【作者】 郭昭亮；

【导师】 刘仓理；

【作者基本信息】 中国工程物理研究院 ， 工程力学， 2010， 硕士

【摘要】 材料或结构的断裂位置及断裂形式一直是工程界和学术界关注的焦点。由于材料的样本个性,在动载作用下,裂纹的萌生往往会表现出特有的随机性,不会出现在某一个特定的位置。为了观测材料在高应变率加载下局部的断裂特征,本文采用预置缺陷的方法,使裂纹自然萌生于观测的视场范围内。利用爆炸膨胀环加载技术来实现对试样环的准一维加载,采用高速分幅相机观测膨胀环中预置圆孔的变形以及萌生裂纹的扩展。并利用LS-DYNA有限元程序再现整个过程。(1)系统回顾调研了断裂研究的历史,分析了目前研究的难点、关注点以及研究方法,针对本课题实际,选择爆炸膨胀环作为加载手段,采用分幅相机作为测试手法,以获得膨胀环在高应变率下的局部断裂特征。(2)为了清晰获得局部断裂特征,对膨胀环预置缺陷,并在试样前放置放大镜。无氧铜的实验发现,不同尺寸的缺陷会导致完全不同的断裂特征。通过改换材料20号钢,成功拍摄到预置圆孔应力集中最大位置处的裂纹萌生,及后期的扩展、止裂现象。(3)针对含预置圆孔的20号钢膨胀环,起爆能量较低时,萌生于圆孔应力集中最大位置的裂纹会在一段时间之后止裂于某一裂纹长度；起爆能量较高时,萌生的裂纹失稳扩展,表现出特有的不稳定性。(4)对于边缘预置缺口的实验,除了预置缺陷位置的裂纹萌生扩展之外,其它位置在内壁或者内部萌生缺陷,随着载荷穿透厚度方向,形成穿透裂纹,共同诱导整体的断裂破坏。(5)实际测量中由于尺度原因往往会将裂纹断面的细节抹平,针对此,提出将裂纹扩展速度区分为测量速度与真实速度。通过这一理念上的区别,可以很简捷的解释极限裂纹扩展速度对其理论极限速度CR的较大偏离。(6)通过对预置圆孔的20号钢膨胀环实验图像处理,得到了可视裂纹尖端的位移及速度随时间的变化。裂纹扩展速度随时间呈现一定的振荡,这体现出其特有的不稳定性。并以此估算裂纹的动态应力强度因子的变化。(7)通过数值模拟探讨预置有圆孔的膨胀环在动态载荷作用下,圆孔的变形、裂纹的萌生以及扩展止裂的全过程。装药的多少决定了膨胀环内壁的载荷历史,由此决定膨胀环是否完全断裂。选择合适的参数,再现了膨胀环中圆孔诱导裂纹的萌生、扩展以及止裂现象。发现裂纹萌生于圆环的内壁,随时间穿透厚度方向,最终形成轴向裂纹。更多还原

【Abstract】 Fracture position and fracture forms of materials and structures have been intensively researched by the academic and engineering circulars. Caused by sample personality, cracks usually initiate at different location under dynamic loading, showing its own randomness。In order to observe the local fracture characteristics of materials under high strain rate, a flaw is preset and the crack initiates naturally in the field range。The technique of expanding ring loading is used to achieve quasi one dimension tensile loading. The high-speed photography technique is serviced to gain the preset hole’s deformation, the crack initiation and crack propagation。At last the whole fracture process is numerically simulated with LS-DYNA 3D finite element code.(1) A review of fracture study is carried out。Current difficulties, focus and methods of the fracture mechanics are researched。The techniques of expanding ring loading and high-speed photography are used to observe the dynamic fracture characteristics for the subject。(2) To obtain the local fracture characteristics, a flaw is preset。In addition, a magnifying glass is placed before the sample。From the experiments of copper, it is found that different flaw sizes caused different fracture forms。The circular hole’s deformation, crack initiation and crack propagation process are obtained, by changing the material.(3) At low explosive power, the crack stopped after a few of time in 20 steel expanding ring with a preset circular hole, while at high explosive power the crack propagating unstably。(4) In addition to the location of crack initiation where preset an extended defect, other cracks initiate in the inner wall or inside for the experiment with a edged notch。As the load changes, defects penetrate through the thickness direction to form a through crack. The fracture induced the destruction of the whole.(5) Details of the crack section are often ignored in actual measurement because of scale effects。It is recommended that the crack velocity can be distinguished as measuring velocity and real velocity。By the difference between these concepts, it is simple to explain why the limit velocity of crack propagation deviated to its theoretical limit speed。(6) According to the image of the 20 steel expanding ring with a preset circular hole, the displacement and the velocity of crack tips are obtained。It is observed that the crack velocity oscillates with time because of the instability。And thus we estimate the dynamic stress intensity factors of the cracks。 (7) The circular hole’s deformation, crack initiation, crack propagation and crack stop are numerically simulated with LS-DYNA 3D finite element code. Macro-crack can be found at the location of the hole under dynamic loading caused by stress concentration. Simulated results are in good agreement with experiments. It is found that crack initiation in the inner wall of the ring, which through the thickness direction。And axial cracks eventually are formed。更多还原