3D synchrotron laminography assessment of damage evolution in blanked dual phase steels

Abstract : The mechanical performance of automotive structures made of advanced high strength steels (AHSS) is often seen reduced by the presence of cut-edges. Here an attempt is made to gain insight into the initial damage state and the damage evolution during loading of a cut-edge. This is assessed in 3D and in-situ by synchrotron laminography observation during simultaneous tensile and bending loading of a cut-edge produced by stamping. Laminography is a technique that allows to observe regions of interest in thin sheet-like objects. It is found for the DP600 laboratory steel grade that the fracture zone is very rough and that needle voids from the surface and in the material bulk follow ferrite-martensite flow lines. During loading the needle voids grow from the fracture zone surface and coalesce with voids in the bulk. The needle cracks coalesce with the burnish zone though narrow zones, called void sheets. The formed cracks are inclined by 45° compared to the load direction.