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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)
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.