BASIC RESEARCH OF SIZE EFPECT BY SHEAR BANDING

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Several plasticity phenomena display a size effect where the smaller the size is the stronger its response. In the deformation of polycrystal, the mismatch of slip at the boundaries of the grains may include gradients of plastic strain, as is discussed previously. This project contains an experimental meso-scale study using FCC polycrystal of pure Aluminum and OFHC Copper in both of tension and compression loading. Experimental measurements are carried out to investigate thickness effects and grain size effects together with geometrical size effects by changing its geometrical sizes of cross sectional area, thickness and grain size. It may be concluded from the stress-strain curves that material response becomes stiffener for smaller, thinner and finer-grain specimens. SEM observation can clarify the internal mechanism of plastic flow in the specimens. The direction of micro shear band may change or the slip itself stops at the grain boundaries, and double-slip may occur in free edge surface. The cut-off surfaces show the typical ductile and shearing fracture processes for large and small specimens, respectively, which relates to the size effects appeared in stress-strain curve. Phenomena dominant of shear deformation process may produce strain gradient, leading to size effect. Continuum 3-dimensional FEM analysis and crystal plasticity analysis using Voronoi tessellation also discuss mechanical feature of shear banding.