• Journal of the Korea Concrete Institute
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• v.27 no.3
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• pp.215-227
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• 2015

In this study, twenty four steel-fiber reinforced concrete (SFRC) beams using recycled coarse aggregates (RCA) were manufactured to examine the shear behavior of SFRC and to determine the beams' ultimate shear strengths. The RCA replacement ratio was fixed at 30%. The variables studied in this investigation are: (1) shear span-to-depth ratios (a/d) of 2, 3 and 4; (2) longitudinal reinforcement ratio (${\rho}$) of 0.008 and 0.0127; and (3) steel fiber volume fractions ($V_f$) of 0, 0.5, 0.75 and 1%. Test results were analyzed and then compared with the findings and proposals of various other researchers. Based on the test results, the more steel fiber volume fraction is increased, the large crack resistance and shear strength are exhibited. Most of the experimental data is higher than the theoretical value. Therefore, steel-fiber reinforced concrete beams using recycled coarse aggregates are suggested to be applied for building structures.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.2A
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• pp.401-409
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• 2006

This study presents basic information on the mechanical properties and long-term deformations of high-strength steel fiber reinforced concrete(HSFRC). The Influence of steel fiber on modulus of elasticity, compressive, splitting tensile and flexural strength, and drying shrinkage and creep of HSFRC are investigated, and flexural fracture toughness is evaluated. Test results show that Test results show that the effect of steel fibers on the compressive strength is negligible, and the modulus of elasticity of HSFRC increased with the increase of fiber volume fraction. And the effect of fiber volume fraction($V_f$) and aspect ratio($l_f/d_f$) on tensile strength, flexural strength and toughness is extremely prominent. It is observed that the flexural deflection corresponded to ultimate load increased with the increase of $V_f$ and $l_f/d_f$, and due to fiber arresting cracking, the shape of the descending branch of load-deflection tends towards gently. Also, the effect of addition of various amounts of fiber on the creep and shrinkage is obvious. Especially, the effect of adding fibers to high-strength concrete is more pronounced in reducing the drying shrinkage than the creep.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.10a
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• pp.190-194
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• 2007

The forging process design and microstructure evolution for gas turbine disk of a Waspaloy is investigated in this study. Parameters related to deformation are die and preform geometry, and forging temperature of die and workpiece. Die and preform design are considered to reduce the forging load, and to avoid the forging defects. Blocker and finisher dies for multistage forging are designed and the initial billet geometry is determined. The control of hot forging parameters such as strain, strain rate and temperature also is important because the microstructure change in hot working affects the mechanical properties. The dynamic recrystallization evolution has been studied in the temperature range 900-$1200^{\circ}C$ and strain rate range 0.01-1.0s-1 using hot compression tests. Modeling equations are required represent the flow curve, recrystallized grain size, recrystallized volume fraction by various tests. In this study, we used to thermo-viscoplastic finite element modeling equation of DEFORM-2D to predict the microstructure change evolution during thermo-mechanical processing. The microstructure is updated during the entire thermal and deformation processes in forging.

• Journal of Periodontal and Implant Science
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• v.52 no.4
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• pp.338-350
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• 2022

Purpose: Various studies have investigated 3-dimensional (3D)-printed implants using Ti6Al-4V powder; however, multi-root 3D-printed implants have not been fully investigated. The purpose of this study was to explore the stability of multirooted 3D-printed implants with lattice and solid structures. The secondary outcomes were comparisons between the 2 types of 3D-printed implants in micro-computed tomographic and histological analyses. Methods: Lattice- and solid-type 3D-printed implants for the left and right mandibular third premolars in beagle dogs were fabricated. Four implants in each group were placed immediately following tooth extraction. Implant stability measurement and periapical X-rays were performed every 2 weeks for 12 weeks. Peri-implant bone volume/tissue volume (BV/TV) and bone mineral density (BMD) were measured by micro-computed tomography. Bone-to-implant contact (BIC) and bone area fraction occupancy (BAFO) were measured in histomorphometric analyses. Results: All 4 lattice-type 3D-printed implants survived. Three solid-type 3D-printed implants were removed before the planned sacrifice date due to implant mobility. A slight, gradual increase in implant stability values from implant surgery to 4 weeks after surgery was observed in the lattice-type 3D-printed implants. The marginal bone change of the surviving solid-type 3D-printed implant was approximately 5 mm, whereas the value was approximately 2 mm in the lattice-type 3D-printed implants. BV/TV and BMD in the lattice type 3D-printed implants were similar to those in the surviving solid-type implant. However, BIC and BAFO were lower in the surviving solid-type 3D-printed implant than in the lattice-type 3D-printed implants. Conclusions: Within the limits of this preclinical study, 3D-printed implants of double-rooted teeth showed high primary stability. However, 3D-printed implants with interlocking structures such as lattices might provide high secondary stability and successful osseointegration.

