Richard A. Regueiro
Professor
Department of Civil, Environmental, and Architectural Engineering
University of Colorado Boulder
richard.regueiro@colorado.edu
ph: 303.492.8026
fx: 303.492.7317
1111 Engineering Dr.
428 UCB, ECOT 441
Boulder, CO 803090428
curriculum vitae
OrcID Webpage



Summary 

Our teaching and research focus on computational multiscale multiphysics materials modeling for simulating inelastic
deformation and failure in heterogeneous porous media, including saturated and partially saturated soils and
rock, unbonded
particulate materials (e.g., sand, gravel, or metallic powders), bonded
particulate materials (e.g., sandstone, asphalt, concrete, ceramics, energetic materials,
...), soft biological tissues (e.g., ocular lens tissue), and thin
deformable porous materials and membranes, for instance.
Scales of interest range from the microstructural/histological to the continuum.
Accounting for microstructural features and response at the pore/particle/grain scale is critical to
understanding and modeling predictively a material's inelastic deformation and transition to failure at the
continuum scale (engineering scale of interest).
Accounting for histological features and response at the cellular/extracellular matrix (ECM) scale likewise is
critical to
understanding and modeling predictively a biological tissue's range of response under physiological
and surgical
influences as well as those encountered in the presence of prosthetic materials.


Courses 


Current Research Projects 

Center for Micromorphic Multiphysics Porous and Particulate Materials Simulations within Exascale Computing Workflows
micromorph.gitlab.io
(DOE NNSA PSAAP, 9/209/25)
The overall objective of the Multidisciplinary
Simulation Center (MSC) is to
simulate with quantified uncertainty, from
poreparticletocontinuumscales, a class
of problems involving granular flows, large
deformations, and fracture/fragmentation of
unbonded and bonded particulate materials.
The overarching problem is the processing
and thermomechanical behavior of
compressed virgin and recycled mock
high explosive (HE) material subjected to
quasistatic and high strainrate, confined
and unconfined compression, insitu
quasistatic Xray computed tomography,
and dynamic (impact) experiments with
ultrafast synchrotron Xray imaging at the
Advanced Photon Source (APS) at Argonne
National Laboratory.
To accomplish the objective, a
micromorphic, multiphysics, multiscale
computational framework will be
developed, verified, and validated with
quantified uncertainty and executed on
exascale computing platforms seamlessly
through a scientific software workflow
to reduce the fulltimeequivalent effort
on handling data from the beginning
to the end of simulation. Machine
Learning (ML) algorithms will be
applied to fill the gaps in multiscale,
constitutive modeling via coordinated
poreparticlescale experiments and
Direct Numerical Simulations (DNS).
Integrated experimental testing at quasistatic
and dynamic rates (including
ultrafast synchrotron xray imaging at the
APS and proton imaging at Los Alamos
National Laboratory's pRad facility) at
length scales ranging from poreparticle
to continuum scales will be conducted to
validate heterogeneous, poreparticletocontinuum
scale, computational models,
calibrate model parameters, and validate
the overall computational framework.
Exascale computing is needed to simulate
these more sophisticated micromorphic,
multiphysics, bridgedDNS simulations,
with offline ML training of micromorphic
constitutive relations to DNS. Furthermore,
for validation and uncertainty quantification
(UQ) requiring multiple instances of these
simulations over statistical distributions of
inputs (such as particle size distribution),
with high and low fidelity, Exascale
computing is a necessity.
(CoDirectors: J. Brown, A. Clarke (Mines), A. Doostan, R. Regueiro (PI), H. Tufo, 9/209/25)

Range of length scale versus time scale for experiments and modeling, and spatial and temporal resolutions of simulations.


Tahoe development:
For some of these projects, we
used a researchoriented, opensource, versioncontrolled, parallel
execution, modularized, highly flexible C++ code called
Tahoe


Group Members 
 Visiting Scientists
 Research Associates (and Postdoctoral Researchers)
 PhD Students

Thomas Allard: TBD, Fall 2020  present.

