  With unique distorted 1T structure and the associated in-plane anisotropic
properties, mono- and few-layer ReX2 (X=S, Se) have recently attracted
particular interest. Based on experiment and first-principles calculations, we
investigate the fracture behavior of ReX2. We find that the cleaved edges of
ReX2 flakes usually form an angle of ~120{\deg} or ~60{\deg}. In order to
understand such phenomenon, we perform comprehensive investigations on the
uniaxial tensile stress-strain relation of monolayer and multi-layer ReX2
sheets. Our numerical calculation shows that the particular cleaved edges of
ReX2 flakes are caused by unique anisotropic ultimate tensile strengths and
critical strains. We also calculate the stress-strain relation of WTe2, which
explains why their cleaved edges are not corresponding to the principle axes.
Our proposed mechanism about the fracture angle has also been supported by the
calculated cleavage energies and surface energies for different edge surfaces.
