Effect of non-linear strain paths on the forming limit diagram of AHSS

The determination of forming limit diagrams is a common method for sheet metal forming processes to provide material formability for the failure prediction. The extent to which the material-specific forming limit curve can be applied is somewhat restricted as its strain dependent failure criterion is based on a linear strain path and therefore has a limited usage for multi-stage forming processes. The overall objective of this work is to identify the effect of non-linear strain paths on the FLD for three different AHSS materials. The materials include a Dual-Phase steel, a TRIP steel and a TWIP steel. Different levels of pre-deformation in biaxial tension, in plain strain and in uniaxial tension are considered. The experimental setup for the forming tool is based on the Nakajima method using a hemispherical punch of 400mm in diameter. An optical 3D deformation measuring system was integrated within the test tool, consisting of two high frequency CCD cameras.