ELEVATED-TEMPERATURE FATIGUE-CRACK GROWTH UNDER DWELL CONDITIONS IN WASPALOY

Growth of corner cracks at temperatures up to 700 degrees C under cycl ic (0.25 Hz), 300 s dwell at peak load, both at R = 0.1, and constant load conditions has been studied in the nickel base turbine disc alloy Waspaloy. No significant effect of dwell on fatigue crack growth rate (da/dN) is found at 550 degrees C, where transgranular fatigue cracki ng predominated. At 600 and 650 degrees C, under dwell cycling a trans ition to entirely intergranular cracking is accompanied by a severe in crease in cyclic crack growth rate. Contrary to the trend for 600-650 degrees C, dwell and constant load data at 700 degrees C shows lower c rack growth rates than for 650 degrees C, indicating that a time depen dent microstructural change occurs during prolonged exposure at 700 de grees C generating an increased resistance to intergranular cracking. Using the stress intensity range (Delta K) as a correlating parameter, fatigue crack growth rates are successfully predicted by linear summa tion of pure fatigue and time dependent data. (C) 1997 Elsevier Scienc e Ltd.