BASIC RESEARCH OF SIZE EFPECT BY SHEAR BANDING

一些塑性现象表现出尺寸效应，尺寸越小，其响应越强。如前所述，在多晶变形中，晶界滑移的失配可能包括塑性应变的梯度。本项目使用纯铝和OFHC铜的面心立方多晶在拉伸和压缩载荷下进行了一项实验性的介观研究。通过改变其截面面积、厚度和晶粒度的几何尺寸，对厚度效应、晶粒度效应以及几何尺寸效应进行了实验研究。从应力-应变曲线可以得出结论，较小、较薄和细晶的试件的材料响应变得更坚硬。扫描电子显微镜观察可以阐明试件中塑性流动的内在机理。微观剪切带的方向可能发生改变或滑移本身停止在晶界，在自由刃面可能发生双滑移。大、小试件的截止面分别呈现典型的延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。剪切变形占主导地位的现象会产生应变梯度，导致尺寸效应。利用Voronoi网格进行了连续体三维有限元分析和晶体塑性分析，并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.