• Transactions of Materials Processing
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• v.5 no.4
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• pp.327-336
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• 1996

The effect of dynamic strain aging on high temperature deformation behaviour of the A-Mg alloy was investigated by strain rate change tests and stress relaxation tests between 20$0^{\circ}C$and 50$0^{\circ}C$. Yield point, short stress transient and periodic discontinuities on the stress-strain curve were considered as an evidence of the effect of dynamic strain aging. With this criterion two distinct strain rate-temperature regimes could be manifested. Dynamic strain aging was considered to be effective in the high temperature-low strain rate regime, whereas dynamic recovery was a dominant deformation mechanism in the low temperature-high strain rate regime. It was found that dynamic strain aging in the high temperature deformation was governed by the mechcanism of diffusion-controlled, viscous dislocation movement.

• Journal of the Korean Geosynthetics Society
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• v.3 no.4
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• pp.21-27
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• 2004

Stress relaxation behaviors of nonwoven geotextile and geomembrane which have protection, filtration and drainage, water barrier functions, respectively were analyzed. 'Theory of transition phenomena' was applied to interpretate the stress relaxation behaviors of two geosynthetics. The initial and later relaxation times for stress relaxation behaviors of geosynthetics were derived from the constitutive equations. The initial relaxation behaviors of these geosynthetics were dependent on the additional strains and were especially faster with temperature. Finally, both relaxation times of geosynthetics were shorter with additional strain and temperature.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.815-818
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• 2002

The elliptic relaxation model(ERM) with the inhomogeneous correction intermediate between near wall with and far from the wall. The source of the ERM usually was appled quasi-homogeneous pressure-strain correlation in homogeneous situations. This formulation was easily applied to the linear model or non-linear pressure-strain model. It is observed that the boundary conditions of the relaxation operator dominate the homogeneous pressure-strain model in the near wall region. While looking at high-Reynolds number flows, it was found necessary to modify the effect of the relaxation operator throughout the log region by accounting for gradients of the flatness variable and turbulent length scales. These effects are kinematic blocking of the wall normal velocity fluctuation and pressure reflections from the surface. This model is wall distances and unit vectors which make the model applicable to flows boundary by a complex geometry. Inhomogeneous correction model is computed inertial and non-inertial channel flow These are compared DNS(Kim et at., Kristofffrsen & Andersson) for channel flow. The present model could be predicted well for rotating flows.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2000.04a
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• pp.76-79
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• 2000

Polymeric composites exhibit highly nonlinear and rate dependent behavior during loading and unloading in off-axis directions. The equilibrium state of stress during loading is lower than the state of stress produced at finite strain rates. The amount of stress relaxation during loading decreases. Interestingly, however, the stress goes up to reach to the equilibrium state of stress for a fixed displacement during unloading. The unloading behavior is quite similar to the loading behavior. The stress relaxation patterns during loading and unloading is also similar, and those depend on the fiber orientation angles and the loading and unloading rates. The AS4/PEEK thermoplastic composite is used to characterize the relaxation behavior for different off-axis angles and loading rates. There exists a transient loading region at the beginning of unloading. The effective stress and effective plastic strain concept is used to establish a master curve of stress recovery pattern for different off-axis angles and unloading rates.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1996.03b
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• pp.48-54
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• 1996

The continuous deformation , multistage deformation and stress relaxation were carried out to investigate the strain induced procipitation by torsion tests in the range of 1000∼800$^{\circ}C$, 0.05/sec∼5/sec for V-microalloyed steel. The starting temperature and time for the initiation of precipitation were determined by stress relaxation tests and the distribution of percipitates increased at higher strain rate and the mean size of precipitates was found to be about 50nm. The precipitation starting time decreased with increasing strain rate from 0.05/sec to 5 /sec and pre-strain. The effect of deformation conditions on the no-recrystallization temperature(Tnr) was determined in the multistage deformation with declining temerature. The Tnr decreased with increasing strain and strain rae. In the controlled rolling, grain refinement and precpitation hardening effects could be achieved by the alternative large pass strain at the latter half pass stage under the condition of low temperature and high strain rate.

