Laboratory Study of Rock Deformation and Fracture - Ultra and Extremely Low Frequency Electromagnetic Fields - page 350

9.2.5 Electron and Ion Emissions Under Solid Failure

In most cases the total charge of fragments of fractured samples has the positive sign
(e.g., Kornfeld 1975 ). It is generally believed that this phenomenon is due to the
short-term emission of the electrons during the fracture of samples. The electrons
emitted in vacuum under dynamical fracture of the samples have been observed by
Deryagin et al. ( 1973 ), Krotova et al. ( 1975 ), Wollbrandt et al. ( 1975 ), Vladikina
et al. ( 1980 ), Dickinson et al. ( 1985 ), Bykova et al. ( 1987 ). The rock fracture in the
atmosphere is also accompanied by the electron emission (Dickinson et al. 1981 ;
Enomoto and Hashimoto 1990 , 1992 ; Enomoto et al. 1993 ).
Under the quasi-static loading of ionic crystals the electron emission takes place
when the strain reaches the threshold value of 0.02-0.1. The mechanoemission
begins still before the occurrence of large cracks and splitting off (Zakrevskiy et al.
1979 ). This effect was assumed to be associated with intersection of the slipbands
followed by the large local strain. Generation of the excited electron states, like
excitons (pair of an excited electron and an associated hole) in these regions could
result in electrons output into vacuum. The triboluminescence and the sharp increase
of conductivity have been observed simultaneously with particle emissions.
The high energy electron emission has been observed under the dynamical frac-
ture of alkali halide crystals in vacuum (Krotova et al. 1975 ). The photomultiplier
with threshold response of 20 keV has detected about 30 impulses under the fracture
of a single crystal of LiF. The average energy of the emitted electron was about
30 keV though sometimes the electron energy reached the value of 100 keV. During
the splitting of mica the energy of mechanoelectrons varies from 10 to 100 keV.
The emission of electrons with energy about 100 keV can be due to the excitation
and ionization of inner electron shells of atoms, which are located in the surface
layer of the crack. This assumption can be supported by the fact that the emission
of mechanoelectrons is often accompanied by not only Roentgen radiation but also
the bremsstrahlung (Slabkiy et al. 1973 ; Lipson et al. 1986 ).
The simultaneous emission of electrons and positive ions has been observed
under both brittle failure of solid dielectrics (Lipson et al. 1986 ; Klyuev et al. 1986 ;
Martelli et al. 1989 ) and plastic deformation of metals (Tupik and Valuev 1985 ). The
peak of ion emission intensity has been shown to fall on the moment of the cracks
formation and growth. Klyuev et al. ( 1986 ) have observed the emission of atomic
hydrogen under the fracture of hydrogenous material. About 10 neutrons per one act
of the rupture have been detected during the impact rupture of cubic monocrystals
of LiD with sizes of 3-4 mm.

9.2.6 Theory of Electric Field Formation in a Crack

The laboratory tests have shown that the electrical charges on the fresh surfaces
of fractured samples decrease in time due to both the relaxation processes and the