Methods: The failure modes of bilayer composites were confirmed under cyclic spherical fatigue loading by dynamic microcosmic observation. Crack mode and evolvement of bilayered composites were explored inflecting with fatigue load, cycles, frequency and water environment. Microcosmic morphology of the samples was observed by high depth of field stereomicroscope to determine degree and modes of fatigue damage.
Results: The appearance of critical crack of porcelain surface was almost consistent in both of zirconia and alumina bilayer composites.The pattern and process of the damage are different under cycle fatigue load. Radial crack line traversed the interface of alumina bi-layer composites，otherwise, crack line was surrounding the load location and prolonged with the interface, with delamination of zirconia bi-layer composites. Process of fatigue damage of bilayered composites were accelerated by water environment .
Conclusions: Zirconia bilayer composites presented cone crack during more load cycles indicated safer from fatigue damage than alumina bilayer composites under same loads and environments.