Fracture Toughness of Advanced Structural Ceramics: Applying ASTM C1421

Abstract

The three methods of determining the quasi‐static Mode I fracture toughness (KIc) (surface crack in flexure—SC, single‐edge precracked beam—PB, and chevron‐notched beam—VB) found in ASTM C1421 were applied to a variety of advanced ceramic materials. All three methods produced valid and comparable KIc values for the Al2O3, SiC, Si3N4, and SiAlON ceramics examined. However, not all methods could successfully be applied to B4C, ZrO2, and WC ceramics due to a variety of material factors. The coarse‐grained microstructure of one B4C hindered the ability to observe and measure the precracks generated in the SC and PB methods while the transformation toughening in the ZrO2 prevented the formation of the SC and PB precracks and thus made it impossible to use either method on this ceramic. The high strength and elastic modulus of the WC made it impossible to achieve stable crack growth using the VB method because the specimen stored a tremendous amount of energy prior to fracture. Even though these methods have passed the rigors of the standardization process there are still some issues to be resolved when the methods are applied to certain classes of ceramics. It is recommended that, when appropriate, at least two of these methods be employed to determine the KIc, especially when a new or unfamiliar ceramic is being evaluated.