Study On Theory And Experiment Of Mechanical Behaviors Of Materials Under High Temperature And Complex Environment

The metals and ceramic coating is the commonly used structural and protective material in high temperature and complex environment. Under the complicated high temperature environment, mechanical behavior such as strength and constitutive relation of metal materials are very different from that of under room temperature, which have obvious temperature dependence, ceramic material used as used as TBC is subjected to oxidation, ablation and thermal shock, which are complex thermo-chemo-mechanical coupling process. In order to ensure the reliability and efficiency of high temperature structures, it is important to study on the special mechanical behavior and improvement of mechanical performance of materials under high temperature and complex environment. Focus on the theory of elastic modulus and strength theory of metal materials under high temperature, the thermal shock of ceramic materials and the in situ testing technique subjected to high temperature and complex environment, this paper research on the following topics:First,we proposes a model based on thermal vibration theory to predict the temperature-dependent modulus with respect to isothermal and isentropic assumption. The lattice vibration free energy is expressed as a function of the two independent scalars from the strain tensor and temperature. By using the Einstein theory, we present the analytical expression for the temperature-dependent Young's modulus, bulk modulus, shear modulus and Poisson's ratio. The theoretical prediction agrees well with the experimental data. The proposed model is further degenerated to Wachtman's empirical equation and provides the physical meaning to the parameters in Wachtman's equation.Second,A generalized strength theory as a function of temperature and state of stresses for metals is developed. Based on the fracture in the hydrostatic stress, we derived a generalized strength model, in which the fracture strength decreases almost linearly with the increasing of the temperature. Furthermore this generalized strength model was extended to the general state of stresses by replacing the equivalent hydrostatic stresses with the temperature effect based on the general thermodynamics principles. Molecular dynamics(MD) simulation was also conducted to simulate the fracture evolution at high temperature and to explain the mechanism of temperature-dependent strength at atomic scale. The proposed model was also verified by experiment of Mo-10 Cu alloy at elevated temperature.Moreover, we provide a large-scale and low-cost method, based on metal-organic frameworks, to fabricate nanostructured conformal coatings with the same material as the substrate. Such design can reduce the thermal mismatch between coatings and substrate and massively change the local thermal conductivity and interfacial heat transfer coefficient. Therefore, the thermal shock resistance(i.e. critical temperature difference) can be enhanced about 75%. We describe all aspects of such nanostructured conformal coatings from fabrication to characterization.Finally, an experimental technique and system to simultaneously measure the full-field temperature and deformation at high temperature is developed using the technique of image processing. The principle of the synchronous measurement of temperature and deformation field is demonstrated and discussed. Experiment on carbon fiber reinforced silicon carbide(C/SiC) composite was conducted to validate this method. The temperature was calculated using an improved two-color method, while the displacement field and strain field were calculated using the digital image processing method with correction of intensity. Results show that the proposed method is applicable for synchronous measurement of temperature and displacement by using one camera, and the mutual interference between the radiation and reflected light can also be effectively eliminated. For subject to high temperature environment with oxidation and thermal ablation, a directly calculates the deformations is further developed. The algorithm is based on the change in contour of feature regions on the surface of the specimens. The feature regions are extracted by either region or edge detectors, and then transformed into ellipse shapes. The deformation gradient and strains can be obtained by calculating the eigenvalues of these ellipses. Both simulation and experimental results validate the reliability and robustness of this directly method.