• Transactions of Materials Processing
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• v.6 no.2
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• pp.102-109
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• 1997

The multistage deformation and stress relaxation were carried out to investigate the strain induced precipitation by torsion tests in the range of 1000~80$0^{\circ}C$, 0.05~5/sec for V-microalloyed steel. The starting temperature and time for the initiation of precipitation were determined by stress relaxation tests. The distribution of precipitates increased, as the strain rate increased and the mean size of precipitates was found to be about 10~30nm. The precipitation starting time$(P_s)$ decreased with increasing strain rate and the amount of pre-strain. The effect of deformation conditions on the no-recrystallization temperature$(T_nr)$ was also determined in the multistage deformation. $T_nr$ Tnr decreased with increasing the strain and strain rate. In the controlled rolling simulation, grain refinement and precipitation hardening effects could be achieved by the alternative large pass strain at the latter half pass stage under the condition of low temperature and high strain rate.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.12
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• pp.2056-2061
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• 2001

Non-proportional loading tests of Nylon 66 at room temperature exhibit path dependent behavior and plasticity-relaxation interactions. The uniaxial formulation of the viscoplasticity theory based on overstress (VBO), which has been used to reproduce the nonlinear strain rate sensitivity, relaxation, significant recovery and cyclic softening behaviors of Nylon 66, is extended to three-dimensions to predict the response in strain-controlled, comer-path tests. VBO consists of a flow law that is easily written for either the stress or the strain as the independent variable. The flow law depends on the overstress, the difference between the stress and the equilibrium stress that is a state variable in VBO. The evolution law of the equilibrium stress in turn contains two additional state variables, the kinematic stress and the isotropic stress. The simulations show that the constitutive model is competent at modeling the deformation behavior of Nylon 66 and other solid polymers.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.1103-1108
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• 2003

The fatigue strength of welded joint is influenced by the welding residual stress which is relaxed depending on local stress distributed in vicinity of stress raisers, eg. under cut, overlap and blow hole. To evaluate its fatigue life the geometry of the stress raisers and the welding residual stress should be taken into account. The several methods based on notch strain approach have been proposed in order to consider the two factors above mentioned. These methods, however, have shown considerable differences between analytical and experimental results. It is due to the fact that the amount of the relaxed welding residual stress evaluated by the cyclic stress-strain relationship do not correspond with that occurred in reality. In this paper the residual stress relaxation model based on experimental results was used in order to reduce the discrepancy of the estimated amount of the relaxed welding residual stress. Under an assumption of the superimposition of the relaxed welding residual stress and the local stress, a modified notch strain approach was proposed and verified to the cruciform welded joint.

• Food Science and Biotechnology
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• v.18 no.4
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• pp.1013-1018
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• 2009

Several rheological terms were introduced to estimate the properties of cooked noodles with different starch content using tensile tests. Ring-shaped specimens were prepared by connecting both ends of the noodle strip before cooking. Hencky strain and rate, as well as true stress were applied in constant deformation tests. The elastic region on the curves of strain vs. stress was not clearly identified. Strain hardening in the subsequent plastic region was more prominent in low-starch noodles. Elongational viscosities at lower strain rates were used to differentiate noodles with different starch content, representing the dominant effect of protein content in the range of lower strain rates. In stress relaxation tests, the reciprocal of Peleg's constant $K_1$ (initial decay rate) and $K_2$ (asymptotic level) increased and decreased respectively, with an increase in starch content. This indicated that addition of starch contributed to the noodles becoming viscous liquid rather than elastic solid.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.22 no.4
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• pp.331-334
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• 2009

In this paper, we present a surface relaxation model in atomistic calculations for thin nanofilms. This surface relaxation model is very simple model which have only two degrees of freedoms to determine the atomic positions of nanofilms. Whereas in conventional molecular statics simulations, the same number of degrees of freedoms at all atom positions are used as unknown variables. In order to prove the reliability of the presented model, we present the results of self-equilibrium strain calculations with the surface parameters obtained from this model.