NUMERICAL STUDY OF ANISOTROPY AND DUCTILITY ENHANCEMENT IN SHEET METALS

Abstract

GTN is analyzed and extended to be suitable for anisotropic matrix materials on the basis of Hill’s quadratic anisotropic yield theory. An effective coefficient is defined in the extension, and it replaces Hill’s constants to provide the elastic–plastic behavior of metals. In this model, three dimensional (3D) and plane stress elements are considered, containing ellipsoidal and spherical microvoids in matrix materials. The effect of each case is studied and compared with the stress–strain curve of a typical Al alloy. Superimposed hydrostatic pressure delays the growth and coalescence of microvoids. Thus, bendability is significantly enhanced by superimposing hydrostatic pressure. The effect of hydrostatic pressure on tensile test simulation is determined and compared with that obtained under bending. This test is simulated under the plane strain state. Sheet metals have higher deformation under bending than under tension. The sensitivity of ductile fracture parameters in the GTN model to bendability is analyzed and used as the basis for exploring the bending response of sheet metals given by this model. Cladding sheet metals slows down the development of stress triaxiality, which has a significant effect on the fracture of sheet metals. Consequently, the growth and coalescence of microvoids are delayed, and this condition leads to high bendability and increases fracture strain. A transition zone in the location of failure initiation is observed. This transition zone changes with increasing cladding thickness ratio (Γ). Fracture initiation changes from the core material close to the clad–core interface to the clad material on the outer surface of the specimen at increased Γ. The effect of mandrel span length as well as stress–strain curves for clad materials on bendability is investigated and used as the basis for understanding changes in transition zones.