Toughness and Flaw Responses in Nontransforming Ceramics: Implications for NDE

Recent developments in the characterization of the strength of ceramics have made it necessary to re-examine several “tradition”, long-standing definitions and assumptions that form the modern-day fracture mechanics basis of NDE. Ceramics are very brittle materials. They are highly susceptible to failure from small scale (1–100 μm) “flaws”. These flaws may be in the form of machining damage, grain boundary fissures, processing defects (pores or inclusions), etc. Theoretically, flaws have been represented as scaled-down versions of large cracks, so that the macroscopic “laws” of fracture might be assumed to apply at the microsale. This philosophy is embodied in the Griffith strength formalism, σm = Τo/Yc1/2 where c is the flaw size, To is the toughness (KIC in metallurgical terminology) and Y is a geometrical constant. Implicit in Eq. 1 are two major conclusions which dictate the entire approach to NDE in ceramics: (1) Failure occurs spontaneously at the critical stress (σm);; (2) Toughness (To) is single-valued