### Research Interests and Recent Projects

**Multiscale Regional Liquefaction Hazard Mapping**

The liquefaction of soil deposits during earthquake events has caused some of the most spectacular examples of earthquake damage. Quantitative assessment and mapping of liquefaction hazard across large area necessitate the integration of solution models and heterogeneous source of information across multiple scales

Our research in this area aims to develop an integrated multiscale framework for accurately evaluating liquefaction potential and its effects across a region. The proposed framework provides a critical linkage between the local site-specific liquefaction analysis and the regional liquefaction hazard mapping through integration of novel multiscale random field models, simplified procedure for liquefaction evaluation, and the latest results from the probabilistic seismic hazard analysis research.

*Liquefaction potential mapping in Marina District, San Francisco *

**Related publications**:

C. Wang, **Q. Chen**, M. Shen and C.H. Juang, On the spatial variability of CPT-based geotechnical parameters for liquefaction potential evaluation, *Soil Dynamics and Earthquake Engineering*, doi:10.1016/j.soildyn.2017.02.001, 2017.[PDF] [BibTex]

W. Liu, **Q. Chen**, C. Wang, C.H. Juang and G. Chen, Spatially correlated multiscale Vs30 mapping and a case study of the Suzhou site, *Engineering Geology*, doi:10.1016/j.enggeo.2017.01.026, 2017.[PDF] [BibTex]

**Q. Chen**, C. Wang and C.H. Juang, Probabilistic and spatial assessment of liquefaction-induced settlements through multiscale random field models, *Engineering Geology*, 211:135–149, doi:10.1016/j.enggeo.2016.07.002, 2016.[PDF] [BibTex]

M. Shen, **Q. Chen**, J. Zhang, W. Gong and C.H. Juang, Predicting Liquefaction Probability based on Shear Wave Velocity: an Update, *Bulletin of Engineering Geology and the Environment*, 75(3):1199–1214, doi:10.1007/s10064-016-0880-8, 2016.[PDF] [BibTex]

**Q. Chen** C. Wang, Z. Luo and C.H. Juang, Probabilistic evaluation of liquefaction-induced settlement mapping through multiscale random field models, *Proceedings of the 6th Asia-Pacific Symposium on Structural Reliability and Its applications*, Shanghai, China, 2016.[PDF] [BibTex]

**Q. Chen**, C. Wang and C.H. Juang, CPT-based evaluation of liquefaction potential accounting for soil spatial variability at multiple scales, *Journal of Geotechnical and Geoenvironmental Engineering*, 142(2), 04015077, doi:10.1061/(ASCE)GT.1943-5606.0001402, 2015.[PDF] [BibTex]

**Multiphysics Problems in Porous Geomaterials**

The fully coupled diffusion-deformation process occurring within porous media, such as sand, clay, and rock, are of interest to numerous geotechnical engineering applications. The presence of fluid inside the pores and in between the interconnected grains may induce excess pore pressure, limit volumetric deformation, and introduce rate dependence to the mechanical response of the solid skeleton due to the transient nature of fluid diffusion.

Moreover, for porous geomaterials such as sandstone, depending on the porosity level, they may be vulnerable to both shear and compaction failures. In addition to accumulating plastic dilation due to micro-crack growth, grain rotation, and sliding, these materials may exhibit significant inelastic compaction due to pore collapse or grain crushing when the confining pressure is sufficiently high.

To capture the complicated hydro-mechanical interactions of fluid-infiltrating porous rock, we propose a stabilized enhanced strain finite element procedure for poromechanics and fully integrate it with an elasto-plastic cap model for porous rocks. Extensive fully-coupled finite element analysis are presented to study how macroscopic plastic volumetric response caused by pore collapse and grain rearrangement affects the seepage of pore fluid, and vice versa.

