An official website of the United States government
Abstract The operation of fracture, diffusion, and intracrystalline‐plastic micromechanisms during semibrittle deformation of rock is directly relevant to understanding mechanical behavior across the brittle‐plastic transition in the crust. An outstanding question is whether (1) the micromechanisms of semibrittle flow can be considered to operate independently, as represented in typical crustal strength profiles across the brittle to plastic transition, or (2) the micromechanisms are coupled such that the transition is represented by a distinct rheology with dependency on effective pressure, temperature, and strain rate. We employ triaxial stress‐cycling experiments to investigate elastic‐plastic and viscoelastic behaviors during semibrittle flow in two distinctly different monomineralic, polycrystalline, synthetic salt‐rocks. During semibrittle flow at high differential stress, granular, low‐porosity, work‐hardened salt‐rocks deform predominantly by grain‐boundary sliding and wing‐crack opening accompanied by minor intragranular dislocation glide. In contrast, fully annealed, near‐zero porosity salt‐rocks flow at lower differential stress by intragranular dislocation glide accompanied by grain‐boundary sliding and opening. Grain‐boundary sliding is frictional during semibrittle flow at higher strain rates, but the associated dispersal of water from fluid inclusions along boundaries can activate fluid‐assisted diffusional sliding at lower strain rates. Changes in elastic properties with semibrittle flow largely reflect activation of sliding along closed grain boundaries. Observed microstructures, pronounced hysteresis and anelasticity during cyclic stressing after semibrittle flow, and stress relaxation behaviors indicate coupled operation of micromechanisms leading to a distinct rheology (hypothesis 2 above).
more »
« less
Relating strain localization and Kaiser effect to yield surface evolution in brittle rocks
SUMMARY The yield surfaces of rocks keep evolving beyond the initial yield stress owing to the damage accumulation and porosity change during brittle deformation. Using a poroelastic damage rheology model, we demonstrate that the measure of coupling between the yield surface change and accumulated damage is correlated with strain localization and the Kaiser effect. Constant or minor yield surface change is associated with strong strain localization, as seen in low-porosity crystalline rocks. In contrast, strong coupling between damage growth and the yield surface leads to distributed deformation, as seen in high-porosity rocks. Assuming that during brittle deformation damage occurs primarily in the form of microcracks, we propose that the measured acoustic emission (AE) in rock samples correlates with the damage accumulation. This allows quantifying the Kaiser effect under cyclic loading by matching between the onset of AE and the onset of damage growth. The ratio of the stress at the onset of AE to the peak stress of the previous loading cycle, or Felicity Ratio (FR), is calculated for different model parameters. The results of the simulation show that FR gradually decreases in the case of weak coupling between yield surface and damage growth. For a strong damage-related coupling promoting significant yield surface change, the FR remains close to one and decreases only towards the failure. The model predicts that a steep decrease in FR is associated with a transition between distributed and localized modes of failure. By linking the evolving yield surface to strain localization patterns and the Kaiser effect, the poroelastic damage rheology model provides a new quantitative tool to study failure modes of brittle rocks.
more »
« less
- Award ID(s):
- 1761912
- PAR ID:
- 10227364
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 221
- Issue:
- 3
- ISSN:
- 0956-540X
- Page Range / eLocation ID:
- 2091 to 2103
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The onset of brittle failure in rocks includes dilatancy and strain localization. To better understand this nucleation process, we analyze the evolution of the local three‐dimensional strain tensor using X‐ray tomograms acquired during triaxial compression experiments on granite and sandstone. The onset of the localization of the compaction, dilation, and shear strain occurs when ∼65% of the rock volume experiences dilation. Tracking the locations of the high strains throughout loading suggests that the deformation that occurs early in loading influences the location of the system‐sized fracture network that produces macroscopic failure. This influence is larger in the sandstone experiments than the granite experiments, likely due to the microstructure of the sandstone. These results have important implications for detecting precursors to catastrophic failure.more » « less
-
Abstract The spatial organization of deformation may provide key information about the timing of catastrophic failure in the brittle regime. In an ideal homogenous system, deformation may continually localize toward macroscopic failure, and so increasing localization unambiguously signals approaching failure. However, recent analyses demonstrate that deformation, including low‐magnitude seismicity, and fractures and strain in triaxial compression experiments, experience temporary phases of delocalization superposed on an overall trend of localization toward large failure events. To constrain the conditions that promote delocalization, we perform a series of X‐ray tomography experiments at varying confining pressures (5–20 MPa) and fluid pressures (0–10 MPa) on Westerly granite cores with varying amounts of preexisting damage. We track the spatial distribution of the strain events with the highest magnitudes of the population within a given time step. The results show that larger confining pressure promotes more dilation, and promotes greater localization of the high strain events approaching macroscopic failure. In contrast, greater amounts of preexisting damage promote delocalization. Importantly, the dilative strain experiences more systematic localization than the shear strain, and so may provide more reliable information about the timing of catastrophic failure than the shear strain.more » « less
-
Background Porosity and other defects resultant by additive manufacturing processes are well-known to affect mechanical properties. However, there remains limited understanding regarding how the internal defect structure influences the evolution of the local strain field, as experimental investigations have not presented direct measurements of the evolving internal strain field in the presence of defects. Objective Interrupted in-situ tensile tests in a lab-based X-ray computed tomography machine were used to investigate the evolution of the strain field around internal defects. The evolution of the internal strain field facilitated examination of the role of specific defects in the macroscopic deformation of additively manufactured tensile coupons. Methods Samples were produced in 316L stainless steel by laser powder bed fusion. An in situ loading device was utilized to subject the samples to tensile failure within a tomographic scanning environment. Digital volume correlation was utilized to directly determine local strain levels within the additively manufactured components in the vicinity of porosity defects. Results Effects of porosity on strain localization and eventual failure of the samples were evaluated. Characteristics of the porosity distribution, including presence of porosity at the surface or near-surface of components, as well as the proximity of pores to each other were found to influence the evolution of failure. Early onset of failure was found to be associated with the availability of neighboring porosity that allowed for rapid progression of the fracture path. Conclusions The direct measurements of strain field evolution in the present study established understanding regarding how internal defect structure characteristics influence the evolution of the local strain field for additively manufactured components. This high fidelity characterization and the associated phenomenological observations have bearing for supporting validation of numerical modeling frameworks for describing failure in these materials.more » « less
-
Abstract Forecasting the onset of a volcanic eruption from a closed system requires understanding its stress state and failure potential, which can be investigated through numerical modeling. However, the lack of constraints on model parameters, especially rheology, may substantially impair the accuracy of failure forecasts. Therefore, it is essential to know whether large variations and uncertainties in rock properties will preclude the ability of models to predict reservoir failure. A series of two‐dimensional, axisymmetric models are used to investigate sensitivities of brittle failure initiation to assumed rock properties. The numerical experiments indicate that the deformation and overpressure at failure onset simulated by elastic models will be much lower than the viscoelastic models, when the timescale of pressurization exceeds the viscoelastic relaxation time of the host rock. Poisson's ratio and internal friction angle have much less effect on failure forecasts than Young's modulus. Variations in Young's modulus significantly affect the prediction of surface deformation before failure onset when Young's modulus is < 40 GPa. Longer precursory volcano‐tectonic events may occur in weak host rock (E< 40 GPa) due to well‐developed Coulomb failure prior to dike propagation. Thus, combining surface deformation with seismicity may enhance the accuracy of eruption forecast in these situations. Compared to large and oblate magma systems, small and prolate systems create far less surface uplift prior to failure initiation, suggesting that more frequent measurements are necessary.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/gji/ggaa130
