Indentation tests have been used to measuring the strength and hardness of materials. Moreover, micro and nanoindentation have become major tools for investigating the micromechanical properties of small scale volumes. However, it is well-known that the micro and nanoindentation hardness of materials shows the strong size effect. But the classical continuum plasticity can’t predict these size effects in micro/nano scale, since the constitutive equation of the classical mechanics doesn’t include the internal length as a parameter for the deformation. In this paper, modified strain gradient theory is proposed based on the nonhomogeneity of polycrystalline metallic materials. When the grains of crystalline metals deform, overlaps and voids appear at the grain boundary. These overlaps and voids can be corrected by the GNDs. By taking into account the nonhomogeneity of polycrystalline materials, the density of the GNDs due to the deformation is calculated. Consideration of the GNDs on the grain boundary gives a relationship between the size effect and the hardness. This relationship can explain the indentation size effects in micro/nano scale. Using the proposed model, analysis of the effect of indent size and grain size under the nanoindentation test of polycrystalline materials is
Recent experiments with non-uniform plastic deformation have shown the size effects in micro/nano scale. But the
classical continuum plasticity can't predict these size effects in micro/nano scale, since the constitutive equation of the
classical mechanics doesn't include the internal length as a parameter for the deformation.
The mechanism based strain gradient plasticity is one of the methods to analyze non-uniform deformation behavior in
micro/nano scale. The MSG plasticity is the multi-scale analysis connecting the micro-scale notion of the statistically
stored dislocations and the geometrically necessary dislocation to meso-scale deformation using the strain gradient.
In this paper, modified strain gradient theory is proposed based on the nonhomogeneity of polycrystalline metallic
materials and free surface effect. Consideration of the geometrically necessary dislocations on the grain boundary and
the free surface effect suggests a relationship between the characteristic length, specimen size and grain size. This
relationship can explain the size effects and flow stress in micro/nanoscale structures. We will propose a new model for
bending tests using the modified strain gradient plasticity theory. Using the proposed model, bending behavior of
polycrystalline materials in micron-scale structures is investigated, and compared with experimental results from other
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.
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