Quantification of feature variability in porous ablators

A crucial component of space vehicles is the thermal protection system (TPS), which protects the vehicle from extreme heat loads. Morphological variations in TPS materials arising from manufacturing processes affect the effective properties of TPS materials. To quantify the variability, property distribution functions are obtained through a combination of X-ray computed tomography (XRCT) and a mesoscale property toolkit that is developed to rapidly process hundreds of structures from primary XRCT scans. Ten different primary volumes of FiberForm, a fibrous carbon-based ablative TPS material, are scanned using XRCT to generate primary volumes. The toolkit extracts, cleans, and repairs extracted volumes from the full primary volumes to generate material property statistics. A novel algorithm to calculate fiber diameters is also developed as part of the toolkit. The property distribution functions are calculated over a range of extracted cubic volumes. The geometrical material properties are found to not converge to a single value even after the length of the extracted cubic volume reaches a length of 300 µm, the representative elementary volume (REV) of FiberForm. Rather, the properties converge to a distribution function. The distribution functions of extracted volumes with a length of 300 µm or larger converge with 100 samples and seven primary XRCT scans. The property distribution functions for surface area, closed volume, porosity, volume-by-area ratio, mean, and standard deviation of the fiber diameters of the material are fit to a beta, log-normal, normal, Weibull minimal, triangular, and gamma functions, respectively. These property distribution functions provide more representative effective properties of the material, instead of a single value that would not capture any variability in the material. The material property distribution functions obtained from this study can be used to understand the performance of materials and improve the reliability and quantification of numerical simulations used for the design of TPS materials.