Enhancing Thermal Insulation of EPDM Ablators via Constructing Alternating Planar Architectures

A Novel Fused SiO2 and h-BN Modified Quartz Fiber/Benzoxazine Resin Ceramizable Composite with Excellent Flexural Strength and Ablation Resistance

Hypersonic vehicles encounter hostile service environments of thermal/mechanical/chemical coupling, so thermal protection materials are crucial and essential. Ceramizable composites have recently attracted intensive interest due to their ability to provide large-area thermal protection for hypersonic vehicles. In this work, a novel ceramizable composite of quartz fiber/benzoxazine resin modified with fused SiO2 and h-BN was fabricated using a prepreg compression molding technique. The effects of the fused SiO2 and h-BN contents on the thermal, mechanical, and ablative properties of the ceramizable composite were systematically investigated. The ceramizable composite with an optimized amount of fused SiO2 and h-BN exhibited superb thermal stability, with a peak degradation temperature and residue yield at 1400 °C of 533.2 °C and 71.5%, respectively. Moreover, the modified ceramizable composite exhibited excellent load-bearing capacity with a flexural strength of 402.2 MPa and superior ablation resistance with a linear ablation rate of 0.0147 mm/s at a heat flux of 4.2 MW/m2, which was significantly better than the pristine quartz fiber/benzoxazine resin composite. In addition, possible ablation mechanisms were revealed based on the microstructure analysis, phase transformation, chemical bonding states, and the degree of graphitization of the ceramized products. The readily oxidized pyrolytic carbon (PyC) and the SiO2 with a relatively low melting point were converted in situ into refractory carbide. Thus, a robust thermal protective barrier with SiC as the skeleton and borosilicate glass as the matrix protected the composite from severe thermochemical erosion and thermomechanical denudation.

A Visco-Hyperelastic Constitutive Model to Characterize the Stress-Softening Behavior of Ethylene Propylene Diene Monomer Rubber

Uniaxial and biaxial cyclic tensile tests and stress relaxation tests were performed on the ethylene propylene diene monomer (EPDM) material to investigate its stress-softening effect. The experimental results reveal that the EPDM material presents a significant Mullins effect during the cyclic stretching processes. Furthermore, it is found that the deformation of the EPDM material does not return to zero simultaneously with the stress, due to the viscoelasticity of the EPDM material. Therefore, this study combines pseudo-elasticity theory and viscoelastic theory to propose a visco-hyperelastic constitutive model. The proposed model is used to fit and analyze the uniaxial and biaxial cyclic test results of EPDM and a comparison is conducted with the corresponding hyper-elastic constitutive model. The results show that the proposed model is in good agreement with the experimental data and superior to the hyper-elastic constitutive model, especially when it comes to the stress-softening unloading process. This work is conducive to accurately characterizing the stress-softening behavior of rubber-like materials at large deformation and can provide some theoretical guidance for their widespread application in industry.

Improving the Heat and Ablation Resistance of Silicone Rubber Composites by Incorporating Hollow Microspheres

For thermal protection materials (TPMs) which are used to protect space vehicles from extreme thermomechanical environments, the thermal conductivity of the original material and the char layer that has formed during ablation plays a significant role in determining the ablation performance. In order to investigate this, in this study, we introduced glass hollow microspheres (GHMs), phenolic hollow microspheres (PHMs), and acrylonitrile-methyl methacrylate copolymer hollow microspheres (AMHMs) into silicone rubber (SR), and the ablation performance of these composites was systematically studied. The thermogravimetric results showed that the residue yield of the SR composites was increased with the incorporation of the hollow microspheres. Compared to the SR composites without the hollow microspheres, the residue weight values under 800 °C (R₈₀₀) of the SR composites with the 30 parts of fumed silica per hundred of the SR (phr) addition of GHMs, PHMs, and AMHMs were increased from 10.11% to 21.70%, 18.31%, and 20.83%, respectively. The ablation tests showed that the addition of the AMHMs enhanced the ablation performance of the SR composites because the linear ablation rates and the backplane temperature were clearly decreased when compared to the SR composites without the hollow microspheres. This work provides an effective and potential method for preparing thermal protection materials with an improved ablation performance.