Key Engineering Materials Vols. 345-346

Abstract: The diamond structure of single crystal silicon transforms to other structures under mechanical stress. We investigate the structural transformation of diamond cubic structure to betatin structure in silicon
under uniaxial stress using atomistic simulation on the basis of the Tersoff potential. As a result, under extensive compressive strain, the structural transformation from Si-I to Si-II is found.

Abstract: A maximum spread model dependent on rolling speed is proposed for rod rolling process following the analysis of deformation behavior of steel under compression and four-pass continuous high speed rod rolling test. Results reveal that amount of spread depends on a region which restricts plastic flow of workpiece during deformation. Increasing rolling speed (strain rate) leads to decrement of friction coefficient and, in turn, enlarge deformation-restricted region in the workpiece and consequently reduce amount of spread during deformation. Hence, the rolling speed dependent spread model could be expressed in terms of flow stress, which is a function of strain, strain rate and temperature and finally the model is proposed in a concise form.

Abstract: NIL(Nano Imprint Lithography) is one of the most promising lithography techniques. There are many variants of NIL, and two major techniques of them are thermal NIL and UV NIL. Here, we focus ourselves on the thermal NIL. During the thermal NIL, the polymeric patterns experience large mechanical strain and high temperature, and this often leads to malformation of polymeric patterns. So it is needed to improve the pattern fidelity and contrast, and these are believed to be closely related to the process condition and mechanical properties. In thermal NIL, PMMA is widely used and chosen as target polymer. Generally, mechanical properties in nano scale are really hard to acquire. In this study, we estimate the mechanical properties of PMMA by molecular dynamic simulation. These properties will be used as input of continuum simulation. We will estimate stress-strain relationship of PMMA. This stress-strain relationship depends on strain rate and temperature. So we will study about strain rate and temperature effect.

Abstract: Both micro- and meso-scale structures are involved in the analyses of many composite materials such as filament-wound tubes. In this paper, a unified approach for applying periodic boundary conditions to micro/meso-scale repeated unit cell models in the finite element analysis is presented. As an application example, a two-scale analysis of a ±θ helical filament-wound tube is provided.

Abstract: The objective of this study is to quantify and understand the effectiveness of a hexavalent chrome replacement ion to inhibit environmentally assisted fatigue crack propagation (EFCP) in high strength aluminum alloys. Addition of molybdate (MoO4 2-) to bulk-low chloride solution effectively inhibits EFCP in peak aged 7075, comparable to that of CrO4 2-. The effectiveness of inhibition depends strongly on loading variables: .K, R, and frequency as explained qualitatively by mechanical instability of a crack tip passive film that otherwise hinders production and uptake of embrittling hydrogen. The critical loading frequency (and crack tip strain rate), below which film stability and inhibition occur, increases with increasing inhibitor concentration, but only for low stress ratio loading, perhaps due to occluded crack transport and reaction considerations. Molybdate could be a beneficial replacement for chromate and a candidate for inhibitor release from a tailored coating.

Abstract: The objective of this study is to characterize the influence of notch effect on the high-cycle corrosion fatigue properties of AISI 347 stainless steel in various environments, namely, air, water, NaCl, NaCl plus inhibitor, and H2SO4 solutions. For smooth-surface and semi-circular notch specimens, the rank of fatigue strength in all of the given environments generally took the following order: air ≈ salt water plus inhibitor > deionized water > salt water > sulfuric acid solution. For V-notch specimens, the S-N curves were separated into two groups, i.e. one with air and 3.5% NaCl plus inhibitor and the other with deionized water, 3.5% NaCl and H2SO4. This was attributed to a greater effect of localized acidification occurring at the root of V-notch as compared to the smooth surface and semi-circular notch.

Abstract: Organic acids (acetic and formic acid) are produced by the decomposition of ETA (Ethanol Amine, C2H7NO) used as pH controller of secondary water in nuclear power plants. Corrosion Fatigue (CF) tests (R=0.2, 0.1Hz) were conducted to evaluate the effect of acetic acid on the CF crack growth rate in high temperature water at 150°C. Acetic acid significantly influenced the environmental cracking behavior of turbine disc steels in high temperature water. The CF crack growth rates of turbine disc steels increase as the organic acid concentration increase to a critical saturation pH value (~pH 4). Beyond the saturation value of pH, the CF crack growth rates decrease significantly. The higher CF crack growth rate of the higher pH solution in water of intermediate content range (pH 4~pH 5) of acetic acid is due to the higher content of H+ enhancing the reduction reactions. Crack tip blunting prevents the CF cracks from growing with increasing rate in the solution of organic acid concentrations beyond the critical value.

Abstract: The new market penetration using the distributed generation technology is linked to a large number of factors like economics and performance, safety and reliability, market regulations, environmental issues, or grid connection standards. KEPCO, a government company in Korea, has performed the project to identify and evaluate the performance of Micro Gas Turbine (MGT) technologies focused on 30, 60kW-class grid-connected optimization and combined Heat & Power performance. This paper describes the results for the mechanical, electrical, and environmental tests of MGT on actual grid-connection under Korean regulations. As one of the achievements, the simulation model of Exhaust-gas Absorption Chiller was developed, so that it will be able to analyze or propose new distributed generation system using MGT.