Jeongwon Park

Stretchable Loudspeaker using Liquid Metal Microchannel

Considering the various applications of wearable and bio-implantable devices, it is desirable to realize stretchable acoustic devices for body-attached applications such as sensing biological signals, hearing aids, and notification of information via sound. In this study, we demonstrate the facile fabrication of a Stretchable Acoustic Device (SAD) using liquid metal coil of Galinstan where the SAD is operated by the electromagnetic interaction between the liquid metal coil and a Neodymium (Nd) magnet. To fabricate a liquid metal coil, Galinstan was injected into a micro-patterned elastomer channel. This fabricated SAD was operated simultaneously as a loudspeaker and a microphone. Measurements of the frequency response confirmed that the SAD was mechanically stable under both 50% uniaxial and 30% biaxial strains. Furthermore, 2000 repetitive applications of a 50% uniaxial strain did not induce any noticeable degradation of the sound pressure. Both voice and the beeping sound of an alarm clock were successfully recorded and played back through our SAD while it was attached to the wrist under repeated deformation. These results demonstrate the high potential of the fabricated SAD using Galinstan voice coil in various research fields including stretchable, wearable, and bio-implantable acoustic devices.

Surface characteristics of a porous-surfaced Ti-6Al-4V implant fabricated by electro-discharge-compaction

Electro-discharge-compaction (EDC) is a unique method for producing porous-surfaced metallic implants. The objective of the present studies was to examine the surface characteristics of the Ti-6Al-4V implants formed by EDC. Porous-surfaced Ti-6Al-4V implants were produced by employing EDC using 480 μF capacitance and 1.5 kJ input energy. X-ray photoelectron spectroscopy was used to study the surface characteristics of the implant materials. C, O, and Ti were the main constituents, with smaller amounts of Al and V. EDC Ti-6Al-4V also contained N. Titanium was present mainly in the forms of mixed oxides and small amounts of nitride and carbide were observed. Al was present in the form of aluminum oxide, while V in the implant surface did not contribute to the formation of the surface oxide film. The surface of conventionally prepared Ti-6Al-4V primarily consists of TiO2, whereas, the surface of the EDC-fabricated Ti-6Al-4V consists of complex Ti and Al oxides as well as small amounts of titanium carbide and nitride components. However, preliminary studies indicated that the implant was biocompatible and supports rapid osseointegration.

Determination of effective mass density and modulus for resonant metamaterials.

This work presents a method to determine the effective dynamic properties of resonant metamaterials. The longitudinal vibration of a rod with periodically attached oscillators was predicted using wave propagation analysis. The effective mass density and modulus were determined from the transfer function of vibration responses. Predictions of these effective properties compared favorably with laboratory measurements. While the effective mass density showed significant frequency dependent variation near the natural frequency of the oscillators, the elastic modulus was largely unchanged for the setup considered in this study. The effective mass density became complex-numbered when the spring element of the oscillator was viscoelastic. As the real part of the effective mass density became negative, the propagating wavenumber components disappeared, and vibration transmission through the metamaterial was prohibited. The proposed method provides a consistent approach for evaluating the effective parameters of resonant metamaterials using a small number of vibration measurements.

Measurement of viscoelastic properties from the vibration of a compliantly supported beam

A laboratory method is presented by which the viscoelastic properties of compliant materials are measured over a wide frequency range. The test setup utilizes a flexible beam clamped at one end and excited by a shaker at the free end. A small specimen of a compliant material is positioned to support the beam near its midpoint. The deformation from gravity is minimized since the specimen is not loaded by an attached mass. Forced vibration responses measured at two locations along the beam are used to derive a transfer function from which the dynamic properties of compliant materials are directly determined by use of a theoretical procedure investigating the effects of specimen stiffness on the propagation of flexural waves. Sensitivity of the measured properties to experimental uncertainties is investigated. Youngs moduli and associated loss factors are determined for compliant materials ranging from low-density closed-cell foams to high-density damping materials. The method is validated by comparing the measured viscoelastic properties to those from an alternative dynamic test method.

