Application of Video Image Correlation Techniques to the Space Shuttle External Tank Foam Materials

Results that illustrate the use of a video-image-correlation-based displacement and strain measurement system to assess the effects of material nonuniformities on the behavior of the sprayed-on foam insulation (SOFI) used for the thermal protection system on the Space Shuttle External Tank are presented. Standard structural verification specimens for the SOFI material with and without cracks and subjected to mechanical or thermal loading conditions were tested. Measured full-field displacements and strains are presented for selected loading conditions to illustrate the behavior of the foam and the viability of the measurement technology. The results indicate that significant strain localization can occur in the foam because of material nonuniformities. In particular, elongated cells in the foam can interact with other geometric or material discontinuities in the foam and develop largemagnitude localized strain concentrations that likely initiate failures. Furthermore, some of the results suggest that continuum mechanics and linear elastic fracture mechanics might not adequately represent the physical behavior of the foam, and failure predictions based on homogeneous linear material models are likely to be inadequate. I.Introduction Maintaining the structural integrity of the sprayed-on foam insulation (SOFI) materials used on the Space Shuttle External Tank (ET) is a key element of maintaining flight safety and mission success until retirement of the Space Shuttle in 2010. To perform this task, a better understanding of the mechanical properties of the SOFI materials and improved structural verification measures are needed. The SOFI materials used on the Space Shuttle ET are closed-cell foams that exhibit a significant amount of nonuniformity in their microstructure. An in-depth description of these materials are presented in Ref. 1 and an example of the nonuniformity in the microstructure is shown in figure 1.This nonuniformity produces localization effects in the material response when subjected to thermal and mechanical loads. These localization effects are similar to random point-wise couples that prevent the development of smoothly varying strain fields that are commonly encountered in metals. In addition, these SOFI materials exhibit a significant amount of irregular anisotropy that is caused by the propensity of these materials to rise in the direction of least resistance during application and the presence of irregular structural features such as a bolted flange. Specifically, the cells elongate in the rise direction, which produces a directional bias in properties such as stiffness, strength, and fracture toughness. For most of the Space Shuttle Program (SSP), the emphasis was on thermal protection and not structural integrity of the SOFI materials. As a result, material properties such as modulus and fracture toughness, have been determined mostly by using relatively low-fidelity, bulk engineering measurements and generally exhibit a substantial amount of scatter in the test data. The nonuniformity of the SOFI microstructure is likely to be a major contributor to this scatter, and is likely to affect the onset and progression of failures in the SOFI materials. Thus, a three-dimensional displacement and strain measurement system that yields high-fidelity, full-field measurements is very useful for providing insight into the fundamental material behavior and for assessing and refining materialproperty characterization tests. Moreover, this type of measurement system is useful for reconciling the test-data scatter and for providing insight into unexpected and unexplained specimen failures. Structural verification procedures for structures made of well-defined engineering materials (e.g., aluminum) generally rely heavily on validated structural analysis tools to augment structural testing. For example, combined mechanical, thermal, and aerodynamic loads usually exist that cannot be simulated adequately in a laboratory environment and analyses are conducted to help assess and put test results into perspective. For SOFI materials,