• Journal of Mechanical Science and Technology
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• 제20권2호
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• pp.262-270
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• 2006

Direct numerical simulations (DNS) of turbulent channel flows up to $Re_{\tau}=1270$ are performed to investigate an elliptic feature and strain rate field on cross sections of coherent fine scale eddies (CFSEs) in wall turbulence. From DNS results, the CFSEs are educed and the strain rate field around the eddy is analyzed statistically. The principal strain rates (i.e. eigenvalues of the strain rate tensor) at the CFSE centers are scaled by the Kolmogorov length $\eta$ and velocity $U_k$. The most expected maximum (stretching) and minimum (compressing) eigenvalues at the CFSE centers are independent of the Reynolds number in each $y^+$ region (i. e. near-wall, logarithmic and wake regions). The elliptic feature of the CFSE is observed in the distribution of phase-averaged azimuthal velocity on a plane perpendicular to the rotating axis of the CFSE $(\omega_c)$. Except near the wall, phase-averaged maximum $(\gamma^{\ast}/\gamma_c^{\ast})$ and minimum $(\alpha^{\ast}/\alpha_c^{\ast})$ an eigenvalues show maxima on the major axis around the CFSE and minima on the minor axis near the CFSE center. This results in high energy dissipation rate around the CFSE.

• 한국기계가공학회지
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• 제5권2호
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• pp.3-9
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• 2006

HSM(High-speed Machining) is widely used in rapid manufacturing of precision products and molds of various materials. Improvement in cutting efficiency is one of the important subjects in the HSM process. To analyse the dynamic behavior during a very short cutting time, the computational analysis code, LS-DYNA3D, was employed for the simulation of the mechanism of HSM for aluminium 7075. This cutting mechanism includes some difficult points in simulation, for example, material and geometrical non-linearity, high-speed dynamic impact, contact with friction, etc. In this paper, a finite element model considering the strain rate effect is proposed to predict the cutting phenomena such as chip deformation, strain and stress distributions, which will help us to design the HSM process.

• 한국분말재료학회지
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• 제11권2호
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• pp.149-157
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• 2004

High temperature deformation behavior of activated sintered W powder compacts was investigated. The compression tests were carried out in the temperature range between 900 and 110$0^{\circ}C$ at the strain rate of $10^{-3}s^{-1}$. The sintered specimens of Ni-doped submicron W powder compacts showed decrease in W grain size with increasing the Ni content. As the result, the flow stress was significantly increased with increasing the Ni content. We obtained Ni-activated sintered W compacts with the relative density of 94 $\pm$ l％and the average grain size of less than 5${\mu}{\textrm}{m}$. A moderate true strain up to 0.60 was obtained without fracture even at 110$0^{\circ}C$ with the strain rate of $10^{-3}s^{-1}$ for the activated W compact despite adding the 1.0 wt％Ni to submicron W powder.

• 한국재료학회지
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• 제28권8호
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• pp.435-439
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• 2018

MA Al alloys are examined to determine the effects of alloying of Mg and Cu and rolling on tensile deformation behavior at 748 K over a wide strain rate range($10^{-4}-10^3/s$). A powder metallurgy aluminum alloy produced from mechanically alloyed pure Al powder exhibits only a small elongation-to-failure(${\varepsilon}_f$ < ~50%) in high temperature(748 K) tensile deformation at high strain rates(${\acute{\varepsilon}}=1-10^2/s$). ${\varepsilon}_f$ in MA Al-0.5~4.0Mg alloys increases slightly with Mg content(${\varepsilon}_f={\sim}140%$ at 4 mass%). Combined addition of Mg and Cu(MA Al-1.5%Mg-4.0%Cu) is very effective for the occurrence of superplasticity(${\varepsilon}_f$ > 500%). Warm-rolling(at 393-492 K) tends to raise ${\varepsilon}_f$. Lowering the rolling-temperature is effective for increasing the ductility. The effect is rather weak in MA pure Al and MA Al-Mg alloys, but much larger in the MA Al-1.5%Mg-4.0%Cu alloy. Additions of Mg and Cu and warm-rolling of the alloy cause a remarkable reduction in the logarithm of the peak flow stress at low strain rates (${\acute{\varepsilon}}$< ~1/s) and sharpening of microstructure and smoothening of grain boundaries. Additions of Mg and Cu make the strain rate sensitivity(the m value) larger at high strain rates, and the warm-rolling may make the grain boundary sliding easier with less cavitation. Grain boundary facets are observed on the fracture surface when ${\varepsilon}_f$ is large, indicating the operation of grain boundary sliding to a large extent during superplastic deformation.

• Entomological Research
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• 제48권5호
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• pp.339-347
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• 2018

Seven different strains of Beauveria bassiana were used in a bioassay on Pieris rapae larvae. The results showed that an B. bassiana strain showed relatively high pathogenicity towards P. rapae larvae. The adjusted mortality rate was 92.86 %, and the infection rate was 85.71 % in 10 days post inoculation. Molecular identification was performed to identify the unknown strain. Internal Transcribed Spacer sequence analysis showed that the polymerase chain reaction amplicon length of the unknown strain of Beauveria sp. was 573 bp, and sequence similarity to the known B. bassiana sequences in the NCBI database was 99 %. The B. bassiana strain was named Bb01. The changes of proteins and PPO of P. rapae larvae infected by B. bassiana Bb01 strain at different times was determined. The activity of PPO increased in 1-6 d and decreased in 7 d again after inoculation. The B. bassiana invaded into the insect body affected the balance of the proteins and PPO.

