@article {220, title = {Strength Homogenization of Double-Porosity Cohesive-Frictional Solids}, journal = {Journal of Applied Mechanics}, volume = {80}, year = {2013}, month = {Mar-2013}, pages = {Article Number: 020902}, abstract = {

The strength homogenization of cohesive-frictional solids influenced by the presence of two pressurized pore spaces of different characteristic sizes is addressed in this study. A two-scale homogenization model is developed based on limit analysis and the second-order method (SOM) in linear comparison composite theory, which resolves the nonlinear strength behavior through the use of linear comparison composites with optimally chosen properties. For the scale of the classical configuration of a porous solid, the formulation employs a compressible thermoelastic comparison composite to deliver closed-form expressions of strength criteria. Comparisons with numerical results reveal that the proposed homogenization estimates for drained conditions are adequate except for high triaxialities in the mean compressive strength regime. At the macroscopic scale of the double-porosity material, the SOM results are in agreement with strength criteria predicted by alternative micromechanics solutions for materials with purely cohesive solid matrices and drained conditions. The model predictions for the cohesive-frictional case show that drained strength development in granularlike composites is affected by the partitioning of porosity between micro-and macropores. In contrast, the drained strength is virtually equivalent for single-and double-porosity materials with matrix-inclusion morphologies. Finally, the second-order linear comparison composite approach confirms the applicability of an effective stress concept, previously proposed in the literature of homogenization of cohesive-frictional porous solids, for double-porosity materials subjected to similar pressures in the two pore spaces. For dissimilar pore pressures, the model analytically resolves the complex interplays of microstructure, solid properties, and volume fractions of phases, which cannot be recapitulated by the effective stress concept.

}, issn = {0021-8936}, doi = {10.1115/1.4007905}, author = {J. Alberto Ortega and Franz-Josef Ulm} } @article {303, title = {Nanochemomechanical assessment of shale: a coupled WDS-indentation analysis}, journal = {Acta Geotechnica}, volume = {7}, year = {2012}, month = {Dec-2012}, pages = {271 - 295}, abstract = {

Establishing the links between the composition, microstructure and mechanics of shale continues to be a formidable challenge for the geomechanics community. In this study, a robust methodology is implemented to access the in situ chemomechanics of this sedimentary rock at micrometer length scales. Massive grids of coupled wave dispersive spectroscopy (WDS) and instrumented indentation experiments were performed over representative material surfaces to accommodate the highly heterogeneous composition and microstructure of shale. The extensive datasets of compositional and mechanical properties were analyzed using multi-variate clustering statistics to determine the attributes of active phases present in shale at microscales. Our chemomechanical analysis confirmed that the porous clay (PC) mechanical phase inferred by statistical indentation corresponds to the clay mineral phase defined strictly on chemical grounds. The characteristic stiffness and hardness behaviors of the PC are realized spatially in regions removed from silt inclusions of quartz and feldspar. At the microscale shared by indentation and WDS experiments, a consistent chemomechanical signature for shale emerges in which the heterogeneities of the PC are captured by the standard deviations of indentation properties and concentrations of chemical elements. However, these local behaviors are of second order compared to the global trend observed for mean mechanical properties and the clay packing density, which synthesizes the relative volumes of clay and nanoporosity in the material. The coupled statistical indentation and WDS technique represents a viable approach to characterize the chemomechanics of shale and other natural porous composites at a consistent scale below the macroscopic level.

}, issn = {1861-1125}, doi = {10.1007/s11440-012-0185-4}, author = {Deirieh, A. and J. Alberto Ortega and Franz-Josef Ulm and Younane N. Abousleiman} } @article {258, title = {The nanogranular origin of friction and cohesion in shale-A strength homogenization approach to interpretation of nanoindentation results}, journal = {International Journal for Numerical and Analytical Methods in Geomechanics}, volume = {35}, year = {2011}, month = {Dec-10-2011}, pages = {1854 - 1876}, abstract = {

An inverse micromechanics approach allows interpretation of nanoindentation results to deliver cohesive-frictional strength behavior of the porous clay binder phase in shale. A recently developed strength homogenization model, using the Linear Comparison Composite approach, considers porous clay as a granular material with a cohesive-frictional solid phase. This strength homogenization model is employed in a Limit Analysis Solver to study indentation hardness responses and develop scaling relationships for indentation hardness with clay packing density. Using an inverse approach for nanoindentation on a variety of shale materials gives estimates of packing density distributions within each shale and demonstrates that there exists shale-independent scaling relations of the cohesion and of the friction coefficient that vary with clay packing density. It is observed that the friction coefficient, which may be interpreted as a degree of pressure-sensitivity in strength, tends to zero as clay packing density increases to one. In contrast, cohesion reaches its highest value as clay packing density increases to one. The physical origins of these phenomena are discussed, and related to fractal packing of these nanogranular materials. Copyright \© 2010 John Wiley \& Sons, Ltd.

}, doi = {10.1002/nag.v35.1710.1002/nag.984}, author = {Bobko, Christopher P. and Gathier, Benjamin and J. Alberto Ortega and Franz-Josef Ulm and Borges, Lavinia and Younane N. Abousleiman} } @proceedings {347, title = {STRENGTH HOMOGENIZATION FOR COHESIVE-FRICTIONAL COMPOSITES: A LINEAR COMPARISON COMPOSITE APPROACH}, journal = {International Workshop on Multiscale and Multiphysics Processes in Geomechanics}, volume = {Springer Series in Geomechanics and Geoengineering}, year = {2011}, month = {Jan-2011}, pages = {57 - 60}, publisher = {Springer Berlin Heidelberg}, address = {JUN 23-25 2010 Stanford Univ, Stanford, CA}, abstract = {

In this work, we address the homogenization of cohesive-frictional strength properties of composite materials through the application of the Linear Comparison Composite (LCC) theory. The LCC methodology is implemented in a multiscale strength model relevant to geomaterials with heterogeneities associated with pore spaces and rigid-like grains. The proposed micromechanics solutions offer quantitative assessments of the effects of grain-scale properties and microstructure on the overall strength of the composite material Keywordscohesive-frictional materials\–linear comparison composite

}, isbn = {978-3-642-19629-4}, issn = {1866-8755}, doi = {10.1007/978-3-642-19630-0_15}, author = {J. Alberto Ortega and Gathier, Benjamin and Franz-Josef Ulm}, editor = {Wu, Wei and Borja, Ronaldo I.} }