Predicting plasticity of amorphous solids from instantaneous normal modes

TL;DR

This paper extends the concept of instantaneous normal modes to deformed amorphous solids, enabling analytical predictions of their plasticity and stress-strain behavior with good agreement to simulations.

Contribution

It introduces a strain-dependent INM framework for amorphous solids, linking vibrational modes to deformation and yielding without adjustable parameters.

Findings

Accurately predicts stress-strain curves and yield points.

Reveals soft and unstable modes are preserved under strain analysis.

Shows strong agreement with simulation data.

Abstract

We present a mathematical description of amorphous solid deformation and plasticity by extending the concept of instantaneous normal modes (INMs) to deformed systems, which allows us to retain the effect of strain on the vibrational density of states (VDOS). Starting from the nonaffine lattice dynamics (NALD) description of elasticity and viscoelasticity of glasses, we formulate the linear response theory up to large deformations by considering the strain-dependent tangent modulus at finite values of shear strain. The (nonaffine) tangent shear modulus is computed from the vibrational density of states (VDOS) of affinely strained configurations at varying strain values. The affine strain, found analytically on the static (undeformed) snapshot of the glass, leads to configurations that are rich of soft low-energy modes as well as unstable modes (negative eigenvalues) that are otherwise completely "washed out" and lost, if one lets the system relax after strain. This procedure is fully consistent with the structure of NALD. The INM spectrum of deformed states allows for the analytical prediction of the stress-strain curve of a model glass, including the prediction of the yield point. Good parameter-free quantitative agreement is shown between the prediction and simulations of athermal quasi-static shear of a coarse-grained polymer glass.