Strain localization in planar shear of granular media: the role of porosity and boundary conditions

Pařez, Stanislav; Trávníčková, Tereza; Svoboda, Martin; Aharonov, Einat

doi:10.1140/epje/s10189-021-00138-2

Cited by 3 publications

(7 citation statements)

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“…The role of grain‐scale properties in macroscopic shear characteristics has been explored in a number of studies. For stiff enough particles with Young's modulus much greater than stresses in the system,

E\gtrsim 1{0}^{3}{\sigma }_{\mathrm{n}}^{\prime }

, the influence of Young's modulus on friction and dilatation becomes small (da Cruz et al., 2005; Favier de Coulomb et al., 2017; Parez et al., 2021). The grain surface friction coefficient appreciably affects shear characteristics only for μ g ≤ 0.2 (da Cruz et al., 2005), corresponding to almost frictionless grains.…”

Section: Discussionmentioning

confidence: 99%

“…Many works have also documented that fault activation is more complex than described by the effective stress law and depends also on pressure gradients (Dahm et al., 2010; Koehn et al., 2020; Rozhko, 2010; Yang & Juanes, 2018) and on the rate of pressurization (French et al., 2016; L. Wang et al., 2020). Finally, fault dilatation also affects shear localization (Bos et al., 2000; Parez et al., 2021; Rattez et al., 2018), which is frequently associated with rate‐weakening behavior (Marone et al., 1990).…”

Section: Introductionmentioning

confidence: 99%

“…Wang et al, 2020). Finally, fault dilatation also affects shear localization (Bos et al, 2000;Parez et al, 2021;Rattez et al, 2018), which is frequently associated with rate-weakening behavior (Marone et al, 1990).…”

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confidence: 99%

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Pore Pressure Drop During Dynamic Rupture and Conditions for Dilatancy Hardening

2023

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Geological faults are often saturated with fluids. Pore fluid pressure controls fault strength and stability of slip. The effect of pore fluid pressure, P, on the shear strength, τ, is commonly expressed by the effective stress law (Hubbert & Rubey, 1959)where μ is the friction coefficient, and σ n is fault-normal (tectonic) stress. Increasing pore pressure reduces the effective normal stress, 𝐴𝐴 𝐴𝐴 ′ n = 𝐴𝐴n − 𝑃𝑃 , and thus the resistance to sliding. Fluid-induced changes in the effective stress have been invoked to explain the earthquake cycle (R.

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E\gtrsim 1{0}^{3}{\sigma }_{\mathrm{n}}^{\prime }

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Pore Pressure Drop During Dynamic Rupture and Conditions for Dilatancy Hardening

2023

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“…For the four case studies, shearing occurs under quasi-static conditions in which dynamic effects are not significant. As a result of high confining pressures and small particle sizes, the inertial number is lower than 10 −3 , even if the seismic shear rates considered in the models are high (Parez et al 2021). Additionally, the frictional strength in quasi-static shear flow grows very slowly with the inertial number (Peyneau & Roux 2008).…”

Section: Phase 2 Of Shear-zone Growth At Submelting Temperaturesmentioning

confidence: 97%

“…Moreover, the relative contribution of effective cohesion to the shear strength decreases significantly at high confining pressure (Estrada & Taboada 2013). Thus, for increasing confining pressure, the particle model tends to behave as a cohesionless frictional material, in which the average shear strain is distributed within a thicker gouge zone to reduce the inertial number and the shear strength (Parez et al 2021).…”

Section: Wear Ratementioning

confidence: 99%

Rheology and breakdown energy of a shear zone undergoing flash heating in earthquake-like discrete element models

