Modeling the Effect of Backstress on Springback Predictions in AA 6016-T4 as a Function of Pre-strain

This paper explores springback levels in AA6016-T4 sheets that were first pre-strained to various levels in uniaxial, plane-strain, and biaxial tension, then subjected to a pure bending operation and released. Finite element modeling of the pre-strain and bending steps was performed using both isotropic and elasto-plastic self-consistent (EPSC) crystal plasticity approaches. Because the EPSC model incorporates backstresses informed by GND content, the springback predictions were far more accurate than those of the isotropic model. In the unstrained base material, the EPSC predictions had a maximum error of 7% versus experiment, compared to 49% for the isotropic approach. The EPSC model accurately predicted the transition from springforward to springback as a function of applied pre-strain, but only when the backtress term was included in the hardening law. The backstress contribution was also found to be increasing influential in predicting springback as pre-strain levels increased, especially in the case of a uniaxial tension pre-strain.