Factors Determining Shear‐Parallel Versus Low‐Angle Shear Band Localization in Shear Deformations: Laboratory Experiments and Numerical Simulations

Abstract

The mechanics of shear failure is a key to understanding a wide range of geological processes, from landslides on the surface to faulting in the Earth's interior. It is now a well-established fact that ductile materials yield under a threshold stress condition, forming shear bands of finite length, which eventually influence the paths of their bulk failure. Compression test experiments show mechanically homogeneous isotropic solids undergo shear failure to produce a conjugate set of shear bands or fractures symmetrically oriented to the applied compression direction (Anand & Spitzig, 1980;Bowden & Raha, 1970;Hutchinson & Tvergaard, 1981;Tvergaard et al., 1981). Employing various yield criteria, many workers have provided theoretical solutions to predict the band orientation as a function of material parameters, such as coefficient of internal friction, dilatancy factor, and strain hardening and softening properties (Anand & Spitzig,

On the origin of shear-band network patterns in ductile shear zones

“…The open-source geodynamic code Underworld is available at www.underworldcode.org, and model parameters required to replicate the results are detailed in the manuscript and in [18]. The data are provided in electronic supplementary material [66].…”

“…The numerical modelling utilizes the open-source, particle-in-cell, finite element code Underworld2 [45], which solves the incompressible Stokes equations for viscoplastic materials, combining the energy conservation equation. Shear zones are modelled as mechanically weak zone, bounded by two relatively stronger [45] and non-deformable walls represented by the model shear zone boundary [18]. The shear zone with thickness W is divided into a zone of effective shear zone (thickness=Wc) sandwiched between two zones of drag zone (thickness=Wd) such that W=Wc+2Wd (figure 6 a ).…”

“…The heterogeneity models, however, do not account for the mechanisms of band networking in mechanically homogeneous shear zones, at least on the scale of macroscopic observations. Numerical simulations based on viscoplastic rheology suggest that shear band networking is possible to occur in homogeneous materials [18,19]. None of the earlier studies, however, systematically investigated the progressive growth of shear band network in an evolving shear zone within homogeneous bulk materials.…”

“…Note that we use the term R type shear band, as used by several previous workers in the foregoing descriptions, to denote this type of low-angle shear bands. Ductile shear zones often form a third type of low-angle shear band, called P-bands, with vergence opposite to the R type [18].

Figure 1A synthetic diagram of secondary shear bands: R (Riedel), C and P and their geometrical relationship with the bulk shear sense (dextral) observed in a ductile shear zone.

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“…anastomosing network formation as a result of various factors, such as thickness variations and mechanical heterogeneities in shear zones [2,[12][13][14][15][16]. A more serious concern is that field observations [1,[17][18][19] and numerical modelling [18][19][20] suggest that simple shear kinematics can produce similar band networks. These divergent views imply that the band networking mechanisms and their controlling factors are yet to be fully understood, and the problem demands a systematic study.…”