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

应变局部化是粒状材料在非均匀变形过程中普遍存在的特征。局部变形通常伴随着整体强度的降低，因此会对材料和结构行为产生重大影响。由于在土壤中经常观察到剪切带，因此能够在分析和设计的预测模型中捕捉应变局部化的全部影响对岩土工程界来说是相当重要的。关键的相关性是预测剪切带何时形成的能力，这个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，即使是最先进和校准良好的数值模型也无法预测局部化的开始，因为控制局部变形的机制仍然没有得到很好的理解。因此，发展更精确的土壤行为数学模型需要对局部化现象有更基本的理解；特别是局部变形产生和发展的重要因素。本研究的目的是将最先进的岩土试验技术与先进的有限元模型相结合，以更全面地了解砂土中的应变局部化过程。将使用中尺度建模方法，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）结合起来，将试样响应作为边值问题进行分析。在实验上，广泛应用于医学应用的x射线计算机断层扫描（CT）技术将被用于捕捉砂土平面应变试样的中尺度密度变化。数字图像处理技术将有助于将CT结果输入到有限元模型中。最后，数字图像相关（DIC）将用于在整个变形过程中跟踪局部平面内位移。建模将考虑强缺陷和弱缺陷的影响，无论是强加的还是自然发生的。通过捕获所有这些缺陷，现有应变局部化模型应用于实际边值问题的潜力最终可以得到真正的评估。美国国家科学基金会的这一奖项将有助于识别导致砂土中应变局部化的重要因素，从而深入了解为什么在颗粒材料中通常会形成持久的剪切带。2002年，美国国家科学基金会（NSF）地质力学和岩土工程系统部赞助了第六届地质力学分岔和不稳定性国际研讨会（IWBI），这突显了在这一活跃研究领域对工程投入的需求。认识到标准有限元方法的根本缺陷和开发技术来规避这些困难，对岩土工程师在实践中如何分析边值问题，特别是在不稳定和软化的情况下，具有巨大的意义。此外，在其他领域使用先进的扫描和数据成像技术，例如在医学和材料科学中使用的技术，将使岩土工程领域与现有技术并驾齐驱。数值和实验研究之间的拟议伙伴关系将点燃更彻底的方法来研究局部化现象。第二位PI是JHU最近的一名教员，拟议的研究将帮助她在高级岩土工程实验方面建立一个强大的研究小组，为妇女和少数民族提供指导。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，其中一个是女性，在她的研究小组中。这两所学院都非常有利于院系对本科生参与研究的支持，以及对代表性不足的学生的支持。通过与当地高中的研究交流项目，所提议的研究的实验室和模拟组件将成为吸引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.