Micro shear bands: Precursor for strain localization in sheared granular materials

Recent studies have shown that detection of the onset and evolution of micro shear bands (MSBs) in granular materials can be improved using measurements of the kinematic behavior of particles. Different methods such as the discrete-element method (DEM) or threedimensional (3D) imaging techniques have been used to measure the kinematics of individual particles within triaxial specimens. However, conventional kinematic techniques that use particle translation and/or rotation cannot detect the onset and growth of MSBs during the hardening phase of axisymmetric triaxial experiments. In order to expose the localized shearing and particle-scale behavior of triaxial specimens, a relative particle translation gradient (RPTG) concept is used to detect and expose the onset of strain localization before the peak principal stress ratio (PSR). RPTG measurements for four different granular materials are reported in this paper. The RPTG concept is used to expose the onset of MSBs during the hardening phase of the experiments. In addition, the contact number of individual particles is quantified and discussed in relation to particle rotation to investigate a particle-scale relationship between particle contacts and rotation. The effects of density, confining pressure, and particle shape on contact number are examined.The research is funded by the US National Science Foundation (NSF) under Grant Nos. CMMI-1266230 and CMMI-1362510. Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the NSF. The SMT images presented in this paper were collected using the X-Ray Operations and Research Beamline Station 13-BMD at Argonne Photon Source (APS), a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The authors acknowledge the support of GeoSoilEnviroCARS (Sector 13), which is supported by NSF-Earth Sciences (EAR-1128799), and DOE, Geosciences (DE-FG02-94ER14466). The authors thank Dr. Mark Rivers of APS for help performing the SMT scans

Surface permeability of porous media particles and capillary transport

We have established previously, in a lead-in study, that the spreading of liquids in particulate porous media at low saturation levels, characteristically less than 10% of the void space, has very distinctive features in comparison to that at higher saturation levels. In particular, we have found that the dispersion process can be accurately described by a special class of partial differential equations, the super-fast non-linear diffusion equation. The results of mathematical modelling have demonstrated very good agreement with experimental observations. However, any enhancement of the accuracy and predictive power of the model, keeping in mind practical applications, requires the knowledge of the effective surface permeability of the constituent particles, which defines the global, macroscopic permeability of the particulate media. In the paper, we demonstrate how this quantity can be determined through the solution of the Laplace-Beltrami Dirichlet problem, we study this using the well-developed surface finite element method

A Pre-Landing Assessment of Regolith Properties at the InSight Landing Site

This article discusses relevant physical properties of the regolith at the Mars InSight landing site as understood prior to landing of the spacecraft. InSight will land in the northern lowland plains of Mars, close to the equator, where the regolith is estimated to be ≥3--5 m thick. These investigations of physical properties have relied on data collected from Mars orbital measurements, previously collected lander and rover data, results of studies of data and samples from Apollo lunar missions, laboratory measurements on regolith simulants, and theoretical studies. The investigations include changes in properties with depth and temperature. Mechanical properties investigated include density, grain-size distribution, cohesion, and angle of internal friction. Thermophysical properties include thermal inertia, surface emissivity and albedo, thermal conductivity and diffusivity, and specific heat. Regolith elastic properties not only include parameters that control seismic wave velocities in the immediate vicinity of the Insight lander but also coupling of the lander and other potential noise sources to the InSight broadband seismometer. The related properties include Poisson’s ratio, P- and S-wave velocities, Young’s modulus, and seismic attenuation. Finally, mass diffusivity was investigated to estimate gas movements in the regolith driven by atmospheric pressure changes. Physical properties presented here are all to some degree speculative. However, they form a basis for interpretation of the early data to be returned from the InSight mission.Additional co-authors: Nick Teanby and Sharon Keda

Discrete particle translation gradient concept to expose strain localisation in sheared granular materials using 3D experimental kinematic measurements

It is well known that the constitutive behaviour of granular materials is influenced by strain localisation into zones of intensive shearing, known as shear bands. The failure mode of specimens tested under axisymmetric triaxial compression is commonly manifested through single or multiple shear bands, or diffuse bifurcation (bulging). The ability to monitor and detect the evolution of strain localisation has been enhanced by measuring particle kinematics using discrete-element methods or three-dimensional imaging techniques such as X-ray computed tomography. However, conventional particle kinematic techniques cannot expose intricate localised shearing during the hardening, before the peak principal stress ratio. This paper presents the concept of particle translation gradient to expose strain localisation in granular materials using experimental measurements of particle translation in three dimensions. Individual silica sand particles were identified and tracked through multiple strains and particles' translations were calculated. Each particle's neighbouring particles were identified and translation fields for each of the neighbouring particles were calculated. The second-order norms between a particle translation vector and the neighbouring particles' translation vectors were averaged, resulting in a relative translation value for each particle. The translation gradient concept is effective in uncovering the onset of strain localisation within sheared granular materials.ACKNOWLEDGEMENTS This material is partially funded by the US National Science Foundation (NSF) under grant no. CMMI-1266230 and Office of Naval Research (ONR) grant no. N00014-11-1-0691. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF or ONR. The SMT images presented in this paper were collected using the X-Ray Operations and Research Beamline Station 13-BMD at Argonne Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. The authors acknowledge the support of GeoSoilEnviroCARS (Sector 13), which is supported by the National Science Foundation � Earth Sciences (EAR-1128799), and the Department of Energy, Geosciences (DE-FG02-94ER14466). They also thank Dr Mark Rivers of APS for help in performing the SMT scans.Scopu

Comprehensive literature review on CH4-CO2 replacement in microscale porous media

Gas production studies from natural gas hydrate reservoirs have been the subject of extensive research in recent years. Although CH4-CO2 replacement production method from gas hydrates has many advantages, the number of the studies related to this production method are less than depressurization production method studies, especially in microscale porous media. Hence, this paper presents a comprehensive literature review on CH4-CO2 replacement to better understand the associated processes and mechanisms in microscale porous media with emphasis on micromodel experiments, 3D imaging, other visualization testing method and pore network modelling. Moreover, the advantages and disadvantages of currently available CH4-CO2 replacement studies were investigated. The critical issues of the replacement method were underlined and new suggestions have been offered for future investigations. � 2018 Elsevier B.V.Scopu