Theoretical Characterization Methods On The High Temperature Strength Of Ceramic Fibers And Their Reinforced Composites

Ceramic fibers have been widely used in the fields such as aerospace,petrochemical engineering,nuclear energy,and automobile industry,due to their excellent mechanical and thermal properties.They are important strengthening phases for temperature-resistant composites.Ceramic fiber reinforced ceramic matrix composites have high specific strength,high specific modulus and good thermal stability,etc.,and show the great potential applications in the field of thermal protection materials and structures.They have become the hotspot of the research and development of high temperature ceramics,and attract great attention from military powers of the world.Short ceramic fiber reinforced magnesium alloy matrix composites have the characteristics of light weight,good wear resistance and good heat resistance,which can be used as temperature-resistant structural components in the combustion engines and brakes for automobiles.For these ceramic fiber reinforced composites which service under high temperature environment,how to characterize and improve their high temperature strength is the focus and difficulty in the research of temperature-resistant composites.It is of great significance in theoretical research and engineering applications to study their failure mechanisms under high temperature environment and establish the corresponding physics-based strength models.In this thesis,the following research work on ceramic fibers and their reinforced composites was carried out:(1)Professor Weiguo Li proposed a modeling idea which can quantitatively characterize the effect of temperature on the mechanical properties of materials,namely the force-heat equivalence energy density principle.Based on the principle,theoretical models without any fitting parameters for the high temperature fracture strength of polycrystalline ceramic fibers were established,which considers the influence of phase composition and oxidation.The quantitative relationship between fiber strength,heat capacity,phase composition,fiber melting point,oxidation parameters,and temperature-dependent fiber Young's modulus was given.In addition,based on Griffith fracture theory,slow crack growth theory and the temperature-dependent fracture surface energy model,the temperature and testing rate dependent fracture strength models for ceramic single crystal fibers were established.Model predictions are in good agreement with the experimental results.In addition,based on the established models,the influencing factor analysis for the high temperature fracture strength of ceramic fibers was conducted.The key controlling factors for their high temperature fracture strengths were found and the methods to improve their high temperature load capacity were proposed.(2)Based on the temperature-dependent fracture strength models for ceramic fibers and Cao and Thouless' theory,the temperature-dependent fracture strength model for unidirectional ceramic fiber reinforced ceramic matrix composites was established.Model predictions agree well with the experimental results.Based on this model,the influencing factor analysis for their temperature-dependent fracture strengths was conducted.Then,some effective ways to improve their high temperature fracture strengths were provided.In addition,a temperature-dependent fracture strength model for whisker reinforced ceramic matrix composites was established,which considers the temperature dependence of the fracture strength of whisker and ceramic matrix,as well as the evolution of residual thermal stress with temperature.Model predictions are in good agreement with the experimental results.Furthermore,the evolution law and the key controlling factors for the temperature-dependent fracture strength of whisker reinforced ceramic matrix composites were systematically analyzed.Some suggestions to improve their high temperature fracture strengths were provided.(3)A physics-based temperature-dependent compressive yield strength model for short ceramic fiber reinforced magnesium alloy matrix composites was established.The model considers the temperature-dependent compressive yield strength of matrix and the combined effects of load transfer strengthening,dislocation density strengthening,residual thermal stress and grain refinement on the composite compressive yield strength.Model predictions agree well with the experimental results.In addition,based on this model,the effect of key factors on the high temperature compressive yield strength of the composite was analyzed in detail.These provide the theoretical guidance and suggestions to improve the high temperature compressive mechanical properties for short ceramic fiber reinforced magnesium alloy matrix composites.