• The Journal of the Acoustical Society of Korea
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• v.39 no.2
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• pp.87-97
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• 2020

In this paper, the design of piezoelectric Micro-machined Ultrasonic Transducer (pMUT) for wideband ultrasonic radiation in air was investigated. One of the methods to achieve wide frequency bandwidth in single device is modeling the transducer to multi-resonance system. The new pMUT was designed as a multi-resonance system with the addition of a suitable acoustic structure to the front and back of a thin film structure. A new pMUT consisting of thin film parts, radiation parts, and packaging parts is designed with a Lumped Parameter Model (L.P.M). Finally, it was validated as a Finite Element Method (FEM) simulation. The final designed pMUT achieved a frequency band of 102 kHz ~ 132 kHz (-3 dB).

• Journal of Advanced Marine Engineering and Technology
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• v.40 no.4
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• pp.314-319
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• 2016

Flange bolt has a circular flange under the head that acts like a washer to distribute the clamping load over a large area. Flange bolt has usually been manufactured by cold forging. Flower shape defect occurs in the flange forging stage. This defect causes lack of dimensional accuracy and low quality. So it is needed to improve these forging defects. In this study, die design method for flower shape defect of flange bolt was suggested. In order to improve flower shape defect, inner diameter of the addition die in conventional forging process was modified. The forging process with applied modified die was simulated by commercial FEM code DEFORM-3D. The simulated results for modified die were confirmed by experimental trials with the same condition.

• The Journal of the Acoustical Society of Korea
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• v.20 no.6
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• pp.57-65
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• 2001

In this paper, we designed the acoustic horn for magnetostrictive ultrasonic transducers in a theoretical manner, and validity of the analysis was verified through comparison with the results of finite element analysis. Results of the two analysis methods showed good agreement with each other. The theoretical method can fairly quickly determine the horn length that satisfies given frequency specification, but also has the drawback that it is applicable only to the frequency range over the cut-off frequency. According to the results, the catenoidal horn can provide larger amplification than the exponential horn. It was also found that it is more desirable for the region having the catenoidal curvature to be as short as possible to achieve larger amplification of the transducer deformation. Based on the analysis results, a magneto-strictive transducer sample was fabricated and its performance was evaluated experimentally. The transducer has the resonance frequency of 19.3 ㎑ as well as the maximum SPL of 199 dB, and shows the omni-directional radiation pattern.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.41 no.11
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• pp.1125-1130
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• 2017

The tooth shapes of serration-type and spline-type reactors are optimized using finite element methods to improve the working life of the part and to lower the stress concentration during rotation resulting from contact with the outer race for a reactor operating with $170^{\circ}C$ transmission oil. The results of thermal expansion analyses between an Al reactor and the steel outer race indicate that, before optimization, the gap between the two parts increases further as the serration-type reactor expands by 0.1 mm and the spline-type one strains by 0.08 mm. Because of shape optimization, a trapezoidal shape is obtained from the initial triangular serration and the rectangular spline of the two reactors. The maximum von Mises stress of the serration-type convertor decreased by 24.5 ％, and by 9.3 ％ for the spline-type convertor. In addition, there is a 13 ％ reduction in the axial thickness, as compared to the initially designed model.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.40 no.2
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• pp.191-195
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• 2020

In this work, tensile tests, one of the most common test method to assess the condition of a corroded pipe, were conducted. According to ASTM E8 method, the use of flat or curved uni-axial tension test is allowed under the recommendation with the usage of grips corresponding to a curvature of the pipe. However, this method is not for corroded specimen. Furthermore, in the case of performing the multiple tensile tests with various curvatures, it is desirable not to produce zigs that fit each curvatures, if merely processing the specimen grip with curvature into the flat grip can show almost identical tensile behavior. Therefore, various tension simulations were conducted first to check if there exist any differences. Also, experiments on corroded tensile specimen were conducted and compared with the FEM simulation that reflects the actual geometry acquired from the 3D scanner.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.43 no.10
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• pp.920-927
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• 2015

In this paper, numerical results of sectional analysis and stress recovery were compared with the results of VABS through the blade analysis library. The results of recovery analysis for one-dimensional model including the stiffness matrix is compared with the calculated three-dimensional stress results of three-dimensionial FEM based on the principle of virtual work. We discuss the configuration of the blade analysis library and compare verifications of numerical analysis results of VABS. Blade analysis library through dimensional reduction and stress recovery is intended to be utilized in conjunction with pre- and post-processing of the analysis program of the composite blade, high-altitude uav's wing, wind blades and tilt rotor blade.

