Civil Engineering Reference
In-Depth Information
frequency, and pulse duration). It is expected that both, model predictions and
experiments, will bring a better understanding of the electroplastic deformation
mechanism in general, and of EAB in particular.
11.1.1 Analysis of an EA Bending Process
Due to the punch motion in a bending process, the geometry, the bending
moments, and force directions change continuously; thus, the bending is a non-
steady-state process, and the stresses and strains on the material change through-
out the process. At the beginning of the process, the deformation is elastic. As the
punch advances, the stresses in the outer fiber at mid-span increase and exceed the
elastic limit; thus, the plastic deformation starts and propagates toward the inner
fiber and into the rest of the material. The punch motion and the amount of mate-
rial springback will dictate the final bending angle.
In an electrically assisted test, a power source provides electricity and the cur-
rent flows through the dies. Figure
11.1
shows an experimental schematic of an
EAB test. Insulation is positioned between the dies and machinery to guide the
flow of the electricity into the workpiece to assist in deformation.
11.1.2 Assumptions of the EAB Model
The following major simplifications and assumptions are used throughout the deri-
vations in this study:
• The workpiece material is homogeneous and isotropic, and the thickness and
width of the sheet are uniform. This is due to the fact that the sheet metal used
for this research was in the annealed state.
• The width of the specimens is at least 10-times larger relative to the thickness;
thus, plane strain conditions exist.
• In the moment that plastic deformation starts, the elastic deformation is negligi-
ble and the power law is used for determining the flow stress of the material.
• The strip is subdivided into three sections: two linear and a circular section. The
bending line is a circular arc with the same radius as the punch itself. The sec-
tions normal to the sheet surface are assumed to remain plane at all times.
• The friction at the workpiece/die interfaces follows Coulomb's friction law. The
lubrication regime is assumed unaffected by the presence of electricity.
• The speciic heat and resistivity of the material are assumed independent of
temperature.
• The strain-rate sensitivity is neglected since previous experimental investiga-
tions found that the temperature rise is not high enough for the process to be
considered as hot forming.
• Volume constancy is valid throughout the deformation process; thus, the change
in sheet thickness during the bending process can be neglected.
