• Journal of Ocean Engineering and Technology
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• v.25 no.3
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• pp.53-65
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• 2011

This is the third of several companion papers dealing with the derivation of material constants for ductile failure criteria under hydrostatic stress. It was observed that the ultimate engineering stresses and elongations at fracture from tensile tests for round specimens with various notch radii tended to increase and decrease, respectively, because of the stress triaxiality. The engineering stress curves from tests are compared with numerical simulation results, and it is proved that the curves from the two approaches very closely coincide. Failure strains are obtained from the equivalent plastic strain histories from numerical simulations at the time when the experimental engineering stress drops suddenly. After introducing the new concept of average stress triaxiality and accumulated average strain energy, the material constants of the Johnson-Cook failure criterion for critical energies of 100%, 50%, and 15% are presented. The experimental results obtained for EH-36 steel were in relatively good agreement with the 100% critical energy, whereas the literature states that aluminum fits with a 15% critical energy. Therefore, it is expected that a unified failure criterion for critical energy, which is available for most kinds of ductile materials, can be provided according to the used materials.

• International Journal of Concrete Structures and Materials
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• v.4 no.2
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• pp.89-96
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• 2010

This paper focuses on the effect of creep on the critical compressive strain (CCS) of concrete. The strain of concrete corresponding to the peak compressive stress is crucial in the selection of the ultimate yield strength of the reinforcing bar used in reinforced concrete columns. Among the various influencing factors, such as the creep, shrinkage, loading rate and confinement, the effect of creep and shrinkage is the most significant. So far, investigations into how these factors can affect the CCS of concrete have been rare. Therefore, to investigate the effect of creep and shrinkage on CCS, an experimental (part I) and a parametric study (part II) were conducted, as presented in these papers (part I considers creep effect, part II considers effect of creep and shrinkage). In part I, experiments pertaining to the loading age, loading rate, loading duration and loading and creep levels were conducted to study the effect of these variables on the CCS of concrete. It was found that the effects of the loading rate, loading age, and level and duration on the CCS of concrete were negligible. However, it is very important to consider the effect of creep.

• Journal of Magnetics
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• v.20 no.1
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• pp.11-20
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• 2015

In this work, Monte Carlo simulation was employed to model the stretchable Ising monolayer film to investigate the effect of the spatial distance variation among magnetic atoms on magnetic behavior of the film. The exchange interaction was considered as functions of initial interatomic distance and the stretched distance (or the strain). Following Bethe-Slater picture, the magnetic exchange interaction took the Lennard-Jones potential-like function. Monte Carlo simulations via the Wolff and Metropolis algorithms were used to update the spin systems, where equilibrium and dynamic magnetic profiles were collected. From the results, the strain was found to have strong influences on magnetic behavior, especially the critical behavior. Specifically, the phase transition point was found to either increase or decrease depending on how the exchange interaction shifts (i.e. towards or away from the maximum value). In addition, empirical functions which predict how the critical temperatures scale with initial interatomic distance and the strain were proposed, which provides qualitatively view how to fine tune the magnetic critical point in monolayer film using the substrate modification induced strain.

• Journal of Korean Tunnelling and Underground Space Association
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• v.10 no.2
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• pp.129-137
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• 2008

This paper intends to develop an assesment technique for the rapid and quantitative evaluation of tunnel safety during tunnel excavation by using displacement measurements. Control criteria for the field measurements are provided at tunnel construction sites in Korea and other countries. However, it was known that the criteria were not clear and varied depending on the construction sites. In order to make a reasonable support for guidelines, critical strain concept is introduced in this study. And the engineering meanings of the critical strain concept are investigated precisely. In order to do this, at first, the engineering meanings of the original concept from the previous researchers was investigated theoretically for the evaluation of tunnel safely. Subsequently displacement data were obtained by using the commercial program, then the evaluation of tunnel safely was conducted with the view point of previous researches. Additionally, strains are determined from the feedback analyses program by inputting measured displacements that were obtained from the commercial program, then the evaluation of tunnel safety was discussed with the critical strain concept. Consequently it can be concluded that the evaluation of tunnel safety can be determined quantitatively and rapidly in the field by introducing the critical strain concept.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.10a
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• pp.809-814
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• 2002

