• Journal of the Korean Geotechnical Society
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• v.28 no.12
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• pp.17-25
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• 2012

FEM requires the stress-strain relationship equations for numerical analyses. However, most formulations for the stress-strain relationship published up to the present are not satisfactory enough to properly express all the levels from the small strain to the peak. Tatsuoka and Shibuya (1991) suggested a new single formulation applicable not only to a wide range of geo-materials from soft clay to soft rock, but also to a wide range of strain levels from $10^{-6}$ to $10^{-2}$. The plain strain compression test is carried out to seven samples of research standard sand specimens and two samples of glass beads, which have been used at world-renowned research institutes. In this study, strains of the maximum principal stress (${\sigma}_1$) and the minimum principal stress (${\sigma}_3$) were thoroughly measured from $10^{-6}$ to $10^{-2}$, and the result, applied to Tatsuoka and Shibuya's new formulation, coincided closely with the measured data of the stress-strain relationship from the small strain to the peak.

• Proceedings of the Korean Geotechical Society Conference
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• 2001.03a
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• pp.315-322
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• 2001

Underground flexible pipes for electric cables are subject to external loads and surrounding soil pressure. Particularly, strain of flexible pipes is of great concern in terms of safety and maintenance for electric cables. In this paper, stress and strain of flexible pipes with various depth are calculated using traditional formula and FEM analysis. The results show that theoretical values are more conservative in strain whereas FEM analysis gives larger stress. Considering the strain criteria - 3.5 %, maximum, flexible pipes can be buried at the range of 50cm to 5m in depth without additional soil improvement.

• Journal of the Korean Geotechnical Society
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• v.26 no.8
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• pp.5-14
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• 2010

To simulate two-dimensional plane-strain conditions in the laboratory model test, the side frictional resistance between the soil and thick glass or plastic sheet of the soil container should be reduced as much as possible. However, in fact this side friction cannot be removed completely. In this paper, the ground model simulated as a multi-sized aluminium rod mixture was introduced to get rid of the side frictional resistance and applied to the laboratory shear box test. In addition, an application of the close range photogrammetric technique to the shear box test was validated. As a result, it was found that a mean value of dilation angle from the close range photogrammetry was close to the dilation angle defined by the curve of shear strain vs. volumetric strain.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.05a
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• pp.190-193
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• 2001

Hot deformation behavior of Udimet 720Li was characterized by compression tests in the temperature range of $1025^{\circ}C\;to\;1150^{\circ}C$ and the strain rate rage of $0.0005s^{-1}\;to\;5s^{-1}$. In order to characterize the dependence of flow stress on strain, strain rate and temperature, a constitutive equation based on hyperbolic sine formation was used. Isothermal forging of Udimet 720Li was performed in the temperature range $1050-1150^{\circ}C$ at strain rates of $0.05s^{-1}\;and\;0.005s^{-1}$. FE simulation was also carried out to predict deformation microstructures during isothermal forging.

• Transactions of Materials Processing
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• v.3 no.4
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• pp.415-426
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• 1994

Multistage deformation behavior of Nb-microalloyed steel and carbon steel was studied by torsion test with declining temperature T, constant pass strain $\varepsilon_i$, interrupt time $(t_i)$, and varying strain rate $(\.{\varepsilon})$. In the range of $1000^{\circ}C~790^{\circ}C$ and 4.00/sec~0.38/sec, the flow stress at each pass was correlated to the deformation variables. As the finished deformation temperatures are decreased to the range of $790^{\circ}C~900^{\circ}C$, Nb precipitates play an important role on the grain refinement of Nb-microalloyed steel. the flow stress of Nb-microalloyed steel was higher than the carbon steel's while the grain size of Nb-microalloyed steel was smaller than carbon steel below the temperature of $900^{\circ}C$.

