  Elucidating the interplay of defect and stress at the microscopic level is a
fundamental physical problem that has strong connection with materials science.
Here, based on the two-dimensional crystal model, we show that the instability
mode of vacancies with varying size and morphology conforms to a common
scenario. A vacancy under compression is fissioned into a pair of dislocations
that glide and vanish at the boundary. This neat process is triggered by the
local shear stress around the vacancy. The remarkable fractionalization of
vacancies creates rich modes of interaction between vacancies and other
topological defects, and provides a new dimension for mechanical engineering of
defects in extensive crystalline structures.
