NASA Technical Reports Server (NTRS) 20070008202: Porous Media Approach for Modeling Closed Cell Foam

1.2 MB·English

by  NASA Technical Reports Server (NTRS)

Porous Media Approach for Modeling Closed Cell Foam

Louis J. Ghosn
Ohio Aerospace Institute
Brook Park, Ohio

Roy M. Sullivan
NASA Glenn Research Center
Cleveland, Ohio

Proposed Abstract for the
43rd Annual Technical Meeting
Society of Engineering Science
August 13-16th, 2006

In order to minimize boil off of the liquid oxygen and liquid hydrogen and to
prevent the formation of ice on its exterior surface, the Space Shuttle External Tank (ET)
is insulated using various low-density, closed-cell polymeric foams. Improved analysis
methods for these foam materials are needed to predict the foam structural response and
to help identify the foam fracture behavior in order to help minimize foam shedding
occurrences. This presentation describes a continuum based approach to modeling the
foam thermo-mechanical behavior that accounts for the cellular nature of the material and
explicitly addresses the effect of the internal cell gas pressure.

A porous media approach is implemented in a finite element frame work to model
the mechanical behavior of the closed cell foam. The ABAQUS general purpose finite
element program is used to simulate the continuum behavior of the foam. The soil
mechanics element is implemented to account for the cell internal pressure and its effect
on the stress and strain fields. The pressure variation inside the closed cells is calculated
using the ideal gas laws. The soil mechanics element is compatible with an orthotropic
materials model to capture the different behavior between the rise and in-plane directions
of the foam.

The porous media approach is applied to model the foam thermal strain and calculate the
foam effective coefficient of thermal expansion. The calculated foam coefficients of
thermal expansion were able to simulate the measured thermal strain during heat up from
cryogenic temperature to room temperature in vacuum. The porous media approach was
applied to an insulated substrate with one inch foam and compared to a simple elastic
solution without pore pressure. The porous media approach is also applied to model the
foam mechanical behavior during subscale laboratory experiments. In this test, a foam
layer sprayed on a metal substrate is subjected to a temperature variation while the metal
substrate is stretched to simulate the structural response of the tank during operation. The
thermal expansion mismatch between the foam and the metal substrate and the thermal
gradient in the foam layer causes high tensile stresses near the metal/foam interface that
can lead to delamination.
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