Results 1 - 10 of 19
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[en] Interfaces often play a significant role in many physical properties and phenomena of nanocrystalline materials (NcMs). In the present paper, the interface effects on the effective elastic property of NcMs are investigated. First, an atomic potential method is suggested for estimating the effective elastic modulus of an interface phase. Then, the Mori-Tanaka effective field method is employed to determine the overall effective elastic moduli of a nanocrystalline material, which is regarded as a binary composite consisting of a crystal or inclusion phase with regular lattice connected by an amorphous-like interface or matrix phase. Finally, the stiffening effects of strain gradients are examined on the effective elastic property by using the strain gradient theory to analyze a representative unit cell. Our analysis shows two physical mechanisms of interfaces that influence the effective stiffness and other mechanical properties of materials. One is the softening effect due to the distorted atomic structures and the increased atomic spacings in interface regions, and another is the baffling effect due to the existence of boundary layers between the interface phase and the crystalline phase
[en] A simplified analysis method for leak-before-break (LBB) analysis of pressured vessels and pipes in nuclear reactors is introduced. It is based mainly on the linear elastic fracture mechanics. The main procedures of LBB analysis are formulated analytically, which mainly include the calculation of the stress intensity factor, the crack opening area and the leakage rate. This method meets the requirements of national standard and international rules, and is very convenient to be adopted for the LBB analysis of some pressure vessels and piping with simple stress distribution and geometrical shape or for the estimation of LBB features of components
[en] With the background of leak-before-break (LBB) analysis of pressurized vessels and pipes in nuclear plants, the fatigue growth problem of either circumferential or longitudinal semi-elliptical surface cracks subjected to cyclic loading is studied by using a continuum damage mechanics method. The fatigue damage is described by a scalar damage variable. From the damage evolution equation at the crack tip, a crack growth equation similar to famous Paris' formula is derived, which shows the physical meaning of Paris' formula. Thereby, a continuum damage mechanics approach is developed to analyze the configuration evolution of surface cracks during fatigue growth
[en] Microcantilever-based techniques can be used to explore the autonomy and property of molecules (e.g. DNA and single actin filaments) adsorbed on a surface. A theoretical model is presented here to predict the resonance frequency of a cantilever induced by physically adsorbed atoms/molecules. The cantilever is modelled as a sandwich beam containing two surface layers of a finite thickness and a bulk layer between them. It is found that the resonance frequency shift depends sensitively on both the mass and bending stiffness variations of the cantilever induced by the adsorbed atoms/molecules. The adsorptions of O atoms on Si(1 0 0), of O atoms on Au(1 0 0) and of H atoms on Au(1 0 0) are taken as three representative examples. We demonstrate that physisorption can induce distinctly different resonant responses of cantilevers, depending not only on the adatoms but also on the substrate material. This study is helpful for the optimal design of microcantilever-based measurement techniques
[en] Using the linear stability analysis method, we investigate the surface wrinkling of a thin polymer coating on a cylinder in an externally applied electric field. It is demonstrated that energy competition between surface energy, van der Waals interactive potential energy and electrostatic interaction energy may lead to ordered patterns on the film surface. The analytical solutions are derived for the critical conditions of both longitudinal and circumferential instabilities. The wavelengths of the generated surface patterns can be mediated by changing the magnitude of the electric field. Our analysis shows that the surface morphology is sensitive to the curvature radius of the fiber, especially in the micrometer and nanometer length scales. Furthermore, we suggest a potential approach for fabricating hierarchical patterns on curved surfaces.
[en] As a result of capillary forces, animal hairs, carbon nanotubes or nanowires of a periodically or randomly distributed array often assemble into hierarchical structures. In this paper, the energy method is adopted to analyse the capillary adhesion of microsized hairs, which are modelled as clamped microcantilevers wetted by liquids. The critical conditions for capillary adhesion of two hairs, three hairs or two bundles of hairs are derived in terms of Young's contact angle, elastic modulus and geometric sizes of the beams. Then, the hierarchical capillary adhesion of hairs is addressed. It is found that for multiple hairs or microcantilevers, the system tends to take a hierarchical structure as a result of the minimization of the total potential energy of the system. The level number of structural hierarchy increases with the increase in the number of hairs if they are sufficiently long. Additionally, we performed experiments to verify our theoretical solutions for the adhesion of microbeams
[en] In this paper, surface effects on the axial buckling and the transverse vibration of nanowires are examined by using the refined Timoshenko beam theory. The critical compression force of axial buckling and the natural frequency of nanowires are obtained analytically, in which the impacts of surface elasticity, residual surface stress, transverse shear deformation and rotary inertia have been included. The buckling and vibration behaviour of a nanowire is demonstrated to be size dependent, especially when its cross-sectional dimension reduces to nanometres. The surface effects with positive elastic constants tend to increase the critical compression force and the natural frequency, especially for slender nanowires, while the shear deformation lowers these values for stubby nanowires. This study may be helpful to accurately measure the mechanical properties of nanowires and to design nanowire-based devices and systems.
[en] The size-dependent elastic property of nanowires induced by the surface effect is investigated by using the core-shell model. The overall effective elastic moduli of nanowires with regular polygonal cross-sections are unified into a simple and explicit relation. It is found that the effect of surface elasticity on the elastic moduli can be well characterized by two dimensionless material and geometric parameters with clear physical meaning. Finite element simulations demonstrate that the derived theoretical relation is applicable for all the vibration, bending, and buckling test methods for measuring the mechanical properties of nanowires. The analytical result is also validated by comparing it with relevant experimental measurements. This study is helpful not only for interpreting various phenomena associated with size-dependent mechanical properties of nanowires but also for developing and evaluating test techniques for material characterization at the nanoscale.
[en] This paper studies the surface instability of an elastic thin solid film lying on a rigid substrate and subjected to van der Waals-like surface interactions. The effect of film-substrate interfacial slippage is accounted for by using a simplified linear cohesive interface model. It is found that the interfacial slippage generally plays a destabilizing role in the surface instability of the thin film. For highly compressible films with Poisson's ratio smaller than 0.25, the surface wrinkling behaviour previously inconceivable in the case of a perfectly bonded interface is now feasible if film-substrate interface slipping is permitted. In addition, our linear perturbation analysis shows that the critical conditions for the onset of surface instability can be modulated by adjusting the slippery stiffness of the interface. The result might be helpful for developing novel techniques to create micro-/nanosized surface patterns.
[en] Carbon nanotube (CNT) probes offer improved imaging resolution in atomic force microscopy (AFM) and nanomanipulating devices due to their excellent mechanical properties and high aspect ratios. The basis of ascertaining scanning image quality using CNT probes is often centered on whether axial buckling has occurred or not. Here we explore the mechanical behavior and applicability of CNTs in surface scanning using molecular dynamics simulations in which the influence of van der Waals interactions is accounted for. Our results indicate the possible deleterious effects from van der Waals interaction dominated buckling of the probe, which is exacerbated by surface corrugations at the atomic scale. Under the premise that these issues can be surmounted, a cantilever model developed under known requirements for the structural characteristics of CNT probes is shown to be able to assess imaging fidelity. This model offers an effective guide to the selection and design of CNT probes for AFM. (paper)