Research on Internal Structural Evolution driven by Collective Defects Field Dynamics for Multiscale Modeling

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This research aims at developing the new methodology to catch the multiscale evolution of the internal structural change due to the collective defects. In the process of growing the internal defects under the applied loads, it is the most important to understand the nanoscale irreversible mechanical behaviors, so called as the nanoplasticity, as the commencement of the macroscopic plastic phenomena. We have taken much interests on the nanoindentation problem because the micron-scale indent load and the nano-scale indent depth can be controlled precisely.We have found the new facts that the critical indent load at the first displacement burst, which has been recognized as the unstable indent depth jump keeping the load, and the burst width have the linear relation. To understand what is happening during the displacement burst, we have performed the large-scale molecular dynamics simulations. In the series of these analyses, the useful mechanical properties like the ideal resolved shearing stress at the dislocation emission have been obtained and then transferred to the mesoscopic methodology like the discrete dislocation method. We understand the formation mechanics of the prismatic dislocation loop which is the most potential events under the indentation and the collective dislocation emission which is the key to the burst mechanics.We have also developed the new technique of the nondestructive observation at the mesoscale. The scanning electron-indued acoustic microscopy(SEAM) can detect a few micron-scale defects placed at the subsurface without destruction of the sample. We had the big expectations to apply the our own method to catch the internal collective defects, however it is found to need the more high chopping frequency of the electron beam and the charge amplifier circuit for the higher frequency.