Collaborative research:
Experimental imaging-finite element modeling of strain localization in granular
soils
Principal Investigators: Ronaldo
I. Borja (
Project Sponsor: National Science Foundation
Project Summary
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 Sixth
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
.