Ryosuke Matsumoto

Size and volume-fraction effects of dispersed nano-crystalline particles on the elastic constants and flow stress of metallic glass

The particle size and crystal volume fraction effects on Youngs modulus and the flow stress of metallic glass with nano-crystalline particles are studied in this paper using molecular dynamics simulations. The investigated volume fraction ranges from 0% (amorphous) to 100% (nano-crystalline metal), and the average particle diameters are 6 and 12 nm. The Youngs modulus of the nano-composite is increased as the average particle size and the crystal volume fraction increase. We also demonstrate that the elastic constants can be accurately estimated by using a self-consistent compliance model based on the equivalent inclusion method. The flow stress is not affected by the particle size when the crystal volume fraction is lower than 60%. The particle size effects appear gradually for higher crystal volume fraction materials; this phenomenon is relevant to an increase in the fraction of the grain boundary in the entire interface.

Atomistic Study on Ideal Strength of Nanocrystal and Deformation Induced Nanostructures

Molecular dynamics simulations are carried out to obtain fundamental knowledge of the mechanical properties of nanocrystalline metal and mechanisms of grain refinement. First, tensile tests of aluminum nanocrystals with different grain sizes are conducted. As the result, an ideally high flow strength is shown in the case of 40 nm grain size. The deformation mechanisms are discussed based on the detail investigation of atomic configuration and the ideal grain size corresponds to the threshold, at which the deformation mode is switched from the dislocation controlled deformation to the grain boundary controlled deformation and vice versa. Second, single- and poly-crystalline iron nanowires in torsion are examined by using twisted periodic boundary condition (TPBC) which is newly proposed. Various mechanical properties of nanowires, e.g. anisotropy, grain size dependence, and specimen size effect, are studied through torque versus twist relationship, and the process of polycrystallization from single crystal and internal structural change of nanocrystals are clarified. Third, an tensile test of double-notched test-piece of amorphous iron is examined for the deformation induced crystallization mechanism.

Development of Deformation Measurement System Consisting of High‐Speed Camera and Digital Image Correlation, and Its Application to the Measurement of Large Inhomogeneous Deformations Around the Crack Tip

Over the last few decades, many experimental methods using digital image correlation (DIC) have been developed as displacement measurement techniques because DIC offers a simple, noncontact procedure for fullfield measurement. However, in addition to high spatial resolutions, it is now necessary to measure deformation at higher temporal resolution to understand in more detail the time evolution of microscale deformation, and to evaluate high-speed microscale phenomena such as crack growth. Therefore, we developed a system for measuring deformation fields at high spatial and temporal resolutions using a high-speed camera, zoom lens, DIC, and incremental evaluation method. We evaluated its accuracy, and used it to measure the deformation field around a crack tip in carbon steel. First, we performed three types of accuracy estimation for rigid displacement. Second, we applied this system to a smooth specimen of carbon steel subjected to tensile loading, and verified the measurement accuracy. Third, we applied this system to a Mode I crack in carbon steel and obtained the evolution of the strain distribution around the crack tip, where a large inhomogeneous deformation had occurred.

Molecular Dynamics Analysis on Crack Growth Behavior in Single and Nano-Crystalline Fe by the Use of FS-2NNMEAM Hybrid Potential

In recent years, nano-crystalline materials have attracted many researchers’ attention, but the fracture mechanism has not been fully clarified. In a molecular dynamics (MD) simulation, grain size and crystal orientation can be chosen, and their effects on the mechanical properties of nano-crystalline materials can be evaluated clearly. This research first compares the results of crack growth behavior in single crystalline Fe for three typical interatomic potentials (Embedded Atom Method (EAM), Finnis Sinclair (FS), and Second Nearest Neighbor Modified EAM (2NNMEAM) potentials) and a Hybrid potential method, which uses FS potential for bcc structure atoms and 2NNMEAM potential for non-bcc structure atoms. The 2NNMEAM potential is accurate, but the computation time is dozens of times that of FS potential, which is the simplest of the three interatomic potentials. Therefore, the 2NNMEAM potential requires too much calculation for the purpose of this research that analyzes the crack growth behavior in nano-crystalline metals. However, Hybrid potential is able to give results similar to those of the 2NNMEAM potential, and the calculation time is close to that of the FS potential. From these results, the crack extension behavior in relatively large nano-crystalline models is analyzed using the Hybrid potential, and we demonstrate the grain-size dependency of the fracture behavior.

Access Control Architecture Separating Privilege by a Thread on a Web Server

In Web hosting services, hosting systems use access controls like suEXEC on apache Web servers to separate privilege by each virtual host. However, existing access control architectures on Web servers have a problem in their low performance and are not appropriate for dynamic contents like Web API since these architectures require termination of the process after each HTTP session. System developers are not easy to install existing access controls since these are provided by each interpreter and program execution methods conventionally. In this paper, we propose the access control architecture gmod_process_securityh. In this architecture a server process creates a new thread on the server process when accepting a request. Then, the web server separates privilege by the thread and processes the contents on the thread. The server process installed gmod_process_securityh executes programs faster. System developers can easily install it on web servers since we replace it with the complicated existing access controls. gmod_process_securityh can be installed for Apache HTTP Server on Linux as Apache Module which is widely used.

