• Journal of the Optical Society of Korea
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• v.16 no.1
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• pp.63-69
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• 2012

This paper presents a method to derive the theoretical requirements for the development of a 400 mm optical window that transmits dual-band wavelengths and had a stable structure. We also present design and fabrication results. Among the required specifications, the surface figure error was defined by the transmitted wavefront deformation (TWD), ${\lambda}$/15 rms at 632.8 nm. This value was derived by estimating the predicted performances with respect to five independent items that could cause system performance degradation and then calculating the required wavefront error (WFE) to satisfy the performance goals. We measured the image resolution at each performance level to trace and verify the requirements. The article also describes a design optimization process that could minimize the weight and volume of the optical window attached to the payload securing the FOV of the camera. In addition, we accurately measured the deformation that occurred in the series of fabrication steps including processing, coating, assembly, bonding and bolting, and investigated the effects by comparing them to the results of a simulation performed in advance to derive the predicted performance.

• Geomechanics and Engineering
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• v.34 no.1
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• pp.1-15
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• 2023

This study conducts uniaxial compression tests on intact and single crack-contained rocks to investigate the time-frequency domain characteristics of acoustic emission (AE) signals monitored during the deformation failure process. A processing approach, short-time Fourier transform (STFT), is performed to obtain the evolution characteristics of time-frequency domain of AE signals. The AE signal modes at different deformation stages of rocks are different. Five modes of AE signal are observed during the cracking process of rocks. The evolution characteristics of time-frequency domain of AE signals processed by STFT can be utilized to evaluate the damage process of rocks. The difference of time-frequency domain characteristics between intact and cracked rocks is comparatively analyzed. The distribution characteristics of frequency changing from a single band-shaped cluster to multiple band-shaped clusters can be regarded as an early warning information of damage and failure of rocks. Meanwhile, the attenuation of frequency enables the exploration of rock failure trends.

• Journal of the Korean Society for Heat Treatment
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• v.18 no.1
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• pp.29-40
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• 2005

Formation of nanocrystalline ferrite was investigated using milled powders obtained by planetary ball milling of chips, which were made by high speed mechanical cutting of a low carbon steel(0.15%C-1.1%Mn-0.01%Ti). After 4 hour milling the chips were changed to powders of $50{\mu}m$ in average size, and with increasing milling time the powders were refined to about $3{\mu}m$ for 128 hour and showed more equiaxed shapes. Nanocrystalline(nc) region appeared in the surfaces of powders milled for 1 hour, and the 4 hour milled powders were almost filled with nc region. Hardness of nc region was much higher than that of work-hardened(WH) region. With increasing milling time, ferrite and cementite in pearlite were severely deformed and lamellar spacing was decreased, and then cementites began to disappear after 4 hour milling due to dissolution into ferrite. Deformation bands formed in lightly work-hardened region showed large width and similar crystallographic orientations. Spacing of deformation bands was decreased with deformation and the layered microstructure consisting of narrow deformation bands subdivided into variously oriented small grains was formed by more deformation, and eventually this structure seemed to be evolved to the nc structure by further deformation. It is also conjectured the growth of nc ferrite grains occurred through the coalescence of nanocrystalline ferrites rather than the nucleation and growth of recrystallized grains.

• Journal of the Korea institute for structural maintenance and inspection
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• v.22 no.2
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• pp.1-7
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• 2018

Suspension bridge cables made of high strength steel wires require periodical maintenance in accordance with the axial force of cable-band bolts, since the bolts in suspension bridges can undergo tension decrease due to creep of cable wires, bolt relaxation, load fluctuation, and cable re-arrangement, etc. Consequently, this study is aimed at investigating and subsequently evaluating the critical factors with respect to the bolt tension-decrease phenomenon in SR suspension bridge in Korea, based on field monitoring, theoretical studies, and field record management works. From the observation, it is interesting to note that the decrease in the bolt tension force is typically accompanied by plastic deformation of the zinc plating layers in the cable wires. In addition, a framework corresponding to generic methodologies to characterize the deformation in terms of the bolt tension-decrease and long-term history management has been developed in this exploratory study.

• Journal of the Korea institute for structural maintenance and inspection
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• v.6 no.1
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• pp.213-221
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• 2002

This paper presents the experimental result of Reinforced Concrete column-steel beam joint connected by Band Plates(BP). Main parameters in the test are the shape of BP and thickness of plate. Ten interior and exterior RC column-steel beam joint specimens are designed. Cyclic loads are applied to the beam end of eight specimens (four interior specimens and four exterior specimens). To evaluate the cyclic effect, monotonic loads are acted for two specimens. All specimen showed similar failure pattern such as the plate of BP get torn after the large deformation. Even though the specimen with double cross type BP has lower strength than the specimen with single cross type BP, the energy dissipation capacity of the specimen turned out high. Thus, provided the strength of joint with double cross type to be designed to have suitable strength by increasing the thickness of plate, the joint system may show higher seismic capacity.

