Numerical analysis for formability boundaries definition in sheet metal forming operations

Formability limits in sheet metal forming have been widely investigated through different approaches. The main aim of such approaches was to develop tools able to prevent or predict fracture occurrence in order to improve production quality. Forming limit diagrams (FLDs) and fracture criteria are probably the most utilised tools in this field; nevertheless, some drawbacks of such tools were demonstrated. In particular, forming limit curves are characterised by ineffective prediction capability if non linear deformation paths are taken into account. Moreover, the importance of stress paths was demonstrated in order to predict fracture, and forming limit stress diagrams are, nowadays, deeply studied in the technical literature. In this paper, the authors present the results of experimental and numerical investigations aimed to analyse the fracture prediction capability of forming limits defined by stress variables. Moreover, the possibility to define a three dimensional stress surface which represents a fracture boundary is discussed.