• 대한건축학회논문집:구조계
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• 제33권12호
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• pp.45-52
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• 2017

The precast concrete (PC) method has many advantages in fast construction, quality control, etc. In domestic construction market, however, its application has been quite limited because of the concerns about structural integrity and seismic performances due to the discrete connections between precast concrete members. By applying the post-tensioning method, the precast beam-column connection can be well tightened, allowing improved structural integrity, and proper seismic performances can be also achieved. In this study, reversed cyclic tests have been conducted on the beam-column connection specimens, where the test variables included the compressive strength of grouting mortar and the tensile strengths of prestressing strands, based on which their seismic performances have been examined in detail. The post-tensioned PC beam-column connections showed good seismic performances comparable to that of the monolithic reinforced concrete connection specimen. When 2400 MPa prestressing strands are applied to the beam-column connection, it is preferable to adjust the prestress level similar to that applied for the 1860 MPa prestressing strands to avoid premature local crushing failures at the beam-column connections.

• Earthquakes and Structures
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• 제20권2호
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• pp.149-160
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• 2021

Buildings made using the locally available clay materials are amongst the least expensive forms of construction in many developing countries, and therefore, widely popular in remote areas. It is despite the fact that these low-strength masonry structures are vulnerable to seismic forces. Since transporting imported materials like cement and steel in areas inaccessible by motorable roads is challenging and financially unviable. This paper presents, and experimentally investigates, adobe masonry structures that utilize the abundantly available local clay materials with moderate use of imported materials like cement, aggregates, and steel. Shake-table tests were performed on two 1:3 reduce-scaled adobe masonry models for experimental seismic testing and verification. The model AM1 was confined with vertical lightly reinforced concrete columns provided at all corners and reinforced concrete horizontal bands (i.e., tie beams) provided at sill, lintel, and eave levels. The model AM2 was confined only with the horizontal bands provided at sill, lintel, and eave levels. The models were subjected to sinusoidal base motions for studying the damage evolution and response of the model under dynamic lateral loading. The lateral forcedeformation capacity curves for both models were developed and bi-linearized to compute the seismic response parameters: stiffness, strength, ductility, and response modification factor R. Seismic performance levels, story-drift, base shear coefficient, and the expected structural damages, were defined for both the models. Seismic performance assessment of the selected models was carried out using the lateral seismic force procedure to evaluate their safety in different seismic zones. The use of vertical columns in AM1 has shown a considerable increase in the lateral strength of the model in comparison to AM2. Although an R factor equal to 2.0 is recommended for both the models, AM1 has exhibited better seismic performance in all seismic zones due to its relatively high lateral strength in comparison to AM2.

• 대한금속재료학회지
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• 제56권12호
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• pp.845-853
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• 2018

To understand the effect of microstructural factors (i.e., the size of ${\alpha}$ phase, equiaxed vs bimodal structure) on high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of mill-annealed Ti-6Al-4V (Ti64) alloy, three specimens of EQ (equiaxed)-8 (8 indicates the size of ${\alpha}$ grain), BM (bimodal)-8, and BM-16 were studied. The uniaxial HCF and FCP tests were conducted at an R ratio of 0.1 under sinusoidal fatigue loading. The microstructural influence (i.e., EQ vs BM) was not significant on the tensile properties of mill-annealed Ti64 alloy, and showed an increase in tensile strength and elongation with decreasing gauge thickness from 50 mm to 1.3 mm. The microstructure, on the other hand, affected the resistance to HCF substantially. It was found that the EQ structure in mill-annealed Ti64 has better resistance to HCF than the BM structure, as a result of different crack initiation mechanism. Unlike HCF behavior, the effect of microstructural features on the FCP behavior of mill-annealed Ti64 was not significant. Among the three specimens, BM-16 specimen showed the highest near-threshold ΔK value, probably because it had the greatest slip reversibility with large ${\alpha}$ grains. The effect of microstructural factors on the HCF and FCP behaviors of mill-annealed Ti64 alloy are discussed based on fractographic and micrographic observations.

