Indentation stress fields in brittle materials: A micro-photoelastic investigation in silicate glasses

Abstract

Indentation experiments have helped understand non-linear mechanical properties of brittle materials such as plasticity, damage and fracture. However, our understanding of stress fields under indentation remains limited due to the general lack of direct measurements. This study introduces a novel approach to characterizing indentation stress fields in silicate glasses by combining high sensitivity birefringence measurements, photoelastic calculations and finite element analysis (FEA). We extensively investigated the elastoplastic response of soda–lime–silicate (SLS) and silica glasses under different indentation conditions, highlighting the effects of composition and indenter geometry on the photoelastic distributions. To predict the photoelastic response under indentation, we carefully calibrated an elastoplastic constitutive relation for silicate glasses using a combination of high-pressure (up to 25 GPa) and nanoindentation experiments. Computed indentation stress fields can then be validated through the comparison of full 3D photoelastic calculations and measured birefringence patterns. One key finding is that the residual stresses arising from the calibrated constitutive relations offered a far more realistic representation of the indentation stress fields than a commonly used approximate analytical elastoplastic model. With this method, stress fields can be investigated not only in oxide glasses but also in any other transparent isotropic material. While the calculated stress fields were generally satisfactory for both glass compositions, they also evidenced that improvements in the constitutive relation are needed for amorphous silica, which is known to undergo significant densification.