• Journal of the Korean Society of Safety
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• v.17 no.2
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• pp.58-62
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• 2002

Longitudinal wave tests with finite elastic medium were performed to investigate the difference between measured values and theoretical values of propagation velocity and elasticity modulus. Each accelerometer was attached on finite elastic medium with same phase and different positions to check the particle motion. The results show that measured values of elasticity moduli from both time domain and frequency domain were similiar to theoretical value. Polarity of signal depends entirely on the phase of accelerometer. It proved that the propagation velocity and the particle motion are in the same direction when a compressive stress is applied. And also the propagation velocity and the particle motion depend on the intensity of the stress and material properties respectively.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.26 no.6
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• pp.367-380
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• 2014

Tsunami take away life, wash houses away and bring devastation to social infrastructures such as breakwaters, bridges and ports. The targeted coastal structure object in this study can be damaged mainly by the tsunami force together with foundation ground failure due to scouring and liquefaction. The increase of excess pore water pressure composed of oscillatory and residual components may reduce effective stress and, consequently, the seabed may liquefy. If liquefaction occurs in the seabed, the structure may sink, overturn, and eventually increase the failure potential. In this study, the solitary wave was generated using 2D-NIT(Two-Dimensional Numerical Irregular wave Tank) model, and the dynamic wave pressure acting on the seabed and the estimated surface boundary of the vertical revetment. Simulation results were used as an input data in a finite element computer program(FLIP) for elasto-plastic seabed response. The time and spatial variations in excess pore water pressure, effective stress, seabed deformation, structure displacement and liquefaction potential in the seabed were estimated. From the results of the analysis, the stability of the vertical revetment was evaluated.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.17 no.1
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• pp.75-82
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• 2004

With an aim at eliminating the numerical dispersion error arising from the numerical simulation of stress wave propagation, numerical dispersion characteristics of the wave equation based one-dimensional finite element model are analyzed and some dispersion control scheme are proposed in this paper The dispersion analyses are carried out for two types of mass matrix, namely the consistent and the lumped mass matrices. Based on the finding of the analyses, dispersion correction techniques are developed for both the implicit and explicit schemes. For the implicit scheme, either the weighting factor for the spatial derivatives of each time level or the lumping coefficient for mass matrix is adjusted to minimize the numerical dispersion. In the case of the explicit scheme an artificial dispersion term is introduced in the governing equation. The validity of the dispersion correction techniques proposed in this study is demonstrated by comparing the numerical solutions obtained using the Present techniques with the analytical ones.

• Smart Structures and Systems
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• v.20 no.2
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• pp.175-180
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• 2017

The load-carrying capacity and structural behavior of concrete-filled steel tube (CFST) structures is highly influenced by the grouting compactness in the steel tube. Due to the invisibility of the grout in the steel tube, monitoring of the grouting progress in such a structure is still a challenge. This paper develops an active sensing approach with combined piezoceramic-based smart aggregates (SA) and piezoceramic patches to monitor the grouting compactness of CFST bridge structure. A small-scale steel specimen was designed and fabricated to simulate CFST bridge structure in this research. Before casting, four SAs and two piezoceramic patches were installed in the pre-determined locations of the specimen. In the active sensing approach, selected SAs were utilized as actuators to generate designed stress waves, which were detected by other SAs or piezoceramic patch sensors. Since concrete functions as a wave conduit, the stress wave response can be only detected when the wave path between the actuator and the sensor is filled with concrete. For the sake of monitoring the grouting progress, the steel tube specimen was grouted in four stages, and each stage held three days for cement drying. Experimental results show that the received sensor signals in time domain clearly indicate the change of the signal amplitude before and after the wave path is filled with concrete. Further, a wavelet packet-based energy index matrix (WPEIM) was developed to compute signal energy of the received signals. The computed signal energies of the sensors shown in the WPEIM demonstrate the feasibility of the proposed method in the monitoring of the grouting progress.

• Journal of the Korean Institute of Navigation
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• v.8 no.2
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• pp.51-70
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• 1984

In the North Pacific Ocean a lot of large waves set up in winter, affected by continued winds and swells owing to severe extratropical cyclones. Under this sea condition, if the ship is about 100,000L/T (in deadweight capacity tonnage), we can't find the danger involved in the ship at sea apparently. But when we compare the seaworthiness of ship's building strength with the stress given to the hull by waves, we can't insist that the former be more stronger than the latter. As a result, VLCC is in danger of destroying and cutting for lack of longitudinal strength in heavy weather. Up to this time, Naval Architects have actively studied the relation between ship's longitudinal strength and waves as a ship's projector; however, actually, they have never made more profound study on the problem of longitudinal strength in relation to navigation. The main puprpose of this thesis is to clarify these vivid actual states of ship's trouble unknown to ship's masters. In this thesis we picked up VLCC Pan Yard, a vessel of Pan Ocean Bulk Carrier company's, as a model ship. And in the North Pacific Ocean, we have chosen for this research the basins where the wind speed and the wave height are greater than average. The data used this thesis are quotes from the "winds and waves of the North Pacific Ocean('64-'73)", and wind speed more than 30 knots was made use of as an ocject of this study. By usinh the ITTC wave spectrum, we found out the significant waves for every 5 knots within the range of 20 knots to 45 knots of wind speed. According to this H1/1000 was calculated. The stress of ship's hull is determined by ship's speed and wave height. We compared the ship's longitudinal strength with a planned wave height by rules of several famous classification societies in the world. In the last analysis, we found out that ship's present planned strength in heavy weather is not enough. Finally we made a graph for avoiding heavy weather, with which we studied safe ship's handling in the North pacafic Ocean in winter.

