Ab-initio density functional theory (DFT) calculations on the elastic coefficients at no-load equilibrium, (001) surface energy and stability limit under the  static tension are implemented for eight fcc, four bcc and four hcp metals. The elastic coefficients and surface energy are compared with DFT database of Materials Project, Zhou’s EAM potential, and experimental data available. The stability limit is evaluated with the eigenvalues of the elastic stiffness matrix, Bij = Δσi/Δεj . Here Δσi and Δεj are the change in the stress and strain in Voigt notation. Most elements show instability for the eigenvalues of ηBorn(= B11 − B12) or ηspinodal of the 3x3 partial matrix of Bij . Pt and W show instability for shear component B44 while Fe does for B66. Then the separation energy of bimetal interface for 120 combinations of these 16 metal elements are estimated from the energy deference of adhered/separated interface in a supercell of stacked unit lattices.
In order to investigate the synergistic effect of different deformation modes (dislocations and local atomistic rear-rangements) on the strength of mixed crystalline and amorphous materials, tensile and compressive deformation analyses of a two-dimensional binary system with various microstructures were carried out using molecular dynamics simulations. The binary system is composed of atoms with two different atomic radii. By varying the mixing ratio and the interac-tion force between different atoms, 66 binary system models with various structures are represented, and each model is classiﬁed into three categories: crystalline, amorphous, and mixed crystalline/amorphous models by structural analysis. Deformation analysis shows that the strength of the mixed crystalline/amorphous models tends to be weaker than that of the crystal and amorphous models. The reason for this is that the edge of the force chain in the amorphous phase appears at the crystalline/amorphous interface, where dislocation release from the interface to the crystalline phase occurs easily.
We have investigated the peeling behavior between pressure-sensitive adhesive and silicon wafer by using molecular dynamics method. Our aim is to clarify the effect of the molecular weight of adhesives to the peeling behavior. We have conducted peeling simulations with various molecular weight of adhesives and various surface roughness of silicon substrate as modeled by amorphous SiO2. We focus on the stress-displacement curve, the maximum value of peeling stress and the energy required for the peeling. It was found that the larger molecular weight and larger surface roughness induced larger peeling energy. Peeling energy is dependent on the interface bonding status. On the contrast, maximum value of peeling stress partially correlates with the molecular weight. From the detailed observation of peeling process, it was found that peeling stress is dependent on the network of adhesive molecule chains. The maximum value of the peel stress increases as the number of partial molecular chains constrained to the substrate, and the number of the partial molecular chains connected to the constrained molecular chain via increase of cross-links. In addition, the number of interfaces of the connected partial molecular chain increases the maximum peeling stress. Our results provide a new insight to the design of adhesive.
Polycarbonate ﬁnds a wide range of applications as structural materials due to the prominent mechanical and physical properties. It is urged to clarify the molecular origin of its mechanical properties for better designing the molecular structure. In this study, we investigated the effect of entanglement and spatial distribution of molecules in the initial structure on fracture stress by means of coarse-grained molecular dynamics (CGMD) simulation. By altering the method of creation of initial molecular structures, we successfully obtained wide variations of the number of entanglements (Ne) and the radius of gyration (Rg). Using the obtained structures as the initial conﬁguration, we performed uniaxial tension simulations to evaluate the maximum stress (σmax) after yielding. We found that σmax cannot be described by either Ne or Rg alone but is expressed as a function of both of them (σmax = f(Ne,Rg)).
A multi-scale analysis of elasto-plastic deformation and dislocation movement and accumulation of tri-crystalline models during cyclic deformation was performed. The accumulation of dislocations in geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) was investigated in detail. Experimental observations have shown that fatigue life and fatigue crack propagation are also deteriorated when the material is subjected to overload or underload. However, in the microscopic and mesoscopic regions, where the effects of the crack tip shape change are not related to the size of the crack tip, a different phenomenon is essentially occurring. In this paper, the effects of overload and underload are investigated using a simple model without a crack. From the analysis results, it was found that the dislocation structure generated in the material subjected to overload or underload affects the subsequent deformation behavior. However, even though the absolute values are the same, the effects of overloading and underloading on the subsequent deformation are different, and underloading increases the non-uniformity of the subsequent deformation, which is on the dangerous side, while overloading decreases with increasing the number of repetitions.
