A3 Static Fatigue and Subcritical Crack Growth

Topic

Slow crack propagation—the phenomenon described by this model—occurs when a constant stress acts upon a ceramic material that is also exposed to a reactive environment, such as moisture. Neither factor is sufficient on its own; rather, the combined action of constant stress and a reactive environment on the same crack causes it to begin growing subcritically—that is, without yet reaching the size that would trigger immediate fracture.

This subcritical crack growth does not proceed uniformly. Even a slight increase in the driving conditions causes the crack's propagation speed to rise disproportionately, following a power law within what is known as Region I of this growth process.

The accumulation of this subcritical growth over the period during which the constant stress and reactive environment act on the crack is precisely what determines the time to fracture. As the crack grows faster at any given moment, it takes less time to reach the critical size that leads to final fracture.

Thus, the slow crack propagation addressed by this model depends on an entire chain of factors: the constant stress and reactive environment that together drive subcritical crack growth; the Region I power law governing that growth; and the accumulation of growth over time, which determines when fracture occurs. Furthermore, this model contrasts with cyclic fatigue in metals—which relies on repeated load cycles rather than sustained constant stress combined with a reactive environment. Because it depends on a reactive environment rather than load cycles, the underlying fracture mechanism differs fundamentally from that of cyclic fatigue in metals.

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