This thesis is concerned with two topics that are fundamental to understanding the mechanical reliability of ceramics and ceramic matrix composites. The first topic is a theoretical formulation of evaluation approaches to linear-elastic fracture (characteristic of monolithic ceramics) and nonlinear fracture with large-scale bridging (characteristic of fiber-reinforced ceramic matrix composites). The second topic is an experimental study of high temperature fracture in a monolithic ceramic, a particulate-reinforced ceramic and a continuous fiber-reinforced ceramic matrix composite.; In monolithic ceramics and particulate-reinforced ceramic matrix composites, which may be described by linear elastic fracture mechanics, an evaluation approach is proposed for the determination of fracture resistance based solely on measurements of crack opening displacement and crack length. Results using this displacement-based approach were found to agree closely with results using the traditional load-based approach. This approach is also used to analyze crack growth stability in fracture tests under different control modes.; In fiber-reinforced ceramic matrix composites, which can not be described by linear-elastic fracture mechanics due to a large fracture process zone, a nonlinear model is proposed. Large-scale bridging effects are treated by describing a crack using three phases: the external response, the fracture process zone response and the material parameter. The bridging law is considered to be a fundamental material property. The material is assumed to exhibit linear-elastic bulk behavior beyond the fracture process zone. In formulating the relations between the phases, two semi-infinite cracks are defined and used with the stress intensity theory employing the weight function method. An evaluation approach based on this nonlinear model is given. No specific computational approach, such as finite element analysis or the weight function method, is involved in the evaluation procedures; only geometry factors and large-scale bridging coefficients associated with the tested specimen are required. Finally, the evaluation approach is verified by finite element analysis and by analysis of experimental data from the literature.; A novel experimental technique was developed for the present study--elevated temperature fracture test using optical methods (ETFOM). This technique permits in-situ observations of micromechanical fracture processes at temperatures to 1500{dollar}spcirc{dollar}C. The apparatus employs a heating stage, a temperature controller and a standard optical microscope. Loading of the crack occurs by differential thermal expansion between the modified chevron-notched test specimen and a thermal loading wedge.; The ETFOM technique was used to study temperature dependent fracture toughness in a monolithic ceramic (sintered {dollar}alpha{dollar}-SiC) and a particulate-reinforced ceramic matrix composite (sintered (TiB{dollar}sb2)sb{lcub}rm P{rcub}{dollar}/SiC) in an inert gas atmosphere over a temperature range of 25-1250{dollar}spcirc{dollar}C. The technique was also used to investigate interlaminar fracture processes in a unidirectional Nicalon fiber reinforced zirconia titanate matrix composite. Relative toughness associated with cracks propagated in different material orientations were evaluated and related to observed toughening mechanisms, including crack bridging and crack branching. A thermal fatigue effect, which significantly degraded the material reliability under thermal cycling, was observed. The mechanism of the thermal fatigue crack growth was attributed to the temperature-induced phase transformation of free zirconia in the matrix. This result demonstrates the usefulness of in-situ observations of microscopic fracture processes at high temperature.
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