Zach Irwin (Mechanical Engineering): TBD, Summer 2020  present.

YuHsuan Lee: TBD, Spring 2019  present.

Arash Mehraban (Computer Science):
Efficient Residual and Matrixfree Jacobian Eval
uation for ThreeDimensional Hexahedral Finite Elements with NearlyIncompressible
NeoHookean Hyperelasticity as applied to Soft Materials,
Fall 2017  Spring 2021. Primary advisor, H. Tufo.

Dhafer Jadaan:
Poromechanical Cohesive Interface Element for Crack Propagation in
Fluid Saturated Porous Media,
Spring 2017  Fall 2021. Coadvisor, J.H. Song.

Nathan Miller:
A Micromorphic LengthScale Coupling Framework for the Determina
tion of HigherOrder Constitutive Models and the MultiScale Simulation of Heterogeneous Materials, Fall 2016  Fall 2021.

Douglas Fankell (Mechanical Engineering): A ThermoPoromechanics Finite Element Model for Predicting Arterial Tissue
Fusion, Fall 2013  Summer 2017. Primary advisor: M. Rentschler.

Farhad Shahabi: Finite strain micromorphic elasticity, elastoplasticity, and
dynamics for multiscale finite element analysis, Fall 2012  Spring 2017.

Erik Jensen: Hierarchical multiscale modeling to inform continuum
constitutive models of soils, Spring 2013  Spring 2017.

Boning Zhang: Grainscale computational modeling of quasistatic and
dynamic loading on natural soils, Fall 2012  Fall 2016.

Michael Stender (Mechanical Engineering):
Modeling the Osteochondral Interface, Fall 2012  Summer 2015.
Primary advisor: V. Ferguson.

Wei Wang: Coupled thermoporomechanical axisymmetric finite
element modeling of soilstructure interaction in partially
saturated soils, Fall 2009  Spring 2014.

Volkan Isbuga: Finite strain micromorphic finite element analysis of elastoplastic geomaterials, Spring 2008  Spring
2012.

Jaehong Kim: Plasticity modeling and finite element analysis of partiallysaturated soils, Spring 2006  Fall 2010.

Beichuan Yan: 3D discrete element modeling of granular materials and its coupling with finite element method, Spring 2006
 Summer 2008.
 MS Students

Bharath Goda (Mechanical Engineering): Dynamic large deformation finite element analysis of soft biological tissues modeled as multiphase
mixtures, 5/18  8/19.

Erik Jensen: Analysis of Drain Effectiveness and Implications for Failure Probability for Concrete Gravity Dams, 5/11 
3/13.

John Sweetser (Mechanical Engineering): Fracture Model for Fluid Saturated Geomaterials Implemented Via a
PoroElastoPlastic Cohesive Surface Finite Element, 6/11  11/12.

Christopher Bay (Mechanical Engineering): Biomechanics and Ultrastructure of the Ocular Lens, 9/10  7/12.

Yevgeniy Kaufman: The Influence of Representative Volume Element Size, Soil Fabric, and Interparticle ElastoPlasticity in
Threedimensional Ellipsoidal Discrete Element Modeling of Granular Assemblies, 9/11  6/12.

Michael Turnquist: Investigation of NonLinear Ductile Tearing Methodology as Applied to a LeakBeforeBreak Assessment,
9/11  12/11, Master's Report.

Garrett Sutley: Computational modeling of shock loading of soil cylinders. Fall 2006  Summer 2009.

Davoud Ebrahimi: Threedimensional finite element implementation for a dynamic solidfluid mixture at finite strain.
Spring 2006  Summer 2007.

Logan Williams (Mechanical Engineering): Optimized finite element analysis of unconfined compression of porcine ocular lenses, 11/067/07.
 Undergraduate Students

Chad Hotimsky (Aerospace Engineering Sciences): Experiments and finite element analysis of unconfined compression for
porcine ocular lenses, Undergraduate Researcher/Discovery Learning Apprentice, 5/125/13.

