On the mechanisms of shear banding

Muhammad Reza Hassani
Dr. Alexandra Lagogianni

Shear banding controls the plasticity of many materials, such as metallic glasses, since it constitutes one of the most prominent shear-induced instability that is macroscopically observed upon external loading.

The group investigates the conditions, the parameters and the underlying mechanisms, upon which a homogeneous plastic deformation switches to the heterogeneous deformation mode of shear banding via monitoring the spatial and temporal correlations of stress, strain and structure. To this aim, we study the phenomenon by starting from its microscopic precursors, the Shear Transformation Zones (STZs). The STZs are randomly distributed regions, consisting of a few tens of atoms, that undergo local yielding and appear from the early elastic regime under external stimuli. Finally, what is macroscopically observed as a shear band is caused by the coalescence and alignment of STZs in narrow bands after the material exceeds the yielding point.

Molecular dynamics simulations of a generic binary Lenard Jones glass in three dimensions.
a) Stress-strain curve extracted under a simple shear protocol with a finite strain rate at zero temperature. b) Spatial distribution of non-affine strain, highlighting the localization phenomenon (shear banding).

The study is performed by employing molecular dynamic simulations with a generic binary Lenard Jones mixture and semi-empirical interatomic potentials, e.g. EAM. The deformation accommodation and the dynamic heterogeneity in metallic glasses are studied by varying parameters such as deformation protocols, temperatures, system geometries and strain rates.