Yutaro Arai

An ablative thermal protection system (ablator) can protect re-entry vehicles from severe aerodynamic heating during re-entry. Ablators are carbon-based materials with an impregnated resin, and the decomposition of the resin during re-entry prevents a temperature increase in the components. The phenolic-impregnated carbon ablator is a well-known ablator. However, its low strength makes the design of re-entry vehicles difficult. Recently, a high-strength three-dimensional networked porous carbon called a carbon monolith (CM) was developed. No significant difference was found in the recession behavior of CMs with or without resin. In this study, a B ₄ C particle–dispersed CM (BCM) was prepared as an advanced ablator without resin. The results obtained by heat exposure simulating atmospheric re-entry conditions (arc jet test at 1.9 and 6.1 MW m ⁻² ) show that the melting of B ₄ C contributes to a decrease in surface and internal temperatures compared with CM. Because the phase change (melting of B ₄ C) acts as an endothermic agent, BCM is recognized as a type of advanced ablator termed a phase-change shield system.

This study presents a comprehensive quantitative investigation into the material properties and damage behavior of unidirectional carbon fiber reinforced plastic laminates with ply discontinuities. Laminates with varying resin gap lengths were evaluated. In situ edge observations and X-ray computed tomography were employed to assess the damage processes in the area of the ply discontinuity. These methods examined initially occurring cracks, secondary cracks, and delamination. Some unexpected cracking behavior was observed in this study. For instance, initial cracks were observed to form not at the interface but at a distance from the resin–ply interface, likely due to a constraint condition caused by the chemical shrinkage of the resin during the curing process. Furthermore, all cracks in the resin region curved toward the center, with successively formed cracks having higher curvatures. This behavior can be attributed to the redistribution of principal stresses within the resin pocket following the initial cracks. The matrix cracking was followed by delamination that started from the tips of the matrix cracks and progressed toward the ends of the specimen. Additionally, the number and locations of cracks in the resin pocket were influenced by the length of the resin gap and the uniformity of the thickness and width of the specimen. In addition to the experimental investigation, a stress-based variational analysis was employed to predict crack onset stresses in the laminates. The analysis successfully calculated the energy release rate for laminates with varying resin pocket lengths and provided predictions for crack onset stresses. The analytical predictions demonstrated good agreement with the experimental observations, enhancing the understanding of resin pocket behavior in CFRP laminates.

The oxidation and recession of carbon-fiber-reinforced ultrahigh-temperature ceramic matrix composites (C/UHTCMCs) fabricated via reactive melt infiltration (RMI) using Zr-Ti alloys with three different compositions are evaluated via an arc-jet tunnel test at temperatures above 2000 °C for 60 s. Thermodynamic evaluations show that the recession of the UHTCMCs is prevented by the formation of a solid solution of ZrTiO₄ on their exposed surface. Because an increase in the Zr content increases the melting temperature of ZrTiO₄, the recession of the composites increases as the Zr content in the infiltrated alloys decreases. UHTCMCs fabricated with Zr-20at%Ti showed the least recession (<5%).

To achieve Si‐free refractory ceramic matrix composites exposed to an oxidizing atmosphere at approximately 2000°C, a Ti–Zr–Mo ternary alloy melt‐infiltration (MI) method was developed for the production of carbon fiber‐reinforced refractory high‐entropy ceramic matrix composites (C/RHECs), with high‐entropy carbides serving as the matrix. This approach was designed using calculation phase diagrams (CALPHADs) and the calculation of thermodynamic parameters. The combination of CALPHAD and the calculation of alloy and carbon reactivity enabled the prediction of reaction and infiltration behavior into a preform comprising carbon fiber, carbon black, and transition metal carbide powders. Furthermore, C/RHECs featuring a (Ti, Zr, Hf, Nb, Ta, Nb, Mo)C matrix were experimentally fabricated through the Ti–Zr–Mo alloy MI method in accordance with the design. The arc‐wind tunnel tests on the C/RHECs conducted at approximately 2000°C revealed surfaces covered with complex oxides. The apparent oxidation rate of the C/RHECs was similar to that of Si‐containing ceramics and composites. These results indicate that complex oxides act as a barrier to oxygen diffusion toward unoxidized regions, making Si no longer essential for protecting materials from oxidation above 2000°C.