Microstructures in solids:
their critical role in governing material mechanical behavior

Katia Bertoldi
Postdoctoral Associate
Department of Mechanical Engineering
Massachusetts Institute of Technology

Abstract
One of the most exciting technological advancements of recent decades is the ability to probe, understand, and design novel materials at the small scale, such as nano-composites and biological composites. These novel materials are particularly interesting, since their morphology is tailored at the nanometer and the micrometer length scales to enable a wide range of macroscopic level functions and they often exhibit superior mechanical properties.

The microstructure of small-scale materials can now be accurately measured using a variety of scale appropriate microscopy tools (from optical to electronic to atomic force microscopy) as well as x-ray and spectroscopic techniques. This knowledge of structural details enables rigorous micromechanics-based development of constitutive equations for the macroscopic behavior of such materials. The constitutive equations are of crucial importance, since they help us to better understand, explain and predict experimental results as well as provide a framework to probe the underlying physics of the mechanical behavior of the materials.

The development of microstructurally-informed macroscopic constitutive equations for the deformation of a selection of novel materials will be presented, including lattice structures, structural interfaces, fiber-bridged elliptical voids. The critical role of microstructure in governing specific features of mechanical behavior will be discussed.
Specifically, in the field of lattice structures, periodic elastomeric solids are subjected to uniaxial compression and novel transformations of the patterned structures are found upon reaching a critical value of applied load. Numerical analyses clearly show the mechanism of the pattern switch to be a form of microstructural elastic instability, giving reversible and repeatable transformation events as confirmed by experiments. This behaviour provides opportunities for transformative phononic/photonic crystals which can switch in a sudden, but controlled manner.

In the field of structural interfaces, a new model of structure embedded in a continuum has been introduced and systematically investigated. The interest in the proposal is that the incorporation of a specific structure in general introduces non-local effects and that these follow from the description in a natural and rational way.

An unexpected outcome from the structural interface model has been the possibility of analyzing elliptical voids bridged by fibers, demonstrating the effects of fibers geometry and stiffness.

Tuesday, March 4th
Wu & Chen Auditorium
10:30AM