Fig. 2

Strain diagram for contact bond (scheme): F _f is the value of “applied” force; u _f is the deviation of an atom from the equilibrium position due to mechanical load; F _un and u _un are the force and displacement of a contact bond instability, respectively; δ _un is the fluctuation of the bond instability; u _c and δ _c are the critical displacement and critical fluctuation, respectively; u _br is the displacement of bond break.