• Smart Structures and Systems
• /
• 제20권6호
• /
• pp.709-728
• /
• 2017

This disquisition proposes a nonlocal strain gradient beam theory for thermo-mechanical dynamic characteristics of embedded smart shear deformable curved piezoelectric nanobeams made of porous electro-elastic functionally graded materials by using an analytical method. Electro-elastic properties of embedded curved porous FG nanobeam are assumed to be temperature-dependent and vary through the thickness direction of beam according to the power-law which is modified to approximate material properties for even distributions of porosities. It is perceived that during manufacturing of functionally graded materials (FGMs) porosities and micro-voids can be occurred inside the material. Since variation of pores along the thickness direction influences the mechanical and physical properties, so in this study thermo-mechanical vibration analysis of curve FG piezoelectric nanobeam by considering the effect of these imperfections is performed. Nonlocal strain gradient elasticity theory is utilized to consider the size effects in which the stress for not only the nonlocal stress field but also the strain gradients stress field. The governing equations and related boundary condition of embedded smart curved porous FG nanobeam subjected to thermal and electric field are derived via the energy method based on Timoshenko beam theory. An analytical Navier solution procedure is utilized to achieve the natural frequencies of porous FG curved piezoelectric nanobeam resting on Winkler and Pasternak foundation. The results for simpler states are confirmed with known data in the literature. The effects of various parameters such as nonlocality parameter, electric voltage, coefficient of porosity, elastic foundation parameters, thermal effect, gradient index, strain gradient, elastic opening angle and slenderness ratio on the natural frequency of embedded curved FG porous piezoelectric nanobeam are successfully discussed. It is concluded that these parameters play important roles on the dynamic behavior of porous FG curved nanobeam. Presented numerical results can serve as benchmarks for future analyses of curve FG nanobeam with porosity phases.

• Steel and Composite Structures
• /
• 제46권4호
• /
• pp.471-483
• /
• 2023

This paper presents an analytical hyperbolic theory based on the refined shear deformation theory for mechanical stability analysis of the simply supported advanced composites plates (exponentially, sigmoidal and power-law graded) under triangular, trapezoidal and uniform uniaxial and biaxial loading. The developed model ensures the boundary condition of the zero transverse stresses at the top and bottom surfaces without using the correction factor as first order shear deformation theory. The mathematical formulation of displacement contains only four unknowns in which the transverse deflection is divided to shear and bending components. The current study includes the effect of the geometric imperfection of the material. The modeling of the micro-void presence in the structure is based on the both true and apparent density formulas in which the porosity will be dense in the mid-plane and zero in the upper and lower surfaces (free surface) according to a logarithmic function. The analytical solutions of the uniaxial and biaxial critical buckling load are determined by solving the differential equilibrium equations of the system with the help of the Navier's method. The correctness and the effectiveness of the proposed HyRPT is confirmed by comparing the results with those found in the open literature which shows the high performance of this model to predict the stability characteristics of the FG structures employed in various fields. Several parametric analyses are performed to extract the most influenced parameters on the mechanical stability of this type of advanced composites plates.

• Korean Chemical Engineering Research
• /
• 제53권5호
• /
• pp.597-602
• /
• 2015

직경이 0.03 m이고 높이가 1.5 m인 마이크로 액적/기포 유동반응기에서 ZnO 입자의 연속제조 특성을 검토하였다. 마이크로 액적을 운반하는 기체의 속도는 6.0 L/min, 전구체 중 Zn이온의 농도는 0.4 mol/L로 유지하였다. ZnO 입자의 합성을 위한 반응의 온도(973 K~1,273 K)와 마이크로 기포의 유속 (0~0.4 L/min)이 합성된 ZnO 입자의 기공 특성에 미치는 영향을 고찰하였다. 본 연구의 실험범위에서 ZnO 입자의 합성온도는 1,073 K가 합성된 ZnO 입자의 기공을 극대화하는데 최적이었다. 또한, 반응기에서 연속 제조되는 ZnO 입자의 평균크기는 반응온도가 증가함에 따라 감소하였으며 입자의 표면은 점점 매끄럽게 단순화되었다. 반응기 내부에 마이크로 기포를 유입함으로써 유입하지 않는 경우와 비교하여 합성된 ZnO 입자 내부에 기공을 효과적으로 형성시킬 수 있었으며, 평균 BET면적을 58%까지 증가시킬 수 있었다. 마이크로 액적/기포 유동반응기를 사용하여 연속 합성한 ZnO 입자의 평균입도는 반응온도에 따라 $1.25{\sim}1.75{\mu}m$이었다.

