• Tribology and Lubricants
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• 제39권6호
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• pp.228-234
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• 2023

In this study, reciprocating wear tests are performed on AISI 52100 bearing steel to investigate its tribological behavior in a hexagonal boron nitride (h-BN) water solution. The h-BN-based aqueous lubricant is prepared using an atoxic procedure called ultrasonic sonication in pure water. Ball-on-flat reciprocating sliding experiments are conducted, where the ball is slewed on a fixed flat at 50-㎛ displacement. The lubricating behavior of h-BN is compared with that of deionized (DI) water. Results show that the friction coefficient is higher in h-BN testing than that in DI tests, but the results are equalized as the friction coefficient reaches a stable level. Scanning electron microscopic images reveal significant material loss in the center and mild abrasion on the edge of the wear scar in h-BN tests. However, these effects are minor in DI water situations. The results of energy-dispersive X-ray spectroscopy show that considerable oxidation occurs in the central zone of the wear scar in h-BN cases with strong adhesion and material removal. These findings reveal the importance of determining ideal circumstances that can tolerate material friction and wear.

• 센서학회지
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• 제29권1호
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• pp.33-39
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• 2020

Typically, photoacoustic images are obtained in water or gelatin because the impedance of these mediums is similar to that of the human body. However, these acoustic mediums can have an additional mass effect that changes the resonance frequency of the transducer. The acoustic radiation impedance in air is negligible because it is very small compared to that of the transducer. However, the high acoustic impedance of mediums such as the human body and water is quite large compared to that of air, making it difficult to ignore. Specifically, in a case where the equivalent mass is very small, such as with a micro-machined ultrasound transducer, the additional mass effects of the acoustic medium should be considered for an accurate resonance frequency design. In this study, a piezoelectric micro-machined ultrasonic transducer (pMUT) was designed to have a resonance frequency of 10 MHz in the acoustic medium of water, which has similar impedance as the human body. At that time, the resonance frequency of the pMUT in air was calculated at 15.2 MHz. When measuring the center displacement of the manufactured pMUT using a laser vibrometer, the resonance frequencies were measured as 14.3-15.1 MHz, which is consistent with the finite element method (FEM) simulation results. Finally, photoacoustic images of human hair samples were successfully obtained using the fabricated pMUT.

• 한국음향학회지
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• 제24권5호
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• pp.282-293
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• 2005

음향 탄성영상법은 외부 진동을 조직에 인가하고 조직 운동을 측정함으로써 조직의 탄성을 영상화하는 초음파영상기법이다. 본 논문에서는 음향 탄성영상법에서 표면 진동자에 의해 연조직 내에 파동이 발생되는 특성과 모드패턴이암과 같은 병변 검출에 미치는 영향을 조사였다. 이를 위해 반공간, 두께가 일정한 무한평판, 그리고 유한 크기 조직에서 발생된 진동패턴을 이론과 유한요소법으로 계산하고 분석하였다. 유한 너비 진동원에 의해 조직에는 특정한 방향으로 강하게 전달되는 횡파가 발생하였으며, 그 특성은 진동자 너비, 주파수 및 진동자로부터의 거리에 의존하였다. 유한 크기 조직에서 병변의 검출가능성은 변위영상에서는 조직내 모드패턴에 큰 영향을 받았으며, 이에 비해 변형률영상에서는 모드패턴에 덜 민감하고 검출가능성도 아주 높은 것으로 나타났다.

