McMat 2007 (ASME Applied Mechanics and Materials Conference)
                University of Texas at Austin, June 3-7, 2007 


   NONLINEAR DEFORMATION MECHANISMS IN METALLIC SPONGES WITH HOLLOW STRUTS

                                T. Daxner

An interesting topology for cellular materials can be obtained by coating a
polymer precursor structure with a layer of metallic material and subsequently
removing the polymer template skeleton by chemical or thermal treatment.  The
resulting micro-structure can be described as a metallic sponge with hollow
struts.

In this paper periodic finite element unit cell models are presented which are
suitable for predicting the deformation mechanisms in such materials. In
analogy to the real process, a Weaire-Phelan type structure representing an
open-cell network of Plateau borders is surrounded by a layer of continuum
solid and continuum shell elements giving a geometrically accurate
representation of the real microstructure.  The principal geometrical
parameters that characterize the microstructure are the relative density of the
template structure and the coating thickness; the influence of these
parameters can be captured well.
  
With respect to the predicted deformation mechanisms the focus is put on
nonlinear effects such as elastic bifurcation buckling as well as on
self-contact of the interior surface of the hollow struts.  The character of
the predicted buckling modes is shown to depend on the thickness and the
curvature of the cell walls, with global, beam-like buckling of the struts or
local, shell-like buckling of the cell wall being possible.  The self-contact
on the interior surface in the vicinity of the ribs, which form at the edges of
the plateau borders, is shown to influence the nonlinear deformation behavior
of the structure.