Use of a d-c Potential Drop Crack Monitoring Technique in the Development of Defect Tolerant Disk Alloys

Abstract

A direct current (d-c) potential drop technique can be used to monitor the growth of cracks from short 0.1-mm (0.004-in.)-deep electric discharge machined (EDM) defects. Two defect geometries have been developed, a chord-shaped defect approximately 1.6 mm (0.063 in.) wide on the surface and a semicircular notch approximately 0.2 mm (0.008 in.) wide. Due to its higher potential-crack depth sensitivity, the chord defect is used in totally elastic load control tests; however, it suffers from multiple nucleation, crack front tunneling at short crack lengths, and notch root radii dependent crack nucleation. These limitations restrict absolute determination of near-threshold crack growth rates, but this test still provides a relative measure of these properties. Cracks grown from semicircular EDM defects grow in a uniform fashion, and crack depths can be predicted from an analytic model. Using this type of test under a strain cycling mode provides an experimental determination of the residual fatigue life and the Region II fatigue crack growth rates. The crack growth rate curves from semicircular defect, strain control, and chord defect load control tests overlap, even though the net section stress often varies by 60 percent. This test method has been used as a screening test in the development of nickel-base superalloy disk materials with improved defect tolerance, that is, resistance to cyclic crack initiation and growth. The materials evaluated include René 95, IN718, and new experimental compositions. Using these tests, processing conditions that improved the defect tolerance of powder metallurgy Rene 95 by 30 percent have been identified. Study of fatigue crack growth curves of IN718 over a range of temperatures has helped to understand variations in the shape of conventional low-cycle fatigue curves.