Study On Temperature-dependent Mechanical Properties And Theoretical Characterization Methods Of Fiber Reinforced Ceramic Matrix Composites

Due to the excellent mechanical and thermal properties,high temperature ceramics have been gradually applied in various high-tech fields,such as sophisticated weapons,aerospace and nuclear industry.The current research shows that adding the reinforcing phase(such as fiber and particulate)into the ceramic matrix is an effective approach to overcome its intrinsic brittleness and improve the mechanical properties such as the fracture toughness and strength.Owing to the excellent properties,such as high specific strength,high specific modulus,high temperature resistance and corrosion resistance,fiber reinforced ceramic-matrix composites(FRCMCs)have been used in the thermal protection and propulsion system of hypersonic vehicles.FRCMCs are usually subjected to extremely high temperature environment during service.Mechanical properties are directly related to the safety and reliability of materials and components.Reasonable characterization and improvement of high-temperature mechanical properties of FRCMCs have become the key and difficult points in the field of ceramic-matrix composites.It is of great significance for material development,application,reliability evaluation and life prediction to fully understand and scientifically evaluate the evolution of mechanical properties with temperature and their key control factors.In view of the above scientific problems,the following research work on FRCMCs has been carried out in this thesis:(1)Based on the force-heat equivalence energy density principle,a temperature-dependent fracture surface energy model without fitting parameters was established for ceramic materials,by developing the equivalent relationship between fracture surface energy and thermal energy.Based on this model and the energy balance method,the temperature-dependent steady-state first matrix cracking stress models were established for unidirectional FRCMCs.The combined effects of temperature,residual thermal stress and interface debonding were included.In addition,a temperature-dependent interfacial shear strength model without fitting parameters was established,by developing the equivalent relationship between shear strain energy and thermal energy.The combined effects of crack length and temperature on first matrix cracking stress was further studied.A temperature-dependent non-steady state first matrix cracking stress model was established for unidirectional FRCMCs by using the stress intensity factor analysis.The above models reveal the mechanism of matrix cracking and the evolution law of key control factors with temperature.These models were well validated by the experimental results,which provide effective theoretical ways to reasonably characterize and predict the temperature-dependent first matrix cracking stress of unidirectional FRCMCs.In addition,the quantitative influence of various key material parameters on their first matrix crack stress at different temperatures was analyzed systematically by using the established models,which provides guidance for improving the first matrix crack stress.(2)A temperature-dependent thermal shock failure criterion for unidirectional FRCMCs was developed.Further,through considering the sensitivity of related material parameters to temperature,a thermal shock resistance model over a wide range of cooling environment temperatures was established.The model predictions achieved good agreement with the experimental results by water quenching method.Moreover,the sensitivity of critical temperature difference to matrix Young's modulus,thermal expansion coefficient and interfacial shear strength and its evolution with cooling environment temperature were studied by using the proposed model.The results showed that the sensitivity of critical temperature difference to matrix Young's modulus and interfacial shear strength increases with increasing cooling environment temperature,and the sensitivity to thermal expansion coefficient of matrix decreases with increasing cooling environment temperature.These results provide theoretical guidance and suggestions for improving the thermal shock resistance of materials.(3)A temperature-dependent fracture strength model without specific heat capacity and fitting parameters for ceramics was established,by developing the equivalent relationship among the strain energy,the kinetic energy and potential energy of atoms.This model has the advantages of easy parameter determination,simplicity and practicality.Further,the theoretical characterization models of temperature-dependent fracture strength including macro-and micro-structural characteristics of materials for short and unidirectional FRCMCs,2D woven and cross-ply ceramic matrix composites were developed respectively.The quantitative relationship between the temperature-dependent fracture strength of FRCMCs and the thermodynamic properties of composite constituent is clarified.The models reveal the evolution law of key control factors of fracture strength of composites with temperature.The above models were well validated by the experimental results,which provide effective means for characterizing and predicting the temperature-dependent fracture strength of FRCMCs.They also provide theoretical support for the development,design of materials,and also for the evaluation and optimization of their mechanical properties at high temperatures.