A new experimental investigation into the effects of reinforcing mortar beams with superelastic SMA fibers on controlling and closing cracks

Choi, Eunsoo,Mohammadzadeh, Behzad,Kim, Dongkyun,Jeon, Jong-Su Elsevier 2018 Composites Part B, Engineering Vol.137 No.-

<P><B>Abstract</B></P> <P>This study assessed the performance of short fibers made of superelastic shape memory alloy (SE SMA) to close cracks in mortar beams. Four types of fibers were considered. Five types of cement mortar beams were manufactured. Four of these specimen types included superelastic SMA fibers such that each one was made with only one type of SMA fiber as well as steel bar, and the other specimen type was made with only a steel bar. The self-crack-closing capacity of the beams was investigated by comparing deflections upon loading and unloading through studying the post-cracking behavior. Results showed that the mortar beams resist flexural force until a crack is generated. Thereafter, the flexural load is sustained by the SE SMA fibers. Spearhead (SH) fiber provides the best self-centering behavior. A good crack recovery capacity cannot be achieved because there is a lake of flag-shaped behavior as the stress induced in the fiber does not hit the upper plateau stress due to fiber slip. The beam with only a steel bar showed a better recovery ratio than other types. For flexural crack control, a perfect bond between the SE SMA short fibers and the mortar matrix is needed to generate self-centering capacity.</P>

Vibration tests of precompressed rubber springs and a flag-shaped smart damper

Choi, Eunsoo,Youn, Heejung,Park, Kyoungsoo,Jeon, Jong-Su Elsevier 2017 ENGINEERING STRUCTURES Vol.132 No.-

<P><B>Abstract</B></P> <P>The aims of this study were to introduce a smart damper with flag-shaped behavior using friction of magnets and self-centering capacity of precompressed rubber springs and to conduct vibrational tests to verify its performance. A damper exhibiting flag-shaped behavior is considered the most effective to sufficiently provide energy dissipation and self-centering after earthquakes or strong vibrations. The damper suggested in this study used practical materials to realize a smart damper. For these purposes, hexahedron Neodymium (NdFeB) magnets with dimensions of 50mm×50mm×25mm (B×L×H) and polyurethane rubber cylinders of 80mm in length and 80mm in diameter with a central hole of 20mm were used. The experimental program in this study consists of three tests; magnetic friction and precompressed rubber springs were first tested separately, and, the damper comprising the two components was tested. For the test of the magnet, two parameters of frictional force and loading frequency were considered. The loading frequency was varied from 0.1 to 2.0Hz, and the frictional force was controlled by the number of magnets, namely, 0, 4, 8, and 12. From the separating tests, frictional coefficients of magnets of were estimated; the value was 0.65. The rigid force of the rubber springs was assessed according to precompression; the rigid force increased linearly with an increasing strain of precompression up to the strain of 20%. From the rubber tests, the Young’s modulus of the polyurethane rubber was estimated as 52.8MPa, which was much greater than that of natural rubber. The vibration test for a combined damper produced flag-shaped behavior of the damper, and their damping ratios and stiffnesses were assessed from the hysteretic curves. The damping ratio of only rubber springs was 2.78%, and the damping ratios with 12 magnets increased to 7.12%. The estimated stiffness after the rigid behavior was 17.2kN/mm. The suggested damper showed good performance of flag-shaped behavior.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Develop a flag-shaped smart damper using magnets and precompressed rubber springs. </LI> <LI> Perform vibration tests with variation of frictional force and loading frequency. </LI> <LI> Exhibit good flag-shaped behavior including self-centering and energy dissipation. </LI> <LI> Rubber springs and magnets are relatively cheaper than shape memory alloys. </LI> <LI> The proposed smart damper can be applied to a knee brace in building frames. </LI> </UL> </P>

Behavior of NiTiNb SMA wires under recovery stress or prestressing

Choi, Eunsoo,Nam, Tae-hyun,Chung, Young-Soo,Kim, Yeon-Wook,Lee, Seung-yong Springer 2012 Nanoscale research letters Vol.7 No.1

