Program > Browse abstracts by speaker > Jacob Jean-Baptiste

Understanding brittle rock failure mechanisms at the grain to sub-grain scale is a key challenge in geosciences. While triaxial compression experiments combined with dynamic X-ray microtomography offer valuable insights into the three-dimensional strain field evolution, they fail to capture the heterogeneous internal stress field before failure, which is critical for predicting microfracture initiation and propagation. Recent development in synchrotron X-ray diffraction techniques, such as high-energy diffraction microscopy, three-dimensional X-ray diffraction and diffraction contrast tomography, enable non-destructive in situ measurement of crystal lattice orientation, elastic strain, and stress at grain to intra-grain scales. Originally developed for materials science, these methods are now applied to geomaterials, providing unprecedented micromechanical insights.
We used scanning three-dimensional X-ray diffraction to study stress evolution in 5 mm-diameter Berea and Fontainebleau sandstone cores deformed under triaxial compression. Experiments were conducted at beamline ID11 of the European Synchrotron Radiation Facility (ESRF) using the HADES apparatus, which allows simultaneous triaxial compression testing and X-ray data acquisition. Stepwise axial loading was applied to the samples while maintaining a constant 10 MPa confinement. Diffraction scans in quartz, acquired using a pencil beam, provided time-series stress maps across a core transect with a 50 µm resolution. Results reveal progressive internal stress buildup consistent with macroscopic loading, accompanied by reorientation of local stress tensors that increasingly align with the average macroscopic stress. Stress distributions follow an exponential law, with widening distribution tails showing increasing stress heterogeneity as loading progresses. These observations indicate that stress is not uniformly distributed within the samples and may be accommodated by localized structures, potentially resembling force-chain networks observed in granular materials. The increasing heterogeneity in stress distribution may play a role in the development of tensile microfractures initiating perpendicular to the macroscopic principal stress σ₁, ultimately leading to macroscopic failure.