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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In this study, we have investigated the surface morphology and the formation features of microcracks in rock samples (coal, sandstone, granite, and quartz) under the nonthermal influence of high-power nanosecond electromagnetic pulses (HPEMP). For this purpose, we used analytical electron microscopy (SEM  EDX), confocal laser scanning microscopy (CLSM), computer X-ray microtomography, Vickers microhardness ( HV ) testing, and other methods. Possible mechanisms of microcracks formation are discussed. The results obtained during this study are compared with experimental data, see, Victorov and Kochanov (2015), Victorov et al. (2019), on the formation of microcracks systems in rocks under explosive impact.

2. Experimental

2.1. Rocks, minerals, specimens, and HPEMP treatment conditions

In our experiments, we used the coal, sandstone, granite, and quartz samples extracted from rock formation and ores of Russia mineral deposits. The sandstone and granite samples were represented by individual grains and plane parallel polished sections with ~15×15×5 mm in size. The coal samples were represented by the pieces of irregular shape several mm (up to 5 mm) in size, taken from outburst (Berezovsky's mine of Kemerovo region) and non outburst (Komsomolskaya mine of AO Vorkutaugol) coal seams. The granite samples were a polymineral aggregates consisting predominantly of quartz, plagioclase, and potassium feldspar that have different strengths and structural properties. Additional studies were performed on the polished thin sections of vein milk-white quartz ~12 × 12 × 5 mm in size. Chemical composition of quartz samples, presented as a result of a chemical analysis of crushed (powdered) mineral, using an ARL ADVANT’X X-ray (XRF) fluorescence spectrometer (Thermo Fisher SI), was as follows, wt.%: SiO 2 99.11, Al 2 O 3 0.60, C 0.08, K 2 O 0.07, Na 2 O 0.05, CaO 0.03, Fe 2 O 3 0.03, TiO 2 0.03. The parameters and conditions for treatment of mineral samples with high-power nanosecond electromagnetic pulses are detailed in our work, see, Bunin et al. (2015). The nanosecond pulse generator with capacitive storage energy operates at a frequency of 100 Hz (pulse repetition rate), the output pulse amplitude is ~25 kV, the duration of the leading edge of the pulse varies from pulse to pulse within 2  5 ns, and the pulse duration varies within 4  10 ns. Video pulses of a bipolar shape are generated, pulse energy is  0.1 J, electric field strength in the inter-electrode gap is (0.5  1)  10 7 V  m –1 , time range of the pulsed treatment of the mineral samples is t treat. =10  300 s. The rock and mineral samples were placed directly on the working surface of the grounded electrode such that there was a small (~0.1–0.5 mm) air gap between the upper surface of the samples and the surface of the active electrode. The linear sizes of the working and grounded electrodes of the pulse generator significantly exceeded the sizes of the pulse processed samples. The morphological features of the minerals surface and the microdamages (pores and microcracks) of the surface samples were studied by means of analytical scanning electron microscopy (SEM) on a LEO 1420VP microscope equipped with an INCA Oxford 350 EDX  analyzer. We also used a JEOL JSM–6610 LV electron microscope in the low-vacuum mode of operation (LV), which allowed us to study rock samples with no preliminary coating of conducting layers on their surfaces. For coal samples, using the method of X-ray computer microtomography (XCMT, X-ray high-resolution scanner SkyScan 1172), the features of the internal microdefects substructure of the mineral substance were studied. The morphological features of the quartz polished sections surfaces were studied using confocal laser scanning microscopy (CLSM) on a Keyence VK–9700 3D microscope. The VK Analyser software package to analyze CLSM-images, obtained in the laser scanning mode with a short-wave violet laser (wavelength, 408 nm) and a source of white light, was used. We attempted to estimate the changes in the roughness parameters (Ra, Rq) of the pulse-modified surface of quartz samples. The microhardness of the quartz (Mohs hardness 7) polished sections ~15×15×5 mm in size determined according to Vickers ( HV , kgf/mm 2 ) on a PMT-3М microhardness tester (LOMO, Russia) using the technique described in our works (see, e.g., Bunin et al. (2015)). The indenter load was 200 g, and the load duration was 10 s. 2.2. Analysis techniques