PSI - Issue 13

Igor Golovnev et al. / Procedia Structural Integrity 13 (2018) 1632–1637 Author name / Structural Integrity Procedia 00 (2018) 000–000

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the initial cooled structure which was not under the external loading:

, t fr fr U U U ∆ = − 0,

The following Figures (Fig.3) show the position of the atoms in the rightmost atomic plane (in the cut-off structure) at different time instants under the cyclic extension, and atoms position in the same plane after the cooling of this nonequilibrium structure.

Fig.3 The right atomic plane at different time instants t after the cyclic extension beginning (black circles) and after cooling of this disturbed structure (grey circles). (а) t = 2 ps; (b) t = 6 ps; (c) t = 8 ps.

From these three Figures it is evident that, starting from the time instant of 8 ps, after the disturbed structure cooling, the atom position begins to deviate severely from the position of the atoms in the disturbed structure before the cooling. This phenomenon is presented by the dramatic jump of the dispersion fr D at this very instant (see the peak in the graph Fig. 5). Above data correlate also with the graph of the varying potential energy of the cooled disturbed nanostructure as compared with the potential energy of the initial state (prior to the external loading) (Fig. 4). Up to the 6 th ps, the structure does not show any irreversible changes, i.e. the processes in it are of elastic character. The dispersion value of the rightmost plane in the disturbed structure at the time moment 6 ps is 0.0362 . Let us refer this value the critical dispersion value in the dynamic state of the structure, i.e. beginning from this point, the irreversible changes of the crystal structure are observed. Beginning from the 7 th ps (Fig. 4), the changes are significant, i.e. the processes are already irreversible. The connection between these structure changes and dispersion of the rightmost plane is clearly seen in Fig. 5.

Fig.4 Changing potential energy of the cooled disturbed nanostructure against the с potential energy of the initial state.

Fig.5 Dispersion of the right plane coordinate versus external loading time. The black line is the dispersion at the external loading off, the grey one – after the disturbed structure cooling.