저자명 김진성 
년도 2010 
The material properties of OFHC(Oxygen Free High thermal Conductivity) film with a thickness of 0.1 mm was evaluated at the strain rates ranging from 0.001/s to 1000/s using High-Speed Micro Material Testing Machine(HSMMTM). The high strain-rate material properties of thin films are important especially for evaluation of structural reliability of micro-formed parts and MEMS products. The high strain-rate material testing methods of thin films, however, are not yet thoroughly established while testing methods of larger specimens for electronics, auto-body, train, ship and ocean structures has been well-established. For evaluation, a HSMMTM has been newly developed to conduct high-speed tensile tests of thin films. The machine developed has a capacity of sufficiently high tensile speed with an electromagnetic actuator, a novel gripping mechanism and an accurate load measurement system. The electromagnetic actuator has the maximum acceleration of 60 G and the maximum tensile velocity of 4 m/s. For gripping of micro specimen, a novel gripping system, a slack-adapter type, was suggested for high-speed material testing. The load ringing frequency of HSMMTM currently developed is 25.5 kHz which is quite high frequency and the tensile load signal has good signal quality at high strain rates. The axial strain during high-speed tensile tests was measured by digital image processing of sequential deformation images from high speed camera. The material selected for high-speed micro tensile tests is OFHC copper film with a thickness of 0.1 mm. Specimen quality such as surface roughness is one of the important factors for accurate measurement of tensile material properties of micro samples. Micro specimens were fabricated by micro photo etching technique which is a process used in microfabrication to remove parts selectively from a thin film or a bulk of substrate. The specimen prepared showed good surface quality and dimensional accuracy. The OFHC copper film shows high strain-rate sensitivity in terms of the flow stress, the fracture elongation and strain hardening. They increase as the tensile strain rate increases. The strain hardening and flow stress in relation to the strain rate were analyzed with Swift equation. The strength coefficient and strain hardening exponent of Swift equation increases as the strain rate increases. The necking instability strain at elevated strain rates was derived from Swift equation. The necking instability strain derived was closely related to the fracture elongation in relation to the strain rates. Therefore the micro formability of OFHC copper film increases as the micro forming speed increases. The rate-dependent material properties of an OFHC copper film are also compared with those of a bulk OFHC copper sheet with a thickness of 1 mm. The flow stress of an OFHC copper film is relatively lower than that of a bulk OFHC copper sheet in the entire range of strain rates while the fracture elongation of an OFHC copper film is much larger than that of a bulk OFHC copper sheet. For grain size investigation of OFHC coppers, OFHC copper samples were prepared for EBSD(Electron BackScattered Diffraction). The grain sizes of two OFHC coppers were compared and the number of grains in the gauge cross-section was evaluated. The OFHC copper film with smaller averaged grain size showed larger strain hardening than the OFHC copper sheet with larger averaged grain size. The yield strength of the OFHC copper film with larger portion of surface grains in the gauge cross-section was lower than the OFHC copper sheet with smaller portion of surface grains in the gauge cross-section. Rate-dependent material properties of OFHC coppers were fitted with the Johnson??Cook constitutive model. The stress-strain curves from the experiment and the Johnson??Cook constitutive model showed large discrepancies as the plastic strain increases since the Johnson??Cook constitutive model implies no rate-dependent strain hardening. New constitutive model was suggested in consideration of strain hardening increment as the strain rate increases. The strain rate hardening term in the new constitutive model consists of: the strain rate sensitivity coefficient of the yield strength; and the strain rate sensitivity coefficient of the strain hardening. The new constitutive model currently suggested can well approximate the rate-dependent stress-strain curves from the experiment. The HSMMTM currently developed can successfully obtain the rate-dependent material properties of OFHC copper film. The experimental stress-strain curves obtained were well-fitted by the new constitutive model. The materials properties of micro samples can provide indispensible mechanical properties for the computer aided simulation of micro forming and crash analysis of micro-parts and micro structures.  
