Abstract
Mining accidents are sometimes preceded by high levels of crackling noise, which follow universal rules for the collapse of minerals. The archetypal test cases are sandstone and coal. Their collapse mechanism is almost identical to earthquakes: the crackling noise in large, porous samples follows a power law (Gutenberg-Richter) distribution P ~ E −e with energy exponents e for near critical stresses of e = 1.55 for dry and wet sandstone, and e = 1.32 for coal. The exponents of early stages are slightly increased, 1.7 (sandstone) and 1.5 (coal), and appear to represent the collapse of isolated, uncorrelated cavities. A significant increase of the acoustic emission, AE, activity was observed close to the final failure event, which acts as “warning signal” for the impending major collapse. Waiting times between events also follow power law distributions with exponents 2+ξ between 2 and 2.4. Aftershocks occur with probabilities described by Omori coefficients p between 0.84 (sandstone) and 1 (coal). The “Bath’s law” predicts that the ratio between the magnitude of the main event and the largest aftershock is 1.2. Our experimental findings confirm this conjecture. Our results imply that acoustic warning methods are often possible within the context of mining safety measures, although it is not only the increase of crackling noise that can be used as early warning signal but also the change of the energy distribution of the crackling events.
Highlights
Earthquakes and the collapse of porous materials are related phenomena deeply connected by the emission of crackling noise (Baró et al 2013; Salje and Dahmen 2014; Sethna et al 2001) where systems under slow perturbation respond through discrete events, so-called “jerks,” with a huge variety of sizes and energies
The energy of the acoustic emission (AE) signal, the number of AE hits per second (AE activity), and the cumulative AE activity are shown as function of the run time in for dry sandstone (Fig. 2a), wet sandstone (Fig. 2b), and coal (Fig. 2c)
AE energies and activities are smaller in the early stages of the compression experiment compared with the late stages
Summary
Earthquakes and the collapse of porous materials are related phenomena deeply connected by the emission of crackling noise (Baró et al 2013; Salje and Dahmen 2014; Sethna et al 2001) where systems under slow perturbation respond through discrete events, so-called “jerks,” with a huge variety of sizes and energies. External loading is applied to the samples and the system’s response is obtained by recording acoustic emission (AE). Baró et al (2013) reported a very complete parallel between the acoustic emissions produced by a porous material under uniaxial compression and earthquakes. The same experimental techniques are widely used for the investigation of device materials such as ferroelectric, ferromagnets, and ferroelastics (Bolgár et al 2016; Dul’kin et al 2015; Guyot et al 1988; Hoffmann et al 2001; Salje et al 2014, 2015; Skal’s’kyi et al 2009; Vives et al 1994) with a significant increase of published data over recent years on the acoustic emission during force-induced changes of microstructures
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