| As human constantly discover and explore the field of space, supersonic flight technology has become a forward-looking and strategic technology. Definitely, the ever-changing field of aerospace technology is inseparable from the development of ultra-high temperature materials. At present, ultra-high temperature materials and applications studied by domestic and foreign scholars are under a rapid development and reach their maturity. Hypersonic aircraft will service a long time in the atmosphere, so the sharp-shaped nose cone and wing leading edge and other structures will experience severe aerodynamic heating process, which will be a very tough challenge for the heat-resistant aircraft materials and structures, especially for the reliability of materials under the condition of the thermal coupling.Ultra high temperature materials (UHTMs) refers to a special material which can maintain the physical and chemical stability in the high temperature (> 2000℃) and the reaction atmosphere (for example, atomic oxygen environment). Ultra-high temperature materials with high strength and high oxidation resistance is capable of long time flight in high speed of sound, re-entry, cross-atmospheric flight and rocket propulsion systems and other extreme environments, commonly used in high temperature conditions in the aircraft nose cone, wing sharp wedge edge, port and other key parts of the engine.The materials which meet the need of the ultra-high temperature mainly include borides, carbides, oxides and nitrides of some transition metal compounds, for example:TaC, ZrB2, ZrC, HfB2, HfC, etc. The melting point of these materials is than 3000℃which means these compounds have good thermal chemical stability and are sufficient in extreme environments. With the high melting point, the borides, carbides, and oxides make up of the multiple composite ceramic materials known as "ultra-high temperature ceramic materials". They showed good antioxidant properties in the temperature of more than 2000℃. In recent years such materials receive much concern and have been used for military, aerospace engineering purposes in many countries.This paper sets the ZrB2-SiC based ultra-high temperature ceramic materials as the research object, under the conditions of re-entry the thesis launches the structural thermal analysis of ultra-high temperature ceramics; stimulates the model in re-entry conditions and carries out the thermal stress analysis of the shuttle wing leading edge structure, gives out the main factors which could influence the thermal stress analysis; considers the ultra-high temperature ceramic materials with initial crack and treats them as the research focus. Through the extension (XFEM) thermodynamic analysis of the ABAQUS finite element method in the finite element analysis software, this thesis simulates the thermal shock resistance of brittle materials in the atmospheric reentry under extreme conditions, and tries to give the evolution of damage of the ultra-high temperature ceramics with defects under the thermal impact load.First, the experiment simulates the transient heat transfer behavior of the sharp wedge-shaped pieces of ultra-high temperature ceramic materials, establishes of a thermal shock re-entry heat transfer model and the whole process of loading curve of heat flux, through the simulation structural analysis of the parameters of ZrB2 based ceramic materials and FRCL-12 material works out the temperature contours. After analyzing the numerical example of two different materials in the structure of heat transfer and observing changes in cloud temperature the experiment obtains the model of heat transfer and the changes of the temperature of each part. Thus adequate data is provided to the follow-up thermal stress analysis.By analyzing the results it is easy to find that the three parameters change including the heat, thermal conductivity and density of the ultra-high temperature ceramic materials will have a certain impact on the heat transfer and temperature changes. As the basic material parameters, their mutual constraints and interactions play a vital role in the heating process of thermal protection structure. Therefore, the choice of appropriate materials used in special parts of the thermal protection structure design of the aircraft is particularly important.Second, the experiment simulates the thermal deformation and thermal stress conditions of the leading edge structure consisted of sharp wedge-shaped wing tip caused by aerodynamic heating duringthe atmospheric re-entry flight. Accordingly, the experiment establishes a mechanical model based on the analysis in heat transfer, through the indirect coupling method calculates the thermal stress and deformation of the ZrB2 based ultra-high temperature ceramic structure and obtains the transient thermal stress and strain distribution trend. According to the results, in the early stage of thermal shock loads of the model, the temperature increased sharply over time. Accordingly, the heat generated by the thermal shock stress also increases rapidly in a short period. For the brittle ultra-high temperature ceramics, it is the most vulnerable stage to cause damage and failure. At the same time, the thesis introduces the thermal stress analysis of the major factors which is made for the simulation results of error analysis.Finally, the experiment simulates the temperature distribution and thermal stress of the ultra-high temperature ceramic with cracks in the initial stage of heat shock and describes the evolution of crack propagation. In the thermal shock conditions, the increased speed of temperature leads to a very high thermal stress on the model. In the model with initial imperfections, the thermal stress concentration phenomenon occurs and with the heat flow increases the thermal stress also increases. In this case, the initial crack begins to expand. Along the wing leading edge the crack spreads to the inside and when the stress reaches to a certain value the model immediately fractures and then loses effectiveness. In addition, apart from the crack the material itself remains intact and there is no damage or holes appear. The failure process is in full compliance with the characteristics of brittle fracture.Comparison of the simulation results of the crack propagation behavior of two materials, the thesis obtains that the thermal shock resistance of materials, to some extent, increase as the SiC content increases. So the thermal shock performance of the ZrB2-30vol%SiC is better than several other family kinds of material.The numerical results obtained in this thesis has some reference value for predicting the damage evolution and material life of the ceramic structure and provides a theoretical basis to further improve the performance of materials and design of materials. |
PDF Full Text Request