Extension of flow stress–strain curves of aerospace alloys after necking

To define accurately the expansion limits of aerospace materials, determination of the material behavior before and after the onset of necking, as well as the failure threshold are essential requirements. The plastic flow behavior before necking (pre-necking phase) has been fully identified by various mathematical models, such as Hollomon and Swift constitutive equations, but a criterion to satisfy the material behavior after necking (post-necking phase) is lacking. To obtain or calibrate accurately the damage constants in coupled and decoupled damage models, a precise stress-strain behavior after necking is required to reduce significantly the error in predicting fracture in metal forming processes. A tool was developed to determine the true stress-true strain curve for the post-necking regime of different aerospace alloys, such as inconel 718 (IN 718), stainless steel 321 (SS 321), and titanium (Ti6Al4V). Uniaxial tensile tests based on the ASTM E8M-11 standard were performed to determine the true stress-true strain behavior before necking. Two different methods, a weighted average method and a new hardening function, were utilized to extend the true stress-true strain curve after necking. The two methods resulted in similar post-necking curves for the different materials, with consideration that the new hardening function could be used for more complicated hardening laws. The flow curves were employed in the simulation of the dome height test and then validated through experimentation. The simulation results were compared with the experimental data to verify the accuracy of the proposed methods in this work.