*Displacement (left) and Pore pressure (right) of 3D punch loading on water-saturated limestone*

**Related publications**:

A.G. Salinger, R. Bartlett, A. Bradlye, **Q. Chen**, I. Demeshko, X. Gao, G. Hanson, A. Mota, R.P. Muller, E. Nielsen, J.T. Ostien, R. Pawlowski, M. Perego, E. Phipps, W. Sun, I.K. Tezaur, Albany: Using Component-Based Design to Develop a Flexible, Generic Multiphysics Analysis Code, *International Journal for Multiscale Computational Engineering*, doi: 10.1615/IntJMultCompEng.2016017040, 2016.[PDF] [BibTex]

W. Sun, **Q. Chen **and J.T. Ostien, Modeling the hydro-mechanical responses of strip and circular punch loadings on water-saturated collapsible geomaterials, *Acta Geotechnica*,
doi:10.1007/s11440-013-0276-x, 2014.[PDF] [BibTex]

**Q. Chen**, W.C. Sun and J.T. Ostien, Finite element analysis of hydro-mechanical coupling effects on shear failures of fully saturated collapsible geomaterials, *Soil Behavior and Geomechanics*, 688-698, doi: 10.1061/9780784413388.072, 2014.[PDF] [BibTex]

**Q. Chen** and W. Sun, Finite element analysis of hydro-mechanical coupling of water saturated porous geomaterials, *Proceedings of the 17th U.S. National Congress on Theoretical and Applied Mechanics*, Michigan State University, 2014.

**Q. Chen**, J.T. Ostien and G. Hansen, Forward automatic differentiation for numerically exact computation of tangent operators in small- and large-deformation computational inelasticity, *Supplemental UE: TMS 2014 Conference Proceedings*, San Diego, CA,
doe:10.1002/9781118889879.ch38, 2014.[PDF] [BibTex]

Z. Lai, **Q. Chen**, C. Wang and X. Zhou, Modeling dynamic responses of heterogeneous seabed with embedded pipeline through multiresolution random field and coupled hydromechanical simulations, in review, 2017.

**Discrete Element and Finite Element Modleing of Failure in Granular Materials**

The term granular media embraces a wide variety of materials both in nature and in engineering applications. Examples of granular media include sand, sandstone, pharmaceutical pills, and so on. Because of the abundant appearance, understanding and modeling of failure phenomena in granular materials can be of great practical importance. For instance, the design of a foundation/footing resting on granular soils requires the knowledge of the bearing capacity of the underlying media; sequestration of CO2 into reservoir requires the understanding of deformation band formation in sandstone that serves as flow barrier.

In this research, we propose multiscale approaches for modeling failure of granular media, where material descriptions at continuum scales are enhanced by information from finer scales. In particular, classical elasto-plasticity models are used to describe material behavior at the continuum scales and are cast within non-linear finite element programs through computational plasticity procedures. At the granular scale, discrete element method and high-fidelity local measurement data from physical experiments are used as micromechanical model to provide material response.

*Multiscale-nature of granular media (shear band image from Alshibli et al. 2003). *

*Discrete element simulation of triaxial compression experiment (left) and granular flow (right). *

*Experiment (left, image from Alshibli et al. 2003) and numerical simulation through multi-scale Assumed Enhanced Strain method (right) of plane strain compression test on sand.*

**References**: Q. Chen, J.E. Andrade and E. Samaniego, *CMAME*, 2010; X. Tu, J.E. Andrade and Q. Chen, *CMAME*, 2009; Q. Chen, A.E. Seifried, J.E. Andrade and J.W. Baker, IJNAMG, 2012; J.E. Andrade, Q. Chen, P.H. Le, C.F. Avila and T.M. Evans, *JMPS*, 2012

**Mechanical Model for Used Nuclear Fuel Cladding**

At the completion of the used nuclear fuel drying process, used fuel Zircaloy (Zry4) cladding typically exhibits a significant population of circumferentially- and radially-oriented hydride inclusions. These hydride inclusions are formed during reactor operation, when water coolant that is in continual contact with the Zircaloy fuel cladding under elevated temperatures and pressures decomposes and the disassociated hydrogen goes into solid solution within the Zry4.