Srinidhi Radhakrishnan (Chemical and Biological Engineering): Confocal laser scanning microscopy imaging of ocular
lens fiber cells, Discovery Learning Apprentice, 8/115/12.

Saikripa Radhakrishnan (Chemical and Biological Engineering):
Cryoelectron microscopy and tomography imaging of type IV collagen meshwork in ocular lens capsules, Discovery Learning
Apprentice, 8/115/12.

Austin Nossokoff: Threedimensional ellipsoidal discrete element modeling of sand, NSF REU student and Discovery Learning
Apprentice, 6/115/12.

Thomas Buzbee (Applied Math and Computer Science): Threedimensional discrete element simulations of unbound
particulate materials, Discovery Learning Apprentice, 8/085/09.

Laura Hatanaka (Mechanical Engineering): Experiments for understanding ocular lens tissue mechanics, Discovery Learning
Apprentice, 8/085/09.

Kauai Alpha: Threedimensional discrete element modeling of granular materials, 10/0712/07.

Kristin Constancio (Chemical and Biological Engineering): Unconfined compression experiments on the ocular lens,
UROP, 8/065/07.

Adam Blanchard (Aerospace Engineering): Experiments and computations for simulating unconfined compression of the
ocular lens, 1/068/06.
 Visiting Students


Graduate Student Recruitment 

We are always looking for talented and motivated graduate students to join our research group.
Depending on your background, you can apply either to the (1)
Engineering Science
graduate program, or
(2)
Geotechnical Engineering and Geomechanics
graduate program. Check the main
CEAE webpage
for further
information on these programs and how to apply. Feel free to
email me
if you are interested in working
in our research group. As you can see from this webpage, the research is in computational multiscale multiphysics
mechanics, so you should have a strong interest (and, ideally, background) in mathematics, computer programming, constitutive
modeling, continuum mechanics, and in general
solving challenging engineering analysis problems, in order to work in our group.


Publications


Journal Articles (refer to
OrcID Webpage)
 Conference/Workshop Papers (peerreviewed, and nonpeerreviewed)

Regueiro, R.A., Zhang, B., Shahabi, F. (2014)
Micromorphic continuum stress measures calculated from threedimensional ellipsoidal discrete element simulations on granular media.
Geomech. Micro Macro  Proc. TC ISSMGE Int. Symp. Geomech. Micro Macro, ISCambridge, p195200.

Kim, J., Park, S.W., Regueiro, R.A. (2014)
Implementation of coupled finite element analysis for partially saturated
soil slope stability.
Unsaturated Soils: Res. Appl.  Proc. Int. Conf. Unsaturated Soils, UNSAT2014.

Wang, W., Regueiro, R.A., McCartney, J.S. (2014) Coupled ThermoPoroMechanical Finite Element Analysis of an
Energy Foundation Centrifuge Experiment in Partially Saturated Silt, pg 26752684, GeoCongress 2014, Atlanta, GA, Feb 2014.

Regueiro, R.A., Pak R., Sture, S., Vasilyev, O., McCartney, J., Yan, B., Duan, Z., Kasimov, N., Hansen, C., Svoboda, J., Mun, W.J.,
BrownDymkoski, E., Li, S., Ren, B., Alshibli, K., Druckrey, A., Lu, H., Luo, H., Brannon, R., BonifasiLista, C., Yarahmadi, A.,
Ghodrati, E., Colovos, J. (2013) ONR MURI Project on Soil Blast Modeling and Simulation, pg 341353,
B. Song et al. (eds.), Dynamic Behavior of Materials, Volume 1: Proceedings of the 2013 Annual Conference on Experimental and
Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series.

Regueiro, R., Sweetser, J., Jensen, E., and Wang, W. (2013) Poromechanical cohesive surface element with
elastoplasticity for modeling cracks and interfaces in saturated geomaterials. Poromechanics V: pp. 891898.

Regueiro, R. (2013) Dynamic finite strain biphasic poromechanics of a `thin' poroelastic layer. Poromechanics V: pp. 21842192.

