• Elastomers and Composites
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• v.58 no.1
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• pp.44-56
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• 2023

Additive manufacturing (AM) or three-dimensional (3D) printing of metals has been drawing significant attention due to its reliability, usefulness, and low cost with rapid prototyping. Among the various AM technologies, fused deposition modeling (FDM) or fused filament fabrication is receiving much interest because of its simple manufacturing processing, low material waste, and cost-effective equipment. FDM technology uses metal-filled polymer filaments for 3D printing, followed by debinding and sintering to fabricate complex metal parts. An efficient binder is essential for producing polymer filaments and the thermal post-processing of printed objects. This study involved an in-depth investigation of and a fabrication route for a novel multi-component binder system with steel alloy powder (45 vol.%) ranging from filament fabrication and 3D printing to debinding and sintering. The binder system consisted of polyvinyl pyrrolidone (PVP) as a binder and thermoplastic polyurethane (TPU) and polylactic acid (PLA) as a carrier. The PVP binder held the metal components tightly by maintaining their stoichiometry, and the TPU and PLA in the ratio of 9:1 provided flexibility, stiffness, and strength to the filament for 3D printing. The efficacy of the binder system was examined by fabricating 3D-printed cubic structures. The results revealed that the thermal debinding and sintering processes effectively removed the binder/carrier from the cubic structures, resulting in isotropic shrinkage of approximately 15.8% in all directions. The scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) patterns displayed the microstructure behavior, phase transition, and elemental composition of the 3D cubic structure.

• Journal of the Korean Geotechnical Society
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• v.22 no.12
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• pp.89-98
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• 2006

In this paper the behavior of saturated sand deposits where liquefaction occurred before is studied for successive earthquakes. The relationship between past pore pressure generation and reliquefaction resistance is examined by using cyclic direct simple shear tests. If the soil sample in direct simple shear produced nearly 90% of excess pore pressure during first time loading, its liquefaction resistance increased during following cyclic loading after consolidation. However, a fully liquefied soil during first time loading has a densely packed condition but shows less liquefaction resistance for the following cyclic loading. UBCSAND model that can account for pore pressure change and stiffness loss of soil during shaking is used to analyze the centrifuge test simulating reliquefaction. The pore pressure rise during first time cyclic loading controls liquefaction resistance. The measurements from reliquefaction centrifuge test are compared with numerical predictions. By considering frequent earthquakes having occurred at the Southern Korea near Japan, such effective stress approach is necessary for reliquefaction study.

• Steel and Composite Structures
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• v.47 no.6
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• pp.709-728
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• 2023

Experimental and analytical studies were conducted to clarify the influencing mechanisms of the longitudinal reinforcement on performance of axially loaded Reinforced Concrete-Filled Steel Tube (R-CFST) short columns. The longitudinal reinforcement ratio was set as parameter, and 10 R-CFST specimens with five different ratios and three Concrete-Filled Steel Tube (CFST) specimens for comparison were prepared and tested. Based on the test results, the failure modes, load transfer responses, peak load, stiffness, yield to strength ratio, ductility, fracture toughness, composite efficiency and stress state of steel tube were theoretically analyzed. To further examine, analytical investigations were then performed, material model for concrete core was proposed and verified against the test, and thereafter 36 model specimens with four different wall-thickness of steel tube, coupling with nine reinforcement ratios, were simulated. Finally, considering the experimental and analytical results, the prediction equations for ultimate load bearing capacity of R-CFSTs were modified from the equations of CFSTs given in codes, and a new equation which embeds the effect of reinforcement was proposed, and equations were validated against experimental data. The results indicate that longitudinal reinforcement significantly impacts the behavior of R-CFST as steel tube does; the proposed analytical model is effective and reasonable; proper ratios of longitudinal reinforcement enable the R-CFSTs obtain better balance between the performance and the construction cost, and the range for the proper ratios is recommended between 1.0% and 3.0%, regardless of wall-thickness of steel tube; the proposed equation is recommended for more accurate and stable prediction of the strength of R-CFSTs.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.3A
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• pp.497-512
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• 2006

In this paper, a simple but effective analysis procedure to estimate seismic capacities of multi-span continuous bridge structures is proposed on the basis of modal pushover analysis considering all the dynamic modes of structure. Unlike previous studies, the proposed method eliminates the coupling effects induced from the direct application of modal decomposition by introducing an identical stiffness ratio and an approximate elastic deformed shape. Moreover, in addition to these two introductions, the use of an appropriate distributed load {P} makes it possible to predict the dynamic responses for all kinds of bridge structures through a simpler analysis procedure. Finally, in order to establish the validity and applicability of the proposed method, correlation studies between rigorous nonlinear time history analysis and the proposed method are conducted for multi-span continuous bridges.

