Lei Xianqi, Li Congling, Shi Xinghua, Xu Xianghong, Wei Yujie
LNM, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, P.R. China.
1] LNM, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, P.R. China [2] School of Civil Engineering, Luoyang Institute of Science and Technology, Luoyang, Henan province, 471023, P.R. China.
Sci Rep. 2015 May 29;5:10537. doi: 10.1038/srep10537.
It is generally observed that the existence of geometrical discontinuity like notches in materials will lead to strength weakening, as a resultant of local stress concentration. By comparing the influence of notches to the strength of three typical materials, aluminum alloys with intermediate tensile ductility, metallic glasses with no tensile ductility, and brittle ceramics, we observed strengthening in aluminum alloys and metallic glasses: Tensile strength of the net section in circumferentially notched cylinders increases with the constraint quantified by the ratio of notch depth over notch root radius; in contrast, the ceramic exhibit notch weakening. The strengthening in the former two is due to resultant deformation transition: Shear failure occurs in intact samples while samples with deep notches break in normal mode fracture. No such deformation transition was observed in the ceramic, and stress concentration leads to its notch weakening. The experimental results are confirmed by theoretical analyses and numerical simulation. The results reported here suggest that the conventional criterion to use brittleness and/or ductility to differentiate notch strengthening or weakening is not physically sound. Notch strengthening or weakening relies on the existence of failure mode transition and materials exhibiting shear failure while subjected to tension will notch strengthen.
一般观察到，材料中诸如缺口之类的几何不连续性的存在会由于局部应力集中而导致强度减弱。通过比较缺口对三种典型材料（具有中等拉伸延展性的铝合金、无拉伸延展性的金属玻璃和脆性陶瓷）强度的影响，我们观察到铝合金和金属玻璃中出现了强化现象：周向缺口圆柱体净截面的拉伸强度随着由缺口深度与缺口根部半径之比量化的约束而增加；相比之下，陶瓷表现出缺口弱化。前两者中的强化是由于产生的变形转变：完整样品中发生剪切破坏，而具有深缺口的样品以正断模式断裂。在陶瓷中未观察到这种变形转变，应力集中导致其缺口弱化。理论分析和数值模拟证实了实验结果。此处报道的结果表明，使用脆性和/或延展性来区分缺口强化或弱化的传统标准在物理上是不合理的。缺口强化或弱化依赖于失效模式转变的存在，并且在拉伸时表现出剪切破坏的材料会出现缺口强化。
