ON THE MECHANICAL BEHAVIOR OF CELLULAR MATERIALS

 

Stelios Kyriakides

Center for Mechanics of Solids, Structures & Materials

The University of Texas at Austin, WRW 110

Austin, TX 78712

E-mail:  skk@mail.utexas.edu

 

Abstract

The compressive response of many cellular materials is characterized by a nearly linear elastic regime which terminates into a limit load. This is followed by an extensive load plateau responsible for their excellent energy absorption characteristics. The plateau usually ends into a second branch of stiff response. Results from a polymeric honeycomb and polyurethane open-cell foams will be used to illustrate these mechanical characteristics. The 2-D geometry and nearly periodic microstructure of honeycombs allows detailed experimental examination of the localized inelastic buckling, and the spreading of collapse which are responsible for these characteristics. It will be shown that, provided the geometry and mechanical properties of the solid are accurately characterized, the crushing response of such honeycombs can be reproduced analytically.

 

The behavior of space-filling open cell foams is generally similar although much more challenging to characterize due to the less regular, 3-D microstructure. Again, experiments coupled with several levels of modeling are used to illustrate these challenges. The experiments include measurement of the complete mechanical response of foams of various cell sizes, characterization of the foam microstructure, and measurement of the mechanical properties of the foam struts. The modeling involves: beam-type models for the elastic properties; numerical, characteristic cell-type models for the elastic properties and the onset of instability; and large-scale models which can reproduce all aspects of the compressive response including the large deformation crushing. The microstructure is first idealized as regular Kelvin cells but with realistic strut geometric characteristics. It will be shown that the onset of instability and its subsequent localization and spreading occur due to elastic buckling of the microstructure.