage – GaLAaD

Silicate glasses are characterized by a remarkable combination of properties: mechanical rigidity and transparency, ease of shaping and low manufacturing costs. However, their use is often limited by a low stress at rupture, even though their intrinsic tensile strength is remarkably high. In fact, as has been known for a century now, surface defects dramatically lower the loading at rupture of typical glass items. However, it has been observed that certain glass compositions are less susceptible to the formation of surface defects, which is often quantified by the resistance to indentation cracking, but we still don't know why. Understanding how defects form on the surface of silicate glasses and how sensitivity to surface damage is related to composition is still a very open question in materials mechanics, and a definite technological challenge. However, it has long been proposed that material damage resulting from plastic shear flow and in particular the formation of shear bands form create the locus of fracture initiation. However, this hypothesis, which has found little echo in the literature, has so far not been systematically tested, in particular because of the difficulties encountered in modelling the plastic response of these materials at the continuum scale: good representations of the elastoplastic strains during indentation were too elusive. In the GaLAaD project, we take advantage of recent experimental and numerical developments to propose a quantitative approach to indentation cracking in two series of glasses of technological interest, respectively sodolime and borosilicate glasses. In this multi-scale study, we will apply recent micromechanical tests to quantify the plastic response of amorphous silicates. The characteristics of the plastic flow and in particular its localization under the form of shear bands during indentation will be measured, and their relationship to fracture initiation will be established. An original technique for measuring birefringence in situ will be developed to allow a very direct experimental measurement of the stress fields that develop during an indentation cycle. On the basis of these experimental results, a model combining the finite element method and phase field will be used and advanced constitutive relations established to model the observed appearance and development of shear bands in amorphous silicates, and how they eventually lead to rupture. Also, using Molecular Dynamics, we will relate the observed effects to the structural characteristics of the glasses under consideration. The coupling between experimental approach and multi-scale simulation allows us to envisage a significant progress in our understanding of indentation cracking in amorphous silicates of technological interest, while providing original academic tools in the field of damage and rupture.