Characterization of forming limits at fracture from shear to plane strain with a dedicated cruciform specimen

Abstract

Biaxial tensile test has been drawing attention as an effective approach to characterize sheet metal formability. In this research, a new cruciform specimen is provided to determine the forming limits at fracture from shear to plane strain. AA6061-O sheet with a thickness of 2 mm is employed as a target material. Firstly, three different contours (Type I—rectangular contour, Type II—circular contour and Type III—elliptical contour) of the specimen thickness reduction zone are proposed, and the effect of the contour on strain path, stress state and strain distribution is numerically investigated. It is found that the cruciform specimen with the elliptical notch contour has high potential to be used for determining the fracture strains. Then, the design is experimentally verified under shear, shear-tension and plane strain conditions. Based on the analyses of the experimental strain distributions, strain paths and initial fracture locations, the performance of the new cruciform specimen is illustrated and discussed. Finally, the finite element model of the new cruciform specimen coupled with the calibrated DF2014 damage criterion is employed to predict the FLCF of AA6061-O.