Wang, W., Regueiro, R.A., Stewart, M., McCartney, J.S. (2012) Coupled ThermoPoroMechanical Finite Element
Analysis of an Energy Foundation Centrifuge Experiment in Saturated Silt, pg 44064415, GeoCongress 2012, Oakland, CA, March, 2012.

Regueiro, R.A., Bammann, D.J., Marin, E.B., Johnson, G.C. (2011)
Finite deformation elastoplasticity for rate and temperature dependent polycrystalline metals.
ASME Int. Mech. Eng. Congr. Expo., IMECE. 8:111123.

Regueiro, R.A., Yan, B. (2011) Coupling discrete elements and micropolar continuum through an
overlapping region, GeoFrontiers, Dallas, TX, March, 2011.

Regueiro, R.A., Yan, B. (2011) Concurrent multiscale
computational modeling for dense dry granular materials interfacing
deformable solid bodies, pg 251273. Bifurcations, Instabilities
and Degradations in Geomaterials, Eds. R. Wan, M. Alsaleh, J. Labuz,
Springer Series in Geomechanics and Geoengineering, SpringerVerlag,
Berlin.

Regueiro, R.A., Yu, S.Y. (2010) Comparison between elastoplastic and rigidplastic cohesive surface elements and embedded strong discontinuity finite element implementation of rock fracture, 44th US Rock
Mechanics Symposium and 5th U.S.Canada Rock Mechanics Symposium, Salt Lake City, UT, June 2730, 2010.

Regueiro, R.A. (2009)
Dynamic strain localization in a simple saturated geomaterial at finite strain. PoroMechanics IV,
DEStech Pub, Inc., 11151120.
 Regueiro, R.A. (2007)
Coupling particle and continuum regions of particulate materials. 2007 ASME International Mechanical
Engineering Congress and Exposition, Seattle, WA.
IMECE200742717.
 Regueiro, R.A. (2006)
Embedded discontinuity finite element modeling of threedimensional strong discontinuities in rocks.
Golden Rocks 2006, Golden, CO. ARMA/USRMS 061069.
 Manzari, M.T., and Regueiro, R.A. (2004) Gradient plasticity modeling of geomaterials in a
meshfree environment, Proceedings of the 16th ASCE Engineering Mechanics Conference, University of Delaware.
 Regueiro, R.A., Foster, C.D., Fossum, A.F., Borja, R.I. (2004)
Bifurcation analysis of a threeinvariant, isotropic/kinematic hardening cap
plasticity model for geomaterials. Gulf Rocks 2004, Houston, TX.
ARMA/NARMS 04520.
 Regueiro, R.A. (2002) A finite deformation coupled isotropic damage anisotropic plasticity model and its numerical implementation in explicit finite element and finite difference codes,
Plasticity, Damage, and Fracture at Macro, Micro, and Nano Scales, A.S. Kahn and O.
LopezPamies, eds., NEAT Press, 741743.
 Regueiro, R.A., Foster, C.D., Borja, R.I. (2002) Three dimensional modeling of slip surfaces in geomaterials, Proceedings of the 15th ASCE Engineering Mechanics Conference, Columbia University,
New York, NY, CDROM.
 Regueiro, R.A., and Horstemeyer, M.F. (2000) CTH analysis of Tantalum EFP formation using the BCJ model, Advances in Computational Engineering & Sciences, S.N. Atluri and F.W. Brust, eds., Tech
Science Press, 384389.
 Regueiro, R.A., Lai, T.Y., and Borja, R.I. (1998)
Computational modeling of strain localization in soft rock,
The Geotechnics of Hard Soils  Soft Rocks, Evangelista and Picarelli,
eds., Balkema, 789797.
 Regueiro, R.A., and Borja, R.I. (1997) Continuum finite element analysis of
strain localization in slopes,
Numerical Models in Geomechanics (NUMOG VI), G.N. Pande and S. Pietruszczak, eds., A.A. Balkema,
213219.
 Reports

