Study Of Size Effects On The Mechanical Behaviors And Properties Of Micro-nano Structures

With the advances of science and technology, much more attentions have been paid on the properties and applications of micro-nano structures. As a result of size effects, micro-nano structures’mechanical properties and behaviors are usually quite different with those of the macro structures. The differences not only help the micro-nano structures have broad applying prospects in many fields, but also bring lots of challenges. Within the thesis, we modeled and analyzed the mechanical behaviors and properties of three important micro-nano structures:micro-nano tubes, nanoparticles and crystalline surface.(1) Wave propagations in micro-nano tubes. We built a wave propagation model of orthotropic shell based on the nonlocal elastic theory, and then obtained a series of analytical solutions of phase velocity, group velocity and frequency. The model has been used on the propagation of mechanical signals in microtubules. It found that the nonlocal effects will reduce the phase velocity of mechanical signals in microtubules, and the reduced degree increases with growing circular half wave number. When the axial wavelength is smaller than 80nm, the signals are strongly impacted by nonlocal effects and sensitive to the value of nonlocal parameters. Also, the signals are under the influence of temperature. There are two zones of high and low temperature in which the signals keep at certain stability. In addition, the model is used to check the frequency spectrum of micro-nano tubes. It discussed the forbidden higher frequency due to the limit of wave number, and further theoretical results predict that forbidden bands of elastic wave exist in some special tubes. It is a new probable mechanism of forbidden bands. We provided some ideas from the view of designing materials.(2) Radial vibration of nanoparticles. A surface elastic theory depending on surface curvature is utilized to develop a surface elastic model of sphere symmetric problems. With the elastic mechanics, we obtained the radial vibration of nanoparticles under the influence of surface effects. We calculated the fundamental frequency with both modified elastic model and high-accuracy atomic theory and their results is in good agreement. The frequency governing equation indicates surface factor in particles’radial vibration is surface elasticity rather than surface stress in the strict sense. When the radius of a particle is smaller than 2.5nm, the effects of surface are strong and size-dependent. The effects of surface curvature cannot be ignored when the radius is smaller than 2nm. In addition, numerical results show surface effects are significant in the fundamental frequency but implicit in the radial vibrations whose order is larger than three. By contrast, the nonlocal effects are significant in the higher order vibration but implicit in fundamental vibration.(3) Geometric modeling and layer simulation of surface layers on nanocrystal. We reviewed the geometric modeling ideas in present nano-solids surface mechanics theory and proposed a method to distinguish surface atomic layer from the inner bulk with molecular dynamics simulations. We examined the layered simulations of FCC mental and halite crystals and indicated their surface thickness. The calculated results also reflect depth-dependence of surface layers’ properties visually. Further observations of the results present that long range interactions between atoms can disturbance the properties of inner surface layers, which are not large but reach a great range than our common sense on surface thickness. Due to the results of the thesis, we suggested that the application of Gurtin-Murdoch surface theory on the characteristic dimension that is larger than 5nm.