講座題目：提高抵抗裂紋擴展能力：韌性、粗糙度與微結構設計

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

報 告 人：Alan Needleman

時 　 間：2019年10月21日(周一)15:00-17:00

地 　 點：中關村校區研究生教學樓101報告廳

主辦單位：研究生院、先進結構技術研究院

報名方式：登錄北京理工大學微信企業號---第二課堂---課程報名中選擇“【百家大講堂】第247期：提高抵抗裂紋擴展能力：韌性、粗糙度與微結構設計”

【主講人簡介】

  Alan Needleman，美國德克薩斯A&M大學材料科學與工程學院特聘教授。1966年于賓夕法尼亞大學獲得學士學位，1971年于哈佛大學在J.W. Hutchinson教授指導下獲得固體力學博士學位。Needleman教授曾先后于麻省理工學院、布朗大學等任教，并于2015年加入美國德克薩斯A&M大學擔任特聘教授。其已發表學術論文300余篇，涉及結構材料的變形與斷裂模擬，孔洞形核、生長和交匯引起的延性斷裂，晶體材料塑性變形的多尺度模擬，時間相關和率相關的塑性流動模擬，塑性材料中裂紋擴展以及動態裂紋擴展等領域。他于1989年當選美國機械工程學會會士、1995年當選美國力學學會會士，并于2000年當選美國工程院院士、2006年當選美國人文與科學院院士，于2018年當選 ASME榮譽會員。

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.

【講座信息】

  在斷裂的力學與物理領域中存在兩個基本問題：一、材料微觀結構特征與其抵抗裂紋擴展的能力之間存在怎樣的關系？二、材料微觀結構特征與斷裂表面的粗糙度之間存在怎樣的關系？而由此可以提出另外一個問題：材料抵抗裂紋擴展的能力和斷裂表面粗糙度之間是否存在對應關系？1984年，Mandelbrot及其同事發現斷裂表面表現出自仿射、類分形的特征。基于這一觀測結果以及圖像分析的進展，物理學界進行了大量對斷裂表面粗糙度的定量表征工作，試圖將分形維數與抵抗裂紋擴展能力聯系起來。盡管這一嘗試在當時沒有成功，但從它出發可以提出一個基本問題，即斷裂表面粗糙度的何種度量（如果存在的話）可以與材料抵抗裂紋擴展能力建立聯系。基于對延性斷裂問題的模擬，主講人Needleman教授提出了一種斷裂表面粗糙度的統計度量，該度量可以與抵抗裂紋擴展能力建立定量關系，并且也可以與可測、可控的微觀結構特征建立聯系。模擬中考慮了兩種理想化的微觀結構：一種涉及穿過分布式第二相顆粒的裂紋擴展，另一種涉及沿晶界的裂紋擴展。最后將討論通過材料微觀結構設計來提高其抵抗裂紋擴展的能力。

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.