A modified strain gradient plasticity theory is proposed based on the mechanism-based strain gradient (MSG) plasticity.
This study is motivated by nonhomogeneity of polycrystalline materials. We believe that the geometrically necessary
dislocations (GND) are generated on slip system as well as grain boundary to accommodate the deformation shape with
internal stress. The new theory differs from the MSG plasticity in consideration of the GND on grain boundary and free
surface effect of polycrystalline materials. A model describing the size effect on the tensile strength of crystalline
metallic materials is investigated. Using the nonhomogeneity of polycrystalline materials and free surface effect, the
density of the geometrically necessary dislocations during tension is derived. Using the proposed model, an analysis of
the effect of both specimen size and grain size on the tensile strength of the polycrystalline materials is carried out.
The mechanical properties of silicon nitride films are investigated. Freestanding films of silicon nitride are fabricated using the MEMS technique. The films were deposited onto (100) silicon wafers by LPCVD (Low Pressure Chemical Vapor Deposition). Square and rectangular membranes are made by anisotropic etching of the silicon substrates. Then the bulge test for silicon nitride film was carried out. The thickness of specimens was 0.5, 0.75 and 1μm respectively. By testing both square and rectangular membranes, the reliability and valiant-ness of bulge test with regard to the shape of specimens was investigated. Also considering residual stress in the films, one can evaluate the Young's modulus from experimental load-deflection curves. Young's modulus of the silicon nitride films was about 232GPa. The residual stress is below 100MPa.
Micro-wires have used widely in microelectronic devices. In order to support the high performance of microsystems, it is
important to measure the mechanical properties of the micro-wires. In this study, we measured the mechanical properties
of micro-wire to investigate the size effect behavior experimentally. Specimens used in this study are platinum micro-wires
that have various diameters such as 15, 25, 40, 50, 125, 200, and 250 &mgr;m. The platinum micro-wires with the
purity of 99.99 % were annealed to remove the residual stress. We carried out the tensile test to measure the mechanical
properties using the nanoUTM and TYTRON 250. Many researches in micro scale structures have shown that the
deformation is dependent on the size of specimen as well as grain size. To evaluate the size effect, we focused on the
relation between the strength and the specimen diameter. Our result shows that the strength of the specimen changes as
its diameter changes. We are trying to investigate micro-structure of the specimen such as grain boundary to explain this