CAREER: Multi-Scale Experiments of Fracture in Elastic-Plastic Materials

职业：弹塑性材料断裂的多尺度实验

• 批准号: 0134226
• 负责人: Jeffrey Kysar
• 金额: $ 37.42万
• 依托单位: Columbia University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-07-01 至 2007-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0134226&HistoricalAwards=false
• 关键词: CAREER; Multi; Scale; Experiments; Fracture

AbstractThe research and educational program consists of developing a multi-scaleexperimental program to study the mechanics of fracture in elastic-plasticmaterials. The research is intended to complement and guide multi-scalesimulations of fracture in elastic-plastic materials. The experiments willfocus on understanding the behavior of cracks that exist along the grainboundary of symmetric tilt bicrystals of either pure aluminum or purecopper. In such a specimen there is a well known, but poorly understood,phenomenon known as directional dependence of fracture. If a crack isintroduced along the grain boundary, the amount it grows macroscopicallydepends upon the direction within the grain boundary that it propagates. Ajudicious choice of crystallographic orientation and loading techniqueswill ensure that the mechanical properties at the macroscopic length scaleare identical for both cracks, yet the amount of macroscopic growth willbe different. Thus the explanation for the directional dependence mustdepend upon deformation mechanisms at the smaller length scales as well asthe interaction of the deformation mechanisms across length scales.In the experiments, cracks will be introduced and propagated in oppositedirections within the grain boundary of symmetric tilt bicrystals. Thestructure of the asymptotic deformation fields will be measured underplane strain conditions to investigate a certain type of straindiscontinuity, known as kink shear discontinuity, that is predicted bytheory and observed in some materials under certain conditions. Theexistence of kink shear discontinuities depends critically upon plasticconstitutive relations at very small length scales. The next set ofexperiments will be to measure the directional dependence of fractureunder both a monotonic and cyclic loading to document the degree of crackgrowth in different directions along a grain boundary. The final set ofexperiments will measure the lattice curvature of the crystal close to thecrack tip to ascertain the density of geometrically necessary dislocationswhich play an important role in strain gradient plasticity. All theexperiments will be simulated with the discrete dislocation plasticitytechnique in collaboration with other researchers.The educational component is to reach out to secondary schools in theneighborhoods around Columbia University, particularly in Harlem, todevelop a science module that is suitable for students in their final twoyears of high school with the goal of inspiring them to continue theireducations. The module will demonstrate that materials are made ofdiscrete atoms by discussing the phenomenon of diffraction. To do so, thestudents will first gain intuition into the diffraction phenomenon byidentifying and matching symmetries in diffraction gratings and therelated diffraction patterns that are created with a standard laserpointer. Then the crystallographic aspects of face-centered cubic metalsand the symmetries of a cube will be introduced. Finally the students willbe asked to identify the two-fold, three-fold, and four-fold rotationsymmetries in separate Laue back reflection x-ray diffraction patterns andcorrelate the patterns with the crystallographic orientations that exhibitthe same sets of symmetries. Thus the students will identify thesymmetries inherent in crystals without having to understand any of thedetails of the diffraction process.

该研究和教育项目包括开发一个多尺度实验项目，以研究弹塑性材料的断裂力学。该研究旨在补充和指导弹塑性材料断裂的多尺度模拟。实验将着重于了解纯铝或纯铜的对称倾斜双晶晶界上存在沿着裂纹的行为。在这样的试样中，有一个众所周知的，但了解甚少的现象，称为断裂的方向依赖性。如果裂纹是沿着晶界引入的，那么它的宏观增长量取决于它在晶界内传播的方向。晶体学取向和加载技术的明智选择将确保两种裂纹在宏观长度尺度上的力学性能相同，但宏观增长量不同。因此，方向依赖性的解释必须依赖于在较小的长度尺度的变形机制，以及跨长度尺度的变形机制的相互作用。渐近变形场的结构将在平面应变条件下测量，以研究某种类型的应变不连续性，称为扭结剪切不连续性，这是由理论预测的，并在某些材料中观察到在某些条件下。扭结剪切不连续面的存在主要取决于塑性本构关系在很小的长度尺度。下一组实验将是测量在单调和循环载荷下裂纹的方向依赖性，以记录裂纹沿沿着晶界在不同方向上的扩展程度。最后一组实验将测量靠近裂纹尖端的晶体的晶格曲率，以确定在应变梯度塑性中起重要作用的几何必要位错的密度。所有的实验都将与其他研究人员合作，用离散位错塑性技术进行模拟。教育部分将深入到哥伦比亚大学周边的中学，特别是哈莱姆，开发一个适合高中最后两年学生的科学模块，目的是激励他们继续接受教育。该模块将通过讨论衍射现象来证明材料是由离散原子组成的。要做到这一点，学生将首先获得直观的衍射现象，通过识别和匹配对称性的衍射光栅和相关的衍射图案，是用标准的激光笔创建。然后介绍面心立方金属的晶体学方面和立方体的对称性。最后，学生将被要求在不同的劳厄背反射X射线衍射图中识别两重、三重和四重旋转对称性，并将这些衍射图与具有相同对称性的晶体学取向相关联。因此，学生将确定晶体中固有的对称性，而不必了解衍射过程的任何细节。