Correlating Stress Ratio Effects on the Fatigue Crack Growth Rate of a Nickel Base Superalloy IN718

Constant amplitude fatigue crack growth rate tests were conducted on a nickel base superalloy IN718 at various stress ratios, R ranging from R = 0.1 to 0.7. Tests were conducted at room temperature and in lab air atmosphere. Tests were performed in a 100 KN computer-controlled servo-hydraulic test machine using compact tension specimens with sinusoidal waveform at 10 Hz. Crack length was monitored by compliance technique using COD gage. Increasing stress ratio was observed to increase crack growth rates and also decrease threshold stress intensity factor range, ∆Kth. Stress ratio effects on crack growth rates were correlated by using a two-parameter crack driving force, \(\Delta K*\). This approach was observed to provide a reasonably good correlation which can further be employed in modeling crack growth behavior under service loads.