Richard A. Regueiro
Department of Civil, Environmental, and Architectural Engineering
University of Colorado Boulder

ph: 303.492.8026
fx: 303.492.7317

1111 Engineering Dr.
428 UCB, ECOT 441
Boulder, CO 80309-0428

curriculum vitae

OrcID Webpage

  • 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.
Current Research Projects
  • Center for Micromorphic Multiphysics Porous and Particulate Materials Simulations within Exascale Computing Workflows (DOE NNSA PSAAP, 9/20-9/25) The overall objective of the Multidisciplinary Simulation Center (MSC) is to simulate with quantified uncertainty, from pore-particle-to-continuum-scales, 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 thermo-mechanical behavior of compressed virgin and recycled mock high explosive (HE) material subjected to quasi-static and high- strain-rate, confined and unconfined compression, in-situ quasi-static X-ray computed tomography, and dynamic (impact) experiments with ultrafast synchrotron X-ray 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 full-time-equivalent 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 pore-particle-scale experiments and Direct Numerical Simulations (DNS). Integrated experimental testing at quasistatic and dynamic rates (including ultrafast synchrotron x-ray imaging at the APS and proton imaging at Los Alamos National Laboratory's pRad facility) at length scales ranging from pore-particle to continuum scales will be conducted to validate heterogeneous, pore-particle-tocontinuum- scale, computational models, calibrate model parameters, and validate the overall computational framework. Exascale computing is needed to simulate these more sophisticated micromorphic, multiphysics, bridged-DNS 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. (Co-Directors: J. Brown, A. Clarke (Mines), A. Doostan, R. Regueiro (PI), H. Tufo, 9/20-9/25)

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 research-oriented, open-source, version-controlled, parallel execution, modularized, highly flexible C++ code called Tahoe
Group Members
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.
Completed Research Projects

Biphasic solid-fluid mixture porous continuum B0 with discontinuity domain Sdomain0 in the reference configuration.

2D cross-sectional illustrations of 3D simulation setups for geotechnical centrifuge validation experiments. Scale I: pore-grain-scale numerical modeling of soil with clay, silt/sand grains, and pore air and water; concurrent multiscale coupling of continuum with open pore-grain-scale domain around buried explosive. Scale II: hierarchical continuum constitutive model informed from Scale I; high-strain-rate, large deformation MPM implementation, and triphasic continuum formulation and implementation.
  • Analysis of Drain Effectiveness and Implications for Failure Probability for Concrete Gravity Dams. (USACE W912P8-08-D-002, PI R. Regueiro, Co-PIs 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 length-scale mechanics for simulating deformation and flow of dense dry particulate materials (NSF-CMMI 0700648, PI R. Regueiro, Co-PIs K. Alshibli, Y. Hammi; completed). Development of a computational multiscale modeling approach and its calibration/validation against micro to macro-scale 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 particle-scale 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 non-uniform 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 in-situ synchrotron micro-computed 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 (cross-sectional view). Particles popping in and out of screen image are crossing the plane of the cross-section, which is used for better viewing of the particle motion around the pile within the container of particles, otherwise not seen if cross-section not shown.

Preliminary particle-finite-element-facet 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 proof-of-concept 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.
  • Grain-to-macro-scale modeling resolution of dynamic failure in bound particulate materials (ARO Solid Mechanics W911NF-09-1-0111, PI R. Regueiro; completed). Using solely grain-scale physics-based simulation methods, it is too computationally intensive to account for both (I) global initial boundary value problem (IBVP) conditions, and (II) grain-scale 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 grain-scale physics and macro-scale continuum IBVP conditions. To achieve the proposed objective, a concurrent computational multi-scale 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 micro-structure where localized deformation nucleates and an interface with a deformable solid body could exist; (2) converting to discrete element fragmentation modeling in micro-structural regions; and (3) adapting numerically grain-scale resolution over the material domain. The desired result is to enable a more complete understanding of the role of grain-scale physics on the thermo-mechanical properties and performance of heterogeneous bound particulate materials of interest to the Army.

2D illustration of concurrent computational multi-scale 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).
  • Soil-Structure Interaction in Geothermal Foundations (NSF-CMMI 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 heated-cooled concrete piles (cast-in-place) and saturated and partially-saturated soil. The thermal cycling currently only accounts for heating and then cooling back to ambient temperature, and does not account for freeze-thaw cycles. The modeling involves thermo-poro-mechanics for partially-saturated and saturated soils, with initial axisymmetric FE implementation to model the centrifuge experiments where cylindrical concrete piles are spun to a certain g-level to investigate scaling of pile size, heated and loaded, all currently in silt. Thermo-poro-mechanical interface elements are developed to model the soil-pile interface conditions (by W. Wang). Extension to three-dimensions will allow analysis of more complex geothermal foundation geometries.

Three-phase mixture theory (solid-liquid-gas) following solid phase motion.

Thermo-poromechanical axisymmetric finite element formulation and implementation.

Multiscale finite element modeling approach for simulating traumatic ocular lens tissue mechanics, with lens ultrastructure characterization. Multiphysics, multiscale finite strain solid-shell 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 age-related 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 potentially-improved 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 custom-designed IOLs to restore accommodative vision.

Diagram of eye (

Unconfined compression stress relaxation tests (A. Blanchard, K. Constancio, L. Hatanaka, C. Bay) conducted to estimate hyper-viscoelastic 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 three-dimensional strong discontinuity finite element modeling of fracture and slip in pressure-sensitive 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 mesh-independent manner.

Hexahedral finite element with strong discontinuity. Various cutting cases.

Demonstration of post-localization 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 multi-phasic 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.

27-node 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 porosity-dependent permeability (Darcy2), permeability decreases as the solid skeleton is compressed, and thus the solution lags the solution without porosity-dependent 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.