• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2004.11a
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• pp.633-636
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• 2004

Acoustic response control of a corner-pinned plate using piezoelectric wafers was studied, both theoretically and experimentally. Three different sizes of aluminum alloy plates were used and available ball joints were employed to hold the plate at the four comers. The plate with the largest aspect ratio showed the largest and most clear responses to the acoustic excitation in the range of frequencies $(0\sim200Hz)$, and sound pressure levels $(80\sim100dB)$ as predicted. The reduction of the acoustic response of the plate by piezoelectric actuator was very significant, more than expected, but abatement of the sound transmission through the plate was only slightly altered by the piezoelectric actuator. This work is an original work extending earlier work with doors excited by acoustic fields. The important difference is the used of ball joints to simulate the joints.

• Proceedings of the Korean Society of Medical Physics Conference
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• 2002.09a
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• pp.192-194
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• 2002

We present the results on the calibration of iso-center positions using the quality assurance system established at PMRC for determination of center position in X-ray and proton irradiation fields. Details on the system are presented in another presentation in this session. The equipment in the system is mounted on a patient treatment bed in each proton exposure room, G1 or G2. A center of a stainless ball on the equipment is set at a cross of laser markers located around the iso-center and fixed on the room and on the snout in the gantry. A proton beam or an X-ray beam is exposed onto the ball through a brass collimator of 100 mm ${\times}$ 100 mm and projected onto the imaging plate set at I cm behind the ball. On the axis perpendicular to the thrust axis of the gantry on the imaging plate, a distance between a center of the collimator image and a center of the ball image varies as a cosine function of gantry angles unless the ball is set on the iso-center. An amplitude of the cosine curve shows the distance between the ball and the iso-center, an offset the offset of the collimator, and a phase shift at a zero crossing point the ball direction viewed from the iso-center. We present the relation among the iso-center position, the laser maker position, and the center of proton and X-ray irradiation fields. Its stability and its reproducibility are discussed.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2002.05a
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• pp.865-868
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• 2002

This study investigated the shape deformation of ball projectile(5.56mn) under the low energy impact by the use of the drop weight impact tester. ball projectile(5.56mm) consisted of the copper face with a lead core. The impact conditions were changed with the variations of the mass and the drop height of the impact tup. Shape deformation of ball projectile(5.56mm) after low velocity impact was measured using a video microscope and CCD camera. The test result showed that impact energy by changing of drop height of the impact tup affected shape deformation of ball projectile(5.56mm). So, it is important to study the relativity between shape deformation of ball projectile(5.56mm) and ballistic protection of plate(such as hybrid composite laminates) under the high velocity impact.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.24 no.8
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• pp.651-658
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• 2014

Notification 2013-611 of MOLIT has come into effect. It relates primarily to new standard impact source. In this study, an in-depth experimental analysis of the difference between a bang machine and an impact ball was performed via field testing of shear wall and flat plate structure at 51 sites. This paper focuses on the difference in single number quantities between a bang machine and an impact ball. At wall thicknesses of 180 and 210 mm in shear wall structure, the single number quantities exhibited differences of 3.1 and 4.5 dB, respectively, and at thicknesses exceeding 250 mm in flat plate structure, the difference was constant at 4.6 dB. With regard to flat plate structures, the single-index difference increased up to 11 dB as the thickness of the floor slab increased. In general, the highest level of contribution for the bang machine was 63 Hz, irrespective of thickness determining bandwidth. The highest level for the impact ball were 63 Hz and 125 Hz. In future research, when reviewing additional field performance measurement data, it will be necessary to consider a detailed examination instead of the current method of uniformly adding 3 dB for all thicknesses and types of structures.

• Carbon letters
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• v.8 no.4
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• pp.326-334
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• 2007

In this study, graphite composites were fabricated by warm press molding method to realize commercialization of PEM fuel cells. Graphite composites have been considered as alternative economic materials for bipolar plate of PEM fuel cells. Graphite powder that enables to provide electrical conductivity was selected as the main substance. The graphite powder was mixed with phenolic resin and the mixture was pressed using a warm press method. First of all, the graphite powder was pulverized with a ball mill for the dense packing of composite. As the ball milling time increases, the average size of particles decreases and the size distribution becomes narrow. This allows for improvement of the uniformity of graphite composite. However, the surface electrical resistivity of graphite composite increases as the ball milling time increases. It is due to that graphite particles with amorphous phase are generated on the surface due to the friction and collision of particles during pulverizing. We found that the contact electrical resistivity of graphite particles increases as the particle size decreases. The contact electrical resistivity of graphite powders was reduced due to high molding pressure by warm press molding. This leads to improvement of the mechanical properties of graphite composite. Hydrogen gas impermeability was measured with the graphite composite, showing a possibility of the application for bipolar plate in fuel cell. And, I-V curves of the graphite composite bipolar plate exhibit a similar performance to the graphite bipolar plate.