A Mail Transfer System Selectively Restricting a Huge Amount of E-Mails

With the wide use of e-mail, it has become commonly expected that e-mails sent are delivered to recipients immediately. It is not rare that a huge amount of emails are posted to a mail transfer system in very short time unintentionally, due to misconfiguration such as an e-mail loop. Such e-mail flood increases the load of the mail transfer system, and causes serious delays in e-mail delivery of the system. We propose a mail transfer system with resiliency against e-mail flood. In the proposed system, the number of each pair of envelope-from and envelope-to is counted. If the number of sessions including a specific pair (envelope-from, envelope-to) during a short period exceeds a pre-specified threshold, we regard this pair as a cause of a huge amount of e-mails, and reject e-mails including this pair after that. By this way, a huge amount of e-mails are selectively restricted without affecting transfer of ordinary e-mails.

Design of Cooperative Load Distribution for Addressing Flash Crowds Using P2P File Sharing Network

The flash crowd is a network phenomenon where a network or host suddenly receives a lot of traffic. It is a serious problem for web servers because it causes the websites temporarily unavailable. However, conventional load distribution methods could not address this problem effectively by reason of that it needs a reliable projection for a peak of loads. Preparing enough infrastructures for addressing flash crowds, it is especially difficult for the administrators such as persons, not-for-profit organizations, and SOHO business operators. In many cases, these websites are not protected from flash crowds. In this paper, we propose a load distribution method for addressing flash crowds effectively by cooperating with other web servers and exchanging data in a P2P file sharing network. In our approach, web servers lent their idle resources each other, and increase the number of replicas automatically according to high demand by using P2P file sharing networks.

Molecular Dynamics Analyses of Crack Growth Behaviors in Single Crystalline Fe by the Use of Hybrid Potential Method

In Molecular Dynamics (MD) simulations, interatomic potentials have significant effects on the results and calculation time, i.e. analysis sizes. In this paper, first, the crack growth behaviors in α-Fe calculated by three representative interatomic potentials, the Embedded Atom Method (EAM) potential fitted by Simonelli, G. et al., Finnis-Sinclair (FS) potential and Second Nearest-neighbor Modified EAM (MEAM) potential, are compared, and the reliability of the results obtained by the MEAM potential is confirmed based on the reproducibility of the important physical properties. Although the MEAM potential is reliable, we show that the computational time required by the MEAM potential is more than 50 times of the FS potential. Here, we propose “Hybrid Potential Method” which selectively uses different interatomic potentials according to the local atomic structure, and connects those potentials by the use of handshake method. We use FS potential, which is efficient in calculation, for bcc structure and the MEAM potential, which is accurate but time consuming, for non-bcc structures. The availability of the Hybrid Potential Method is demonstrated, and then the method is applied to crack growth simulations using large calculation models. We report the phase transformations and grain nucleation near the crack tip during crack growth.

Molecular Dynamics Analysis on Initial Texture and Processing Route Influences on Grain Refinement Behavior of α-Fe by Equal Channel Angular Pressing

Fine-grained polycrystalline metals have a very high yield stress and excellent workability. Hence, numerous researchers are trying to develop an efficient process to obtain such materials. Our goal is to develop an efficient severe plastic deformation (SPD) process through investigating grain-refinement mechanisms in Equal Channel Angular Pressing (ECAP). In this paper, a series of molecular dynamics (MD) simulations of severe simple-shear deformations, which are ideally equivalent to SPD applied by typical ECAP processing routes, is performed using three-dimensional models that are thin and have a square shape with a periodic-boundary condition. We analyze the influences of the processing route and initial texture on the microstructural evolution. It is shown that twinning deformations are dominant under the calculated conditions, and that the structural evolution is notably affected by the relationship between the applied simple-shear direction and the characteristic crystal orientation, which can easily cause a twinning deformation. We conclude that Route A, without a rotation of the billet between processes, is the most efficient route. This is because twinning deformations along the simple-shear direction interact with the twin boundaries developed by the stress-component conjugate to the simple-shear. Furthermore, we demonstrate that the influence of the initial texture difference remains in force during multiple processes that have the same sliding plane.

Predicting Speed of Crack Running in Functionally Graded Material

A theoretical and computational methodology for the analysis of the functionally graded material (FGM) is introduced, and its application is made to the problem of a dynamically propagating crack running transversely in the FGM, where the intensity of the estimated crack-tip severity is managed to keep in valance with the graded material toughness in the FGM during the propagation. To detect the crack-tip severity, an integral fracture parameter, T*, is used. The crack is propagated so that the value of T* is equated to the prescribed varying critical values of T* for the graded material. Emphasis is placed on the use of a fuzzy inference technique in order to control the crack speed, which is deduced from a few T* values immediately preceding the current crack position. As to describing the constitutive law for the FGM, micro-spherical particles of arbitrary size in mesoscale are considered to be randomly dispersed in the matrix medium. By assuming that the volume fraction of the inclusion is continuously varied from 0 to 100 percent in the material, the grading is modeled. For modeling the constitutive law for the FGM composite media of thermo-elastoplasticity, a closed form SCC-LRM constitutive model describing the nonlinear material mechanics of the particle-dispersed medium is used. The model is based on the self-consistent scheme and uses Eshelby’s equivalent inclusion method. Unprecedented analytical results of predicting the crack speed of a crack running transversely in the FGM plate are obtained. In some cases of material grading, apparent crack arresting is observed as the crack runs into the metal rich area of the FGM.Copyright