Regueiro, R.A. (2013) Computational overlap coupling between micropolar linear elastic continuum finite elements and nonlinear
elastic spherical discrete elements in one dimension, Army Research Laboratory, ARLCR0710, January 2013, 58 pgs.
 Regueiro, R.A. (2010) Nonlinear micromorphic continuum mechanics and finite strain
elastoplasticity, Army Research Laboratory, ARLCR0659, November 2010
 Regueiro, R.A., Fossum, A.F., Jensen, R.P., Foster, C.D., Manzari, M.T., and Borja, R.I.
(2005) Computational modeling of fracture and fragmentation in geomaterials, SAND20055940, Sandia
National Laboratories.
 Regueiro, R.A. (1998)
Finite Element Analysis of Strain Localization in Geomaterials taking a Strong
Discontinuity Approach, Ph.D. Thesis, Department of Civil and Environmental Engineering, Stanford
University, advisor: Prof. R.I. Borja.


Completed Research Projects 

Large Deformation Total Lagrangian Biphasic Poromechanical Cohesive Interface Element (CIE) for Crack
Propagation in FluidSaturated Porous Media,
(Sandia National Laboratories, 10/199/21)
The work shall use a wellknown finite element analysis code (Abaqus) to provide
benchmarking data for ongoing efforts at SNL to couple together Peridigm and PFLOTRAN
simulations for modeling of dynamically evolving fractures in the presence of water.
Work at the
university will involve a
twodimensional
(2D), plane strain, biphasic (solid skeleton, pore fluid), cohesive interface element (CIE)
method along with bulk quadrilateral element at small strain using the User Element (UEL) in Abaqus
Standard (monolithicallycoupled, NewtonRaphson nonlinear iterative solution) developed by D. Jadaan. Critically, the CIE
formulation and implementation allow naturally for variable aperature cracks (initial, nucleated, and
evolving) that significantly affect the poromechanical properties of the porous solid (e.g., geologic
media including jointed rock, or built infrastructure materials such as fractured concrete).
(Sandia National Laboratories, NM (Jessica Rimsza), CA (Reese Jones), CU Boulder (R. Regueiro), 10/199/21)

Biphasic solidfluid mixture porous continuum B0 with discontinuity domain Sdomain0 in the reference
configuration.


Integrated Experimental  Computational Multiscale Immersed ParticleContinuum Approach to
Modeling and Simulation of Multiphase Soil Failure Mechanics under Buried Explosive Loading
(ONRMURI N000141110691, 8/119/21).
Current computational modeling methods for simulating blast and ejecta in soils resulting
from the detonation of buried explosives rely heavily on continuum approaches such as
Arbitrary LagrangianEulerian (ALE) and pure Eulerian shockphysics techniques. These
methods approximate the soil as a Lagrangian solid continuum when deforming (but not flowing)
or an Eulerian nonNewtonian fluid continuum when deforming and flowing at high strain rates.
These two extremes do not properly account for the transition from solid to fluidlike
behavior and vice versa in soil, nor properly address advection of internal state variables
and fabric tensors in the Eulerian approaches. To address these deficiencies on the
modeling side, we will develop a multiscale multiphase hybrid Lagrangian particlecontinuum
computational approach, in conjunction with coordinated laboratory experiments for parameter
calibration and model validation.
MURI Team: University of Colorado Boulder (PI Regueiro, CoPI Pak, McCartney, Sture,
Vasilyev); University of California, Berkeley (Li); University of Texas, Dallas (Lu);
University of Tennessee, Knoxville (Alshibli); University of Utah (Brannon)

2D crosssectional illustrations of 3D simulation setups for geotechnical centrifuge
validation experiments.
Scale I: poregrainscale numerical modeling of soil with clay, silt/sand grains, and pore
air and water; concurrent multiscale coupling of continuum with open poregrainscale
domain around buried explosive. Scale II: hierarchical continuum constitutive model
informed from Scale I;
highstrainrate, large deformation MPM implementation, and triphasic continuum formulation
and implementation.


