Effect of thickness ratio on load-bearing capacity of bilayered dental ceramics.

The aims of this study were to reveal the mechanism of failure of bilayered beams and to assess the thickness ratio effect on the load-bearing capacity of the bilayered beams. Both analytical and finite element analysis methods were used to analyze the stress distributions of bilayered beams subjected to three-point bending test and the residual thermal stresses due to coefficient of thermal expansion mismatch. Then, the ideal load-bearing capacity of the beams as a function of core thickness was evaluated based on the mechanical models. Finally, three-point bending tests were performed on Cercon Zirconia /IPS e.max Ceram beams to verify the mechanical models. The failure load as a function of core thickness was obtained. For the materials employed in this study, the thickness ratio did not significantly affect the load-bearing capacity of bilayered beams when the thickness ratio changed from 1:2 to 2:1. The residual thermal stresses in the core layer have slightly beneficial effects on the strength of the beams. The first strength theory can be used to explain the mechanism of failure, which can be described as the failure is interpreted by tensile stress and ultimate strength of the material. Based on the relationship between the thickness ratio and load-bearing capacity, the core/veneer thickness ratio of the connector of a fixed partial denture could be relatively small to about 1:2 to obtain a good appearance.