Prediction of maximum crack length in longitudinal varestraint testing

High speed, high resolution video observations of solidification cracking in longitudinal Varestraint tests of Alloy 718 reveal a number of important features of the test. For large augmented strains, crack initiation occurs at a liquid fraction of approximately 0.2, and crack growth proceeds in both advancing (growth in the torch travel direction) and retreating (growth away from the trailing edge) directions. For the advancing crack, the average growth velocity is identical to the travel speed, indicating that the advancing tip grows at isothermal temperature and consequently, a fixed liquid fraction. Termination of the advancing crack tip occurs as the augmented strain field diminishes. The retreating crack tip grows in an increasing strain field and decreasing temperature, and appears to terminate when the crack tip intersects the solidus. These observations define criteria for the major events governing the maximum crack length at high augmented strains. Coupling these criteria with models of strain development, temperature distribution, and solidification behavior allows for a priori estimation of the maximum crack length. In the present work, the maximum crack length for a variety of Nb-containing Fe and Ni-based superalloys are estimated by using the above models and criteria, and compared with experimental results.