• Journal of the Korea Institute of Building Construction
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• v.15 no.1
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• pp.25-33
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• 2015

In this study, initial crack index was evaluated by FEM analysis to find the crack propagation from hydration heat in precast concrete. As results, as the usage of hardening accelerator increased, initial compressive strength increased and setting time was shortened. Additionally, as amounts of hardening accelerators increased, the central temperature of concrete increased and the time to reach the highest temperature was shortened. It was demonstrated that the hardening accelerators accelerated the hydration reaction of cement, and caused the increase of hydration heat within the short period of time. Furthermore, the crack index for evaluating the heat level was performed by FEM. As results, there was no problem about the cracks, despite of the growth of initial high hydration heat. This is because of the increased tensile strength that is large enough to sustain the thermally induced-stress.

• Journal of the Korean Society for Railway
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• v.18 no.6
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• pp.543-551
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• 2015

Quick-hardening track (QHT) is a construction method which is used to change from old ballast track to concrete track. Sufficient time for construction is important, as the construction should be done during operational breaks at night. Most of the time is spent on exchanging the ballast layer. If it is possible to apply the ballast non-exchange type of quick-hardening track, it would be more effective to reduce the construction time and costs. In this paper, pouring materials with high permeability are suggested and a construction method involving a layer separation pouring process considering the void condition is introduced in order to develop ballast non-exchange type of QHT. The separate pouring method can secure the required strength because optimized materials are poured into the upper layer and the lower layer for each void ratio condition. To ensure this process, a rheology analysis was conducted on the design of the pouring materials according to aggregate size, the aggregate distribution, the void ratio, the void size, the tortuosity and the permeability. A polymer series was used as the pouring material of the lower layer to secure the void filling capacity and for adhesion to the fine-grained layer. In addition, magnesium-phosphate ceramic (MPC) was used as the pouring material of the upper layer to secure the void-filling capacity and for adhesion of the coarse-grained layer. As a result of a mechanics test of the materials, satisfactory performance corresponding to existing quick-hardening track was noted.

• Journal of the Korea Concrete Institute
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• v.14 no.5
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• pp.660-667
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• 2002

The effects of polymer-cement ratio and antifoamer content on the durability of ultrarapid-hardening polymer-modified mortars using redispersible polymer powder are examined. As a result, regardless of the antifoamer content, the setting time of the ultrarapid-hardening polymer-modified mortars using redispersible polymer powder tend to delay with increasing polymer-cement ratio. The water absorption and chloride ion penetration depth of the ultrarapid-hardening polymer-modified mortars using redispersible polymer powder decrease with increasing polymer-cement ratio and antifoamer content. The resistance of freezing and thawing and chemicals improvement is attributed to the improved bond between cement hydrates and aggregates because of the incorporation of redispersible polymer powder

• Transactions of the Korean Society of Mechanical Engineers A
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• v.40 no.2
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• pp.207-212
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• 2016

High-frequency induction hardening (HFIH) is used in many industries and has a number of advantages, including reliability and repeatability. It is a non-contact method of providing energy-efficient heat in the minimum amount of time without using a flame. Recently, HFIH has been actively studied using the finite-element method (FEM), however, these studies only focused on the accuracy of the analysis. In this paper, we analyzed HFIH by using a variable frequency based on the conditions of the same shape and input power then comparing the analysis results to experimental results. The analysis and experimental results indicate that the hardening depths are approximately the same using the optimal frequency of 3kHz.

• Journal of the Earthquake Engineering Society of Korea
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• v.12 no.6
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• pp.65-71
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• 2008

A one-dimensional site response analysis program (KODSAP) was developed using cyclic soil behavior model by using the modified parallel IWAN model. The model is able to predict the cyclic hardening and degradation of soil through the adjustment of the internal slip stresses of its elements beyond the cyclic threshold, and satisfies Bauschinger's effect and the Masing rule in terms of its own behavior characteristics. The program (KODSAP) used the direct integration method in the time domain. The elasticity of the base rock was considered as a viscous damper boundary condition. The effects of cyclic hardening or degradation of soil on site response analysis were evaluated through parametric studies. Three types of analyses were performed to compare the effect of analysis and cyclic parameter on site response. The first type was equivalent linear analysis, the second was nonlinear analysis, and a third was nonlinear analysis using the cyclic hardening or degradation model.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.6
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• pp.1455-1464
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• 1994

