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Abstract Critical processes including seismic faulting, reservoir compartmentalization, and borehole failure involve high‐pressure mechanical behavior and strain localization of sedimentary rocks such as sandstone. Sand is often used as a model material to study the mechanical behavior of poorly lithified sandstone. Recent studies exploring the multi‐scale mechanics of sand have characterized the brittle, low‐pressure regime of behavior; however, limited work has provided insights into the ductile, high‐pressure regime of behavior viain‐situmeasurements. Critical features of the ductile regime, including grain breakage, grain micromechanics, and volumetric strain behavior therefore remain under‐explored. Here, we use a new high‐pressure triaxial apparatus within‐situx‐ray tomography to provide new insights into deformation banding, grain breakage, and grain micromechanics in Ottawa sand subjected to triaxial compression under confining pressures between 10 and 45 MPa. We observed strain‐hardening at pressures above 15 MPa and strain‐neutral responses at pressures below 15 MPa. Compacting shear bands and grain breakage were observed at all pressures with no significant variation due to grain size, except for minor increases in breakage in less‐rounded sands. Grain breakage emerged at stress levels lower than the assumed yield threshold and more intense breakage was associated with thinner deformation bands. Contact sliding at inter‐grain contacts demonstrated a bifurcation into a bimodal distribution, with intense sliding within deformation bands and reduced but non‐negligible sliding outside of deformation bands, suggesting that off‐band zones remain mechanically active during strain hardening. 

Critical state and continuum plasticity theories have been used in research and engineering practice in soil and rock mechanics for decades. These theories rely on postulated relationships between material stresses and strains. Some classical postulates include coaxiality between stress and strain rates, stress–dilatancy relationships, and kinematic assumptions in shear bands. Although numerical and experimental data have quantified the strains and grain kinematics in such experiments, little data quantifying grain stresses are available. Here, we report the first-known grain stress and local strain measurements in triaxial compression tests on synthetic quartz sands using synchrotron X-ray tomography and 3D X-ray diffraction. We use these data to examine the micromechanics of shear banding, with a focus on coaxiality, stress-dilatancy, and kinematics within bands. Our results indicate the following: 1) elevated deviatoric stress, strain, and stress ratios in shear bands throughout experiments; 2) coaxial principal compressive stresses and strains throughout samples; 3) significant contraction along shear bands; 4) vanishing volumetric strain but nonvanishing stress fluctuations throughout samples at all stages of deformation. Our results provide some of the first-known in situ stress and strain measurements able to aid in critically evaluating postulates employed in continuum plasticity and strain localization theories for sands. 

Triaxial compression experiments are commonly used to characterize the elastic and inelastic behavior of geomaterials. In situ measurements of grain kinematics, particle breakage, stresses, and other microscopic phenomena have seldom been made during such experiments, particularly at high pressures relevant to many geologic and man-made processes, limiting our fundamental understanding. To address this issue, we developed a new triaxial compression device called HP-TACO (High-Pressure TriAxial COmpression Apparatus). HP-TACO is a miniaturized, conventional triaxial compression apparatus permitting confining pressures up to 50 MPa and deviatoric straining of materials, while also allowing in situ x-ray measurements of grain-scale kinematics and stresses. Here, we present the design of and first results from HP-TACO during its use in laboratory and synchrotron settings to study grain-scale kinematics and stresses in triaxially compressed sands subjected to 15 and 30 MPa confining pressures. The data highlight the unique capabilities of HP-TACO for studying the high-pressure mechanics of sands, providing new insight into micromechanical processes occurring during geologic and man-made processes. 

Granular materials are found throughout nature and industry: in landslides, avalanches, and river beds, and also in pharmaceutics, food, and mineral processing. Many behaviors of these materials, including the ways in which they pack, deform, flow, and transmit energy, can be fully understood only in the context of inter-particle forces. However, we lack techniques for measuring 3D inter-particle force evolution at subsecond timescales due to technological limitations. Measurements of 3D force chain evolution at subsecond timescales would help validate and extend theories and models that explicitly or implicitly consider force chain dynamics in their predictions. Here, we discuss open challenges associated with force chain evolution on these timescales, challenges limiting such measurements, and possible routes for overcoming these challenges in the coming decade. 

{"Abstract":["Data used for the two-hour photoelasticity lesson on September 29, 2022 at the 2022 ALERT Doctoral School in Aussois, France.  <\/p>\n\nIn the Data directory, you will find the PEGS-master, PhotoelasticDisks, and Results subdirectories. You will also find the Jupyter notebook ALERTPhotoelasticity_220929_v1.ipynb.<\/p>\n\nPhotoelasticity data is in the PhotoelasticDisks subdirectory. N_Image and P_Image contain a sequence of images of 511 bidisperse birefringent disks in simple shear as viewed with unpolarized light and polarized light, respectively. The Positions subdirectory contains the position and radii of the disks in disks in each image. The G2images and radii_highlighted subdirectories contain, respectively: (1) images of each particle colored by G^2 as computed from the photoelasticity images via methods described in (Daniels, et al., Review of Scientific Instruments, 88, 051808 (2017)); (2) images of deformation of the particle with the outlines of each particle highlighted. Computations are performed in the accompanying ALERTPhotoelasticity_220929_v1.ipynb Jupyter notebook, which may be opened on any computer supporting jupyter notebooks or through Google Colab.<\/p>\n\nWithin PEGS-master, you can open PeGSDiskSolve.m to solve for inter-particle forces using methods described in Sec. V of (Daniels, et al., Review of Scientific Instruments, 88, 051808 (2017)) and in the thesis of James Puckett (thesis titled "State Variables in Granular Materials: an Investigation of Volume and Stress Fluctuations" and completed at North Carolina State University in 2012). You can also find a script titled "PlotExpVsSynth.m" that compares results from G^2 calculations; results are put into the Results subdirectory.<\/p>\n\nPaths may need to be changed in all scripts.<\/p>\n\nRelated content from the doctoral school can be found here: https://github.com/alert-geomaterials/2022-doctoral-school. <\/p>"]} 

{"Abstract":["This dataset contains tomography images and stress-strain curves used in the publication titled "Micromechanics and Strain Localization in Sand in the Ductile Regime" in the Journal of Geophysical Research: Solid Earth<\/p>"]} 

We have developed and employed a 3D particle stress tensor and contact force inference technique that employs synchrotron X-ray tomography and diffraction with an optimization algorithm. We have used this technique to study stress and force heterogeneity, particle fracture mechanics, contact-level energy dissipation, and the origin of wave phenomena in 3D granular media for the past five years. Here, we review the technique, describe experimental and numerical sources of uncertainty, and use experimental data and discrete element method simulations to study the method’s accuracy. We find that inferred forces in the strong force network of a 3D granular material are accurately determined even in the presence of noisy stress measurements.