• Journal of the KSME
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• v.44 no.6
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• pp.34-39
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• 2004

형상기억합금(SMA : Shape Memory Alloy)은 일반적인 금속이나 합금에서는 찾아볼 수 없는 형상기억효과(shape memory effect)와 초탄성 (superelasticity) 거동을 보이고 있다. 이러한 특성은 1951년에 금-카드뮴(Au-Cd) 합금에서 처음으로 발견되었으며, 1963년에 미국 해군병기연구소(Naval Ordnance Laboratory)에서 니켈-티타늄 (Ni-Ti) 합금에서 형상기억효과를 발견한 후로 널리 상용화되었다. 니티놀(nitinol)이라고 불려지는 니켈-티타늄 계열의 형상기억합금은 단위 부피당 많은 에너지를 낼 수 있고, 내 부식성(corrosion resistance)과 생화학적 적합성(bio-compatibility)이 뛰어나다. 또한 100,000사이클 이상의 긴 사용수명을 갖기 때문에 작동기(actuator)로서 우수한 특징을 갖는다. (중략)

• Proceedings of the KSR Conference
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• 2004.10a
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• pp.611-615
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• 2004

To obtain material properties of NiTi shape memory alloy showing pseudoelastic or shape memory effect, tensile test was conducted for various temperatures. Transformation temperature also was measured by using DSC(Differential Scanning Calorimeter), and crystallographic feature of transformation was observed by XRD(X-ray Diffraction).

• 한국지구물리탐사학회:학술대회논문집
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• 2004.06a
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• pp.132-138
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• 2004

탄성파 탐사는 인공지진파를 이용하여 지표면 하부의 물성을 알아내는 지구물리탐사로서 20 세기 초부터 석유탐사와 공학적 지반조사에 가장 널리 사용되었다. 굴절법 탄성파 탐사는 지층의 탄성파 속도를 알아내는 방법으로서 최근에는 석조문화재 등의 지반특성 조사에서 사용된 예가 있다. 이번 연구에서는 공주 공산성의 쌍수정 광장에 위치하는 공산성 원형연못 주변의 지반에 대하여 굴절법 탐사를 실시하였다. 쌍수정 광장은 기존의 발굴조사를 통하여 백제 추정왕궁지가 위치한 곳으로 알려졌으며, 광장 남쪽에 원형연못(상면직경 7.3 m, 바닥직경 4.78 m, 높이 3 m)도 발굴되었다. 원형연못 주변에 5개 탄성파 측선을 설치하였고, 해머 타격점과 수신기의 배열을 3가지 다른 방식을 적용하여 24 m, 31 m, 48 m 측선깊이의 굴절법 자료를 얻었다. 대체로 공산성 원형연못 주변의 지반은 3개 층으로 구성되어 있다. 각 층의 겉보기 속도는 약 261${\~}$391 m/s, 약 591${\~}$992 m/s, 약 1950${\~}$3230 m/s이며, 첫 번째와 두 번째 층의 두께는 각각 약 2${\~}$2.4 m 와 4.6${\~}$8.6 m이다. 일반적으로 최하부 층의 속도는 기반암, 상부층들의 속도는 풍화토에 대응한다. 그러나 두 번째 층의 주시곡선 형태와 속도범위는 국내 석탑 문화재 하부의 것과 유사한 것으로 보아 공산성 연못주변은 인공적인 기초지반의 가능성을 제기하며, 그렇다면 공산성 원형연못은 파내려 간 것보다는 쌓아 올렸을 것이다.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.25 no.6
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• pp.559-567
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• 2012

A level set based topological shape optimization method for nonlinear structure considering hyper-elastic problems is developed. To relieve significant convergence difficulty in topology optimization of nonlinear structure due to inaccurate tangent stiffness which comes from material penalization of whole domain, explicit boundary for exact tangent stiffness is used by taking advantage of level set function for arbitrary boundary shape. For given arbitrary boundary which is represented by level set function, a Delaunay triangulation scheme is used for current structure discretization instead of using implicit fixed grid. The required velocity field in the actual domain to update the level set equation is determined from the descent direction of Lagrangian derived from optimality conditions. The velocity field outside the actual domain is determined through a velocity extension scheme based on the method suggested by Adalsteinsson and Sethian(1999). The topological derivatives are incorporated into the level set based framework to enable to create holes whenever and wherever necessary during the optimization.

• The korean journal of orthodontics
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• v.33 no.6 s.101
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• pp.465-474
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• 2003

The great variety of commercial brands of orthodontic wires available on the market, stimulated by the so called superior wires (nickel titanium with shape memory effect and superelastic nickel titanium), makes the professional choice for a suitable and less expensive material difficult. The in vitro study of the mechanical properties of the orthodontic wires acts as an auxiliary tool for the professional. In this paper, a comparative study of mechanical properties was made, using stress strain tests for 4 types of orthodontic wires (conventional stainless steel, multistranded steel, superelastic nickel titanium and thermoactivated nickel titanium) separated into 5 groups. A series of 6 tests were tested for each group of wires. Initially, each group was tested 3 times until the wires broke. Furthermore, 3 more tests for each group were performed, stretching the wires under standardized activation loads, for a reliable comparison of their mechanical properties, during loading and unloading. 1 tests were applied to check differences among the groups. In vitro, the results suggest that regarding the mechanical properties supposedly desirable for physiological teeth movement, such as resilience, elasticity modulus, strength liberated during unloading, and the way that strength is liberated, thermoactivated nickel titanium wires, acting under mouth temperature, seems to be a good choice, fellowed by superelastic nickel titanium, multistranded stainless steel, and conventional stainless sleet. Superelasticity was demonstrated for superelastic nickel titanium wires. When at $37^{\circ}C$, thermoactivated nickel titanium wires showed shape memory effect, showing that temperature is important for enhancing the mechanical properties.

