• Smart Structures and Systems
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• v.4 no.3
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• pp.319-342
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• 2008

Identification of the nonlinear hysteretic behavior of a reinforced concrete (RC) bridge pier subjected to earthquake loads is carried out based on acceleration measurements of the earthquake motion and bridge responses. The modified Takeda model is used to describe the hysteretic behavior of the RC pier with a small number of parameters, in which the nonlinear behavior is described in logical forms rather than analytical expressions. Hence, the modified extended Kalman filter is employed to construct the state transition matrix using a finite difference scheme. The sequential modified extended Kalman filter algorithm is proposed to identify the unknown parameters and the state vector separately in two steps, so that the size of the problem for each identification procedure may be reduced and possible numerical problems may be avoided. Mode superposition with a modal sorting technique is also proposed to reduce the size of the identification problem for the nonlinear dynamic system with multi-degrees of freedom. Example analysis is carried out for a continuous bridge with a RC pier subjected to earthquake loads in the longitudinal and transverse directions.

• Journal of the Society of Naval Architects of Korea
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• v.36 no.3
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• pp.15-21
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• 1999

The flow characteristics around KRISO 300K VLCC double-body model have been experimentally investigated in a closed-type subsonic wind tunnel. The local mean velocity and turbulence statistics including turbulent intensity. Reynolds shear stress and turbulent kinetic energy were measured using a x-type hot-wire probe. The measurements were carried out at several transverse stations of the stern and near wake regions. The surface flow was visualized using on oil-film technique to see the flow pattern qualitatively. The flow in the stern and near wake region revealed complicated three-dimensional flow characteristics. The VLCC model shows a hook-shaped wake structure behind the propeller boss in the main longitudinal vortex region. The thin boundary layer at midship was increased gradually in thickness over the stern and evolved into a full three-dimensional turbulent wake.

• Steel and Composite Structures
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• v.28 no.6
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• pp.759-766
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• 2018

Friction Stir Welding (FSW) is a solid-state process, where the objects are joined together without reaching their melting point. It has been shown that this method is a suitable way to join dissimilar aluminium alloys. The current article employed hole drilling technique to measure the residual stress distribution experimentally in different zones of dissimilar aluminium alloys AA6061-T6 and AA7075-T6 Butt welded using FSW. Results are compared with those of similar AA6061-T6 plates joined using a conventional fusion welding method called tungsten inert gas (TIG). Also, the evolution of the residual stresses in the thickness direction was investigated, and it was found that the maximum residual stresses are below the yield strength of the material in the shoulder region. It was also revealed that the longitudinal residual stresses in the joint were much larger than the transverse residual stresses. Meanwhile, Vickers micro hardness measurements were performed in the cross-section of the samples. The largest hardness values were observed in the stir zone (SZ) adjacent to the advancing side whereas low hardness values were measured at the HAZ of both alloys and the SZ adjacent to the retreating side.

• Korean Journal of Optics and Photonics
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• v.7 no.1
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• pp.37-43
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• 1996

Two type interference air refractometers have been developed by using a frequency stabilized He-Ne laser in a transverse magnetic field. The refractometers were based on symmetric multiple pass interferometer. In this system, One part of the beams passed through air whose refractivity is to be measured and the other part of the beams passed through a vacuum champer to be used as a reference. Several measurements were performed under normal air condition. Maximum difference between two interference refractometers was ${\pm}2{\times}10^{-8}$.

• Structural Engineering and Mechanics
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• v.67 no.3
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• pp.233-244
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• 2018

This study aims at evaluating the performance of repairing technique with CFRPs in recovering cyclic performance of damaged columns in flexure in terms of structural response parameters such as strength, dissipated energy, stiffness degradation. A 2/3 scaled substandard reinforced concrete frame was constructed to represent the substandard RC buildings especially in developing countries. These substandard buildings have several structural deficiencies such as strong beam-weak column phenomenon, improper reinforcement detailing and poor material properties. Flexural plastic hinges occurred at the columns ends after testing the substandard specimen under both constant axial load and reversed cyclic lateral loading. Afterwards, the damaged columns were externally wrapped with CFRP sheets both in transverse and longitudinal directions and then retested under the same loading protocol. In addition, ambient vibration measurements were taken from the undamaged, damaged and the repaired specimens at each structural repair steps to identify the effectiveness of each repairing step by monitoring the change in the natural frequencies of the tested specimen. The ambient vibration test results showed that the applied repairing technique with external CFRP wrapping was proved to recover stiffness of the pre-damaged specimen. Moreover, the lateral load capacity of the pre-damaged substandard RC frame was restored with externally bonded CFRP sheets.

