Civil Engineering Reference
In-Depth Information
magnitude than the mechanical stresses. Additionally, this energy from the electrons
cannot solely cause dislocation motion or plastic deformation, but rather it acts as
a supplement to the mechanical stresses on the dislocations. When the electricity is
applied by itself without being coupled with deformation, the energy from the elec-
trons to the dislocations will assist in stress relaxation by breaking and reforming
bonds in the immediate area around the dislocations. This effect is similar to a pro-
cess anneal, where heat is added to a material to assist in breaking and reforming of
bonds to lower their energy states. This will result in elastic relaxation, or spring-
back reduction. The work by Green et al. [
23
] showed that a short, single pulse of
electricity was able to eliminate elastic springback in sheet specimens that were
already formed around a die. The electrons were able to accommodate breaking and
reforming of the bonds to eliminate the tension in the bonds above the neutral axis
and to eliminate the compression in the bonds below the neutral axis.
As the electrons impact the dislocations, they increase the energy (i.e., increase
the temperature) around the dislocations and this can provide the energy needed
to assist the dislocations past the lattice obstacles that are holding them up.
Kravchenko [
8
], in his explanation of electroplasticity, stated this effect when he
explained that, if there is an electric current flowing, the energy from the electrons
is transferred to the dislocations, thus making the plastic flow easier.
Percent cold work research shows that as the amount of preexisting disloca-
tions within the metal's lattice increase (by way of cold-working), the efficiency of
the applied electrical power is increased [
4
]. This is because the larger number of
dislocations within the lattice allows for the applied electrons to impact them and
cause regions of localized heating. The increased temperatures of these regions
also increase the energy of the atoms in these regions, thus allowing for disloca-
tion motion to occur much easier than at lower temperatures.
3.4 Electroplastic Theory Conclusions
The main conclusions drawn from this chapter are as follows:
• The low of electrical current or the movement of valance electrons within the
material is limited by the electrical resistivity. The resistivity is derived from the
atomic structure, bond type, atomic spacing, and the material defects present in the
lattice. As a result of electron scattering within the lattice, areas of greater vibra-
tional energy exist around defects due the increased amount of electron/ion core
interaction. Additionally, the defect-free lattice has some resistance to electric flow
and the entire material begins to heat. This phenomenon is known as Joule heating.
• Joule heating differs from conventional convection heating of a material (i.e., in an
oven). This is due to the convection heating only providing a uniform increase in
vibrational energy throughout the lattice (i.e., both defect and defect-free regions).
In contrast, Joule heating creates areas of increased vibrational energy at defects
as compared to the defect-free region. Thus, the energy is more directed to the
critical areas (i.e., dislocations and defects) in the lattice for material deformation.
