CHARACTERIZATION OF ENVIRONMENT-DEPENDENT FATIGUE CRACK GROWTH IN ALLOY 718 AT 650°C H. Ghonem and D. Zheng Mechanics of Solids Laboratory Department of Mechanical Engineering and Applied Mechanics University of Rhode Island, Kingston, R. I. 0288 1, USA Abstract The elevated temperature fatigue crack growth behavior in Alloy 718 when subjected to a loading frequency lower than the transitional frequency of this alloy, is viewed to be fully environment-dependent. In this process,the crack growth increment per loading cycle is assumed to be equal to the intergranular depth ot oxygen diffusion taking place at the crack tip during the cycle effective oxidation time. In order to identify the trend of this diffusion depth an experimental program was carried out on compact tension specimensmade of Alloy 718 at 650°C in which fatigue crack growth measurementswere made for two different cyclic load conditions, one including hold time periods at minimum load level while the second includes these periods at the maximum load level. This work resulted in establishing a relationship correlating the intergranular depth of oxygen diffusion and the instantaneous value of the stress intensity factor K. This relationship when integrated over the cycle effective oxidation time results in a close form solution describing the environment-dependent fatigue crack growth rate. A comparison is made between the results of this solution when applied to different loading frequencies and the corresponding experimental results. This comparison indicates the validity of the proposed model, in particular at low AK values and lower loading frequencies where environment fully dominates the crack growth process. Superalloys ‘718,625 and Various Derivatives Edited by Edward A. Imria The Minerals, Metals & Materials Society, 1991 477