Effects of material microstructure on blunt projectile penetration of a nickel-based super alloy

Engine fan-blade containment systems, required in many aviation applications, are frequently manufactured from high-temperature superalloys, such as Inconel-718. As in many other applications, there is an incessant desire to maximize mechanical properties of the containment component while minimizing its weight. However, a thorough understanding of the impact behavior of the various heat treatments of these alloys in engine fan-blade containment applications does not currently exist. Due to this incomplete state of knowledge, a combined experimental–analytical investigation was conducted at CWRU in collaboration with researchers at NASA GRC. As a part of this investigation, thin plates of Inconel-718, in both annealed and precipitation hardened conditions were subjected to semi-quantitative high speed penetration tests. Dynamic compression and top hat shear localization tests using a split Hopkinson pressure bar were also conducted as part of a more fundamental assessment of this material. The measured dynamic material response in compression was used to develop a material model which adequately described the dynamic behavior of IN-718 in both the annealed and precipitation hardened states. Moreover, a transient large deformation thermo-elastic-viscoplastic finite element code is used to understand the local thermo-mechanical fields during impact in both the annealed and precipitation hardened microstructures. The results from these studies show that the annealed material demonstrated superior penetration resistance when compared with the hardened material. The annealed IN-718 absorbed more energy through multiple deformation modes and did not show any susceptibility to shear localization, which was in contrast to the precipitation hardened material. These results, therefore, suggest a precipitation hardened condition may not be optimal for impact energy absorption applications, such as, in engine fan-blade containment.