Development and validation of a fatigue reliability method for bridging materials Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/4j03d246r

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  • Toughened ceramics, or bridging ceramics, are being developed in an attempt to widen the application base of traditional ceramics, which are brittle and have low resistance to crack extension. The crack growth resistance of these extrinsically toughened ceramics has been shown to increase with crack length, and unlike traditional ceramics, toughened ceramics are susceptible to fatigue failure (failure below fracture strength after repeated loading and unloading). Many ceramic applications, especially those in the aerospace or automotive industries, require components to be designed for infinite life since fatigue crack growth can lead rapid failure. The current methods for predicting and preventing fatigue failures are unsuitable; most were developed for metal alloys, which behave very differently than toughened ceramics. Damage tolerant design takes into account that most engineering structures are inherently flawed, thus a method is needed that ensures an existing flaw will not propagate to failure within foreseen in-service lifetimes. Such a method has been proposed for making fatigue reliability predictions in bridging ceramics. Fatigue crack growth experiments were performed on 99.5% pure polycrystalline alumina. The fatigue behavior was characterized as a function of crack size using a fatigue threshold R-curve, similar in principal to a fracture toughness Rcurve. A bridging stress profile, the relevant material property, was both calculated using the R-curve and measured directly with fluorescence spectroscopy. The fatigue threshold R-curve was used to make predictions of fatigue endurance strength based on an initial minimum detectable flaw size. These endurance strength predictions were validated with small crack fatigue failure experiments.
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