Weiguo Li

Unified Thermal Shock Resistance of Ultra-High Temperature Ceramics Under Different Thermal Environments

The thermal shock resistance (TSR) of the ultra-high temperature ceramic (UHTC) plate under convective environments is studied. The critical failure temperature difference has a danger temperature range about the thermal shock initial temperature. However, the critical failure dimensionless time decreases as the thermal shock initial temperature increases. The TSR of UHTCs is susceptible to thermal environments. The heat transfer condition shows its advantage in representing the TSR of UHTCs under different thermal environments. Universal conclusions about the TSR of UHTCs under different thermal environments are drawn using heat transfer condition. Three types of critical heat transfer condition that respectively correspond to the first, second, and third type thermal boundary conditions are introduced to characterize the TSR of UHTCs under different thermal environments similar to using various types of strength in representing the fracture-resistance abilities of the materials under different loads. The critical heat transfer condition is applied to the TSR of the UHTC plate under active cooling. The critical heat transfer condition is susceptible to the difference of the thermal shock initial temperature and the coolant temperature.

Thermal Shock Resistance of Ultra-High-Temperature Ceramic Thermal Protection System

Nomenclature ai−1, ai, ai 1, a 0 i = coefficients in the discrete equation, KW · m−2 · °C−1 B0, B1, B2 = coefficients in Young’s modulus expression Cp = specific heat at constant pressure, J · kg−1 · K−1 E = Young’s modulus, GPa E0 = Young’s modulus at room temperature, GPa fj = term in Eq. (4), where j is 1–5 (f1 and f4 in MPa · m, f2 and f5 in KPa · m , and f3 in Pa · m) h = plate thickness, mm K = coefficient matrix in Eq. (1), KW · m−2 · °C−1 k = thermal conductivity,W · m−1 · °C−1 kFi = thermal conductivity at face Fi, W · m−1 · °C−1 m = ratio of compressive-tensile strength n = number of nodes P = load vector in Eq. (1), MW · m−2 qs = surface heat flux, MW · m −2

Modeling of the temperature-dependent ideal tensile strength of solids

To reveal the fracture failure mechanisms of single crystals at elevated temperatures, a new temperature-dependent ideal tensile strength model for solids has been developed, based on the critical strain principle. At the same time, the uniaxial tensile strength model, based on the critical failure energy density principle for isotropic materials that was presented in the previous study, is generalized to multi-axial loading and to cubic single crystals. The relationship between the two models is discussed, and how to obtain the material properties needed in the calculations is summarized. The two well-established models are used to predict the temperature-dependent ideal tensile strength of W, Fe and Al single crystals. The predictions from the critical strain principle agree well with the predictions from the critical failure energy density principle. The theoretical values from the critical strain principle at 0 K is in reasonable agreement with the ab initio results. The study shows that the temperature dependence of the ideal tensile strength is similar to that of Young’s modulus; that is, the ideal tensile strength firstly remains approximately constant and then decreases linearly with the temperature. The fracture failure for single crystals at elevated temperatures has been identified, for the first time, as a strain-controlled criterion.

Numerical Simulation for Thermal Shock Resistance of Ultra-High Temperature Ceramics Considering the Effects of Initial Stress Field

Taking the hafnium diboride ceramic as an example, the effects of heating rate, cooling rate, thermal shock initial temperature, and external constraint on the thermal shock resistance (TSR) of ultra-high temperature ceramics (UHTCs) were studied through numerical simulation in this paper. The results show that the external constraint has an approximately linear influence on the critical rupture temperature difference of UHTCs. The external constraint prepares a compressive stress field in the structure because of the predefined temperature field, and this compressive stress field relieves the tension stress in the structure when it is cooled down and then it improves the TSR of UHTCs. As the thermal shock initial temperature, a danger heating rate (or cooling rate) exists where the critical temperature difference is the lowest.

