Numerical Analysis of Time-Dependent Springback in L-Bending of Advanced High-Strength Steel Sheet Based on Unified Constitutive Model

Springback of sheet metals consists of two components: springback caused by elastic recovery during unloading and time-dependent springback caused by viscoplastic phenomena over time after unloading. In this study, time-dependent springback in the L-bending of an advanced high-strength steel sheet was analytically investigated using unified constitutive equations based on kinematic hardening and overstress theory. The material parameters for the constitutive equations were determined by referring to the data from the stress relaxation tests in addition to the uniaxial tension and tension-compression tests. The calculated results for stress relaxation and time-dependent springback are compared with the corresponding experimental data. The calculated results captured the trend of the time-dependent springback over time reasonably well but overestimated the influence of the bend radius and underestimated the influence of bend holding. The obtained results suggest that unified constitutive equations based on kinematic hardening and overstress theory can qualitatively describe the time-dependent springback behavior through a stress relaxation-based approach.