A practical constitutive model for AZ31B Mg alloy sheets with unusual stress–strain response

• A practical constitutive model representing yielding asymmetry and anisotropy of Mg alloys is proposed.
• The model is formulated and implemented in the sheet forming oriented FE software by user subroutine.
• The unique concave-up and sigmoidal stress–strain curves during in-plane cyclic loadings of AZ31B Mg sheets could be exactly reproduced.

Magnesium (Mg) and its alloys have been increasingly used in the automotive industry owning to their superior mechanical compared to other metals. Sheet forming of Mg alloys requires not only a good understanding of the materials but also an appropriate material model. The hardening behavior of Mg alloy sheets during plastic deformation is the main concern in tool design and product manufacturing processes. A simple but computationally effective constitutive model is suggested to predict the mechanical behavior of Mg alloy sheets. The proposed material model is implemented as a user-defined material subroutine in the commercial finite element code PAM-STAMP®, then calibrated and validated using in-plane cyclic loading tests with different values of prestrain. The continuous tension–compression–tension and compression–tension–compression tests are conducted on a newly developed testing system. After calibration, the model can accurately predict the yielding asymmetry behavior and the stress–strain curve featured by the unusual sigmoidal shape which is the result of the activation of twinning and untwinning deformation mechanisms.