Understanding Fracture Toughness

[removed][removed] The International Organization of Standardization recognizes clinical outcomes appear related to the core ceramic's fracture toughness. Values range from 0.8 MPa*m(1/2 for veneering porcelain to more than 5.0 for transformation-toughened zirconia note: metals start @~14).
In bilayer restorations fractures generally start from the intaglio (cemented) surface with the fact being stress in the strongest layer being most important for fracture toughness. Further to note the load-bearing strength of a crown increases by the square of the total thickness. Bilayer prostheses are as durable as full-thickness prostheses.

(Thomas Hill, PhD, DT Sept 2016; ref Dibner & Kelly, J Prosthet Dent, 2016)