Collaborative Research: Experimental Imaging-finite Element Modeling of Strain Localization in Granular Soils

应变局部化是颗粒材料非均匀变形的普遍特征。 局部变形通常伴随着整体强度的降低，因此可能对材料和结构行为产生重大影响。 由于剪切带经常在土壤中观察到，因此能够在分析和设计的预测模型中捕获应变局部化的全部影响对岩土工程界具有相当大的兴趣和重要性。 关键的相关性是预测剪切带何时形成的能力，这种狭窄的不连续区域如何在材料内定向，以及剪切带的传播如何影响局部化后的本构响应。 目前，即使是最先进的和良好的校准的数值模型不能预测的局部化的开始，因为控制局部化变形的机制仍然没有得到正确的理解。因此，更准确的土壤行为的数学模型的发展需要更基本的理解的局部化现象，特别是，重要的因素负责的局部化变形的开始和发展。 本研究的目的是结合联合收割机国家的最先进的土工试验技术与先进的有限元建模，以获得更深入的了解应变局部化过程中的砂。 将使用一种中尺度建模方法，该方法将试样响应视为结构响应，并将测量的空间密度变化和其他缺陷（自然和强加的）作为边界值问题来分析试样响应。 在实验上，X射线计算机断层扫描（CT）技术，广泛用于医疗应用，将被用来捕捉平面应变试样的砂中尺度密度变化。 数字图像处理技术将有助于将CT结果作为输入传输到有限元模型中。 最后，数字图像相关（DIC）将用于跟踪整个变形过程中的局部面内位移。 建模将考虑强缺陷和弱缺陷的影响，包括强加的和自然发生的。通过捕捉所有这些缺陷，现有的应变局部化模型应用于实际的边界值问题的潜力，最终可以真正地进行评估。这个NSF奖将使识别的重要因素，有助于启动应变局部化的沙子，产生巨大的洞察力，为什么持久的剪切带形成在他们形成的粒状材料一般。 美国国家科学基金会的地质力学和岩土工程系统部在2002年赞助了第六届地质力学分叉和不稳定性国际研讨会（IWBI），这突出了在这一活跃的研究领域中工程投入的必要性。 认识到标准有限元方法的根本缺陷和技术的发展，以规避这些困难有巨大的影响，岩土工程师如何分析边界值问题在实践中，特别是在政权的不稳定和软化。 此外，使用其他领域中可用的先进扫描和数据成像技术，如医学和材料科学中使用的技术，将使岩土工程领域与当前技术相媲美。 数值和实验研究之间的拟议伙伴关系将点燃一个更彻底的方法来调查本地化现象。第二个PI是最近在JHU的教师成员，拟议的研究将帮助她发展一个强大的研究小组，在先进的岩土工程实验，可以提供指导妇女和少数民族。 目前，第一个PI支持两个代表性不足的研究生（黑人和西班牙裔），而第二个PI支持两个本科生，其中一个是女人，在她的研究小组。 这两所学校都非常有利于部门支持本科生参与研究，并支持代表性不足的学生。 通过与当地高中的研究交流计划，拟议研究的实验室和模拟部分将成为让高中生参与岩土工程和一般研究过程的理想途径。数值和实验研究的结合将为斯坦福大学和约翰霍普金斯大学的研究生提供一种更多方面的研究生教育方法。

Strain localization is a ubiquitous feature of granular materials undergoing nonhomogeneous deformation. Localized deformation typically is followed by a reduction in the overall strength, and thus can have a significant impact on material and structural behavior. Because shear bands are quite often observed in soils, it is of considerable interest and importance to the geotechnical community to be able to capture the full effects of strain localization in predictive models for analysis and design. Of key relevance are the ability to predict when a shear band forms, how this narrow zone of discontinuity is oriented within the material, and how the propagation of the shear band influences the post-localization constitutive response. Currently, even the most advanced and well-calibrated numerical models cannot predict the onset of localization, as the mechanisms governing localized deformation still are not properly understood.The development of more accurate mathematical models of soil behavior thus requires a more fundamental understanding of the localization phenomena; in particular, the important factors responsible for the inception and development of localized deformation. The objective of this research is to combine state-of-the-art geotechnical experimental techniques with advanced finite element modeling to obtain a more thorough understanding of the strain localization process in sands. A meso-scale modeling approach will be used, which will treat specimen response as a structural response and will incorporate the measured spatial density variation and other imperfections (natural and imposed) to analyze the specimen response as a boundary-value problem. Experimentally, the technique of X-Ray Computed Tomography (CT), widely used in medical applications, will be used to capture meso-scale density variations in plane strain specimens of sand. Digital Image processing techniques will aid in transferring of the CT results as input into the finite element models. Finally, Digital Image Correlation (DIC) will be used to track local, in-plane displacements throughout deformation. The modeling will consider effects of both strong and weak imperfections, both imposed and naturally occurring. By capturing all of these imperfections, the potential of existing strain localization models for application to practical boundary-value problems can, finally, truly be assessed.This NSF award will enable identification of important factors that contribute to the initiation of strain localization in sands, yielding tremendous insight as to why persistent shear bands form where they form in granular materials in general. That the Geomechanics and Geotechnical Systems Division of NSF contributed to sponsoring the 6th International Workshop on Bifurcations and Instabilities in Geomechanics (IWBI) in 2002 highlights the need for engineering input in this active research area. Recognition of the fundamental deficiencies of the standard FE method and development of techniques to circumvent these difficulties have immense implications to how geotechnical engineers analyze boundary-value problems in practice, particularly in the regime of instability and softening. Furthermore, the use of advanced scanning and data imaging techniques available in other fields, such as those used in medical and materials sciences, will put the field of geotechnical engineering at parity with current technology. The proposed partnership between numerical and experimental research will ignite a more thorough approach to investigating the localization phenomenon.The second PI is a recent member of the faculty at JHU, and the proposed research will help her to develop a strong research group in advanced geotechnical experimentation that can provide mentoring to women and minorities. Currently the first PI supports two underrepresented graduate students (Black and Hispanic) while the second PI supports two undergraduates, one of whom is a woman, in her research group. Both schools have been very conducive to departmental support of undergraduate involvement in research, and to support of underrepresented students. Through research exchange programs with local high schools, the laboratory and simulation components of the proposed research will serve as ideal avenues to engage high school students in geotechnical engineering and the research process in general. The union of numerical and experimental research will offer Stanford University and Johns Hopkins University graduate students a more multifaceted approach to graduate education.