An extensive study explores spallation mechanisms in TPS materials under high-enthalpy environments, using pressure measurements and mass spectrometry.
Overview of the Study
An extensive research project was conducted to explore the spallation mechanisms of thermal protection system (TPS) materials under high-enthalpy environments. This study aimed to elucidate the chemical and mechanical processes responsible for the evolution of gases and internal pressure buildup within TPS materials, which are crucial for predicting their degradation and failure modes.
Experimental Approaches and Data Analysis
### Chemical Evolution and Pressure Buildup
In-depth pressure measurements, conducted within the Hypersonic Materials Environmental Test System (HyMETS), provided quantitative insights into the dynamic pressure changes as gases evolved during testing. Figure 1 details the process, illustrating the initial phase where the pressure increases due to gas release and subsequent pressure buildup.
### Mass Spectrometry Analysis
Complementary to the pressure measurements, mass spectrometry was utilized to identify and characterize volatile species released as TPS materials decomposed under heat. This analysis differentiated between lower-temperature desorbing species, such as water, and those produced during the high-temperature pyrolysis of the polymer backbone.
Establishing the Link Between Chemical Decomposition and Mechanical Response
By combining the results from mass spectrometry and HyMETS testing, researchers established a quantitative link between the chemical decomposition of TPS materials and their mechanical response. This foundational understanding is critical for interpreting the microscale chemical processes that manifest as macroscale material instability.
Insights into Spallation Mechanisms
The study revealed that initial heating of TPS materials leads to the release of absorbed water from microballoons and the surrounding matrix before significant pyrolysis. This early water release can generate localized stresses, potentially leading to crack formation, even before extensive pyrolysis. As heating continues, the pyrolysis front advances, releasing a substantial amount of gas and causing a rapid pressure buildup. If the internal pressure exceeds the local material strength, spallation events occur, characterized by the sudden ejection of fragments. This sequence underscores the interplay between early-stage volatile release, gas evolution during pyrolysis, and stress generation, all of which influence the stability of TPS materials under entry conditions.
For further information, contact Dr. Brody K. Bessire at brody.k.bessire@nasa.gov.
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