• 한국산학기술학회논문지
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• 제20권12호
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• pp.94-101
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• 2019

본 연구에서는 초음파 용접기의 진동에너지를 모재에 전달하는 금속 용접용 혼의 형상설계를 위해 해석적 기법을 적용하였으며, 20kHz 영역에서 사용되는 한파장 바 형상을 갖는 용접용 혼의 형상설계를 위해 FEM(Finite Element Method)을 사용하였다. 봉의 종진동 이론을 Ansys APDL(Ansys Parametric Design Language)에 적용하여 혼의 형상을 최적화하였다. 적절한 혼의 형상을 도출하기 위해 진동의 입력면과 출력면의 면적비와 혼의 축방향 길이비를 이용하여 총 25가지의 모델을 설계하였으며, Modal 해석과 Harmonic 해석을 통해 균일한 진동특성을 확보하고자 하였다. 스텝 바혼의 끝단부(팁)의 균일도 90%이상을 확보하기 위해 Harmonic 해석을 통해 길이비, 면적비를 결정하였고 이에 따라 혼의 총길이 130mm, 스텝 길이 65mm, 출력단 면적 28.79mm의 혼을 설계하였다. 설계치수를 바탕으로 Titanium (Ti-6Al-4V)-GR5 재료를 이용하여 혼을 제작하였다. 또한, 별도로 구성된 초음파 용접시스템을 이용하여 혼의 끝단부의 진동 진폭률 및 변위특성을 평가하였다. 최적화된 혼의 진폭률은 51%까지 향상됨을 확인하였다. 최종적으로 균일한 종방향 스텝 바혼을 설계하였으며, 끝단부(팁)의 균일도 97.4%이상을 확보하였다.

• 한국소음진동공학회논문집
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• 제19권9호
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• pp.935-942
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• 2009

In this paper, the purpose is to investigate the natural frequency of a cracked Timoshenko cantilever beams by FEM(finite element method) and experiment. In addition, a method for detection of crack in a cantilever beams is presented based on natural frequency measurements. The governing differential equations of a Timoshenko beam are derived via Hamilton's principle. The two coupled governing differential equations are reduced to one fourth order ordinary differential equation in terms of the flexural displacement. The crack is assumed to be in the first mode of fracture and to be always opened during the vibrations. The detection method of a crack location in a beam based on the frequency measurements is extended here to Timoshenko beams, taking the effects of both the shear deformation and the rotational inertia into account. The differences between the actual and predicted crack positions and sizes are less than 6 % and 23 % respectively.

• Transactions on Electrical and Electronic Materials
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• 제11권2호
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• pp.65-68
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• 2010

The operation principle of a traveling wave rotary type ultrasonic motor can be successfully applied to the fluidic transfer mechanism of the micro-pump. This paper proposes an innovative valveless micro-pump type that uses an extensional vibration mode of a traveling wave as a volume transportation means. The proposed pump consists of coaxial cylindrical shells that join the piezoelectric ceramic ring and metal body, respectively. In order to confirm the actuation mechanism of the proposed pump model, a numerical simulation analysis was implemented. In accordance with the variations in the exciting wave mode and pump body dimension, we analyzed the vibration displacement characteristics of the proposed model, determined the optimal design condition, fabricated the prototype pump from the analysis results and evaluated its performance. The maximum flow rate was approximately $595\;{\mu}L/min$ and the highest back pressure was 0.88 kPa at an input voltage of $130\;V_{rms}$. We confirmed that the peristaltic motion of the piezoelectric actuator was effectively applied to the fluid transfer mechanism of the valveless type micro pump throughout this research.

• 전기학회논문지
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• 제62권3호
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• pp.395-400
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• 2013

In this paper, we introduce the implementation of high accurate level sensor system using the pulse wave type magnetostriction sensor. When a current pulse flows along the waveguide, the magnetic field also propagates towards the end of waveguide. When this magnetic field just passes the position of the magnet for level detection, the resultant magnetic field by these two magnetic fields makes a torsional reflected signal. This is used to calculate the time difference between a interrogation pulse wave and this torsional reflected signal. The key elements and characteristics were investigated to implement level sensor system based on this principle. We introduce a method to calculate the speed of ultrasonic reflected signal and how to make a model of sensing coil. In particular, we experiment with the characteristics of the torsional reflected signal according to the changes of the interrogation voltage and displacement. To make high accurate level sensor system, two methods were compared. One is to use the comparator and time counter, the other is STFT(Short Time FFT) which is capable of the time-frequency analysis.

