Illumination of Damage and Critical Transition During Time-Dependent Deformation in Herrnholz Granite Using Distributed Strain Sensing

Brittle creep in rock that results from time-dependent subcritical crack growth often plays a fundamental role in the emergence of precursory phenomena of impending catastrophic events in the upper crust. Laboratory has been used to investigate time-dependent cracking in brittle rocks, and signals of acoustic emission (AE) and X-ray tomography were employed as proxies for damage accumulation, which increase non-linearly towards failure (Heap et al., 2009; Renard et al., 2020). Despite these studies, the evolution of damage in real time and especially the potential impact of strain localization on dynamic critical transition of failure yet are understudied.

We study brittle creep in a dry Herrnholz granite (with an initial porosity of 2.2%) under triaxial stress conditions. The test procedures consisted of (i) confining the sample to P_c =10 MPa and (ii) then applying and holding a differential stress σ_d = 234 MPa. This stress was held constant and a standard creep response was observed exhibiting a clear trimodal behavior that culminated with the formation of a shear fracture and catastrophic failure of the sample. We used the distributed strain sensing (DSS) fiber optic technology to obtain local estimations of strain and calculate volumetric strain on the surface of the sample using an interpolation strategy. During the primary creep phase, deformation mapped with DSS was found to be sparsely distributed in the form of volumetric deformation in general uniform throughout the sample and expressed on the surface. The transient acceleration of creep (creep burst) was only identified in local strain measurements near the final faulting position and occurred during the steady-state creep phase. This clearly indicates that strain began to localize around the ultimate location of fracture, which was also confirmed by the postmortem 3D optical scanning.

Using this better understanding of progressive strain localization, we searched for indications of damage evolution and critical behavior. During the creep phases, changes in certain properties of DSS array were examined for potential precursory signatures. We analyzed the statistics of damage rate and incremental strain and detected a significant breaking of scaling during the creep phase which led to a critical interpretation of fracture. Prior to the critical point, creep bursts correlated with the nucleation and growth of the main fault, which likely indicates the onset of scaling divergence where damage began to self-organize toward failure. These results show that strain localization which drives the fracture development can be captured by DSS technology and the brittle creep processes in Herrnholz granite follow a critical point transition which can be attributed to a self-adjustment of local strains after creep burst.

 

References:

Heap, M. J., Baud, P., Meredith, P. G., Bell, A. F., & Main, I. G. (2009). Time-dependent brittle creep in Darley Dale sandstone. Journal of Geophysical Research, 114(B7), B07203. https://doi.org/10.1029/2008JB006212

Renard, F., Kandula, N., McBeck, J., & Cordonnier, B. (2020). Creep burst coincident with faulting in marble observed in 4‐D synchrotron X‐ray imaging triaxial compression experiments. Journal of Geophysical Research: Solid Earth, 125, e2020JB020354. https://doi.org/ 10.1029/2020JB020354