Formability of sheet metal flowing through drawbead–an experimental investigation

Abstract

A phenomenon that when the sheet metal flows through drawbead, the limit strains of sheet metal lie above the forming limit diagram indicating fracture in theory, while it is safe in practice, violates the normal forming limit curve has been found. In order to explain the phenomenon, a series of tests were conducted. In the first part, the conventional uniaxial tensile test was employed as a reference. In the second part, specimens were pulled through various drawbead inserts several times, and then cut to the uniaxial tensile test specimens and tested by the conventional uniaxial tensile test. In the third part, specimens were pulled through drawbead inserts many times until the fracture occurs. The results indicate that the percentage of elongation of pre-strained specimens with several times are smaller than that of the conventional uniaxial tensile test. The transition process from the plane strain path to the uniaxial tensile strain path leading to the additional strain hardening is the main reason for the reducing formability of sheet metal. While the percentage of elongation of specimens pulled through drawbead inserts many times until the fracture occurs are evidently higher than that of the conventional uniaxial tensile test and could reach close to 100%. Bending and unbending under tension has a significant effect on the increased percentage of elongation.