• Journal of the Korean Ceramic Society
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• v.24 no.6
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• pp.586-592
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• 1987

The role of the strain energy and phase stability in the diffuse phase transition have been investigated in the highly disordered solid solution, (Pb1-xBax)(Zr0.4Ti0.6)O3 (0.2 x 0.4). X-ray diffraction analysis indicates that tetragonality (c/a) decreases with the increasing Ba content. Also as the Ba content increases, phase transition becomes more diffuse and at the same time dielectric relaxation as a function of measured frequencies in the 1KHz-10MHz range occurs very pronouncedly. In the Ba content range, 0.2 x 0.35, hysteresis loops are routinely observed and the loop is observed to narrow shape as the Ba content increases but becomes very slim at 40mol% Ba content. Moreover thermal analysis shows that there is no abrupt change in the thermal expansion coefficient below the apparent transition temperature at which dielectric constant becomes maximum. From the above results, it has been concluded that creation of the strain energy due to the distorthion that occurred during the phase transition suppresses diffuse phase transition.

• Proceedings of the Korean Reliability Society Conference
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• 2000.04a
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• pp.213-222
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• 2000

In present study, a stochastic model is developed for the low cycle fatigue life prediction and reliability assessment of 316L stainless steel under variable multiaxial loading. In the proposed model, fatigue phenomenon is considered as a Markov process, and damage vector and reliability are defined on every plane. Any low cycle fatigue damage evaluating method can be included in the proposed model. The model enables calculation of statistical reliability and crack initiation direction under variable multiaxial loading, which are generally not available. In present study, a critical plane method proposed by Kandil et al., maximum tensile strain range, and von Mises equivalent strain range are used to calculate fatigue damage. When the critical plane method is chosen, the effect of multiple critical planes is also included in the proposed model. Maximum tensile strain and von Mises strain methods are used for the demonstration of the generality of the proposed model. The material properties and the stochastic model parameters are obtained from uniaxial tests only. The stochastic model made of the parameters obtained from the uniaxial tests is applied to the life prediction and reliability assessment of 316L stainless steel under variable multiaxial loading. The predicted results show good accordance with experimental results.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1999.05a
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• pp.692-695
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• 1999

This paper presents annealing characteristics of CrN thin-film strain gauges, which were deposited on glass by DC reactive magnetron sputtering in an argon-nitrogen atmosphere)Ar-(5-~25%)$N_2$. The physical and electrical characteristics of these films investigated with the thickness range 3500$\AA$ of CrN thin films, annealing temperature (100~30$0^{\circ}C$) and annealing time (24-72hr) . The optimized condition of CrN thin-film strain gauges were thickness range of 3500$\AA$ and annealing condition(30$0^{\circ}C$ , 48hr) in Ar-10%$N_2$ deposition atmosphere. Under optimum conditions, the CrN thin-films for strain gauge is obtained a high resistivity, $\rho$=1147.65$\Omega$cm a low temperature coefficient of 11.17. And change in resistance after annealing for the CrN thin film were quitely linear and stable.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.5
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• pp.1349-1355
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• 1990

By examining the fatigue deformation properties of 12% Cr rotor steel which has been proved to have high fatigue and creep rupture strength around 600deg. C, authors reviewed major fatigue life prediction models such as Manson, Langer and Morrow equations, and following results were obtained. (1) A simple life prediction model for 12% Cr rotor steel was obtained as follows : DELTA..epsilon.$_{t}$ =2.18+.sigma.$_{u}$ /E+ $N^{-0.065}$+ $e^{0.6}$ $N^{-0.025}$ This equation shows that fatigue life, N, can be easily determined when total strain range, DELTA..epsilon.$_{t}$ and ultimate tensile strength, .sigma.$_{u}$ are known by simple tension test on the given test conditions. (2) Life prediction equation with equivalent maximum stress, DELTA..sigma./2, corresponding maximum strain in one cycle at room temperature is as follows: DELTA..sigma./w=-7.01logN+96.69+96.69

• Transactions of Materials Processing
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• v.7 no.6
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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.