20
10
0
10
20
Figure 30. Typical signal captured from a thermo-
crack grown in lake ice (after Gluschenkov and
280
0
840
1120
560
Petrenko 1993). T = 5C.
t (ms)
3.3
V, volts
3
2.7
2.4
2.1
1.8
1.5
I
II
1.2
0.9
Figure 31. Electromagnetic emission from a crack in
sea ice (after Petrenko and Gluschenkov 1995). The
0.6
crack was 60 cm long and 10 cm wide. Area I corre-
0.3
t, s
sponds to the crack propagation and area II corre-
0
sponds to electrical relaxation of the electric field after
0
0.005
0.01
0.015
crack arrest.
t (s)
range, from about 103 Hz to 1 MHz. Additional filter-
ELECTROPLASTIC EFFECTS IN ICE
ing was used to cut off the common industrial (60 Hz)
and ionospheric (ƒ ≥ 100 kHz) noise.
Several electromechanical phenomena in ice are
associated with the motion of electrically charged dis-
Most of the signals captured from identified cracks
locations. They are all of quite small magnitude. Nev-
were very similar to those found in the laboratory (Pe-
ertheless, their significance is determined by their con-
trenko 1992a, Fifolt et al. 1993). A typical signal cap-
tribution to the study of the physical mechanisms that
tured from a thermocrack in freshwater columnar lake
govern motion of dislocations and, hence, plastic de-
ice is depicted in Figure 30. The crack appeared 10 m
formation of ice. Electrically charged dislocations
from the antenna and split several single-crystalline
were found in many materials such as ionic crystals
columns of ice. As in the laboratory experiments, the
(see review by Whitworth 1975); covalent semicon-
oscillation of the electrical field can be interpreted in
ductors (see review by Alexander and Teichler 1991);
terms of an oscillating electrical charge on the crack
and crystals with mixed covalentionic bonding (see
surfaces.
review by Osip'yan et al. 1986). In these materials the
An electrical signal captured by a dipole antenna
from a larger crack splitting a cold (35C) plate of sea
electrical charge on a dislocation core appears when a
dislocation captures either charged point defects (such
ice is shown in Figure 31. The crack surface was nor-
as vacancies and interstices in ionic crystals) or elec-
mal to the direction of the gradient of salinity of the ice,
trons (or holes), as happens in semiconductors or when
and hence the crack grew perpendicular to the frozen-in
both point defects and electrons (holes) are captured.
intrinsic electrical field. Again, as in corresponding lab-
Significant theoretical efforts have been applied to
oratory experiments, the signal increases during crack
clarify the formation of the dislocation charges in ionic
propagation and then relaxes after crack arrest.
22
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