Fundamentals of bending AA6xxx sheet

This paper describes recent experimental results on the strain distributions developed during bending of AA6xxx sheet for automotive applications, together with a new model for the mechanics and metallurgy of strain localization during bending. A detailed microscopic study (optical and SEM/EBSD) shows that damage development during bending to strains of order unity is controlled by through-thickness shear banding at the grain scale. A new finite element microstructure-based model is introduced to predict this strain localization during practical bending. The sheet metal is modelled as a grain aggregate, each grain having its own flow stress. After validation, the model is applied to the experimental results through an analysis of the critical plastic strain at the outer surface during bending of AA6016 sheet alloys. It correctly describes the respective influences of sheet thickness, grain size and shape, and work hardening. In particular the model brings out the primary importance of large-strain hardening and the spread of the flow stress distribution.