基于二维离散单元法研究短纤维/硅橡胶防热涂层的热振衰退机理

Short fiber reinforced silicon rubber coatings (SF/SRACs) have endured severe erosion and degradation as they working under the heat-vibration coupling condition of a new generation launch vehicle. It is always difficult to describe the porousness and heterogeneity of SF/SRACs on a suitable scale using ablation numerical models based on continuity hypothesis, and there are some technology bottlenecks to deal with material interface movements or even surface mechanical erosion. Aiming at these problems, this project innovatively introduces the discrete element method (DEM) in two dimensions to simulate the high-temperature ablation behavior of SF/SRACs. A 2D-DEM numerical model is proposed and its thermodynamic and mechanical parameters are calibrated through comparing measurements of specimen tests and predictions of numerical tests. In order to simulate the coking of pyrolysis gases and thermal expansion behaviors as SF/SRACs high-temperature ablating, the relationships among pore pressures and the diameters of discrete elements, and the relationships among the temperatures and diameters are established, respectively. The internal structure evolution and stress filed variation of SF/SRACs as quartz lamp radiant heating are observed on a meso-scale. Then we study the crack initiation and propagation occurring in the SF/SRACs exposed to the heat-vibration coupling condition. By means of DEM simulations imposed with the heat-vibration condition, the relationships among the surface recession of SF/SRACs and the short fiber formula parameters, such as the weight fraction, length and dispersibility, are studied, and then the typical heat-vibration induced failure modes are determined. Based on above contents, we attend to reveal the heat-vibration recession mechanism, and then provide the theoretical support to the reasonable and reliable TPS design for the new generation launch vehicle.

新一代运载火箭热振耦合服役环境下，短纤维增强硅橡胶防热涂层（SF/SRACs）的烧蚀表面加速剥离，防热性能严重衰减。基于连续性假设的烧蚀模型难以恰当尺度描述SF/SRACs的多孔性、非均质性结构特征，且在处理界面移动、表面剥蚀等行为方面存在技术瓶颈。针对上述问题，本项目将二维离散单元方法引入高温烧蚀数值仿真领域，并以SF/SRACs为研究对象，建立二维离散单元法数值模型，结合试件级实验和数值仿真实验，标定模型的热、力学参数，建立孔隙压力与颗粒直径的关联式模拟碳沉积现象，建立温度与颗粒直径的关联式模拟热膨胀现象，从细观级别观测静态加热下材料的结构演化和应力状态变化，在热振条件下研究材料的裂纹形成与扩展规律，研究纤维质量分数、长度和分散性等配方参数与热振表面后退率之间的相关性，识别典型的热振破坏模式，从而阐明SF/SRACs的热振衰退机理，为合理可靠设计火箭防热方案提供理论支撑。

针对新一代运载火箭热振耦合服役环境下，短纤维增强硅橡胶防热涂层(SCF/SRACs)的体积烧蚀和表面后退演化规律尚不清晰的难题，本项目以SCF/SRACs为研究对象，建立了二维DEM数值模型，通过材料级显微观测、试片级力学测试标定了主要模型参数，通过试片级静态辐照加热和热振实验初步验证了SCF/SRACs二维DEM数值模型在模拟体积烧蚀和热振衰退功能上的可行性和有效性。利用该模型开展了静态加热条件下材料内部的碳沉积和热膨胀行为，从细观级别观测了材料内结构变化和应力演化，开展了热振条件下材料裂纹产生与扩展行为，开展了烧蚀材料配方参数研究，识别了材料典型的热振破坏模式，揭示了SF/SRACs热振衰退机理，该方法所辅助研发的SR107-TA烧蚀材料已成功应用于我国首例固液捆绑火箭的飞行试验。