Dynamics of fractoluminescence, electromagnetic and acoustic emissions upon impact on a granite surface

Abstract

The dynamics of fractoluminescence (FL), electromagnetic (EME) and acoustic emissions (AE) induced in a granite sample by a striker blow was studied. The impact causes strong irregular vibrations in the striker-sample system. About 900 μs after the impact, the amplitude of these vibrations decreases approximately by a factor of 100. As granite possesses piezoelectric properties, the mechanical vibrations induce equally strong electromagnetic oscillations. FL arises about 60 μs after the impact. The FL spectra contain the bands of ≈1.9 and 2.6 eV, which means that under the impact, the SiOSi bonds are broken and SiO·free radicals and Si: centers are formed. FL appears as a series of flashes with a duration of ≈10 ns. Each flash is assumed to correspond to the initiation of a microcrack. The linear sizes of the microcracks estimated from the intensity of the flashes and the velocity of elastic waves range from ≈7 μm to ≈40 μm, with an average of ≈10 μm. The microcracks are grouped in microseries with a duration of ≈4μs, which causes modulation of strong AE oscillations by weak oscillations with a period of ≈8 μs. The formation of microcracks with a linear size exceeding ≈30 μm causes discontinuities in the time dependence of EME.