Fracture Toughness — MFDS Part 1 MCQ
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Correct answer: D — Critical mode I stress-intensity factor at which an existing crack becomes unstable
Option D is correct. KIC is the critical mode I stress-intensity factor under plane-strain conditions and expresses resistance to unstable propagation of a pre-existing crack. This is particularly relevant to brittle dental ceramics, in which fracture commonly initiates at flaws. Option E describes nominal fracture strength, which is influenced by specimen geometry and the flaw population. Option B describes bulk material toughness: the energy absorbed per unit volume before fracture. Option C describes hardness, assessed through resistance to local indentation or scratching. Option A describes elastic modulus, the ratio of stress to elastic strain. KIC therefore characterises crack-growth resistance rather than strength, energy absorption, hardness or stiffness.
Reference: Ryan AK, Orr JF, Mitchell CA. A comparative evaluation of dental luting cements by fracture toughness tests and fractography. Proceedings of the Institution of Mechanical Engineers, Part H. 2001;215(1):65–73. https://pubmed.ncbi.nlm.nih.gov/11323987/