• 한국건축시공학회지
• /
• 제15권3호
• /
• pp.281-289
• /
• 2015

본 연구에서는 $CO_2$ 양생 효과를 극대화 할 수 있는 ${\gamma}-C_2S$와 MgO와 같은 $CO_2$ 흡수 물질을 혼입한 시멘트 페이스트의 $CO_2$ 양생 효과에 관한 연구를 위하여 W/B를 40%로 설정하고 혼화재료 ${\gamma}-C_2S$와 MgO를 90% 다량 치환하여 혼화재료, $CO_2$ 양생 유무에 따른 압축강도 측정, 미세 화학분석을 실시하였다. 그 결과 $CO_2$ 양생으로 인해 Plain의 경우 약 1.08배~1.26배의 압축강도 증가 효과가 나타났으며 ${\gamma}-C_2S$와 MgO를 90% 치환한 ${\gamma}-C_2S$, MgO 실험체의 경우, 각각 14.56배~45.7배, 6.5배~10.37배 향상 효과가 나타났다. 이에 따라 미세 화학분석을 실시하여 다량의 $CaCO_3$, $MgCO_3$가 생성된 것을 확인하고 공극 감소의 효과를 확인하였다. 따라서 $CO_2$ 흡수물질 ${\gamma}-C_2S$, MgO를 다량 혼입한 시멘트 페이스트의 $CO_2$ 양생에 의한 압축강도 발현효과가 검증됨을 확인할 수 있었다.

• 대한토목학회논문집
• /
• 제36권2호
• /
• pp.207-217
• /
• 2016

투기계수는 콘크리트의 공극뿐만 아니라 공극크기, 공극분포, 공극간의 굴곡특성에 의해서도 영향을 받는다. 투기계수는 시멘트 페이스트의 미세구조에 의해 지배되는데, 시멘트 페이스트 및 골재 각각이 콘크리트의 투기성능에 미치는 영향을 다룬 연구는 드물다. 더우기, 탄산화가 투기계수에 큰 영향을 미칠 수 있음에도 불구하고, 탄산화된 콘크리트에 대한 투기계수를 다룬 연구는 더욱 드문 실정이다. 본 연구의 목적은 탄산화 및 비탄산화된 콘크리트의 투기계수를 추정할 수 있는 기초적 접근방법을 개발하는 것이다. 본 연구에서는 미세구조 모델 및 시멘트의 경화특성을 기초로 투기계수를 산정할 수 있는 해석적 기법이 제안되었다. 탄산화된 콘크리트에서 감소된 공극량이 계산되었으며 이는 투기계수의 산정에 이용하여 탄산화된 콘크리트의 투기계수를 계산하였다. 높은 물-시멘트비를 갖는 콘크리트는 탄산화로 인하여 투기계수의 감소가 더욱 뚜렷한 것을 확인되었다. 한편, 시멘트 페이스트에서 투기계수와 투수계수는 Klinkenberg 효과에 의한 선형관계가 성립되나, 콘크리트에서는 성립되지 않았다. 해석결과는 YOON's 실험방법을 수행하여 검증하였다.

• Advances in nano research
• /
• 제10권2호
• /
• pp.115-128
• /
• 2021

In this article, free vibration behavior of electro-magneto-thermo sandwich Timoshenko beam made of porous core and Graphene Platelet Reinforced Composite (GPLRC) in a thermal environment is investigated. The governing equations of motion are derived by using the modified strain gradient theory for micro structures and Hamilton's principle. The magneto electro are under linear function along the thickness that contains magnetic and electric constant potentials and a cosine function. The effects of material length scale parameters, temperature change, various distributions of porous, different distributions of graphene platelets and thickness ratio on the natural frequency of Timoshenko beam are analyzed. The results show that an increase in aspect ratio, the temperature change, and the thickness of GPL leads to reduce the natural frequency; while vice versa for porous coefficient, volume fractions and length of GPL. Moreover, the effect of different size-dependent theories such as CT, MCST and MSGT on the natural frequency is investigated. It reveals that MSGT and CT have most and lowest values of natural frequency, respectively, because MSGT leads to increase the stiffness of micro Timoshenko sandwich beam by considering three material length scale parameters. It is seen that by increasing porosity coefficient, the natural frequency increases because both stiffness and mass matrices decreases, but the effect of reduction of mass matrix is more than stiffness matrix. Considering the piezo magneto-electric layers lead to enhance the stiffness of a micro beam, thus the natural frequency increases. It can be seen that with increasing of the value of WGPL, the stiffness of microbeam increases. As a result, the value of natural frequency enhances. It is shown that in hc/h = 0.7, the natural frequency for WGPL = 0.05 is 8% and 14% less than its for WGPL = 0.06 and WGPL = 0.07, respectively. The results show that with an increment in the length and width of GPLs, the natural frequency increases because the stiffness of micro structures enhances and vice versa for thickness of GPLs. It can be seen that the natural frequency for aGPL = 25 ㎛ and hc/h = 0.6 is 0.3% and 1% more than the one for aGPL = 5 ㎛ and aGPL = 1 ㎛, respectively.

