Observations in Fracture Toughness Testing of Glasses and Optical Ceramics

Fracture toughness is a critical structural design parameter and an excellent metrics to rank materials. Itdetermines fracture strength by way of the flaws, both inherent and induced, and defines the endpoint of the slow crackgrowth curve. The fracture toughness of structural and optical ceramics, and glasses as measured by several techniques is compared. When good metrology is employed, the results are very comparable with two exceptions: materials exhibiting crack growth resistance and those with a low SCG exponents. For materials with R-curves, the result is a function of extension and can be minimized with short cracks. For materials with low SCG exponents, such as glasses, elimination of the corrosive media andor increasing the stress intensity rate minimizes effects. A summary of values is given, and it appears that highly modified glasses exhibit lower fracture toughness and slow crack growth exponent than high purity glasses such as fused silica.