APPLICATION OF FRACTURE MECHANICS PRINCIPLES TO FATIGUE CRACK GROWTH IN A POWDER METALLURGY NICKEL-BASE SUPERALLOY (STRAIN ENERGY DENSITY, SHORT CRACK, ELEVATED TEMPERATURE, ASTROLOGY)

The fatigue crack propagation response of hot isostatically pressed (HIP) L.C. Astroloy was examined as a function of grain size, various loading parameters, crack configurations and temperature. Results for material with average grain sizes ranging from 5 (mu)m to 50 (mu)m are presented. Long crack near threshold FCP behavior obtained using disc compact tension (DC(T)) specimens was found to be relatively insensitive to changes in grain size at a stress ratio of 0.1. Increasing the stress ratio to 0.5 revealed an increase in FCP rates with increasing grain size. Of greater significance, however, was the observation that crack growth rates in region I were highly sensitive to variations in specimen geometry. At a given stress intensity range, fatigue crack growth rates were observed to increase as the loading of a given geometry became more symmetric with respect to the crack plane. It is suggested that this phenomenon is related to differing near field crack tip stress distributions between specimen geometries and their interaction with the material's microstructure.;The application of tensile overloads to fatigue cracks were shown to introduce increasing cyclic delay in crack propagation with decreasing baseline stress intensity range. This delay data may be effectively correlated in terms of the ratio of the baseline plastic zone size to the relevant microstructural parameter.;The "apparent anomalous" growth behavior of short fatigue cracks in this Ni-based superalloy was also examined. Experimental and analytical results indicate a possible breakdown of linear elastic fracture mechanics (LEFM) in the "so-called" short crack regime. Short crack data only appear anomalous when correlated in terms of the stress intensity factor. When short and long crack data are analyzed in terms of a more valid driving force (e.g., strain energy density criterion), no anomalous short crack is observed.;Fatigue crack propagation rates at high cyclic frequency (30 Hz) were observed to increase at elevated temperatures. This increase has been attributed primarily to reduced component stiffness associated with elevated temperature exposure as well as changes in environmental sensitivity and variations in deformation mechanisms. At low cyclic frequency (0.3 Hz) FCP rates were observed to increase further. This latter behavior is suggested to be a consequence of creep-induced grain boundary particle cavitation and triple-junction cracking processes.