Study On High Temperature Fracture Strength And Theoretical Characterization Method Of Ceramic Matrix Composites

High-end technical fields such as aviation,aerospace,nuclear energy,petrochemical industry,etc.are critical to a country's equipment manufacturing,military security,and international voice.Its development level is an important indicator of a country's industrial level,national defense capabilities,and overall national power.Due to high melting point,good high-temperature strength retention rate and excellent resistance to oxidation ablation,ceramic matrix composites have been widely used in the above fields,such as thermal protection system of hypersonic aircraft,high-temperature structural components of engines as well as the key components of nuclear fission reactors.Ceramic matrix composites often face a complex high-temperature environment during service,they usually bring unpredictable catastrophic consequences if failure occurs.Therefore,to clarify the failure mechanism of the materials in service processes,fully understand the influencing factors' evolution and the mechanism's change of the strength control with increasing temperature and to reasonably and effectively characterize the relationship between strength and service temperature is the core to improve the safety and reliability of ceramic matrix composites.And it is one of the basic problems that must be solved in material design,preparation and application,which has very important theoretical significance and engineering application value.This thesis carried out the following research work on the high-temperature fracture strength and theoretical characterization method of ceramic matrix composites,based on the force-heat equivalence energy density principle:(1)Considering the load carrying capacity of the broken fibers and its evolution with temperature,a temperature-dependent tensile fracture strength model of unidirectional ceramic-fiber reinforced ceramic matrix composites was constructed.The model predictions are in good agreement with the available measurements.Further,utilizing the developed temperature-dependent tensile fracture strength model of unidirectional ceramic-fiber reinforced ceramic matrix composites,the quantitative impacts of the strength contribution of broken fibers and the Young's modulus of fibers on the tensile fracture strength of the materials at different temperatures were analyzed.It can provide theoretical basis and guidance for the composites' design,manufacture and application under high temperatures.(2)Based on the relationship between thermal energy and atomic potential energy and kinetic energy,a temperature-dependent fracture strength model for ultra-high temperature ceramic matrix composites was established.The model predictions achieve good agreement with measurement results.It is worth noting that the model has no fitting parameters,revealing the inherent relationship between the fracture strength at different temperatures and the fracture strength at arbitrary reference temperature,the heat of fusion,melting point,thermal expansion coefficient and Young's modulus.All material parameters can be easily obtained from material handbooks or literature,which provides a simple,convenient and effective method to forecast the fracture strength of ultra-high temperature ceramic matrix composites at different temperatures,avoiding a great deal of high-temperature tests.(3)On the basis of the principle of force-heat energy density equivalence,a theoretical characterization model of temperature-dependent fracture toughness for ultrahigh temperature ceramic matrix composites without fitting parameters was developed.Furthermore,on the basis of Griffith fracture theory and considering the temperature dependence of relevant parameters,a theoretical characterization model of thermaldamage fracture strength for ultra-high temperature ceramic matrix composites,which took into consideration damage and residual thermal stress and their evolution with the variation of temperature,was established.And the model is well verified by experimental results.Based on this model,a reasonable definition of the critical crack size of Zr B2-Si C in argon and air was given,and the effect of crack size on the temperature-dependent fracture strength of the composite materials was quantitatively analyzed,which can offer a theoretical support for further improving the fracture strength of ultra-high temperature ceramic composites at high temperatures.