A new and efficient algorithm for the integration of the thermal-elasto-plastic constitutive equation is proposed. While it falls into the category of the return mapping method, the algorithm adopts the three point approximation of plastic corrector within one time increment step. The results of its application to a von Mises-type thermal-elasto-plastic model with combined hardening and temperature-dependent material properties show that the accurate iso-error maps are obtained for both angular and radial errors. The accuracy achieved is because the predicted stress increment in a single step calculation follows the exact value closely not only at the end of the step but also through the whole path. Also, the comparison of the computational time for the new and other algorithms shows that the new one is very efficient.

• Journal of the Korean Society of Food Science and Nutrition
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• v.28 no.3
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• pp.566-571
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• 1999

Effects of storage temperature at 18oC and 5oC for 9 days and heating conditions on textural char acteristics in strong wheat flour gels were investigated using a Instron after microwave (2450MHz, 700W). Heating condition did not affect the texture property in the wheat flour gels, but the frozen storage was very effective to prevent the hardening of the samples. Hardness, adhesiveness, and gumminess in the wheat flour gels decreased in increasing moisture content from 45% to 55%, but there was little change in cohesiveness. The storage at room temperature for 2 hours resulted in increased hardness in the heated wheat flour gels. PE wrap package of the wheat flour gels prevented weight loss by 3~6% and hardening effectively. Sugar, or sugar and shotening resulted in increased hardness in loaf bread. The hardness and weight loss in loaf breads were markedly changed by the heating time with microwave. The most critical time point was around 4 minutes following the heating.

• Journal of the Korean Society for Heat Treatment
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• v.35 no.5
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• pp.255-261
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• 2022

In this work, the strain-amplitude independent and strain-amplitude dependent damping capacities of Mg-5%Sn alloy have been investigated as a function of its age-hardening response. The hardness increased with an increase in aging time, reached a peak value after 48 h, and then it gradually decreased. The damping capacities of the Mg-5%Sn alloy exhibited a decreasing tendency in the order of solution-treated, under-aged, peakaged, and over-aged states in the strain-amplitude dependent region, whereas they increased continuously with aging time in the strain-amplitude independent region. The microstructural examination during aging revealed that the lower concentration of Sn solutes in the α-(Mg) matrix and the lower density of the Mg₂Sn precipitate particles may well be the crucial factors for better damping values in the strain-amplitude independent and strain-amplitude dependent regions, respectively.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.5
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• pp.881-889
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• 2002

Newly developed continuous indentation technique has made nondestructive assessment of tensile properties possible. The present study was undertaken to determine new parameters for indicating time-dependent material degradation of petro-chemical plant. Continuous indentation tests were performed for Cr-Ni steel and Cr-Mo steel which are widely used as facilities of petro-chemical plant. From the results, it was found that yield strength and tensile strength cannot be used as general degradation parameters because the changes in the strengths with aging time didn't show any tendency. On the other hand, work hardening exponent and yield ratio showed consistent tendency with increasing aging time. Therefore, nam attention of this work was paid on them as new degradation parameters, and the in-field applicability of the parameters was evaluated and discussed.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.7 s.94
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• pp.1700-1709
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• 1993

The stepped specimen which is subjected to step loading is modeled to study the initiation and growth of adiabatic shear band using explicit time integration finite element method. Three different clearance sizes are tested. The material model for the stepped specimen includes effects of strain hardening, strain rate hardening and thermal softening. It is found that the material inside the fully grown adiabatic shear band experiences three phase of deformation, (1) homogeneous deformation phase, (2) initiation/incubation phase, and (3) fast growth phase. The second phase of deformation is initiated after sudden shear stress drop which occurs at the same time regardless of the clearance size. The incubation time prior to fast growth phase increases, as the clearance size of the stepped specimen increases. Whereas, after incubation period, the growth rate of the adiabatic shear band decreases, as the clearance size decreases. It is also found that two adiabatic shear band may develop instead of one for the smaller clearance size.