Role of superposition of dislocation avalanches in the statistics of acoustic emission during plastic deformation

Abstract

Various dynamical systems with many degrees of freedom display avalanche dynamics, which is characterized by scale invariance reflected in power-law statistics. The superposition of avalanche processes in real systems driven at a finite velocity may influence the experimental determination of the underlying power law. The present paper reports results of an investigation of this effect using the example of acoustic emission (AE) accompanying plastic deformation of crystals. Indeed, recent studies of AE did not only prove that the dynamics of crystal defects obeys power-law statistics, but also led to a hypothesis of universality of the scaling law. We examine the sensitivity of the apparent statistics of AE to the parameters applied to individualize AE events. Two different alloys, MgZr and AlMg, both displaying strong AE but characterized by different plasticity mechanisms, are investigated. It is shown that the power-law indices display a good robustness in wide ranges of parameters even in the conditions leading to very strong superposition of AE events, although some deviations from the persistent values are also detected. The totality of the results confirms the scale-invariant character of deformation processes on the scale relevant to AE, but uncovers essential differences between the power-law exponents found for two kinds of alloys.

Modeling the complexity of acoustic emission during intermittent plastic deformation: Power laws and multifractal spectra

“…For instance, AE studies on the smooth homogeneous yield phenomenon exhibit intermittent AE signals with its overall envelope exhibiting a peak just beyond the elastic regime decaying for larger strains [5,6]. Recent AE studies for this case throw light on the origin of the pulse like character of AE signals [7][8][9][10]. The smooth σ − curves are then interpreted as resulting from the averaging process of the dislocation activity in the sample.…”

“…While the AE associated with both the continuous yield and the discontinuous yield consist of a sequence of intermittent bursts of acoustic energy, the AE technique is not only able to distinguish these two types of AE patterns but also to quantify them. Indeed, the recent progress in experimental techniques with high degree of resolution and accuracy have helped to establish quantitative characterization of the AE signals corresponding to the different types of stress-strain curves [7,9,10,14,15,23,24].…”

“…One characteristic feature of AE signals is the scale free statistics of the amplitude (or energy) in widely different physical systems such as volcanic activity [25], microfracturing process [26], thermal cycling of martensites [27][28][29], peeling of an adhesive tape [30][31][32] and plastic deformation of materials exhibiting homogeneous yield [7,9,10] as well as discontinuous flows [10,23,24]. In the context of plastic deformation, the statistics of the recorded intermittent AE signals obtained from an uniaxial compressive creep of ice crystals, single and polycrystalline samples of hcp and fcc structures [10] follow power law distributions.…”

“…In the context of plastic deformation, the statistics of the recorded intermittent AE signals obtained from an uniaxial compressive creep of ice crystals, single and polycrystalline samples of hcp and fcc structures [10] follow power law distributions. Surprisingly, even in the case of the PLC effect, power law distributions for the AE energies are reported for all the PLC bands [10,19,24].…”

“…In the context of plastic deformation, the statistics of the recorded intermittent AE signals obtained from an uniaxial compressive creep of ice crystals, single and polycrystalline samples of hcp and fcc structures [10] follow power law distributions. Surprisingly, even in the case of the PLC effect, power law distributions for the AE energies are reported for all the PLC bands [10,19,24]. Interestingly, while the scaling region for the AE energies corresponding to the type A bands has the least scaling regime, it is only for the type A band that power law distributions for the magnitudes and durations of stress drops have been reported [23,[33][34][35].…”