Comparing High Strain Rate Shear Response and Resistance to Adiabatic Shear Bands in 3D Printed and Wrought Inconel 718

With the increased demand for additively manufactured components in the aerospace industry, the necessity for characterization of 3D printed materials under nominal and off-nominal loading conditions grows rapidly. In this research, the resistance to adiabatic shear band (ASB) formation in 3D printed and wrought Inconel 718 will be compared. Reliable turbine fan-blade containment systems, which are required on many aircrafts for safe operation of gas turbine engines, are commonly made from high-strength titanium or nickel-based superalloys. When engine fan blades fail, the fragments impact the containment systems walls at high velocities and either cause global plastic deformation, which is preferred, or localized plastic deformation, in the form of shear plugging. During plugging, intense plastic shear strains may cause ASBs, which is the localization of shear strain into a very narrow band of material, which can cause premature catastrophic failure. In a region of intense shear strain, both work hardening and thermal softening occur causing a thermo-mechanical instability. At a critical value of shear strain, thermal softening effects dominate and any increased hardness from work hardening effects is completely mitigated, thus forming an ASB. Dynamic shear tests were carried out using a split Hopkinson pressure bar to compress “top-hat geometry” shear samples and high strength steels stopper rings were attached to the samples to limit their deformation. After mechanical testing, each sample was sectioned and prepared for microscopy to characterize the ASB formation based on the strain in the sample. A shear strain value of 3.23 was observed to result in ASB formation across the entirety of the shear region. The samples that experienced a higher strain clearly showed cracks that traveled partially along the ASB. One sample was tested without the shear stopper ring, and this allowed the crack to travel along the ASB across the entire shear region breaking the sample into two pieces. The ASBs observed in the printed samples were approximately 4-microns wide which are markedly narrower than the 7-micron wide bands in the wrought material. Microhardness tests conducted on the material surrounding the ASB revealed that the material hardness increases around the ASB with a peak in microhardness at the center of the ASB. This hardness peak in the ASB center is often attributed to nanoscale grains that formed by dynamic recrystallization occurring when ASB forms. Determining at what shear strain an ASB may form will help future fan-blade containment system designs ensure the safety for future aircraft.