• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2003.05a
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• pp.109-114
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• 2003

Wireless communication systems are inevitable for efficient underwater activities. Because of the poor propagation characteristics of light and electromagnetic waves, acoustic waves are generally used for the underwater wireless communication. Although there are many kinds of information type, visual images take an essential role especially for search and identification activities. For this reason, we developed an acoustic-based underwater image transmission system under a dual use technology project supported by MOCIE (Ministry of Commerce, Industry and Energy). For the application to complicated and time-varying underwater environments all-digital transmitter and receiver systems are investigated. Array acoustic transducers are used at the receiver, which have the center frequency of 32kHz and the bandwidth of 4kHz. To improve transmission speed and quality, various algorithms and systems are used. The system design techniques will be discussed in detail including image compression/ decompression system, adaptive beam- forming, fast RLS adaptive equalizer, ${\partial}/4$ QPSK (Quadrilateral Phase Shift Keying) modulator/demodulator, and convolution coding/ Viterbi. Decoding.

• Geomechanics and Engineering
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• v.17 no.5
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• pp.429-438
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• 2019

Microbially induced carbonate precipitation (MICP) is an innovative soil improvement approach utilizing metabolic activity of microbes to hydrolyze urea. In this paper, the shear response and the erodibility of MICP-treated sand under axial compression and submerged impinging jet were evaluated at a low confining stress range. Loose, poorly graded silica sand was used in testing. Specimens were cemented at low confining stresses until target shear wave velocities were achieved. Results indicated that the erodibility parameters of cemented specimens showed an increase in the critical shear stress by up to three orders of magnitude, while the erodibility coefficient decreased by up to four orders of magnitude. Such a trend was observed to be dependent on the level of cementation. The treated sand showed dilative behavior while the untreated sands showed contractive behavior. The shear modulus as a function of strain level, based on monitored shear wave velocity, indicated mineral debonding may commence at 0.05% axial strain. The peak strength was enhanced in terms of emerging cohesion parameter based on utilizing the Mohr-Coulomb failure criteria.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2022.04a
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• pp.245-246
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• 2022

Recently, research on the self-healing of concrete using bacteria has been actively conducted. The self-healing method using bacteria has a low self-healing rate and the surrounding environment of the fracture site is very important. A previous study to solve this problem involves the manufacture of capsules using 3D printing. Fracture position control was an important topic in 3D printing-based capsules. In this study, to compensate for the shortcomings of existing studies, a capsule capable of selective destruction location control was produced using PDMS-based molds that are not restricted by the environment. Resin capsules were prepared for each part using several molds and a bonding surface was arranged. In order to verify this on the bonding surface, fracture strength and wave unit values were analyzed through a three-way compression experiment. It can be seen that as the curing time increases, the deviation between samples decreases. In addition, through experiments, it was confirmed that the junction surface and wave unit values coincide in all three directions. It can be used for self-healing research using various solutions.

• Journal of the Society of Disaster Information
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• v.18 no.3
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• pp.646-659
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• 2022

Purpose: In this study, we developed a 4-way sway prevention brace that efficiently reduces the installation area and has excellent stability and seismic performance compared to the conventionl sway prevention brace used in existing firefighting facilities. The performance and reliability of the developed 4-way way prevention brace were analyzed by the tensile, compression tests and seismic tests. Method: As the static test, 4-way sway prevention braces were installed on the horizontal and vertical pipes to perform the tensile and compression tests based on the KFI certification standard and the maximum movement was measured at the rated load. As a dynamic test, 4-way sway prevention braces were installed in the pipes filled with water, and the test response spectrum to the input excitation wave were measured through the acceleration sensors. After the seismic tests, separation, failure, and local deformation of the pipes, and 4-way sway prevention braces were not observed. Result: The results of the tensile and compression tests indicated that the maximum movement of the pipe during tension and compression was 50% to 70% or less compared to the certification values, indicating that the performances of the 4-way sway prevention braces were very excellent. The results of the the seismic tests indicated that the test response spectrum of the 4-way sway prevention braces is within the required response spectrum. Conclusion: In this study, it was found that the 4-way sway prevention braces satisfied the KFI certification standard and were superior compared to the existing sway prevention brace in terms of the stability, cost, and installation area.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.42 no.6 s.336
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• pp.67-74
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• 2005

In this paper, high LO-RF isolation W-band MIMIC single-balanced mixer was designed and fabricated using a branch line coupler and a $\lambda$/4 transmission line. The simulation results of the designed 94 GHz balun show return loss of -27.9 dB, coupling of -4.26 dB, and thru of -3.77 dB at 94 GHz, respectively. The isolation and phase difference were 23.5 dB and $180.2^{\circ}$ at 94 GHz. The W-band MIMIC single-balanced mixer was designed using the 0.1 $\mu$m InGaAs/InAlAs/GaAs Metamorphic HEMT diode. The fabricated MHEMT was obtained the cut-off frequency(fT) of 189 GHz and the maximum oscillation frequency(fmax) of 334 GHz. The designed MIMIC single-balanced mixer was fabricated using 0.1 $\mu$m MHEMT MIMIC Process. From the measurement, the conversion loss of the single-balanced mixer was 23.1 dB at an LO power of 10 dBm. Pl dB(1 dB compression point) of input and output were 10 dBm and -13.9 dBm respectively. The LO-RF isolations of single-balanced mixer was obtained 45.5 dB at 94.19 GHz. We obtained in this study a higher LO-RF isolation compared to some other balanced mixers in millimeter-wave frequencies.

