The thermomechanical behavior of damaged space shuttle tile thermal protection system is determined using the finite element method. The relative effects of damage on the thermal protection capability and the induced thermal stresses in the TPS are determined by comparing the thermal and structural response of the damaged configurations with the undamaged configurations. The TPS, consisting of 3 components (the LI-900 tile, the strain isolation pad and the underlying structure), is modeled as a discrete three-layer structure. The TPS is subjected to the re-entry heating and pressure profile of the Access to Space vehicle, and the transient temperature distribution and the resultant thermal stresses in the system are computed. Three different sizes of damage having diameters 0.5", 1" and 1.5", based on hypervelocity impact, are considered. The validity of the simplifying assumptions used in a recent study is systematically examined. Some of these assumptions are relaxed and their effects on the system response are studied in the present work. The effect of damage location on the overall behavior of the TPS is also examined. Damage changes the surface properties of the tile and increases the surface area exposed to heating. It significantly reduces the radiation heat loss from the surface of the tile, resulting in elevated temperatures in the TPS. The elevated temperatures, with the stress concentrations introduced by the damage increases the thermal stresses significantly. Results suggest that the damage considered is capable of raising the maximum temperature in the tile to beyond its melting point and may cause structural failure.
Knowledge of the size effect on the strength of hybrid bimaterial joints of steel and fiber composites is important for new designs of large lightweight ships, large fuel-efficient aircrafts, and lightweight crashworthy automobiles. Three series of scaled geometrically similar specimens of symmetric double-lap joints with a rather broad size range (1:12) are manufactured. The specimens are tested to failure under tensile displacement-controlled loading, and at rates that ensure the peak load to be reached within approximately the same time. Two series, in which the laminate is fiberglass G-10/FR4, are tested at Northwestern University, and the third series, in which the laminate consists of NCT 301 carbon fibers, is tested at the University of Michigan. Except for the smallest specimens in test series I, all the specimens fail by propagation of interface fracture initiating at the bimaterial corner. All the specimens fail dynamically right after reaching the maximum load. This observation confirms high brittleness of the interface failure. Thus, it is not surprising that the experiments reveal a marked size effect, which leads to a 52% reduction in nominal interface shear strength. As far as the inevitable scatter permits it to see, the experimentally observed nominal strength values agree with the theoretical size effect derived in Part II of this study, where the size exponent of the theoretical large-size asymptotic power law is found to be −0.459 for series I and II, and −0.486 for series III.
The thermomechanical behavior of damaged space shuttle tile thermal protection system (TPS) is considered. The effects of damage on the thermal protection capability and the induced thermal stresses in the TPS are examined by comparing the thermal and structural response of the damaged configurations with the undamaged configurations. The TPS is subjected to the re-entry heating and pressure profile, and the transient temperature distribution and the resultant thermal stresses in the system are computed using finite element analysis. Three different damage sizes are considered. The validity of the simplifying assumptions is systematically examined. Certain assumptions are relaxed and their effects on the system response are determined. Thermal loads based on high speed flow past a cavity are also incorporated to provide a more accurate model. Damage changes the surface properties of the tile, which significantly reduces the radiation heat loss from the surface of the tile. It also alters the flow field and thus the thermal loads sustained by the TPS, resulting in elevated temperatures. The elevated temperatures combined with the stress concentrations induced by the damage increases the thermal stresses. Results indicate that damage is capable of elevating the maximum temperature in the tile to beyond its melting point and cause structural failure.
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