Creep, Subcritical Crack Growth, and Fatigue
Introduction
At low and intermediate temperatures, failure typically emanated from a preexisting flaw formed during processing or surface finishing. The condition for failure was straightforward: Fracture occurred rapidly and catastrophically when K1 > K1c. It was tacitly implied that for conditions where K1 < K1c, the crack was stable, i.e., did not grow with time, and consequently, the material would be able to sustain the load indefinitely. In reality, the situation is not that simple — preexisting cracks can and do grow slowly under steady and cyclic loadings, even when KI < KIC. For example, it has long been appreciated that in metals, cyclic loadings, even at small loads, can result in crack growth, a phenomenon referred to as fatigue. In contrast, it has long been accepted that ceramics, because of their lack of crack-tip plasticity or work hardening, were not susceptible to fatigue. More recently, however, this has been shown to be false: Some ceramics, especially those that exhibit R curve behavior, are indeed susceptible to cyclic loading.
Another phenomenon that has been well appreciated for a long time is that the exposure of a ceramic to the combined effect of a steady stress and a corrosive environment results in slow crack growth. In this mode of failure, a preexisting subcritical crack, or one that nucleates during service, grows slowly by a stress-enhanced chemical reactivity at the crack tip and is referred to as subcritical crack growth (SCG). Unfortunately, this phenomenon is also sometimes termed static fatigue, seemingly to differentiate it from the dynamic fatigue
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