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Research Papers

Analysis of Space Shuttle External Tank Spray-On Foam Insulation With Internal Pore Pressure

[+] Author and Article Information
Brett A. Bednarcyk

 NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135brett.a.bednarcyk@nasa.gov

Jacob Aboudi

 Tel Aviv University, Tel Aviv 69978, Israel

Steven M. Arnold, Roy M. Sullivan

 NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135

J. Eng. Mater. Technol 130(4), 041005 (Sep 09, 2008) (16 pages) doi:10.1115/1.2969247 History: Received May 16, 2007; Revised June 02, 2008; Published September 09, 2008

The polymer spray-on foam insulation used on NASA’s Space Shuttle external fuel tank is analyzed via the high-fidelity generalized method of cells micromechanical model. This model has been enhanced to include internal pore pressure, which is applied as a boundary condition on the internal faces of the foam pores. The pore pressure arises due to both ideal gas expansion during a temperature change as well as outgassing of species from the foam polymer material. Material creep and elastic stiffening are also incorporated via appropriate constitutive models. Due to the lack of reliable properties for the in situ foam polymer material, these parameters are backed out from foam thermomechanical test data. Parametric studies of the effects of key variables (both property-related and microstructural) are presented as is a comparison of model predictions for the thermal expansion behavior of the foam with experimental data.

Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

(a) A porous material with triply periodic microstructures. (b) The repeating unit cell, defined in the (y1,y2,y3) coordinate system, is discretized into Nα×Nβ×Nγ subcells. (c) The monolithic (or empty) subcell is defined in the local coordinate system (y¯1(α),y¯2(β),y¯3(γ)).

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Figure 2

Tensile response of the BX-265 foam perpendicular to the rise direction—load up to 0.303 MPa followed by 15 s hold (32)

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Figure 3

Tensile response of the foam polymer material (with no voids) as predicted by the combination of the elastic stiffening model and the Newtonian-viscous creep model

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Figure 4

Micrograph of the BX-265 spray-on foam insulation

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Figure 5

Elongated tetrakaidecahedron representation of the foam microstructure

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Figure 6

HFGMC repeating unit cell (7×7×7 subcells) used to represent the spray-on foam insulation microstructure (not to scale). Subcell dimensions are in microns.

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Figure 7

Dog bone test specimen used to measure the thermomechanical response of the BX-265 foam

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Figure 8

Experimental thermomechanical test stress-strain curve: (1) load to 0.303 MPa at room temperature, (2) hold for 15 s, (3) cool to −196°C under load, (4) unload to 0.034 MPa, and (5) load to failure (32)

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Figure 9

Model-experiment correlation for room-temperature loadup and hold portion of the thermomechanical foam response

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Figure 10

Axisymmetric transient thermal boundary value problem for the foam specimen quenched in liquid nitrogen

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Figure 11

Comparison of finite element and analytical results for the average temperature versus time in the foam specimen during cooldown

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Figure 12

Model-experiment correlation for the full thermomechanical test on the BX-265 foam (perpendicular to rise)

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Figure 13

Predicted effective stress (σeff=3J2) fields (MPa) at three cut locations within the foam microstructure (see Fig. 6) at the end of steps 1–3 of the thermomechanical loading profile

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Figure 14

Predicted equivalent plastic strain (2εijinεijin/3) fields (%) at three cut locations within the foam microstructure (see Fig. 6) at the end of steps 1–3 of the thermomechanical loading profile

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Figure 15

Predicted effective stress (σeff=3J2) fields (MPa) at three cut locations within the foam microstructure (see Fig. 6) at the end of step 5 of the thermomechanical loading profile

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Figure 16

(a) Parametric study showing the effect of a pore prepressure on the predicted tensile response of the BX-265 foam (perpendicular to rise). (b) Parametric study showing the effect of applied strain rate (with and without a pore prepressure) on the predicted tensile response of the BX-265 foam (perpendicular to rise).

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Figure 17

Parametric study comparing the predicted thermal response of the BX-265 foam (perpendicular to rise) with and without pressure increase due to outgassing in addition to the ideal gas expansion of air in the pores

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Figure 18

Comparison of pore pressure versus temperature profile calculated based on thermogravimetric analysis (TGA) of the BX-265 foam (29), which includes the contribution due to outgassing of water, with the applied pore pressure employed in the elevated temperature simulations

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Figure 19

Parametric study showing the predicted temperature-induced creep behavior of the BX-265 foam after a heatup to the indicated temperature followed by a 120 s hold (perpendicular to rise)

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Figure 20

Parametric study showing the effect of the HFGMC repeating unit cell (and pore) aspect ratio on the predicted thermal response of the BX-265 foam

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Figure 21

Comparison of model predictions of the BX-265 foam thermal expansion with experimental data

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