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Ceramic Fracture Mechanism — MFDS Part 1 MCQ

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ModerateDental MaterialsCeramic Fracture MechanismMFDS Part 1

A newly fabricated all-ceramic crown is subjected to a steadily increasing load until it fractures catastrophically at a load below the ceramic's theoretical strength. Fractographic examination identifies the origin at a small intaglio surface flaw in a region of tensile stress. Which mechanism best explains the failure?

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Correct answer: E — Unstable brittle crack propagation from the pre-existing flaw

The correct answer is E. Dental ceramics are linear-elastic, brittle materials with limited ability to dissipate stress by plastic deformation. Tensile stress is concentrated at a pre-existing surface or internal flaw; when the crack-tip stress intensity reaches the material's fracture toughness, unstable crack propagation produces catastrophic fracture. This explains failure below the theoretical strength of a flawless ceramic. Plastic yielding and ductile rupture are characteristic of metals, while creep is not the principal response under this steadily increasing load. Electrochemical section loss is also a metallic failure mechanism. Cyclic fatigue can promote subcritical crack growth during repeated loading, but the stem describes a monotonic load-to-fracture test rather than repeated bending.

Reference: Kelly JR et al. ADM guidance—ceramics: Fatigue principles and testing. Dental Materials. 2017;33(11):1192–1204. https://pubmed.ncbi.nlm.nih.gov/29017761/