Unloading Modulus on Springback in Steels

Springback is one of the major concerns as more advanced high strength steels are used in the automotive industry. Unlike FEA forming simulation, FEA springback simulation is not widely used in the production due to its poor accuracy. One of the reasons for this is that some complex material behaviors are not fully considered in the FEA simulations.

In this study, a number of experiments were conducted to study the unloading behavior of steels with different pre-strain levels. Unloading modulus and related damage parameters at 1%, 2% and 5% pre-strain are calculated for 14 different steel grades from the experimental data to consider the inelastic effects during unloading. The unloading modulus was found to be 10% to 20% lower than the Young's modulus. FEA springback simulations were conducted for a benchmark test with two steel grades. Simulated springback results are compared with the experimental data. Results show that the simulation accuracy is significantly improved by incorporating the unloading modulus concept instead of conventional elastic-plastic model.