Finite element and analytical study of plane strain draw-bend failure of advanced high strength steels

The inability to predict shear failure during sheet stamping operations has been a major barrier limiting the widespread use of advanced high strength steels (AHSS) in the automotive industry. Based on recent research results obtained by the authors, finite element and analytical procedures were developed to predict shear failure during plane strain draw-bending. The material model used in this work is capable of representing reduced strain hardening and softening of materials deformed under adiabatic condition. This capability was required because the mechanical properties (in particular, strain hardening) of AHSS change significantly with time and location during sheet forming processes because significant deformation heating occurs associated with the large plastic work of these materials. In the finite element procedure, a finite element model of the plane strain draw-bend fracture test was constructed using Abaqus Standard 6.7. In the analytical procedure, the equilibrium equation was solved to obtain the tensile force required to bend a sheet in plane strain over a radius. In both procedures, localized necking begins when the tensile force reaches maximum. Stress (and strain)-based failure criteria, (failure stress (and strain) vs. radius plots) were obtained using the finite element and analytical procedures, which showed good agreement with each other. The criteria obtained by these models can be used to design the forming processes that exploit the real formability of AHSS.