Collaborative Research: ThreeDimensional Assessment of Stresses and Fracture Behavior in Sand
(NSFCMMI 1361267, PI K. Alshibli, University of
Tennessee, Knoxville, CoPIs R. Regueiro, P. Kenesei; started August 2014, ended July 2018).
My involvement in this research project is to develop a computational model at finite strain of the crystal elasticity and fracture
mechanics of individual silica quartz particles to estimate interparticle forces, and upscaling interparticle force estimates and particle
geometry information (contacts, centroids, branch vectors) to approximate granular stress and strain measures at finite strain, for
generality.


Analysis of Drain Effectiveness and Implications for Failure Probability for Concrete Gravity Dams.
(USACE W912P808D002, PI R. Regueiro, CoPIs B. Amadei, R. Pak; completed).
The U.S. Army Corps of Engineers (USACE) has many large concrete gravity structures associated with dams and related works. Some fail to
meet or marginally meet design guidelines. This is particularly true when tensile stress is calculated on the upstream face for elevated
reservoir levels and the "cracked base" analysis is invoked where the drains are assumed to be ineffective. While appropriate for new
structures, design criteria may not be appropriate for evaluating existing structures. It is important to evaluate actual failure
probabilities in a reasonable fashion so that limited resources can be targeted to the structures that pose the largest risk. Therefore,
the realistic probability of cracking, the effectiveness of a drainage curtain following cracking, and the limit state after cracking
propagates must all be taken into account. The specific goal of this research is to evaluate the probability of failure for concrete
gravity dam structures including the effects of potential cracking and drainage. Work done by E. Jensen.


Collaborative Research: Bridging and coupling particle to continuum lengthscale mechanics for
simulating deformation and flow of dense dry particulate materials
(NSFCMMI 0700648, PI R. Regueiro, CoPIs K. Alshibli, Y. Hammi; completed).
Development of a computational multiscale modeling approach and its
calibration/validation against micro to macroscale experiments. To date, there
is no single computational multiscale modeling approach that can simulate the
deformation and flow of dense dry particulate materials accounting for their
discrete particlescale mechanics across several orders of magnitude in length
scale. If successful, the research will (among other applications) (1) provide
physical insight into the deposition and compaction of metallic powders into
complex die shapes and the potential mechanisms leading to nonuniform density
after compaction; and (2) determine the disturbed state of sand particles
(fabric, porosity, strength, ...) in the vicinity of a rigid penetrating object
(e.g., steel pile, cone penetrometer, or earth penetrator). My collaborators
include an expert (K. Alshibli, UTenn, Knoxville) in insitu synchrotron microcomputed
tomography of deforming particulate materials, capable of tracking individual
particle motion during overall specimen compression, generating validation data
for the multiscale model. Relation to metallic powders comes through the
collaboration with Y. Hammi from CAVS at MSU.

Three containers with different number of particles and fixed boundary particles
to demonstrate boundary effects during penetration (B. Yan).

Pile resistance force versus penetration displacement plots demonstrating expected higher
resistance by smaller containers of particles compared to larger (boundary further from penetrator) containers (B. Yan).

Concurrent multiscale modeling approach.

Click here
to run movie (by B. Yan) of pile penetration (crosssectional view).
Particles popping in and out of
screen image are crossing the plane of the crosssection, which is used for
better viewing of the particle motion around the pile within the container
of particles, otherwise not seen if crosssection not shown.

Preliminary particlefiniteelementfacet coupling (B. Yan) in
Tahoe

Pile resistance force versus penetration displacement plots (B. Yan) demonstrating reduced resistance
as elastic stiffness of finite element mesh coupled to smaller container is adjusted
to match pile resistance in
larger container with rigid/fixed particle boundary (no coupling). This is a
proofofconcept code coupling
(Tahoe
and ellipsoidal DEM code), whereas the
eventual true concurrent scheme (see figure to upper left) will involve an overlapping region and a higher
order continuum plasticity model in this region to couple properly the kinematics
and forces/stresses, and thus make the pile resistance independent of container size.