• 한국재료학회지
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• 제22권1호
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• pp.42-45
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• 2012

The mechanical behavior and microstructural evolution during high temperature tensile deformation of recrystallized Ni3Al polycrystals doped with boron were investigated as functions of initial grain size, tensile strain rate and temperature. In order to obtain more precise information on the deformation mechanism, tensile specimens were rapidly quenched immediately after deformation at a cooling rate of more than $2000Ks^{-1}$, and were then observed by transmission electron microscopy (TEM). Mechanical tests in the range of 923 K to 1012 K were carried out in a vacuum of less than $3{\times}10^{-4}$ Pa using an Instron-type machine with various but constant cross head speeds corresponding to the initial strain rates from $1.0{\times}10^{-4}$ to $3.1{\times}10^{-5}s^{-1}$. After heating to deformation temperature, the specimen was kept for more than 1.8 ks before testing. The following results were obtained: (1) Flow behavior was affected by initial strain size; with decreasing initial grain size, the level of a stress peak in the true stress-true strain curve decreased, the steady state region was enlarged and elongation increased. (2) On the basis of TEM observation of rapidly quenched specimens, it was confirmed that dynamic recrystallization certainly occurred on deformation of fine-grained ($3.3{\mu}m$) and intermediate-grained ($5.0{\mu}m$) specimens at an initial strain rate of $3.1{\times}10^{-5}s^{-1}$ and at 973 K. (3) There were some dislocation-free grains among the new recrystallized grains. The obtained results suggest that both dynamic recrystallization and grain boundary sliding are operative during high temperature deformation.

• 한국해양공학회지
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• 제16권3호
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• pp.59-64
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• 2002

In the present study, we examined the influence of prestrain on creep strength of Class M alloy(STS310S) and Class A(STS310J1TB) alloys containing precipitates. Prestrain was given by prior creep at a higher stress than the following creep stresses. Creep behaviour before and after stress change and creep rate of pre-strained specimens were compared with that of virgin specimens. Pre-straining produced the strain region where the strain rate was lower than that of a virgin specimen both for STS310J1TB and STS310S steels. The reason for this phenomenon was ascribable to the viscous motion of dislocations, the interaction between dislocations and precipitates in a STS310J1TB steel, and the interaction of dislocations with sub-boundaries in a STS310S steen which has the higher dislocation density and smaller subgrain size resulted from pre-straining at higher stress.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.419-423
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• 2001

It is important to know the mechanical properties of the materials under dynamic load. The mechanical properties of most materials are influenced to some extent by strain rate. One of the reliable test device for determining the mechanical properties of materials at high strain rate is Split Hopkinson Pressure bar. In this paper, we conducted the mechanical properties test for the aluminium alloy 6063 and 6061 using the SHPB device.

• 대한기계학회논문집
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• 제19권3호
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• pp.858-867
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• 1995

Characteristics of premixed flames in counter-flow system are numerically studied using a detailed chemical reaction mechanism including gas phase radiation. Without radiation effect accounted, low CO and high NO$_{x}$ emission indices are observed, when strain rate decreases, due to increased residence time and higher flame temperature. Higher NO$_{2}$ production has been also observed when two premixed flames are interacting or cold air stream is mixed with burned gas. The rate of NO$_{x}$ production and destruction is dependent upon the diffusional strength of H and OH radicals, the existence of NO and the concentration of HO$_{2}$. For radiating flames, the peak temperature and NO$_{x}$ production rate decreases as the strain rate decreases. At high strain rate, it is found that the effect of radiation on flame is little due to its negligible radiating volume. It is also found that NO$_{x}$ production from the interacting premixed flame is reduced due to reduced temperature resulting from radiation heat loss. It is concluded that the radiation from gas has significant effect of flame structure and on emission characteristics.ristics.

• 소성∙가공
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• 제7권6호
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• pp.620-630
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• 1998

Conventional hot forging and thixoforging of Al 7075 alloy have been compared with respect to microstructures, formability and hardness. Two distinctive temperature-strain rate ranges for hot forging of Al 7075 alloy were observed from the results of simple compression tests with strain rates of 10-3∼101 sec-1 in the temperatures between $250^{\circ}C$ and $500^{\circ}C.$ In the dynamic recovery range (low temperature-high strain rate range) multi-stage forging was necessary to form a complex shape part due to the lack of formability. In the high temperature-low strain rate range, in which dynamic recrystallization takes place a complex shaped park could be formed by single-stage forging. About 50% cold working in the SIMA process was necessary to get a fine and homogeneous microstructures. Microstructural study suggest that thixoforged Al 7075 part has fine grains and homogeneous microstructures. Its hardness number is almost same to that of conventional hot forged part after aging treatment.