Crack growth under constant sustained load at elevated temperature in IN718 superalloy

Crack growth rate ( da/dt) in IN718 nickel base superalloy has been studied at 600 and 700 degrees C, in both compact tension and corner crack specimens, under sustained constant load conditions, generating an intergranular fracture mode. Using the stress intensity factor, K, as the correlating parameter, slower growth rates are found for corner cracks than for the through cracks in CT specimens together with a greater degree of crack tunneling. In the CT specimens, the introduction of lateral side grooves increases the crack growth rate. Disparities found between growth rates in the CC geometry are found to correlate with the incubation time for re-initiation of crack growth, which is attributed to a crack tip blunting and creep mechanism. A limited assessment of the correlation of growth rates using the creep crack growth parameter C* is made.