A Design Approach for Correlating High Temperature Fatigue Crack Growth Over a Wide Range of Parameters

High temperature fatigue crack growth was studied in several materials under various environmental conditions. At temperatures less than one-half of the melting point, creep effects were found to be negligible in comparison to environmental effects. The general fatigue behavior is a result of both time dependent and cycle dependent processes. A model is formulated which combines linearly the separate time and cycle dependent behaviors to correlate the general fatigue behavior. The cycle dependent behavior in the model is based upon the premise that the relative corrosion rate controls the crack growth rate. The few constants in the cycle dependent portion of the model are based upon the fatigue crack growth in the environment with zero load ratio and no dwell. The time dependent behavior is determined from static crack growth data in the environment. The resulting Kinetic Model correlates with good success the effects of temperature, frequency, dwell time and load ratio on the high temperature air fatigue crack growth of IN-100.