The industrial manufacturing methods for ceramics are powder mixing, molding, and firing. Ceramics are fired at a higher temperature than metal sintering. For this reason, in the ceramics manufacturing process, a large amount of energy is consumed, and a large amount of carbon dioxide is also emitted, especially in the firing process. Therefore, attention is focused on the non-firing solidification process of ceramics. In this method, after the molding process, there is a solidification process using a solvent instead of firing. In order to realize this solidification process, a grinding process is required to increase the activation energy of the surface of the raw ceramics particle. Therefore, in this study, we set up a molecular dynamics model that simulated grinding and calculated the activation of the silica surface. The grinding of the material surface was modified by the cylindrical indenter of LAMMPS, the material surface was constantly activated by passing multiple indenters continuously instead of a single indenter. As a result, a clear increase in energy was observed. It was suggested that continuous energy input is more effective than local energy input to the surface when reproducing surface activity. Furthermore, activation of the internal structure was observed as in the experiment. Adding water molecules in the relaxation calculation on the activated surface, binding through and without water molecules was observed. It was clarified that there are hydrogen bonds and siloxane bonds in this bond.
This study investigates the multiaxial creep rupture life and creep properties of Type 304 austenitic stainless steel by conducting multiaxial creep tests at 923 K. As a multiaxial testing method, axial load and torsional torque were applied to a hollow cylindrical specimen using a combined tension-torsion testing machine. The time-strain curve was consistent up to about 10%, using the equivalent von Mises strain, independently of the difference in the multiaxial stress conditions. The creep rupture life in the pure torsion was longer than that in tensile testing, which may be because the specimen cross-sectional area remained almost unchanged in the pure torsion. The multiaxial creep rupture lives are evaluated to be within a factor of 2 scatter band using the equivalent von Mises stress which has the lowest scatter among the stress parameters. The observations of the longitudinal microstructure after the creep test showed that the crystal grains were deformed in the principal stress direction with the creep duration. In this experimental condition, the fracture was dominated by stress deformation, and the equivalent von Mises type would have been appropriate for creep deformation analysis.
Water saturation conditions in underground rock masses vary with place and time. Hence for estimating the deformation, failure, and time dependent behavior of rock around underground openings, it is essential to consider water saturation in rock mechanical models. In this study, on the basis of recent experimental results, the effect of water saturation was implemented in the variable-compliance-type model based on non-linear viscoelastic theory. The previous variable-compliance-type model possessed the three constants of which values change with a change of water saturation and have been unformulated. In contrast, the constants for water saturation and for time dependence were separated in the proposed model with adopting a new function of stress. Uniaxial compression strength tests and creep tests of andesite were simulated with this proposed model. The calculated results of the stress-strain curves and their water- and time-dependence were coincident with the experimental results. It was found from the calculated and experimental results that the effects of water saturation and strain rate on strength are convertible and that the effects of water saturation and creep stress on creep lifetime are also convertible. These results indicate that creep lifetime in arbitrary water saturation conditions is estimated from the strain rate dependence of strength using the model.
Thermal barrier coatings (TBCs) are important in improving the efficiency of gas turbine engines. TBCs are generally produced by atmospheric plasma spraying, resulting in a lamellar microstructure with pores and cracks. The stress-strain relations at high temperatures are one of the most important mechanical properties for the stress analysis of TBC. The previous studies by curvature measurement during thermal cycling revealed that TBC shows in-plane nonlinear behavior attributed to the unique microstructure. However, there has been little discussion on temperature-dependence of non-linear stress-strain relationship. In this study, we propose a new method to determine the nonlinear stress-strain relation at each temperature. This method evaluates the stress-strain responses based on the temperature- and stress-dependence of coating secant modulus of elasticity, where the nonlinearity was quantified by the stress-dependent decrease rate of secant modulus. The experiments were made on two-layered specimens, consisting of ceramic top coats and Ni-based superalloy substrate, with the different ratios of coating thickness to substrate thickness. The temperature range of thermal cycling was between room temperature and 915℃. Specimens in as-sprayed and heat-treated at 1000℃ conditions were tested to investigate the effects of sintering. It was found from the results of as-sprayed specimens that the nonlinearity of stress-strain relation decreased above 600℃ due to sintering densification. For heat-treated specimens, the nonlinearity of stress-strain relation became high with rising temperature.