Optimization on Cruciform Specimen Geometries of AA5052 under Equi-Biaxial Loading: Acquisition of Ultimate Fracture Strain

Background The evaluation of the formability for aluminum alloy under complex loading conditions is particularly significant. Based on the characteristics of conveniently achieving multiple strain path states, the cruciform specimen has been widely used in the experiment of evaluating formability. But there is no general specimen design scheme to realize proportional strain path history in initial fracture point. Objective This paper was aiming at obtaining the equi-proportional strain path history and exploring the ultimate fracture strain of AA5052 aluminum alloy sheets under the equi-biaxial tension. Methods Firstly, two cruciform specimen schemes suitable for 1.2 mm sheet materials were proposed. The strain localization characteristics were estimated and thinning critical value in the central region was determined by simulations. Subsequently, through the 3D digital image correlation (DIC) test system, the strain path history of the initial fracture point was obtained. Finally, the fracture morphology characteristics were observed through SEM. Results The results showed that the initial fracture point could be located in the center of the specimen in both designs (Design-straight and Design-flaring). The specimens with slits showed a smaller strain gradient existed in the central region. Meanwhile, the initial point of fracture was closer to the equi-biaxial tensile strain path, and the strain limit was larger. Conclusions The two design methods could optimize forming limit value and realize the equi-proportional strain path history in central fracture point. The Design-straight and Design-flaring was more suitable for the AA5052-O and AA5052-H material respectively. It was related to the local stress concentration effect of the material.