Pore defects’ influence on the local, near threshold fatigue crack growth behavior of additively manufactured Ti-6Al-4V

Pore defects can exist in additively manufactured (AM) components, even with optimized process parameters and post processing techniques. Lack of fusion (LOF) defects can be detrimental to fatigue, and understanding their influence on near threshold behavior is necessary for the damage tolerant design of aerospace components. This work presents a modeling framework to predict an indicator for the near threshold, local growth of a crack in the vicinity of a pore in AM materials. Three statistically equivalent virtual microstructure (SEVM) models were generated based on the crystallographic orientation and morphology of $\alpha$ laths, given the prior $\beta$ grain structures of AM Ti-6Al-4V. Each SEVM was simulated with a small semicircular crack, constituting the baseline case, as well as with five experimentally characterized LOF defects positioned at variable distances from the small crack. Cyclic crystal plasticity simulations were performed with several applied stress intensity factors, and a methodology based on the accumulated plastic strain energy density, $w^P$, was developed to postulate crack growth rates from these static simulations. Multiple simulations lead to the construction of crack growth rate curves, from which a threshold stress intensity factor, $\Delta K_{th}$, can be defined. The findings demonstrated that LOF morphology and crack-pore distance are the most influencing factors resulting in a decreased value of $\Delta K_{th}$, while crack shielding and crack blunting can increase the $\Delta K_{th}$ value. This modeling approach and near threshold crack behavior can provide important quantification of pore influence on fatigue crack growth rates of AM Ti-6Al-4V, which can support material qualification efforts.