BASIC RESEARCH OF SIZE EFPECT BY SHEAR BANDING

一些塑性现象显示出尺寸效应，其中尺寸越小，其响应越强。在多晶体的变形中，晶粒边界处的滑移失配可包括塑性应变梯度，如前所述。本计画包含一个细观尺度的实验研究，使用纯铝与无氧铜的面心立方多晶在拉伸与压缩两种负载下。通过改变其截面积、厚度和晶粒尺寸的几何尺寸，对厚度效应、晶粒尺寸效应以及几何尺寸效应进行了实验测量。从应力-应变曲线可以得出结论，对于较小、较薄和晶粒较细的试样，材料响应变得更硬。扫描电镜观察可以阐明试样塑性流动的内在机制。微观剪切带的方向可能发生变化或滑移本身停止在晶界处，自由边表面可能发生双滑移。大、小试样的截断面分别呈现出典型的韧性断裂和剪切断裂过程，这与应力-应变曲线中出现的尺寸效应有关。剪切变形过程中的主导现象可能产生应变梯度，导致尺寸效应。通过三维有限元分析和晶体塑性Voronoi网格分析，探讨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.