A2 Static Fatigue (Moisture Assisted Subcritical Crack Growth)

Topic

This model describes subcritical crack growth in silica assisted by ambient moisture—a phenomenon known as static fatigue—by directly adopting the framework established by another model in this set, which describes static fatigue and subcritical crack growth in ceramics: a constant stress acting on a crack, combined with a reactive environment, causes the crack to grow subcritically without yet reaching the size that would trigger immediate fracture. Because this model directly reuses that framework, there is no need to redefine from scratch the mechanism by which constant stress and the reactive environment drive crack growth.

In this specific system, ambient moisture acts as the reactive environment and accelerates subcritical crack growth in the silica; this growth follows the Region I power law established in the other model, meaning that a small increase in ambient moisture causes a disproportionately large increase in the crack's propagation rate. This is, in fact, the system in which Wiederhorn first characterized this phenomenon of moisture-assisted static fatigue in 1967.

However, that same subcritical growth power law does not rely on a fixed constant in this model: the ambient relative humidity empirically modifies the subcritical growth constant. Higher relative humidity increases the subcritical growth constant—causing the crack to grow faster under the same level of constant stress—while lower relative humidity decreases the constant, causing the crack to grow more slowly under that same stress. For this reason, this model—incorporating the ambient-moisture-assisted subcritical crack growth that initiated the process—directly utilizes the causal chain already established by another model in this set: the accumulation over time of subcritical growth (accelerated by ambient moisture and modified by relative humidity according to a specific constant) under the influence of sustained stress is precisely what determines the time to silica fracture.

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