• Structural Engineering and Mechanics
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• v.75 no.2
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• pp.193-209
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• 2020

This paper presents a new hyperbolic shear deformation plate theory including the stretching effect for free vibration of the simply supported functionally graded plates in thermal environments. The theory accounts for parabolic distribution of the transverse shear strains and satisfies the zero traction boundary conditions on the surfaces of the plate without using shear correction factors. This theory has only five unknowns, which is even less than the other shear and normal deformation theories. The present one has a new displacement field which introduces undetermined integral variables. Material properties are assumed to be temperature-dependent, and graded in the thickness direction according to a simple power law distribution in terms of the volume power laws of the constituents. The equation of motion of the vibrated plate obtained via the classical Hamilton's principle and solved using Navier's steps. The accuracy of the proposed solution is checked by comparing the present results with those available in existing literature. The effects of the temperature field, volume fraction index of functionally graded material, side-to-thickness ratio on free vibration responses of the functionally graded plates are investigated. It can be concluded that the present theory is not only accurate but also simple in predicting the natural frequencies of functionally graded plates with stretching effect in thermal environments.

• Nuclear Medicine and Molecular Imaging
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• v.42 no.6
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• pp.435-443
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• 2008

Purpose: Gated myocardial perfusion SPECT provides not only myocardial perfusion status but also various functional parameters of left ventricle. We compared left ventricular ejection fraction, end-diastolic volume, LV mass by cardiac SPECT using Quantitative Gated SPECT (QGS), 4D-MSPECT software and standard 2D-echocardiography. Materials and Methods: One hundred fourteen patients (male 51, female 63; 29-85 years old, mean $61.3\;{\pm}\;13.3$ years old) with normal perfusion status on Tc-99m tetrofosmin gated myocardial perfusion SPECT were analyzed retrospectively. Ejection fraction (LVEF), End-diastolic volume (LVED), LV mass (LVM) were calculated using QGS, 4D-MSPECT, and LVEF, LVM using 2D-echocardiography. Statistical analysis including Bland-Altman plot was performed using $MedCalc^{(R)}$ (MedCalc software, Mariakerke, Belgium). Results: The correlation of LVEF between methods was good: 0.95/0.96 (stress/rest) between QGS and 4D-MSPECT, 0.79 between QGS and echocardiography, 0.79 between 4D-MSPECT and echocardiography (p<0.001). Using Bland-Altman plot, the 95% confidence interval of agreement between QGS and 4D-MSPECT ranged from -12.7% to 7.3% / from -12.2% to 6.5% (stress/rest). The agreement between QGS and echocardiography, 4D-MSPECT and echocardiography ranged from -17.4% to 24.0%, and -14.8% to 27.0% respectively. The correlation of LVM between methods was also good: 0.95 between QGS and 4D-MSPECT, 0.76 between QGS and echocardiography, 0.73 between 4D-MSPECT and echocardiography (p<0.001). The 95% confidence interval of agreement between QGS and 4D-MSPECT ranged from -33.8g to 14.1g (stress/rest), The 95% confidence interval of agreement between QGS and echocardiography, 4D-MSPECT and echocardiography ranged from -148.7 g to 21.8. g, and -142.8 g to 35.5 g, respectively. Conclusion: There was a good correlation for LVEF, LVEO, LVM among methods (QGS, 4D-MSPECT, echocardiography), but the variance between methods was big. Therefore, the functional parameters by each method cannot be used interchangeably.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.75.1-75.1
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• 2011