• Computers and Concrete
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• v.21 no.5
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• pp.539-546
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• 2018

Making a transverse opening in concrete beams in order to accommodate utility services through the member instead of below or above of that, sometimes may be necessary. It is obvious that inclusions of an opening in a beam decreases its flexural and shear strengths. Fabricated steel bars are usually used to increase the capacity of the opening section, but details of reinforcements around the opening are dense and complex resulting in laborious pouring and setup process. The goal of this study was to investigate the possibility of using steel fibers in concrete mixture instead of complex reinforcement detailing order to strengthen opening section. Nonlinear finite element method was employed to investigate the behavior of steel fiber reinforced concrete beams. The numerical models were validated by comparison with experimental measurements tested by other investigators and then used to study the influence of fiber length, fiber aspect ratio and fiber content on the shear performance of SFRC slender beams with opening. Finally, it was concluded that the predicted shear strength enhancement is considerably influenced by use of steel fibers in concrete mixture but the effect of fiber length and fiber aspect ratio wasn't significant.

• Journal of information and communication convergence engineering
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• v.14 no.4
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• pp.252-257
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• 2016

An edge-located electromagnetic bandgap (EL-EBG) structure using a defected ground structure (DGS) is proposed to suppress resonant modes induced by edge excitation in a two-dimensional planar parallel plate waveguide (PPW). The proposed EL-DGS-EBG PPW significantly mitigates multiple transverse-magnetic (TM) modes in a wideband frequency range corresponding to an EBG stopband. To verify the wideband suppression, test vehicles of a conventional PPW, a PPW with a mushroom-type EBG structure, and an EL-DGS-EBG PPW are fabricated using a commercial process involving printed circuit boards (PCBs). Measurements of the input impedances show that multiple resonant modes of the previous PPWs are significantly excited through an input port located at a PPW edge. In contrast, resonant modes in the EL-DGS-EBG PPW are substantially suppressed over the frequency range of 0.5 GHz to 2 GHz. In addition, we have experimentally demonstrated that the EL-DGS-EBG PPW reduces the radiated emission from -24 dB to -44 dB as compared to the conventional PPW.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.5
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• pp.625-634
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• 1986

The strain distribution on a welded aluminum alloy transition joint produced by a static tensile load has been measured using a moire method combined with photoelastic coating method. The test specimens were made of aluminum alloy 6061-T6 and 2014-T6 butt welded with ER-4043 filler metal, and were post welded heat treated (solid solution heat treatment 502.deg. C 70min.) and precipitated (artificial aging 171.deg. C 600min.) to cause an abrupt change of mechanical properties between the base metals and weld metal. The photoelastic epoxy rubber was cemented on the specimen grating which had been reproduced on the specimen surface by using an electropolishing. The measurements were compared with strains computed by Finite Element Analysis. The following results were abtained. (1) The maximum strain were distributed along the center line in the transverse directiion of the weld metal. (2) The strain gradient along the fusion line increased approaching the V-groove tip and the maximum value was observed at a quarter of width from the V-groove tip. (3) The moire method combined with photoelastic coating was proved very useful for real time strain measurement in the welded aluminum alloy transition joint.

• Tribology and Lubricants
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• v.34 no.1
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• pp.9-15
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• 2018

Modern high-performance automotive turbochargers (TCs) implement ceramic hybrid angular contact ball bearings in series with squeeze film dampers (SFDs) to enhance transient responses, thereby reducing the overall emission levels. The current study predicts the rotordynamic responses of the commercial automotive TCs (compressor wheel diameter = ~53 mm, turbine wheel diameter = ~43 mm, and shaft diameter at the bearing locations = ~7 mm) supported on ball bearings and SFDs for various design parameters of SFDs, including radial clearance, axial length, lubricant viscosity, and rotor imbalance conditions (i.e., amplitudes and phase angles) while increasing rotor speed up to 150 krpm. This study validates the predictive rotor finite element model against measurements of mass, polar and transverse moments of inertia, and free-free mode natural frequencies and mode shapes. A nonlinear rotordynamic model integrates nonlinear force coefficients of SFDs to calculate the transient responses of the TC rotor-bearing system. The predicted results show that SFD radial clearances, as well as phase angles of rotor imbalances, have the paramount effect on the dynamic responses of TC shaft motions.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2006.05a
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• pp.421-427
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• 2006

This paper presents a novel method to determine sound power level(PWL) emitted by a travelling vehicle for road traffic noise simulation. The PWL is evaluated by the equivalent sound pressure level(SPL) measured by close proximity method and the sound power correction factor derived from the maximum SPL measured by pass-by method and the propagation attenuation of vehicle noise during the pass-by measurement. Using the method, we derive the empirical formula for PWL estimation in 1/1-octave and overall frequency bands for 8 vehicles(automobile, SUV, small truck, large bus, trailer, 3 dump trucks) tested at two road surfaces(dense graded asphalt, 30mm transverse tinning concrete) of Korean highway test road. The suggested approach, if securing sufficient data to represent the acoustic characteristics of au vehicle types, has a strong merit to be able to evaluate sound power levels for any combination of vehicle categories and traffic volumes.