• Ocean Systems Engineering
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• v.4 no.4
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• pp.327-342
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• 2014

The tension leg platform (TLP) is one of the compliant structures which are generally used for deep water oil exploration. With respect to the horizontal degrees of freedom, it behaves like a floating structure moored by vertical tethers which are pretension due to the excess buoyancy of the platform, whereas with respect to the vertical degrees of freedom, it is stiff and resembles a fixed structure and is not allowed to float freely. In the current study, a numerical study for square TLP using modified Morison equation was carried out in the time domain with water particle kinematics using Airy's linear wave theory to investigate the effect of changing the tether tension force on the stiffness matrix of TLP's, the dynamic behavior of TLP's; and on the fatigue stresses in the cables. The effect was investigated for different parameters of the hydrodynamic forces such as wave periods, and wave heights. The numerical study takes into consideration the effect of coupling between various degrees of freedom. The stiffness of the TLP was derived from a combination of hydrostatic restoring forces and restoring forces due to cables. Nonlinear equation was solved using Newmark's beta integration method. Only uni-directional waves in the surge direction was considered in the analysis. It was found that for short wave periods (i.e., 10 sec.), the surge response consisted of small amplitude oscillations about a displaced position that is significantly dependent on tether tension force, wave height; whereas for longer wave periods, the surge response showed high amplitude oscillations that is significantly dependent on wave height, and that special attention should be given to tethers fatigue because of their high tensile static and dynamic stress.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2004.10a
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• pp.634-637
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• 2004

This paper presents a Split Hopkinson Pressure Bar(SHPB) technique to obtain compressive stress-strain data for rubber materials. An experimental technique that modifies the conventional Split Hopkinson Pressure Bar(SHPB) has been developed for measuring the compressive stress-strain responses of materials with low mechanical impedance and low compressive strengths such as rubber. This paper introduces an all-polymeric pressure bar which achieves a closer impedance match between the pressure bar and the specimen materials. In addition, we are a pulse shaper to lengthen the rising time of the incident wave to ensure stress equilibrium and homogeneous deformation of a rubber materials. It is found that the modified technique can be determine the dynamic deformation behavior of an NBR rubber more accurately.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.06a
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• pp.1185-1191
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• 2000

In general, the strength of hull structures can be estimated from stress evaluation considering static and hydro-dynamic load due to sea-wave. However, war ships such as submarine, have frequently experienced the underwater explosion and local structures of ship as well as hull girder can be damaged by the dynamic response excited from underwater non-contact explosion. When explosion happens at underwater, shock wave is radiated In early short time, then gas bubbles are generated, and expansion and contraction are repeated as they float to the surface. The shock wave causes the damage of equipment and its supporting structures, on the other hand, the hull girder strength can be lost by resonance between bubble pulsation and lowest ship natural vibration period. In this paper, the hydro-Impulse force due to bubble was calculated. Based on these results the hull girder strength of submarine was estimated from transient response analysis by using NASTRAN. Also, shock analysis for some equipment supporting structures was carried out by using DDAM. In order to evaluate the strength of these local structures due to shock wave.

• Korean Journal of Health Education and Promotion
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• v.24 no.1
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• pp.75-91
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• 2007

Objectives: It is known that the majority of middle schoolers spend the substantial amount of time using the Internet. Despite the fact, little is known about which factors are related to an increase in Internet use and what could be the consequences of excessive Internet use. The aim of this study was to explore factors affecting an increase in Internet use among elementary schoolers representing the population of South Korean elementary schoolers. Method: Secondary data obtained from the Korea Youth Panel Survey were analyzed longitudinally. Results: The average minutes spent using the Internet was 73.43 in the first wave, and was 92.17 in the second wave. Also, the percentage of those who experienced illegal behaviors on the Internet was 22.86% in the first wave, and was 30.92% in the second wave. Second, increased Internet use was strongly associated with an attack tendency, depression, suicidal ideation, stress, and delinquent behaviors. Finally, gender(being female), and increased attacking tendency, and smoking experiences successfully predicted an increase in Internet use one year later. Conclusions: It is necessary to provide education programs to young people and to develop preventive strategies for them. Through providing education programs, children can form desirable attitudes regarding the Internet use. Through providing intervention strategies, children can prevent from experiencing negative consequences due to excessive Internet use.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.7
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• pp.1759-1772
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• 1994

In this study, we re-examined the Tate's modified Bernoulli equation to study penetration phenomena for long rod projectile into single or multi-layered finite thickness plates. We used the force equlibrium equation at mushroomed nose/target interface instead of conventional pressure equation at the stagnation point. In our penetration model, we considered the velocity dependent $R_t$ value for semi-infinite target and considered only the back face effect for finite target. To compensate for $R_t$ value according to target's thickness and back face effect, we used the spherical cavity expansion theory for semi-infinite plate and used the cylindrical cavity expansion theory for finite plate. Also we developed the experimental technique using make screen to measure the penetration duration time at each layered plate. In 3-layered laminated RHA/mild steel/ A1 7039 plate, we observed that spall had occured around the back face of A1 7039 plate by the stress wave interaction. Through the comparison between theoretical and experimental data including Lambert's results, we conform that our study has good confidences.