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오충석(Chung-Seog Oh),송지호(Ji-Ho Song) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.3
Fatigue crack growth and closure behaviour of surface cracks as well as through-thickness cracks in aluminium alloy 7075-T65l are investigated under pure bending loading. We have placed a special focus on the comparison of the surface crack closure behaviour under two loading cases, axial and bending, and also with the through-thickness cracks. The ratio of crack opening ratio in the crack length direction to that in the crack depth direction is close to 0.9 regardless of the applied loading types and the stress ratios. Growth rates of surface cracks can be successfully explained in terms of the effective stress intensity factor range, independently of the loading type, the crack shape and the stress ratio.
금 박막의 기계적 물성에 미치는 온도 및 변형률 속도의 영향
오충석(Chung-Seog Oh),Nikhil Karanjgaokar,Ioannis Chasiotis 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.11
The effect of strain rate and temperature on the inelastic properties of nanocrystalline Au films was quantified with microscale tensile specimens tested over six decades of strain rate and RT to 120 C. The full field strain and temperature distributions are measured with DIC and microscopic IR camera. Two kinds of heating techniques of resistive and hot plate heating are developed and compared. The elastic modulus was independent of the strain rate and temperature, but the inelastic mechanical response was clearly rate sensitive. The yield strength and the ultimate tensile strength increased with the strain rate but decreased with the temperature.
김상교(Sang-Kyo Kim),오충석(Chung-Seog Oh),이학주(Hak-Joo Lee) 한국비파괴검사학회 2010 한국비파괴검사학회지 Vol.30 No.5
박막은 마이크로 전자 장치, 자기 기록 매체, 미세 기전 시스템 및 표면 코팅과 같은 다양한 응용에 있어서 매우 중요한 역할을 수행하는 재료이다. 이러한 박막의 재료 물성 값은 상응하는 거시 재료의 물성 값과 다를 수 있기 때문에 박막의 기계적 물성 값들을 신뢰성 있게 측정할 수 있는 시험법의 개발이 요구되어져 왔다. 본 연구에서는 종래의 약 처짐 시험법과 단축 인장 시험법의 한계성을 극복하기 위해 나노미터 이하의 면외 변위 측정 분해능을 갖는 백색광 간섭계를 채택한 새롭고 간편한 시험법을 개발하였다. 개발된 시험법의 유효성을 검증해 보기 위하여 스퍼터링을 포함한 마이크로 공정에 의해 자유지지 알루미늄 박막 시험편을 제작한 뒤 이를 이용하여 인장 물성 값을 측정하였다. 폭 0.5 mm, 두께 1㎛인 시험편을 실리콘 다이상에 1~5개 제작하여 사용하였다. 모터 구동 팁, 하중계 및 6 자유도 정렬 장치로 구성된 시험기를 자체적으로 제작한 뒤 막 처짐 시험을 수행하였다. 시험기는 가능한 작게 제작하여 상용 백색광 간섭 현미경 아래에 설치 가능하도록 하였다. 백색광 간섭무늬를 이용하여 시험편과 시험기 사이의 정렬 맞춤을 수행하였다. 영 계수는 62 GPa, 항복점은 247 MPa로 측정되었다. Thin films play an important role in many technological applications including microelectronic devices, magnetic storage media, MEMS and surface coatings. It is well known that a thin film"s material properties can be very different from the corresponding bulk properties and thus there has been a strong need for the development of a reliable test method to measure the mechanical properties of a thin film. We have developed an alternative and convenient test method to overcome the limitations of previous membrane deflection experiment and uniaxial tensile test by adopting a white light interferometer having sub-nanometer out-of-plane displacement resolution. The freestanding aluminium specimens are tested to verity the effectiveness of the test method developed and get the tensile properties. The specimens are 0.5 rum wide, 1 ㎛ thick and fabricated through MEMS processes including sputtering. 1 to 5 specimens are fabricated on Si dies. The membrane deflection experiments are carried out by using a homemade tester consisted of a motor-driven loading tip, a load cell, and 6 DOF alignment stages. The test system is compact enough to set it up beneath a commercial white light interferometric microscope. The white light fringes are utilized to align a specimen with the tester. The Young"s modulus and yield point stress of the aluminium film are 62 GPa and 247 MPa, respectively.
양원존(Won-Jon Yang),오충석(Chung-Seog Oh) Korean Society for Precision Engineering 2022 한국정밀공학회지 Vol.39 No.5
The main shaft of a mechanical press inevitably includes significant stress concentrations that can trigger severe mechanical damage and finally lead to failure under repetitive use. In this study, an efficient procedure to quantitatively evaluate the fatigue life of the shaft system including the main shaft and its support bearings, based on the macroscopic failure analysis of the main shaft broken during actual use, was investigated. For this purpose, the bearing support was modeled as an elastic foundation, and the elastic foundation stiffness value was varied to determine the optimal value that best simulates the failure behavior, especially with respect to the failure location and failure sequence, of an actual shaft. While the finite element mesh size was kept the same, only the effect of elastic foundation stiffness was investigated. The optimum value for the main shaft investigated in this study was approximately 60 N/mm³, and the fatigue life of the shaft was evaluated based on the conventional maximum principal stress theory. Based on this, two modified designs to enhance the fatigue life of the existing shaft are proposed.