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，因此可能会对材料和结构行为产生重大影响。由于在土体中经常观察到剪切带，因此能够在用于分析和设计的预测模型中捕捉应变局部化的全部影响是岩土工程界非常感兴趣和重要的事情。关键的关联是预测剪切带何时形成的能力，这个狭窄的间断区在材料中如何定向，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不能预测局部化的开始，因为局部化变形的机制还没有得到很好的理解。因此，发展更精确的土体行为数学模型需要对局部化现象有更基本的了解，特别是局部化变形发生和发展的重要因素。这项研究的目的是将最先进的岩土试验技术与先进的有限元建模相结合，以更深入地了解砂土中的应变局部化过程。将使用中尺度建模方法，将试件的响应视为结构响应，并将测量的空间密度变化和其他缺陷(自然的和强加的)结合起来，将试件的响应作为边值问题进行分析。在实验上，被广泛应用于医学应用的X射线计算机层析成像(CT)技术将被用于捕捉砂土平面应变样品的细观密度变化。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，将使用数字图像相关(DIC)来跟踪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，既有强加的，也有自然发生的。通过捕捉所有这些缺陷，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。这一NSF奖将使人们能够识别有助于在砂子中启动应变局部化的重要因素，从而对为什么持续剪切带在颗粒材料中形成的地方产生巨大的洞察力。国家科学基金会的岩土力学和岩土系统司在2002年为第六届国际岩土力学分叉和不稳定性研讨会(IWBI)提供了赞助，这突出表明在这一活跃的研究领域需要工程投入。认识到标准有限元方法的根本缺陷，并发展规避这些困难的技术，对岩土工程师在实践中分析边值问题，特别是在不稳定和软化区域的边值问题具有巨大的影响。此外，在其他领域使用先进的扫描和数据成像技术，例如用于医学和材料科学的技术，将使岩土工程领域与目前的技术平起平坐。拟议的数值研究和实验研究之间的合作将点燃一种更彻底的方法来研究本地化现象。第二个Pi是JHU最近的一名教员，拟议的研究将帮助她在高级岩土实验方面发展一个强大的研究小组，可以为女性和少数族裔提供指导。目前，第一个公共投资支持两名代表性不足的研究生(黑人和西班牙裔)，而第二个公共投资支持她的研究小组中的两名本科生，其中一名是女性。这两所学校都非常有利于院系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。数值研究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.