Influence of transverse normal stress on sheet metal formability

Since the late 1960ʹs, the strain space forming limit diagram, FLDϵ, has been nearly universally employed as a method to help predict sheet metal failure for the open die stamping process. Traditional FLDϵʹs in use today are based upon a plane stress assumption. However, a plane stress assumption may not always be valid for many sheet metal forming operations. Through the employment of a strain-to-stress space mapping procedure, a new sheet metal formability model that takes into account the through-thickness normal stress (σ3) is proposed. Good agreement with a limited set of experimental data is found.