Transient phase in shear zone formation in sands

Abstract

An experimental study has been made to study the emergence of narrow shear zones (“shear bands”), in uniformly graded quartzitic sand under dry conditions using the model setup of orthogonal cutting. A fixed region of interest around the stationary tool is imaged using a high-resolution and high-speed camera and further analyzed using Particle Image Velocimetry (PIV) algorithm to obtain the flow (velocity) fields. The velocity field maps of the near tool tip region demonstrate sharp change in the motion of sand particles along with the formation of a “dead zone” – a region of material stagnation or no flow. This extent of the dead zone was found to fluctuate in time for a given cutting speed, and no significant effect of the cutting speed (or strain rate) is observed in the propagation of dead zone size. The dynamic repose angle of the evolving sand pile in front of the tool increases initially and then attains a steady value of around 40°. The regions of intense straining in the sands were revealed in the effective strain rate fields. The shear localization was seen to emerge from the tooltip and propagate toward the free surface. The inclination angle of these bands evolves periodically with time and showed a decreasing trend due to an increase in the surcharge. The oscillatory force signatures suggested that every undisturbed ensemble of sands, on reaching the tool vicinity, undergoes material softening (dilation) followed by hardening(compaction), which is also reflected in the periodic formation and destruction of shear bands.