Anisotropy parameter, α, after unloading (or σ~0) versus maximum strain, εmax, imposed on the system for two different strain rates 10-2 ps-2 (red circles) and 10-4 ps-1 (blue squares). After shear unloading, normal stresses persist due to the plane deformation geometry and amplify the structural anisotropy (see insert). After relaxation of these stresses with an additional NPT step of 20 ps, a stress free state is obtained, while the silica structure still exhibits a significant anisotropy index.
Recently many scientists have dedicated an enormous number of research hours to understanding the structure, the dynamics and the mechanical properties of amorphous material, but despite this effort they remain ill-understood. Unlike the plasticity of crystalline materials, the plasticity of glasses cannot be defined unless a completely novel description is introduced. We have employed the fabric tensor which is commonly used in granular physics and more recently in foams to reveal irreversible deformation in glasses. This work shows how the structure of silica can be irreversibly modified by the application of an external shear stress: under shear plasticity a non-reversible anisotropy sets in and appears stable. Thus providing a new interpretation of the small scale plasticity of glass; as revealed by the fabric tensor, which, to the best of our knowledge, has never been used in this context.
C. L. Rountree, D. Vandembroucq, M. Talamali, E. Bouchaud and S. Roux Plasticity-Induced Structural Anisotropy of Silica Glass Physical Review Letters 102, 195501 (2009)