The 23rd International Congress of Theoretical and Applied Mechanics (ICTAM2012) will be held in Beijing, China, from August 19 to 24, 2012. The Congress (ICTAM), as the most authoritative and comprehensive conference in mechanics in the world and regarded as the “Olympic games” of mechanicians, is organized by the International Union of Theoretical and Applied Mechanics (IUTAM) every four years. It brings together scientists and students from all over the world to exchange state-of-the-art achievements in the field of mechanics to promote the research, application and education of mechanics. The ICTAM2012, hosted by the Chinese Society of Theoretical and Applied Mechanics (CSTAM), is expected to promote research and applications of mechanics in China as well as throughout the world. CSTAM has decided to launch a series of activities in 2012 named “Mechanics Year 2012”, with the theme of “Mechanics · Achievement · Future”.

The dynamics equation for each individual atom is established directly around the equilibrium state of the system of N atoms based on the inter-atomic potential energy of EAM model. Using the theory of lattice dynamics and periodical boundary condition, the 3N×3N stiffness matrix in eigen equations of vibration frequencies for a parallelepiped crystal is reduced to a 3n×3n matrix of eigen equations of vibration frequencies for a unit lattice. The constitutive relation of the crystal at finite temperature is extracted based on the quantum-mechanical principle. The thermodynamic properties and the stress-strain relationships of crystal Cu with large plastic deformation at different temperatures are calculated, the calculation results agree well with experimental data.

The diffusion, viscosity and thermal conductivity coefficients of gases between two parallel solid walls have been obtained analytically based on the Green-Kubo relation under a hard-sphere model. They decrease nonlinearly as the Knudsen number defined as the ratio of the mean free path to the wall distance increases. This theoretical prediction was in good agreement by the DSMC results.

Since Mr. Tsien brought up his idea of physical mechanics, as a new field in engineering science, to public attention in the early 50’s of the 20th century, innumerable application examples of physical mechanics approach in diverse fields have manifested its strong vitality increasingly. One of important aspects in applications of physical mechanics is to appropriately choose the microscopic quantity for the system in consideration and build a bridge to connect its relevant microscopic information to its desired macroscopic properties. We present two unique cases of using the physical mechanics approach to study colloidal stability. In the first case we measured the outcomes from artificially induced collisions at individual particle levels, by means of directly observing artificially induced collisions with the aid of optical tweezers. In the second case, by using T-matrix method, the microscopic quantity extinction cross section of the doublet can be accurately evaluated and therefore the measurement range and accuracy of the turbidity methodology for determining the CRC are greatly improved.

Based on molecular force fields, a new finite element model is constructed for multi-walled carbon nanotubes where the interlayer interactions and C—C bonds are simulated by the elements of piece-wise linear spring and rectangular cross section beam, respectively. For high computation efficiency and atomic reification, the radial breathing modes of multi-walled carbon nanotubes are studied systemically using this model. The results show the correspondence between carbon nanotube structures and vibrational modes, which provide unequivocal data for the experimental characterization of carbon nanotubes. An empirical relationship of radial breathing modes frequencies with the nanotube radius are also obtained for two-layer carbon nanotubes.

Ignition delay times of China No. 3 aviation kerosene were measured behind reflected shock waves using a heated high-pressure shock tube. Experimental conditions covered a wider temperature range of 820-1500 K, at pressures of 5.5, 11 and 22 atm, equivalence ratios of 0.5, 1.0 and 1.5, and oxygen concentration of 20%. Adsorption of kerosene on the shock tube wall was taken into account. Ignition delay times were determined from the onset of the excited radical OH emission in conjunction with the pressure profiles. The experimental results of ignition delay time were correlated with the equations: τ = 3.2×10^-11[Kerosene]^0.22 [O₂]^-1.09 exp(69941/RT) and τ = 4.72×10^-7P^-0.88φ0.23 exp(62092/RT). The current measurements provide the ignition delay behavior of China No. 3 aviation kerosene at high pressures and air-like O₂ concentration.

