Civil Engineering Reference
In-Depth Information
Fig. 3.10
Ductile fracture stages.
a
Initial Necked Region.
b
Formation of Micro-Voids.
c
Coalescence to Larger Void
stress induces separation of the material at grain boundaries or at small impurity
particles (Fig.
3.10
b). As the local stress in the material increases, the micro-voids
grow and coalesce into larger voids (Fig.
3.10
c). Over time, crack initiation begins
at the void and the crack grows till the material ultimately fractures.
The high stresses within the material that causes micro-voids to form can be
a result of pinned dislocations within the lattice. As a result of the applied cur-
rent providing energy to the dislocations, the added energy can allow for pinned or
stuck dislocations to continue moving again. Consequently, this reduces the local
stresses within the material's lattice and delays the process of void formation and
fracture. This theorized ability of the electric current to supply sufficient energy to
allow for pinned dislocations to be mobile can explain the observed effects seen in
experimental testing [
6
,
21
].
3.3.4 Supporting Experimental Results
There has been various experimental works performed, which support the new
electroplastic theory. Specifically, the experimental conclusions will discuss the
heating effects of applied electricity and will address the increased atomic-level
vibrations generated by the increase in temperature at lattice “hot spots.”
Heating and increased atomic-level vibrations
• Threshold versus resistivity relation [
22
]
• Dislocation density versus temperature relation (stationary-electrical tests) [
4
]
• Springback elimination in sheet bending [
23
]
• Dislocation density versus EEC (EAF tests) [
4
]
