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Study on the Strain-Rate Dependent Tensile Properties of Micro-Scale Aluminum Specimens(알루미늄 마이크로시편의 변형률속도에 따른 인장 특성 연구)
The material properties of A1100 film with a thickness of 96 µm has been evaluated at strain rates ranging from 0.001/s to 100/s using Static Micro Material Testing Machine (SMMTM) and High-Speed Micro Material Testing Machine (HSMMTM). The high strain-rate material properties of thin films are important especially for the evaluation of structural reliability of micro-formed parts and micro-electromechanical systems (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, and ocean structures has been well-established.
In the present study, a Micro-tester has been newly developed for micro-tensile tests at strain rates ranging from 0.001/sec to 0.1/sec. The new machine has a gearhead type linear motor and an accurate load measurement system. The gearhead type linear motor has the maximum tensile load of 70 N and the maximum tensile speed of 1.5 mm/s. Also, a new jig system for HSMMTM has been newly developed for easy installation of a specimen and accurate alignment between a specimen and the jig system to enhance the reproducibility of tests. Digital image correlation (DIC) method is employed to measure the axial strain of micro-specimens. To enhance the accuracy of the strain measurement, a novel technique is introduced such that very fine speckle patterns can be made. For A1100 thin film with the thickness of 96 µm, micro-tensile tests have been performed using Micro-tester at strain rates from 0.001/s to 0.1/s, and using HSMMTM at strain rates from 1/s to 100/s. The dimension of a specimen is 300 µm in gauge width and 1500 µm in gauge length, which were fabricated by micro-photo etching technique.
To examine the effect of grain size and specimen size on the tensile properties of A1100, the tensile tests have been carried out at the strain rates ranging from 0.001/s to 100/s using Instron5583 and high speed material testing machine (HSMTM) for A1100 with the thickness of 1.0 mm. Electron Back Scattered Diffraction(EBSD) and Focused Ion Beam(FIB) images were obtained for both micro and macro specimens and analyzed to measure the grain size. The grain size of two A1100 has been compared and the number of grains in the gauge cross-section has been evaluated. The A1100 film with smaller average grain size shows larger strain hardening than the A1100 sheet with larger average grain size. The flow stress of the A1100 film with smaller grains was higher than the A1100 sheet with larger grains. This result shows the opposite result with those of OFHC copper film.