• 대한금속재료학회지
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• 제46권9호
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• pp.545-553
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• 2008

Dynamic deformation behavior of ultra-fine-grained pure coppers fabricated by equal channel angular pressing (ECAP) was investigated in this study. Dynamic torsional tests were conducted on four copper specimens using a torsional Kolsky bar, and then the test data were analyzed by their microstructures and tensile properties. The 1-pass ECAP'ed specimen consisted of fine dislocation cell structures elongated along the ECAP direction, which were changed to very fine, equiaxed subgrains of 300~400 nm in size as the pass number increased. The dynamic torsional test results indicated that maximum shear stress increased with increasing ECAP pass number. Adiabatic shear bands were not found at the gage center of the dynamically deformed torsional specimen of the 1- or 4-pass ECAP'ed specimen, while some weak bands were observed in the 8-pass ECAP'ed specimen. These findings suggested that the grain refinement according to the ECAP was very effective in strengthening of pure coppers, and that ECAP'ed coppers could be used without serious reduction in fracture resistance under dynamic torsional loading as adiabatic shear bands were hardly formed.

• Smart Structures and Systems
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• 제26권6호
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• pp.693-701
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• 2020

The conventional Kalman Filter (KF) provides a promising way for structural state estimation. However, the physical parameters of structural systems or models should be available for the estimation. Moreover, it is not applicable when the loadings applied to the structures are unknown. To circumvent the aforementioned limitations, a two-stage KF with unknown input approach is proposed for the simultaneous identification of structural parameters and unknown loadings. In stage 1, a modified observation equation is employed. The structural state vector is estimated by KF on the basis of structural parameters identified at the previous time-step. Then, the unknown input is identified by Least Squares Estimation (LSE). In stage 2, based on the concept of sensitivity matrix, the structural parameters are updated at the current time-step by using the estimated structural states obtained from stage 1. The effectiveness of the proposed approach is numerically validated via a five-story shearing model under random and earthquake excitations. Shaking table tests on a five-story structure are also employed to demonstrate the performance of the proposed approach. It is demonstrated from numerical and experimental results that the proposed approach can be used for the identification of parameters of structure and the external force applied to it with acceptable accuracy.

• 한국항공우주학회지
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• 제49권7호
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• pp.549-555
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• 2021

원공이 있는 복합재 적층판의 강도는 복합재 항공기기 설계 시 충격하중이 가해지는 부위의 설계 허용치로 사용된다. 일반적으로 BVID (Barely Visible Impact Damage)에 의한 강도저하는 24.0 mm 폭의 시편에 6.0 mm 지름의 원공이 있는 시편의 강도로 가정한다. 본 연구에서는 원공이 있는 복합재 적층판의 강도에 적층각이 미치는 영향을 조사하기 위해 잔류강도 시험을 수행하였다. 원공이 있는 적층판의 강도를 예측하기 위해 특성길이를 이용한 점응력 파손기준을 사용하였고, 이론해를 검증하기 위하여 유한요소해석을 수행하였다. 실험결과로부터 특성길이는 0°, ±45°와 90°층의 비율과 관련이 있음을 보였다. 또한 회귀분석을 통하여 임의의 적층 패턴에 대한 특성길이와 원공이 없는 시편의 강도를 결정하였다.

• 한국의류학회지
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• 제45권3호
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• pp.453-463
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• 2021

The lengthwise and widthwise deformation of warp-knitted fabrics with different sizes and loading modes were evaluated. Moreover, five tricot samples cut in three directions were compared under four test conditions (A-D). In tests A and B, 500 and 250 g loads were applied on a layer of 20 × 20 and 5 × 10 cm² samples, respectively. In test C, a 20 × 20 cm² sample was folded in half over a rod, and 500 g load was applied to each half. In test D, a 20 × 20 cm² sample was sewn in a loop and subjected to a 500 g load. The lengthwise extension and widthwise contraction analysis results indicate that test B affords the largest values. However, analysis results of the warp-knitted fabric normalized through conversion to a 1 g load and 1 cm sample width indicate that the largest values are afforded for test D. Therefore, pattern reduction may vary depending on the measurement method and properties of the knitted fabric used for the compression wear production, causing variations in the finished product. Thus, an appropriate measurement method must be adopted based on the compression wear design and knitted fabric to be used.

