• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.2D
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• pp.227-234
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• 2008

This study examines which models are more suitable for representing mechanical property of unbound materials to analyze behavior of asphalt pavement structure. Results from FWD (Falling Weight Deflectometer) test were used to apply to nonlinear elastic model. The new method which can deduct material constants of nonlinear elastic model is suggested from FWD test data rather than laboratory resilient modulus ($M_R$) test. It is confirmed that the material constants are within the common range in subbase. Test output from FWD and MDD (Multi-Depth Deflectometer) was used to verify reliability of the model. From the results of verification, this study shows that a non-linear elastic model agrees to MDD test data more than a linear elastic model does.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.6A
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• pp.625-639
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• 2009

Experimental and analytical studies are performed on the mechanical behavior of concrete-filled tubular(CFT) truss girders for different f/L ratios. Bending tests are conducted on two CFT truss girder specimens to determine fundamental structural characteristics such as the strength and deformation properties. Nonlinear material models for CFT members subjected to an axial compressive force are compared in this paper by using the nonlinear finite element program, ABAQUS. Previous researchers have proposed several nonlinear stress-strain models of confined concrete. In this study, the nonlinear analyses are performed applying several stress-strain models for confined concrete proposed by Mander, Sakino, Han, Susantha and Ellobody, and the results are compared with the experimental results in terms of load-deflection and load-strain relationships. Based on the comparisons of the load-deflection relationships, the models proposed by Mander and Susantha provide a maximum load about 12.0~13.8% higher and that by Sakino gives a maximum load about 7.6% higher than the experimental results. The models proposed by Han and Ellobody give a maximum load only about 0.2~1.2% higher than the test results, showing the best agreement among the proposed stress-strain models. However, the load-strain relations predicted by the existing models generally provide conservative results exhibiting larger strains than the experimental data.

• Progress in Superconductivity and Cryogenics
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• v.25 no.4
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• pp.19-23
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• 2023

The Second-generation high-temperature superconducting (HTS) Rare-Earth Barium Copper Oxide (REBCO) wire is a composite laminate having a multi-layer structure (8 or more layers). HTS wires will undergo multiple loads including the bending-tension loads during winding, high current density, and high magnetic fields. In particular, the wires are subjected to bending stress and magnetic field stress because HTS wires are wound around a circular bobbin when making a high-field magnetic. Each of the different laminated wires inevitably exhibits damage and fracture behavior of wire due to stress deformation, mismatches in thermal, physical, electrical, and magnetic properties. Therefore, when manufacturing high-field magnets and other applications, it is necessary to calculate the stress-strain experienced by high-temperature superconducting wire to present stable operating conditions in the product's use environment. In this study, the finite element model (FEM) was used to simulate the strain-stress characteristics of the HTS wire under high current density and magnetic field, and bending loads. In addition, the result of obtaining the neutral axis of the wire and the simulation result was compared with the theoretical calculation value and reviewed. As a result of the simulation using COMSOL Multiphysics, when a current of 100 A was applied to the wire, the current value showed the difference of 10^-9. The stress received by the wire was 501.9 MPa, which showed a theoretically calculated value of 500 MPa and difference of 0.38% between simulation and theoretical method. In addition, the displacement resulted is 30.0012 ㎛, which is very similar to the theoretically calculated value of 30 ㎛. Later, the amount of bending stress by the circular mandrel was received for each layer and the difference with the theoretically obtained the neutral axis result was compared and reviewed. This result will be used as basic data for manufacturing high-field magnets because it can be expanded and analyzed even in the case of wire with magnetic flux pinning.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.36 no.5
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• pp.323-330
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• 2023

The development of an analysis model that reflects the microstructure characteristics of polyvinyl alcohol (PVA) fiber-reinforced cementitious composites, which have a highly complex microstructure, enables synergy between efficient material design and real experiments. PVA fiber orientations are an important factor that influences the mechanical behavior of PVA fiber-reinforced cementitious composites. Owing to the difficulty in distinguishing the gray level value obtained from micro-CT images of PVA fibers from adjacent phases, fiber segmentation is time-consuming work. In this study, a micro-CT test with a voxel size of 0.65 ㎛³ was performed to investigate the three-dimensional distribution of fibers. To segment the fibers and generate training data, histogram, morphology, and gradient-based phase-segmentation methods were used. A U-net model was proposed to segment fibers from micro-CT images of PVA fiber-reinforced cementitious composites. Data augmentation was applied to increase the accuracy of the training, using a total of 1024 images as training data. The performance of the model was evaluated using accuracy, precision, recall, and F1 score. The trained model achieved a high fiber segmentation performance and efficiency, and the approach can be applied to other specimens as well.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.34 no.2
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• pp.66-72
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• 2024

