Forming limit of sheet metals based on mixed hardening model

Abstract

The calculation method of forming limit of sheet metals based on M-K instability theory is proposed, and the method is applicable to different yield criterions and hardening models. The forming limit diagrams of AA5754-O, AA6111-T4 aluminum alloy sheet and DP steel sheet under combined loading paths are obtained based on mixed hardening model with YLD2000-2D yield criterion proposed by Barlat in 2003 and L-C nonlinear kinematic hardening model proposed by Lemaitre and Chaboche. The results show that the forming limit diagram made up of limit strain (FLD-strain) is evidently influenced by the loading path. The forming limit diagram made up of limit stress (FLD-stress) is also influenced by loading path and it is not an only curve, which differs from the conventional view. The degree of the influence of loading path on FLD-stress is related with pre-strain. The larger the pre-strain is, the greater the influence of loading path on FLD-stress will be. The change of FLD-stress is small only when pre-strain is small. In addition, the hardening behavior of the material will influence the path-dependence of FLD-stress: The larger the proportion of kinematic hardening in the whole hardening is, namely the more obvious Bauschinger effect of the material, the greater the influence of loading path on FLD-stress will be.