CREEP CRACK INITIATION AND GROWTH IN INCONEL 718 ALLOY AT 650°C

Abstract

AbstractCreep cracking tests were performed on a nickel base superalloy, Inconel 718, using CT type specimens at 650°C in an air environment. Both time to initiate creep crack propagation (T1) and creep crack growth rate (ȧ) were measured as a function of the stress‐intensity factor, K. It is shown that crack initiation behaviour and early crack growth (stage I) are strongly dependent on test procedures. Furthermore there is no unique correlation between ȧ and K, except in the regime corresponding to relatively high crack growth rates (stage II). This conclusion was reached by conducting tests both under increasing and under decreasing K. The reasons for this behaviour are briefly discussed in relation to environmental effects. A simple model (based upon a theoretical stress‐strain field for creeping cracked solids and the ductility exhaustion concept) is used in an attempt to explain the results. The limitations for the application of this model are discussed. It is shown that this approach can explain a number of experimental results provided that it is accepted that creep ductility is time‐dependent because of environmental effects.Finally, the results of these crack propagation tests are used in conjunction with the theoretical results derived from the Riedel and Rice analysis to model both crack initiation and crack growth behaviour of Inconel 718 under sustained load.