Angle of contact between sheet and die during stretch–bend deformation as determined on the bending-under-tension friction test system

Laboratory tests, designed to assess the frictional behavior and formability of sheet metal, rely on direct measurement of pulling and contact forces for friction evaluation. The bending-under-tension friction test subjects a strip to bending as it slides over a cylindrical die surface and can evaluate sheet forming behavior during stretch–bend deformation. To gain a better understanding of sheet–die interaction, the bending and unbending response of the sheet as it contacts the cylindrical die was investigated. The existence of bending and unbending pressure peaks as well as the resulting extent of the angle of contact between the sheet and the die was observed experimentally using a Kynar® piezoelectric sensor. Direct evidence of pressure peaks during stretch–bend deformation of sheet steels is presented. The extent of the contact angle is less than the geometrical angle-of-wrap and increases with strip tension. The experimental findings for contact angles compare favorably to the predicted results from a simplified vector analysis of strip tension and strip-bending resistance.