번호 제목 저자명 날짜 조회 수
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26 Forming Limit Diagram of Auto-body Steel Sheets at High Strain Rates for Sheet Metal Forming and Crashworthiness (박판성형 및 충돌성능 향상을 위한 고변형률속도에서의 차체강판 성형한계도) 김석봉  2010.07.13 22996
25 A New Ductile Fracture Criterion for the Formability Prediction of Steel Sheets and Its Application to Finite Element Analysis (강판의 성형성 예측을 위한 새로운 연성 파괴 조건 및 유한 요소 해석에의 응용) [1] Yanshan Lou  2012.12.10 22989
24 Analysis of Elasto-Plastic Stress Waves by a Time Discontinuous Variational Integrator of Hamiltonian with a Second-Order Integration Scheme of the Constitutive Model (해밀토니안의 시간 불연속 변분적분기와 구성방정식의 2차 정확도 적분법을 이용한 탄소 조상순  2008.12.15 22985
23 A Study on the Tension/Compression Hardening Behavior of Auto-body Steel Sheets Considering the Pre-strain and the Strain Rate (초기 변형률 및 변형률 속도를 고려한 차체 강판의 인장/압축 경화 거동에 관한 연구) [1] 배기현  2011.01.11 22251
» A Study on Material Properties of OFHC Copper Film at High Strain Rates using High-Speed Micro Material Testing Machine (고속마이크로재료시험기를 이용한 무산소동 박판의 고변형률속도 재료물성치 연구) 김진성  2010.07.13 21747
21 Finite Element Inverse Approach and Initial Guess Generation for Sheet Metal Forming Analysis of Complicated Auto-body Members (복잡한 차체부재의 박판성형공정을 위한 유한요소 역해석 및 초기추측치 계산) 김승호  2005.11.29 20435
20 A Study on the Dynamic Failure Model of a Spot Weld under Combined Loading Conditions for Auto-body Crash Analyses (차체용 부재의 충돌해석을 위한 복합하중조건에서 점용접부의 동적 파단모델 연구) [1] 송정한  2008.07.24 20279
19 Optimum Process Design in Sheet Metal Forming Processes using Finite Element Sensitivity Analysis (유한요소 민감도해석을 이용한 박판금속성형에서의 공정변수 최적설계) [1] 김세호  2005.11.29 20095
18 Strain-Rate Dependent Anisotropic Yield Criteria for Auto-body Steel Sheets (자동차용 강판의 변형률속도 의존 이방성 항복함수에 관한 연구) 허지향  2012.12.11 19810
17 Evaluation of a cast-joining process of dual metal crankshafts for heavy-duty engines with ductile cast iron and high strength forged steel(구상흑연주철과 고강도 단조강의 주조접합 이종금속을 이용한 중대형 엔진 크랭크샤프트의 평가) 한 신  2010.07.13 19212
16 Measurement Uncertainty Evaluation for High Strain Rate Tensile Properties of Auto-body Steel Sheet (자동차용 강판 고속인장물성 데이터의 측정불확도 산출) 정세환  2012.12.10 19127
15 Dynamic Formulation of Finite Element Limit Analysis for Impact Simulation of Structural Members (구조부재의 충돌해석을 위한 유한요소 극한해석의 동적 수식화) 김기풍  2005.11.29 19029
14 Development of a Nonlinear Degenerated Shell Element with the Drilling Degree of Freedom by the Cubic Polynomial Interpolation and the Assumed Strain Method (드릴링 자유도의 삼차 근사법과 대체변형률법을 이용한 비선형 감절점 쉘 요소의 개발) 이형욱  2005.11.29 19023
13 Rate Dependent Hardening Model for Pure Titanium Considering the Effect of Deformation Twinning (쌍정의 영향을 고려한 티타늄의 변형률속도 의존 경화 모델) 안광현  2012.12.12 18785
12 Microscopic investigation of the strain rate hardening for auto-body steel sheet(차체강판 변형률속도 경화의 미시적 관찰) 윤종헌  2010.07.13 18401
11 Finite Element Simulation of 3-dimensional Superplastic blow forming with diffusion bonding (유한요소법을 이용한 초소성 재료의 삼차원 확산 접합 및 압력 성형 해석) 이기석  2005.11.29 18293
10 Crash Analysis of Auto-body Structures with an Explicit Finite Element Method ( 외연적 유한요소법을 이용한 차체 구조물의 충돌해석 ) 강우종  2005.11.29 17968
9 Shell Element Formulation for Limit Analysis of Thin-Walled Structures ( 박판부재의 붕괴거동해석을 위한 극한해석의 쉘요소 수식화 ) 김현섭  2005.11.29 16082
8 A Study on a Continuum Damage Yield Function to Predict Ductile Fracture of Materials (재료의 연성파단을 예측하기 위한 연속체 손상 항복 함수에 관한 연구) 고윤기  2012.12.10 15975