An Interpretation of the Yoder-Griffis-Crooker Observations of Sustained-Load Cracking in Ti-6Al-4V

Their recent NRL report documents a substantial degradation in load carrying capability of partially cracked specimens of annealed Ti-6 Al-4V plate in a room air environment. In this paper, the effect is analyzed as a special case of tensile ligament instability model, set up to permit integration of crack growth rate over the path to K_Ic. Results show that the time dependence and magnitude of the load carrying capability and of the sustained load crack growth SLC are predictable from plastic flow characteristics, including the transient stress relaxation rate, and from a slight environmental effect. A simplified diagram for engineering assessment of SLC is presented. Importance is attached to the persistence of stress relaxation, as much as to its rate, in inducing SLC. Little effect is expected from the level of fracture toughness to the extent that it is controlled by the process zone size d_T.