Predictive Modeling of Chemical and Structural Failure of Porous Ablative Materials

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Predictive modeling of chemical and structural failure of porous ablative materials Carbon fiber reinforced materials are commonly used for Thermal Protection Systems (TPS) materials. This class of material, which includes NASA's own PICA, are ideal for this purpose since they are light weight, offers low thermal conductivity, and high thermal resistance. In order to model the oxidation behavior of the fibers, it is crucial to understand the flow near and within the TPS, at the fiber scale. One of the challenges of attempting to model this type of flow at this scale is that the fibers are very small ( ? 1 to 10?m), the mean free path of the molecules and atoms in the gas is often on the order of the fibers themselves. When this is the case, the Navier-Stokes equations, which are based on a continuum assumption, are no longer valid. For this kind of flow, the value of the Knudsen number - the ratio between the mean free path of the gas and a relevant length scale, in this case the average pore diameter - encountered by the material is > 0.02. This suggest that modeling these flows requires a method which can account for a range of Knudsen numbers, and is particularly effective in more rarefied regimes. One such method is Direct Simulation Monte Carlo (DSMC), which can efficiently be used to model the gas phase, and gas-surface interactions phenomena. DSMC is a stochastic particle based method, which is valid -- given sufficient computational resources -- for all Knudsen numbers. Additionally, the method can handle convection, as well as many other relevant physical phenomena. When the high temperature flow interacts with the fibers, chemical reactions -- mainly oxidation -- takes place. This typically occurs in a characteristic surface consumption mode referred to as pitting. Rather than consuming the surface uniformly, oxidation proceeds preferentially through the growth in depth of multiple cavities scattered on the originally smooth surface of the fiber. This phenomenon results in a net increase of the exposed surface, which, in addition, forms highly reactive sites. Both effects accelerate the rate of oxidation, potentially compromising the strength of the fibers. This may lead to the early break-up of the fibers, reducing dramatically the protecting properties of the TPS. Pitting can be traced to the molecular mechanism of carbon oxidation, which is the result of four main processes, namely oxygen adsorption, surface diffusion, surface reaction and products desorption. Currently available kinetic mechanisms account for these processes through effective, global reactions rates fitted to available experimental data. They can predict reasonably well the macroscopic, overall behavior of carbon materials, essentially because they are fine-tuned to do so. However, none of them addresses the microscopic nature of oxidation and therefore cannot account for pitting. Once the flow around the fibers is accurately modeled and the gas surface interactions appropriately accounted for, it is possible to model the thermo-mechanical response of the fibers. This is achieved using a material response code strongly coupled to a steady-state structural mechanics code. The stress acting on the fiber results from a combination of thermal strain, internal pressure and external shear stress, coming from both the near surface flow field and pyrolysis gas blowing. To correctly predict the leading factors of spallation and failure of the TPS, it is critical to accurately model both the distributions and magnitudes of stresses within materials in a strongly coupled approach.
Effective start/end date1/15/181/14/22


  • University of Minnesota: $400,000.00


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