Objectives: Ceramics are susceptible to premature failure under long-term cyclic loading. Therefore, in vitro fatigue tests of dental ceramic restorations have been the subject of interest. Most of the previous studies use dental ceramics as the incipient materials, which inhibits in situ observation of crack initiation/propagation due to the poor transparency of this class of materials. Hence, the actual failure modes of ceramic restorations still remain elusive. This study adopts a curved glass layer on a compliant substrate system, emulating essential features of brittle crowns on tooth dentin, aiming to gain a better understanding of the fracture behavior in brittle materials under cyclic loading. Methods: Fatigue loading is delivered with a tungsten carbide sphere (r = 1.5mm) using a mouth-motion simulator (Elf 3300, EnduraTEC, Minnetonka, MN) in water. Curved specimens are fabricated by bonding curved glass (inner diameter, 4mm) onto epoxy substrate. The epoxy substrate was pre-cured to minimize the shrinkage stresses at the glass/epoxy interface. Repeated loading of spherical indenter on a curved bilayer structure produces cumulative damage. Crack evolution in the glass/epoxy bilayers is followed in situ during cycling. Results: Contact damage in a curved structure is highly dependent upon the contact load. Cone cracks dominate at low loads (< 75N); they form at the contact surface and propagate downward, reaching the glass/epoxy interface. Bottom surface radial cracks initiate at higher loads (> 120N), and they compete with cone cracks in the region between 120-200N. However, above 200N, bottom radial cracks become unstable; they propagate upward to reach the contact surface and extend sideward to intersect with the margin area. Conclusion: Different modes dominate under given conditions of testing; and, that a mode which dominates in single-cycle loading may not continue to dominate in cyclic loading. Supported by the NYU Research Challenge Fund and NIDCR P01 DE10976.

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