Stress-Strain Curves of Sheet Material in High-Rate Forming Processes

Abstract

Electromagnetic forming technologies are based on high-voltage discharge of capacitors through the conductive coil. Two methods of testing and the results of dynamic coefficient kd for aluminum alloys, copper, brass, steel, and some other materials are presented. The first method is based on expansion of rings machined from tubular blanks, which are designated for further stamping operations. The displacement of the ring was registered by using the light shading method. Parameters of the discharge electric current running through the electromagnetic coil were measured with a Rogowski gauge. The acceleration stage of the ring expansion process was used for more accurate calibration of the inductive gauge defining the parameters of electromagnetic pressure. Registering the kinematics of the ring during the inertial stage of the deformation process provided the information on dynamic behavior of the studied material. The second method employed in this paper for dynamic yield stress measurement was based on transverse pulsed loading of a sheet sample clamped by its ends. Shapes of the samples during their deformation were photographed using a high-speed camera. The specifics of the sheet sample deformation under the pulsed transverse load are the following: the sample has near-trapezoidal shape; the middle part of the sample has almost the same velocity ??0 through the whole process; the angle between two inclined parts of the sample and the horizontal middle area ?? has minor variation during the deformation process. In some cases, hybrid stamping processes including conventional forming on the press and final shape calibrating with pulsed forming technique require additional information about the influence of the static preliminary deformation of the sheet on dynamic yield stress. Experiments with different levels of material prestrain were conducted for this purpose.