This research is concerned with the modeling and failure analysis of closed cell
foam with various scales of microstructure that is disordered due to defects. This foam
material is used for the forward bipod closeout on the space shuttle external tank. Three
dimensional finite element simulations of closed cell foams with various microstructures
are performed to study the influence of the geometric character of the microstructure (eg.
defect size and distribution) on the stiffness and failure behavior of the foam. First,
regularly arrayed cells are modeled for a reference to compare with the disordered
microstructure. For studying the effect of cellular microstructure, a discrete model is
developed where in every edge and face of each cell are modeled. Two types of defects,
point defects (void) and area defects (knot), are indicated from the examination of
BX250 and BX265 polyurethane foams. However, this research is focused on the point
defect.
Analyzing a material with such complex microstructure is especially
challenging in terms of computation power as well as required modeling techniques. A
finite element model consisting of only beam and shell elements was developed. Certain complications that arise from using beam and shell elements were resolved using novel
techniques. Stiffness predictions from the model agreed with data from the literature for
a wide range of relative densities. Parametric studies were performed to examine the
effect of different properties, such as relative densities and edge fraction, on the
effective stiffness, Von Mises stress, and buckling stress. The thickness of the face plays
an important role in the behavior of the foam material. Linear buckling and postbuckling
analyses were performed to understand the effect of local buckling on the effective
properties of the foam and stress concentrations.
A distorted multicell model was developed to analyze the effect of point defects
on the foam behavior. In particular, two geometric parameters, the defect size and the
defect density (or the distance between two defects) were varied to find their effect on
the stress concentrations and the effective stiffness of the foam. It is seen that the
discrete model that accounts for the foam microstructure reveals much more about the
foam behavior than a homogenous model.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/4986 |
| Date | 25 April 2007 |
| Creators | Sue, Ji Woong |
| Contributors | Whitcomb, John D. |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | 9053819 bytes, electronic, application/pdf, born digital |
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