• Corrosion Science and Technology
• /
• 제16권5호
• /
• pp.257-264
• /
• 2017

Film of new Ti-15Zr-5Nb alloy formed during galvanic anodizing in orthophosphoric acid solution was characterized by optical microscope, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and Raman micro-spectroscopy. Its anticorrosive properties were determined by electrochemical techniques. The film had a layer with nanotube-like porosity with diameters in 500-1000 nm range. The nano layer contained significant amounts of P and O as well as alloying element. Additionally, Raman micro-spectroscopy identified oxygen as oxygen ion in $TiO_2$ anatase and phosphorous as $P_2O_7{^{4-}}$ ion in phosphotitanate compound. All potentiodynamic polarization curves in artificial Carter-Brugirard saliva with pH values (pH= 3.96, 7.84, and 9.11) depending on the addition of 0.05M NaF revealed nobler behavior of anodized alloy and higher polarization resistance indicating the film is thicker and more compact nanolayer. Lower corrosion rates of the anodized alloy reduced toxicity due to less released ions into saliva. Bigger curvature radii in Nyquist plot and higher phase angle in Bode plot for the anodized alloy ascertain a thicker, more protective, insulating nanolayer existing on the anodized alloy. Additionally, ESI results indicate anodized film consists of an inner, compact, barrier, layer and an outer, less protective, porous layer.

• 한국공작기계학회논문집
• /
• 제15권6호
• /
• pp.32-37
• /
• 2006

Micron size Co-alloy(T800) powder was coated on Inconel 718 by HVOF thermal spraying for the studies of the improvement of durability of high speed spindle by using Taguchi program for the parameters of spray distance, flow rates of hydrogen and oxygen and powder feed rate. The optimal coating process was determined by the studies of coating properties such as micro-structure, porosity, surface roughness and micro hardness. Friction and wear behaviors of coatings were investigated by sliding wear test at room temperature and $1000^{\circ}F(538^{\circ}C)$. At both room temperature and $538^{\circ}C$ the sliding wear debris and friction coefficients of the coating were drastically reduced compared with the surface of non-coated parent material. This shows that Co-alloy powder coating is highly recommendable for the durability improvement surface coating of high speed air-bearing spindle. At high temperature wear traces and friction coefficients of both coating and non-coating were drastically reduced compared with those of room temperature since the brittle oxides were formed easily on the surface, and the brittle oxide phases were attrited by the reciprocating sliding wear according to the complicated mixed wear mechanisms These oxide particles, partially melts and the melts play role as lubricant and reduce the wear and friction coefficient. This also shows that Co-alloy powder coating is highly recommendable far the durability improvement surface coating on the surface vulnerable to frictional heat such as high speed spindles.

• Composites Research
• /
• 제16권3호
• /
• pp.1-8
• /
• 2003

금속복합재료 개발을 위한 고온가압 성형공정은 기지재료의 비탄성거동과 성형체 내부의 기공에 대한 충진 과정을 수반하며 이러한 강화공정은 압력, 온도 그리고 강화재와 모재의 상대부피분률과 같은 공정변수의 영향을 받게 된다. 특히 티타늄금속기 복합재료의 강화공정은 강화재와 모재 사이의 기계적 혹은 열적 특성 차이 및 생산환경으로 인한 다양한 형태의 손상이 발생할 수 있으며 따라서 이들을 극복하기 위한 재료특성, 작용압력, 온도, 시간조건 등과 공정에 따른 조직의 진전 등 미소역학적 연구가 수반된 최적의 고온가압강화공정의 개발이 요구되어진다. 이를 위하여 본 연구는 VHP방식을 이용한 SiC／Ti-6Al-4V 연속섬유강화 금속기 복합재료의 강화공정실험을 수행하였으며 특히 미시역학적 접근에 따른 다공성 재료의 구성방정식을 이용하여 보강재와 기지재료의 변형거동과 고온가압공정에 필요한 다양한 조건들을 실험결과와 비교 연구하였으며 유한요소해석을 통해 공정변수와 그에 따른 결과들을 고찰하였다.

• 한국구조물진단유지관리공학회 논문집
• /
• 제17권6호
• /
• pp.122-129
• /
• 2013

콘크리트의 전기저항은 염소이온 확산계수와 비교하여 빠르고 간단히 측정할 수 있기 때문에, 염소이온 확산계수를 제어하기 위한 간접적인 지표로서 활용될 수 있다. 전기저항은 해양구조물의 유지관리 전략을 수립하는데 매우 중요하다. 본 연구의 목적은 전기저항측정의 프로토콜을 설계하는 것이다. 본 연구는 염소이온과의 관계성을 토대로 콘크리트의 성능을 파악하는 중요한 비파괴 접근방법을 제시하였다. 수분, 공극량, 공극간 굴곡특성 등과 같은 미세구조 특성이 전기저항에 미치는 영향을 배합조건별 수화단계별로 고찰하였다. 전기저항과 염소이온 확산계수의 관계로부터 정규적인 전기저항의 측정을 토대로 염소이온 확산계수를 간접적으로 통제하는데 본 성과가 이용될 수 있다.