Deflected ‘teardrop cracking’ in nickel based superalloys: Sustained macroscopic deflected fatigue crack growth

A sustained macroscopic deflected fatigue crack growth, the so-called ‘teardrop’ crack (TDC), has been observed under mode I (tensile) loading in Udimet 720Li when tested at 300 °C in air and at 300, 600 and 650 °C in vacuum. A definition of the onset of sustained shear crack growth is proposed and applied to different test geometries in different temperatures and environments. SEM analysis of the fracture surfaces have shown a competition between the sustained shear and mode I crack growth modes in both the terraced shear lip and flat teardrop region, which indicates that the macroscopic deflection is triggered by the free surface. The tilt angles measured from the deflections have been used to define a mixed mode plane stress test, attempting to replicate the stress state giving rise to deflected shear growth.