Investigation On Microstructure And Properties Evolution And Thermal Reliability Assessment Of Fibrous Insualiton

Reusable launch vehicles entering the earth's atmosphere at hypersonic speeds encounter aerodynamic heating. The development of reusable thermal protection system (TPS) with low-weight, low-cost, high reliability is one of the key projects in order to limit the maximum temperature of the primary structure of the vehicle during entry. High temperature fibrous insulation is an attractive insulation for the metallic TPS since it provides an excellent combination of low weight, low thermal conductivity and high service temperature. During the service life of fibrous insulation, the fibrous insulation is subject to severe aerodynamic heating. Moreover, many other environmental factors, such as debris impact, contamination, seal leakage, thermal shorts, materials aging, etc., may or may not affect performance. The complexity of various environmental factors and the requirement for high reliability make the materials response problem quite difficult. Little overheating of the primary structure of the vehicle may lead to unexpected disasters.In present paper, thermal response behavior and thermal properties of fibrous insulation subjected to severe aerodynamic heating are investigated from three standpoints, such as materials, engineering thermophysics and reliability. The evolution of microstructures and thermal properties is observed. The determination of conductive and radiative properties for fibrous insulation is discussed. Additionally, probabilistic thermal analysis and thermal reliability assessment are conducted. The major contents and conclusions are described as follows:Firstly, the evolution of microstructures and thermal properties for fibrous insulation before and after heat treatment is investigated by SEM, DSC, XRD and FTIR. It is found that the as-received material is amorphous. And mullite is devitrified from the original material at around 980°C. The devitrification will occur progressively with the increase of the heat treatment temperature, causing the mullite grains to grow and crystallinities increase, which reduces the flexibility of the fibers. Eventually the fibers fuse together and the product becomes brittle. The spectra of the untreated fibers show the presence of broad vibration bands which are characteristics of amorphous materials. However, after heating at 1000°C, one can observe the presence of mullite specific bands. The thermal properties correlated with the microstructure evolution are also investigated. The calculated effective thermal conductivity exhibites increase for samples after heat treatment, although no obvious increase is observed with heat treatment in the measured data. The radiative properties evolution is very complex. The devitrifation may affect the photon thermal conductivity at a certain temperature. The extinction coefficient of the sample after heat treatment at 1000℃for 16h is lower than the data of the as-received material, while the result of the sample after heat treatment at 1200℃for 16h is higher than that of the sample after heat treatment at 1000℃. It is observed that the albedo of scattering decreases after heat treatment, and decreases with the increasing of heat treatment temperatures. It should be stated that the thermal conductivity for the samples after heat treatment obviously increases, and the value for the sample after heat treatment at 1200℃for 16h triples the value for the as-received material. This phenomenon can be explained by the fact that after heat treatment, the devitrification and the increased grain size and crystallinities of the sample after heat treatment increase the mean free path of phonon scattering, thus the solid conduction by fibers increase. Furthermore, the sintering and shrinkage occurres after severe heat treatment increase the contact area among the fibers, leading to the increase in the thermal conductivity.Secondly, in order to investigate the radiative heat transfer in fibrous insulation, spectral extinction coefficients and Rosseland mean extinction coefficients are calculated based on the Beer's law from the measured transmittance data at various temperatures. Taking into account anisotropic scattering in fibrous insulation, the measured radiative properties are modified, and a modified factor of extinction coefficient and an equivalent albedo of scattering are defined. An inverse conduction–radiation analysis in an absorbing, emitting and scattering medium is conducted for the simultaneous estimation of the conductive and radiative properties using the experimentally measured temperature responses for external temperatures up to 980K. For validation purpose, the estimated thermal properties are used to calculate the transient temperature responses and effective thermal conductivities,which are then compared with the measured data. It is found that the calculated results correspond well with the experimental data within an average of 3.1 percent under transient condition and 9.8 percent under steady-state condition. This confirms the good behavior of the model and the validity of results.Thirdly, the definitions for thermal protection system thermal reliability are provided borrowing from machine design approaches. And these definitions are firstly applied to thermal reliability assessment for fibrous insulation materials. The probabilistic thermal analysis of the fibrous insulation subjected to aerodynamic heating conditions is performed to account for the uncertainties such as material properties, loading conditions and geometrical variations. The statistical analysis of thermal response shows that the response temperature is significantly dependent on time and location. Large variations in the statistics parameters are observed at the location where temperature slip occurs for the first one minute of entry. To identify the dominant variables which most influence temperature response scatter, the correlation coefficients of various variables are computed. The results show that the aerodynamic heating temperature and initial temperature have significant effects for the considered locations. However, the extinction coefficient, the specific heat of virgin material and the solid fraction ratio have non-negligible effect on the back side temperature scatter of fibrous insulation. Furthermore, the relationship between the probabilistic thermal reliability and thickness factor of safety is developed. Quantitative as well as qualitative information is provided in the present methodology, which is valuable to the thermal analysis, design and testing of fibrous materials.