By incorporating the contribution of solute atoms to the Helmholtz free energy of solid solution, a linear relation is derived between Young’s modulus and the concentration of solute atoms. The solute atoms can either increase or decrease Young’s modulus of solid solution, depending on the first-order derivative of the Helmholtz free energy with respect to the concentration of solute atoms. Using this relation, a closed-form solution of the chemical stress in an elastic plate is obtained when the diffusion behavior in the plate can be described by the classical Fick’s second law with convection boundary condition on one surface and zero flux on the other surface. The plate experiences tensile stress after short diffusion time due to asymmetrical diffusion, which will likely cause surface microcracking. The results show that the effect of the concentration dependence of Young’s modulus on the evolution of chemical stress in elastic plates is negligible if the change of Young’s modulus due to the diffusive motion of solute atomsis is not compatible in magnitude with Young’s modulus of the pure material. Also, a new diffusion equation is developed for strictly regular binary solid solution. The effective diffusivity is a nonlinear function of the concentration of solute atoms.

Atomic-undercoordination-induced local bond contraction, bond strength gain, and the associated temperature (T)-dependent atomic-cohesive-energy and binding-energy-density are shown to originate intrinsically the exotic paradox of superplasticity, superelasticity, and superrigidity demonstrated by solid sizing from monatomic chain to mesoscopic grain. The paradox follows these relationships: where A, B, η₁, d and ΔT_mk =T_m-T are size (K)-dependent physical parameters. T_m(K) is the melting point. Mechanical work hardening during compressing and self-heating during stretching modulate the measured outcome extrinsically. Superplasticity dominates in the solid-quasimolten-liquid transition state. The competition between the accumulation and annihilation of dislocations activates the inverse Hall-Petch relationship. Therefore, it is essential for one to discriminate the intrinsic competition between the local bond energy density gain and the atomic cohesive energy loss from the extrinsic factors of pressure and temperature in dealing with atomistic mechano-thermo dynamics.  

Unlike nonspecific adhesion of conventional hard materials in engineering commonly described by JKR and DMT type models, the molecular adhesion via specific receptor-ligand bonds is stochastic by nature and has the feature that its strength strongly depends on the medium stiffness surrounding the adhesion. In this paper, we demonstrate in a stochastic-elasticity framework that a type of materials with linearly graded elastic modulus can be designed to achieve “equal load sharing” and enhanced cooperative rebinding among interfacial molecular bonds. Upon modulus gradation, multiple molecular bonds can be elastically decoupled but statistically cooperative. In general, uniform molecular adhesion can be accomplished by two strategies: homogeneous materials with sufficient stiffness higher than a threshold or heterogeneous materials satisfying the criterion on modulus gradation. These results not only provide a theoretical principle for possible applications of functionally graded materials in quantitatively controlling cell-matrix adhesion, but also have general implications on adhesion between soft materials mediated by specific molecular binding.

In this paper, we introduce our finding of the effects of C₆₀ nanoparticles (NP) infiltration on mechanical properties of cell and its membrane. Atomic force microscopy (AFM) is used to perform indentation on both normal and C₆₀ infiltrated red blood cells (RBC) to gain data of mechanical characteristics of the membrane. Our results show that the mechanical properties of human RBC membrane seem to be altered due to the presence of C₆₀ NPs. The resistance and ultimate strength of the C₆₀ infiltrated RBC membrane significantly decrease. We also explain the mechanism of how C₆₀ NPs infiltration changes the mechanical properties of the cell membrane by predicting the structural change of the lipid bilayer caused by the C₆₀ infiltration at molecular level and analyze the interactions among molecules in the lipid bilayer. The potential hazards and application of the change in mechanical characteristics of the RBCs membrane are also discussed. Nanotoxicity of C₆₀ NPs may be significant for some biological cells.

