Square Punch Forming: Section Analysis and Bending Study

Square punch/open die experiments were conducted to study the stretch forming of full-width blanks and plane strain strips. In order to measure bending effects, strains on both surfaces of the sheets were measured for three punch profile radii, using circle grids with 1.27 mm diameter. Two rigid-viscoplastic finite element programs, SHEET-S and SHEET-3, were used to predict plane-strain and 3-D strain distributions. The numerical results were compared with experimental data and good agreement was observed. It was found that the section analysis approach may be valid for a wide range of applications not strictly meeting the plane strain conditions, such as the biaxial stretch case. The bending effect is studied and a geometrical correction of the membrane FEM predictions, through t/2r, was introduced. The membrane section analysis program agreed well with measurements for r/t ( radius of curvature/sheet thickness ) values greater than 3 and when compound curvature was not present. For scan lines over corners with compound curvatures, 3-D membrane FEM was found to be more suitable than the section analysis.