Advanced Materials Research Vols. 33-37

Abstract: In this study, the rate-dependent subloading-friction model, which can rationally describe the reciprocal transition of static-kinetic frictions by the unified formulation, is proposed. Then, the one-dimensional model of spring-mass system is implemented by incorporating the present friction model, and is applied to simulations of stick-slip motion. Further, we verified the validity of the present approach for the stick-slip motion by numerical experiments under various dynamic conditions.

Abstract: With the development of material science, especially as MEMS/NEMS are playing a more and more important role in modern engineering, some mechanical behaviors of materials, e.g., fracture, shear instability, need to be investigated from multidisciplinary perspective. The molecular dynamics (MD) simulations are performed on single-crystal copper block under simple shear to investigate the size and strain rate effects on the mechanical responses of face-centered cubic (fcc) metals. It is shown that the yield stress decreases with the specimen size and increases with the strain rate. Based on the theory of dislocation nucleation, a modified power law is proposed to predict the scaling behavior of fcc metals. In the MD simulations with different strain rates, a critical strain rate exists for each single-crystal copper block of given size, below which the yield stress is nearly insensitive to the strain rate. A hyper-surface is therefore formulated to describe the combined size and strain rate effects on the plastic yield stress of fcc metals.

Abstract: In most engineering applications, solutions derived from the lower bound theorem of limit analysis are particularly valuable because they provide a safe estimate of the load that will cause collapse. In this paper, the lower bound theorem is firstly implemented making use of the meshless local Petrov-Galerkin (MLPG) method with natural neighbour interpolation. In the present MLPG formulation, the natural neighbour interpolation is employed for constructing trial functions, while the three-node triangular FEM shape function is used as the test function over a local sub-domain. The self-equilibrium stress field is expressed by linear combination of several self-equilibrium stress basis vectors with parameters to be determined. These self-equilibrium stress basis vectors can be generated by performing an equilibrium iteration procedure during elasto-plastic incremental analysis. The Complex method is used to solve these nonlinear programming sub-problems and determine the maximal load amplifier. The numerical results show that the present solution procedure for limit analysis is effective and accurate.

Abstract: This paper describes the computer simulation of irradiation growths induced by neutron irradiations in zirconium using a combination of Molecular Dynamics (MD) and Kinetic Monte Carlo (KMC) methods. First, we performed the MD simulation of the displacement cascade on a defect cluster to study the interaction between the defect cluster and the displacement cascade. The MD simulations provide a lot of information on the amount of the defect production and the subsequent morphological change in the defect cluster. The results are used to make simple models that describe the nature of the displacement cascade overlap on the defect clusters. The models are then implemented into the KMC simulation code to extend the length- and time-scale of the simulation, which allows us to evaluate directly the defect cluster accumulations during a long-term irradiation. The irradiation growth strain resulting from the defect cluster accumulations is simply evaluated, and compared to an available experimental data. The comparison suggests that the displacement cascade overlap plays an important role on the irradiation growth, and, consequently, the KMC method with the simple models must be appropriate for the simulations of the irradiation growth.

Abstract: This paper provides the results of the MD simulations of the interaction between a screw dislocation and a copper precipitate in iron. From the results, the screw dislocation has an attractive interaction with the copper precipitate. Also, the dependence of the Critical Resolved Shear Stress (CRSS) for the screw dislocation to break away from the copper precipitate on the size of the precipitate and temperature is studied. Finally, the CRSS obtained by the MD simulations is modeled statistically using a Russel-Brown model. Then we found that an addition of the Peierls stress, which is calculated by the MD simulations, to the Russel-Brown model gives a good prediction of the CRSS.

Abstract: This paper describes the development of a software to perform three-dimensional crack propagation analyses. The software is based on the conventional finite element method with second order tetrahedral element and an automatic mesh generation software. Hexahedral finite elements have historically been used in fracture analyses and methodologies to compute the crack parameters have been developed for the hexahedral elements. In present research, the authors have developed a VCCM (virtual crack closure-integral method) for the second order tetrahedral finite element. Use of the tetrahedral element allows us to utilize an automatic mesh generation software. The direction and rate of crack propagation are predicted based on the stress intensity factors and the shape of crack is updated. Hence, a software package containing the modules for mesh generation, for finite element analysis, for stress intensity factor evaluation, for predicting the rate and the direction of crack propagation and for updating crack configuration, can be developed.

Abstract: We have been developing an advanced general-purpose computational mechanics system, named ADVENTURE, which is designed to be able to analyze a three dimensional finite element model of arbitrary shape over 100 million Degrees Of Freedom (DOF) mesh. The one of main process modules for solid analysis, named ADVENTURE_Solid, is based the hierarchical domain decomposition parallel algorithm and employs the balancing domain decomposition as a solution technique for linearized equations. The ADVENTURE_Solid has been successfully implemented on a single PC, PC clusters and MPPs with high parallel performances. Since the software has become a quite large system, it is not easy to install or operate the system on parallel machines by users. In this paper, to operate such a system from a client PC through the network, the Web-based CAE system is developed.

Abstract: As the size of ferroelectric nanostructures decreases, the fundamental questions of the size dependence of the ferroelectric properties, and of their possible disappearance at a finite critical size, become crucial. In this paper, Landau-Ginzburg-Devonshire(LGD) theory is introduced to study the size-dependent ferroelectric properties of BaTiO3 nanowires. Our results demonstrate that the Curie temperature and mean polarization decrease with the reducing of nanowire diameters. We further show that these size effects are significant only when the nanowire diameters are less than 20nm, Above this, the ferroelectric behaviors of nanowires are almost the same as those of bulk materials and size effect can be neglected.

Abstract: This paper employs static atomistic simulations to investigate the effect of a void on the nanoindentation of Cu(111). The simulations minimize the potential energy of the complete system via finite element formulation to identify the equilibrium configuration of any deformed state. The size and depth of the void are treated as two variable parameters. The numerical results reveal that the void disappears when the indentation depth is sufficiently large. A stress concentration is observed at the internal surface of the void in all simulations cases. The results indicate that the presence of a void has a significant influence on the nanohardness extracted from the nanoindentation tests.

Abstract: The mechanism of fatigue crack nucleation for nanocrystalline (nc) nickel was experimentally investigated in this paper. The samples of electrodeposited nc nickel were loaded cyclically by using a three point bending instrument at first. Then, atomic force microscopy (AFM) was used to scanning the sample surface after fatigue testing. The results indicated that, after fatigue testing, there are vortex-like cells with an average size of 108nm appeared along the crack on nc nickel sample. And, the roughness of sample surface increased with the maximum stress at the surface