Electroporation (EP) is one of the important techniques for the introduction of genes and drugs into cells with intense pulsed electric field to induce nanometer-sized electropores on cell membranes. Recently, micro/nano technology has been applied to many novel micro EP devices which can not only significantly increase uptake of biomolecules, DNA transfection and cell viability, but also enable large-scale single-cell EP. However, most EP theories developed in the past three decades can not precisely predict the experimental results of EP of biological cells. With the advanced micro EP chips for large-scale single-cell EP experiments, more precise EP theoretical models can be developed to describe the complicated multiscale dynamic behavior of EP.

A micromechanical model of double-walled carbon nanotube (DWCNT) pullout from a composite matrix is presented with the interfacial residual stress and van der Waals (vdW) forces taken into consideration. The interfacial residual stress induced by thermal expansion coefficient (TEC) mismatch is introduced via thermo-elastic constitutive relations. The influence of vdW interactions between two layers of DWCNT on the interfacial stress distributions of DWCNT and matrix is analyzed. The analytical expressions of interfacial shear stress and the axial stresses of DWCNT and matrix are derived, respectively. Furthermore, the influences of temperature change, interfacial friction coefficient, DWCNT aspect ratio, DWCNT volume fraction and the relative modulus between DWCNT and matrix are illustrated and discussed.

In the present research, the measurement fluctuations of mechanical properties in nanowires (NWs) are investigated by using the molecular dynamics simulation. The large numbers of simulations are performed to study the yield behaviors of the NWs. The results have shown that the yield behavior of the smaller diameter NW is more sensitive to the presence of vacancies, and the dispersion of the measured mechanical properties for the small scale NW is larger than that for the large scale NW. Present results have also shown that vacancies escape from the bulk to the free surfaces as a result of high stress applied at the small scale systems similar to the dislocation starvation phenomenon observed in the compression test of nano-pillars, and dislocation nucleation induced by surface defect occurs after the vacancy reaches free surface leading to lower yield strength. Moreover, the strong surface vacancy interactions at the nanoscale level are also investigated.

In this paper, the mechanisms of particle movement on an electric curtain are examined. Intermittent bursts in which particles leaping from a surface of an electric curtain are demonstrated both by numerical simulation and experimental observation. The hopping and surfing modes during particle removal are shown to depend on the particle electric charge and its initial position. The particles initially located on the top of electrode have a large electric charge tend to assume a hopping mode. Transverse movement of the particle initially located between the electrodes is analyzed, and the particle traps along the electric curtain surface are also observed.

Motivated by the recent biomimic design of microstructured adhesive surfaces, we study adhesion between a film-terminated fibrillar array and a rigid substrate. Using a two-dimensional model and ignoring the deformation of the fibers and the backing layer, we show that the adhesion behavior is dominated by a dimensionless parameter reflecting the global flexibility of the terminal film. In particular, if the parameter is larger than 0.4, the adhesion is reversible; otherwise one or more hysteresis loops will appear after an approach-retraction cycle, leading to significant increase in the specific separation work. The result is expected to help not only optimal design of the structure, but also other applications such as micro-manipulation in micromechanical systems.

Based on load-displacement curves, indentation is widely used to extract the elastoplastic properties of materials. It is generally believed that such a measure is non-unique and a full stress-strain curve cannot be obtained using plural sharp and deep spherical indenters. In this paper we show that by introducing an additional dimensionless function of ΔA/A (the ratio of residual area to the area of an indenter profile) in the reverse analysis, the elastoplastic properties of several unknown materials that exhibit visually indistinguishable load-displacement curves can be uniquely determined with a sharp indentation.

The fabrication technique of micro/nano-scale speckle patterns with focused ion beam (FIB) system is studied for digital image correlation (DIC) measurement under a scanning electron microscope (SEM). The speckle patterns are fabricated by directly etching the counterpart of the specimen to the black part of a template. Mean intensity gradient is used to evaluate the quality of these SEM images of speckle patterns fabricated based on different templates to select an optimum template. The pattern size depending on the displacement measurement sensitivity is adjusted by altering the magnification of FIB according to the relation curve of the etching size versus magnification. The influencing factors including etching time and ion beam current are discussed. Rigid body translation tests and rotation tests are carried out under SEM to verify the reliability of the fabricated speckle patterns. The calculated values are in good agreement with the imposed ones.

