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 Title: Sub-critical crack growth in a ceramic and a ceramic composite: Effects of monotonic loads, cyclic loads, and high temperatures Author(s): Raghuraman, S. Doctoral Committee Chair(s): Stubbins, James F. Department / Program: Nuclear, Plasma, and Radiological Engineering Discipline: Nuclear, Plasma, and Radiological Engineering Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): Engineering, Mechanical Engineering, Materials Science Abstract: Advanced ceramics have toughened microstructures which resist crack propagation, and also allow considerable sub-critical crack growth prior to failure. The sources of toughening processes are the microstructural elements in the crack wake which induce closure tractions on the crack. This experimental and theoretical study examines the sub-critical crack growth behavior in alumina and SiC$\sb{\rm f}$/SiC composite under conditions which degrade the toughening processes.In alumina, crack growth is a complex and variegated process which depends not only on the microstructural variables and operating parameters but also on geometrical aspects such as specimen shape, size, and crack lengths. This makes it necessary to conduct a large variety of tests in order to elucidate the entire range of its crack growth response. In this study it is attempted to describe much of this spectrum using a limited number of tests which encompass a diverse mixture of testing situations. Mechanical property data was obtained from tests in two loading modes (cyclic and monotonic), ambient and elevated temperatures, two loading methods and specimen shapes (tensile and flexure), and crack domains (short and long). A consolidated theoretical approach was developed based on several separate theories. The calculations were performed established on physically based mechanisms which have been verified by microscopy (SEM, TEM) and failure analysis. The results reproduce the trends observed experimentally and also reveal some facets of sub-critical crack growth that pass undetected in conventional experiments. They also serve to understand the qualitative influences of important microstructural variables and loading parameters. The crack growth behavior was shown to be considerably different in each crack length domain.In SiC$\sb{\rm f}$/SiC, attention was focussed on characterizing the mechanical behavior under service conditions of the applications envisaged for this material, namely cyclic loading, high temperature oxidation and stress-oxidation situations. It was found that the interfacial properties between the fiber and the matrix, and the evolution of the interphases during the tests were the most important factors affecting strength and fracture toughness. The materials behavior could be unambigously correlated with microscopy observations and failure analysis. Separate models for describing stress-strain behavior under cyclic fatigue, ambient and elevated temperature monotonic crack growth are discussed. An oxidation model which is unique to fiber reinforced ceramic composites is discussed and validated using TEM. The phenomenon of stress-oxidation, which has not been considered in literature, is found to be the most performance limiting damage mechanism in this material. Issue Date: 1995 Type: Text Language: English URI: http://hdl.handle.net/2142/22508 Rights Information: Copyright 1995 Raghuraman, S. Date Available in IDEALS: 2011-05-07 Identifier in Online Catalog: AAI9543699 OCLC Identifier: (UMI)AAI9543699
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