The Study Of Stress Computation In Nanomaterial And Mechanical Properties Of The Copper Nanomaterial

The mechanical properties and behaviors of nanomaterials are an important branch innano technology owing to their fascinating properties and potential applications in the newgeneration of high-performance nano electromechanical devices. In particular, someoutstanding properties induce by the novel size, witch are not remarkable in their bulkcounterparts. Molecular dynamics is an important method to research the mechanicalproperties of nano materials because of the imperfection of the theory and the difficultiesof experiment.The molecular dynamics method is elucidated in details. Some key issues, such as theinter-atomic potentials functions, and the integral algorithm, and the control oftemperature. A program is developed based on these theories in this work, for the purposeof facilitating the atomistic simulation.According to the different roles of internal forces and the external forces twodifferent stress definitions are obtained based on the equilibrium differential equation ofelastic static mechanics and the principle of virtual work. The two stress definitions areexternal virial stress and internal virial stress. The external virial stress and the internalvirial stress are not only provided two different stress definitions but also clearly pointedthe physical meaning in the definitions. If the internal forces or external forces are knownthe average stress can be calculated. Combined the virial theorem and these two virialstress definitions a new expression of system temperature is obtained. It reveals theessential principle in heat transfer phenomena. The theoretical analyses verified that theconventional virial stress, with che kinetic energy term is totally identical the internalvirial stress.Molecular dynamics methods are used to investigate the mechanical properties ofcopper nanowires with square cross-section, hexagonal cross-section and circularcross-section. The atomistic simulation scheme for deformation evolution process ofcopper nanowires with different cross-sectional geometries is presented. The stress-straincures are also obtained. The numerical results show that the mechanical properties of square cross-sectional nanowires is the best among the three cross-sectional geometries,the mechanical properties of circular cross-sectional nanowires is better than thehexagonal cross-sectional nanowires. Furthermore, size effects on mechanical propertiesof copper nanowires are obtained.Molecular dynamics methods are used to study the response of Cu nanotubes undertensile strain. The results show that the deformation mechanism of copper nanotubes is thesame with the copper nanowires. The Cu nanotubes show dislocation nucleation andgliding. The necking and break-up phenomena of nanotubes are demonstrated. Themechanical properties dependence on diameter and wall-thickness is investigated. Themechanical properties of Cu nanotubes show less dependence on the diameter, but showmuch dependence on the wall-thickness of nanotubes. The influence of the vacancies isinvestigated. The yielding stress and yielding strain appear an obvious decrease due to thevacancies, vacancies in the inner surfaces induces the largest decrease, followed by thevacancies in the middle of the nanotubes, and then the vacancies in the outer surfaces ofthe nanotubes. The vacancies have little influence in the elastic modulus of nanotubes. TheYoung’s modulus is essentially independent of strain rate, the yielding stress and yieldingstrain are increase with the strain rate.Finally, the molecular dynamics method is used to simulate crack healing in coppernanoplate during heating. During crack healing, the tip of crack is blunted and deforms toround shape, the crack becomes smaller and smaller until it is healed through slip bandsemitting from the pre-crack tip and expanding to the top and bottom of the coppernanoplate. The ability of crack healing is sensitive to the temperature and the size of crack.