Fatigue and Static Crack Growth Rate of Alloy 718 Under Cathodic Polarization

Fatigue crack growth rate was developed on three heats of Alloy 718 (UNS N07718) under cathodic polarization, over a wide range of loading conditions. Fatigue crack growth rate increased with decreasing frequency over a range of K-max and Delta K conditions. In most cases, there was no evidence of a plateau in fatigue crack growth rate at low frequencies. The fatigue crack growth rate over the range of conditions evaluated was influenced by static crack growth rate at K-max, The principle of superposition of fatigue crack growth and static crack growth was used to rationalize the observed crack growth rate response. Static crack growth rate of Alloy 718 measured under constant K conditions was lower than that measured under rising displacement conditions. A crack tip strain rate-based model was used to rationalize the fatigue crack growth rate behavior and the static crack growth rate behavior under constant K. However, the formulation of the model for the rising K was not able to rationalize the crack growth rate under rising displacement conditions.