Recent studies have shown that the triple-phase contact line has critical effect on the contact angle hysteresis of surfaces. In this study, patterned surfaces with various surface structures of different area fractions were prepared by electron etching on a silicon wafer. The advancing angle, receding angle and hysteresis angle of these surfaces were measured. Our experimental results showed that while the geometry of microstructure and contact line have a minor effect on the advancing angle, they have a significant effect on the receding angle and thus the hysteresis angle. We have shown that the effect of microstructure and the contact line can be described by a quantitative parameter termed the triple-phase line ratio. The theoretical predictions were in good agreement with our experimental results.

Effects of agglomerates on the densification behavior and microstructural evolution during solid-state sintering of a cube of copper particles have been studied with discrete element method (DEM). It is found that the densification of the sintering system decreases as the volume fraction of agglomerates increases. At a given volume fraction of agglomerates, the smaller the size of agglomerates, the poorer the densification and more inhomogeneous the compact is. The morphology and distribution of agglomerates have negligible effects on the densification, especially for the case with a low volume fraction of agglomerates. Agglomerates with a smaller average coordination number would have more restriction on the densification of sintering bodies. To our best knowledge, it is the first time to study the effect of agglomerates on sintering behavior using DEM. This study should be useful for further investigations of the effect of various inhomogeneities of microstructure on the complex sintering process by DEM.

In this paper we use an alternative method to study analytically and numerically for a nonlocal elastic bar in tension. The equilibrium equation of the model is a Fredholm integral equation of the second kind. With the aid of an efficient iterative method, we are able to get the approximate analytical solution. For the purpose of comparisons, numerical solutions are also obtained for two types of nonlocal kernels, which show the validity of the analytical solution. The effects of some related parameters are also investigated.

This paper focuses on the interaction between a micro/nano curved surface and a particle located inside the surface (hereafter abbreviated as in-surface-particle). Based on the exponential pair potential (namely 1/R^2k) between particles, the interaction potential between the micro/nano curved surface and the in-surface-particle is derived. The following results are shown: (a) For an even number of exponents in the pair potential, the interaction potential between the micro/nano curved surface and the in-surface-particle can be expressed as a unified function of the mean curvature and Gaussian curvature of the curved surface; (b) the curvatures and the gradients of curvatures of the micro/nano curved surface are the essential factors that dominate the driving force acting on the particle.

We verify the accuracy of the curvature-based potential. By means of the idealized numerical experiment, we show that the curvature-based potential is in good agreement with the numerical experiment, and the errors are within a reasonable range. Based on the curvature-based potential, the equipotential surfaces of particles are derived, and the intrinsic relations between the equipotential surfaces and Weingarten helicoids are shown.

The mechanical behavior of graphene under in-plane shear is studied using molecular dynamics simulations. We show that the shear behavior of chiral graphene is dependent on the loading direction due to its structural asymmetry. The maximum shear failure strain of graphene in one direction may be 1.7 times higher than that in the opposite direction. We discuss also the influence of the cut-off parameters on the calculations. Our findings are useful for the understanding of mechanical behavior of graphene and the potential applications of graphene in nanodevices.

A mode II crack in single-crystal silicon was investigated experimentally using high-resolution transmission electron microscopy. Geometric phase analysis and numerical moiré method were employed to map the deformation fields of the crack-tip area. The normal strain field maps of the crack-tip area indeed showed the deformation occurs primarily in the vicinity of the dislocations and the normal strains are near zero in the crack-tip area. The shear strain field map shows that the relatively large shear strain is in the crack-tip area. The experimental results were compared with the predictions of linear elastic fracture mechanics. The comparison shows that measured strain distribution ahead of the crack-tip agrees with the predictions of linear elastic fracture mechanics up to 1 nm from the crack-tip.