Graintomacroscale modeling resolution of dynamic failure in bound particulate materials
(ARO Solid Mechanics W911NF0910111, PI R. Regueiro; completed).
Using solely grainscale physicsbased simulation methods, it is too computationally intensive to account
for both (I) global initial boundary value problem (IBVP) conditions, and (II) grainscale material
behavior, to understand fundamentally the mechanics of dynamic failure in bound particulate materials. The
objective of the proposed research is to achieve this understanding by accounting simultaneously for
grainscale physics and macroscale continuum IBVP conditions. To achieve the proposed objective, a
concurrent computational multiscale modeling approach will be developed that
involves the following 3 features: (1) coupling regions of micromorphic continuum finite element to an
`open window' on the particulate microstructure where localized deformation nucleates and an interface
with a deformable solid body could exist; (2) converting to discrete element fragmentation modeling in
microstructural regions; and (3) adapting numerically grainscale resolution over the material domain.
The desired result is to enable a more complete understanding of the role of grainscale physics on the
thermomechanical properties and performance of heterogeneous bound particulate materials of interest to
the Army.

2D illustration of concurrent computational multiscale modeling approach in the contact interface region
between a bound particulate material (e.g., ceramic target) and deformable solid body (e.g., refractory
metal projectile).


SoilStructure Interaction in Geothermal Foundations
(NSFCMMI 0928159, PI J. McCartney, CU Boulder; completed).
My involvement in this research project is to develop a soil constitutive model and finite element
implementation to analyze soil structure interaction between heatedcooled concrete piles (castinplace)
and saturated and partiallysaturated soil. The thermal cycling currently only accounts for heating and
then cooling back to ambient temperature, and does not account for freezethaw cycles.
The modeling involves thermoporomechanics for partiallysaturated and saturated soils, with initial axisymmetric FE implementation to model
the centrifuge experiments where cylindrical concrete piles are spun to a certain glevel to investigate
scaling of pile size, heated and loaded, all currently in silt. Thermoporomechanical interface elements
are developed to model the soilpile interface conditions (by W. Wang). Extension to
threedimensions will allow analysis of more complex geothermal foundation geometries.

Threephase mixture theory (solidliquidgas) following solid phase motion.

Thermoporomechanical axisymmetric finite element formulation and implementation.


Simulation of blast loading on an ultrastructurallybased computational model of the ocular lens.
U.S. Army Medical Research and Materiel Command (USAMRMC W81XWH1011036, PI R. Regueiro, started September 2010,
ended September 2016), Telemedicine and
Advanced Technology Research Center (TATRC), Vision Research Program (VRP).
Traumatic cataract in ocular lenses may result from blast loading or blunt trauma, whereby
(i) the lens capsule is perforated by intraocular foreign bodies (IOFBs) which in turn damage the lens
fiber cells, (ii) the lens is loaded fluid dynamically by the surrounding aqueous and vitreous humors,
and/or (iii) the lens internal substance (crystallins lens fiber cells) is stressed by the passing shock
wave. Traumatic cataract can result in a partially or fully clouded lens, complete dislocation of the lens
(floating between aqueous and vitreous humors), or zonule rupture such that partial or full vision loss may
occur. The mechanisms of traumatic cataract formation that may require cataract surgery (implantation of
an intraocular lens (IOL)) are not well understood in comparison to the mature and everimproving surgical
technology and procedures.
The research objective is to establish an ultrastructurallybased computational finite element model of the
ocular lens subjected to blast loading
to attempt to better understand the mechanisms of traumatic cataract formation and how it may be
treated clinically.

Multiscale finite element modeling approach for simulating traumatic ocular lens tissue mechanics, with
lens ultrastructure characterization. Multiphysics, multiscale finite strain solidshell
formulation and implementation.


