  We perform combined X-ray tomography and shear force measurements on a
cyclically sheared granular system with highly transient behaviors, and obtain
the evolution of microscopic structures and the macroscopic shear force during
the shear cycle. We explain the macroscopic behaviors of the system based on
microscopic processes, including the particle level structural rearrangement
and frictional contact variation. Specifically, we show how contact friction
can induce large structural fluctuations and cause significant shear dilatancy
effect for granular materials, and we also construct an empirical constitutive
relationship for the macroscopic shear force.
