Crack Coalescence and Microscopic Crack Growth in the Delayed Fracture of Alumina

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Abstract
Microscopic crack growth in the delayed fracture of alumina is studied. A model for the delayed fracture, based on microscopic cracks and their interaction and coalescence, is proposed as opposed to the conventional single‐worst‐crack model. The proposed analysis gives a more realistic prediction than the conventional one. It is found that an assembly of microscopic cracks, which may not be observable, may be worse than a single macroscopic crack in delayed fracture and that the coalescence of the microscopic cracks may occur in a very short time without being identified because the critical amount of slow crack growth from the individual inherent flaws can be very small. Applicability of proof‐testing concepts is reexamined. It is demonstrated that the existing concepts give nonconservative prediction in time‐to‐failure; but the nonconservative prediction is partially or sometimes excessively compensated by conservative estimation of the initial crack size.

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