Stretch-flangeability characterisation of multiphase steel using a microstructure based failure modelling

For car body design, a reliable prediction of material formability is required. Stretch-flangeability has become an especially important formability parameter in addition to tensile properties, particularly for complicated auto body parts or parts under heavy deformation conditions. The hole expansion ratio of multiphase steel is strongly influenced by microstructures and their distribution. Therefore, failure behaviour by the stretch-flangeability analysis of this steel has to be described as a function of microstructure. In this work, hole expansion tests were carried out for steel sheets with different microstructures and their local fracture surfaces were afterwards investigated. In the failure modelling, an FE approach is presented by means of Representative Volume Elements (RVE) in combination with continuum damage mechanics in order to consider the influence of multiphase microstructure on mechanical properties and complex fracture mechanisms. At the failure moment, the local strain distributions between different phases were studied, and correlated with macroscopic deformability results. The influence of material properties of individual phases and the local states of stress on formability as well as failure behaviour could be examined. In conclusion, a microstructure based stretch-formability analysis for the characterisation of multiphase sheet steels is aimed.