Tensile and uniaxial/multiaxial fatigue behavior of ceramic matrix composites at ambient and elevated temperatures

Increasing use or fiber reinforced ceramic matrix composites (CMC's) materials is needed, especially for hostile environments such as elevated temperatures. However, some fundamental issues regarding how these materials should be made for optimized performance are far from being settled. This study focuses on the modeling of the tensile behavior of unidirectional CMC using statistical methods and micro-mechanical analysis, based on laboratory observations. The model can be used to examine the effect of performance-influencing parameters on the strength of unidirectional CMC, thus shed light on how such material should be put together. The tensile strength model was then modified such that the behavior of unidirectional CMC under cyclic tensile load can be studied. Results from the tensile strength model suggest that the Weibull modulus, m, of the strength of the reinforcing fibers and the fiber/matrix interfacial shear stress both have significant effect on the strength and toughness of the unidirectional composite: a higher m value and a lower interfacial shear stress result in a lower strength; a lower value of m and a higher interfacial shear stress results in a higher strength but lower toughness. Calculations from the tensile fatigue model suggest that a lower m value results in a longer fatigue life.; The performance of "real" components (tubes) made of CMC was also investigated. Experimental procedures and methods were developed and established, base-line data were obtained from testing CMC tubes under uniaxial and multiaxial loads, each for two types of loading history, quasi-static and cyclic; and, at both ambient and elevated temperatures. Damage and failure mechanisms in these components were examined in detail. It is shown that the cross-over regions of the fiber tows in the tube are very critical to the failure of the structure. Data interpretation schemes for structure failure are also suggested, in which a bundle strength approach and a fracture mechanics approach are used to predict the failure of the composite structure under torsion and tension, respectively, with fairly good agreement with experimental data.