Study Of Thermal Conductivity Of Core Materials And Thermal Shock Fracture Behavior Of Face Materials For The Foam Sandwich Structures

Thermal protection system is an essential part of future hypersonic vehicles. It protects the internal components of hypersonic vehicles from being destroyed by aerodynamic heating. In order to reduce the weight of the system, sandwich structures are used extensively. Typically, a sandwich structure is made of a core sandwiched by two outer panels. Due to their low density and good thermally insulated performance, ceramic foams are excellent candidate materials for the core of the sandwich structures. On the other hand, the hypersonic vehicles environment conditions are severe. During their service, the face materials of the sandwich structure may be subjected to particle striking and aerodynamic heating. This causes serious safety issue for the system. Therefore, understanding of the thermal conduction behavior of the core material and the fracture behavior of the face material is essential for improving the reliability of the system. This thesis studies the equivalent thermal conductivities of foam core at high temperatures and the fracture behavior of solid faces subjected to thermal loads. The main contents of the thesis are listed below.The body-centered cubic cell model is applied to the foam structures to obtain their equivalent thermal conductivities at high temperatures. The geometric optic laws and diffraction theories are applied to predict the radiative properties of the foams. The Rosseland approximation is used to calculate the radiative conductivities. Expressions of the equivalent thermal conductivities of foams are given through superposition principle. The influences of structural characteristics and optical properties of the solid matrix on the extinction coefficients are discussed in detail. It is found that the influence of the reflectivity depends on the way the radiation is reflected. Compared to the diffuse reflection, the specular reflection is more favorable to the propagation of radiation. To demonstrate the theory model, the thermal conductivities of Al2O3ceramic foams under different temperatures are measured through hotline method. It is found that the thermal conductivities increase rapidly with temperature and cell diameter, which is consistent with the theoretical results.The fracture mechanics of a material layer with an internal crack are investigated under the framework of non-Fourier heat conduction. Both heated crack and thermally insulated crack are considered. The heated crack can be a source of heating or cooling. This case develops the mode I thermal stress intensity factor at the crack tip. A thermally insulated crack does not allow any penetration of the thermal flow across the crack. This case develops the mode II thermal stress intensity factor at the crack tip. Laplace transform and dual integral equation technique are used to solve the problem. The effects of thermal relaxation time, crack length and layer thickness on thermal stress intensity factor are discussed. Comparisons between the non-Fourier results and the classical Fourier results are made. The results show that the stress intensity factor predicted by the non-Fourier heat conduction model is higher than that predicted by the classical Fourier model. Especially, when the thickness of the material layer becomes smaller, the difference becomes more significant.The thermal shock resistance for a plate subjected to a sudden temperature change is obtained under the framework of non-Fourier heat conduction. First, closed-form solution for the temperature field and the associated thermal stress are obtained for the plate without cracking. Next, the transient thermal stress intensity factors are obtained through a weight function method. Finally, the thermal shock resistance is obtained according to the maximum local tensile stress criterion and the maximum stress intensity factor criterion. The difference between the non-Fourier solutions and the classical Fourier solution is discussed. It is found that the Fourier heat conduction considerably overestimates the thermal shock resistance of the solid. This confirms the fact that introduction of non-Fourier heat conduction model is essential in the evaluation of thermal shock resistance of solids.Strong heating of the sandwich structures may cause significant thermal strain and stress gradients in the face materials. Therefore, the influence of stress gradient non-local effect on the thermal shock resistance behavior of the materials needs to be evaluated. In the fourth part of this thesis, thermal shock fracture of solid material plate is studied under the framework of stress gradient non-local elasticity theory. The temperature field and the associated thermal stress for the un-cracked plate are obtained. The thermal stresses of the un-cracked plate, with opposite signs, are used to obtain the thermal stress intensity factors for the cracked plate through a weight function method. Two types of crack problems are solved: a plate with an edge crack under cold shock and a plate with a center crack under hot shock. The thermal shock resistances of the plate are evaluated according to the maximum local tensile stress criterion and the maximum stress intensity factor criterion. Comparisons between the non-local elasticity theory and the classical elasticity theory are made. The results show that non-local elasticity theory gives a higher thermal shock resistance prediction than the classical elasticity theory.