Hole Expansion Simulation of Steel Sheet Considering Differential Hardening

Finite element analysis (FEA) of hole expansion forming is investigated to clarify the effects of material models on the predictive accuracy of the FEA. The multiaxial plastic deformation behavior of a hot rolled steel sheet with a tensile strength of 440 MPa was measured using biaxial tensile tests with cruciform specimens (ISO 16842, 2014) and tubular specimens subjected to servo-controlled axial force and internal pressure. Many linear stress paths in the first quadrant of stress space are applied to the specimens to measure the contours of plastic work (CPW) and the directions of plastic strain rate (DPSR) up to a reference plastic strain of 0.22. It is found that the Yld2000-2d yield function (Barlat et al., 2003) accurately reproduces both the CPW and DPSR. Isotropic (IH) and differential hardening (DH) models are determined using the Yld2000-2d yield function; in the DH model the values of exponent and eight material parameters of the Yld2000-2d yield function change as functions of reference plastic strain. Moreover, a hole expansion forming experiment was performed. The thickness strain distribution along the hole edge was measured and compared with the FEA results obtained using selected yield functions. The DH model correctly predicts the minimum thickness position that matches the fracture position of the specimen in experiment. It is concluded that the yield function best capturing both the plastic work contours and the directions of plastic strain rates leads to the most accurate FEA results.