Development of the Forming Limit Diagram for AA6016-T4 at Room Temperature Using Uniaxial Tension of Notched Samples and a Biaxial Test

Within the framework of the formability limit assessment in sheet metal forming, testing of notched tensile samples coupled with digital image correlation (DIC) has been analysed as an alternative to overcome the implications of Nakajima testing in relation to times of test preparation, cost of the equipment, presence of friction, and amount of material required for the test. Additionally, the complications of the Nakajima testing at elevated temperatures need to also be considered. In this work, specific notched sample geometries have been investigated to accurately identify the forming limits of Aluminium alloy AA6016 in T4 condition. Once the notched geometry had been defined, experimental tensile testing of the samples coupled with DIC technology allowed us to identify the formability limits of interest. Finally, a comparison at room temperature with the conventional Nakajima testing was performed experimentally. Two different methodologies for strain limit evaluation in notched samples have been investigated in the present analysis. The first one is called a position-dependent method and is based on the inverse best-fit parabola of the “bell-shaped curve”, which is used in the conventional Nakajima test. The second approach referred to a time-dependent method and is based on the strain rate evaluation at the necking zone. This strain-rate-dependent method, which works in combination with DIC measurements, was found to be more accurate to determine the necking limits than the previous one; in addition, it also provides more accurate information for the safe zone of forming.