Synchrotron X-ray imaging in 4D: Spatiotemporal evolution of damage and compaction localization in triaxially compressed porous limestone

Abstract:

The investigation of the failure and strain localization in a porous rock is essential in poromechanics and rock physics. Extensive triaxial compression experiments on porous rocks have revealed a broad spectrum of failure modes. Techniques such as acoustic emission provide spatiotemporal information on the partitioning and localization of damage. Observations on deformed samples using microscopy and microcomputed tomography (CT) can also reveal microscale damage and its distribution. Only by synthesizing such complementary measurements on multiple scales could one infer the dynamics of rock failure phenomena. A conventional triaxial compression experiment on Leitha limestone (with an initial porosity of ~22%) was conducted in dry conditions at confining pressure of 20 MPa and in situ CT imaging was performed (Huang et al., 2019). With increasing differential stress, the sample strain hardened, and two distinct yield points were identified in the stress-strain curve. At an intermediate scale of 10 voxels (65μm), the spatiotemporal evolution of local porosity and damage revealed the development of five discrete compaction bands, the first unambiguous observation of such a bifurcation phenomenon in a porous carbonate rock. The geometric attributes and grain-scale damage processes revealed by voxel-scale CT images are similar to those for compactions bands in porous sandstones. To elucidate the multi-scale development of damage, 3D digital image correlation (DIC) was used to quantify the spatiotemporal development of displacement and strain fields associated with the CT images. The DIC analysis demonstrates that the initiation and propagation of compaction bands were manifested by localization of significant volumetric strain. Whereas geometry of the localization seems more complex than the idealized model of discrete planar zones, the local strain tensor inside a band was inferred to evolve with initiation and propagation, with the maximum principal strain typically aligned subparallel to the axial direction.