  Understanding the relationship between micromechanics and macroscopic plastic
deformation is vital for elucidating the deformation mechanism of amorphous
solids, such as granular materials. In this study, we directly measure T1
events, which are topological rearrangements of particles, and the associated
microscopic stresses in dense packings of photoelastic disks under pure shear.
We observe a remarkable similarity between the evolution of the total number of
T1 events and the global stress-strain curve. The competition between the birth
and death of T1 events establishes a dynamic equilibrium after the yield
strain, contributing to the overall plastic behavior. Despite the erratic
stress fluctuations of individual T1 events, the local stresses decrease on
average once T1 events occur, indicating their soft characteristics.
Furthermore, we demonstrate that the microscopic stress fluctuations exhibit a
long-range anisotropic spatial correlation similar to the Eschelby character,
but with a distinct scaling of $r^{-1.5}$. Interestingly, we also find a
striking similarity in the correlation functions of T1 and non-T1 regions.
These findings establish a significant connection between macroscopic
mechanical behavior and the elementary deformation of T1 events, shedding light
on the fundamental understanding of granular materials as amorphous solids with
contact-scale plastic deformation.
