Experiments on the plastic deformation of LiF ionic monocrystals under uniaxial compression are performed with simultaneous recording of acoustic ͑AE͒ and electromagnetic ͑EME͒ emissions. A strong correlation between AE and EME events has been found, which clearly demonstrates that the observed EME is caused by a dynamical interaction between moving dislocations and charged vacancies in the ionic lattice during work hardening. The mechanism proposed to explain EME is based on the assumption that gliding edge dislocations sweep up the vacancies of a preferable sign. As a result, when a dislocation pileup is formed, a certain nonequilibrium charge density is accumulated at its head, resulting in electric polarization of the deformed crystal. As the external loading increases, a locked dislocation pileup bursts through the stoppers and quickly loses its bounded charge. The relaxation of this charge produces an intrinsic polarization current generating an electric pulse. It is assumed that the relaxation current can be described as an athermic viscous motion of vacancies under the kinetic friction force ϳBv ͑B is the friction coefficient and v is the vacancy velocity͒ in a self-consistent electric field determined by the distribution of the total charge density. A nonlinear integrodifferential equation of motion for the nonequilibrium charge density is derived. For a special form of the initial charge density distribution, an automodel solution of this equation describing the polarization current has been built. The electrical signal generated by an acting slip system has been calculated. By comparing the calculated and experimentally measured electric signal patterns, the friction coefficient for the linear chain of vacancies ͑the analog of an edge dislocation extra plane͒ in LiF has been estimated to be B Ӎ 0.9 ϫ 10 −5 g cm −1 s −1 . This value is in accordance with the corresponding coefficient for dislocations in ionic lattices.
Transient variations of the electric field are detected, prior to the failure of a rock sample which is subjected to uniaxial compression at a variable rate. These precursory electric signals are attributed to the stress induced polarization of the sample and seem to have a form similar to the so called Seismic Electric Signals (SES), which are detected in Greece by the VAN network, prior to earthquakes. The emitted electric signals seem to follow in form the variations of the first time derivative of the externally applied stress. A tentative model for the origin of these signals is also discussed.
Part of Special Issue "Precursory phenomena, seismic hazard evaluation and seismo-tectonic electromagnetic effects" Abstract. A crucial question of the scientific community nowadays, concerns the existence of electric signals preceding earthquakes. In order to give a plausible answer to this question, we carried out two kinds of laboratory experiments of uniaxial deformation of ionic crystals and rock samples: a) In the first kind, stress induced polarization currents are detected and recorded. Our experimental results showed not only the existence of stress induced polarization currents before the fracture of the samples, but the possibility of the propagation of these signals, as well, through conductive channels, for distances much longer than the source dimensions. b) In the second, acoustic and electromagnetic signals are detected and recorded in the frequency range from 1 KHz to some MHz. The mechanism of generation of these signals is shown to be different for those emitted from piezoelectric and from non-piezoelectric materials.A plausible model is also suggested, on the compatibility of our laboratory results with the processes occurring in the earth during the earthquake preparatory stage.
Simultaneous measurements of acoustic emissions (AE) and electromagnetic emissions (EME) during plastic deformation and destruction under uniaxial compression along 〈001〉 direction are made on LiF monocrystals after gamma irradiation by 60Co source. The irradiation doses are 1, 2, and 10 Mrad. The EME measurements in the radio-frequency range are carried out using two types of electromagnetic sensors: (i) a simple electrical stub antenna and (ii) a toroidal inductance coil. Two checking experiments on unirradiated crystals are performed as the starting point to discover the effect of gamma irradiation on acoustic and electromagnetic emissive ability of plastically deformed ionic crystals. Unirradiated LiF monocrystals demonstrate high-intensive EME at easy glide and work hardening stages, as well as at the fracture during destruction of the sample. At radiation doses more than ∼1 Mrad, in the active loading stage the EME of LiF monocrystals vanishes, except few individual electromagnetic pulses (only at 1 and 2 Mrad doses), which are time correlated with well-defined drop-jumps on the loading diagram and therefore can be associated with macroscopic crack openings. Moderate electromagnetic activity in irradiated crystals occurs only in the final stage of deformation at the complete fracture of the sample. Thus, after gamma irradiation the formation of polarization currents due to dynamic interaction between charged vacancies and moving dislocations is suppressed, and only EME connected with the redistribution of the free charge on the crack branches is observed. Acoustic emission diagrams of low-irradiated LiF are typical for the work hardening stage in crystals containing a great amount of strong point stoppers. At larger irradiation doses the AE diagram displays quite different behavior at low- and high-loading regions with a sharp boundary between them. The low-loading region shows poor AE activity, which changes sharply into high-active burst-like emission with an increase in loading. The boundary between two regions shifts to higher loadings with radiation dose. The higher is the radiation dose the lower is the relative intensity of AE in the high-stressed region. The physical mechanisms of EME and AE in gamma-irradiated ionic crystals are discussed.
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