Avalanches in compressed porousSiO2-based materials

Abstract

The failure dynamics in SiO 2 -based porous materials under compression, namely the synthetic glass Gelsil and three natural sandstones, has been studied for slowly increasing compressive uniaxial stress with rates between 0.2 and 2.8 kPa/s. The measured collapsed dynamics is similar to Vycor, which is another synthetic porous SiO 2 glass similar to Gelsil but with a different porous mesostructure. Compression occurs by jerks of strain release and a major collapse at the failure point. The acoustic emission and shrinking of the samples during jerks are measured and analyzed. The energy of acoustic emission events, its duration, and waiting times between events show that the failure process follows avalanche criticality with power law statistics over ca. 4 decades with a power law exponent ε 1.4 for the energy distribution. This exponent is consistent with the mean-field value for the collapse of granular media. Besides the absence of length, energy, and time scales, we demonstrate the existence of aftershock correlations during the failure process.

Are triggering rates of labquakes universal? Inferring triggering rates from incomplete information

“…As expected, the branching ratio, the ratio between the power-law transition and the background rate are essential to understand the results of MASR in terms of the triggering kernel. The existence of a characteristic scale in the temporal triggering kernel offers a plausible explanation to the detection of effective Omori exponents lower than one in unlocalized catalogs of acoustic emission during mechanical processes [7,44,73,10,78,87] and calorimetry in structural phase transitions [4]. In the specific case of the failure of porous materials under compression [7,44] an effective Omori exponent p ∼ 0.7 was observed using MASR, compatible with the short time power-law regime found in localized catalogs [48] and the MASR of the modified ETAS model (Fig.…”

“…In the case of earthquakes, the most reliable methods to identify triggering relations use spatio-temporal correlations between events [35,39,50,51]. In the absence of spatial information -as it is the case for previous rock fracture experiments [7,43,44] -this is not an option and one has to rely on the measurement of the whole activity rate after each event [7,52]. This technique can lead to a strong bias in the estimation of triggering rates in cases where either the number of triggered events or the background rate of events activated by other mechanisms is high, or both, as we show explicitly here.…”

“…We test it against synthetic catalogs generated by a modified Epidemic-Type Aftershock Sequence (ETAS) model with a triggering rate characterized by two power laws, as observed in the most recent rock fracture experiments [48]. We address the possibility that the low experimental OU exponent (p values) and some other inconsistent results observed in the previous rock fracture experiments [7,43,44] are a consequence of the hidden complexity of the triggering process. First, we formulate triggering in terms of branching processes.…”

Increasing power-law range in avalanche amplitude and energy distributions

“…In most cases, these experimental limitations are not sharp due to electronic uncertainties. Recent studies regarding the AE in compression experiments of porous materials [17][18][19], wood [20], ethanol-dampened charcoal [21], confined-granular matter under continuous shear [22], etc.…”

Universality of power-law exponents by means of maximum-likelihood estimation

“…Recent studies regarding the acoustic emission (AE) in compression experiments of porous glasses and minerals [21][22][23][24][25] or wood [26] have focused the attention in the energy distribution of AE events due to the similarities with the GR law for earthquakes [20]. According to the terminology which is used in some of these studies, we will name as labquakes those AE events that occur during the compression of materials.…”