Micropolar effect on the cataclastic flow and brittle‐ductile transition in high‐porosity rocks

Abstract

A micromechanical distinct element method (DEM) model is adopted to analyze the grain‐scale mechanism that leads to the brittle‐ductile transition in cohesive‐frictional materials. The cohesive‐frictional materials are idealized as particulate assemblies of circular disks. While the frictional sliding of disks is sensitive to the normal compressive stress exerted on contacts, normal force can be both caused by interpenetration and long‐range cohesive bonding between two particles. Our numerical simulations indicate that the proposed DEM model is able to replicate the gradual shift of porosity change from dilation to compaction and failure pattern from localized failures to cataclastic flow upon rising confining pressure in 2‐D biaxial tests. More importantly, the micropolar effect is examined by tracking couple stress and microcrack initiation to interpret the transition mechanism. Numerical results indicate that the first invariant of the couple stress remains small for specimen sheared under low confining pressure but increases rapidly when subjected to higher confining pressure. The micropolar responses inferred from DEM simulations reveal that microcracking may occur in a more diffuse and stable manner when the first invariant of the macroscopic couple stress are of higher magnitudes.

Brittle to Semibrittle Transition in Quartz Sandstone: Energetics

“…Hence, we conclude that the slope (the hardening behavior) of σ d -ε a curves is governed by slip along shear bands. Our findings support results from constitutive and discrete element models of granular deformation focusing on intergranular sliding without considering intragranular cracking (e.g., Tengattini et al, 2014;Zheng et al, 2016). These models reproduce the essential features of the stress-strain and compaction behavior associated with the transition to cataclastic flow observed in laboratory.…”

“…The observation that many shear bands in the semibrittle faulting regime show relatively large offsets (with offset to length ratio of up to~3%; Table 2) suggests that the bands are long-lived. This inference agrees with permanent, mesoscale bands predicted from models where grain contacts lose shear strength once slip occurs (Wang et al, 2008;Zheng et al, 2016), but differs from short-lived bands shown in models where the once-slipped contacts regain the strength (Aharonov & Sparks, 2002).…”

“…Modeling studies of microcracking provide a physical basis for the transition from brittle faulting to cataclastic flow in porous rocks-through illustrating how a change in the applied stress state influences the stress state at the grain scale (e.g., Aharonov & Sparks, 2002). An increase in pressure (i.e., a concomitant decrease in the ratio of peak stress to mean pressure; σ d /P m ) results in a reduction in the length scale of stress perturbations (e.g., Zheng et al, 2016). At low ratios of σ d /P m , microfracture propagation is restricted within local stress perturbations around defects (e.g., pores, grain contacts, and microfractures).…”

A Model of Failure and Localization of Basalt at Temperature and Pressure Conditions Spanning the Brittle‐Ductile Transition

“…Nonlocal models (Guy et al., 2018; Tvergaard & Needleman, 1995), micropolar continua (Suh et al., 2020; Zheng et al., 2016), gradient‐damage models (de Borst & Verhoosel, 2016; Nguyen et al., 2018), enhanced‐strain methods (Armero & Garikipati, 1996), viscoplastic regularization (Duretz et al., 2020; Simo & Ju, 1987), or other extended formulations are proven methods to mitigate the mesh‐dependent effects (Parisio et al., 2019; Yoshioka et al., 2019). In this work, we apply a mesh‐adjusted softening parameter α d and all simulations are performed with the same discretization (quadrilateral elements of 0.5 mm size).…”