时间：12月5日16:00—17:30
　　地点：机械馆J3
　　题目：镁合金畸变强化本构模型
　　内容简介：
　　 织构演化引起的变形镁合金宏观各向异性无法通过各向同性强化和随动强化有效模拟。基于率无关塑性力学，显微组织转变导致的屈服面宏观各向异性演化被称为畸变强化，其在非比例加载情况下更加突出。基于超弹塑性框架和本构方程客观时间导数，修改了三个畸变强化模型，并验证了热力学自洽性。其中仅有一个模型在一定条件下满足热力学自洽，但是无法模拟变形镁合金各向异性力学性能。基于塑性势、非关联流动法则和广义正交法则，建立了新的有限变形本构模型，该模型自然满足热力学和变分自洽，其畸变强化由广义 Armstrong-Frederick本构关系定义。新模型可以模拟AZ31板材各向异性，并通过用户自定义程序嵌入ABAQUS有限元模型。
　　报告人： 石宝东
　　2008年获得当年国家留学基金委公派留学德国资格
　　2009年2月至2012年8月，在中国国家留学基金委和德国亥姆霍兹联合会共同资助下，在德国亥姆霍兹格斯塔赫特研究中心攻读博士学位
　　2012年11月，获得德国多特蒙德工业大学工学博士学位，研究方向：镁合金畸变强化本构模型
　　2012年9月至2013年8月，在德国多特蒙德工业大学机械学院从事博士后研究工作

科研与研究生科
2013年12月2日

　　Time: 5th December 16:00—17:30
　　Site: Machinery hall J3
　　Title： Macroscopic modeling of distortional hardening in polycrystals: application to magnesium alloys
　　Abstract：
　　Texture evolution in metals due to rotation of the atomic lattice results in a complex macroscopic mechanical behaviour which cannot in general be reasonably captured by only classical isotropic or kinematic hardening. Focusing on standard rate-independent plasticity, the evolution of microstructure leads to an evolving macroscopic anisotropy of the yield surface, also known as distortional or differential hardening. This effect is very important, particularly if non-radial loading paths such as those associated with forming processes are to be numerically analyzed.
In the present work, different existing distortional hardening models are critically reviewed. They are reformulated into the modern framework of hyperelastoplasticity and the same objective time derivative is applied to all evolution equations for a better comparison. Furthermore, since the original models are based on a yield function not accounting for the different mechanical responses between tension and compression as observed in metals showing a hexagonal close-packed atomic structure, respective generalizations are also discussed. It is shown that only one of the extended models can fulfill the second law of thermodynamics. That model predicts a high curvature of the yield surface in the loading direction, while the opposite region of the yield surface is rather flat. Such a response can indeed be observed for some materials. In the case of magnesium alloys, however, that does not seem to be true. Therefore, a new constitutive model is presented. Its underlying structure is surprisingly simple and the model is not only thermodynamically consistent but also variationally consistent. Conceptually, distortional hardening is described by an Armstrong-Frederick-type evolution equation. The calibrated new model is implemented in a finite element framework and its predictive capabilities are demonstrated.
　　Speaker:Dr. -Ing. Baodong Shi
　　2009.02-2012.08 Study at Helmholtz Zentrum Geesthacht for PhD funded by CSC (China Scholarship Council) and German Helmholtz Associations
　　2012.11 Granted Dr. -Ing. by TU Dortmund
　　2012.09-2013.08 Post-doc research work at TU Dortmund.