Asymmetric Damage Avalanche Shape in Quasibrittle Materials and Subavalanche (Aftershock) Clusters

Abstract

Crackling dynamics is characterized by a release of incoming energy through intermittent avalanches. The shape, i.e. the internal temporal structure of these avalanches, gives insightful information about the physical processes involved. It was experimentally shown recently that progressive damage towards compressive failure of quasi-brittle materials can be mapped onto the universality class of interface depinning when considering scaling relationships between the global characteristics of the microcracking avalanches, such as energy and duration. Here we show, for three concrete materials prepared from three different mixtures, and from a detailed analysis of the acoustic emission waveforms generated by microcracking events, that the shape of these damage avalanches is strongly asymmetric, characterized by a very slow decay. This remarkable asymmetry, at odds with mean-field depinning predictions, could be explained, in these quasi-brittle materials, by retardation effects induced by enhanced viscoelastic processes within a fracture process zone generated by the damage avalanche as it progresses.

The duration-energy-size enigma for acoustic emission

“…These authors clearly identified the discrepancy between x = 2 and x = 3 but offered no reason. We rationalize this observation and argue that avalanche exponents are modified by details of the AE experiments 45 .…”

Change of Acoustic Emission Characteristics during Temperature Induced Transition from Twinning to Dislocation Slip under Compression in Polycrystalline Sn

“…Furthermore, the first parts, belonging to small values of D in Figure 7 , are out of the region where the power fitting was made. The reason of this can be twofold: (i) these small values of S and D are sensitive to the choice of the threshold; and (ii) the effect of intrinsic absorption of the AE signal [ 29 , 30 , 31 ]. Indeed, it was shown in [ 29 ] that the signal intensity (defined as I ( t ) = U 2 ( t )) is influenced by an exponential decay due to intrinsic absorption factor, exp(− t/τ a ), where τ a is the characteristic time of absorption, and it was concluded that for short events ( D < τ a ) the AE duration is meaningless in terms of source mechanism [ 29 ].…”

“…The reason of this can be twofold: (i) these small values of S and D are sensitive to the choice of the threshold; and (ii) the effect of intrinsic absorption of the AE signal [ 29 , 30 , 31 ]. Indeed, it was shown in [ 29 ] that the signal intensity (defined as I ( t ) = U 2 ( t )) is influenced by an exponential decay due to intrinsic absorption factor, exp(− t/τ a ), where τ a is the characteristic time of absorption, and it was concluded that for short events ( D < τ a ) the AE duration is meaningless in terms of source mechanism [ 29 ]. On the other hand, it is expected that in the D >> τ a limit the above distortion effect of the transfer behavior of the system can be moderate or even negligible [ 30 , 48 , 49 ].…”

“…We also analyzed the power law-type scaling relations between the energy, amplitude, size and duration times of AE signals, also taking into account the distortion effect of the AE device. It will be demonstrated that, in accordance with [ 10 , 29 , 30 , 31 ], reliable approximate estimates of the exponents of the above scaling relations can be obtained only at large values of the parameters while, for example, at short duration times the attenuation effects of the AE detecting device dominate.…”

“…The characteristic exponent τ , belonging to the area distribution ( P(S)~S −τ ) has a similar behavior; even in mean filed approximation, τ ≅ γ is predicted [ 24 ]. This also means that the temporal shapes of avalanches [ 10 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ] (i.e., the V(t) function), on a properly reduced scales, are universal within a class of materials with the same deformation mechanism. Indeed, we will show that, similarly to ε, γ also changes: γ = 1.78 ± 0.08 for twinning and γ = 1.35 ± 0.05 for dislocation slip mechanism, respectively, confirming that these characteristic exponents are different for the two deformation mechanisms.…”