讲座题目：提高抵抗裂纹扩展能力：韧性、粗糙度与微结构设计

Toughness, Roughness and the Possibility of Microstructure Design for Improved Crack Growth Resistance

报 告 人：Alan Needleman

时 　 间：2019年10月21日(周一)15:00-17:00

地 　 点：中关村校区研究生教学楼101报告厅

主办单位：研究生院、先进结构技术研究院

报名方式：登录北京理工大学微信企业号---第二课堂---课程报名中选择“【百家大讲堂】第247期：提高抵抗裂纹扩展能力：韧性、粗糙度与微结构设计”

【主讲人简介】

  Alan Needleman，美国德克萨斯A&M大学材料科学与工程学院特聘教授。1966年于宾夕法尼亚大学获得学士学位，1971年于哈佛大学在J.W. Hutchinson教授指导下获得固体力学博士学位。Needleman教授曾先后于麻省理工学院、布朗大学等任教，并于2015年加入美国德克萨斯A&M大学担任特聘教授。其已发表学术论文300余篇，涉及结构材料的变形与断裂模拟，孔洞形核、生长和交汇引起的延性断裂，晶体材料塑性变形的多尺度模拟，时间相关和率相关的塑性流动模拟，塑性材料中裂纹扩展以及动态裂纹扩展等领域。他于1989年当选美国机械工程学会会士、1995年当选美国力学学会会士，并于2000年当选美国工程院院士、2006年当选美国人文与科学院院士，于2018年当选 ASME荣誉会员。

Alan Needleman is the Distinguished Professor of Department of Materials Science & Engineering at Texas A&M University. He earned a B. S. at the university of Pennsylvania. He completed his Ph.D. in solid mechanics at Harvard University under the supervision of Professor J. W. Hutchinson. He used to work at MIT and Brown University and he joined the Texas A&M university as the Distinguished Professor. He has published over 300 scientific papers on such subjects as computational modeling of deformation, fracture processes in structural materials, ductile fracture by void nucleation, growth and coalescence, multi-scale modeling of plastic deformation of crystalline solids, modeling of time and rate dependent plastic flow, crack growth in plastically deforming solids and dynamic crack growth. In 1989, he was advanced to Fellow grade in the American Society of Mechanical Engineers. In 1995, he was elected as fellow of American Academy of Mechanics. In 2000, Needleman was elected to the U.S. National Academy of Engineering. In 2006, he was elected as a member of American Academy of Arts & Sciences. In 2018, he was elected as Honorary Member of ASME.

【讲座信息】

  在断裂的力学与物理领域中存在两个基本问题：一、材料微观结构特征与其抵抗裂纹扩展的能力之间存在怎样的关系？二、材料微观结构特征与断裂表面的粗糙度之间存在怎样的关系？而由此可以提出另外一个问题：材料抵抗裂纹扩展的能力和断裂表面粗糙度之间是否存在对应关系？1984年，Mandelbrot及其同事发现断裂表面表现出自仿射、类分形的特征。基于这一观测结果以及图像分析的进展，物理学界进行了大量对断裂表面粗糙度的定量表征工作，试图将分形维数与抵抗裂纹扩展能力联系起来。尽管这一尝试在当时没有成功，但从它出发可以提出一个基本问题，即断裂表面粗糙度的何种度量（如果存在的话）可以与材料抵抗裂纹扩展能力建立联系。基于对延性断裂问题的模拟，主讲人Needleman教授提出了一种断裂表面粗糙度的统计度量，该度量可以与抵抗裂纹扩展能力建立定量关系，并且也可以与可测、可控的微观结构特征建立联系。模拟中考虑了两种理想化的微观结构：一种涉及穿过分布式第二相颗粒的裂纹扩展，另一种涉及沿晶界的裂纹扩展。最后将讨论通过材料微观结构设计来提高其抵抗裂纹扩展的能力。

Two fundamental questions in the mechanics and physics of fracture are: (i) What is the relation between observable features of a material's microstructure and its resistance to crack growth? and (ii) What is the relation between observable features of a material's microstructure and the roughness of the fracture surface? An obvious corollary question is: What is the relation, if any, between a material's crack growth resistance and the roughness of the corresponding fracture surface? In 1984, Mandelbrot and co-workers showed that fracture surfaces exhibit self-affine, fractal-like scaling properties. This observation, together with advances in image analysis, precipitated a significant body of work in the physics community on the quantitative characterization of fracture surface roughness with the aim of relating the fractal dimension to crack growth resistance. While this effort was not successful, it raised the question of what measure, if any, of fracture surface roughness can be related to crack growth resistance. I will describe work on modeling ductile fracture that reveals a measure of the statistics of fracture surface roughness that can be quantitatively related to crack growth resistance and how this quantity relates to a measurable and (hopefully) controllable microstructural feature. Simulation results for two idealized microstructures will be discussed: one microstructure involves crack growth through a distribution of second phase particles and the other involves crack growth along grain boundaries. The implications for designing material microstructures with improved crack growth resistance will be discussed.