We utilized controlled vertical drying deposition (CVDD) method, which can fabricate a uniform face-center-cubic (FCC) structure film, to investigate the crack formation and the size dependence of shear modulus in a drying particulate film. We found that both crack spacing and shear modulus depend on colloidal particle size. They drop with increase of particle radius (R) in a single range. Furthermore, compared with the shear modulus variation of a dry particulate film, it was found that both solid part and liquid part in a drying particulate film play equivalent roles in the film mechanical behavior.

The prediction of catastrophic rupture of heterogeneous brittle material has been investigated by researchers in the past. In this work, the acoustic emission generated by marble samples under compression was analyzed to verify a power law singularity of index -1/2 as a catastrophe precursor. It is found that prior to catastrophe, the variation of the system response to the controlling parameter follows a power law of negative index, which proves the power law singularity as a common precursor of catastrophe. However, the power indexes vary with variables and samples. The uncertainty reflects sample specificity of an evolution induced catastrophe (EIC) process.

Thermal vibration of single-layered graphene sheets (SLGSs) is investigated using plate model together with the law of equi-partition of energy and the molecular dynamics (MD) method based on the condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) force field. The in-plane stiffness and Poisson ratio of SLGSs are calculated by stretching SLGSs. The effective thickness of SLGSs is obtained by the MD simulations for the thermal vibration of SLGSs through the natural frequency. The root-mean-squared (RMS) amplitudes for SLGSs of differing temperatures and boundary conditions are calculated by the MD, and are compared with the results calculated by the thin plate model together with the law of equi-partition of energy. At the center of SLGSs, the thin plate theory can predict the MD results reasonably well. For the difference of bonding structure of the edge atoms, the deviation between the MD results and plate theory becomes more readily apparent near the edges of SLGSs.

The influence of specimen size on the mechanical behavior of Au pillars is studied by means of molecular dynamics (MD) simulations with the EAM potential. Under compression at 300 K, as the deformation of pillars is in the plastic stage, nucleation of partial dislocations is observed. The coupling effect of surface stress and thermal activation is considered when analyzing the size effect on the yield property of the Au pillars. It appears that both the tensile stress component and the temperature in the surface layer impart significant effect on the mechanical behaviors of the nano-sized Au pillars.

In this study, a theoretical model was established for predicting the equilibrium shape of the droplet on flat and spherical surfaces. The theoretical equilibrium shape of heavy droplets could be obtained once contact angle and volume of droplets were given. It showed that the predictions of the theoretical flat model were in good agreement with the shape obtained by Surface Evolver when the contact angle is below 120° and the droplet size is on the order of capillary length. This available range will decrease and increase when the heavy droplet is on convex and concave spherical surface, respectively, in contrast to that on flat surface. The available range will decrease more for higher curvature of convex spherical surfaces.

In this paper, an atom-continuum coupled model for thermo-mechanical behaviors in micro-nano scales is presented. A representative volume element consisting of atom clusters is used to represent the microstructure of materials. The atom motions in the RVE are divided into two phases, structural deformations and thermal vibrations. For the structural deformations, nonlinear and nonlocal deformation at atomic scales is considered. The atomistic-continuum equations are constructed based on momentum and energy conservation law. The non-locality and nonlinearity of atomistic interactions are built into the thermo-mechanical constitutive equations. The coupled atomistic-continuum thermal-mechanical simulation process is also suggested in this work.

In this paper, the possibility of the monatomic chain (MC) formation for ZnO material was studied by molecular dynamics (MD) simulation. The process of MC formation and the effects of temperature, strain rate and size were studied extensively. The tensile process can be divided to be five stages and the ZnO diatomic chain (DC) can be found. The MD results show that most atoms in MC came from the original surface of ZnO nanowires (NWs). Temperature and strain rate are two important factors affecting the process, and both high temperature and low strain rate in a certain range would be beneficial to the formation of DC. Moreover, the effects of strain rate and temperature could attribute to the Arrhenius model and the energy release mechanism. Furthermore, multi-shell structure was found for the samples under tensile strain and the layer-layer distance was about 3 ?. Our studies based on density functional theory showed that the most stable structure of ZnO DC was confirmed to be linear, and the I-V curve was also got using ATK.