<P>The recovery stress of martensitic shape-memory alloy [SMA] wires can be used to confine concrete, and the confining effectiveness of the SMA wires was previously proved through experimental tests. However, the behavior of SMA wires under recovery stress has not been seriously investigated. Thus, this study conducted a series of tests of NiTiNb martensitic SMA wires under recovery stress with varying degrees of prestrain on the wires and compared the behavior under recovery stress with that under prestressing of the wires. The remaining stress was reduced by the procedure of additional strain loading and unloading. More additional strains reduced more remaining stresses. When the SMA wires were heated up to the transformation temperature under prestress, the stress on the wires increased due to the state transformation. Furthermore, the stress decreased with a decreasing temperature of the wires down to room temperature. The stress of the NiTiNb wires was higher than the prestress, and the developed stress seemed to depend on the composition of the SMAs. When an additional strain was subsequently loaded and unloaded on the prestressed SMA wires, the remaining stress decreased. Finally, the remaining stress becomes zero when loading and unloading a specific large strain.</P>

A Systems Analysis of Korean Lifelong Educational Policies With Special Reference to the Lifelong Education Law

Choi, Eunsoo 한국성인교육학회 2002 Andragogy Today : International Journal of Adult & Vol.5 No.4

이 연구의 목적은 한국의 평생교육법이 교육정책의 주요한 부분으로서 어떻게 역할을 하고 있는가를 분석하는 것이다. 분석의 틀은 David Easton의 체제분석 모형을 따랐다. 체제분석의 분석적 틀에 따라 평생교육법의 출현 조성 환경, 요구, 지지, 투입, 정치적 시스템, 산출, 실행, 결과, 그리고 새로운 투입 요소 등이 분석되었다. 시스템 분석 모형에 의한 평생교육법의 정치적 체제는 기본적으로 순환적 성격이다라는 결론에 도달하면서, 이 연구의 분석 결과는 다음과 같이 요약된다. 우선, 평생교육의 확산과 더불어 조성된 정치적, 교육적, 사회적, 경제적 발전은 하나의 스트레스로 작용하면서 새로운 평생교육체제 및 평생교육법의 출현을 바라는 요구를 필요로 한다. 이러한 요구들은 여러 정부의 지지 활동들과 더불어 투입 요소로 변환하게 된다. 이러한 투입 요소는 교육개혁위원회 특별 활동 팀의 선행 연구로부터 시작하여 국회에서의 평생교육법의 통과에 이르기까지의 하나의 정치적 체제를 거치면서 산출로 변환하게 된다. 평생교육법은 실행 단계를 거치면서 여러 가지 긍정적인 영향과 더불어 개정해야 할 부분을 갖게 되는 결과를 보여주게 된다. 이러한 결과들은 입법부에 개정안의 형태로 제출된 상태에서 새로운 투입 요소를 산출하게 된다. 이러한 순환적 상황은 새로운 정치 체제를 형성하게 되면서 제2의 순환을 야기하게 된다. This study was designed to analyze how the Lifelong Education Law was acted as an important part of education reform in Korea within the framework of Systems Analysis, which is an analytical scheme developed by David Easton. Following the analytical framework of systems analysis the aspects of the environment, demands, support, political system, output, implementation, outcome and new inputs of the Lifelong Education Law in Korea was provided in order. Leading the conclusion that the analytical framework of the Lifelong Education Law in terms of the Systems Analysis will be is basically cyclical, it has shown that the political, educational, social, and economic development along with the prosperity of lifelong education functioned as stresses which generated demands for the transition from the old adult and continuing education system to the new lifelong education structure and also the replacement of the old adult and continuing education law with a new lifelong learning law. These demands were converted into the input, that is, the necessity of a new lifelong learning law with the support of several governmental activities. Through the political system starting from the trial research by the task force team of the Presidential Commission on Education Reform to the passage of the Law at the National Assembly, the input converted to the output, that is, the Lifelong Education Law. The Law, during the process of implementation, brought some positive effects and a need to alter. These impacts would generate new set of inputs with the form of a revised bill to the legislature, which would possibly form the new political system in the next cycle for the framework of the systems model.

