弹性波在纳米单晶铜杆中的传播

PROPAGATION OF ELASTIC WAVE IN SINGLE-CRYSTAL NANO COPPER BAR

采用预弛豫和对试件一个端部进行速度加载(另一端固定)的方法在分子动力学模拟中实现了纳米杆真实的动态单向加载过程,并提出了与宏观一致的应变率定义;对长脉冲和短脉冲在纳米铜杆中的传播过程进行了分子动力学模拟,同时也对短脉冲在纳米铜杆中的传播进行了连续介质力学三维有限元模拟.长脉冲的模拟结果表明,纳米铜杆的弹性模量会随着试件截面的增大而增大.短脉冲的模拟结果显示,分子动力学模拟结果相当于在连续介质力学三维有限元模拟结果上叠加了一个抖动,且两种方法模拟得到的波速和Rayleigh波速基本吻合,这表明边界扰动在纳米尺度试件中的传播仍然具有一维弹性应力波和横向惯性效应引起的几何弥散的特征;而叠加的抖动则体现了表面原子对弹性波传播的影响,且与加载前未能将试件完全弛豫到自然状态有关.

A method of pre-relaxation and velocity loading on one end of specimen （with the other fixed in plane） is used in molecular dynamics （MD） simulation on nano copper bar under uniaxial tensile impact loading. A definition of strain rate in MD simulation accordant with the macro one is brought forward. The propagations of long pulse and short pulse in nano copper bar are studied using MD simulation. 3D finite element method is also used to simulate the propagation of short pulse in nano copper bar. The simulation of long waves shows that the Young＇s module increases with the increase of the size of the specimen＇s cross section. Meanwhile, the difference between the wave shape gained from MD simulation and that gained from FEM simulation is that the former contains vibrations of high and low frequencies. The Rayleigh＇s velocities of elastic wave gained from both simulations are similar. These indicate that the propagation of pulse in nano scale specimen has the same feature and character with that of elastic wave in continuum. And the vibrations in the result of the MD simulation show the effect of the surface atoms and the initial unnatural status in the specimens.