In this study, the applicability of CCMs (Carbondioxide conversion capture materials) manufactured by reacting carbon dioxide gas with DG (Desulfurization gypsum) as a cement substitute for secondary concrete products were evaluated and the basic physical properties of CCMs-mixed mortar and concrete specimens were measured to derive the optimal mixing ratio. The main chemical oxides of CCMs were CaO and SO₃, and the main crystalline phases were CaSO₄·2H₂O, Ca(OH)₂, CaCO₃, and CaSO₄. In addition, by the results of particle size analysis and heavy metal measurement, the applicability of CCMs as a cement substitute for secondary concrete products was confirmed. As a result of measuring the strength behavior using mortar and concrete specimens with CCMs, the compressive and flexural strength decreased as the mix ratio of CCMs increased, but requirements by the standards for interlocking blocks and retaining wall blocks, which are target products in this study, were satisfied up to the optimal mixing ratio of 10 wt.% substitution. Therefore, its applicability as a cement substitute for secondary concrete products was confirmed.

• Advances in nano research
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• v.16 no.6
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• pp.565-577
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• 2024

Vibration investigation of fluid-filled functionally graded cylindrical shells with ring supports is studied here. Shell motion equations are framed first order shell theory due to Sander. These equations are partial differential equations which are usually solved by approximate technique. Robust and efficient techniques are favored to get precise results. Employment of the Rayleigh-Ritz procedure gives birth to the shell frequency equation. Use of acoustic wave equation is done to incorporate the sound pressure produced in a fluid. Hankel's functions of second kind designate the fluid influence. Mathematically the integral form of the Langrange energy functional is converted into a set of three partial differential equations. A cylindrical shell is immersed in a fluid which is a non-viscous one. These shells are stiffened by rings in the tangential direction. For isotropic materials, the physical properties are same everywhere where the laminated and functionally graded materials, they vary from point to point. Here the shell material has been taken as functionally graded material. After these, ring supports are located at various positions along the axial direction round the shell circumferential direction. The influence of the ring supports is investigated at various positions. Effect of ring supports with empty and fluid-filled shell is presented using the Rayleigh - Ritz method with simply supported condition. The frequency behavior is investigated with empty and fluid-filled cylindrical shell with ring supports versus circumferential wave number and axial wave number. Also the variations have been plotted against the locations of ring supports for length-to-radius and height-to-radius ratio. Moreover, frequency pattern is found for the various position of ring supports for empty and fluid-filled cylindrical shell. The frequency first increases and gain maximum value in the midway of the shell length and then lowers down. It is found that due to inducting the fluid term frequency result down than that of empty cylinder. It is also exhibited that the effect of frequencies is investigated by varying the surfaces with stainless steel and nickel as a constituent material. To generate the fundamental natural frequencies and for better accuracy and effectiveness, the computer software MATLAB is used.

• Journal of the Korea institute for structural maintenance and inspection
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• v.28 no.4
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• pp.13-20
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• 2024

In this study, when subjected to in-plane shear such as a shear wall, the behavior characteristics of a concrete member using CFRP rebars were investigated when the longitudinal reinforcement ratio was kept constant at 2.96% and the transverse reinforcement ratio was changed from 0.30 to 2.98%. The evaluation was conducted based on MCFT theory and analyzed by comparison with the case of concrete members using steel rebars. When the reinforcement ratio ranged from 0.30 to 1.19%, concrete members employing CFRP rebars exhibited higher shear strength compared to those using steel rebars. In contrast, at high reinforcement ratios of 1.79 and 2.98%, it was observed that the shear strength of the member with CFRP rebar was lower compared to the member with steel rebar. Maximum shear strain was observed to be higher for members reinforced with steel rebars at lower reinforcing bar ratios, while for ratios of 0.97% and above, CFRP rebars resulted in higher maximum shear strain. As the reinforcement ratio increases, the use of CFRP rebar instead of steel rebar results in a greater increase in maximum shear strain. By analyzing the difference in strain in the reinforcing bar as well as the difference in principal strain in the element caused by differences in the mechanical properties of the steel rebar and CFRP rebar, the shear strength and shear strain when using steel rebar and CFRP rebar with different reinforcement ratios can be compared and analyzed.

• Journal of the Korean Society for Heat Treatment
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• v.37 no.4
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• pp.155-162
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• 2024

SCr420H steel which is commonly utilized for automotive components requires the carburizing heat treatment process. Abnormal grain growth during this treatment significantly affects the mechanical properties of the steel parts. Consequently, a process designed to prevent abnormal grain growth at certain elevated temperatures is essential. For enhanced grain refinement, we considered the addition of Nb in SCr420H steel. The experimental condition of the carburizing heat treatment involved reheating the steel sample to temperatures between 940℃ and 1080℃. Using scanning electron microscopy, we examined the microstructure of specimens treated with the secondary solution, revealing an organization of bainite and ferrite. Transmission electron microscopy was utilized to determine the type, shape, and size of the carbonitrides, showing a high fraction of AlN at the secondary solution treatment temperature of approximately 1050℃ and of (Nb,Ti)(C,N) around 1200℃. AlN particles measured about 100 nm and (Nb,Ti)(C,N) about 50 nm. Optical microscopy was utilized to assess grain size variations at different secondary solution treatment temperatures. It is noted that the temperature at which abnormal grain coarsening occurred rose with increasing secondary solution treatment temperatures, indicating a greater influence of (Nb,Ti)(C,N) with higher heat treatment temperatures. This research provides reference data for preventing abnormal grain growth in Nb-added low alloy steels undergoing carburizing heat treatment.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.37 no.4
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• pp.275-282
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• 2024