Taboada

Renouf

2023

Geophysical Journal International

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Summary We study the initiation and growth of a dry granular shear zone subjected to seismic shearing and flash heating from the perspective of a discrete element method. For this purpose, we created a semi-periodic numerical shear test similar to a rotary shear machine in which a 2 mm ×1.5 mm sample composed of micrometric cohesive disks is sheared in between two rigid walls. The strength of cohesive bonds is defined according to an elasto-brittle contact law calibrated to simulate peak and residual strength envelopes derived from rock mechanics tests. The sample is traversed by a pre-existing fracture and subjected to a vertical confining pressure (e.g., 112.5 MPa) and a velocity step function (e.g., 1 m/s) applied on the top and bottom walls, respectively. Slip along the fracture induces the growth of a shear zone, which thickens by progressive abrasion of damaged material from cohesive blocks. We carried out two parametric studies to determine the rheology and physical properties of the shear zone for slip velocities and confining pressures characteristic of shallow earthquakes and several flash-heating temperatures. According to parametric studies, the mechanical behavior of the shear zone exhibits three distinct phases. The initial phase of rupture initiation is characterized by the propagation of a shear instability generated by the velocity step (phase 1). During this phase, friction and dilatancy curves are approximated by asymmetric peak functions whose amplitude and geometry are controlled primarily by confining pressure. In the intermediate phase of shear-zone growth, the sample displays an initial transient stage that asymptotically approaches steady state at submelting temperatures (phase 2). According to the inertial number, seismic shearing occurs under quasistatic conditions despite high shear rates. Thus, friction and dilatancy observed in all simulations are roughly constant regardless of slip velocity, confining pressure, and gouge zone thickness. In the final phase of shear weakening, the model evolves toward a new steady state at flash-heating temperatures (phase 3). Average friction and dilatancy are represented by sigmoidal decreasing curves that approach steady-state values lower than phase 2. Predictably, the thermally weakened friction in steady state (μss ∼ 0.1) is close to the strength of frictionless granular samples sheared in quasistatic conditions. We calculate breakdown energies for the gouge and damage zones and the fracture energy at intermediate and high confining pressures. We show that breakdown energy fundamentally differs from fracture energy commonly used in seismology. The breakdown energy of the damage zones shows long-period damped oscillations weakly correlated with shear-stress fluctuations around average decaying values. Our results suggest that dilatancy is the primary energy sink within the damage zones at steady-state values. The breakdown energy components of the gouge zone follow a similar decaying trend as the average fracture energy but over a longer critical distance. Decohesion and dilatancy are the major energy sinks linked to gouge formation at intermediate pressures. In contrast, dilatancy and debonding frictional energies predominate at high confining pressures. Breakdown energy is equivalent to a fraction of fracture energy that nearly triples when doubling the confining pressure.

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Particle fluctuations and their effects on the rheological behavior of sheared granular flows

2023

Physics of Fluids

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We present numerical simulations of plane-sheared granular flows in two pressure-controlled configurations and investigate the particle fluctuations over a wide range of the inertial number I. Quantities affecting the velocity fluctuations, including the granular temperature and the stress ratio, are shown to be intrinsically related and to exhibit similar I-dependent characteristics. We first propose a scaling law describing the one-to-one relationship between the granular temperature and I in both the inertial and collisional regimes, where the volume fraction plays an important role. This relation differs from the power-law dependence between the stress ratio and I. However, for low values of I, these two velocity fluctuation quantities deviate from the one-to-one relation, similar to the effective friction coefficient μ. Based on the obtained inverse power law, we propose a new unified model incorporating the stress ratio to describe the rheological behaviors in the quasistatic, inertial, and collisional regimes. The proposed model is more applicable than the one that integrates the granular temperature, allowing nonlocal effects to be roughly eliminated at low values of I and the rheological dependence on the volume fraction to be effectively removed at high values of I. This provides an alternative approach for developing rheological models for granular materials under complex flow conditions.

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Strain localization in planar shear of granular media: the role of porosity and boundary conditions

Cited by 3 publications

References 81 publications

Pore Pressure Drop During Dynamic Rupture and Conditions for Dilatancy Hardening

Pore Pressure Drop During Dynamic Rupture and Conditions for Dilatancy Hardening

Rheology and breakdown energy of a shear zone undergoing flash heating in earthquake-like discrete element models

Particle fluctuations and their effects on the rheological behavior of sheared granular flows

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