Grain scale modeling of the bendability of AA6xxx Al alloy sheet

Bending sheet metal is a common shaping operation but, at high strains, may lead to failure that is difficult to predict from either standard mechanical tests or models. A recent experimental study of bending AA 6xxx sheet for automotive applications has shown that through-thickness strain localization controls damage development. Here, a new finite element microstructure based model of the standard bending test is introduced to predict strain localization during bending. The sheet metal is modeled as a grain aggregate, each grain having its own flow stress. The model is validated by comparison with a standard model and experimental results through an analysis of the critical plastic strain at the outer surface. It is applied to the bending of industrial AA6xxx sheet alloys and 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 flow stress distribution width. It can be used to give simple guidelines for designing highly bendable sheet metal.