Thermal stability enhancement of Cu/WN/SiOF/Si multilayers by post-plasma treatment of fluorine-doped silicon dioxide

The effect of a post-plasma treatment on the dielectric property and reliability of fluorine doped silicon oxide (SiOF) film was studied. Also, the thermal stability of the Cu/WN interconnect system with SiOF interlayer dielectrics was examined by rapid thermal annealing. The surface roughness of SiOF films increased with increasing plasma treatment power due to ion bombardment effect during the plasma treatment. As the plasma treatment power increased, the dielectric constant increased from 3.16 to 3.43, while the change in the relative dielectric constant of the plasma treated films decreased in magnitude after treatment at 100 °C for 30 min in boiling water. Furthermore, the chemical properties of the plasma treated SiOF layers near the top surface tend to resemble those of thermal oxides after plasma treatment with sufficient plasma power, apparently due to the reduction in the Si–F bonding in the films. In the case of a Cu/WN/SiOF/Si multilayer structure, surface oxidation and densification due to th...

Identification of Structural Defects in Rail Fastening Systems Using Flexural Wave Propagation

An experimental method based on flexural wave propagation is proposed for identification of structural damage in rail fastening systems. The vibration of a rail clamped and supported by viscoelastic pads is significantly influenced by dynamic support properties. Formation of a defect in the rail fastening system induces changes in the flexural wave propagation characteristics owning to the discontinuity in the structural properties. In this study, frequency-dependent support stiffness was measured to monitor this change by a transfer function method. The sensitivity of wave propagation on the defect was measured from the potential energy stored in a continuously supported rail. Further, the damage index was defined as a correlation coefficient between the change in the support stiffness and the sensitivity. The defect location was identified from the calculated damage index.

Sensing of fluid viscoelasticity from piezoelectric actuation of cantilever flexural vibration

An experimental method is proposed to measure the rheological properties of fluids. The effects of fluids on the vibration actuated by piezoelectric patches were analyzed and used in measuring viscoelastic properties. Fluid-structure interactions induced changes in the beam vibration properties and frequency-dependent variations of the complex wavenumber of the beam structure were used in monitoring these changes. To account for the effects of fluid-structure interaction, fluids were modelled as a simple viscoelastic support at one end of the beam. The measured properties were the fluids dynamic shear modulus and loss tangent. Using the proposed method, the rheological properties of various non-Newtonian fluids were measured. The frequency range for which reliable viscoelasticity results could be obtained was 10-400 Hz. Viscosity standard fluids were tested to verify the accuracy of the proposed method, and the results agreed well with the manufacturers reported values. The simple proposed laboratory setup for measurements was flexible so that the frequency ranges of data acquisition were adjustable by changing the beams mechanical properties.

Multiscale Simulations for Impact Load–Induced Vibration: Assessing a Structure's Vulnerability

Vibration resulting from high-velocity projectiles impacting a structure was simulated at multiple scales. Local impact simulations were performed to predict the material deformation and penetration phenomena at the location of impact. The resulting penetration behavior of a steel panel was analyzed for various projectile velocities, sizes, and panel thicknesses. Three-layer panels with Kevlar as the core material were simulated to understand the effects of structural layering on the reduction of the impact force. The forces acting on the panel in the longitudinal and transverse directions were calculated from the obtained stress distribution in the local deformation model. Using the estimated force input, transient longitudinal and flexural wave propagations were calculated to analyze the radiation of the impact energy along the structural span. Vulnerable positions with high possibilities of damage to crucial components due to impact loading were identified from the resulting vibration responses.

