Comparison of Johnson-Cook and Cowper-Symonds models for aluminum alloy sheet by inverse identification based on electromagnetic bulge

The strain rates of sheet metal often reach several 103 s−1 in various high speed forming processes, and how to determine the strain-rate-sensitive hardening behavior of sheet metal at such high strain rate is an essential issue. The dynamic bulge experiment of aluminum alloy sheet was performed by using electromagnetic pulse forces. The strain-rate-sensitive parameters of hardening models were inversely identified by minimizing the discrepancy between the simulated and experimental displacements of specimen after electromagnetic bulge. The strain-rate-sensitive hardening models of Johnson-Cook and Cowper-Symonds were determined for 2024-O aluminum alloy sheet. Both the models showed that 2024-O aluminum alloy sheet exhibits a positive strain rate sensitivity when the strain rate rises higher than 500 s−1. The flow stresses of Johnson-Cook model increase very slightly when the strain rate ranges from 500 s−1 to 3500 s−1, and they increase about 10% when compared with the quasi-static ones. Meanwhile, the flow stresses of Cowper-Symonds model increase progressively and more significantly. They increase from about 5–35% when the strain rate changed from 500 s−1 to 3500 s−1. The electromagnetic hole-flanging experiment was also conducted to verify that Johnson-Cook model is more accurate for strain-rate-sensitive hardening model of 2024-O aluminum alloy sheet at high strain rates of 103 s−1.