• Journal of Adhesion and Interface
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• v.9 no.3
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• pp.20-26
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• 2008

It is well know that the structure of shape memory alloy (SMA) can change from martensite austenite by either temperature or stress. Due to their inherent shape recovery properties, SMA fiber can be used such as for stress or cure-monitoring sensor or actuator, during applied stress or temperature. Incomplete superelasticity was observed as the stress hysteresis at stress-strain curve under cyclic loading test and temperature change. Superelasticity behavior was observed for the single-SMA fiber/epoxy composites under cyclic mechanical loading at stress-strain curve. SMA fiber or epoxy embedded SMA fiber composite exhibited the decreased interfacial properties due to the cyclic loading and thus reduced shape memory performance. Rigid epoxy and the changed interfacial adhesion between SMA fiber and epoxy by the surface treatment on SMA fiber exhibited similar incomplete superelastic trend. Epoxy embedded single SMA fiber exhibited the incomplete recovery during cure process by remaining residual heat and thus occurring residual stress in single SMA fiber/epoxy composite.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.33 no.3
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• pp.179-186
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• 2020

In this study, the wall stress and rupture risk for abdominal aortic aneurysms were calculated based on the age and geometry of the examined abdominal aortic aneurysms. The geometry of the abdominal aorta was simulated using computed tomography data from patients with abdominal aortic aneurysms. With regard to material properties, the Gasser-Ogden-Holzapfel model was applied to the analysis to simulate the anisotropic hyperelastic characteristics of the artery. In addition, each material parameter was estimated to consider the properties for age and for normal and aneurysm tissue. Moreover, the correlation between the diameter and angle of the aortic aneurysms was analyzed based on data from patients with abdominal aortic aneurysms, and series simulations were conducted. As a result, the rupture risk for the abdominal aortic aneurysms was evaluated based on the age and geometry of the aneurysm.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.2
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• pp.169-176
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• 2015

The mechanical properties of living tissues have been major subjects of interest in biomechanics. In particular, the characteristics of very soft materials such as the brain have not been fully understood because experiments are often severely limited by ethical guidelines. There are increasing demands for studies on remote medical operations using robots. We conducted compression tests on brain-like specimens made of gelatin to find substitutes with the mechanical properties of brain tissues. Using a finite element analysis, we compared our experimental data with existing data on the brain in order to establish material models for brain tissues. We found that our substitute models for brain tissues effectively simulated their mechanical behaviors.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.6
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• pp.619-628
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• 2011

Human soft tissues, including muscles, ligaments, skin, and blood vessels, are an interesting subject because damage to them can be observed in everyday life. Besides the lack of available experimental data and the large deformation upon loading, the anisotropic and compressible nature of annulus fibrosus makes it more difficult to find a simple material model. A fiber-reinforced hyperelastic material model is used to determine the stress-strain curves upon uniaxial loading. The energy potential function for annulus fibrosus is composed of three different parts: matrix, fibers, and matrix-fiber interaction, which accounts for the angles between two families of fibers. In this paper, two different types of energy potential function for the matrix are considered, and are inserted into the fiber-reinforced model. The calculated results are compared with the Neo-Hookean model and experimental data, and reasonable agreement is observed overall.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2018.06a
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• pp.132-132
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• 2018

NiTi 형상 기억 합금은 형상기억 효과 (Shape memory effect) 또는 초탄성 효과 (superelasticity effect)를 나타낸다고 알려져 있다. 대표적으로 Ni:Ti 조성비가 1:1을 갖는 NiTi(니티놀) 합금은 형상기억 및 초탄성 효과가 우수하여 기계 가공 공정뿐만 아니라 우수한 내마모성을 요구하는 공구에 사용하기 적합하다. 하지만 NiTi 박막은 합금과 같은 Damping capacity를 가지고 있지만 비교적 낮은 물리적 특성을 가지고 있다. 본 연구에서는 NiTi 박막의 낮은 물리적 특성을 향상시키기 위하여 TiN과 NiTi의 2층형 박막을 제조하고 각 층의 두께 변화를 조절하여 특성 향상에 대한 기초연구를 진행했다. 타겟은 NiTi (Ni:Ti=48.2:51.8 at.%) 합금 타겟과 Ti 타겟을 사용하였고, 시편과 타겟 간의 거리는 약 10cm 이며, 시편은 기초분석을 위한 SUS304, 물리적 특성 평가를 위한 초경 을 사용하였다. 초경은 실제 공구에서 사용하고 있는 Co함량이 10% 함유된 시편은 선정했다. 시편 전처리는 알코올과 아세톤으로 세척을 실시한 후 진공챔버에 장착하고 ${\sim}10^{-5}Torr$ 까지 진공배기를 실시하였다. 기판 정청은 글로우 방전 방식으로 약 800 V 전압에서 30분간 실시했다. 공정 가스는 Ar와 $N_2$ 혼합가스를 사용하였으며, UBM(Un-Balanced Magnetron) 스퍼터링 소스를 이용하여 2층형 박막을 제조했다. TiN과 NiTi 층의 두께 비율을 0.5, 1 그리고 2 로 변화시켜 코팅했으며, 박막의 총 두께는 약 ${\sim}3{\mu}m$ 이다. 기초분석은 FE-SEM을 통해 두께와 박막 비율을 확인 및 XRD 분석을 통해 박막 정성분성을 실시했다. 2층형 박막의 물리적 특성은 Nanoindentation test, AFM 및 ball on disc를 이용하여 평가했으며, 그 결과 두께 비율 변화에 따라 물리적 특성 변화가 나타남을 확인했다.