UC Santa Barbara

Mechanical response of ceramic matrix composites is critically dependent on properties of fibers and fiber coatings. In this dissertation, testing methods and analysis techniques for obtaining key constituent properties are developed. First, a critical assessment of existing methods used to determine fiber strength distributions is made, through a combination of established theorems in convolution and uncertainty propagation, Monte Carlo simulations of single fiber tension and fiber bundle tests, and experimental measurements on bundles of SiC fibers. The results reveal that fiber Weibull parameters are most reliably obtained by fitting stress-strain data from fiber bundle tests directly to the functional form predicted by fiber bundle theory. Second, fiber push-in tests are used to probe the mechanical properties of BN fiber coatings and their interfaces with the fibers and the matrix in several prototypical SiC-SiC composites. Push-in results reveal two distinct behaviors: one in which coating rupture occurs suddenly, followed by interface sliding, and another in which yielding of the coating occurs first, followed by rupture and sliding. A new micromechanical model for elastic/plastic coating deformation and subsequent rupture and sliding is developed. The model provides a framework for interpreting push-in results, including ways to ascertain the mechanism that governs the stress for push-in initiation and for extracting pertinent coating properties. Finally, effects of constituent properties on tensile response of unidirectionally-reinforced composites are examined through a combination of analytical models of fiber fragmentation, matrix cracking, and interface debonding and sliding, along with Monte Carlo simulations and experimental measurements on SiC-SiC minicomposites. The results reveal two distinct domains of fiber fragmentation and subsequent pullout; at low stresses, fibers break in a random manner throughout the composite and, at the stress maximum, additional breaks are localized to regions near the eventual fracture plane. Composite rupture occurs when the local fiber bundle response in the most heavily strained regions (within matrix crack planes) reaches a load maximum. The failure stress and strain vary considerably with fiber volume fraction, especially when the matrix strength distribution is broad. In many cases of practical interest, the full potential of the fibers is not realized.