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 Title: Fatigue crack growth in ceramics containing a viscous grain boundary phase at elevated temperatures Author(s): Yao, Daping Doctoral Committee Chair(s): Shang, Jian Ku Department / Program: Engineering, MetallurgyEngineering, Materials Science Discipline: Engineering, MetallurgyEngineering, Materials Science Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): Engineering, Metallurgy Engineering, Materials Science Abstract: Elevated-temperature crack growth behavior in a commercial Al$\sb2$O$\sb3$ and a hot-pressed 30 vol.% TiB$\sb2$-SiC composite was examined under tensile static loading (static fatigue) and tension-tension cyclic loading (cyclic fatigue). The study was carried out at temperatures of 700-900$\sp\circ$C, where the vitreous grain boundary phase flowed viscously. Experimental results have shown the existence of cyclic fatigue in these materials, but the cyclic effect cannot be seen as the consequence of a static fatigue mechanism, although under both cyclic and static loading conditions crack propagation assumed an intergranular fracture mode. The testing temperature, load ratio, and cyclic frequency were found to exert significant effects on cyclic fatigue-crack growth behavior. A damage zone was observed ahead of the crack tip in which grain boundary cavitation (or microcracking) occurred during fatigue-crack growth. An analytical model based upon the damage accumulation in the grain boundary phase was developed that successfully predicted the frequency and load ratio dependencies of crack growth. Values of activation energy for cyclic and static fatigue crack growth were approximately the same. Fracture mechanisms in both cases were also found to be similar. However, crack growth under static loads was faster than that under cyclic loads at the same maximum stress intensity. Such a difference in the growth rate suggested that the damage accumulation in the grain boundary phase differed during cyclic and static fatigue processes. In the TiB$\sb2$-SiC composite, cyclic fatigue-crack growth at elevated temperatures was affected by oxide-induced crack closure and showed an anomalous temperature dependence. After subtracting crack closure, cyclic fatigue-crack growth exhibited a temperature dependence that was governed by the viscous flow of the grain boundary phase. Issue Date: 1995 Type: Text Language: English URI: http://hdl.handle.net/2142/23684 Rights Information: Copyright 1995 Yao, Daping Date Available in IDEALS: 2011-05-07 Identifier in Online Catalog: AAI9624543 OCLC Identifier: (UMI)AAI9624543
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