Application of an Elongated Kelvin Model to Space Shuttle Foams

The space shuttle foams are rigid closed-cell polyurethane foams. The two foams used most extensively on the space shuttle external tank are BX-265 and NCF124-124. Because of-the foaming and rising process, the foam microstructures are elongated In the rise direction. As a result, these two foams exhibit a nonisotropic mechanical behavior. A detailed microstructural characterization of the two foams is presented. Key features of the foam cells are described and the average cell dimensions in the two foams are summarized. Experimental studies are also conducted to measure the room temperature mechanical response of the two foams in the two principal material directions (parallel to the rise and perpendicular to the rise). The measured elastic modulus, proportional limit stress, ultimate tensile strength, and Poisson's ratios are reported. The generalized elongated Kelvin foam model previously developed by the authors is reviewed and the equations which result from this model are summarized. Using the measured microstructural dimensions and the measured stiffness,ratio, the foam tensile strength ratio and Poisson's ratios are predicted for both foams and are compared with the experimental data. The predicted tensile strength ratio is in close agreement with the measured strength ratio for both BX-265 and NCFI124-124. The comparison between the predicted Poisson's ratios and the measured values is not as favorable.