We investigate the connection between the presence of bars and AGN activity, using a volume-limited sample of ~6,000 late-type galaxies with axis ratio b/a > 0.6 and $M_r=-19.5+5logh$ at low redshift (0.02 < z ${\leq}$ 0.055), selected from Sloan Digital Sky Survey Data Release 7. Although bars are believed to play an important role in fueling AGNs, it is still an open question whether AGN activity is connected with the presence of bars. We find that the bar fraction in AGN-host galaxies (44.1%) is 2.5 times higher than in non-AGN galaxies (17.6%), implying an AGN-bar connection. However, this trend is simply caused by the fact that AGN-host galaxies are on average more massive and redder than non-AGN galaxies since the bar fraction increases with u-r color and velocity dispersion. When AGN-host and non-AGN galaxies with fixed u-r color and velocity dispersion are compared, the excess of bar fraction in AGN-host galaxies disappears. Among AGN-host galaxies we find no strong difference of the Eddington ratio distributions between barred and non-barred systems. These results indicate that AGN activity is not directly connected with the presence of bars.

• Computers and Concrete
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• v.7 no.5
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• pp.469-482
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• 2010

This paper investigates the concrete permeability through a numerical and statistical approach. Concrete is considered as a random heterogeneous composite of three phases: aggregates, interfacial transition zones (ITZ) and matrix. The paper begins with some classical bound and estimate theories applied to concrete permeability and the influence of ITZ on these bound and estimate values is discussed. Numerical samples for permeability analysis are established through random aggregate structure (RAS) scheme, each numerical sample containing randomly distributed aggregates coated with ITZ and dispersed in a homogeneous matrix. The volumetric fraction of aggregates is fixed and the size distribution of aggregates observes Fuller's curve. Then finite element method is used to solve the steady permeation problem on 2D numerical samples and the overall permeability is deduced from flux-pressure relation. The impact of ITZ on overall permeability is analyzed in terms of ITZ width and contrast ratio between ITZ and matrix permeabilities. Hereafter, 3680 samples are generated for 23 sample sizes and 4 contrast ratios, and statistical analysis is performed on the permeability dispersion in terms of sample size and ITZ characteristics. By sample theory, the size of representative volume element (RVE) for permeability is then quantified considering sample realization number and expected error. Concluding remarks are provided for the impact of ITZ on concrete permeability and its statistical characteristics.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.10a
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• pp.33-36
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• 2000

The dynamic recrystallization (DRX) of medium carbon steels (SCM 440 and POSMA45) was studied with torsion test in the temperature range of $900-1100^{\circ}C$ and the strain rate range of $5.0x10^{-2}\;-\;5.0x10^0/sec$. To establish the quantitative equations for DRX, the evolution of flow stress curve with strain was analyzed. The critical strain (${\varepsilon}_c$) and strain for maximum softening rate ( ${\varepsilon}^{*}$) could be confirmed by the analysis of work hardening rate ($d{\sigma}/d{\varepsilon}\;=\; \theta$). The volume fraction of dynamic recrystallization ($X_{DRX}$) as a function of processing variables, such as strain rate ( $\dot{\varepsilon}$ ), temperature (T), and strain ( $\varepsilon$ ) were established using the ${\varepsilon}_c$ and ${\varepsilon}^{*}$. For the exact prediction, the ${\varepsilon}_c$, ${\varepsilon}^{*}$ and Avrami' exponent (m') were quantitatively expressed by dimensionless parameter, Z/A respectively. The transformation-effective strain-temperature curve for DRX could be composed. It was found that the calculated results were agreed with the experimental data for the steels at any deformation conditions.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.12
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• pp.2917-2925
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• 2000

Tensile strength and failure process of composite materials depend on the variation in fiber strength, matrix properties and fiber-matrix interfacial shear strength. A Monte-Carlo simulation considering variation in these factors has been widely used to analyze such a complicated phenomenon as a strength and simulated the failure process of unidirectional composites. In this study, a Monte Carlo simulation using 2-D and 3-D(square and hexagonal array) model was performed on unidirectional graphite/epoxy and glass/polyester composites. The results simulated by using 3-D hexagonal array model have a good agreement with the experimental data which were tensile strength and failure process of unidirectional composites.