Problems involving coupled multiple space and time scales offer a real challenge for conventional frame-works of either particle or continuum mechanics. In this paper, four cases studies （shear band formation in bulk metallic glasses, spallation resulting from stress wave, interaction between a probe tip and sample, the simulation of nanoindentation with molecular statistical thermodynamics） are provided to illustrate the three levels of trans-scale problems （problems due to various physical mechanisms at macro-level, problems due to micro-structural evolution at macro/micro-level, problems due to the coupling of atoms/ molecules and a finite size body at micro/nano-level） and their formulations. Accordingly, non-equilibrium statistical mechanics, coupled trans-scale equations and simultaneous solutions, and trans-scale algorithms based on atomic/molecular interaction are suggested as the three possible modes of trans-scale mechanics.
In the present research, microstructure of akind of limnetic shell （Hyriopsis cumingii） is observed and measured by using the scanning electron microscopy, and mechanical behavior experiments of the shell nacre are carried out by using bending and tensile tests. The dependence of mechanical properties of the shell nacre on its microstructure is analyzed by using a modified shear-lag model, and the overall stress-strain relation is obtained. The experimental results reveal that the mechanical properties of shell nacre strongly depend on the water contents of the limnetic shell. Dry nacre shows a brittle behavior, whereas wetting nacre displays a strong ductility. Compared to the tensile test, the bending test overestimates the strength and underestimates the Young＇s modulus. The modified shear-lag model can characterize the deformation features of nacre effectively.
The formation and mechanical properties of amorphous copper are studied using molecular dynamics simulation. The simulations of tension and shearing show that more pronounced plasticity is found under shearing, compared to tension. Apparent strain hardening and strain rate effect are observed. Interestingly, the variations of number density of atoms during deformation indicate free volume creation, especially under higher strain rate. In particular, it is found that shear induced dilatation does appear in the amorphous metal.
In the present study, peel tests and inverse analysis were performed to determine the interracial mechanical parameters for the metal film/ceramic system with an epoxy interface layer between film and ceramic. Al films with a series of thicknesses between 20 and 250 μm and three peel angles of 90°, 135° and 180° were considered. A finite element model with the cohesive zone elements was used to simulate the peeling process. The finite element results were taken as the training data of a neural network in the inverse analysis. The interracial cohesive energy and the separation strength can be determined based on the inverse analysis and peel experimental result
La57.6Al17.5(Cu,Ni)24.9 and La64Al14(Cu,Ni)22 bulk metallic glasses (BMGs) were prepared by copper-mould casting method. Plastic deformation behavior of the two BMGs at various loading rates was studied by nanoindentation. The results showed that the La57.6Al17.5(Cu,Ni)24.9 BMG with a glass transition temperature of 423 K exhibited prominent serrated flow at low loading rates, whereas less pronounced serrated flow at high rates during nanoindentation. In contrast, the La64Al14(Cu,Ni)22 BMG with a glass transition temperature of 401 K exhibited prominent serrated flow at high loading rates. The different rate dependency of serrated flow in the two La-based BMGs is related to the different glass transition temperature, and consequently the degree of viscous flow during indentation at room temperature. A smoother flow occurs in the alloy with relatively lower glass transition temperature, due to the relaxation of stress concentration.
Wei Peng1), Taihua Zhang2), Yuan Liu1), Lei Li1), and Bingchen Wei1) 1) National Microgravity Laboratory, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, China 2) State Key Laboratory of Nonlinear Mechanics (LNM), Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, China
The B- and P-doped hydrogenated nanocrystalline silicon films （nc-Si：H） are prepared by plasma-enhanced chemical vapour deposition （PECVD）. The microstructures of doped nc-Si＇H films are carefully and systematically characterized by using high resolution electron microscopy （HREM）, Raman scattering, x-ray diffraction （XRD）, Auger electron spectroscopy （AES）, and resonant nucleus reaction （RNR）. The results show that as the doping concentration of PH3 increases, the average grain size （d） tends to decrease and the crystalline volume percentage （Xc） increases simultaneously. For the B-doped samples, as the doping concentration of B2H6 increases, no obvious change in the value of d is observed, but the value of Xc is found to decrease. This is especially apparent in the case of heavy B2H6 doped samples, where the films change from nanocrystalline to amorphous.
Recently, people are confused with two opposite variations of elastic modulus with decreasing size of nano scale sample： elastic modulus either decreases or increases with decreasing sample size. In this paper, based on intermolecular potentials and a one dimensional model, we provide a unified understanding of the two opposite size effects. Firstly, we analyzed the microstructural variation near the surface of an fcc nanofilm based on the Lennard-Jones potential. It is found that the atomic lattice near the surface becomes looser in comparison with the bulk, indicating that atoms in the bulk are located at the balance of repulsive forces, and the elastic moduli decrease with the decreasing thickness of the film accordingly. In addition, the decrease in moduli should be attributed to both the looser surface layer and smaller coordination number of surface atoms. Furthermore, it is found that both looser and tighter lattice near the surface can appear for a general pair potential and the governing mechanism should be attributed to the surplus of the nearest force to all other long range interactions in the pair potential. Surprisingly, the surplus can be simply expressed by a sum of the long range interactions and the sum being positive or negative determines the looser or tighter lattice near surface respectively. To justify this concept, we examined ZnO in terms of Buckingham potential with long range Coulomb interactions. It is found that compared to its bulk lattice, the ZnO lattice near the surface becomes tighter, indicating the atoms in the bulk are located at the balance of attractive forces, owing to the long range Coulomb interaction. Correspondingly, the elastic modulus of one-dimensional ZnO chain increases with decreasing size. Finally, a kind of many-body potential for Cu was examined. In this case, the surface layer becomes tighter than the bulk and the modulus increases with deceasing size, owing to the long range repulsive pair interaction, as well as the cohesive many-body interaction caused by
This study investigated the process parameter effects on the structural and optical properties of ZnO thin film using radio frequency (RF) magnetron sputtering on amorphous glass substrates. The process parameters included RF power and working pressure. Results show that RF power was increased to promote the crystalline quality and decrease ZnO thin film defects. However, when the working pressure was increased to 3 Pa the ZnO thin film crystalline quality became worse. At a 200 W RF power and 1 Pa working pressure, the ZnO thin film with an optical band gap energy of 3.225 eV was obtained.
