Perfectly plastic flow in silica glass

The plastic behavior of silicate glasses has emerged as a central concept for the understanding of glass strength. Here we address the issue of shear-hardening in amorphous silica. Using in situ SEM mechanical testing with a high stiffness device, we have been able to compress silica pillars to large strains while directly monitoring radial strain. The sizeable increase of pillar cross-section during compression directly demonstrates the significant role of homogeneous shear flow. From the direct evaluation of the cross section, we have also measured true stress-strain curves. The results demonstrate that silica predominantly experiences plastic shear flow but that there is no shear-induced hardening. The consequence of this finding for our understanding of glass strength is discussed.