• 비파괴검사학회지
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• 제25권4호
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• pp.274-286
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• 2005

본 연구에서는 편측노치를 갖는 단일 hi판 및 유리섬유/알루미늄 혼합 적층판에 대해 인장하중하의 파괴거동에 따른 음향방출(AE) 특성을 살펴보았다. 단일 알루미늄의 파괴시에 발생하는 AE신호는 저주파수 대역을 갖는 신호와 고주파수 대역을 갖는 신호의 2가지 형태로 분류될 수 있었다. 고주파수 대역의 AE는 0.45mm 변위 보다 큰 후반부 하중단계에서 주로 검출되었다. 유리섬유/알루미늄 혼합 적층판의 경우에는, 고진폭의 긴 유지시간의 신호가 FFT 주파수 해석에 의거하여 거시적인 균열진전이나 A1층과 섬유층 사이의 층간분리에 해당하는 신호로 추정되었다. 위와 같은 AE신호의 해석결과와 광학현미경 및 초음파 스캔에 의한 파괴관찰에 의거하여 섬유층 배향에 따라 상이한 섬유/알루미늄 적층판의 파괴거동과 관련된 AE특성을 규명하였다.

• 대한의용생체공학회:의공학회지
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• 제30권6호
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• pp.469-475
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• 2009

Diseases caused by indurate tissues of the internal organs are liver cirrhosis and abdominal sclerosis. The cause of chronic gastro-intestinal disease is a digestive system disorder and a defecation disorder. They impede peristaltic movement and digestive system with the symptom that indurate tissues. The purpose of the present study was to determine the disease grade quantitatively by measuring an indurated standard of tissues and organs. For the measurement of elasticity, we designed the system that measure the displacement of the substance and approved pressure using ultrasound transducer. For verification of developed system, we compared elasticity as results of experiment between the developed system and public elasticity measurement machine at individual plastic phantoms made by plastic hardener and softener. Elasticity of the plastic phantoms is averagely 0.007MPa lower measured by developed system than Micro-indenter, and less than 10% errors. Comparing with economical value and accuracy between developed system and Micro-indenter, the system is significant of measurement for tissue elasticity. Thus, it is possible to measure a elasticity at human tissue and organ. A chronic gastro-disease as well as grade could be decided objective validity using this system.

• Smart Structures and Systems
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• 제25권1호
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• pp.97-104
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• 2020

Due to the superior properties of nanoparticles, using them has been increased in concrete production technology. In this study, the effect of zinc oxide (ZnO) nanoparticles on the mechanical and smart properties of concrete was studied. At the first, the ZnO nanoparticles are dispersed in water using shaker, magnetic stirrer and ultrasonic devices. The nanoparticles with 3.5, 0.25, 0.75, and 1.0 volume percent are added to the concrete mixture and replaced by the appropriate amount of cement to compare with the control sample without any additives. In order to study the mechanical and smart properties of the concrete, the cubic samples for determining the compressive strength and cylindrical samples for determining tensile strength with different amounts of ZnO nanoparticles are produced and tested. The most important finding of this paper is about the smartness of the concrete due to the piezoelectric properties of the ZnO nanoparticles. In other words, the concrete in this study can produce the voltage when subjected to mechanical load and vice versa it can induce the mechanical displacement when subjected to external voltage. The experimental results show that the best volume percent for ZnO nanoparticles in 28-day samples is 0.5%. In other words, adding 0.5% ZnO nanoparticles to the concrete instead of cement leads to increases of 18.70% and 3.77% in the compressive and tensile strengths, respectively. In addition, it shows the best direct and reverse piezoelectric properties. It is also worth to mention that adding 3.5% zinc oxide nanoparticles, the setting of cement is stopped in the concrete mixture.