• Proceedings of the Korean Society of Medical Physics Conference
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• 2002.09a
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• pp.189-191
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• 2002

In the proton therapy using a gantry system, periodical verification of iso-center position is very important to assure precision of patient positioning system at any gantry angles in proton treatment. In the gantry system, there are three different types of iso-center; 1) in a geometrical view, 2) in an X-ray beam's eye view, 3) in a proton beam's eye view. Idealistically, they would be an identical point. They could, however, be different points. It may be a source of errors in patient positioning. At PMRC, we have established a system of verification for iso-center positions using a stainless ball of 2-cm in diameter and an imaging plate. This system provides the relation among a center of a patient target position, a center of proton irradiation field, and/or a center of X-ray field in accuracy of 50$\square$m in the 2) and 3) views, as images of a center of the stainless ball and a center of a 100 mm${\times}$100 mm-aperture brass collimator recorded on the imaging plate, which is setup at 1-cm behind the ball. In addition, it provides simultaneously the images of the ball and the collimator on an imaging intensifier (II), which is setup downstream of the proton or X-ray beam. We present a method of quality assurance (QA) for calibration of iso-center position in a rotation gantry system at PMRC and the performance of this system. A proton beam position on the 1$\^$st/ scatterer in the nozzle of the gantry affects less sensitive (reduced by a factor of 1/5) to the results of the iso-center position. The effect is systematically correctable. The effect of the nozzle (or the collimator) position is less than 0.5 mm at the maximum extraction (390 mm).

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1997.04a
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• pp.160-166
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• 1997

The spindle motor in a computer hard disk drive can be modeled as a rotor-bearing system supported by the base plate. Ball bearing is the crucial element to determine the stiffness of the spindle motor, and its design parameters and operating conditions determine the dynamic characteristics of the spindle motor. In the analysis of a rotor-bearing system with a short shaft like a spindle motor, the stiffness of the base plate as well as ball bearings must be considered accurately to analyze the dynamic charateristics of a spindle motor. In this paper, the lateral and the axial vibration of the spindle motor were analyzed by the transfer matrix method for the dual-shaft rotor-bearing model and by d.o.f lumped parameter model, respectively. The simulation results had good agreements with the experimental modal testing. The dynamic characteristics were fully investigated for the change of the major design parameters of the spindle motor, i.e. the preload of ball bearings and the rotational speed.

• Journal of the Korea Institute of Military Science and Technology
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• v.8 no.3 s.22
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• pp.101-110
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• 2005

In this paper, the experiments have been conducted to measure the residual velocity for 3.5g steel ball perforating light weight metal plates of aluminum alloy and magnesium alloy. Non-contact electro-magnetic sensors were used to measure the velocity of steel ball before/after perforating plates. The thicknesses of specimens used were about 2.8mm and 4.8mm. The impact velocities of steel ball were from 662m/s to 3594m/s. With same conditions, numerical analysis using Autodyn 2D has been conducted. The results of numerical analysis corresponded with those of experiments. Also, It is suggested that the difference between the residual velocity of experiment or numerical analysis and that of THOR experimental equation of BRL grew smaller as the impact velocity were increased.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2002.10b
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• pp.211-212
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• 2002

Friction and wear of pressureless sintered Ti(C,N)-WC ceramics were studied using a ball-on-reciprocating flat apparatus in open air. The silicon nitride ball and the cemented carbide (WC-Co) ball were used against the Ti(C,N)-WC plate samples. The friction coefficients of the Ti(C,N)-WC samples against the silicon nitride ball and the cemented carbide ball were about 0.57 and 0.3, respectively. The wear coefficient of the sample without WC addition was 5 times as large as that of the sample with 10 mole % WC addition when tested against the silicon nitride ball under 98 N. The higher wear coefficient of Ti(C,N)-0WC was explained in part by larger grain size. Wear occurred mainly by grain dislodgment after intergranular cracking mainly caused by the accumulated stress within the grains.

• Journal of Korea Foundry Society
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• v.39 no.5
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• pp.87-93
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• 2019

The effects of mold materials on the microstructure and tensile properties were investigated to develop a mass production technique of aluminum alloy parts with excellent mechanical properties using a lost foam casting method. The microstructures of the plate-shaped cast alloy showed a tendency to be finer in proportion to the thickness of the plate, and a remarkably fine structure was obtained by applying a steel chill or a ball as a mold material compared to general sand. When a steel ball was used, it was observed that the larger the ball, the finer the cast structure and the better the tensile properties. The microstructure and tensile properties of the cast parts with complex shapes were greatly affected by the gating system, but the positive effects of the steel chill and the steel ball as a mold material were clear.