Pullout behavior of superelastic SMA fibers with various end-shapes embedded in cement mortar

Choi, Eunsoo,Mohammadzadeh, Behzad,Hwang, Jin-Ha,Kim, Woo Jin Elsevier 2018 Construction and Building Materials Vol.167 No.-

<P><B>Abstract</B></P> <P>This study investigated the effects of superelastic shape memory alloy (SMA) fibers end-shape on pullout resistance through hysteretic pullout testing. Superelastic NiTi SMA wire of 1.0 mm diameter was employed in manufacturing short fibers. SMAs were fabricated with four end-shapes: 1) prismatic and straight end, 2) L-shaped end, 3) N-shaped end, and 4) crimped end with a spearhead. The embedded length of a fiber into the mortar matrix having compressive strength of 50.0 MPa was 18.0 mm, except for the N-shaped end fiber for which the embedded length was 21.0 mm. The pullout test was conducted with displacement control to obtain hysteretic pullout behavior by four cycling loadings. The results showed that crimped-end fibers considerably magnified the pullout strength, and the deformation was recovered by their superelastic behavior. As N-shaped end fibers could not show flag-shaped behavior, another test was performed through which fibers were annealed to induce superelasticity, so, a perfect strain recovery was obtained. Results showed that additional annealing of the N-shaped fibers after the manufacturing improved superelastic behavior at the bended part.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Effect of superelastic SMA fibers end-shape on pullout resistance was investigated. </LI> <LI> Four end-shapes: straight end, L-shaped N-shaped, and crimped end were considered. </LI> <LI> Pullout test was done by displacement control to obtain hysteretic pullout behavior. </LI> <LI> Four cycling loadings were applied to each specimen during pullout test. </LI> </UL> </P>

Robust SiO₂ gate dielectric thin films prepared through plasma-enhanced atomic layer deposition involving di-sopropylamino silane (DIPAS) and oxygen plasma: Application to amorphous oxide thin film transistors

Choi, Yoo-Jin,Bae, Seung-Muk,Kim, Jae-Hwan,Kim, Eui-Hyun,Hwang, Hee-Soo,Park, Jeong-Woo,Yang, Heesun,Choi, Eunsoo,Hwang, Jin-Ha Elsevier 2018 CERAMICS INTERNATIONAL Vol.44 No.2

<P><B>Abstract</B></P> <P>DIPAS (di-isopropylamino silane, H<SUB>3</SUB>Si[N(C<SUB>3</SUB>H<SUB>7</SUB>)<SUB>2</SUB>]) and O<SUB>2</SUB> plasma were employed, using plasma-enhanced atomic layer deposition (PEALD), to deposit silicon oxide to function as the gate dielectric at low temperature, i.e., below 200°C. The superior amorphous SiO<SUB>2</SUB> thin films were deposited through the self-limiting reactions of atomic layer deposition with a deposition rate of 0.135nm/cycle between 125 and 200°C. PEALD-based SiO<SUB>2</SUB> thin layer films were applied to amorphous oxide thin film transistors constructed from amorphous In-Ga-Zn-O (IGZO) oxide layers, which functioned as channel layers in the bottom-gated thin film transistor (TFT) structure, with the aim of fabricating transparent electronics. The SiO<SUB>2</SUB> gate dielectric exhibited the highest TFT performance through the fabrication of heavily doped n-type Si substrates, with a saturation mobility of 16.42cm<SUP>2</SUP>/V·s, threshold voltage of 2.95V and large on/off current ratio of 3.69 × 10<SUP>8</SUP>. Ultimately, the highly doped Si was combined with the ALD-based SiO<SUB>2</SUB> gate dielectric layers, leading to a saturation mobility of 16.42cm<SUP>2</SUP>/V·s, threshold voltage of 2.95V, S-slope of 0.1944, and on/off current ratio of 3.69 × 10<SUP>8</SUP>. Semi-transparent and transparent TFTs were fabricated and provided saturation mobilities of 22.18 and 24.29cm<SUP>2</SUP>/V·s, threshold voltages of 4.18 and 2.17V, S-slopes of 0.1944 and 0.1945, and on/off current ratios of 9.63 × 10<SUP>8</SUP> and 1.03 × 10<SUP>7</SUP>, respectively.</P>