• Structural Engineering and Mechanics
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• v.84 no.3
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• pp.345-360
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• 2022

In this paper, the impact on seismic performance of an economical effective technique for retrofitting reinforced concrete (RC) columns using high-strength steel strips under high axial compression ratios was presented. The experimental program included a series of cyclic loading tests on one nonretrofitted control specimen and three retrofitted specimens. The effects of the axial compression ratio and spacing of the steel strips on the cyclic behavior of the specimens were studied. Based on the test results, the failure modes, hysteretic characteristics, strength and stiffness degradation, displacement ductility, and energy dissipation capacity of the specimens were analyzed in-depth. The analysis showed that the transverse confinement provided by the high-strength steel strips could effectively delay and restrain diagonal crack development and improve the failure mode, which was flexural-shear failure controlled by flexural failure with better ductility. The specimens retrofitted using high-strength steel strips showed more satisfactory seismic performance than the control specimen. The seismic performance and deformation capacity of the retrofitted RC columns increased with decreasing axial compression ratio and steel strip spacing. Based on the test results, a hysteretic model for RC columns that considers the transverse confinement of high-strength steel strips was then established. The hysteretic model showed good agreement with the experimental results, which verified the effectiveness of the proposed hysteretic model. Therefore, the aforementioned analysis can be used for the design of retrofitted RC columns.

• Steel and Composite Structures
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• v.45 no.1
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• pp.133-145
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• 2022

Steel fiber composite bar (SFCB) is a novel type of reinforcement, which has good ductility and durability performance. Due to the unique pseudo strain hardening tensile behavior of SFCB, different flexural behavior is expected of SFCB reinforced concrete (SFCB-RC) beams from traditional steel bar reinforced concrete (S-RC) beams and FRP bar reinforced concrete (F-RC) beams. To investigate the flexural behavior of SFCB-RC beam, four points bending tests were carried out and different flexural behaviors between S/F/SFCB-RC beams were discussed. An flexural analytical model of SFCB-RC beams is proposed and proved by the current and existing experimental results. Based on the proposed model, the influence of the fiber volume ratio R of the SFCB on the flexural behavior of SFCB-RC beams is discussed. The results show that the proposed model is effective for all S/F/SFCB-RC flexural members. Fiber volume ratio R is a key parameter affecting the flexural behavior of SFCB-RC. By controlling the fiber volume ratio of SFCB reinforcements, the flexural behavior of the SFCB-RC flexural members such as bearing capacity, bending stiffness, ductility and repairability of SFCB-RC structures can be designed.

• Journal of The Korean Society of Integrative Medicine
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• v.11 no.4
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• pp.27-39
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• 2023

Purpose : The aim of this study was to identify the influence of transient isokinetic exercise on cardiac autonomic modulation and muscle properties in healthy male subjects. Methods : Twenty-eight healthy males underwent isokinetic exercise of both knee joints using a Biodex systems 3 isokinetic dynamometer with an angular velocity of 60 °/sec. The changes in activity of the autonomic nervous system, as determined by heart rate variability (HRV), and in muscle properties were evaluated at three times: pre-exercise, immediately post-exercise, and 10 min post-exercise. Results : The time domain analysis of HRV revealed significant changes in the beat count and mean and minimal heart rate (HR) measured at pre-exercise, immediately post-exercise, and 10 min post-exercise (p<.001). The beat count and mean HR were markedly increased immediately post-exercise compared to pre-exercise, but then significantly decreased at 10 min post-exercise (p<.001). All parameters of the frequency domain were significantly altered by isokinetic exercise (p<.01). The low frequency/high frequency (LF/HF) ratio, as an index for the sympathovagal balance, was elevated by exercise and remained at a similarly high level at 10 min post-exercise (p<.01). The muscle properties of rectus femoris were changed as follows: Muscle tone and stiffness were significantly increased between pre-exercise and immediately post-exercise (p<.001), and between pre-exercise and at 10 min post-exercise (p<.001). Whereas, the elasticity showed no significant change. Conclusion : These results demonstrated that transient isokinetic exercise could induce changes in cardiac autonomic control and muscle properties. In particular, up-regulation of LF/HF ratio after exercise signifies thus enhanced sympathetic modulation by isokinetic exercise. Therefore, it is needed to understand the cardiovascular risks that may arise during isokinetic exercise for providing the basic evidence to establish appropriate isokinetic exercise protocols as effective rehabilitation exercises.

