Strength And Thermal Shock Resistance Of Advanced High-temperature Materials

Advanced materials are a broad family of materials, such as newly developed materials or developing materials, which have more excellent properties than traditional materials. Currently, advanced materials are widely used in the defense construction and serve under high-temperature environments. The security and reliability of these materials are key problems urgently to be solved. It is very important to study the high-temperature thermal properties and mechanical properties of these materials. This dissertation studied the high-temperature strength and thermal shock resistance of the chemical vapor deposited zinc sulfide(CVD ZnS), ultra-high-temperature ceramics(UHTCs) and single crystals, according to the status of the research of each material. Theoretical, numerical and experimental methods were adopted according to the feature of each scientific issue. The main work is as follow:(1)A simple, effective and economic apparatus for the direct uniaxial tensile strength of brittle materials was developed based on the traditional fixture for tension at high temperatures. The ratio of the uniaxial tensile strength measured on the devopled apparatus to the three-point bending strength of CVD ZnS at room temperature is close to thoses of other brittle materials reported in literature. Besides, the measured uniaxial tensile strength can be related with the three-point bending strength of CVD ZnS at room temperature using Weibull statistics. The validity of the apparatus was thus proved. The uniaxial tensile strength of CVD ZnS from room temperature to 600°C was measured for the first time. In addition, the compressive strength from room temperature to 600°C was measured. The failure mechanism and failure mode in this temperature range were analyzed from both macro- and micro-scale.(2)Based on the critical failure energy density principle and the critical strain principle, the temperature-dependent ideal tensile strength models for the cubic and hexagonal single crystals were developed. The models relate the ideal tensile strength to the elastic properties, specific heat at constant pressure and thermal expansion of the single crystals. The temperature-dependent ideal tensile strengths of cubic single crystals(W, Al and Fe) and hexagonal single crystals(ZrB2, HfB2 and TiB2) were studied theoretically for the first time. Specifically, the study for Al is from absolute zero to melting point. The theoretical results were compared with that from ab initio(AI), molecular dynamics(MD), and AI MD simulations. The results showed that the theoretical modoels can reasonably predict the temperature dependence of the ideal tensile strength of single crystals. The fracture failure for single crystals at elevated temperatures was identified, for the first time, as a strain-controlled criterion.(3)The transient temperature solution of the semi-infinite solid was successfully applied to predict the transient temperature response of the plate with finite thickness under single thermal environments. Besides, the temperature-dependent model of thermal stress field of free plate was derived based on the thermoelasticity. For complex thermal environments, such as the UHTC thermal protection system with convective cooling, finite volume method was used to obtain the calculation model of thermal shock resistance. All the models were validated by comparing the results given by the models with that calculated in ABAQUS. The thermal shock resistance of UHTCs under aerodynamic thermal environments and convective environments was studied in detail. Heat transfer conditons for the first, second and third type thermal boundary condtions were introduced to better sduty the thermal shock resistance of UHTCs. Universal conclusions about the thermal shock resistance of UHTCs under different thermal environments were drawn using heat transfer conditions. The critical heat transfer conditions were introduced to characterize the thermal shock resistance of UHTCs. The relation between the heat transfer condition and the critical heat transfer condition is similar to that between stress and strength. The critical heat transfer condition emphasizes that materials will not failure because of thermal shock forever. The critical heat transfer condition not only can be used to characterize the thermal shock resistance of materials, but also can be used as parameter for safty design in the thermal structure engineering. The study showed that the thermal upshock resistance of the UHTC thermal protection system caused by aerodynamic heating can be improved by convective cooling, but convective cooling can also lead to thermal downshock at the lower surface. The heat transfer condition should be less than the critical heat transfer condition to ensure that convective cooling will not cause the failure of the UHTC thermal protection system.(4)The effects of nine types of mechanical boundary conditions and the in-plane geometric shapes on the thermal shock resistance of ceramics for the first, second and third type thermal boundary condtions were studied. The nine types of mechanical boundary conditions were ordered based on the thermal shock resistance. How the asymmetries of the in-plane geometric shapes and the mechanical boundary conditions would worsen the thermal shock resistance of ceramics when the components are constrained was expounded.