The focus of this work is to develop a high-fidelity mechanical model for the Zry4-hydride system such that given a particular morphology of hydride inclusions, one can model and predict the response of the hydride cladding under various loading scenarios. The model treats the Zry4 matrix material as J2 elastoplastic, and treats the hydrides as platelets oriented in predefined directions (e.g., circumferentially and radially). Results from numerical modeling are compared well with as-fabricated Zry4 as well as hydride HB Robinson fuel cladding experiments.

*Radial (left) and circumferential (right) damage in hydrided nuclear cladding in ring compression test.*

**References**: Q.Chen, J.T. Ostien and G.Hansen, *Journal of Nuclear Materials*, 2014; G. Hansen, J.T. Ostien and Q. Chen, *Sandia Technical Report, SAND2012-5808*.

**Ductile Failure in Lightweight Metals under Various States of Stress Triaxiality**

Lightweight metals such as aluminum alloy are widely used in engineering applications. Their fracture strains have been shown experimentally to be linked with stress triaxiality, which is defined as the ratio of mean stress over effective (shear) stress.

This work aims to understand and predict ductile failures in lightweight metals under various states of stress triaxiality through a combined numerical and experimental study. We employ a shear-modified Gurson-type damage model to account for damage (void) growth under low triaxiality. The model is formulated in a large-deformation hyper-elastic framework and implemented into our computing environment *Albany* to simulate various boundary value problems, where corresponding experimental efforts are undertaken by our collaboratores at Sandia National Laboratories.

*Tiraxiality (left) and damage (right) contours in combined tension and torsion simulation of a thin-tube Aluminum T6061 specimen*.

**References**: Q. Chen, J.T. Ostien and W.Y. Lu, *SES 2012 Conference Proceedings*; Q. Chen, J.T. Ostien and W. Y. Lu, *ASME IMECE 2012 Conference Proceedings*.

**High-performance Computing Software Packages**

Many of our recent reserch efforts are built on the high-performance computing environment * Albany*, developed at

**Sandia National Laboratories**.

*Albany*is a c++ objective-oriented, parrallel, unstructured-grid, implicit finite element code for solving partial differential equations (PDEs) in various fields of engineering applications. The code is designed for the rapid development of finite-element analysis capabilities enabled through the concept of agile components.

*Albany*has a unique infrastructure that limits the need for programming to just writing the physics residual equations based a generic data type;

*Albany*will then compute the system Jacobian and preconditioner for the Newton nonlinear solver, as well as providing sensitivity and uncertainty information about the simulation and selected input and model parameters. In terms of computational mechanics capability, currently,

*Albany*has multiphysics capabilities (thermal-hydro-mechanical coupling) and includes a extensive library of material models.

Instruction to obtain and compile * Albany* can be found: https://github.com/gahansen/Albany

In addition to *Albany*, there are a number of other software packages we use in our daily research activities:

- Particle Flow Code (PFC): A commercial general purpose discrete element simulation software
- LIGGGHTS : An open source parallel discrete element method particle simulation software
- Trelis : A high-end, commercial-grade geometry and mesh generation software for FEA and CFD simulations
- Paraview: An open source multi-platform data analysis and visualization application
- MATLAB: A commercial scientific computing, data visualization and programming language and environment
- Spin 3D: A Fortran-based three-dimensional multiphysics finite element program

**References**:
A.G. Salinger, R. Bartlett, A. Bradlye, **Q. Chen**, I. Demeshko, X. Gao, G. Hanson, A. Mota, R.P. Muller, E. Nielsen, J.T. Ostien, R. Pawlowski, M. Perego, E. Phipps, W. Sun, I.K. Tezaur, Albany: Using Component-Based Design to Develop a Flexible, Generic Multiphysics Analysis Code, *International Journal for Multiscale Computational Engineering*, doi: 10.1615/IntJMultCompEng.2016017040, 2016.[PDF] [BibTex]

**Research Sponsors:**
We are grateful for the past and present support of our research by the following sponsors: U.S. Department of Energy, U.S. Geological Survey, U.S. Department of Education, NSF, NASA SC Space Grant Consortium, NASA/BWX, American International Group, and Clemson University.