Heat Transfer and Failure Mode Analyses of Ultrahigh-Temperature Ceramic Thermal Protection System of Hypersonic Vehicles

The transient temperature distribution of the ultrahigh-temperature ceramic (UHTC) thermal protection system (TPS) of hypersonic vehicles is calculated using finite volume method. Convective cooling enables a balance of heat increment and loss to be achieved. The temperature in the UHTC plate at the balance is approximately proportional to the surface heat flux and is approximately inversely proportional to the convective heat transfer coefficient. The failure modes of the UHTCs are presented by investigating the thermal stress field of the UHTC TPS under different thermal environments. The UHTCs which act as the thermal protection materials of hypersonic vehicles can fail because of the tensile stress at the lower surface, an area above the middle plane, and the upper surface as well as because of the compressive stress at the upper surface. However, the area between the lower surface and the middle plane and a small area near the upper surface are relatively safe. Neither the compressive stress nor the tensile stress will cause failure of these areas.

Fabrication and Flatwise Compression Property of Glass Fiber-Reinforced Polypropylene Corrugated Sandwich Panel

Composite sandwich structures with cellular cores have wide application in many fields such as aerospace due to their excellent properties. Thermoplastic composite structure has superior impact res...

The mechanical and electronic properties of Al/TiC interfaces alloyed by Mg, Zn, Cu, Fe and Ti: First-principles study

The adhesion and ductility of (100) and (110) Al/TiC interfaces alloyed by Mg, Zn, Cu, Fe, and Ti have been investigated using first-principles methods. Fe and Ti can enhance the adhesion of (100) and (110) interfaces. Mg and Zn have the opposite effect. Interfacial electronic structures have been created to analyze the changes of the work of adhesion. It is found that more charge is accumulated at interfaces alloyed by Fe and Ti compared with pure Al/TiC. There is also an obvious downward shift in the Fermi energy of Fe, Ti at the interface. Furthermore, the unstable stacking fault energies of the interfaces are calculated; the results demonstrate that the preferred slip direction is the direction for (100) and (110) Al/TiC. Based on the Rice criterion of ductility, the results predict that Mg, Fe, and Ti are promising candidates for improving the ductility of Al/TiC interfaces.

Temperature dependent compressive yield strength model for short fiber reinforced magnesium alloy matrix composites

In this paper, based on our previous study regarding the temperature-dependent yield strength for metallic materials and the existing strengthening theories, a physics-based temperature dependent compressive yield strength model for short fiber reinforced magnesium alloy matrix composites was developed. This model was verified by comparison with the experimental data of seven types of magnesium alloy matrix composites. Good agreement between the model predictions and the experimental data was obtained, which fully validates the reasonability of the present model. Moreover, based on the model and the existing material parameters, the influencing factor analysis for short fiber reinforced magnesium alloy matrix composites was systematically conducted. Some novel insights regarding the control mechanism of their temperature dependent compressive yield strengths were provided.

Numerical Simulation for Thermal Shock Resistance of Thermal Protection Materials Considering Different Operating Environments

Based on the sensitivities of material properties to temperature and the complexity of service environment of thermal protection system on the spacecraft, ultrahigh-temperature ceramics (UHTCs), which are used as thermal protection materials, cannot simply consider thermal shock resistance (TSR) of the material its own but need to take the external constraint conditions and the thermal environment into full account. With the thermal shock numerical simulation on hafnium diboride (HfB2), a detailed study of the effects of the different external constraints and thermal environments on the TSR of UHTCs had been made. The influences of different initial temperatures, constraint strengths, and temperature change rates on the TSR of UHTCs are discussed. This study can provide a more intuitively visual understanding of the evolution of the TSR of UHTCs during actual operation conditions.

Theoretical Research on Thermal Shock Resistance of Ultra-High Temperature Ceramics Focusing on the Adjustment of Stress Reduction Factor

The thermal shock resistance of ceramics depends on not only the mechanical and thermal properties of materials, but also the external constraint and thermal condition. So, in order to study the actual situation in its service process, a temperature-dependent thermal shock resistance model for ultra-high temperature ceramics considering the effects of the thermal environment and external constraint was established based on the existing theory. The present work mainly focused on the adjustment of the stress reduction factor according to different thermal shock situations. The influences of external constraint on both critical rupture temperature difference and the second thermal shock resistance parameter in either case of rapid heating or cooling conditions had been studied based on this model. The results show the necessity of adjustment of the stress reduction factor in different thermal shock situations and the limitations of the applicable range of the second thermal shock resistance parameter. Furthermore, the model was validated by the finite element method.