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Contact damage modes in cyclic loading with spheres are investigated in three nominally brittle ceramics, soda-lime glass, porcelain and fine-grain silicon nitride, in moist environments. Initial damage at small numbers of cycles and low loads consists of tensile-driven macroscopic cone cracks (“brittle” mode). Secondary damage at large numbers of cycles and high loads consists of shear-driven distributed microdamage (“quasi-plastic” mode), with attendant radial cracks and a new form of deeply penetrating subsidiary cone cracks. Strength tests on indented specimens are used to quantify the degree of damage. Both damage modes degrade the strength: the first, immediately after cone crack initiation, relatively slowly; the second, after development of radial cracks, much more rapidly. A fracture mechanics model describing the first mode, based on time-integration of slow growth of cone cracks, is presented. This model provides simple power-law relations for the remaining strength in terms of number of cycles and contact load for materials design. Extrapolations of these relations into the quasi-plastic region are shown to be non-conservative, highlighting the need for further understanding of the deleterious quasi-plastic mode in tougher ceramics. Comparison with static contact data indicates a strong mechanical (as opposed to chemical) component in the cyclic fatigue in the quasi-plastic region. |
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