• 한국재료학회지
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• 제31권6호
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• pp.345-352
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• 2021

In this study, Ni nanoparticle supported by graphene oxide (GO) (Ni-GO) is successfully synthesized through hydrothermal synthesis and calcination, and Cr(VI) is extracted from aqueous solution. The morphology and structure of Ni-GO composites are characterized by scanning electron microscopy (SEM), trans mission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). High-resolution transmission electron microscopy (HRTEM) and XRD confirms the high dispersion of Ni nanoparticle after support by GO. Loading Ni on GO can obviously enhance the stability of Ni-GO composites. It can be calculated from TGA that the mass percentage of Ni is about 60.67 %. The effects of initial pH and reaction time on Cr(VI) removal ability of Ni-GO are investigated. The results indicate that the removal efficiency of Cr(VI) is greater than that of bared GO. Ni-GO shows fast removal capacity for Cr(VI) (<25 min) with high removal efficiency. Dynamic experiments show that the removal process conforms to the quasi-second order model of adsorption, which indicates that the rate control step of the removal process is chemical adsorption. The removal capacity increases with the increase of temperature, indicating that the reaction of Cr(VI) on Ni-GO composites is endothermic and spontaneous. Combined with tests and characterization, the mechanism of Cr(VI) removal by rapidly adsorption on the surface of Ni-GO and reduction by Ni nanoparticle is investigated. The above results show that Ni-GO can be used as a potential remediation agent for Cr(VI)-contaminated groundwater.

• 한국지진공학회논문집
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• 제26권1호
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• pp.13-22
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• 2022

This study proposes a low-cycle fatigue life derived from measurement points on pipe elbows, which are components that are vulnerable to seismic load in the interface piping systems of nuclear power plants that use seismic isolation systems. In order to quantitatively define limit states regarding leakage, i.e., actual failure caused by low-cycle fatigue, in-plane cyclic loading tests were performed using a sine wave of constant amplitude. The test specimens consisted of SCH40 6-inch carbon steel pipe elbows and straight pipes, and an image processing method was used to measure the nonlinear behavior of the test specimens. The leakage lines caused by low-cycle fatigue and the low-cycle fatigue curves were compared and analyzed using the relationship between the relative deformation angles, which were measured based on each of the measurement points on the straight pipe, and the moment, which was measured at the center of the pipe elbow. Damage indices based on the combination of ductility and dissipation energy at each measurement point were used to quantitatively express the time at which leakage occurs due to through-wall cracking in the pipe elbow.

• 한국구조물진단유지관리공학회 논문집
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• 제26권5호
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• pp.119-126
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• 2022

본 연구는 내진설계가 되어 있지 않은 필로티 건축물의 부족한 횡력을 보강할 수 있는 외부부착형 내진보강공법(Binding Column Method, BCM)을 제안하였다. 또한, 제안된 내진보강공법을 대상으로 보강실험체 4개, 기준 실험체 1개를 제작하여 반복가력 실험을 통하여 보강 전·후의 내진성능향상 효과를 검토하였다. 실험 결과, 기준 실험체(SC1)는 급격한 강도저하와 함께 취성적인 전단파괴의 양상을 나타낸 반면, BCM 공법을 적용한 실험체(SC2, SC3, SC4, SC5)는 강도 및 강성의 증가와 함께 에너지 흡수 능력이 큰 타원형의 이력특성을 나타내었다. 또한, 간격이 좁고 토크가 크며, L자형 강판의 두께가 두꺼울수록 보강효과가 향상됨을 알 수 있다. BCM공법 중 전단보강간격이 작고, 조임력 값이 크며, 연결철물이 두꺼운 SC4실험체가 가장 뛰어난 내진성능보강 효과를 나타내었다.