Materials of micro-/nanometer dimensions have aroused remarkable interest, motivated by the diverse utility of unconventional mechanical and electronic properties distinguished from the bulk counterpart and various industrial applications such as electronic/optic devices and MEMS/NEMS. The size of their elements is now, ultimately, approaching nanometer and atomic scales. Since the conventional theory of "fracture mechanics" is based on the continuum-body approximation, its applicability to the nanoscale components is questionable owing to the discreteness of atoms. Moreover, for...