Recently, the properties of early-age concrete are increasingly important because these properties directly influence the behavior of early-age concrete structures including stress and cracking behavior. Nevertheless, the studies on early-age concrete are limited to strength and temperature development. The purpose of present study is to propose a simple and rational method which can predict the stress and strain behavior of young age concrete. A series of test have been done to measure the temperature development, strains and stresses in concrete members. The concept of equivalent age was used to define the degree of hydration and this degree of hydration was used to calculate the strength and elastic modulus. The critical degree of hydration and thermal expansion coefficient were calculated using experimental data. It is seen that the critical degree of hydration range from 0.05 to 0.11 based on the measuring method. The thermal expansion coefficient was calculated based on the measured non-mechanical strain and it is found that the coefficient decreases slightly with the increase of age. The consideration of critical degree of hydration in calculating stresses gives more accurate results. The present study provides useful method and data in evaluating early-age behavior of concrete structure.

• Structural Engineering and Mechanics
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• v.48 no.2
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• pp.195-205
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• 2013

The buckling problem of linearly tapered micro-columns is investigated on the basis of modified strain gradient elasticity theory. Bernoulli-Euler beam theory is used to model the non-uniform micro column. Rayleigh-Ritz solution method is utilized to obtain the critical buckling loads of the tapered cantilever micro-columns for different taper ratios. Some comparative results for the cases of rectangular and circular cross-sections are presented in graphical and tabular form to show the differences between the results obtained by modified strain gradient elasticity theory and those achieved by modified couple stress and classical theories. From the results, it is observed that the differences between critical buckling loads achieved by classical and those predicted by non-classical theories are considerable for smaller values of the ratio of the micro-column thickness (or diameter) at its bottom end to the additional material length scale parameters and the differences also increase due to increasing of the taper ratio.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2003.07a
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• pp.573-576
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• 2003

HTS superconducting tapes are now commercially available for practical applications such as magnets and cables. Since superconductors in such applications are subjected to high mechanical loads that can significantly degrade the superconducting properties, mechanical properties and the strain tolerance known as the strain effect on superconducting properties are needed to be estimated for developing superconducting devices. Influences of loading mode on the Ic degradation and the interaction on strain effect were discussed in this study.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2002.04a
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• pp.50-54
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• 2002

Influences of bending strain on the critical current ($I_c$) in Ag-sheathed Bi-2223 superconducting tapes at 77K were investigated. The effect of bending mode on the bending strain characteristics was discussed in viewpoints of sample geometry, n-value and damage morphology. Especially, in this paper, we reported the $I_c$ behavior in Ag alloy sheathed Bi-2223 multifilamentary superconducting tapes under hard bending. As a result, $I_c$ degradation behavior of the hard bending appeared remarkably than the case of easy bending, but it did not influence greatly on the n-value.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2003.05a
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• pp.17-22
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• 2003

HTS superconducting tapes are now commercially available for practical applications such as magnets and cables. Since superconductors in such applications are subjected to high mechanical loads that can significantly degrade the superconducting properties, mechanical properties and the strain tolerance known as the strain effect on superconducting properties are needed to be estimated for developing superconducting devices. The progress in technology achieved in the field of strain effect evaluation on the critical current of HTS tapes in various deformation modes is discussed in this study.

• Korea-Australia Rheology Journal
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• v.15 no.2
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• pp.63-73
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• 2003

An experimental technique was developed to determine the strain-rate in a tensile specimen. Then one can calculate the transient isothermal elongational viscosity. Both shear and elongational viscosities were measured to study the effect of shear and elongational fields on the flow properties. The comparison between these viscosities shows that the onset of rapid viscosity growth as crystallization solidification proceeds occurs at about the same value of time at very small deformation rates (0.0028 and 0.0047 $s^{-1}$). The comparison of these measured viscosities as functions of shear and elongational Hencky strains also reveals that the onset of rapid viscosity growths starts at critical Hencky strain values. The behaviour of steady shear viscosity as function of temperature sweep was also explored at three different low shear rates. Finally, the influence of changing oscillatory frequencies and strain rates was also investigated.