IMECH-IR  > 非线性力学国家重点实验室
非晶合金钝化裂纹前端场动态演化行为
Alternative TitleDynamic evolution behavior of blunt crack-front field in amorphous alloy
宋如月
Thesis Advisor陈艳
2019-05-29
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype硕士
Degree Discipline固体力学
Keyword非晶合金 钝化裂纹 剪切带 塑性区 尺寸效应
Abstract

    长程无序的原子结构特征,在赋予非晶合金优异的力学性能的同时,也使得其室温塑性变形极易集中在纳米尺度的剪切带内,从而造成材料的脆性破坏。室温低延性极大地限制了非晶合金作为工程结构材料的应用。剪切带形成与演化及其诱致的断裂行为是其中的关键科学问题。已有的研究表明,非晶合金的延脆性与其 裂纹前端场力学行为密切相关。裂纹前端塑性区及局部剪切带形成与演化作为关联宏观塑性与微观结构的桥梁,是揭示非晶合金失效机制的重要线索。为此,本文围绕非晶合金钝化裂纹前端场动态演化行为这一关键问题,以纯弯曲状态下双边缺口试样变形、断裂行为为切入点,从实验、有限元模拟、理论三方面开展了系列的研究。
    基于压力敏感材料的滑移线场理论和相关弹性解,推导了纯弯曲状态下双边缺口试样沿裂纹扩展方向应力分布的弹塑性近似解。该理论解可以很好地反映非晶合金的压力敏感特性,预测塑性核随外载的演化情况,同时可以很好地描述不同形状因子(B/R)试样的应力分布情况。经检验,该理论结果与有限元模拟结果吻合地很好。
    通过在扫描电子显微镜下进行双边缺口试样的原位四点弯曲实验,以及相应平面应变条件下的有限元模拟,观察得到了钝化裂纹前端均匀塑性区和局部剪切带的动态演化过程。依据对剪切带和塑性区尺寸的统计与分析,阐明了非晶合金中均匀塑性变形与局域化剪切带相互竞争的失效机理。
    研究揭示了非晶合金的断裂韧性存在强烈的缺口尺寸效应。当试样形状因子(B/R)很小时(0.5~3),剪切带的扩展始终被约束在塑性区内,且随着B/R的减小表现出高韧性;而在一定范围内(B/R=3~18),由于剪切带和塑性区的演化速度呈现不同的变化规律,导致断裂韧性随着B/R的增大而增强;而当缺口锐度增大到一定程度(B/R=30)时,试样韧性再次降低,这归因于试样断裂由剪切主导的韧性断裂转变为微孔洞化主导的脆性断裂。

Other Abstract

    Due to the long-range disordered atomic structure, amorphous alloy have excellent mechanical properties. However, this special structure also makes its plastic deformation easily concentrate in nanoscale shear band at room temperature, leading to brittle failure of the material. This greatly limits the application of amorphous alloys as engineering structural materials. The formation and evolution of shear band, as well as the fracture behavior induced by it are the key points in this problem. Previous studies have shown that the brittleness of amorphous alloys is closely related to the crack-front field. As a bridge linking macroscopical plasticity with microstructure, the plastic zone at the crack front and the evolution of local shear bands are important clues to reveal the failure mechanism of amorphous alloys.Therefore, this article carries out series of studies through theory, experiment and finite element simulation, focusing on the dynamic evolution behavior of blunt crack-front field in amorphous alloy, and taking the deformation and fracture behaviors of double-notched specimens under pure bending as entry points.
    Based on the slip line field theory of pressure sensitive material and the relevant elastic solution, the approximate solution of elastic-plastic stress along the direction of crack propagation in double notched specimen under pure bending is derived. The theoretical solution could reflect the pressure sensitive characteristics of amorphous alloys well, predict the evolution of plastic nucleus with the external load, and describe the stress distribution of samples with different shape factors. The theoretical results are in good agreement with the finite element simulation results. 
    Through the in-situ four-point bending experiment of double-notched specimens under scanning electron microscope, as well as the corresponding plane strain finite element simulation, the dynamic evolution of uniform plastic zones and local shear bands at the blunt crack-front were observed. Based on the statistics and analysis of the size of shear bands and plastic zones, the failure mechanism induced by the competition between uniform plastic deformation and localized shear band is expounded.
    It is found that the fracture toughness of amorphous alloys has a strong notch size effect. When the shape factor (B/R) is small (0.5~3), the expansion of shear band is confined in plastic zone all the time, and the sample shows high toughness with the decrease of B/R. However, within a certain range (B/R=3~18), the fracture toughness increases with the increase of B/R because the propagation velocity of shear band and plastic zone show different rules. Moreover, when the notch sharpness increases to a certain extent (B/R=30), the toughness decreases again. In discussion, this is because the fracture has changed from ductile fracture dominated by shear to brittle fracture mediated by the cavitation process.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/79094
Collection非线性力学国家重点实验室
Recommended Citation
GB/T 7714
宋如月. 非晶合金钝化裂纹前端场动态演化行为[D]. 北京. 中国科学院大学,2019.
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