CRACK CLOSURE EFFECT ON CRACK-GROWTH RATE AT 650-DEGREES-C IN DOUBLE NOTCHED SPECIMENS OF A NICKEL-BASE SUPERALLOY

Creep-fatigue tests were performed at 650 degrees C in air on a N18 ni ckel base superalloy, using double notched and smooth specimens. The d eformation mechanisms observed by TEM at the notch root are shown to b e compatible with the constitutive set of equations used in the finite element analysis which is presented. For a given K-max at the notch r oot, the crack growth rate is much higher in a notched specimen than i n a smooth one. This effect can be explained by a variation of the cra ck closure stress level with the local R ratio and the local stress. A strong accelerating effect of the R ratio, especially for negative va lues, is found in smooth specimens. Introducing a K-op correction in t he experimental results leads to a good agreement between the measured crack growth rate plotted versus K-eff in notched and smooth samples.