Thermomechanical Analysis And Structural Design Of Ultra-high Temperature Ceramics

With the rapid development of aerospace and military technology, hypersonicvehicles qualified for long-time hypersonic flight are gaining more and more attentionand becomes critical technology for all countries at present time and even in the future.Severe requirements are made for thermal protection materials in order to realize thistechnology.UHTC （Ultra-High Temperature Ceramics） becomes one of the candidatesmaterial for thermal protection system of hypersonic vehicles in the future due to theirexcellent properties at high temperature. This paper deals with the fracture behavior ofthe UHTC and thermal-coupling analysis of thermal protection structures consideringthe brittleness and their extreme service environment aiming at putting them forward forthe practical application.The fracture toughness and flexural strength of ZrB₂-SiC-G based UHTC aremeasured by three-point bending test at room temperature and high temperatureconcluding that the fracture toughness tends to decrease with increasing temperatureand the flexural strength shows an inverse relationship.Finite-element analysis has beenperformed on models established according to the specimen obtaining the J-integral andstress intensity factor around the crack tip.The crack propagation has been analyzed bynumerical simulation using extended finite element method and the load-displacementcurve computed shows a good agreement with experiment. The study shows that thefracture process of ZrB₂-SiC-G could be regarded as linear elastic and the fracturecriterion is consistent with power-law criterion.Thermal-coupling analysis has been performed by numerical methods with the3DFEM model simulating the real behavior of the UHTC thermal protection system.Thetemperature and thermal stress distribution has been obtained by numerical computationand also their relationship with time.The numerical analysis indicates that thetemperature on the upper surface and the bottom surface are1215K and719.6Krelatively with a drop of495.4K when a heat flux of1MW/m²was applied on the FEmodel for150s.With the thermal stress predicted we can evaluate the reliability of the component by the margin of safety at different temperature. The evaluation shows thatwhen exposed to thermal load for150s, the maximum principle stress of UHTC frame isup to234.59MPa, which exceeds225.78MPa, the bending strength of the material.The thermal and mechanical response of ZrB₂-SiC leading-edge to aerodynamicheating is analyzed during which we focus on the thermal stress distribution ofstationary point, windward and bolt hole.Calculation indicates that the temperature andthermal stress of the component increase significantly with the distance betweenstationary point decreases.The thermal stress tend of stationary point over heating timeis analyzed.During aerodynamic heating the maximum principle stress reachs its peakvalue421MPa, which exceeds bending strength of the material355MPa,in just2sindicating that the component might crack.the numerical result shows a good agreementwith arc-jet test provided by literature.The impact of the material properties on thethermal and mechanical response of the leeding-edge under thermal load is analyzed.Itshows that a higher thermal conductivity or a lower elastic modulus is benefit for thebearing capacity of the leeding-edge.Structure optimization design has been performed based on the thermal-mechanicalresponse of the UHTC leading-edge concluding that the component achieves itsmaximum safty factor when the bolt whole radius takes the value R=1.877mm.Thethermal shock resistance of ZrB₂-SiC is investigated from the perspective ofoptimization drawing a conclusion that the critical thermal shock temperature differenceis743.75℃.