PSI - Issue 65

Igor Zh. Bunin et al. / Procedia Structural Integrity 65 (2024) 32–38 Igor Zh. Bunin et al. / Structural Integrity Procedia 00 (2024) 000–000

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3.2. The possible mechanisms of microdefects formation in geomaterials exposed to high-voltage nanosecond pulses

The skin effect is one of possible mechanisms of microcracks formation in the pre-surface layers of coal samples under the nonthermal impact of nanoseconds HPEMP. This effect associated with the concentration of total current mainly near the semiconductor surface upon an increase in the frequency of the alternating magnetic field, see, e.g., Landau and Lifshits (2016). This process leads to the fact that the field’s energy dissipates in the surface layer rather than uniformly along the sample’s cross section. In our previous papers, see, Chanturiya et al. (2005), Chanturiya and Bunin (2007), it was shown that, as a result of HPEMP effect on the natural mineral complexes, the essential contrast local heating of microinclusions in semiconducting or dielectric mineral media is possible due to the implementation of skin effect. At this local heating, the size of the fracturing area in a mineral matrix in order of magnitude is the (metal) microinclusion size. It must be noted that the problem of the electrophysical properties of coals is very complicated. This is because the crystal-like graphitized phase in coal is similar to metals in its properties, while the amorphized phase resembles dielectrics, see, Panchenko (2009). Also, for coals, a correlation between electrical resistivity and the outburst hazard has not yet been established. Sandstone is a detrital sedimentary rock, which is a homogeneous or layered aggregate of detrital grains ranging in size from 0.05  0.0625 mm to 2 mm (grains of sand) bound by some mineral substance (cement). For sandstone and granite, the migration (interlayer) and/or electron-relaxation polarization of the mineral media, see, e.g., Poplavko (1980) is a possible mechanism for the microdamages formation in rock samples under the influence of high-power electric fields. Migration polarization manifests itself in solid dielectrics with an inhomogeneous structure, microimpurities, and in the presence of layers with different conductivity. This type of polarization occurs due to the movement of weakly bound ions over a considerable distance, which causes the formation of space charges at the boundary of the structural components of the dielectric minerals (rocks) and can cause the destruction of the mineral substance. It is interesting to note that the minimum value of the microcracks opening in rocks after explosive action and electromagnetic pulse (HPEMP) effect is  ~0.2–0.3  m, and the most typical crack opening value width is 2–3  m. SEM and XCMT data show that most of the microcracks which appear in rocks after the impact of high pulse pressure from explosions and treatment with high-voltage nanosecond pulses are of the type-I normal-rupture crack. We did not observe the formation of the shear-type (sliding II or tearing III modes) defects. The crystal cell of β -quartz (SiO 2 ) contains three silicon ions, each surrounded by four oxygen ones. In each SiO 4 tetrahedron, two oxygen ions lie somewhat above and the other two somewhat below the silicon ion. The formation of crystallographically oriented discharge channels in silicon dioxide, see Emlin et al. (2009), suggests the realization of an specific (certain) electrical breakdown mechanism. The authors of paper, see Emlin et al. (2009), considered the formation of breakdown channels in silicon dioxide in terms of the model of impact ionization of the valence band by fast nonequilibrium electrons, and also, in terms of the model of primary electron generation by cascade Auger transitions in the valence band of the dielectric. The disintegration of mineral media, due to the formation of microchannels of electric breakdown, lie in planes of silicon ions, as a result of charge carriers (primary electrons) being generated by cascade Auger transitions in the valence zone of the dielectric mineral with regard to the crystallochemical stricture of quartz, see, Emlin et al. (2009) is a possible and more suitable mechanism of the quartz surface softening under the influence of high-voltage nanosecond pulses. In this study, using scanning electron microscopy and additionally the computer X-ray microtomography, we have investigated the surface morphology and features of the microdefects formation in geomaterials when exposed to high-power nanosecond electromagnetic pulses. As a result of HPEMP action, the formation of microcracks occurred mainly in near-surface layers of coal samples extracted from outburst-prone seams. In the sandstone and granite we observed the formation and propagation of microcracks, mainly along the grain boundaries (intergranular fracture), as a result of HPEMP effect. The minimum opening of (  min ) and the most characteristic opening (  * ) of microcracks formed in rocks after the explosive impact and electromagnetic pulses effect are almost identical. They have the values  min ~ 0.2–0.3 µm, and  * ~ 2–3 µm, respectively. The following possible mechanisms of microcracks formation are discussed. We have considered such failure mechanisms of geomaterials as the skin 4. Conclusions