New test and interpretation of electromagnetic forming limits of sheet metal

A new method is proposed to determine the electromagnetic forming limits of sheet metal. The method deforms the specimen apex on a constant strain path. The strain path can be varied between uniaxial tension and biaxial tension by changing the specimen and tool shapes. Furthermore, the method breaks the specimen at the apex. In order to ensure an apex failure and avoid bending, a new specimen concept that promotes uniform pressure application is developed.
The proposed method is used to find the electromagnetic forming limit curves for AA1050A, AA5083, and Mg AZ31 sheets. These materials exhibit higher necking limits in the electromagnetic forming, when compared to quasi-static forming. In addition, in the electromagnetic forming, the limits increase with the strain rate. In order to explain the higher forming limits in the electromagnetic forming, fracture surfaces of quasi-static and electromagnetic samples are examined.
Fracture surfaces reveal that the failure in the quasi-static forming is driven by in-plane shear stress, while the failure in the electromagnetic forming is driven by tensile and out-of-plane shear stress. This suggests the existence of out-of-plane shear stress in the electromagnetic forming. Out-of-plane shear stress is shown by Allwood and Shouler (2009) to increase elongation to failure in quasi-static tensile tests. This study proposes out-of-plane shear stress as a reason for the higher limits in the electromagnetic forming. In the electromagnetic forming, out-of-plane shear stresses can arise from the out-of-plane electromagnetic forces. Simulations of the electromagnetic forming limit test shows that they reach considerable magnitudes.