• Journal of the Korean Geotechnical Society
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• v.30 no.1
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• pp.65-74
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• 2014

Cemented soils have been widely used in road and dam construction, and recently ground improvement of soft soils. The strength of such cemented soils can be tested by using cored sample or laboratory-prepared specimen through unconfined compression or triaxial tests. It takes time to core a sample or prepare a testing specimen in the laboratory. In a certain situation, it is necessary to determine the in-situ strength of cemented soils very quickly and on time. In this study, the relation between unconfined compressive strength and shear wave velocity was investigated for predicting the in-situ strength of cemented soils. A small cemented specimen with 5 cm in diameter and 10 cm in height was prepared by Nakdong river sand and ordinary Portland cement. Its cement ratios were 4, 8, 12, and 16% and air cured for 7, 14, and 28 days. For recycling of resources, a blast furnace slag was also used with sodium hydroxide as an alkaline activator. The shear wave velocity for cemented soils was measured and then unconfined compressive strength test was carried out. As a cement ratio increased, the shear wave velocity and unconfined compressive strength increased due to increased density and denser structure. The relation between unconfined compressive strength and shear wave velocity increased nonlinearly for cemented soils with less than 16% of cement ratio.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.20 no.2
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• pp.191-206
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• 2007

The general theoretical solutions for the wavespeed and damping derived in Part 1 of this work, are incorporated into the computer code. In this paper the code is used in a parametric study of the influence of excitation frequency and variations in material properties on propagation velocity and damping. Compressional wave velocity for waves of the first kind is shown to vary as a function of the frequency-permeability product, with a zone where wavespeed transitions from a lower bound value to a higher bound value with increasing values of the product. Damping is seen to be a maximum where the rate of change in wavespeed is greatest. Waves of the second kind also show a transition in wavespeed from near zero at low values of the frequency-permeability product to an upper bound value at higher values of the product.

• Geotechnical Engineering
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• v.12 no.5
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• pp.63-78
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• 1996

Various type of in-situ and laboratary tests were performed in order to evaluate the stiffness of sedimentary soft rock. In triaxial compression tests of sedimentary soft rocks, axial strains from the axial displacement of the loading piston or specimen cap conventionally were considerably larger than those measured. tocally on the lateral surfaces of specimen, due to the bedding errors at the top and bottom ends of a specimen. A local deformation transducer was used to measure axial strains free from the bedding error ranging from 0.001% to about 1%. In ultra-sonic wave tests, the elastic modulus of unconfined spec imens was smaller than that of confined specimens, due probably to microfracks. Young's modulus Ed from ultra-sonic wave tests and those at small local strains from triaxial tests were similar, both of which agreed very well with Young's modulus Er from field shear wave velocities. Young'a modulus from the field behaviour was virtually similar to that obtained by reducing Er based on the strain level-dependency of stiffness evaluated by the triaxial tests.

• Journal of the Korean GEO-environmental Society
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• v.16 no.11
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• pp.5-11
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• 2015

The development of Controlled Low Strength Material (CLSM), which is a highly flowable material, has been performed for the application of backfill. The objective of this study is to compare the compressive strength and compressive wave velocity of CLSM according to the curing time. To investigate the characteristics of the CLSM consisting of sand, silt, water, flyash, and CSA cement, uniaxial compression test and flow test were carried out. For the measurement of compressional waves, a cell and a couple of transducers were used. The test results show that the compressive strength increases with the curing time, while the increment of compressive strength decreases with the curing time. In addition, the compressive wave velocity increases with the curing time, and the correlation between the compressive wave velocity and compressive strength is similar to exponential function. This study suggests that the correlation between the compressive wave velocity and compressive strength may be effectively used for the estimation of compressive strength of the CLSM at early curing time.

• Geophysics and Geophysical Exploration
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• v.14 no.4
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• pp.324-334
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• 2011

Seismic wave velocity change has been monitored due to the accumulation of micro-cracks by uniaxial loads on the rock samples from Seokmo Island with stepwise increase in 5 stages. After the load was applied up to 95% of UCS, P- and S-wave velocities varied in ranges of 0.9 ~ 18.3% and 2.8 ~ 14.8% of fresh rock sample velocities, respectively. Unlike seismic velocity of the dry rock samples that showed overall decreases after the loading, velocity changes of saturated rock samples were much more complicated. These seemed to be due to the mixture of two contradictory mechanisms; i.e. accumulation of micro-crack causes an increase in porosity and a decrease in wave velocity, while saturation causes an increase in wave velocity. Most of tested rocks showed a trend of velocity increase with low axial load and then velocity decrease at later stages. Starting stage of velocity decrease differs from samples to samples. After the failure of rock occurred, noticeable increases of porosity and decreases of wave velocity have been observed. It showed overall trend that the more the quartz contents and the lower the silicate, the higher the Young's modulus.