Richard A. Regueiro
Associate Professor
Department of Civil, Environmental, and Architectural Engineering
University of Colorado at 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



Courses 


Research 

Summary:
My research group is interested in computational multiscale multiphysics materials modeling for simulating inelastic
deformation and failure in heterogeneous porous media, including saturated and partially saturated soils and rock, unbound
particulate materials (e.g. sand, gravel, or metallic powders), bound
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.


Integrated Experimental  Computational Multiscale Immersed ParticleContinuum Approach to
Modeling and Simulation of Multiphase Soil Failure Mechanics under Buried Explosive Loading
(ONRMURI, started August 2011).
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 at 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: Bridging and coupling particle to continuum lengthscale mechanics for
simulating deformation and flow of dense dry particulate materials (NSFCMMI 0700648).
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.

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

Concurrent multiscale modeling approach.


Preliminary particlefiniteelementfacet coupling in
Tahoe

Pile resistance force versus penetration displacement plots 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).
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: John McCartney, CU Boulder)
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. 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), 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 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.

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.


Tahoe development:
For each of these projects, and for future research projects, I and my graduate
students use a researchoriented, opensource, versioncontrolled, parallel
execution, modularized, highly flexible C++ code called
Tahoe


Graduate Student Recruitment 

I am always looking for talented and motivated graduate students to join my research group.
Depending on your background, you can apply either to the (1) Engineering Sciences 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
with me. As you can see from this webpage, my 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 my group.


Publications 
 Journal Articles (email me for more
information)

Isbuga, V., Regueiro, R.A. (2011)
Threedimensional finite element analysis of finite strain micromorphic
linear isotropic elasticity. Int. J. Engr. Sci. 49:13261336

Regueiro, R.A., Isbuga, V. (2011) Length scale effects in
finite strain micromorphic linear isotropic elasticity: Finite element
analysis of threedimensional cubical microindentation.
J. Nanoengineering and Nanosystems.
 Regueiro, R.A., Yu, S.K. (2011) Finite element analysis of
grainmatrix microcracking in shale within the context of a multiscale modeling approach for fracture. in
press, Int. J. Multiscale Comput. Eng.
 Regueiro, R.A., Ebrahimi, D. (2010) Implicit dynamic threedimensional finite element analysis of an
inelastic biphasic mixture at finite strain. Part 1: application to a simple geomaterial. Comput.
Meth. Appl. Mech. Eng. 199:20242049.
 Regueiro, R.A., Foster, C.D. (2010) Bifurcation analysis of a threeinvariant, isotropic/ kinematic
hardening cap plasticity model for geomaterials. in press Int. J. Numer. Anal. Methods Geomech.
 Regueiro, R.A. (2010) On finite strain micromorphic elastoplasticity. Int. J. Solids Struct.
47:786800.
 Yan, B., Regueiro, R.A., Sture, S. (2010)
Three dimensional discrete element modeling of granular materials and its coupling with finite element
facets. Eng. Comput. 27(4):519550.
 Regueiro, R.A. (2009) Finite strain micromorphic pressuresensitive plasticity. ASCE
J. Eng. Mech., 135(3):178191.
 Regueiro, R.A., Voyiadjis, G. (2009) Nonlocal and generalized continuum materials modeling for simulating multiscale behavior. ASCE J. Eng. Mech., 135(3):115116.
 Foster, C.D., Borja, R.I., Regueiro, R.A. (2007) Embedded strong discontinuity finite elements for
fractured geomaterials with variable friction.
Int. J. Numer. Methods Eng., 72:549581.
 Regueiro, R.A., Dixit, P., Garikipati, K. (2007) On standard and vector finite element
analysis of a strict antiplane shear plasticity model with elastic curvature, Comp. Meth. App. Mech.
Engr., 196:26922712.
 Manzari, M.T., Regueiro, R.A. (2005) Gradient plasticity modeling of geomaterials in a meshfree environment, Part I: theory and variational formulation, Mech. Res. Commun. 32:53646.
 Foster, C.D., Regueiro, R.A., Borja, R.I., Fossum, A.F. (2005) Implicit integration of a threeinvariant, singlesurface, isotropic/kinematic hardening, cap plasticity model for geomaterials. Comput. Meth. Appl. Mech. Eng. 194:51095138.
 Creighton, S.L., Regueiro, R.A., Garikipati, K., Klein, P.A., Marin, E.B., and Bammann, D.J. (2004) A variational multiscale method to incorporate strain
gradients in a phenomenological plasticity model. Comput. Meth. Appl. Mech. Eng. 193:545375.
 Li, C., Borja, R.I., Regueiro, R.A. (2004) Dynamics of porous media at finite strain.
Comput. Meth. Appl. Mech. Eng. 193:383770.
 Regueiro, R.A., Bammann, D.J., Marin, E.B., Garikipati, K. (2002) A nonlocal
phenomenological anisotropic finite deformation plasticity model accounting for dislocation
defects, J. Eng. Mater. Technol.Trans. ASME, 124:380387.
 Borja, R.I., and Regueiro, R.A. (2001) Strain localization in frictional materials exhibiting displacement jumps, Comput. Meth. Appl. Mech. Eng., 190: (2021) 25552580.
 Regueiro, R.A., and Borja, R.I. (2001) Plane strain finite element analysis of
pressuresensitive plasticity with strong discontinuity, Int. J. Solids Struct.,
38 (21) 36473672.
 Borja, R.I., Regueiro, R.A., and Lai, T.Y. (2000) FE modeling of strain localization in soft rock, J. Geotech. Geoenviron. Eng., 126 (4) 335343.
 Regueiro, R.A., and Borja, R.I. (1999) A finite element model of localized deformation in
frictional materials taking a strong discontinuity approach, Finite Elem. Anal. Des.,
33 (4) 283315.
 Conference/Workshop Papers

Regueiro, R.A., Bammann, D.J., Marin, E.B., Johnson, G.C.
(2011) Finite deformation elastoplasticity for rate and temperature
dependent polycrystalline metals, pg 113, ASME 2011
International Mechanical Engineering Congress & Exposition,
Denver, CO, November, 2011, accepted.
 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. (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.