• Journal of the Korea Society of Computer and Information
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• v.17 no.9
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• pp.9-16
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• 2012

In this paper, we propose the two dimensional variable-length vector storage format which can be used for efficient storage of sparse matrix in the FEM (finite element method). The proposed storage format is the method storing only actual needed non-zero values of each row on upper triangular matrix with the total rows N, by using two dimensional variable-length vector instead of $N{\times}N$ large sparse matrix of entire equation of finite elements. This method only needs storage spaces of the number of minimum 1 to maximum 5 in 2D grid structure and the number of minimum 1 to maximum 14 in 3D grid structure of analysis target. The number doesn't excess two times although involving index number. From the experimental result, we can find out that the proposed storage format can reduce the memory space more effectively, as the total number of nodes increases, than the existing skyline storage format storing maximum column height.

• Steel and Composite Structures
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• v.17 no.6
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• pp.891-906
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• 2014

This study aims to present a three dimensional finite element model to investigate the wave propagation in a concrete filled steel tubular column (CFSC) due to transient impact load. Both the concrete and steel are regarded as linear elastic material. The impact load is simulated by a semi sinusoidal impulse. Besides the CFSC models, a concrete column (CC) model is established for comparing under the same loading condition. The propagation characteristics of the transient waves in CFSC are analyzed in detail. The results show that at the intial stage of the wave propagation, the velocity waves in CFSC are almost the same as those in CC before they arrive at the steel tube. When the waves reach the column side, the velocity responses of CFSC are different from those of CC and the difference is more and more obvious as the waves travel down along the column shaft. The travel distance of the wave front in CFSC is farther than that in CC at the same time. For different wave speeds in steel and concrete material, the wave front in CFSC presents an arch shape, the apex of which locates at the center of the column. Differently, the wave front in CC presents a plane surface. Three dimensional effects on top of CFSC are obvious, therefore, the peak value and arrival time of incident wave crests have great difference at different locations in the radial direction. High-frequency waves on the waveforms are observed. The time difference between incident and reflected wave peaks decreases significantly with r/R when r/R < 0.6, however, it almost keeps constant when $r/R{\geq}0.6$. The time duration between incident and reflected waves calculated by 3D FEM is approximately equal to that calculated by 1D wave theory when r/R is about 2/3.

• Progress in Superconductivity and Cryogenics
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• v.11 no.1
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• pp.49-54
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• 2009

This paper presents the numerical simulation results on the moving type electrodynamic suspension (EDS) simulator and static type EDS simulator using high-Tc superconducting (HTS) levitation magnet. The levitation force of the EDS system is formed by the reaction between the moving magnet and the fixed ground conductor. The possible two ways to simulate the EDS system were simulated in this paper by using finite element method (FEM). The first way was the moving type simulator which consists of the fixed HTS magnet and the moving ground conductor. The second way was the static type simulator which consists of the fixed magnet, the fixed ground conductor and the ac current supply system. To verify the characteristics of high speed EDS system with the moving type simulator heavy, large and fast moving ground conductor is needed. The static type simulator can get the characteristics of the high speed EDS system by applying equivalent ac current to velocity, therefore it does not need large moving part. The static type EDS simulator, which can consist of an HTS magnet, the fixed ground conductor(s), an AC power supply and the measuring devices, also test the effect of the shape of the ground conductor easily. The plate type ground conductor made stronger levitation force than ring type ground conductor. Although the outer diameter 335 mm ring type ground conductor (Ring3) was larger than the outer diameter 235 mm ground conductor (Ring2), the levitation force by Ring2 was stronger than that by Ring3. From the calculation results on this paper, the consideration of the magnetic flux distribution according to the levitation height should be included in the process of the ground conductor design.

• Composites Research
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• v.22 no.2
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• pp.30-36
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• 2009

In this paper, we propose the design method for the composite torque link of a landing gear for a helicopter. The composite torque link has to be light weighted and very stiff to keep the shock absorber in the landing gear of helicopter. The configuration and structural shape has to be designed in consideration of the RTM (Resin Transfer Molding) manufacturing process which is adopted to minimize the manufacturing cost. The mechanical properties are obtained through the coupon tests with the specimens made by the same manufacturing process for the composite structure. The optimal design process was performed through iterative modifications of the models which were verified by stress analysis using FEM. The composite torque link has lug-shaped parts and is very thick, so 3D Layered solid elements of ABAQUS were used to get the stress field including the stress components in thickness direction and non-linear static analysis using contact B.C. of rigid-deform condition was used to get the optimal design.