Do Discrete Fine-Scale Mechanical Models with Rotational Degrees of Freedom Homogenize Into a Cosserat or a Cauchy Continuum?

TL;DR

This paper investigates whether homogenizing discrete mechanical models with rotational degrees of freedom results in a Cosserat or Cauchy continuum, using asymptotic expansion homogenization to analyze the conditions and validity of each continuum type.

Contribution

It provides a rigorous derivation showing the conditions under which homogenization yields Cosserat or Cauchy continua, including the transition between these limits.

Findings

Homogenization always involves a stationary unit cell problem.

The only inertia appears in the linear momentum balance at the coarse scale.

Significant Cosserat effects are linked to non-physical high bending stiffness.

Abstract

This article answers the question of whether homogenization of discrete fine-scale mechanical models, such as particle or lattice models, gives rise to an equivalent continuum that is of Cauchy-type or Cosserat-type. The study employs the machinery of asymptotic expansion homogenization to analyze discrete mechanical models with rotational degrees of freedom commonly used to simulate the mechanical behavior of heterogeneous solids. The proposed derivation has general validity in both stationary (steady-state) and transient conditions (assuming wavelength much larger that particle size) and for arbitrary nonlinear, inelastic fine-scale constitutive equations. The results show that the unit cell problem is always stationary, and the only inertia term appears in the linear momentum balance equation at the coarse scale. Depending on the magnitude of the local bending stiffness, mathematical homogenization rigorously identifies two limiting conditions that correspond to the Cauchy continuum and the Cosserat continuum. A heuristic combination of these two limiting conditions provides very accurate results also in the transition from one limiting case to the other. Finally, the study demonstrates that cases for which the Cosserat character of the homogenized response is significant are associated with non-physically high fine-scale bending stiffness and, as such, are of no interest in practice.