Insights into Spallation Mechanisms of Thermal Protection System Materials from Mass Spectrometry and HyMETS Testing
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An effort has been undertaken to study the mechanisms responsible for internal pressure buildup in thermal protection system (TPS) materials subjected to high enthalpy environments. Understanding how gases evolve, migrate, and interact with the microstructure of a TPS is essential for predicting degradation and failure modes such as spallation. To this end, complementary experimental approaches have been used to provide both chemical and mechanical insight into subsurface processes.
Chemical evolution and internal pressure buildup were identified using the processes shown in Figure 1. In Part A, extensive pressure measurements obtained during testing in the Hypersonic Materials Environmental Test System (HyMETS) quantified the dynamic buildup of subsurface pressure as the gases evolved. In part B, mass spectrometry was applied to characterize the volatile species released during the decomposition of TPS under heating. This analysis distinguishes between species that desorb at lower temperatures, such as releasing water before significant changes in permeability, and those produced upon disruption of the polymer backbone by high-temperature pyrolysis. Together, these datasets established a quantitative link between chemical decomposition and mechanical response, providing a basis for interpreting how microscale chemical processes manifest as macroscale material instability.
Lessons learned from mass spectrometry and HyMETS testing provided insight into the spallation mechanisms of TPS, as illustrated in Figure 1. Initial heating of TPS induces the release of water absorbed by the microballoons and the surrounding matrix before extensive pyrolysis (I). This early release of cramped water can generate localized stresses when the material is in a state of low permeability and can result in the formation of localized cracks prior to pyrolysis. As heating continues, the pyrolysis front advances, releasing a significant amount of gas and a rapid rise in pressure occurs (II). If the internal pressure exceeds the local resistance of the material, a sudden ejection of fragments ensues, marking a spallation event (III). This sequence highlights the likely interplay between early-stage volatile release, pyrolysis gas evolution, and stress generation, all of which govern the stability of the TPS material under inlet conditions.
For more information, contact Dr. Brody K. Bessire. brody.k.bessire@nasa.gov
