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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Cyclic and static fatigue properties of ceramics

Author: VAUGHAN, Dion

Year: 1990

Supervisor(s): Frank Guiu

A mechanical gripping system has been devised for applying uniaxial tensile and fully reversed loads, to dumbell shaped ceramics specimens in a servohydraulic test machine. Characteristic room temperature, S-N curves have been produced for a range of advanced oxide ceramics (eg 99.5% - Al2O3, 99.9% - Al2O3, ZTA and TZP) using fully reversed loading. Static fatigue curves, produced using direct tensile loading, have been additionally generated for all the quoted materials. The existence of genuine mechanical fatigue damage processes (as opposed to environmentally assisted slow crack growth) has been established in these ceramics by observing that the time to failure at a given peak stress, is consistently smaller under fully reversed than under direct tensile loading. Fractography has been used to establish failure origins, and relevant fatigue crack growth mechanisms in the systems under consideration. The effects of material batch variability and surface finish on both cyclic and static fatigue behaviour, has been elucidated for the 99.9% Al2O3. Annealing of `as received' ground, ZTA specimens at 1250C for 1 hour has additionally been found to have a significant effect on the short term uniaxial tensile strength. A novel technique has been developed to study the propagation of sub-surface cracks under cyclic fatigue conditions. The technique is based on repeated loading of a sharp indenter into a brittle surface. The dependence of the number of indentation cycles needed to produce chipping on indenter load ha been established for a 99.9% Al2O3 And ZTA.