A recently proposed rapid-disassembly , carbon-minimized dismantle connection was tested using cyclic loading. To better understand the behavior of the test specimen, three-dimensional finite element (3D-FE) analyses were conducted using a "tied model" (bolted contact surfaces are tied together) and a "bolt-slip model" (contact surfaces slip and separate). The tied model suggests that plastic hinging of the beam occurs if the proposed connection behaves rigidly. The bolt-slip model suggests that the proposed connection, if manufactured and assembled properly, can dissipate energy to about 0.5 times that experienced by a rigid connection. However, when compared in a test, its moment-rotation hysteresis curve does not match well, which suggests that the low performance of the test specimen is attributable to a manufacturing deficiency. Regardless, the results corroborate the pinching phenomenon observed in the experimental hysteresis and fracture failure of the test specimen.

• Child Health Nursing Research
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• v.4 no.2
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• pp.159-176
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• 1998

In effort to conduct comparative study on the caregiving environment of Neonatal Intensive Care Unit(NICU) in both U. S and Korea, this study was been conducted first in the U.S. Purpose : The purpose of this study is to identify the physical environment and direct caregiving practices to lowbirth weight infants in NICU in the US. It also aims to examine the NICU outcome status and behavioral reponses of lowbirth weight infants. Methods : A study design using descriptive and inferential statistics was been conducted through an observational, field method. A sample of 15 preform infants admitted to NICU were recruited for the study. The subjects were those with birth weight between 1,000 gm to 1,500 gm, born at the gestation period of 27 to 33 weeks, and without any chromosomal or other genetic anomalies, major congenital infections, or maternal illness. Thirty minutes observation(three times of ten minutes of continuous observation)of the infant's behavior and physiological status, and an four-hour observation of the physical environment and direct care giving procedures were been conducted on the 3rd and 10th day after birth, and on the day of discharge from the NICU or at 34weeks postconception. The data to be collected were in four areas : the demograghic characteristics of the infants, the physical environment and care giving procedures, the frequency of the infant's designated behavior and physiological response, and NICU outcome variables. A descriptive analysis and Kruskal-Wallis, Pearson r were been applied according to variable characteristics. Results : 1. Mother's mean age was 29.47. The sample consisted of 6 males and 9 females. Mean gestational ages were 29.17 weeks. Mean birth weight was 1236.33g. Mean Apgar scores at one minute were 6.6, and 7.8 at five minutes. 2. The location for the incubator was in the distance from the light, X-ray screens and nursing station, in proximity to side-lamp, telephone and faucet on the third day after birth. The location for the incubator was in the distance from the light and radio on the tenth day and in proximity to nursing station on the day of dischage from the NICU or at 34weeks postconception. 3. Nesting was the most applying aids to the infants. And foot roll, shielding and plastic frame were frequently using by nurses for facilitating well modulated restful posture. 4. There were statistically significant changes in the patterns of physical environment included locating the infant's incubator and bedding, specific aids to self regulation on the 3rd and 10th day after birth, and on the day of discharge from the NICU or at 34weeks postconception. 5. Statistically significant changes were not appeared in the patterns of direct caregiving procedure to the infants included stress inducing or reducing manipulations on the 3rd and 10th day after birth, and on the day of discharge from the NICU or at 34weeks postconception. 6. The stress response of the infants in NICU were significantly reduced as the infants grow older. 7. There were not statistically significant correlation between the physical envronment and the stress responses of the infants in NICU. 8. There were statistically significant correlation between the direct caregiving procedure to the infants and the stress response of the infants in NICU in the second and third observation on the day three. 9. Average weight gain per day from birth to discharge was 38.73g, number of days in the hospital was 42.60, number of days before bottle feeding was 3.6. Postconception age starting bottle feed ing was 31/sup +5/ weeks. Number of days on mechanical ventilator was average 7.64, 11.42 was an average number of days of oxygen need. Conclusion : It, thus, appears that to minimize the sensorymotor stimulation for the low birthweight preterm infant in NICU, manipulation of care giving practices to the babies whatever the stress inducing or reducing procedures, have to be limited in the immediate early stage after birth. And it needed to be reexamine to identify the appropriate and specific physical environment and the patterns of direct caregiving to the low birthweight preform infant as the infants grow older in NICU.