Experimental micro–macromechanics: shear-induced fabric evolution can make denser assemblies less stiff

Granular materials exhibit path-dependent stiffness following preshearing; however, the micromechanical origins of this behaviour remain insufficiently understood. While numerical simulations have provided valuable insights, experimental investigation of both micro- and macroscopic responses under different shearing histories is essential for a comprehensive understanding of real granular behaviour. In this study, biaxial shear tests under constant mean stress were conducted on assemblies of aluminium rods subjected to distinct shearing histories, combined with advanced image analysis to track microstructural evolution directly. A counterintuitive response was observed: the apparently densest specimen exhibited the lowest initial stiffness and dilatancy, attributed to preshearing-induced fabric anisotropy. Particle rotations were highly localised within the shear band, predominantly irreversible and played a major role in plastic deformation. Significant and largely irreversible reorientation of the contact network occurred only when the stress ratio exceeded a threshold or upon load reversal, whereas microstructural changes remained limited and largely reversible below this level. These results demonstrate that void ratio alone cannot fully explain macroscopic behaviour; instead, stiffness and dilatancy are governed by both the connectivity and orientation of the contact network. The findings highlight the critical importance of microstructural evolution in constitutive modelling of granular materials subjected to different shearing histories.