Geoscience Reference
In-Depth Information
9.3
Conclusions
The laboratory tests have shown that the effect of shock polarization is practically
observed in all materials: dielectrics, metals, semiconductors and etc. The medium
polarization occurs at different structural levels ranging from defects of crystal
lattice to grain boundaries, cracks, pores, and other microscopic inhomogeneities.
In monocrystals with ionic bond the shock polarization effect is due to the spatial
selection of charged edge dislocations and point defects of lattice. This effect has the
strain threshold associated with transition from the thermofluctuational mechanism
of defect displacement to the over-barrier one. It appears that the strain threshold for
dislocations is lower than that for the point defects. As the threshold is exceeded, the
diffusion coefficient and mobility of the defects increase abruptly that result in the
accumulation of electric charges at the SW front while the opposite charges leave at
the sample surfaces.
Experimental evidence for the linear dependence of surface charge density on
the SW front on strain amplitude has been observed under the compression of
ionic crystals sandwiched between plates of short-circuited condenser. According
to the theory this tendency follows the linear dependence between the rates of
defect production and plastic strain. Drastic changes in signal polarity, as the shock
pressure exceeds the certain threshold, are supposed to be due to the electron
breakdown of ionic crystals.
A variety of electromagnetic effects has been observed under the fracture of
rocks and other natural nonuniform materials. The polarization nuclei are mainly
concentrated in plastic/craze zones which are located at the tips of growing cracks
and in the vicinity of pores and inclusions. High strain rate inside these zones gives
rise to intensive production of mobile charged linear and point defects of lattice that
results in the generation of electric current. The electromagnetic variations caused
by the rock fracture have a wide band spectrum in the range from 10 Hz to 1 MHz
depending on the spatial scale of the fractured area. In laboratory conditions the
maximum of intensity lies in the range 1-50 Hz. The typical signal generated by
an individual microcrack looks as a burst of damped oscillations with total duration
about 1-10 s.
The fresh crack surfaces contain the fluctuation mosaic areas with positive and
negative charges. The electrometric probe measurements have shown that the charge
density can reach a value about 10 4 -10 2 C/m 2 at certain sites while the mean
value on the cleavage surface is about 10 7 -10 8 C/m 2 . However, the surface
charge density can be much greater for the dynamical stage of crack growth.
The indirect measurements are evidence for the generation of transient electric
field 10 8 -10 9 V/m inside the cracks and pores. This is quite consistent with
the theory which predicts that the cumulation of electric field in the vicinity of
collapsing pore can result in local breakdown of dielectrics. Moreover, the short-
term electron emissions with energies 10-100 keV have been observed during the
fracture of dielectrics. This phenomenon is usually accompanied by Bremsstrahlung
and even by characteristic X-ray radiation. The simultaneous emissions of electrons,
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