Ocular lens tissue mechanics:
Understanding the mechanics of lens accommodation (ability of the eye
dynamically to focus near to far, or far to near) can assist in the diagnosis of
early presbyopia as well as in the development of new potential clinical
treatments and intraocular lens (IOL) design and implantation strategies.
Related to the mechanism of focusing, presbyopia is an ocular disease that stems
from agerelated loss of lens accommodation leading to loss of focusing range
and near vision. This is attributed to changes in ciliary muscle function, as
well as changes in the elastic and deformable properties of the lens substance
and the lens capsule. The precise relationship of these changes, however, is not
well described. During cataract surgery, the process of removing a circular
portion latitudinally of the anterior lens capsule can lead to tearing of the
capsule longitudinally, propagating to the posterior capsule and loss of the
full lens and zonules. A fundamental understanding of lens capsule mechanics
can lead to potentiallyimproved surgical procedures to avoid such tears during
cataract surgery (now the most common surgery among the elderly population, with
projections on surgeries to increase as the population ages) as well as
customdesigned IOLs to restore accommodative vision.

Diagram of eye (http://www.nei.nih.gov/).

Unconfined compression stress relaxation tests (A. Blanchard, K. Constancio, L. Hatanaka, C. Bay) conducted to estimate
hyperviscoelastic material properties of lens capsule and internal substance.
Axisymmetric nonlinear finite strain finite element analysis conducted within
optimization program to fit parameters to data generated by students.


Embedded threedimensional strong discontinuity finite element modeling of
fracture and slip in pressuresensitive materials:
Such materials include stiff clays, rocks, and concrete.
The embedded strong discontinuity finite element formulation is based upon the
assumed enhanced strain method, which has the advantage over other embedded
discontinuity finite elements in that it does not require additional global
degrees of freedom associated with a nucleating and propagating crack.
These computational techniques should resolve fractures and slip surfaces,
their constitutive response, and the
associated loss of strength (softening), in a meshindependent manner.

Hexahedral finite element with strong discontinuity. Various cutting cases.

Demonstration of postlocalization softening along strong discontinuity.

Reconstruction of total displacement field u^h in right figure showing
strong discontinuity, whereas
the compatible displacement field
\tilde{u}^h in the left figure is the deformation seen in the mesh deformation. Hence, the
approach is called an "embedded" discontinuity approach because the crack or
slip plane is not seen explicitly in the mesh deformation, but exists in the
enhanced finite element formulation.

Plane strain slope stability analysis with embedded discontinuity element. Slip line drawn through localized enhanced elements.


Implicit three dimensional finite element analysis of dynamic inelastic biphasic porous media at finite strain:
Simulating the mechanical response of porous materials, such as geologic
materials and biological tissues. These materials are a mixture of solid
constituents (e.g., collagen fibers, sand grains) and interstitial liquid and/or gas. Biological tissues are more complex than this simple definition, and thus require care in their modeling using mixture and porous media theory. The finite strain, implicit dynamic, finite element implementation and analysis provides the overlaying framework
in which to develop multiscale materials models of heterogeneous porous
materials. Extension to triphasic or multiphasic mixtures may be needed.
Finite strains allow the modeling of large deformation in soils and biological
tissues, and implicit dynamics the efficient simulation of long period motions
encountered during earthquakes and running or jumping. Higher rate impact
during car crash or otherwise requires an explicit dynamic analysis, which
reduces readily from an implicit implementation.
Research conducted by D. Ebrahimi.

Motion of solid and fluid phases, with respect to solid skeleton phase motion.

27node mixed hexahedral finite element for 3D biphasic mixture implementation.

1D column consolidation mesh using 3D element.

Vertical settlement, showing stiffer response of finite strain formulation.
For porositydependent permeability (Darcy2),
permeability decreases as the solid skeleton is compressed,
and thus the solution lags the solution without porositydependent
permeability (Darcy1).

2D impulse loading and mesh.

Traveling speeds of primary waves between solid (single phase) analysis and
biphasic analysis are nearly the same, except amplitudes are different.