Seismic Protection for Multiple Span Continuous Steel Bridges using Shape Memory Alloy-Restrainer-Dampers

Eunsoo Choi,Haksoo Kim 한국지진공학회 2004 한국지진공학회논문집 Vol.8 No.1

본 논문은 강교좌장치를 가지고 있는 연속 강교량을 지진으로부터 보호하기 위해서 형상기억합금을 이용한 장치를 제안했다. 예로 사용하고 있는 연속 강교량은 고정단을 가진 교각에 지진하중이 집중하고 상당히 큰 상부구조의 중량으로 인하여 교각에 손상을 입기 쉽고 상판의 교대에 대한 충돌로 교대의 수동변위가 크게 나타날 수 있다. 본 연구에서는 두 가지 종류의 restrainer-damper가 제안되었으며 지진해석을 통해서 효과를 검증하였다. 또한 미주에서 일반적으로 사용되고 있는 강재 케이블의 restraoiner와 성능을 비교하였다. This paper introduces a shape memory alloy-restrainer-damper(SMA-RD) to protect multiple span continuous steel bridges from seismic loads. The type of bridges has only one fixed bearing condition on a pier and expansion bearings are located on the other piers and abutments. Due to this state and a big mass of the deck, these bridges are usually very vulnerable to column's damage on which fixed bearings are located and large deformation of abutments in passive action. Two types of SMA-RDs are developed, and their effect is inspected for protecting the bridges through seismic analyses. Conventional steel restrainer cables are also used to reduce the seismic vulnerability of the bridge and the results are compared to those of the SMA-RDs.

Prestressing effect of cold-drawn short NiTi SMA fibres in steel reinforced mortar beams

Choi, Eunsoo,Kim, Dong Joo,Hwang, Jin-Ha,Kim, Woo Jin Institute of Physics Publishing 2016 Smart materials & structures Vol.25 No.8

<P>This study investigated the prestressing effect of cold-drawn short NiTi shape memory alloy (SMA) fibres in steel reinforced mortar beams. The SMA fibres were mixed with 1.5% volume content in a mortar matrix with the compressive strength of 50 MPa. The SMA fibres had an average length of 34 mm, and they were manufactured with a dog-bone shape: the diameters of the end- and middle-parts were 1.024 and 1.0 mm, respectively. Twenty mortar beams with the dimensions of 40 mm?×?40 mm?×?160 mm (<I>B</I>?×?<I>H</I>?×?<I>L</I>) were prepared. Two types of tests were conducted. One was to investigate the prestressing effect of the SMA fibres, and the beams with the SMA fibres were heated at the bottom. The other was to assess the bending behaviour of the beams prestressed by the SMA fibres. The SMA fibres induced upward deflection and cracking at the top surface by heating at the bottom; thus, they achieved an obvious prestressing effect. The beams that were prestressed by the SMA fibres did not show a significant difference in bending behaviour from that of the SMA fibre reinforced beams that were not subjected to heating. Stress analysis of the beams indicated that the prestressing effect decreased in relation to the cooling temperature.</P>

Cyclic tests of SMA Rings in Austenitic or Martensitic State

Choi, Eunsoo,Nam, Tae-Hyun,Park, Seungjin,Woo, Daeseung,Kwon, Eunsol 대한토목학회 2017 대한토목학회 학술대회 Vol.2017 No.10

Mechanical Properties of Cold Drawn NiTi SMA Wire

Choi, Eunsoo(최은수),Alireza Ostadrahimi 한국콘크리트학회 2020 한국콘크리트학회 학술대회 논문집 Vol.32 No.2