• Korea-Australia Rheology Journal
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• v.13 no.2
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• pp.67-81
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• 2001

The development of new techniques for the dynamic measurement of linear viscoelastic properties is an active area of rheometry, and this paper surveys some novel deformation geometries which have been recently reported e.g. oscillating probe-type devices which are imbedded in or placed on the surface of the sample. Small amplitude band-limited pseudorandom noise is used for the displacement signal, with Fourier analysis of the complex waveform of the resistance force yielding the frequency dependent viscoelastic material functions (e.g. storage and loss moduli G", G"). Theoretical calculations of the fundamental equations relating force to displacement and instrument geometry, were carried out with the aid of the correspondence principle of linear viscoelasticity. The rapidity of the tests and flexibility in terms of sample preparation and stiffness mean that this basic technique should find many applications in rheometry. Three examples of oscillatory tests are presented in detail squeeze flow, imbedded needle and concentric sliding cylinder geometries.eometries.

• Journal of the Earthquake Engineering Society of Korea
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• v.3 no.3
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• pp.33-44
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• 1999

An energy balance procedure is developed to incorporate the effects of earthquake duration which involves the effect of cyclic loading and the corresponding cumulative plastic deformation. Particular emphasis is given to the flexural failure of non-seismically designed columns of reinforced concrete frames. For this, conceptual strength deterioration models for columns, governed by concrete, anchorage failure and longitudinal steel fracture due to low-cycle fatigue, are proposed. It is evident that the energy-based method has good agreement with the experimental data and is able to predict the failure mode.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1999.08a
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• pp.163-171
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• 1999

The abnormal grain coarsening in wire rolling induces detrimental defects, such as jagged size tolorance, severe bending after heat treatments and drawing troubles, in the following secondary processes. Neishi et al observed that there is a band type region where grain coarsenting occurs in the plastic strain vs. deformation temperature plot. Based on the finding, we have investigate whether grain sizes and ferrite volume fractions are correlation to deformation strain with three kinds of wire rod diameters as for the different average deformation conditions. The samples were chosen from the No.2 Wire Rod Mill of POSCO where 3-roll type of finishing mill stand are used. It was found in the present work that the grain size and ferrite volume fraction of the rolled and cooled microstructure were changed with rolling reduction and rolling temperature. Abnormally grown grains at various observed points were also found. To have homogeneously fine grains of microstructure from the No. 2 Wire Rod Mill, it will be easier to control finish rolling temperature at around 750$^{\circ}C$ rather than to find another rolling schedule.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2004.04a
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• pp.237-240
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• 2004

High strain-rate deformation behavior of NiAl/Ni micro-laminated composites was characterized by split hopkins on pressure bar(SHPB). When the strain rate increased, the compressive stress of micro-laminated composites were increased a little. When the intermetallic volume fraction increased, the compressive stress of micro-laminated composites increased linearly irrespective of strain rate. Absorbed energy during the quasi-static and SHPB tests was calculated from the integrated area of stress-strain curve. Absorbed energy of micro-laminated composites deviated from the linearity in terms of the intermetallic volume fraction but merged to the value of intermetallic as the strain rate increased. This was due to high tendency of intermetallic layer for the localization of shear deformation at high strain rate. Microstructure showing adibatic shear band(ASB) confirmed that the shear strain calculated from the misalignment angle of each layer increased and ASB width decreased when the intermetallic volume fraction. Simulation test impacted by tungsten heavy alloy cylinder resulted that the absorbed energies multiplied by damaged volume of micro-laminated composites were decreased as the intermetallic volume fraction increased. Fracture mode were changed from delamination to single fracture when the intermetallic volume fraction and this results were good matched with previous results[l] obtained from the fracture tests.

• Korean Journal of Materials Research
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• v.27 no.12
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• pp.679-687
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• 2017

A strain-gradient crystal plasticity finite element method(SGCP-FEM) was utilized to simulate the compressive deformation behaviors of single-slip, (111)[$10{\bar{1}}$], oriented FCC single-crystal micro-pillars with two different slip-plane inclination angles, $36.3^{\circ}$ and $48.7^{\circ}$, and the simulation results were compared with those from conventional crystal plasticity finite element method(CP-FEM) simulations. For the low slip-plane inclination angle, a macroscopic diagonal shear band formed along the primary slip direction in both the CP- and SGCP-FEM simulations. However, this shear deformation was limited in the SGCP-FEM, mainly due to the increased slip resistance caused by local strain gradients, which also resulted in strain hardening in the simulated flow curves. The development of a secondly active slip system was altered in the SGCP-FEM, compared to the CP-FEM, for the low slip-plane inclination angle. The shear deformation controlled by the SGCP-FEM reduced the overall crystal rotation of the micro-pillar and limited the evolution of the primary slip system, even at 10 % compression.