Effect of the static compressive load on vibration propagation in multistory buildings and resulting heavyweight floor impact sounds

Experiments were performed to identify the mechanism of heavyweight floor impact sound transmission through floors in a high-rise apartment building. Vibration and sound levels on each floor of the multistory building were measured. The vibration generated at a given floor was transferred to multiple adjacent floors with decreasing amplitudes proportional to the distance from the excited floor. This vibration transfer introduced significant sound transmissions. The structural static load varied depending on the floor location due to differences in the weight of the structure above the floor, especially for wall construction buildings. The static load at the wall of the bottom floor was the largest among the different floors. The influence of this static load on the impact sound generation was investigated through tests in the actual building and the scale model, respectively. The results were numerically analyzed using the spectral element method. With the increasing static load, the resonance frequencies of the floor increased due to the change in the vibration modes of the structure. The modulated sound generation from the floor vibrations transmitted to multiple layers with larger magnitudes due to this static load.

Vulnerability Assessment for a Complex Structure Using Vibration Response Induced by Impact Load

This work presents a vulnerability assessment procedure for a complex structure using vibration characteristics. The structural behavior of a three-dimensional framed structure subjected to impact forces was predicted using the spectral element method. The Timoshenko beam function was applied to simulate the impact wave propagations induced by a high-velocity projectile at relatively high frequencies. The interactions at the joints were analyzed for both flexural and longitudinal wave propagations. Simulations of the impact energy transfer through the entire structure were performed using the transient displacement and acceleration responses obtained from the frequency analysis. The kill probabilities of the crucial components for an operating system were calculated as a function of the predicted acceleration amplitudes according to the acceptable vibration levels. Following the proposed vulnerability assessment procedure, the vulnerable positions of a three-dimensional combat vehicle with high possibilities of damage generation of components by impact loading were identified from the estimated vibration responses. § 이 논문은 2014년도 대한기계학회 신뢰성부문 춘계학술대회 (2014. 2. 26.-28., 제주대) 발표논문임 † Corresponding Author, parkj@hanyang.ac.kr C 2014 The Korean Society of Mechanical Engineers 박정원 · 구만회 · 박준홍 1126 대한 재료의 손상 정도를 파악하거나 내충격성 이 향상된 복합소재 개발을 위한 다양한 이론 적, 실험적 연구가 진행되어 왔다. 나아가 외부 위 협에 대한 전투 생존성 분석 기법과 상용 프로그 램이 개발되기도 하였으며 다양한 시스템에 대한 취약성 평가가 이루어져 왔다. 하지만 구조물 전체로 전달되는 충격에너지를 고려하지 않고 피 격 부위에서 국부적인 구조물 관통 정도에 따른 제한적인 취약 확률을 제시하였다. 충격에 대한 전투 차량과 같은 복합구조물의 취 약성 평가를 수행하기 위해서는 피격 시 전달되는 충격력에 의한 구조물 전체의 충격 거동을 예측해 야 한다. 이를 위해 복잡한 구조물의 충격 해석에 일반적으로 사용되는 유한요소모델을 이용할 경 우 다양한 피격 상황에 따라 많은 해석 시간이 소 요되므로 구조물의 충격 거동을 빠르게 예측할 수 있는 해석 기법이 필요하다. 충격 해석에 스펙트 럴요소법(Spectral element method)을 적용하게 되 면 주파수에 따라 변화하는 동적강성행렬로부터 구조물의 동적 응답을 주파수 및 시간 영역에서 빠르고 정확하게 계산할 수 있다. 이 방법은 유한 요소법에 비해 요소 수를 증가시키지 않고도 고주 파 영역에서 응답의 정확성을 향상시킬 수 있는 장점이 있다. 본 논문에서는 충격 응답을 이용한 3차원 복합 구조물의 취약성 분석 모델을 제안하였다. 충격 해석 시간을 단축할 수 있도록 스펙트럴요소법을 적용하여 피격 부위로부터 구조물을 통해 전파되 는 충격에너지 특성을 파악하였다. 주요시스템으 로 전달되는 충격파에 의한 부품의 취약확률 함수 를 정의하고 시스템의 취약 확률을 예측하였다. 제안된 취약성 분석 모델을 이용해 피격 조건에 따라 전투차량 모델을 구성하는 주요 시스템의 취 약성을 비교하고 충격에 취약한 구조물 위치를 파 악하였다.