• Journal of the Korean Geosynthetics Society
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• v.23 no.2
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• pp.29-42
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• 2024

The 2017 Pohang earthquake caused damage to quay structures due to liquefaction. Liquefaction occurs when effective stress is lost due to an increase in excess pore water pressure during an earthquake. As a result, the damage caused to the pier-type quay wall was identified and the damage caused by liquefaction was analyzed. In addition, in the case of improved ground, damage occurred due to liquefaction of the lower sand layer due to the difference in stiffness from the soft rock layer, so additional numerical analysis was performed assuming non-liquefaction ground. There are several factors that affect the increase in excess pore water pressure ratio, such as the relative density of the ground and the magnitude of the input seismic acceleration. Therefore, this study performed numerical analysis for Cases 1 to 3 by increasing the magnitude of the input acceleration, and in the case of improved ground, damage occurred due to liquefaction of the lower sand layer, so the analysis was performed assuming non-liquefaction ground. As a result, the improved ground requires additional reinforcement when there is liquefied ground below, and the horizontal displacement of the pier-type quay piles was reduced by about two times.

• PNF and Movement
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• v.22 no.1
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• pp.71-80
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• 2024

Purpose: The purpose of this study is to compare the effects of proprioceptive neuromuscular facilitation (PNF) stretching, based on ballistic stretching and the contract-relax technique, on hip joint flexibility and muscle tone in adults with shortened rectus femoris muscles. Methods: The study involved 40 adults with shortened rectus femoris muscles, identified using the modified Thomas test. Participants were randomly divided into two groups: PNF stretching, employing the contract-relax technique, and ballistic stretching. Measurements included muscle tension, hip joint range of motion, and muscle characteristics. The rectus femoris muscle shortening effect was confirmed by the modified Thomas test, while the flexibility effect was assessed through hip joint motion range. The muscle tension effect was determined using Myoton-PRO. Results: Both stretching methods resulted in significant improvements in modified Thomas test angles and frequency, with the PNF stretching group showing notably greater changes. However, neither stretching method significantly affected decrement or stiffness measurements. These findings suggest that PNF stretching may be more effective for certain outcomes compared to ballistic stretching. Conclusion: In summary, both stretching methods positively influenced flexibility and muscle tension, with PNF stretching showing a greater impact. These findings highlight the importance of selecting the appropriate stretching technique for achieving functional improvements in muscles, which could serve as valuable indicators for preventing and treating muscle injuries in both sports and daily activities.

• Korean Circulation Journal
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• v.53 no.6
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• pp.406-417
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• 2023

Background and Objectives: Pathophysiological changes of right ventricle (RV) after repair of tetralogy of Fallot (TOF) are coupled with a highly compliant low-pressure pulmonary artery (PA) system. This study aimed to determine whether pulmonary vascular function was associated with RV parameters and exercise capacity, and its impact on RV remodeling after pulmonary valve replacement. Methods: In a total of 48 patients over 18 years of age with repaired TOF, pulmonary arterial elastance (Ea), RV volume data, and RV-PA coupling ratio were calculated and analyzed in relation to exercise capacity. Results: Patients with a low Ea showed a more severe pulmonary regurgitation volume index, greater RV end-diastolic volume index, and greater effective RV stroke volume (p=0.039, p=0.013, and p=0.011, respectively). Patients with a high Ea had lower exercise capacity than those with a low Ea (peak oxygen consumption [peak VO₂] rate: 25.8±7.7 vs. 34.3±5.5 mL/kg/min, respectively, p=0.003), while peak VO₂ was inversely correlated with Ea and mean PA pressure (p=0.004 and p=0.004, respectively). In the univariate analysis, a higher preoperative RV end-diastolic volume index and RV end-systolic volume index, left ventricular end-systolic volume index, and higher RV-PA coupling ratio were risk factors for suboptimal outcomes. Preoperative RV volume and RV-PA coupling ratio reflecting the adaptive PA system response are important factors in optimal postoperative results. Conclusions: We found that PA vascular dysfunction, presenting as elevated Ea in TOF, may contribute to exercise intolerance. However, Ea was inversely correlated with pulmonary regurgitation (PR) severity, which may prevent PR, RV dilatation, and left ventricular dilatation in the absence of significant pulmonary stenosis.