The diffusion behavior of interstitial hydrogen in steel should be clarified to reveal the mechanism of hydrogen embrittlement. In this study, we performed molecular dynamics (MD) simulations to elucidate the relationship between various stress conditions and the diffusion coefficients of interstitial hydrogen in body-centered-cubic (bcc) iron. The results revealed that the diffusion coefficients are independent of isotropic stress and exhibit strong anisotropic stress de-pendence under uniaxial stress along the 〈100〉 direction. The deformation of the atomic structure around the octahedral interstitial site (O site) resulting from stress was examined to elucidate the origin of the anisotropy of the stress depen-dence. Moreover, a model that predicts the activation enthalpy was derived by quantitatively evaluating the deformation around the O site. The activation enthalpy predicted by the model was consistent with the results of MD simulations when the deformation around the O site was not significant.
Graphite is carbon nanomaterial with laminated structure as perfect crystal, which has excellent mechanical properties. In this work, our purpose is to obtain the fundamental information about controlling the out-of-plane deformation of nano layered graphene with lattice defects. We investigate simulation of multi-layer graphene which is controlled by the arrangement of lattice defects under compression using molecular dynamics method. The results of simulation show that out-of-plane deformation such as delamination and kink deformation occur in the multi-layer graphene with dislocations under compression. In addition, we can also observe that delamination usually occurs near the dislocations. This characteristical deformation is affected by the position and misorientation angle of dislocation array. Compressive stress-compressive strain curves of the simulation show that maximum compressive stress becomes relatively higher as increasing the number of dislocations. From these results, there is some possibility of controlling compressive deformation of graphene by arranging dislocations regularly.
Engineering applications of high-strength martensitic steels have been limited because of concerns regarding delayed fracture. Recent experimental and theoretical studies revealed that the presence of mobile hydrogen moving toward cleavage crack surfaces in these steels plays an important role in delayed fracture. Although the accumulation rate of hydrogen is affected by its own concentration and diffusion coefficient, it is still unclear how these properties are influenced by varying carbon content in martensitic steel. In this study, we estimated the hydrogen accumulation rate in martensitic steel under different carbon contents (0.395 and 0.787 mass %) and temperatures (300, 700, and 1000 K) by calculating its hydrogen solubility and diffusion coefficient using computational methods such as the density functional theory, molecular dynamics simulation, and finite element method. Results revealed that the hydrogen accumulation rate tended to increase with the carbon content and vice versa.
High-Entropy Alloys (HEAs) are alloys in solid solution with five or more elements and exhibit excellent mechan-ical properties. However, the mechanism has not been fully understood yet. Because general grain boundaries (GBs) contain various size of atomic free volume, a deviation of atomic compositions at GBs may appear in HEAs by replacing atoms with different atomic sizes to reduce atomic free volume at GBs. In this study, the influence of the deviation of atomic compositions at GBs on dislocation emission from GBs in HEAs is investigated through two dimensional atomic simulations. Various equiatomic HEAs with five elements are modeled by a modified Morse (two body atomic) potential. Thermal equilibrium GBs at finite temperatures are obtained by hybrid Monte Carlo-molecular dynamics simulations. As results, GBs in HEAs mainly consist of two elements with the minimum and maximum atomic size and the critical stress to emit dislocations from the GBs increases as the deviation of atomic compositions becomes large.
Appropriate ceramic materials with strong adhesion to B-DNA, which is an eco-friendly ( biodegradable and biocompatible) organic material considered to be used for electronics devices, were selected by using a combination of an orthogonal array and a response-surface method. In this technique, at the first stage, the important factors that significantly influence the adhesion strength were selected from various factors that characterize ceramic materials by using an orthogonal array with molecular dynamics simulations. As a result, the short-side and long-side lattice constants (a and b ) were selected from four ceramic-material factors (a, b, the surface energy, S, and the cohesive energy, C ). At the second stage, the adhesion strength was described as a function of the selected important factors by using a response-surface method. From this function, the optimal solution (the best values for a and b) that made the adhesion strength maximum was obtained. The best values for a and b were obtained as 0.340 nm and 0.589 nm, respectively. At the third stage, the best ceramic material whose lattice constants were equal to the best values (a =0.340 nm and b =0.589 nm), which are B-DNA’s lattice constants, was selected by use of the simulation results of lattice constants. As a result, ZrO2/CaO/HfO2, HfO2/CaO/SrO and CaO/SrO/HfO2, whose lattice constants were a =0.340 nm and b =0.589 nm, were selected as the best ceramic materials with the strongest adhesion to B-DNA. The results show that lattice matching is important in the adhesion between B-DNA and ceramics.
The micropolar body with an additional microscopic degree of freedom (DOF) can diversely extend the deformability by modeling the rotational material point instead of the classical rigid one. It contains three DOFs at any point in addition to the macroscopic three DOFs. Based on totally 6 DOFs, two kinds of strain tensors, which total number of components reaches 18, are defined and the conjugate stress and couple stress tensors are obtained via the constitutive laws. In order to make clear of the macroscopic nonlocal elastic properties of micropolar body, two-scale homogenization method based on such microcontinuum theories has been formulated in the finite element method. The orthogonal and oblique lattice framework structures consisting of micropolar beams are analyzed to summarize the size dependence of unit cell. The framework structure consisting of beams, which deformation is expressed by the angle of deflection as well as the deflection, is strongly affected by the surrounding deformation state. Therefore, the beam has been recognized as a pseudo model of nonlocality. The results point out some noticeable facts. First, even if the microscopic framework structure doesn’t have any nonlocality, that is, it just contains the conventional local elastic constants, the nonlocal elastic constants of the homogenized macroscopic body exist because the rotation DOF in the microcontinuum is the same dimension as the angle of deflection in beam. Also, the micropolar body clearly illustrates the size effect for the different size of unit cell with providing the micrometer sized characteristic length.
This paper clarifies the flow analysis method of fluidization-treated soils using 3D granules analysis by distinct element method (DEM) and the validity of the method. In recent years, fluidization-treated soils have been widely used as a material for landfilling such as underground space and so on in Japan. However, in the design and construction of fluidization-treated soils, it is based on the existing rule of thumb. In this study, the authors considered the applicability of 3D granules analysis by DEM in the flowability evaluation of fluidization-treated soils. As a result, it was clarified that flowability of fluidization-treated soil can be well reproduced by using 3D granules analysis by DEM. In addition, flowability was compared and evaluated by simulating pumping of two types of fluidization-treated soils using 3D granules analysis by DEM.
SEC method is one type of two-stage-mixing method of concrete. It had been found that SEC concrete produced by this mixing method had reduced bleeding, improved segregation resistance and high flowability under vibration, and improved strength characteristics and durability. Also, the fact that SEC concrete exhibited excellent properties after hardening was related to the bleeding reduction effect of this concrete. Therefore, in order to improve the bleeding characteristics of this concrete, the authors examined the influence of differences in concrete mixing method, procedure of materials charging, and material temperature. As a result, it was found that the bleeding of cement paste, mortar and concrete produced by the SEC (Sand Enveloped with Cement) method were reduced to about 40% compared to the case of normal mixing of the same proportion. The authors proposed a model on the composition of cement particles in concrete, which can explain the reduction of bleeding, the improvement of material segregation resistance and the fluidity under vibration in such fresh concrete in which differ the mixing method. In the future, the authors will plan to clarify the certainty of proposed model produced by this methods such as trying to visualize cement particle structure.