BASIC RESEARCH OF SIZE EFPECT BY SHEAR BANDING

几种塑性现象表现出尺寸效应，尺寸越小响应越强。如前所述，在多晶变形过程中，晶界滑移的失配可能包括塑性应变的梯度。本项目采用FCC纯铝多晶和OFHC铜多晶在拉伸和压缩载荷下进行了中尺度实验研究。通过改变其横截面积、厚度和晶粒尺寸的几何尺寸，进行了厚度效应、晶粒尺寸效应和几何尺寸效应的实验测量。从应力-应变曲线可以看出，越小、越薄、越细的试样，材料响应越强。扫描电镜观察可以阐明试件塑性流动的内在机理。微剪切带方向可能发生改变或滑移本身在晶界处停止，自由边缘面可能出现双滑移。大试样和小试样的断口分别表现为典型的韧性断裂和剪切断裂，这与应力-应变曲线中出现的尺寸效应有关。以剪切变形过程为主的现象会产生应变梯度，导致尺寸效应。连续体三维有限元分析和基于Voronoi镶嵌的晶体塑性分析也讨论了剪切带的力学特征。

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.