Hiroyuki Y. Suzuki

Maximizing interval reliability in operational software system with rejuvenation

Software aging often affects the performance of a software system and eventually causes it to fail. A novel approach to handle transient software failures is called software rejuvenation which can be regarded as a preventive and proactive solution that is particularly useful for counteracting the phenomenon of software aging. In this paper, we consider the optimal software rejuvenation policy maximizing the interval reliability in the general semi-Markov framework. We derive analytically the optimal software rejuvenation timing which maximizes the limiting interval reliability or the interval reliability with exponentially distributed operation times. Further, we examine numerically the transient behavior of the interval reliability at an arbitrary operation time. Our results under the interval reliability criteria are extentions of some earlier work, since the interval reliability can be specialized to the pointwise availability and the common reliability function.

Net shape formation of sub-micron alumina with reduced flaws by high-speed centrifugal compaction process

A new compacting method for powder materials is presented. High-Speed Centrifugal Compaction Process (HCP) utilizes centrifugal force of about 10,000 g for the compaction. HCP is suitable for net shape compaction of fine sub-micron and nano-powders. HCP possesses a unique compacting mechanism that is different from other colloidal processes including Pressure Casting (PC), and has a number of useful characteristics. HCP has a higher compacting speed than PC, wide applicability for net shape formation, as well as a defect removing function. Because of homogeneous and flawless green microstructure, HCP alumina shows superior sinterability and higher strength and hardness than most of other aluminas.

Sedimentation Behavior of Mixed Powder Slurry under High-Speed Centrifugal Force

Compaction behavior of two component slurry during High-speed Centrifugal Compaction Process (HCP) was observed. Slight amount of iron oxide powder is mixed into alumina slurry, then the slurry was sedimented in a centrifuge under rotational speed of up to 11,500 rpm. A “Y” letter shaped flow pattern was emerged in the cross section of the compact. The pattern was clearer with higher rotational speed, but indifferent to acceleration rate of rotation. A similar pattern was simulated when we presume bidirectional initial flow in centrifugal field, which indicated that the combination of Corioli’s force and bidirectional flows of powder and dispersing medium caused such flow pattern.

Production of Cemented Carbide-Alumina Composite Material by Wet-Shaping Process

In this study, the characteristics of the production process of cemented carbide-alumina composite material made using the wet-shaping process were investigated. The production process in this study produced a green compact of composite material by repeating the wet-shaping process for the molding of each material, and it made possible the sintering of plural materials with varying sintering conditions at the same time, a process that was difficult until now. By using wet-shaping and ultra-fine powder, which have superior sintering characteristics, sintering conditions were found in which it was possible to sinter cemented carbide and alumina at the same time, with a sintering temperature of 1723 K and a sintering time of 5.4 ks. With these sintering conditions, the relative densities of the sintered compact of cemented carbide and alumina were 99.0 % and 98.9 %, respectively. It is clear that the characteristics of sintered compact made with these sintering conditions are superior. When the cemented carbide slurry and the alumina slurry were layered by repeating the wet-shaping process, a composite material was able to be produced by inserting an active brazing filler metal in the interface to improve the bondability of the cemented carbide and the alumina during the sintering. However, it was observed that the active brazing filler metal and the cobalt in the cemented carbide flowed out from the interface between the cemented carbide and the alumina in the sintered compact of the composite material.

Production Processes and Characteristics of Porous Alumina with Air Bubbles Introduced into Slurry

In this study, production processes for porous alumina, and the characteristics of the material, were investigated. Porous alumina was produced by a wet-shaping process in which air bubbles were introduced into the slurry. The feature of this production process is that many pores are produced by slip casting carried out using whipped slurry, where only the conditions of the slurry are adjusted. The advantage of this process is its simplicity. From the results, it is made clear that a green compact of porous alumina can be produced by changing the amount of solvent and binder, and also that a sintered compact of porous alumina can be produced by a low sintering temperature, such as 1473 K. The four point bending strength of porous alumina is about 515 MPa when the porosity is about 30 %. The excellent characteristics of the sintered compact of porous alumina are shown by the observation results of the fracture surface in this production process. The dense alumina body is sintered while maintaining the fine grains, and with the micro pores remaining in the grain boundary.

Compaction of Ultra-Fine WC Powder by High-Speed Centrifugal Compaction Process

High-Speed Centrifugal Compaction Process (HCP) is one of slip-using compacting method originally developed for processing of oxide ceramics. In this study, we are going to apply the HCP to ultra-fine (0.1 micron) WC powder. Organic liquid of heptane was chosen as dispersing media to avoid possible oxidation of WC. For slip preparation, addition of sorbitan-monostearate (SMS) dramatically improved state of dispersion. The mixing apparatus also was in consideration. The slips mixed by conventional ball mill or turbula mill were scarcely densified by the HCP. Only the slips mixed by high energy planetary ball mill were packed up to 55% by the HCP, and sintered to almost full density at 1673 K without any sintering aids. This sintered compact marked Vickers hardness of Hv 2750 at maximum.

Flow patterns in green bodies made by high-speed centrifugal compaction process

High-Speed Centrifugal Compaction Process (HCP) is a wet compacting method, in which powders are compacted under a huge centrifugal force. The HCP was well applied to small alumina specimens, but the compact easily cracked when we applied the HCP to other materials. In the present study we clarify the mechanism that introduces such cracks. Firstly, we observed HCP alumina, dyeing with iron oxide powder or by the Immersion Liquid Technique, and found that there was a kind of flow pattern generated during the HCP. A simple simulation also revealed that the formation of such a flow pattern was related to the Colioli’s force in the centrifugal field, and therefore was hard to suppress. Nonetheless, the actual introduction of inhomogeneity along the flow was largely affected by another factor. Die releasing oil was dragged in the flows and formed low density regions along the flow.

Development of High-Speed Centrifugal Compaction Process of Alumina

High-Speed Centrifugal Compaction Process (HCP) is one of slip-using compacting method in which the compacts are obtained as sediments under huge centrifugal force. Compacting mechanism of the HCP differs from other slip using compacting methods. The unique compacting mechanism of the HCP leads a number of characteristics such as a higher compacting speed, wide applicability for net shape formation, etc. Moreover, the most outstanding characteristics of the HCP can be found in homogeneous and flawless microstructure of the green compacts, because the process possesses an intrinsic defect removing function. That is, the defect inducing matters in slips (powder aggregates, inclusions, bubbles) are removed from the main part of the compact by the classification effect of centrifuge, and therefore the HCP alumina exhibits superior sinterabiliy and higher strength and hardness than most other aluminas. Many of the advantageous characteristics of the HCP can be improved with a higher centrifugal force.