Abstract
A kinetic model is developed that considers the relative contributions of creep and environmental degradation to the enhanced cyclic crack growth rate at high temperatures. It is shown that the apparent activation energy for crack growth is dependent on the stress intensity factor range ΔK and the ramp-up time. When environmental degradation is the main contribution, the apparent activation energy decreases with increasing ΔK. However, the activation energy is about half that for diffusion of any detrimental species in the matrix. This mechanism is accompanied by a decrease in the Paris exponent with increase in temperature. When creep effects dominate, the reverse holds and the activation energy corresponds to that for growth of grain boundary voids. It is posulated that these phenomena are the consequence of the stress field at the crack tip, which causes diffusion in this region to be position dependent.
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