Large Deformation and Instability of Soft Hollow Cylinder With Surface Effects

Surface stress, which is always neglected in classical elastic theories, has recently emerged as a key role in the mechanics of highly deformable soft solids. In this paper, the effect of surface stress on the deformation and instability of soft hollow cylinder is analyzed. By incorporating surface energy density function into the constitutive model of a hyper-elastic theory, explicit solutions are obtained for the large deformation of soft hollow cylinder under the uniform pressure loading and geometric everting. The surface tension and the residual surface stress have a significant effect on the large deformation and instability of the soft cylinder. When the pressure loading and geometric everting are applied on the soft hollow cylinder, significant changes in the critical condition of the creases are found by varying the surface parameters. Two models of instability, surface crease and global buckling behavior, will be generated on the soft hollow cylinder with the uniform pressure, and the formed instability model is dependent on the ratio of the thickness to the radius. The results in this work reveal that surface energy obviously influences both the deformation and the instability of soft hollow cylinder at finite deformation and will be helpful for understanding and predicting the mechanical behavior of soft structures accurately.

Surface Effects on Large Deflection of Nanobeams Subjected to a Follower Load

As a basic element of the micro/nanodevices, nanobeams have remarkable physical properties and have attracted considerable attention in the previous studies. However, previous publications did not study the large deformation problem of nanobeams under follower loading when the surface effect becomes significant and especially for the influence of surface effect on mechanical behaviors of the nanobeams under follower loading remains unclear. In this paper, we investigated the large deformation behavior of nanobeams subjected to follower loads in consideration of the surface effects. The mechanical model of large deflection of extensible cantilever nanobeams under follower loading is presented in combination with the surface elasticity and residual surface stress, and then a MATLAB program of shooting method with a technique for determining the initial value was developed to solve the problems. The results indicate that the surface effects have an important influence on the large deflection of nanobeams under follower loading: when the surface residual stress is positive, the maximums of displacement in horizontal and vertical directions and the rotation angle of the free end become lager, but the corresponding follower force related to those maximums becomes smaller. When the residual surface stress is negative, the results are the opposite. In addition, the influence of the cross-sectional dimension of the nanobeams under follower loading on surface effects was discussed. This work is beneficial to understand the mechanism of large deformation of nanobeams with surface effects subjected to follower loads, and can also provide inspirations to design advanced nanomaterials and nanoscaled devices.

Surface Effects on Effective Young’s Modulus of Nanoporous Structures

Nanoporous materials and structures have attracted widespread attention due to their excellent mechanical properties. Based on the surface elasticity, the effective Young’s moduli are derived for four typical nanoporous structures with periodic unit cells. When the cross-sectional size reduces to nanoscale, the effective Young’s modulus is revealed to be strongly size-dependent. Both the effects of residual surface stress and effective-surface Young’s modulus are examined. The results indicate that negative effective Young’s modulus can be achieved when the residual surface stress is less than zero. The influences of the cross-sectional shape on the relationship between the overall deformation and applied loads are examined. The relative density also plays an important role to the mechanical characteristics not only at macroscales, but also at nanoscales.

Elastohydrodynamic Lubrication Line Contact Based on Surface Elasticity Theory

Using surface elasticity theory, this article first analyzes the surface effect on the elastohydrodynamic lubrication (EHL) line contact between an elastic half-plane and a rigid cylindrical punch. In this theory, the surface effect is characterized with two parameters: surface elastic modulus and residual surface stress. The density and viscosity of the lubricant, considered as Newtonian fluid, vary with the fluid pressure. A numerical iterative method is proposed to simultaneously deal with the flow rheology equation, Reynolds equation, load balance equation, and film thickness equation. Then, the fluid pressure and film thickness are numerically determined at the lubricant contact region. Influences of surface elastic modulus, residual surface stress, punch radius, resultant normal load, and entraining velocity on the lubricant film thickness and fluid pressure are discussed. It is found that the surface effect has remarkable influences on the micro-/nano-scale EHL contact of elastic materials.

Material-Oriented Regularization Toward Solving Manufacturing Inverse Problem in Ion Beam Microprocessing

A material-oriented regularization (MOR) methodology is developed to solve manufacturing inverse problem of estimating the manufacture input process parameters for a required output performance, demonstrated by ion beam microprocessing of tungsten components in future fusion reactors. The MOR methodology is explored as following steps: forward problem modeling, identification of characteristic material loading, and solving the inverse problem via the characteristic material loading. A thermodynamic model is established in forward problem scheme by comprehensively incorporating material constraints of tungsten, to simulate the output of residual surface stresses in top layer of several μm that determines fatigue performance of the microprocessed tungsten component. With the experimentally verified model, all material loading variables, i.e., thermal, elastic strain, and plastic strain energies can be explicitly described under the processing load of thermal energy input. Among the material loading variables, stored elastic strain energy is identified as characteristic material loading with a highest sensitivity in correlation to residual surface stresses, as process signature. The processing load of 2.1–4.2 J/cm2 is derived for a required residual surface stress in range of 0–1500 MPa within 15 μm depth, with an upper bound of the relative error of 4.7–11.7% for the inverse problem solution. The MOR enables comprehensive incorporation of material constraints with a self-convergence effect to effectively relax the ill-posedness of manufacturing inverse problems, otherwise in conventional regularizations such constraints have to be empirically adjusted in compromise with data fitting.

Effect of Microstructure and Surface Energy on the Static and Dynamic Characteristics of FG Timoshenko Nanobeam Embedded in an Elastic Medium

This paper presents an investigation of the size-dependent static and dynamic characteristics of functionally graded (FG) Timoshenko nanobeams embedded in a double-parameter elastic medium. Unlike existing Timoshenko nanobeam models, the combined effects of surface elasticity, residual surface stress, surface mass density and Poisson’s ratio, in addition to axial deformation, are incorporated in the newly developed model. Also, the continuous gradation through the thickness of all the properties of both bulk and surface materials is considered via power law. The Navier-type solution is developed for simply supported FG nanobeam in the form of infinite power series for bending, buckling and free vibration. The obtained results agree well with those available in the literature. In addition, selected numerical results are presented to explore the effects of the material length scale parameter, surface parameters, gradient index, elastic medium, and thickness on the static and dynamic responses of FG Timoshenko nanobeams.

Studies on thermo-electro-mechanical coupling bandgaps of a piezoelectric phononic crystal nanoplate with surface effects

Applying surface piezoelectricity theory and plane wave expansion (PWE) method to the model of Kirchhoff plate, the calculation method of band structure of a piezoelectric phononic crystal (PC) nanoplate with surface effects is proposed and formalized. In order to investigate the bandgap properties of first order in the nanoplate in detail, the corresponding influence rules of thermo-electro-mechanical coupling fields, surface effects and geometric parameters on bandgaps are studied. During the researches, temperature variation, electrical voltage and external axial force are picked as the influencing parameters corresponding to thermo-electro-mechanical coupling fields. Residual surface stress and material intrinsic length are chosen as the influencing parameter related to surface effects. Lattice constant, radius of PZT-4 hole and thickness of nanoplate are picked as the influencing parameters of geometric parameters. All the results are expected to be helpful for the design of micro and nanodevices based on piezoelectric periodic nanoplates.