Strain Localization Above the Yielding Point in Cyclically Deformed Glasses

Abstract

We study the yielding behavior of a model glass under cyclic athermal quasistatic deformation and at finite rate and temperature, computationally, and show that yielding is characterized by the discontinuous appearance of shear bands, whose width is about ten particle diameters at their initiation, in which the strain gets localized. Strain localization is accompanied by a corresponding change in the energies and a decrease in the density in the shear band. We show that the glass remains well annealed outside the shear band, whereas the energies correspond to the highest possible energy minima at the given density within the shear band. Diffusive motion of particles characterizing the yielded state are confined to the shear bands, whose mean positions display movement over repeated cycles. Outside the shear band, particle motions are subdiffusive but remain finite. Despite the discontinuous nature of their appearance, shear bands are reversible in the sense that a reduction in the amplitude of cyclic deformation to values below yielding leads to the healing and disappearance of the shear bands.

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Plain Language Summary

Understanding how solid materials respond to stress is critical to their characterization and is useful in understanding a variety of phenomena, ranging from earthquakes to the flow of toothpaste. Under large-enough stress, these materials “yield” and exhibit permanent deformation or plasticity. Researchers would like to better understand how this transition occurs when such stresses, or deformations, are oscillatory. Using computer simulations, we study how a glasslike solid responds to cyclic deformation.

We find that beyond some threshold amount of deformation, yielding occurs as a sharp transition and is accompanied by the sudden appearance of narrow regions of intense shearing strain, known as shear bands. Within these bands, there is an increase in interaction energy even while the energy decreases in the rest of the glass. We show that despite their discontinuous appearance, the formation of the shear bands is reversible—a reduction of the applied deformation below the yielding threshold leads to the healing and disappearance of the shear bands.

Our work highlights the role of mechanically induced annealing, which should be taken into account in attempts to understand conditions for shear-band formation.